To see another comparison of the difference between wide aperture and narrow aperture at similar focal ratios, follow this link to the Sky Story Astrobin. The image is NGC 5194, shot on a 203 mm SCT and on an 81 mm refractor. The refractor image was shot in better conditions and had almost twice as much integration time. The wide aperture SCT still substantially outperformed the refractor. www.astrobin.com/bvs7mz/C/
Great video! This is something that I have realized also that focal ratio is not the main driver if you get a good image. With my 8" RC I can see that images build up as quickly as with my 70mm Apo even though the focal ratios are f/8 against f/6 meaning that the Apo should be 2/3 quicker from the focal ratio but in fact there is no difference. The small Apo was for long time my most used and most liked telescope but I am changing most of my imaging to larger aperture telescopes. I just realize that I have to use it again. Not that I don't like my small Apo anymore as it has its use-cases. So each of my telescopes is for a specific case except two which are very similar in focal length. I would not try to capture the Spaghetti Nebula with my 8" RC, my 6" Newtonian or 4" Apo. I would use the 70mm Apo as it is the use-case for such small telescope. Same with the Rosette or Seagull. Sure I could capture details with the others, do masaics but having the right scope for the target size is always more convenient. The limited number of nights with clear skies is the other downside why it does not make much sense to start a project like the Rosette in 9 or 12 panels. So have the right telescope for the use case if you are limited with clear nights. Don't look too much at the focal ratio, choose what fits your needs of the framing that you like to achieve. A larger aperture with a reducer is able to gather more photons because of is size than a smaller scope with just a flattner.
Very good points, and clear nights are perhaps the most precious resource for many people. A clear night with good seeing, low turbulence and ideally free of moonlight--they are hard to get. For myself, I decided a long time ago that images will just take as long as they take to get. Though I do intend to compensate soon by building a second observatory.
How many hours did you expose with both telescopes? I have the C8HD with x0.7 a F7 and the william optics pleiades 68mm F3.8. Both got Asi294mcpro. I did both on the Crescent for 3h with a dualband HA-OII filter. And i have more detail with my F3.8 than the C8. I need more than 12h to get a good detail in the Crescent
The refractor version of the Bubble Nebula has about 720 minutes of integration. It has a focal ratio of 5.5 with its reducer. The SCT version of the Bubble Nebula has about 750 minutes of integration. It has a focal ratio of 6.1 with its reducer.
Still thinking about the information you presented in this and the last couple of videos. To me it makes more sense to think about the unobstructed light gathering area of a telescope. Because yes resolution is by physics depending on aperture but any obstruction (secondary mirror) leads to little light loss. It also doesn’t make sense to me that two telescopes with different aperture but same light collecting area should achieve different brightness (lets say a 5 inch apochromatic refractor and a 6 inch schmidt cassegrain that have different aperture but same unobstructed area).
this is why i love your channel, this info goes right in the face of all the youtube channels, peddling the redcat 51 and all its derivatives as if its alpha and omega of astro photography
I decided a long time ago I was going to follow the science, not the trends. Though I have nothing against the Redcat. But narrow aperture is something that can't be worked around. The resolving capacity of aperture is a physical property of every telescope.
SNR and Image scale! Have a long - slow focal length big scope? Pair it with a large pixel camera or bin 2x2. Have a fast large telescope, Pair it with smaller pixels.. An F10 8" SCT and an 8" F4 Newt can image at the same speed and image scale if the pixels sizes are adjusted accordingly. They would also have the same SNR. It really comes down to camera choice-preference. I prefer my ASI533mm to other camera choices. Thus, my chosen scope would be an 8" F4 newt (or 10" F4) to image at 1 arc second (or a bit smaller)..
I agree with what you are saying in this video, but i think why people like telescopes with fast focal ratio is because you improve SNR quicker. As you said, the light is less spread out, so each pixel on the sensor gathers more light in a given amount of time, resulting in less noisy images.
You may well be right, and it may be that this is much more of an issue under light polluted skies. I honestly don't know. Living in the Canadian backwoods, I have effectively no experience dealing with manmade light pollution. The video isn't against low focal ratio telescopes, though. Rather, it attempts to rectify the idea that focal ratio is what really matters. For example: Imagine two telescopes with the same focal ratio. Double the aperture on one. That telescope will now gather information four times faster and double the SNR (if I remember my SNR math correctly).
Great debunking of misconceptions. Every astronomy fan from the times, when there were no digital cameras, knows that both the resolving power in seconds and the limiting magnitude of the telescope depend only on the entry diameter of the scope and nothing else. So it would worth mentioning not only the resolution formula, but also limiting magnitude formula which is 5*logD + const, where const depends on the receiver, whether it is basically an eye or photo sensor.
