Agreed, working on a mount for testing. I have gotten good results with our acrylic second surface parabolic mirrors and I am currently having first surface versions mirrored. They experience minor aberration due to temperature changes but still work with an aperture added.
The reason for a good image with the smallest aperture is the long focal ratio and not the 'quality' of the mirror near the center. With the full aperture, your focal ratio is 'short' (maybe f3) but fully stopped down, your focal ratio is more like 15 and at longer ratios like that, surface defects matter much less; the apparatus becomes more forgiving. That is why it is much, much easier to make an f15 telescope than an f3 telescope of the same mirror size. Why then do observatories and amateur telescope makers go to all the trouble of producing fast optics? Because slow telescopes with long focal ratios become huge as the mirror diameter increases. A 24 inch f/15 mirror requires a nearly 30 foot-long tube assembly whereas a 24" f3 would only be 6 feet long. That means a lighter, easier to mount and use telescope---but only if the optics are very close to perfect---and that requires time and expertise and often, some pretty elaborate testing apparatus. A really, really good 24 inch telescope might cost $50,000 or more. The reason people aren't making telescopes from trashcan lids is that a $50 Chinese telescope at Costco will perform far better.
@@chanakyasinha8046 Absolute perfection is an asymptotic goal. The interesting thing is that virtually all telescope mirror are still made by skilled artisan's using bare hands. Fully automated production of highly accurate aspheric optics is still an elusive goal. Centuries old manual polishing and testing techniques remain the preferred method. That could change in the future through the use of something like magnetorheological fluid polishing, but for now a pitch lap in the hands of an expert optician produces the most accurate optical surfaces achievable.
I built the trashcan lid mirror and was able to turn cardboard on fire in less than 30 seconds. Works really awesome and my next step it to rig up a galvanized steel or copper pipe with my pool hose to build a water heater to warm up the pool for winter swimming. Thank you thank you thank you, you're awesome! Keep up the great work.
You are simply amazing DAN. I looked into the Schlieren photography you simplified and realized only one modification since the 1800s; you modified something no one else has in 150 years. Now good zoom images from a trash can. Hearts and happy new years to you both!
Thank you:-) This video series covers a lot: watch?v=QpkWTdXwTnw I am planing some classroom styled videos that cover optics from the basics up to advanced. Including photograph and film/video as they all tie together. Any specific topics, I am open to suggestions. I will also be uploading a DIY very powerful 100x to near unlimited laser microscope in a future video. Thank you again. Dan
I can't imagine this would be a replacement for an amateur astronomer who just wanted something they put together, and it just works. But part of me loves the concept. I imagine you could make a piece of kit that has temperature parameters and small pumps, you get the right shape every-time automatically. But I wonder if you could make an adaptive optics like system using a more complex rig, with a flexible mirror. I would have to see regulations, and the power involved. But I know that you can get some pretty powerful lasers for way cheaper then some of these five grand mirrors online.
Acrylic is the best option for an inexpensive permeant mirror. We are producing huge first surface mirrors that are CnC cut. The focal length is fixed. I like the vacuum as you end up with a zoom telescope and you can make a Cassegrain OR Gregorian via a vacuum concave or pressure convex SECONDARY mirror. The back would be clear optical glass and the center aluminum of the primary mirror can be removed with the process at 6 minutes here: watch?v=FyCLOXF1188 Future video:-)
I would think putting in sufficient epoxy then applying vacuum and swishing it around could solidify the Mylar thus making a more stable mirror for a telescpe.
Wow, very cool video! So by placing the cardboard circle on the mirror, you 'stopped down' the aperture of your lens effectively. I guess I never thought about exactly WHY a camera lens is sharper when closed down a couple stops from wide open. Now it's obvious why, you're not using the edges of the lens at that point, so your image is even sharper. Awesome stuff
reminds me of making a pin hole camera with baking paper and an empty bean can. the pin hole makes nearly everything focus on the paper obviously upside down, good vids Dan
Dan ! Holy crap ! Telescope mirrors are mirrored on the surface and should not be touched on the reflective surface. I saw a lot of finger marks on that mirror !
This is so smart. Dan, thank you for making this video. I am going to try to build one for a telescope project. By the way my tributes ti The King of Random, RIP as loosing him felt like loosing a friend.
The Gregorian telescope mirror was like that and the reason I got it for 70 bucks:-) The trash can lid mirror is roughed up from some solar tests but still provides a good image. A clean one should be even better.
My one big question about the large scale applications of this type of mirror is how much gravitational distortion would diminish the image quality? Larger (much larger) mirrors of this type would distort as they rotate away from being aligned vertically. It would be interesting to know how large the skin of a mirror would have to be before made unusable because of this.
It looks like you can get some pretty good images with this mirror. Has anyone tried to make this into a telescope? My main concern in astigmatism. It may look pretty good on the wall, but what does it look like through an eyepiece in a telescope?
