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Reuse of alternative optical components for objectives

Optics Refractor
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#1 Spinwiz

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Posted 17 January 2025 - 11:34 PM

Hi star gazers;

 

This is related to me looking to build various scopes from finders on up - on the cheap!    So far I haven't found similar threads but with 530 some odd threads I thought it might be faster to just get some feed back.  I'd like to know how successful people have been with salvaged optics from things like projectors, binoculars and such to build anything from finders on up.

 

Some of this stuff can be found so cheap in thrift shops and even junk streams that it seems to be a good idea.

 

The alternative is to buy achromats from Edmunds or similar vendors.   However I really don't know about the possible vendors for such and the price is likely high especially for experimental work.



#2 RichA

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Posted 18 January 2025 - 01:32 AM

Buy Chinese achromats off aliexpress.  Dirt cheap, some are decent.


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#3 luxo II

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Posted 18 January 2025 - 01:50 AM

 I'd like to know how successful people have been with salvaged optics from things like projectors, binoculars and such to build anything from finders on up.

Welcome to CN, I see you are new here !

 

Your question indicates you don't know the fundamental equations that define the relationships between the curves of lenses and their focal length, plus the basic aberrations. To work well lenses are designed for the specific application, not picked at random.

 

I tried, as a kid... ultimately I threw all that stuff out as being useless - I had cannibalised old slide projectors, an overhead projector, plus a box of prisms and doublets left over from binocular production.

 

I'd have to say don't waste your money on junk optics - there really is a reason they are junk. Save your cash and buy something more useful that actually works well.

 

1. These days you can buy cheap Chinese objectives on AliExpress and by all accounts they work quite well, and are coated (which you cannot DIY). You can even buy a cell.

 

2. if you have aperture fever you can grind and polish your own mirrors as I and many other older members have done; all it requires is two glass blanks (from the CN classifieds), a place to work (its a bit messy), the abrasives and polishing materials (sold as a kit) and a Foucault or Ronchi tester (you make that yourself). Borrow a copy of the SciAm ATM books or Texereau, the whole process is amply described.

 

3. As for eyepieces... dont bother.

 

While you can cobble a 40mm Plossl using a pair of PCX achromats from binoculars, even the cheap eyepieces on AliExpress will work better than what you can make at home without access to a workshop.

 

Once you've made a decent 6" or 8" mirror, learnt how optics work, and seen the views from it you won't look at the junk again. NB I started at the age of 12 and by 17 had made a 6" f/8, 6" f/5, 8" f/7, 4" f/4, and a 6" f/19. a few years later a 12" f/4 and cassegrain secondary followed.


Edited by luxo II, 18 January 2025 - 02:09 AM.

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#4 andycknight

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Posted 18 January 2025 - 04:34 AM

Hi,

 

Technically any lens can work (even a simple magnifying glass - and long ago that was my first telescope!), but the problem is their limitations. HOWEVER don't let that stop you from experimenting! It can be a lot of fun to make your own telescope and you will learn a lot in the process.

 

For a non achromatic (simple lens) you want a long focal length. Normal practice is to stop down the lens to a smaller diameter to improve the image quality and keep the focal length of the eyepiece at least as large as the Lens diameter. i.e. a 25mm diameter lens - use a 25mm or longer Eyepiece, as this helps to keep false colour and spherical aberration under control.

 

From what you list, Binocular lenses are about the best bet... But at F/3.5~F/4 (50mm objectives) are typically only useful up to around 30~50x magnification (or the image becomes too blurred). You can stop them down (and it does help, but then you limit the light throughput). Low power (wide field), suffers from the edges of the FOV being out of focus, but you can correct this by placing a simple negative lens near the eyepiece field stop.

 

The really good thing about an old busted up Binocular, is that you get everything you need... The main lens, eyepiece and prism (for the star diagonal).

 

The problem with projector lenses is that they are too fast (typically F/2 of faster). Again they will work, but eyepieces really don't like this speed and the (cheaper) projector lenses provide very soft images anyway. As before stopping them down is the normal solution.

 

Cheap compact fixed focus cameras are normally a disaster due to the slow lens, short focal length and tiny size.

 

SLR/DSLR Camera lenses can work very well, but they need to have a long focal length to be really useful. They have a flat field of view, which is ideal for low power. The problem with camera lenses is the limited back focus (the distance between the back of the lens and the focal plane). This prevents the use of a star diagonal, but a solution here is to put a Barlow lens between the camera lens and diagonal mirror. From experimentation I've found that a 10mm eyepiece is generally about the limit for high magnification, or the image gets a little too soft.

