Finally. I now understand why I don't understand. This whole fast/slow/gathers more light nonsense has had me confused for ages. Of course it's gathering more light, it's gathering more sky. So when the "experts" tell me my C8 is too slow, and I need a refractor that's fast because imaging will be easier, because it gathers more light... it's actually all nonsense. Because the object I want to look at is only emitting the same amount of light, and it's not going at a different speed. The lower f-ratio scope is gathering more light... but it's light that I didn't even want anyway, because that stuff is not the stuff I'm looking at! That's peripheral stuff. Why would I want to gather more light, that's not the light coming from the thing I want to look at?!
Ah yes and no. You need to differentiate between two different types of light harvesting. One is the light you collect from a single pointsource. For this the rule is simple: the bigger the aperture of the telescope the more light you gather :-) If it comes to areal luminosity - for example if you image a huge nebula - the "peripheral" photons are actually what you want to image :-) here, too, the aperture plays into account because it's a focal RATIO including the aperture. For your case: if you image a tiny galaxy on f20 this will for sure result in a low areal luminosity BECAUSE there are less photons coming from this direction. If you keep the aperture and reduce the focal length of your scope (so lower the f ratio to say F5) you will collect additional photons coming from the sides (see my video about "science behind telescopes) BUT not more from that same (now small and nearly point like) source that is your tiny galaxy. How should it, your aperture is still the same. To put it into a nutshell: the bigger picture is always brighter than the rabbit hole. Cheers and clear skies!!
@@catchingphotons Thanks! Someone told me that telescopes don't magnify, that's what the eyepiece does, but isn't focussing the image onto the sensor actually magnifying it?
This dude here (Chris) should be a called "the professor" for kind of astro tutorials, you will never find any tutorial in the whole youtube such as this series to get into astro and astrophotography... really methodological and very well structured videos and easy to understand.... forget all dudes out there who will confuse you and mostly sell you stuff, this is way ahead and at least will give you a huge boost to start... thanks Chris.
Nice video ! It makes sense but it's basically still the same with photography that the wider the aperture the more light you get (plus depth). I always like to think about aperture as just being able to see a object from different positions but mashed up in one single and so the wider the more of an objects light can be summed up.
Great video! really great explanation. it does seem you do a good job of explaining advanced concepts, but not dumbing it down to much. thanks! i'll be watching all your other videos now..
Also a longer focal length can help add more detail to the image that a short f ratio wont give. Its the same as say looking at Orion Nebula, a lower power will show a brighter image, but lower detail add some magnification you start getting more detail but smaller field of view. Works the same way imaging
Thank you - beautifully explained. I hope you cover the third important element in this field, namely resolution, in a later episode - in fact I'll check now!
Changing the focal ratio will not change the brightness when viewed at the same magnification. People think it gets a brighter or dimmer if you use the same eyepiece because you are changing the magnification as you change focal length. The brightness will remain constant if you keep the same magnification as you change focal length either up or down. You will have to change eyepieces to do this but the same relative brightness at the same Madden magnification will be constant regardless of what ratio.
I'm not team visual but please help me out: If I use two scopes: A) 750mmFL and 80mmAP and B)750mmFL and 250mmAP. Using the same eyepiece the magn. should be same because FL is equal. But with scope B I collect like 250^2/80^2~10x the amount of light. Why shouldn't the image be brighter? It's the whole trick of large mirrors... Clear skies!
@@catchingphotons: the brightness of the image depends on two things. How much life the telescope is collecting overall and how far out you were spreading the light by meaning how much are magnifying the light. If you were using to telescopes that is that is the same temperature, the same objects when viewed at the same magnification in each scope will be the same brightness. The only difference will be the size of the eyepiece you have to use to get the same magnification in a scope. If you have a scope that is that has a 1000 mm focal length and you magnify say Jupiter 200 times, it's going to be the same brightness as if you magnify it 200 times in a scope that is that has only a 500 mm focal length if both telescopes have the same Adventure. If they have different African Earth, to attain the same brightness with the same image, you will have to magnify them different amounts. For example if you are using an 8 inch telescope I'm side by side with a 12-inch telescope, you will have to use less magnification with the eight biggest hits ringing in left light. You will have to use the 4-inch magnification then. If you say I'm going to magnify 30x per inch, that would be 240x for the 8 (30x8) and 360x for the 12 (30x12) to get the same brightness. Somerset that's focal Ratio or a relative focal length determines brightness but that isn't the case. That gesture spreads out the lighting more or less but when viewed at the same magnifications relative to aperture the brightness will be the same.
Also, the easiest way to compare the brightness level between two size telescopes is to divide the larger aperture by the smaller aperture and then Square the answer.
Nice insight...keep exploring. One question, in the Cadioptric telescope the effective distance of focus gets shortened. How do you calculate F ratio there.
Do they? Scopes like SCT "fold" the light path but then it's just a matter of addition. It's only the tube that is shortened to my understanding. Please correct me if I'm wrong. Cheers 🥂!
Thanks for the explanation! It was really helpful. I'm making a Newtonian reflector telescope at home. I got two options for buying primary mirror. Both with diameter 203mm but different focal lengths, 750mm and 1200mm. That makes f ratios f/3.7 and f/5.9. Surprisingly both mirrors are having same price. Confused... which one I should buy?
HalfMVsquared might be right. Also: The f3.7 mirror might be of slightly lower quality because it's harder to grind a low-f-ratio mirror (steeper slopes with demanding for high precision). But that's just educated guessing. Clear skies!
Visual observations only it is very hard to beat a 150mm F/15 Refractor on the Solar system objects At 150mm the seeing remains more stable most of the time. The visual images in my Long Focus Refractor look very nice indeed. Afocal images with an iphone 8 are very nice and no stacking of images. However what the Human Eye sees is still better the most cameras. Now days it is all about letting the camera have all the fun. Very nice presentation.