Refreshing explanatoin. Some 40 years a go, had a lesson in school about this stuf. Strange how all is still in your head and that is comes to the surface when you watch a video that stirs in your brain ;) (I have a Altair Starwave 110ED-R F7)
@@SKYST0RY Thanks, it's my first and only on. Have it for 8 years now. But first 4 years in the netherlands don't use it much. Now in Spain everytime I get the chace.
I get asked that a lot. I capture video of moving through the stars in a virtual planetarium called Space Engine, then screen composite the stars over the image in my video editor, Davinci Resolve.
@SKYST0RY all you have to do to prove that is look at the focal ratios the HST and JWST. I'm no professional, but I'm seriously starting to think that for the small objects I am interested in I really need to get myself an 8 inch f/10 and just roll with the native focal ratio. Might to get a camera with larger pixels to go with it though
@@iamjessieray I remember the very first time I ever looked at an image I had shot with a 203 mm SCT compared to my 81 mm refractor. It was eye opening. I realized that day I was pretty much done with refractors. That say, "Aperture is king," is entirely true.
@@MazzifLOL I have seen some INCREDIBLE results with 5" maks so I'm starting to thing that f-ratio is really irrelevant if you are willing to put some more time in and can fit your object of interest in your FoV
I think the main reason people think and like to say a low f ratio is faster and therefore better than a higher f ratio is to make it simpler for beginners.
Not sure what you are saying, this video is accurate enough with explanations where simplifications are made. If you doubt it, do the etendue calculations and test it in your yard if you can. It will all make sense eventually if you do. This doesn't mean that a small telescope isn't useful, but under the right conditions a large scope will out perform a small one with a million caveats of course. Etendue shows that on a per pixel level an 8" scope is an 8" scope if you match the sampling in arc seconds.
![CAMPปลิ้น | EP.82[1/2] แก๊ง 3 สาวเพื่อนซี้บุก CAMPปลิ้นครั้งนี้ห้ามเสียอาการ](http://i.ytimg.com/vi/_6WUjXHe1TM/mqdefault.jpg)
To see another comparison of the difference between wide aperture and narrow aperture at similar focal ratios, follow this link to the Sky Story Astrobin. The image is NGC 5194, shot on a 203 mm SCT and on an 81 mm refractor. The refractor image was shot in better conditions and had almost twice as much integration time. The wide aperture SCT still substantially outperformed the refractor. www.astrobin.com/bvs7mz/C/
Great video! This is something that I have realized also that focal ratio is not the main driver if you get a good image. With my 8" RC I can see that images build up as quickly as with my 70mm Apo even though the focal ratios are f/8 against f/6 meaning that the Apo should be 2/3 quicker from the focal ratio but in fact there is no difference.
The small Apo was for long time my most used and most liked telescope but I am changing most of my imaging to larger aperture telescopes. I just realize that I have to use it again. Not that I don't like my small Apo anymore as it has its use-cases. So each of my telescopes is for a specific case except two which are very similar in focal length.
I would not try to capture the Spaghetti Nebula with my 8" RC, my 6" Newtonian or 4" Apo. I would use the 70mm Apo as it is the use-case for such small telescope. Same with the Rosette or Seagull. Sure I could capture details with the others, do masaics but having the right scope for the target size is always more convenient. The limited number of nights with clear skies is the other downside why it does not make much sense to start a project like the Rosette in 9 or 12 panels.
So have the right telescope for the use case if you are limited with clear nights. Don't look too much at the focal ratio, choose what fits your needs of the framing that you like to achieve. A larger aperture with a reducer is able to gather more photons because of is size than a smaller scope with just a flattner.
Very good points, and clear nights are perhaps the most precious resource for many people. A clear night with good seeing, low turbulence and ideally free of moonlight--they are hard to get. For myself, I decided a long time ago that images will just take as long as they take to get. Though I do intend to compensate soon by building a second observatory.
How many hours did you expose with both telescopes?
I have the C8HD with x0.7 a F7 and the william optics pleiades 68mm F3.8.
Both got Asi294mcpro.
I did both on the Crescent for 3h with a dualband HA-OII filter. And i have more detail with my F3.8 than the C8. I need more than 12h to get a good detail in the Crescent
The refractor version of the Bubble Nebula has about 720 minutes of integration. It has a focal ratio of 5.5 with its reducer. The SCT version of the Bubble Nebula has about 750 minutes of integration. It has a focal ratio of 6.1 with its reducer.
Still thinking about the information you presented in this and the last couple of videos. To me it makes more sense to think about the unobstructed light gathering area of a telescope. Because yes resolution is by physics depending on aperture but any obstruction (secondary mirror) leads to little light loss. It also doesn’t make sense to me that two telescopes with different aperture but same light collecting area should achieve different brightness (lets say a 5 inch apochromatic refractor and a 6 inch schmidt cassegrain that have different aperture but same unobstructed area).
this is why i love your channel, this info goes right in the face of all the youtube channels, peddling the redcat 51 and all its derivatives as if its alpha and omega of astro photography
I decided a long time ago I was going to follow the science, not the trends. Though I have nothing against the Redcat. But narrow aperture is something that can't be worked around. The resolving capacity of aperture is a physical property of every telescope.