 

Regards

 

Andy.



#5 Oregon-raybender

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Posted 18 January 2025 - 04:48 PM

I get second optics from second hand stores and such.

But I know what to look for and how to use.

Surplus Shed is another good place to get stuff.

There are other sites like that about. American Surplus

 

Starry Nightswaytogo.gif

 

https://sciplus.com/


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#6 sunrag

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Posted 18 January 2025 - 06:14 PM

Buy Chinese achromats off aliexpress.  Dirt cheap, some are decent.

I have bought several Achromats from AliExpress - all have worked very well for me (or may be I don't know any better). Definitely put a Want ad in CN. I saw a couple of ads for D152/F8 objectives for about $300.



#7 Spinwiz

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Posted 24 January 2025 - 01:30 PM

Welcome to CN, I see you are new here !

 

Your question indicates you don't know the fundamental equations that define the relationships between the curves of lenses and their focal length, plus the basic aberrations. To work well lenses are designed for the specific application, not picked at random.

 

I tried, as a kid... ultimately I threw all that stuff out as being useless - I had cannibalised old slide projectors, an overhead projector, plus a box of prisms and doublets left over from binocular production.

 

I'd have to say don't waste your money on junk optics - there really is a reason they are junk. Save your cash and buy something more useful that actually works well.

 

1. These days you can buy cheap Chinese objectives on AliExpress and by all accounts they work quite well, and are coated (which you cannot DIY). You can even buy a cell.

 

2. if you have aperture fever you can grind and polish your own mirrors as I and many other older members have done; all it requires is two glass blanks (from the CN classifieds), a place to work (its a bit messy), the abrasives and polishing materials (sold as a kit) and a Foucault or Ronchi tester (you make that yourself). Borrow a copy of the SciAm ATM books or Texereau, the whole process is amply described.

 

3. As for eyepieces... dont bother.

 

While you can cobble a 40mm Plossl using a pair of PCX achromats from binoculars, even the cheap eyepieces on AliExpress will work better than what you can make at home without access to a workshop.

 

Once you've made a decent 6" or 8" mirror, learnt how optics work, and seen the views from it you won't look at the junk again. NB I started at the age of 12 and by 17 had made a 6" f/8, 6" f/5, 8" f/7, 4" f/4, and a 6" f/19. a few years later a 12" f/4 and cassegrain secondary followed.

Thanks for your response.

 

The optics I was looking at was not exactly junk.   In this case it appeared to be a projector for medium format slides.   Sadly I let that one slip buy.   I've actually started buying quality eyepieces, well one to start, but I just have a itch to build something.

 

As for optics I'm not totally a newbie here, what I was looking for and you certainly provided a perspective is how well this has worked our for others.  Interestingly In my youth, almost 50 years ago now, i did have a couple of telescope making books.   Sadly not long after getting the books, the $$$$$ to even think about following through evaporated.  



#8 Spinwiz

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Posted 24 January 2025 - 01:43 PM

Hi,

 

Technically any lens can work (even a simple magnifying glass - and long ago that was my first telescope!), but the problem is their limitations. HOWEVER don't let that stop you from experimenting! It can be a lot of fun to make your own telescope and you will learn a lot in the process.

 

For a non achromatic (simple lens) you want a long focal length. Normal practice is to stop down the lens to a smaller diameter to improve the image quality and keep the focal length of the eyepiece at least as large as the Lens diameter. i.e. a 25mm diameter lens - use a 25mm or longer Eyepiece, as this helps to keep false colour and spherical aberration under control.

 

From what you list, Binocular lenses are about the best bet... But at F/3.5~F/4 (50mm objectives) are typically only useful up to around 30~50x magnification (or the image becomes too blurred). You can stop them down (and it does help, but then you limit the light throughput). Low power (wide field), suffers from the edges of the FOV being out of focus, but you can correct this by placing a simple negative lens near the eyepiece field stop.

 

The really good thing about an old busted up Binocular, is that you get everything you need... The main lens, eyepiece and prism (for the star diagonal).

 

The problem with projector lenses is that they are too fast (typically F/2 of faster). Again they will work, but eyepieces really don't like this speed and the (cheaper) projector lenses provide very soft images anyway. As before stopping them down is the normal solution.

 

Cheap compact fixed focus cameras are normally a disaster due to the slow lens, short focal length and tiny size.