Heavy light pollution limits your raw exposure time but filters - especially narrow band - can give you a huge advantage! One way or the other: a faster scope will give you a chance for shorter exposures with the same amount of data captured. I wish you the best! Clear skies!!
@@catchingphotons Yes, I ended up subscribing last night after watching more of your videos. I'm making a cheat sheet and reference guide later today. Keep up the good work 👍🏻
If you want to do deep sky visual you are looking for the widest aperture and the lowest f/ratio you can get. I'd go with the f5 @650mm FL scope. The 1000mm FL will give you a much smaller field of view (~magnification) but for deep sky magnification is not what we want. Fire planetary sure thing but but with deep sky stuff. With shorter FL you can find objects easier, too (because you don't get "lost in space" all the time :o) Cheers and clear skies!!
Say I had two telescopes scope one and scope two. Scope one has an apperture of 300mm with a focal length of 1200mm and scope two has an apperture of 150mm with a focal length of 600mm, they both have the same f ratio of 4. What would the difference be between these two scopes? Would they be the same brightness but the 300mm would be more "zoomed in"? If I had eye pieces that let me get 60x magnification on each one would the 300mm appeture be brighter when at 60x magnification?
Hey! There is a new video online where we talk about different telescopes ;-) maybe give it a try? My suggestion: visual: Dobson Photography: nebula -> short refractor All-rounder -> intermedium Newton Clear skies!!
😂 LOL As a big coffee lover, I definitely recognized that sound. I can't blame you for needing one while making this video, which helped me immensely. It was so good I took notes for the quick reference guide I'm making. I'm a very critical person and you passed my test. Thank you so much.
I'm a beginner so I may just lack knowledge, but when would an amateur astronomer *specifically* want a high f ratio? F/10 or above either means a small refractor, like an 80/900 or a 90/900, or a maksutov like the 90/1250. Planets are supposedly the strong suit of these, but they can't really even get the best magnifications for that (200+) because that's just way over the aperture's theoretical limit. So why not get a shorter refractor or better yet, a newtonian? Something like a 130/650 tabletop dobsonian is still very cheap but actually capable of high magnifications at good quality, plus it shows much more of the deep sky. I guess I understand an MC if compactness is number one priority, but those start getting much more expensive than newtonians of the same aperture very quickly.
You are very right. For beginner's it's mainly about money and simplicity. A cheap refractor (with simple glass or even plastic lenses) NEEDS the long focal length to avoid chromatic aberration. In the same instance the refractors are more easy because they don't need to be collimated and people like that. I'm totally on your side: never use a cheap - thus long - refractor - just choose the Dobson. Collimation is no Wizards work and the bigger aperture is worth every penny. And with as decent Barlow lens your gain the same focal length with still more aperture. Clear skies!
I am new in the astrophotography, I have nexstar 5slt with asi 294. That scope has a F/10. I discovered I couldn't get all nebulas or galaxies close to my camera, they're far away. So I use Barlow, but that makes my F ratio higher. Do I have to capture longer? 🤔 like 1 hour?
Hey Stefan, is it the scope with the 1270mm focal length? This should be more than enough - most times even too much - for nebula. I don't really know what you mean by "close to the camera" but I highly recommend installing "Stellarium" for PC and enter the specs of your scope and the camera. I did a tutorial about Stellarium. It's a cool software. You can then look at all the objects in the night sky and see if your focal length and camera fits the object. F10 is fairly slow. Faint DSOs are faint - not necessarily small. Imaging those require focal lengths about 500-1000 and minute long exposures stacked to hours of integrated time for each object. My tip: your scope is great for lunar and planetary especially if you have a Barlow to increase. You can use an eyepiece and your smartphone to take great images. Look my latest video about smartphone photography. Clear skies!!
@@catchingphotons okay thx for the info. The scope is 1270mm yes, but I am going to buy this year a skywatcher eq6 r pro mount and the William optics scope with f5 ratio
But in the meantime go ahead and take some cool images of the moon, the craters and the planets (if accessible from your position). I'd love to see some of your images with the 1270mm scope!
Oh No! Why did you show a RASA! Now I have to get one!!! I added a focal reducer to my C90 and now I hardly ever want to take it off! I believe it turns a f14 to a f9.
Haha :-) I know the feeling. The RASA (or equivalent) will be on my shopping list one day. But so is a good planetary scope (c11 or MAK or something) and a small traveller scope (
im sorry fot dumb question mby... I am not english speaker and i am new to telescopes (watching these videos to teach my dad who is looking for telescope, and planning to translate him these important informations). Anyway, is this video about taking pictures with thelescope (mentioning image)...do you actually just mean the view that is being reflected for him to just look "into" telescope. I am sorry if i dont make any sense while writing it, but i hope you can help me out. also quick little question, he has been looking at one second hand telescope that seller is saying is used one "season", its Seben Big Boss 1400-150 6" EQ3 Reflector Telescope, its selling for very cheap 150 EUR (175 USD)
Hey Selina, Your question is not dumb because the nomenclature can be confusing at first. When talking about focal length and aperture in this video we are talking about basic properties of telescopes no matter whether you a look through the telescope visually or use this telescope for astrophotography. The thumb rule is: the longer the focal length the narrower is the Field of view. That means a higher magnification with the same eyepiece when using visuall or a smaller patch of the night sky when using for imaging. About your second question: I can't judge the pricing because I don't know the brand but the focal length will restrict this telescope when using with a cheap amount for planetary imaging/ visual observation "only". That means: watching the moon, watching craters on the moon, watching the planets, watching Saturn's rings, watching Jupiter's moons: all that is fine. For deep sky photography, so imaging nebulous or huge galaxies in the night sky, you will need a shorter focal length scope and a robust and capable tracking mount. That said: it is possible to get first results using a DSLR lens and a sturdy tripod only to get a first grip on this hobby. Just give it a go and see what's possible!
