Large format Lenses (Individual elements) to build a refracting telescope

[Steven Ruttenberg]

I am looking for individual elements to build a Cook Triplet. Preferably around 5-10 in in diameter. Building a large format refractor telescope. Right now is to do something called eye piece projection using a 4x5 or 8x10 camera with a lens and attach it to the focuser of the telescope to get a large enough image circle to cover 4x5/8x10 dry plates or film.

[jnantz]

Have you found anchor optical or the surplus shed yet? they sell lens glass and “stuff “…

[Steven Ruttenberg]

Have you found anchor optical or the surplus shed yet? they sell lens glass and “stuff “…

I will check them out.

[Mark J]

I wish you the best of luck but it's going to be tough to find the right sizes, shapes and powers. Do you have a design on paper ?
Back around 30 years ago, our company made a 14" aperture Cooke triplet for a camera obscura that was built in Aberystwyth, mid Wales.

[Dan Fromm]

Hmm. OP, it seems you're fixated on a large front element. 5" - 10" in diameter. I'm not sure you can afford any of them, but start looking here: https://web.archive.org/web/20210517230952/http://alag3.mfa.kfki.hu/astro/giantlenses/200mm.htm

[reddesert]

The point of a Cooke triplet or similar lens design is to provide a flat field and good image quality over some area of film without intermediate optics. If you are doing eyepiece projection, you're interposing a lens that accepts rays over a much smaller area - basically whatever angle of rays you can fit through the focuser and the lens aperture - and images onto the film. Getting this set up so that the image is focused correctly on the detector can be somewhat challenging even for a normal amateur-size telescope and eyepiece.

For the proposed large format telescope, I think getting it to illuminate a 4x5 or 8x10 area will be quite difficult; there will likely be a lot of vignetting. IOW, the Cooke triplet is the least of your worries. Start with a large single element positive lens or achromatic doublet as the telescope objective, set that up on a bench with your planned large format lens/camera, and see if you can come up with a setup (focal length of the "eyepiece" lens, spacing between objective, eyepiece, and film, etc) that illuminates an acceptable area of film. You can then think about the mechanical constraints this places on the telescope design.

Astronomical telescopes that illuminate a large field of view tend to use optically fast systems (short f-ratio), either prime focus with a corrector/field flattener, or a camera-specific design like a Schmidt camera.

[Corran]

Perhaps something to read through:
https://jackdoerner.net/exposition/2016/02/Large_Format_Astrophotography/

I still would recommend spending a good few years learning astrophotography with simpler tools. It's hard enough with a modern digital camera and decent telescope.

Or at the very least, start with a 4x5 and standard 210mm or 300mm lens and work from there.

[Corran]

Of course if you're an oil baron or whatever you could always just pick this up:

https://planewave.com/products/rc1000/

I'm sure they will build you something to cover 4x5.

[LabRat]

The "old school" way to build telescopes was using war surplus aerial camera lenses (cheap and plentiful)... Not the fastest lenses, heavy (often), but have irises to reduce light... Longer FL's were mostly used with 9"X9" formats...

Steve K

[Photomagica]

Steve,
I've built several refracting telescopes and am very familiar with all aspects of astrophotography. I was a lead planner on the Giovale Open Deck Observatory at Lowell Observatory and helped them select the PlaneWave telescope for the Dyer Observatory. Feel free to contact me at bill@billpeters.ca for specific help.

If you are eyepiece projecting on a 4x5 inch or larger film, the system will be slow, with a large effective f-number and that will limit your to brighter objects like the Sun (with a filter) and the Moon. For larger fields of view it is best to leave out the eyepiece and acquire a lens that covers the plate.

Tell me more about the types of objects you want to photograph or the size of the area of the sky you want to cover and I may be able to make some suggestions.
Best regards,
Bill