 

SLR/DSLR Camera lenses can work very well, but they need to have a long focal length to be really useful. They have a flat field of view, which is ideal for low power. The problem with camera lenses is the limited back focus (the distance between the back of the lens and the focal plane). This prevents the use of a star diagonal, but a solution here is to put a Barlow lens between the camera lens and diagonal mirror. From experimentation I've found that a 10mm eyepiece is generally about the limit for high magnification, or the image gets a little too soft.

 

Regards

 

Andy.

Thanks Andy.

 

The projectors that happened to show up at the local thrift store, in one case was an F7 acromat and I forget what the other was.   I'm now kicking my self for not buying as they wanted like $8 for the one.   It just seemed like a complete waste of optics.

 

As for camera lenses I'm familiar with a lot of the problems associated with the short focal length.   Sadly I've yet to see and telephoto lenses.   That said 35mm cameras that would have cost me a small fortune in my youth are often to be had for less that $30.   Those are fully working cameras too.  



#9 luxo II

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Posted 25 January 2025 - 05:35 AM

As for optics I'm not totally a newbie here, what I was looking for and you certainly provided a perspective is how well this has worked our for others.

I should add in my glass-pushing period I did make an oversized 40mm Ramsden eyepiece for projecting an image of the sun during a solar eclipse, to be observed by a crowd of students. It had to be oversized to avoid the sun striking the walls of the barrel and frying anything over time; such a thing simply wasn't available in the 1970's so I had to make it.

 

Working with just a motorized spindle, micrometer and a spherometer the optical glass blanks are quite a bit softer than the old-school Pyrex, so it was quite quick and easy to make the small lenses involved in a few days, and I had access to a lathe for centering, trimming the edges and making a cell to hold them turned from brass plumbing parts. It worked out well and did for the eclipse nicely.

 

These days you'd have to be nuts to make an eyepiece as just about anything you could possibly want exists, and is available at very reasonable prices.


Edited by luxo II, 25 January 2025 - 05:39 AM.


#10 geovermont

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Posted 25 January 2025 - 11:13 AM

I think that for the best chances of success you may want to stick with repurposing binoculars. Often all that is wrong with a pair of "junked" binoculars is the collimation. That is, there are still two nice achromatic objectives and two nice eyepieces and maybe the focuser works. Find an old pair of 7 x 35 or 7 x 50 binoculars and you are off and running. For one thing, you may find that one half of the binocular can function as a pretty nice monocular. I have half of an old "Telstar" 7x50 binocular that works just fine that way. Try for the half with the diopter focus. I haven't had mine out for a long time, but I think I just sawed off the left half and kept the focuser wheel assembly. And if you find an old pair with individual focus (not center focus wheel), then you're all set--just disassemble the center joint and you have two instant telescopes. With some ingenuity involving scrap wood you could mount it on a scope as an erect image finder. Or, remove objective cell and eyepiece and mount in a plumbing parts special as an inverting finder. Add flock paper on the inside or glare stops to reduce glare. Or, get a small set of 7 x 20s or some such and remove the two objectives, mount them together with masking tape and plumbing parts wizardry and you have a nice "Plossl" (more or less) eyepiece. The experiments I am describing are actually pretty likely to be optically satisfying if you start with decent old binoculars. You don't need Zeiss here. Old Sears, Tasco, Bushnell, Bausch & Lomb etc. can work (I'd skip plastic bodies and plastic lenses to start with anyway). Have fun--it can't hurt to try.


Edited by geovermont, 25 January 2025 - 11:14 AM.


#11 BlueMoon

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Posted 25 January 2025 - 11:25 AM

 

This is related to me looking to build various scopes from finders on up - on the cheap!

From experience, as one who loves to tinker and invent, unless you have a bunch of stuff laying around that you can scavenge parts from, I think you'll find it to your advantage both cost and time wise, to simply purchase an inexpensive achromat.  It's a lot of fun to improvise, but without a "junk drawer" available, it can be surprisingly costly. Cheers and good luck.



#12 geovermont

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Posted 25 January 2025 - 11:39 AM

And yeah, you might not save much money, but it's fun to repurpose stuff. I personally dislike buying an individual item from China and having it shipped halfway around the world to my door--seems wasteful. Every junk shop, Resource, Goodwill place I go in has at least one pair of binoculars gathering dust, so that would be my personal choice.



#13 geovermont

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Posted 25 January 2025 - 12:23 PM

And it's never a bad idea to fill the optical junk drawer so you can try random fun projects at a later date!




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