Correct me if i'm wrong, but a fast and a slow telescope collect exactly the same amount of light at the same magnification if their aperture are the same? The only difference would be your ability to have lower mag with a fast telescope (and higher with a slow, if your aperture is large enough).
Yea I thought and discussed about that a lot. If you pic one single point on the sensor say directly in the middle your point is valid. Light rays come from straight ahead and hence your aperture is the same the same amount of light rays will enter the scope. But if you're factor in the focal length and areal luminosity the situation gets different. Imagine a long focal length scope: a certain patch of Sky will be projected onto your camera sensor. From this patch of Sky a certain amount of photons will hit your scope. If you decrease the focal length then you project a larger patch of Sky onto your cameras sensor. There are just more photos coming from this larger area. Your telescopes aperture stays the same but the angle from where the photons can still hit your sensor will get bigger the shorter your focal length gets. So the aerial luminosity will increase with decreased focal length. This is actually still true for stars I think because even stars do not appear point like in our telescope but are slightly smeared. Please correct me if I do have any misconceptions. Anyways: clear skies!!
@@catchingphotons I don't have enough expertise in this area to be certain, but my thought process is that aperture rules the light collected. In your example you say a fast scope will capture light from a larger area. Is that true even with the same magnification (not the same eyepice)?
Nice video except for the coffee gulping. A blemish for sure on an otherwise smooth production. Surely in your next video you can wait till your done. 👍
To get down the f-ratio you will need a reducer since the aperture is fixed. 0.5x reducer will cut the focal length in half and turn f13 to f6.5 and so on. Downside: the more you bend optical physics the larger any imperfections will get. Aberrations will increase and 60mm scopes will come to the edge of possible "area/circel of illumination" and the corners will get dark. So there is a limit on how far you can push that. 800mm f14 is for every level planetary I'd say. I'd suggest you look into that field first. Clear skies!!
Hey dude! Do you mean you create your own mirror?? Epic! I have no idea but rule of thumb: the lower the F-ratio the more precise the mirror needs to be. So if I'd create a mirror from scratch (crazy though) I'd keep it slightly higher (>f7 or something)... Do you have experience in grinding mirrors? Will it be parabolic or spherical?
Sry for the late response, was "out of business". 4.5 is considered "fast" but it all depends on what you want to do with it..what's your main use case?
So I have a Cassegrain focal length - 80" (2000mm) and Clear aperture - 8" (200mm) so an f/10 is this OK ? its an Old Celestron Super C8 Plus I want to be able to see galaxies, nebulas, planets, but I don't seem to be able to thus far. I have Magnification - 77.9x (26mm eyepiece).
Hey Chuck! Frankly: with your scope you won't to be able to view any fancy DSO other than maybe the core of the Andromeda Galaxy or the blurry centers of globular clusters. People tend to over-rate what's possible within visual astronomy. BUT I'm team imaging, so please take my words with a grain of salt. 2000 mm is a very long focal length. Please download stellarium for PC and enter your scope specification in order to get a better understanding of what's possible with this focal length. Most deep sky objects of very large and won't fit into your field of view. The exceptions though: right now it's Galaxy season and those tiny dots or best suited for long scopes. Are there very dim you won't see them visually. With your scope you have two options: A) choose a very (!!!) Sturdy and expensive mount like the eq6 ora eq8 from skywatcher and use your scope for smaller deep sky objects. F10 when will mean that you you have to spend multiple hours worth of exposure time for each target because your scope is considered to be slow. B) my recommendation is is to use a modest mount and target the moon and the planets with your scope. The wide aperture of 200 mm and the large native focal length in combination with a high quality barlow lens will enable you to zoom into to the tiny planets in the night sky and squeeze out impressive details. If you don't want to invest into a planetary camera you can use an adaptor for your smartphone. I did a video about smartphone adaptors. Planetary imaging does not require a highly precise moment and hence might be easier for beginners with a long focal length scope. Deep Sky photography with high focal length is a topic for astro experts. As a beginner I wouldn't choose a scope over 750mm focal length as my first choice for deep sky imaging. Using your scope visually you will be delighted to observe high-quality details of the moon and the planets. I will never forget seeing the rings of Saturn and the cassini Division or the moons of Jupiter and Jupiter's swirling bands of cloud for the first time. Please keep in mind that deep sky and planetary are two very distinct fields of this hobby. I did a video about the differences between planetary and deep Sky, maybe you'll give it a go. Anyway: I wish you lot of fun with your scope whatever you choose to do and if your eyeball to do some imaging please take me on Twitter, I'd be delighted to see your results. Clear skies and see you around next time! -Chris
Hi dude, Team visual observations or team astro-photography? So: F750 is the same focal length as I have. It will give you (depending on the sensor) a intermediate range of sight. For example. The moons fits into my FOV approximately twice, using a old 4/3' sensor (Olympus). For planets I use a 3x barlow to get up tp 2.2m focal length. My tip: Use "Stellarium" and enter your scope and your camera specs. Than go through the Messiere catalog and other famous objects (horse head and stuff) and see how the objects appear. That gives you a first orientation. D203 is slightly bigger than my 6" scope (150) and will enable you to gather light even faster. Downside (!): You are @ f/3.7 and that is a pretty fast system! Make sure your optics are of best quality. You will definitely need a coma corrector with this setup - the faster the system the higher is the impact of optical imperfections... Go and play :-) Greetings Chris
Thanks Chris its my first telescope that I'm building. So just wanted your opinion as I don't know much about this game yet. Can't wait to try it out in a couple of days. Made the tube from fibreglass and the spider from aluminium.