Love it, Dan! I'd thought about doing this many years ago when I was looking at building inexpensive telescopes. If memory serves, I remember someone doing this with one of those mylar balloons for the reflective material, hehe.
Dan, I have thought to use a kid's plastic wading pool to do this. I wonder if Mylar comes wide enough to do this? To fine tune the shape of the lens I figured to use one of those brake bleeding pumps. Another way to bend the Mylar might be to make a hole in the back of the pool and attach a string to the Mylar. Pulling the string back for the desired bend should cause a parabolic shape. Using a winding mechanism like a fishing reel might make it more precise.
Not sure if you noticed but with the Gregorian mirror he was definitely trying not to touch its surface too much with his fingers. Go to about 3:40 and you can see that he's definitely keeping his fingers away from it, palming the edges. Granted, it was in rough shape but I think subconsciously he didn't want to mistreat it as much as the $5 mirror. Probably has some experiments left to do with it!
If the back of mylar mirror had a thin fiberglass coating, enough to maintain the shape of mirror, plus a provision to provide a push/pull on center back of mirror, then the spherical surface could be altered to a parabola by deepening the center, this would be manual figure control, the idea is to have the light reflecting off the primary mirror fall "Inside" the "Airy disc" in a star test, I believe it is possible to achieve a 1/2 or maybe 1/4 wave or better figure using this principle on a vacuum formed mylar mirror..., about 20 years ago I met a guy who worked for JPL , he worked with this other guy on fiber composite mirrors, they showed me a 48 inch fiber composite mirror with shape memory, they folded it in half then let it fold back to original shape and said it could be folded up inside a tube then unfurled in space for use in a space telescope, they also had active figure control systems with these mirrors for ground based telescopes, this is a viable alternative to the expensive pyrex and other low expansion glass type primary mirrors...
holy shit, how did you get ahold of that telescope mirror? Even in that condition its still worth thousands unless its heavily chipped. And even chipped, it might be able to be refigured...
Hey, I know your videos are already instructional, but have you ever considered doing straight-up optics lessons on this or another channel? I would LOVE to get a comprehensive optics education from you. You're a terrific teacher. Anyone with me on this?
dan the man..hi dan,i am a great fan of you and denise..i liked all your videos and haven't missed one..you've inspired me greatly to go green..thanks for the videos.. warm regards from india raju
Either a big syringe, or a kind of membrane, that can be pulled by a spring or weight. I could imagine making something from hoses and 2 bottles and some water that could pull a constant pressure for the mirror to be in the same focal length despite changing temperature of the trapped air. For a telescope mirror this would be useful...
I would love to make myself a 12 inch telescope. Thinking using a glass 12 inch sauce pot cover.... would that be any good? Then.... how to make it reflective.? I've seen aerosol aluminum spray can paint with impressive results..... Please. Your input on that, or those...lol Thanks
Relying on air pressure seems too unstable. Here's an idea: what about pouring a layer of cement through an opening in the trashcan lid, then use a vacuum pump to shape it while it sets. I should try this one day...
Another good informative video. Watching you hold up the mirror with the hole in the middle gave me a thought. Now that the era of CD's is fading, I wonder if there is an easy method to form a CD into a parabolic mirror. What do you think?
why not gluing a syringe (without the needle) to the hole in the trash can lid? add a little ratchet (cut in the plastic shaft) and you have an adjustable vacuum pump (plus you can be consistent if you want to do several adjustments over time)
Wow, I didn't realize the impact aperture size has on the sharpness of the image. Is it because it limits the surface area of the mirror so you get less light "interfering" since it is never reflacted off the imperfect more outer parts of the mirror?
"This takes the cost of the mirror out of the picture." The catch is it also removes the quality. Would love to see you attempt to get anything even close to the quality image you get with of a quarter wave length mirror with an old trash can lid and foil.
Not really. Planar glass (i mean really planar) is even more expensive than parabolic, in larger sizes. If it is not flat or not parallel enough that will make aberrations all around the mirror. I made a 1,2 m diameter household glass mirror like this and the glass seems a bit wobbly around the edges, but most importantly it is kinda bumpy all over. Not much, you would not notice it otherwise just if looking at the reflections from a sharp angle. This makes the image a bit fuzzy.
You can't keep the vacuum long enough for something like that, Dan. Not for astronomical use. The vibration would mess it up too (it sounds like a drum, because it is a drum... that means vibration.) For an image, that is a killer. That said, it makes a fine mirror for non-astronomical use. The shape of even a bad telescope mirror is measured in fractions of wavelengths of light - 5*10 ^ -11 meters. The smoothness of the glass far exceeds anything you could ever do with mylar.