Hence my first proper telescope was 6 inch reflector 🪞 and love every imperfections and need to faf with collimation and all.
SNR and Image scale!
Have a long - slow focal length big scope? Pair it with a large pixel camera or bin 2x2.
Have a fast large telescope, Pair it with smaller pixels..
An F10 8" SCT and an 8" F4 Newt can image at the same speed and image scale if the pixels sizes are adjusted accordingly. They would also have the same SNR.
It really comes down to camera choice-preference.
I prefer my ASI533mm to other camera choices. Thus, my chosen scope would be an 8" F4 newt (or 10" F4) to image at 1 arc second (or a bit smaller)..
A good point. A bit down the line, I plan to go more in depth into that aspect of things.
I would rather have an off axis f/20 than an f/2 hyperstar
Another great instruction video! Thank you.
I agree with what you are saying in this video, but i think why people like telescopes with fast focal ratio is because you improve SNR quicker. As you said, the light is less spread out, so each pixel on the sensor gathers more light in a given amount of time, resulting in less noisy images.
You may well be right, and it may be that this is much more of an issue under light polluted skies. I honestly don't know. Living in the Canadian backwoods, I have effectively no experience dealing with manmade light pollution. The video isn't against low focal ratio telescopes, though. Rather, it attempts to rectify the idea that focal ratio is what really matters. For example: Imagine two telescopes with the same focal ratio. Double the aperture on one. That telescope will now gather information four times faster and double the SNR (if I remember my SNR math correctly).
Great debunking of misconceptions. Every astronomy fan from the times, when there were no digital cameras, knows that both the resolving power in seconds and the limiting magnitude of the telescope depend only on the entry diameter of the scope and nothing else.
So it would worth mentioning not only the resolution formula, but also limiting magnitude formula which is 5*logD + const, where const depends on the receiver, whether it is basically an eye or photo sensor.
I should delve into limiting magnitude some time.
this is best explanation about the f ratio i have ever seen .
Very didactic! Thanks for sharing
Refreshing explanatoin. Some 40 years a go, had a lesson in school about this stuf. Strange how all is still in your head and that is comes to the surface when you watch a video that stirs in your brain ;) (I have a Altair Starwave 110ED-R F7)
To my knowledge, short of brain damage we don't really forget things. They just get lost in "filing" lol I had to look up that telescope. Looks nice.
@@SKYST0RY Thanks, it's my first and only on. Have it for 8 years now. But first 4 years in the netherlands don't use it much. Now in Spain everytime I get the chace.
hello,
very good video.
how do you make the tracing stars on the photo?
thank you
I get asked that a lot. I capture video of moving through the stars in a virtual planetarium called Space Engine, then screen composite the stars over the image in my video editor, Davinci Resolve.
@@SKYST0RY merci 😊
My "slow" Explore Scientific 127ED F/7.5 produces some of the best images for me.
I'm not surprised. I find f/5.5 to f/10 to be a sweet spot. Professional telescopes may have focal ratios in excess of 20.
@SKYST0RY all you have to do to prove that is look at the focal ratios the HST and JWST. I'm no professional, but I'm seriously starting to think that for the small objects I am interested in I really need to get myself an 8 inch f/10 and just roll with the native focal ratio. Might to get a camera with larger pixels to go with it though
@@iamjessieray I remember the very first time I ever looked at an image I had shot with a 203 mm SCT compared to my 81 mm refractor. It was eye opening. I realized that day I was pretty much done with refractors. That say, "Aperture is king," is entirely true.
@@iamjessieray I have a local astro buddy who loves to image with his f/13 102mm mak. Crazy. But he makes good results!
@@MazzifLOL I have seen some INCREDIBLE results with 5" maks so I'm starting to thing that f-ratio is really irrelevant if you are willing to put some more time in and can fit your object of interest in your FoV
I think the main reason people think and like to say a low f ratio is faster and therefore better than a higher f ratio is to make it simpler for beginners.
Possibly. The idea has become entrenched.
I also just love spreading misinformation 👍
Not sure what you are saying, this video is accurate enough with explanations where simplifications are made. If you doubt it, do the etendue calculations and test it in your yard if you can. It will all make sense eventually if you do. This doesn't mean that a small telescope isn't useful, but under the right conditions a large scope will out perform a small one with a million caveats of course. Etendue shows that on a per pixel level an 8" scope is an 8" scope if you match the sampling in arc seconds.
more troll food?