Wow! That's quite an undertaking! I wish you success and clear skies! As I said: Stellarium is a great way to get a feeling for the FOV of systems. Watch out to set the focus point in such a way that you are able to mount cameras such as DSLRs. Want to link some images of your scope? I'd love to see some! Greetings Chris
Hi Crhis your a great tutor. Would love to share a link. I'm actual recording my build which should be finished by next week for my channel. With regards to the focuser placement, I'm doing the following and your input would be appreciated. The focal length is 750mm and the radius of my tube is 130mm; the hight of my viewing plan (focuser) is 80mm there for im putting my fucuser 210mm back from my focal lenth as the secondary mirror will bend the light 90 degrees. This should put it in its optimal position, if I'm not mistaken?
@@catchingphotons Hey Chris, Its been along time but I finally managed to complete my video on how i built my Newtonian telescope. come check it out. I'm quit happy with it and used alot of information from your channel also. th-cam.com/video/IubRsQdxwxg/w-d-xo.html
Hello, I am looking to buy either a TS Optics Telescope N 154/600 Photon or a Skywatcher Telescope N 150/750 PDS Explorer BD OTA. Both are parabolic The TS is an f/4 and the Skywatcher is an f/5 as its got a focal length of 750. I do not understand this and would like to know hat is the best scope to pick What are the pros and cons between an f/4 and f/5 and having different focal lengths? I plan on using an EQ5 goto mount
Hey Wayne, I recommend installing the free software "Stellarium" and use the FOV tool to try different focal lengths, for example with a Canon APSC sensor... This way you can try the effect of different focal lengths on different objects. astronomy.tools/calculators/field_of_view/ This website is another good option for exploring FOVs. In any case: F4 means slightly faster light gathering capabilities, but demands for higher quality of the optics. 600 FL can be useful sometimes because with my 750 some larger objects just won't fit into the FOV. Anyway: I own the Skywatcher 150/750 PDS and I am very happy with this mount. Clear skies!!
Hi, Thank you for your video. All of them were informative. I learn a lot. Although my doubt is: I was thinking of buying 8" f/5 1000mm focal length astrograph, I am interested in observation as well as for photography. I saw f/5 is best for both with less problem of coma and collimation but I am confused on its focal length as per your video. As you said for DSO 750 mm is best as FOV is getter in it than in 1000mm. Can't I use reducer for it? or please advise me pros and cons for going with 1000mm focal length with above telescope. Thank you in advance.
Hey Raju, yes of course: F/5 is fine for you if you want to go imaging anyway. The aperture will give you access to lots of photos :-) Concerning the focal length: use Stellarium or an "astro FOV tool" like www.skyatnightmagazine.com/astronomy-field-view-calculator/ and enter your scopes specs. You can compare different focal lengths on different deep sky objects (M-catalog). You will see that most objects will be way too big for 1000mm. Also: Using a 1000mm focal length scope demands for a VERY solid mount (EQ6 at least) in order to keep it steady. The longer the FL the heavier the scope and the more demanding precision is... Feel free to ask any other question! Clear skies!!
@@catchingphotons thank you for your instant reply. Can you suggest me some scope with 8" f/5 750mm focal length or similar? What do you say f/4 for observing? And what difference will be there for 6" and 8" from its lens?
If you want to get started with astro you will need to set your focus. Planetary or deep sky? If your main interest will be imaging deep sky objects then I'd recommend a short 400mm refractor. If you want to image planets get yourself a 1500mm Dobson or a MAK. My Skywatcher 750 (6") is something like a middle ground with benefits and disadvantages in both directions: too long for some DSO and too short for really good planetary. Anyway: I like my scope but advice you that you NEED at least an EQ6 (1500€ upwards) to hold it stable. That's a benefit with short refractors: small scope small mount. Anyway: clear skies!!
Your so limited to eyepiece upgrades in fast scopes like my orion spaceprobe 130st and it's a 1.25 focuser and just in case there's coma, I cant get a 1.25 paracorr like you could before. Aye that rhymed!
Audience is split in have. Please be aware that there is a remastered version of every "drinking" video without any distracting sounds available. Thanks for the feedback though. Clear skies!
Finally. I now understand why I don't understand.
This whole fast/slow/gathers more light nonsense has had me confused for ages. Of course it's gathering more light, it's gathering more sky.
So when the "experts" tell me my C8 is too slow, and I need a refractor that's fast because imaging will be easier, because it gathers more light... it's actually all nonsense.
Because the object I want to look at is only emitting the same amount of light, and it's not going at a different speed. The lower f-ratio scope is gathering more light... but it's light that I didn't even want anyway, because that stuff is not the stuff I'm looking at! That's peripheral stuff.
Why would I want to gather more light, that's not the light coming from the thing I want to look at?!
Ah yes and no.
You need to differentiate between two different types of light harvesting.
One is the light you collect from a single pointsource. For this the rule is simple: the bigger the aperture of the telescope the more light you gather :-)
If it comes to areal luminosity - for example if you image a huge nebula - the "peripheral" photons are actually what you want to image :-) here, too, the aperture plays into account because it's a focal RATIO including the aperture.
For your case: if you image a tiny galaxy on f20 this will for sure result in a low areal luminosity BECAUSE there are less photons coming from this direction. If you keep the aperture and reduce the focal length of your scope (so lower the f ratio to say F5) you will collect additional photons coming from the sides (see my video about "science behind telescopes) BUT not more from that same (now small and nearly point like) source that is your tiny galaxy. How should it, your aperture is still the same.
To put it into a nutshell: the bigger picture is always brighter than the rabbit hole.
Cheers and clear skies!!
@@catchingphotons Thanks! Someone told me that telescopes don't magnify, that's what the eyepiece does, but isn't focussing the image onto the sensor actually magnifying it?
This dude here (Chris) should be a called "the professor" for kind of astro tutorials, you will never find any tutorial in the whole youtube such as this series to get into astro and astrophotography... really methodological and very well structured videos and easy to understand.... forget all dudes out there who will confuse you and mostly sell you stuff, this is way ahead and at least will give you a huge boost to start... thanks Chris.
Thanks dude for your encouraging words! I hope my videos will be of any help!
Greetings and clear skies! - Chris
Great explanation. Finally starting to understand scope specs.