Oh yes you can maintain the vacuum!, then build up fiber glass layers, thick but not too think, want to maintain very slight flexibility, then apply a push/pull screw to center back of mirror as to alter figuere from a sphere to a parabola, I know this will work as i have ground/polished and figuered pyrex mirrors ranging in diameter from 4 to 13 inch, and have acheived 1/10 wave of accuracy...The requirement that an astronomical mirror be made of a low E substrate glass is utterly ridiculous, remember your allways dealing with atmospheric distortion anyhow, and in this day of ccd imaging with sophisticated image stacking and enhancement a 1 wave optical system could yield images which rival that of a 1/20 wave system after processing...
Also the glass mirror i made like that is limited to more than 8000 mm safely and probably can be risked to go down to 7000 mm. That is too much for a practical telescope. I used 6 mm thick mirror i believe. The thinner you can go the shorter you can make the focal length, but it is more difficult to cut and handle. I have a 1.5 mm thick and 150mm diameter mirror that i can make to be like 500mm focal length easily. It is like a shaving mirror, not like serious optics.
A Gregorian primary is generally NOT parabolic (there ARE special cases where it is, but those are rare). Also, I think it is false to assert that generally, optical surfaces are better at their center than at their periphery. The obverse is often the case too. What is true is that longer focal RATIO optics produce better images than shorter ratio optics do. That is why stopping down an optic clarifies an image (and not necessarily because one is masking off the putatively less accurate outer areas of an optic surface.). You can prove this by stopping down the edge of the mirror rather than the center. Pressure deformed mirrors are interesting though and this is a good demonstration of how useful they can be.
Excellent video! Informative and well described! Now to scour the neighbourhood for garbage can lids...muhahaha... But seriously damn good information. Thanks
PLEASE ANSWER! This is A M A Z I N G! 1) What are you doing at @7:46 There is a second piece of cardboard, but we never see much of what you do with it, how far away, etc. And wgat is thd Transformer you nentikn, ? Is that the firts cutout! WHY TWO CUTOUTS, IF THEY ARE RIGHT NEXT TO EACH OTHER?,.. (I suppose its a matter of experimentation too), 2) Can you suggest anything other than an 'Oscar the Grouch' trashcan lid? I haven't seen one of those in California in over a decade,... :) 3) Any ideas on getting the 'vacumn' or Shape to stay? I am considering something like filling the space with foam insulation, then using my wet dry vac to pull suction, maybe experiment, with letting thd foam every so slightly harden before pulling vacumn etc,... seems like this could scale up well. I see myself creating a 6 foot version with my wet dry vac permantly attached. You taught more in a few minutes than all the negative nancys at cloudy nights and that other site,. Always saying what Can't be done. I Really need to build a BIG scope, I have found miles tall spires on the moon with my 6" Maksutov Cassegrain and correlated the spot on Google Moon sure enough, in the elevation data, anomalies, right where I saw in real world,.. a Spire 7700 meters high, sure right near the spot, , less than 100 meters wide Not naturally possible, the shadows I saw in my scope were sooo thin but must have been ten miles long, I also found a VERY Square Structure with 90° walls, I HAVE to revisit those with more magnification! THANK YOU!
I made one three years ago and i saw 5-6 moon pictures 😁 and enjoyed seeing it.. Enlarges and all.. I was not able to point out the mistake but that was water lens with tightly filled. But still the holding transparent part was not able to give like stable optical clarity.. 😢
Interesting and informative. I was looking for info on how to record my 10 spherical mirror. Like your Gregorian it is filthy and scratched but still produces pretty good views as evidenced by all the oo's and ah's at the public viewing event I attended last night. Three other guys had expensive commercial telescopes but I brought my 16 year old home built Dobsonian. Each of us concentrated on a different object and due to my location I chose the Orion Nebula. While they weren't hostile in any way I could feel a bit of cold shoulder from them, maybe because I am not a member of their astronomy club. But the line for my telescope was just as long as theirs. I don't mean to sound arrogant but all of my non-astronnomer friends are amazed by my Dobsonian even if they've never looked through it. By the way, I'm subscribing and 👍.
I knew a guy who was on the team than made the 4 meter Mayall reflector at Kitt peak observatory many years ago, he said that mirror gets covered in dust throughout most of the year before cleaning and really does not affect optical quality as much as one would think...
2 things, what you are making is a reflecting type of "telescope" that is similar to that of a mirror, not a lense, in a lense, the light goes through the optics, not bounce off. Second, with all that light aberration occurring in the total light path, the final image it will make will be truly blury. Not the kind of quality an astronomer or amateurs alike would enjoy when it's 32 degrees out side on top of a 10 foot ladder in complete darkness now is it?
I have perhaps an unusal question. Is it possible and are the avantages to "mirroring" ( making reflective) one side of a magnifying lens for astronomy?
Hi, It would need to be perfectly flat on one side an there would be tons of optical aberration because the light would need to enter glass then bounce back through it. First surface is needed for true optical applications. Traditional lenses only have one light pass so light filtering is reduced.