Thank you Jim!
Turn it up to 11! Lol thank you for the video. I show this to everyone who tries to understand telescopes.
Haha, thanks so much for your encouraging words! Much appreciated!
Clear skies!
This was incredibly helpful. Thank you.
Thank you very much, Andrew!
Greetings
Chris
Schönes Video, vor allem für Einsteiger ist das sicher hilfreich. LG Justus
Thanks! I hope beginners can take away some basic thumbrules from this videos! That would be awesome!
Greetings
Chris
Best tutorials I've seen... Please keep up the awesome work.
Thanks a lot for those encouraging words!
Nice video ! It makes sense but it's basically still the same with photography that the wider the aperture the more light you get (plus depth). I always like to think about aperture as just being able to see a object from different positions but mashed up in one single and so the wider the more of an objects light can be summed up.
Thanks that's definitely a way of thinking about the problem. Cheers 🥂
thank you foe making the video , no onw was willing to make
Thanks Yash, you are very welcome!
Thank you, Chris!
Thank you, Jon!! ;-)
Great video! really great explanation. it does seem you do a good job of explaining advanced concepts, but not dumbing it down to much.
thanks! i'll be watching all your other videos now..
Thank you Lance for that encouraging comment!
Also a longer focal length can help add more detail to the image that a short f ratio wont give. Its the same as say looking at Orion Nebula, a lower power will show a brighter image, but lower detail add some magnification you start getting more detail but smaller field of view. Works the same way imaging
Well framed Dennis. Clear skies!
best breakdown of f ratio on youtube
Thank you Chico Rebelp! That's very kind of you :-)
Greetings, Chris
great work and presentation, many thanks
You are very welcome!
Thank you - beautifully explained. I hope you cover the third important element in this field, namely resolution, in a later episode - in fact I'll check now!
I will - thanks for your comment!
What a great video! Perfect for a newbie like me! Thanks for the series of tutorial videos
Thanks dominique for the encouraging comment! The series will be continued :-)
Changing the focal ratio will not change the brightness when viewed at the same magnification. People think it gets a brighter or dimmer if you use the same eyepiece because you are changing the magnification as you change focal length. The brightness will remain constant if you keep the same magnification as you change focal length either up or down. You will have to change eyepieces to do this but the same relative brightness at the same Madden magnification will be constant regardless of what ratio.
I'm not team visual but please help me out:
If I use two scopes: A) 750mmFL and 80mmAP and B)750mmFL and 250mmAP.
Using the same eyepiece the magn. should be same because FL is equal.
But with scope B I collect like 250^2/80^2~10x the amount of light. Why shouldn't the image be brighter? It's the whole trick of large mirrors...
Clear skies!
@@catchingphotons: the brightness of the image depends on two things. How much life the telescope is collecting overall and how far out you were spreading the light by meaning how much are magnifying the light. If you were using to telescopes that is that is the same temperature, the same objects when viewed at the same magnification in each scope will be the same brightness. The only difference will be the size of the eyepiece you have to use to get the same magnification in a scope. If you have a scope that is that has a 1000 mm focal length and you magnify say Jupiter 200 times, it's going to be the same brightness as if you magnify it 200 times in a scope that is that has only a 500 mm focal length if both telescopes have the same Adventure. If they have different African Earth, to attain the same brightness with the same image, you will have to magnify them different amounts. For example if you are using an 8 inch telescope I'm side by side with a 12-inch telescope, you will have to use less magnification with the eight biggest hits ringing in left light. You will have to use the 4-inch magnification then. If you say I'm going to magnify 30x per inch, that would be 240x for the 8 (30x8) and 360x for the 12 (30x12) to get the same brightness. Somerset that's focal Ratio or a relative focal length determines brightness but that isn't the case. That gesture spreads out the lighting more or less but when viewed at the same magnifications relative to aperture the brightness will be the same.
Also, the easiest way to compare the brightness level between two size telescopes is to divide the larger aperture by the smaller aperture and then Square the answer.
This is really helpful
Thanks a lot!!
Nice insight...keep exploring. One question, in the Cadioptric telescope the effective distance of focus gets shortened. How do you calculate F ratio there.
Do they? Scopes like SCT "fold" the light path but then it's just a matter of addition. It's only the tube that is shortened to my understanding. Please correct me if I'm wrong. Cheers 🥂!
thanks, first time I really understood this.
Thanks for the comment J Barry! Glad the video was of any help! Clear skies!
Amazingly helpful! Cheers boss!
Thanks a lot!!
Great video mate!
Well spoken and edited!
Thanks a lot Rick!
A great explanation ...thank you 🌹
You are very welcome! Thanks for this encouraging comment!
Great videos, truly helping
Thanks! Glad it was of any use. Clear skies!!
Thanks for the explanation! It was really helpful. I'm making a Newtonian reflector telescope at home. I got two options for buying primary mirror. Both with diameter 203mm but different focal lengths, 750mm and 1200mm. That makes f ratios f/3.7 and f/5.9. Surprisingly both mirrors are having same price. Confused... which one I should buy?
HalfMVsquared might be right.
Also: The f3.7 mirror might be of slightly lower quality because it's harder to grind a low-f-ratio mirror (steeper slopes with demanding for high precision). But that's just educated guessing.
Clear skies!
the drinking parts really kills this otherwise great tutorial
Thanks! There are drink free versions available. Cheers!
Visual observations only
it is very hard to beat a
150mm F/15 Refractor on the
Solar system objects
At 150mm the seeing remains more stable most of the time.
The visual images in my
Long Focus Refractor look very nice indeed.
Afocal images with an iphone 8 are very nice and no stacking of images.
However what the Human Eye sees is still better the most cameras.
Now days it is all about
letting the camera have all the fun.
Very nice presentation.
Thanks for your insights! Visual autonomy is a very rewarding field. Cheers 🥂
I wait a f10 C8 edge hd I live in red zone so f10 is good for that beacause my sky is pollued
Heavy light pollution limits your raw exposure time but filters - especially narrow band - can give you a huge advantage!
One way or the other: a faster scope will give you a chance for shorter exposures with the same amount of data captured.
I wish you the best! Clear skies!!
Wow, this is really helpful video.
Glad you liked it! Clear skies!
@@catchingphotons Yes, I ended up subscribing last night after watching more of your videos. I'm making a cheat sheet and reference guide later today. Keep up the good work 👍🏻
Thank you. Shoul i buy a 130/650 or a 130/1000 if I want to watch deep sky objects?
If you want to do deep sky visual you are looking for the widest aperture and the lowest f/ratio you can get. I'd go with the f5 @650mm FL scope. The 1000mm FL will give you a much smaller field of view (~magnification) but for deep sky magnification is not what we want. Fire planetary sure thing but but with deep sky stuff. With shorter FL you can find objects easier, too (because you don't get "lost in space" all the time :o)
Cheers and clear skies!!
Say I had two telescopes scope one and scope two. Scope one has an apperture of 300mm with a focal length of 1200mm and scope two has an apperture of 150mm with a focal length of 600mm, they both have the same f ratio of 4. What would the difference be between these two scopes? Would they be the same brightness but the 300mm would be more "zoomed in"? If I had eye pieces that let me get 60x magnification on each one would the 300mm appeture be brighter when at 60x magnification?
*sips coffee* ☕ This is very helpful!
Looking for a deep sky telescope for beginners
any suggestions? O.o
Hey! There is a new video online where we talk about different telescopes ;-) maybe give it a try?
My suggestion:
visual: Dobson
Photography: nebula -> short refractor
All-rounder -> intermedium Newton
Clear skies!!
View the new version with enhanced audio and no coffee:
th-cam.com/video/ylKU-dVHnaE/w-d-xo.html
😂 LOL As a big coffee lover, I definitely recognized that sound. I can't blame you for needing one while making this video, which helped me immensely. It was so good I took notes for the quick reference guide I'm making. I'm a very critical person and you passed my test. Thank you so much.
@@spamcan9208 Thanks for this encouraging comment! Means a lot to me. Clear skies man!
No coffee = no fun. Coffee must stay. :) BTW, really great tutorial
I'm a beginner so I may just lack knowledge, but when would an amateur astronomer *specifically* want a high f ratio? F/10 or above either means a small refractor, like an 80/900 or a 90/900, or a maksutov like the 90/1250. Planets are supposedly the strong suit of these, but they can't really even get the best magnifications for that (200+) because that's just way over the aperture's theoretical limit. So why not get a shorter refractor or better yet, a newtonian? Something like a 130/650 tabletop dobsonian is still very cheap but actually capable of high magnifications at good quality, plus it shows much more of the deep sky. I guess I understand an MC if compactness is number one priority, but those start getting much more expensive than newtonians of the same aperture very quickly.
You are very right.
For beginner's it's mainly about money and simplicity. A cheap refractor (with simple glass or even plastic lenses) NEEDS the long focal length to avoid chromatic aberration. In the same instance the refractors are more easy because they don't need to be collimated and people like that.
I'm totally on your side: never use a cheap - thus long - refractor - just choose the Dobson. Collimation is no Wizards work and the bigger aperture is worth every penny. And with as decent Barlow lens your gain the same focal length with still more aperture.
Clear skies!
I am new in the astrophotography, I have nexstar 5slt with asi 294. That scope has a F/10. I discovered I couldn't get all nebulas or galaxies close to my camera, they're far away. So I use Barlow, but that makes my F ratio higher. Do I have to capture longer? 🤔 like 1 hour?
Hey Stefan,
is it the scope with the 1270mm focal length? This should be more than enough - most times even too much - for nebula.
I don't really know what you mean by "close to the camera" but I highly recommend installing "Stellarium" for PC and enter the specs of your scope and the camera. I did a tutorial about Stellarium. It's a cool software. You can then look at all the objects in the night sky and see if your focal length and camera fits the object.
F10 is fairly slow. Faint DSOs are faint - not necessarily small. Imaging those require focal lengths about 500-1000 and minute long exposures stacked to hours of integrated time for each object.
My tip: your scope is great for lunar and planetary especially if you have a Barlow to increase. You can use an eyepiece and your smartphone to take great images. Look my latest video about smartphone photography.
Clear skies!!
@@catchingphotons okay thx for the info. The scope is 1270mm yes, but I am going to buy this year a skywatcher eq6 r pro mount and the William optics scope with f5 ratio
But in the meantime go ahead and take some cool images of the moon, the craters and the planets (if accessible from your position). I'd love to see some of your images with the 1270mm scope!
Oh No! Why did you show a RASA! Now I have to get one!!! I added a focal reducer to my C90 and now I hardly ever want to take it off! I believe it turns a f14 to a f9.
Haha :-) I know the feeling. The RASA (or equivalent) will be on my shopping list one day. But so is a good planetary scope (c11 or MAK or something) and a small traveller scope (
@@catchingphotons Well now I am just waiting for a C6 to be available again so I can put a HyperStar on it. just imagine a portable F/2 setup!
im sorry fot dumb question mby... I am not english speaker and i am new to telescopes (watching these videos to teach my dad who is looking for telescope, and planning to translate him these important informations). Anyway, is this video about taking pictures with thelescope (mentioning image)...do you actually just mean the view that is being reflected for him to just look "into" telescope.
I am sorry if i dont make any sense while writing it, but i hope you can help me out.
also quick little question, he has been looking at one second hand telescope that seller is saying is used one "season", its Seben Big Boss 1400-150 6" EQ3 Reflector Telescope, its selling for very cheap 150 EUR (175 USD)
Hey Selina,
Your question is not dumb because the nomenclature can be confusing at first.