Hi, this is good work but still a conclusion can't be made. First there is a minimum surface accuracy of mirrors for telescope let's say lambda divided by 4 or less. So I would like to request you for a Foucault test of mirror. It can be done by an iPhone and pinhole and a light source. If that goes well we can find a way to harden the mirror and also decide it's shape. Could you please try either this test or looking at moon through a magnifying lens which gets reflected of the mirror. This simple test will tell if it is really upto as a telescope mirror at this stage. I am also looking into different ways to make cheap telescope mirror.
How about using a real glass surface that has been mirrored and then deforming it with a vacuum? The ability to flex will be diminished. But the quality should be vastly better.
Are you even familiar with the precision requirements for the telescope mirrors? Or why don't you show the results of a Foucault test, or any optical test, instead? Even the image on the wall is terrible. Stop misleading people!
his point is that this was made for like $2 instead of an $800 mirror. Obviously the precision isn't there, but it would do well for images of solar system objects.
criticism is easy. Idea to make cheap telescope mirror for every one is good. So make brinstrom. Can we make a cheap parabolic helper to force the good form ?
Mylar, not foil. You know, space age stuff. In space. This wouldn't have been possible way back when, and I'm sure this is far more effective than nothing, for almost as much money as nothing.
Excellent way to make a cheap reflector for a solar furnace, but as a telescope mirror - useless. The frame (trashcan lid) rim will not be planar which will result in outrageous astigmatism. Also, the Mylar film itself is not uniform enough in it's tensile properties to produce a predictably accurate curve.
A vacuum-pulled film will produce a spherical mirror, not good for telescope making because of spherical aberration. You need a parabolic mirror for telescopes.
How do you reckon it is spherical? I mean, it is obviously not parabolic, but why spherical? Id suspect something like a catenary. Also, maybe it is possible to correct the error through a certain shape of the secondary mirror.
Skandalos I'm thinking it is spherical because air pressure is evenly distributed in all directions and the plastic film's tensile strength is (presumably) also equal in all directions. But I may of course be wrong. Depending on the mirror shape a secondary mirror may not be able to correct. Grossly misaligned rays might miss the secondary altogether, for instance.
Bruno Lobo In a 2-dimensional situation even pressure perpendicular on a chain yields the catenary curve,so Im pretty sure it will be the same in a 3-dimensional case. After a bit of googling ("catenary mirror") Im fairly certain that Im right. The catenary is very close to a parabola as long as the curvature is very shallow which is why the guy in the video gets much better results when reducing the aperture. As for correctors, there are correctors for spherical mirrors, the Schmidt-corrector but I didnt find any correctors for a catenary mirror.
Skandalos Remember that in the chain example, it is being pulled by gravity that acts in the same direction (vertical) along the chain's length, which is not the case with the plastic film, where pressure is always perpendicular to the surface. This might significantly change the surface shape from a catenary.
Agreed, working on a mount for testing. I have gotten good results with our acrylic second surface parabolic mirrors and I am currently having first surface versions mirrored. They experience minor aberration due to temperature changes but still work with an aperture added.
Dan. You are awesome and I learn from you constantly!!
Grant Thompson - "The King of Random" hi
Dude you are awesome seen your vids...
The reason for a good image with the smallest aperture is the long focal ratio and not the 'quality' of the mirror near the center. With the full aperture, your focal ratio is 'short' (maybe f3) but fully stopped down, your focal ratio is more like 15 and at longer ratios like that, surface defects matter much less; the apparatus becomes more forgiving. That is why it is much, much easier to make an f15 telescope than an f3 telescope of the same mirror size. Why then do observatories and amateur telescope makers go to all the trouble of producing fast optics? Because slow telescopes with long focal ratios become huge as the mirror diameter increases. A 24 inch f/15 mirror requires a nearly 30 foot-long tube assembly whereas a 24" f3 would only be 6 feet long. That means a lighter, easier to mount and use telescope---but only if the optics are very close to perfect---and that requires time and expertise and often, some pretty elaborate testing apparatus. A really, really good 24 inch telescope might cost $50,000 or more. The reason people aren't making telescopes from trashcan lids is that a $50 Chinese telescope at Costco will perform far better.
'Camera Obscura' . This is great stuff. Very impressive.
Mylar is not foil, it is vacuum deposited aluminum. While it will not equal a million dollar mirror, it does work.
still waiting on video 2 where you show pinpoint star images with this trash mirror
hahahahahahahahaha!
Its not possible.. Perfection is indeed something human hand lacks 😐
@@chanakyasinha8046 Absolute perfection is an asymptotic goal. The interesting thing is that virtually all telescope mirror are still made by skilled artisan's using bare hands. Fully automated production of highly accurate aspheric optics is still an elusive goal. Centuries old manual polishing and testing techniques remain the preferred method. That could change in the future through the use of something like magnetorheological fluid polishing, but for now a pitch lap in the hands of an expert optician produces the most accurate optical surfaces achievable.
kek
Very impressive, much better results than I was expecting. Gotta build my own now!