When talking about focal length and aperture in this video we are talking about basic properties of telescopes no matter whether you a look through the telescope visually or use this telescope for astrophotography.
The thumb rule is: the longer the focal length the narrower is the Field of view. That means a higher magnification with the same eyepiece when using visuall or a smaller patch of the night sky when using for imaging.
About your second question: I can't judge the pricing because I don't know the brand but the focal length will restrict this telescope when using with a cheap amount for planetary imaging/ visual observation "only".
That means: watching the moon, watching craters on the moon, watching the planets, watching Saturn's rings, watching Jupiter's moons: all that is fine.
For deep sky photography, so imaging nebulous or huge galaxies in the night sky, you will need a shorter focal length scope and a robust and capable tracking mount.
That said: it is possible to get first results using a DSLR lens and a sturdy tripod only to get a first grip on this hobby.
Just give it a go and see what's possible!
Correct me if i'm wrong, but a fast and a slow telescope collect exactly the same amount of light at the same magnification if their aperture are the same? The only difference would be your ability to have lower mag with a fast telescope (and higher with a slow, if your aperture is large enough).
Yea I thought and discussed about that a lot.
If you pic one single point on the sensor say directly in the middle your point is valid. Light rays come from straight ahead and hence your aperture is the same the same amount of light rays will enter the scope.
But if you're factor in the focal length and areal luminosity the situation gets different. Imagine a long focal length scope: a certain patch of Sky will be projected onto your camera sensor. From this patch of Sky a certain amount of photons will hit your scope. If you decrease the focal length then you project a larger patch of Sky onto your cameras sensor. There are just more photos coming from this larger area. Your telescopes aperture stays the same but the angle from where the photons can still hit your sensor will get bigger the shorter your focal length gets. So the aerial luminosity will increase with decreased focal length. This is actually still true for stars I think because even stars do not appear point like in our telescope but are slightly smeared.
Please correct me if I do have any misconceptions. Anyways: clear skies!!
@@catchingphotons I don't have enough expertise in this area to be certain, but my thought process is that aperture rules the light collected. In your example you say a fast scope will capture light from a larger area. Is that true even with the same magnification (not the same eyepice)?
Nice video except for the coffee gulping. A blemish for sure on an otherwise smooth production. Surely in your next video you can wait till your done. 👍
Thanks Eastern,
I will continue to improve quality along the way. Everything is still a bit new to me.
Clear skies!
So what do I need to speed up a 60mm refractor? Doublet. F800mm. f/13.3. Can I get it down to a f/10? Maybe faster?
To get down the f-ratio you will need a reducer since the aperture is fixed.
0.5x reducer will cut the focal length in half and turn f13 to f6.5 and so on.
Downside: the more you bend optical physics the larger any imperfections will get. Aberrations will increase and 60mm scopes will come to the edge of possible "area/circel of illumination" and the corners will get dark.
So there is a limit on how far you can push that.
800mm f14 is for every level planetary I'd say. I'd suggest you look into that field first.
Clear skies!!
Please tell me
what should be my focal length if my aperture is 12 inches and primary mirror is also 12 inches and I want to do deep sky astronomy
I'm making my own telescope
Hey dude! Do you mean you create your own mirror?? Epic!
I have no idea but rule of thumb: the lower the F-ratio the more precise the mirror needs to be. So if I'd create a mirror from scratch (crazy though) I'd keep it slightly higher (>f7 or something)...
Do you have experience in grinding mirrors? Will it be parabolic or spherical?
@@catchingphotons actually I would be buying it from my friend
@@catchingphotons it would be parabolic
I have a Bresser 102mm ED refractor with a 460mm focal length which is f/4.5. Fast?
Oh, I asked too soon.
Sry for the late response, was "out of business".
4.5 is considered "fast" but it all depends on what you want to do with it..what's your main use case?
So I have a Cassegrain focal length - 80" (2000mm) and Clear aperture - 8" (200mm) so an f/10 is this OK ? its an Old Celestron Super C8 Plus I want to be able to see galaxies, nebulas, planets, but I don't seem to be able to thus far. I have Magnification - 77.9x (26mm eyepiece).
Hey Chuck!
Frankly: with your scope you won't to be able to view any fancy DSO other than maybe the core of the Andromeda Galaxy or the blurry centers of globular clusters.
People tend to over-rate what's possible within visual astronomy. BUT I'm team imaging, so please take my words with a grain of salt.
2000 mm is a very long focal length. Please download stellarium for PC and enter your scope specification in order to get a better understanding of what's possible with this focal length. Most deep sky objects of very large and won't fit into your field of view. The exceptions though: right now it's Galaxy season and those tiny dots or best suited for long scopes. Are there very dim you won't see them visually.
With your scope you have two options:
A) choose a very (!!!) Sturdy and expensive mount like the eq6 ora eq8 from skywatcher and use your scope for smaller deep sky objects. F10 when will mean that you you have to spend multiple hours worth of exposure time for each target because your scope is considered to be slow.
B) my recommendation is is to use a modest mount and target the moon and the planets with your scope. The wide aperture of 200 mm and the large native focal length in combination with a high quality barlow lens will enable you to zoom into to the tiny planets in the night sky and squeeze out impressive details. If you don't want to invest into a planetary camera you can use an adaptor for your smartphone. I did a video about smartphone adaptors.
Planetary imaging does not require a highly precise moment and hence might be easier for beginners with a long focal length scope.
Deep Sky photography with high focal length is a topic for astro experts. As a beginner I wouldn't choose a scope over 750mm focal length as my first choice for deep sky imaging.
Using your scope visually you will be delighted to observe high-quality details of the moon and the planets. I will never forget seeing the rings of Saturn and the cassini Division or the moons of Jupiter and Jupiter's swirling bands of cloud for the first time.
Please keep in mind that deep sky and planetary are two very distinct fields of this hobby. I did a video about the differences between planetary and deep Sky, maybe you'll give it a go.