I built the trashcan lid mirror and was able to turn cardboard on fire in less than 30 seconds. Works really awesome and my next step it to rig up a galvanized steel or copper pipe with my pool hose to build a water heater to warm up the pool for winter swimming. Thank you thank you thank you, you're awesome! Keep up the great work.
I'd love to see this in a telescope. Maybe you can build a wooden dobsonian one and use this as the mirror.
You are simply amazing DAN. I looked into the Schlieren photography you simplified and realized only one modification since the 1800s; you modified something no one else has in 150 years. Now good zoom images from a trash can. Hearts and happy new years to you both!
Thank you:-) This video series covers a lot:
watch?v=QpkWTdXwTnw
I am planing some classroom styled videos that cover optics from the basics up to advanced. Including photograph and film/video as they all tie together. Any specific topics, I am open to suggestions.
I will also be uploading a DIY very powerful 100x to near unlimited laser microscope in a future video. Thank you again.
Dan
I can't imagine this would be a replacement for an amateur astronomer who just wanted something they put together, and it just works. But part of me loves the concept. I imagine you could make a piece of kit that has temperature parameters and small pumps, you get the right shape every-time automatically. But I wonder if you could make an adaptive optics like system using a more complex rig, with a flexible mirror. I would have to see regulations, and the power involved. But I know that you can get some pretty powerful lasers for way cheaper then some of these five grand mirrors online.
Any cool ideas to do with the lenses from a rear projection tv....I saved them i before blew it up.
Wow this is just what I was looking for thank you so much Dan
Once again great stuff, Dan! I did suspect silicone fumes might not be as cool as I think! Haha!
Acrylic is the best option for an inexpensive permeant mirror. We are producing huge first surface mirrors that are CnC cut. The focal length is fixed. I like the vacuum as you end up with a zoom telescope and you can make a Cassegrain OR Gregorian via a vacuum concave or pressure convex SECONDARY mirror. The back would be clear optical glass and the center aluminum of the primary mirror can be removed with the process at 6 minutes here:
watch?v=FyCLOXF1188
Future video:-)
How to make cassigarian telescope from this
This works for schelieren setup?
I would think putting in sufficient epoxy then applying vacuum and swishing it around could solidify the Mylar thus making a more stable mirror for a telescpe.
Wow, very cool video! So by placing the cardboard circle on the mirror, you 'stopped down' the aperture of your lens effectively. I guess I never thought about exactly WHY a camera lens is sharper when closed down a couple stops from wide open. Now it's obvious why, you're not using the edges of the lens at that point, so your image is even sharper. Awesome stuff
Amazing an inspiring! Would like to see you making this into a telescope and looking at stars!
reminds me of making a pin hole camera with baking paper and an empty bean can. the pin hole makes nearly everything focus on the paper obviously upside down, good vids Dan
Dan ! Holy crap ! Telescope mirrors are mirrored on the surface and should not be touched on the reflective surface. I saw a lot of finger marks on that mirror !
This is so smart. Dan, thank you for making this video. I am going to try to build one for a telescope project. By the way my tributes ti The King of Random, RIP as loosing him felt like loosing a friend.
The Gregorian telescope mirror was like that and the reason I got it for 70 bucks:-) The trash can lid mirror is roughed up from some solar tests but still provides a good image. A clean one should be even better.
My one big question about the large scale applications of this type of mirror is how much gravitational distortion would diminish the image quality? Larger (much larger) mirrors of this type would distort as they rotate away from being aligned vertically. It would be interesting to know how large the skin of a mirror would have to be before made unusable because of this.
It looks like you can get some pretty good images with this mirror. Has anyone tried to make this into a telescope? My main concern in astigmatism. It may look pretty good on the wall, but what does it look like through an eyepiece in a telescope?
I agree with you and I am wondering the same thing.
Dan, please show us a HD video of this when you can. Outstanding idea and very cheap.
Beautiful. You had me at "off axis reflector".
Love it, Dan! I'd thought about doing this many years ago when I was looking at building inexpensive telescopes. If memory serves, I remember someone doing this with one of those mylar balloons for the reflective material, hehe.
Dan, I have thought to use a kid's plastic wading pool to do this. I wonder if Mylar comes wide enough to do this? To fine tune the shape of the lens I figured to use one of those brake bleeding pumps. Another way to bend the Mylar might be to make a hole in the back of the pool and attach a string to the Mylar. Pulling the string back for the desired bend should cause a parabolic shape. Using a winding mechanism like a fishing reel might make it more precise.
Great explanation of camera optics.
Multiple exhaust ports on the reverse side might make it useful for an adaptive optics system similar to newer 'scopes.
Looking forward to the video about turning this into a spotlight!
Not sure if you noticed but with the Gregorian mirror he was definitely trying not to touch its surface too much with his fingers. Go to about 3:40 and you can see that he's definitely keeping his fingers away from it, palming the edges. Granted, it was in rough shape but I think subconsciously he didn't want to mistreat it as much as the $5 mirror. Probably has some experiments left to do with it!
If the back of mylar mirror had a thin fiberglass coating, enough to maintain the shape of mirror, plus a provision to provide a push/pull on center back of mirror, then the spherical surface could be altered to a parabola by deepening the center, this would be manual figure control, the idea is to have the light reflecting off the primary mirror fall "Inside" the "Airy disc" in a star test, I believe it is possible to achieve a 1/2 or maybe 1/4 wave or better figure using this principle on a vacuum formed mylar mirror..., about 20 years ago I met a guy who worked for JPL , he worked with this other guy on fiber composite mirrors, they showed me a 48 inch fiber composite mirror with shape memory, they folded it in half then let it fold back to original shape and said it could be folded up inside a tube then unfurled in space for use in a space telescope, they also had active figure control systems with these mirrors for ground based telescopes, this is a viable alternative to the expensive pyrex and other low expansion glass type primary mirrors...
holy shit, how did you get ahold of that telescope mirror? Even in that condition its still worth thousands unless its heavily chipped. And even chipped, it might be able to be refigured...
Hey, I know your videos are already instructional, but have you ever considered doing straight-up optics lessons on this or another channel? I would LOVE to get a comprehensive optics education from you. You're a terrific teacher. Anyone with me on this?
dan the man..hi dan,i am a great fan of you and denise..i liked all your videos and haven't missed one..you've inspired me greatly to go green..thanks for the videos..
warm regards from india
raju
Either a big syringe, or a kind of membrane, that can be pulled by a spring or weight.
I could imagine making something from hoses and 2 bottles and some water that could pull a constant pressure for the mirror to be in the same focal length despite changing temperature of the trapped air. For a telescope mirror this would be useful...
great to see his,i want watch more of your work..
I would love to make myself a 12 inch telescope.
Thinking using a glass 12 inch sauce pot cover.... would that be any good?
Then.... how to make it reflective.?
I've seen aerosol aluminum spray can paint with impressive results.....
Please. Your input on that, or those...lol
Thanks
Did you try it? I'd like to know more about the aluminum spray, in surprised that worked.
Hey Dan, you could knock up a light bucket newtonian with this. You might get to see jupiter way better than I can with my 5 1/2" scope.
Hi, what is the current progress. I am gonna try 👍
Relying on air pressure seems too unstable. Here's an idea: what about pouring a layer of cement through an opening in the trashcan lid, then use a vacuum pump to shape it while it sets. I should try this one day...
Another good informative video. Watching you hold up the mirror with the hole in the middle gave me a thought. Now that the era of CD's is fading, I wonder if there is an easy method to form a CD into a parabolic mirror. What do you think?
Your videos are great! Nice job.
why not gluing a syringe (without the needle) to the hole in the trash can lid? add a little ratchet (cut in the plastic shaft) and you have an adjustable vacuum pump (plus you can be consistent if you want to do several adjustments over time)
Did you made the telescope?? We need to know about you
Wow, I didn't realize the impact aperture size has on the sharpness of the image. Is it because it limits the surface area of the mirror so you get less light "interfering" since it is never reflacted off the imperfect more outer parts of the mirror?
I should develope the habit of listening. Good video.
if you made the support structure an inflateable ring you could make the whole thing lighter than air
Holly cow, this trash can mirror is damn good!
How much vacuum did you draw out of that lid. It must have been very slight
"This takes the cost of the mirror out of the picture."
The catch is it also removes the quality. Would love to see you attempt to get anything even close to the quality image you get with of a quarter wave length mirror with an old trash can lid and foil.
Not really. Planar glass (i mean really planar) is even more expensive than parabolic, in larger sizes. If it is not flat or not parallel enough that will make aberrations all around the mirror.
I made a 1,2 m diameter household glass mirror like this and the glass seems a bit wobbly around the edges, but most importantly it is kinda bumpy all over. Not much, you would not notice it otherwise just if looking at the reflections from a sharp angle. This makes the image a bit fuzzy.
You can't keep the vacuum long enough for something like that, Dan. Not for astronomical use. The vibration would mess it up too (it sounds like a drum, because it is a drum... that means vibration.) For an image, that is a killer. That said, it makes a fine mirror for non-astronomical use. The shape of even a bad telescope mirror is measured in fractions of wavelengths of light - 5*10 ^ -11 meters. The smoothness of the glass far exceeds anything you could ever do with mylar.
Oh yes you can maintain the vacuum!, then build up fiber glass layers, thick but not too think, want to maintain very slight flexibility, then apply a push/pull screw to center back of mirror as to alter figuere from a sphere to a parabola, I know this will work as i have ground/polished and figuered pyrex mirrors ranging in diameter from 4 to 13 inch, and have acheived 1/10 wave of accuracy...The requirement that an astronomical mirror be made of a low E substrate glass is utterly ridiculous, remember your allways dealing with atmospheric distortion anyhow, and in this day of ccd imaging with sophisticated image stacking and enhancement a 1 wave optical system could yield images which rival that of a 1/20 wave system after processing...