Anyway: I wish you lot of fun with your scope whatever you choose to do and if your eyeball to do some imaging please take me on Twitter, I'd be delighted to see your results.
Clear skies and see you around next time!
-Chris
Hi a quick question! What could a newtonian with D203F750 be used for? Is it any good?
Hi dude,
Team visual observations or team astro-photography?
So: F750 is the same focal length as I have. It will give you (depending on the sensor) a intermediate range of sight. For example. The moons fits into my FOV approximately twice, using a old 4/3' sensor (Olympus). For planets I use a 3x barlow to get up tp 2.2m focal length.
My tip: Use "Stellarium" and enter your scope and your camera specs. Than go through the Messiere catalog and other famous objects (horse head and stuff) and see how the objects appear. That gives you a first orientation.
D203 is slightly bigger than my 6" scope (150) and will enable you to gather light even faster. Downside (!): You are @ f/3.7 and that is a pretty fast system! Make sure your optics are of best quality. You will definitely need a coma corrector with this setup - the faster the system the higher is the impact of optical imperfections...
Go and play :-)
Greetings
Chris
Thanks Chris its my first telescope that I'm building. So just wanted your opinion as I don't know much about this game yet. Can't wait to try it out in a couple of days. Made the tube from fibreglass and the spider from aluminium.
Wow! That's quite an undertaking! I wish you success and clear skies!
As I said: Stellarium is a great way to get a feeling for the FOV of systems.
Watch out to set the focus point in such a way that you are able to mount cameras such as DSLRs.
Want to link some images of your scope? I'd love to see some!
Greetings
Chris
Hi Crhis your a great tutor. Would love to share a link. I'm actual recording my build which should be finished by next week for my channel. With regards to the focuser placement, I'm doing the following and your input would be appreciated. The focal length is 750mm and the radius of my tube is 130mm; the hight of my viewing plan (focuser) is 80mm there for im putting my fucuser 210mm back from my focal lenth as the secondary mirror will bend the light 90 degrees. This should put it in its optimal position, if I'm not mistaken?
@@catchingphotons Hey Chris, Its been along time but I finally managed to complete my video on how i built my Newtonian telescope. come check it out. I'm quit happy with it and used alot of information from your channel also. th-cam.com/video/IubRsQdxwxg/w-d-xo.html
Hello, I am looking to buy either a TS Optics Telescope N 154/600 Photon or a Skywatcher Telescope N 150/750 PDS Explorer BD OTA.
Both are parabolic
The TS is an f/4 and the Skywatcher is an f/5 as its got a focal length of 750.
I do not understand this and would like to know hat is the best scope to pick
What are the pros and cons between an f/4 and f/5 and having different focal lengths?
I plan on using an EQ5 goto mount
Hey Wayne,
I recommend installing the free software "Stellarium" and use the FOV tool to try different focal lengths, for example with a Canon APSC sensor... This way you can try the effect of different focal lengths on different objects.
astronomy.tools/calculators/field_of_view/
This website is another good option for exploring FOVs.
In any case: F4 means slightly faster light gathering capabilities, but demands for higher quality of the optics. 600 FL can be useful sometimes because with my 750 some larger objects just won't fit into the FOV.
Anyway: I own the Skywatcher 150/750 PDS and I am very happy with this mount.
Clear skies!!
@@catchingphotons Thank you so much!!
Clear skies!
Hi,
Thank you for your video. All of them were informative. I learn a lot.
Although my doubt is:
I was thinking of buying 8" f/5 1000mm focal length astrograph, I am interested in observation as well as for photography. I saw f/5 is best for both with less problem of coma and collimation but I am confused on its focal length as per your video. As you said for DSO 750 mm is best as FOV is getter in it than in 1000mm. Can't I use reducer for it? or please advise me pros and cons for going with 1000mm focal length with above telescope.
Thank you in advance.
Hey Raju,
yes of course:
F/5 is fine for you if you want to go imaging anyway. The aperture will give you access to lots of photos :-)
Concerning the focal length:
use Stellarium or an "astro FOV tool" like www.skyatnightmagazine.com/astronomy-field-view-calculator/ and enter your scopes specs.
You can compare different focal lengths on different deep sky objects (M-catalog). You will see that most objects will be way too big for 1000mm.
Also: Using a 1000mm focal length scope demands for a VERY solid mount (EQ6 at least) in order to keep it steady. The longer the FL the heavier the scope and the more demanding precision is...
Feel free to ask any other question!
Clear skies!!
@@catchingphotons thank you for your instant reply.
Can you suggest me some scope with 8" f/5 750mm focal length or similar?
What do you say f/4 for observing? And what difference will be there for 6" and 8" from its lens?
If you want to get started with astro you will need to set your focus. Planetary or deep sky? If your main interest will be imaging deep sky objects then I'd recommend a short 400mm refractor.
If you want to image planets get yourself a 1500mm Dobson or a MAK.
My Skywatcher 750 (6") is something like a middle ground with benefits and disadvantages in both directions: too long for some DSO and too short for really good planetary. Anyway: I like my scope but advice you that you NEED at least an EQ6 (1500€ upwards) to hold it stable.
That's a benefit with short refractors: small scope small mount.
Anyway: clear skies!!
Your so limited to eyepiece upgrades in fast scopes like my orion spaceprobe 130st and it's a 1.25 focuser and just in case there's coma, I cant get a 1.25 paracorr like you could before. Aye that rhymed!
Fast scopes are indeed somewhat tricky to use, no question. I feel highly comfortable with my f5 scope. Clear skies!
Nice video :) I learned again a lot. But please first drink after recording the video. It is distracting.
Hey Marc, thanks for your comment!!
There is a re-uploaded version of this video as "coffee-free" available :-) clear skies!!
Don't talk and drink, super annoying
Audience is split in have. Please be aware that there is a remastered version of every "drinking" video without any distracting sounds available.
Thanks for the feedback though.
Clear skies!