Also the glass mirror i made like that is limited to more than 8000 mm safely and probably can be risked to go down to 7000 mm. That is too much for a practical telescope.
I used 6 mm thick mirror i believe. The thinner you can go the shorter you can make the focal length, but it is more difficult to cut and handle.
I have a 1.5 mm thick and 150mm diameter mirror that i can make to be like 500mm focal length easily. It is like a shaving mirror, not like serious optics.
A Gregorian primary is generally NOT parabolic (there ARE special cases where it is, but those are rare). Also, I think it is false to assert that generally, optical surfaces are better at their center than at their periphery. The obverse is often the case too. What is true is that longer focal RATIO optics produce better images than shorter ratio optics do. That is why stopping down an optic clarifies an image (and not necessarily because one is masking off the putatively less accurate outer areas of an optic surface.). You can prove this by stopping down the edge of the mirror rather than the center.
Pressure deformed mirrors are interesting though and this is a good demonstration of how useful they can be.
Excellent video! Informative and well described! Now to scour the neighbourhood for garbage can lids...muhahaha... But seriously damn good information.
Thanks
jason del Thank you Jason!
Thank you so much for this vid!
I will by trying this out ASAP
Thank you. Great video! I appreciate the information very much! Awesome.
PLEASE ANSWER! This is A M A Z I N G! 1) What are you doing at @7:46 There is a second piece of cardboard, but we never see much of what you do with it, how far away, etc. And wgat is thd Transformer you nentikn, ? Is that the firts cutout! WHY TWO CUTOUTS, IF THEY ARE RIGHT NEXT TO EACH OTHER?,.. (I suppose its a matter of experimentation too), 2) Can you suggest anything other than an 'Oscar the Grouch' trashcan lid? I haven't seen one of those in California in over a decade,... :) 3) Any ideas on getting the 'vacumn' or Shape to stay? I am considering something like filling the space with foam insulation, then using my wet dry vac to pull suction, maybe experiment, with letting thd foam every so slightly harden before pulling vacumn etc,... seems like this could scale up well. I see myself creating a 6 foot version with my wet dry vac permantly attached. You taught more in a few minutes than all the negative nancys at cloudy nights and that other site,. Always saying what Can't be done. I Really need to build a BIG scope, I have found miles tall spires on the moon with my 6" Maksutov Cassegrain and correlated the spot on Google Moon sure enough, in the elevation data, anomalies, right where I saw in real world,.. a Spire 7700 meters high, sure right near the spot, , less than 100 meters wide Not naturally possible, the shadows I saw in my scope were sooo thin but must have been ten miles long, I also found a VERY Square Structure with 90° walls, I HAVE to revisit those with more magnification! THANK YOU!
I made one three years ago and i saw 5-6 moon pictures 😁 and enjoyed seeing it.. Enlarges and all.. I was not able to point out the mistake but that was water lens with tightly filled. But still the holding transparent part was not able to give like stable optical clarity.. 😢
Fascinating! Thank you!
hand held vacuum pump used for bleeding brakes
Ok I know how expensive those mirrors could be. they could be re-coated though if its been polished properly.
We have some professional criticism on this trash can lid mirror. Reckon what Hecht's criticism would be?
Interesting and informative. I was looking for info on how to record my 10 spherical mirror. Like your Gregorian it is filthy and scratched but still produces pretty good views as evidenced by all the oo's and ah's at the public viewing event I attended last night. Three other guys had expensive commercial telescopes but I brought my 16 year old home built Dobsonian. Each of us concentrated on a different object and due to my location I chose the Orion Nebula. While they weren't hostile in any way I could feel a bit of cold shoulder from them, maybe because I am not a member of their astronomy club. But the line for my telescope was just as long as theirs. I don't mean to sound arrogant but all of my non-astronnomer friends are amazed by my Dobsonian even if they've never looked through it. By the way, I'm subscribing and 👍.
I knew a guy who was on the team than made the 4 meter Mayall reflector at Kitt peak observatory many years ago, he said that mirror gets covered in dust throughout most of the year before cleaning and really does not affect optical quality as much as one would think...
awesome, I didn't know that about aperture... I hope its accurate =D
so then your tracking mechanism could consist of two teeny motor-gearboxes winding two bits of thread. no matter how big the mirror.
Could make a decent size Cassegrain with that mirror.
Can you use a chrome painted satelite tv dish antenna as parabolic reflector for an optical telescope ?
haha...no
Why no company is preparing huge concave mirrors.
Why hole in the middle.
Good example how we can do the same job with less resource spending.
2 things, what you are making is a reflecting type of "telescope" that is similar to that of a mirror, not a lense, in a lense, the light goes through the optics, not bounce off. Second, with all that light aberration occurring in the total light path, the final image it will make will be truly blury. Not the kind of quality an astronomer or amateurs alike would enjoy when it's 32 degrees out side on top of a 10 foot ladder in complete darkness now is it?
I have perhaps an unusal question. Is it possible and are the avantages to "mirroring" ( making reflective) one side of a magnifying lens for astronomy?
Hi,
It would need to be perfectly flat on one side an there would be tons of optical aberration because the light would need to enter glass then bounce back through it. First surface is needed for true optical applications. Traditional lenses only have one light pass so light filtering is reduced.
I suspected that, you confirmed it...thanks.
Hi, this is good work but still a conclusion can't be made. First there is a minimum surface accuracy of mirrors for telescope let's say lambda divided by 4 or less. So I would like to request you for a Foucault test of mirror. It can be done by an iPhone and pinhole and a light source. If that goes well we can find a way to harden the mirror and also decide it's shape. Could you please try either this test or looking at moon through a magnifying lens which gets reflected of the mirror. This simple test will tell if it is really upto as a telescope mirror at this stage. I am also looking into different ways to make cheap telescope mirror.
Hey would like to take some advice from you if you can contact me
great vid. lol at trying to find a good excuse not to inhale any fumes xD
I made a lens cover from mylar for my slr still camera to take pictures of the solar eclipse
"It's not as cool as you think."
LOL!
Excellent.!
can be made to weigh next to nothing. simplifies the problem of making a tracking mechanism
you know that you can resurface that mirror and re-aluminize the surface and you have a mirror you can sell for a couple grand.
Awesome!
That's unbelievable.
How about using a real glass surface that has been mirrored and then deforming it with a vacuum? The ability to flex will be diminished. But the quality should be vastly better.
Are you even familiar with the precision requirements for the telescope mirrors? Or why don't you show the results of a Foucault test, or any optical test, instead? Even the image on the wall is terrible. Stop misleading people!
Boris Zaprudin Makes a good sun cooking device, perhaps a bit unpractical with a rather long focus distance.
his point is that this was made for like $2 instead of an $800 mirror. Obviously the precision isn't there, but it would do well for images of solar system objects.
What was the name of that telescope on the satellite?, millions of dollars, they fucked up the mirror.
Ahhhh Amateur Telescope Making.... now you are talking about my hobby.
criticism is easy. Idea to make cheap telescope mirror for every one is good.
So make brinstrom.
Can we make a cheap parabolic helper to force the good form ?
does this mirror work for schliren photography
Derek would be proud you're researching this ;)
muy bueno
Mylar, not foil. You know, space age stuff. In space.
This wouldn't have been possible way back when, and I'm sure this is far more effective than nothing, for almost as much money as nothing.
Excellent way to make a cheap reflector for a solar furnace, but as a telescope mirror - useless. The frame (trashcan lid) rim will not be planar which will result in outrageous astigmatism. Also, the Mylar film itself is not uniform enough in it's tensile properties to produce a predictably accurate curve.
hello captain america!
A vacuum-pulled film will produce a spherical mirror, not good for telescope making because of spherical aberration. You need a parabolic mirror for telescopes.
How do you reckon it is spherical? I mean, it is obviously not parabolic, but why spherical? Id suspect something like a catenary. Also, maybe it is possible to correct the error through a certain shape of the secondary mirror.
Skandalos I'm thinking it is spherical because air pressure is evenly distributed in all directions and the plastic film's tensile strength is (presumably) also equal in all directions. But I may of course be wrong. Depending on the mirror shape a secondary mirror may not be able to correct. Grossly misaligned rays might miss the secondary altogether, for instance.
Bruno Lobo In a 2-dimensional situation even pressure perpendicular on a chain yields the catenary curve,so Im pretty sure it will be the same in a 3-dimensional case. After a bit of googling ("catenary mirror") Im fairly certain that Im right. The catenary is very close to a parabola as long as the curvature is very shallow which is why the guy in the video gets much better results when reducing the aperture.
As for correctors, there are correctors for spherical mirrors, the Schmidt-corrector but I didnt find any correctors for a catenary mirror.
Skandalos Remember that in the chain example, it is being pulled by gravity that acts in the same direction (vertical) along the chain's length, which is not the case with the plastic film, where pressure is always perpendicular to the surface. This might significantly change the surface shape from a catenary.
Bruno Lobo good point!
Ok pantulan image tu dimensi sisi perlu ikut dngan zoom
But image tu serasi dngn zoom
Have a look at the videos I have posted. They are all about mirror making and testing.
I'm speechless...It took you 30 mins to do what the smartest minds in astronomy were to dumb to figure out. Thank you for sharing...
WOW!
maybe not the best for a telescope, but for a buck you can't beat the price. a little more research into this could yield some surprising results. +1