telescope as a lens?

[GPS]

Why vignetting? The exit pupil of a telescope like the celestron f/10, focal-lenght 2000mm and entrance pupil 200mm, is in the area of 5 to 7mm. And the entrance pupil of a LF-lens f/8 150mm for an 4x5" camera is 18,75mm. So the rays from the telescope leave the telescope's eyepiece parallel and if the LF-camera without telescope is focussed at infinity the image produced by the telescope with eyepiece fills the whole 4x5".

Of course if the mechanical problems to mount the camera are solved.

Peter

Unfortunately Peter, the things are much more complicated in the afocal photography than you see it. I mistakingly took you for someone who knew it...
In the afocal photography, the vignetting depends on many factors, such as the eyepiece eye relief, the position of the diaphragm of the taking lens in relation to the exit pupil of the eyepiece, the film format, the focal length of the camera lens etc.
If you want to study it more I suggest you see the easy but good book of "Astrophotography for amateurs" by Michael A. Covington. Cheers!

[Peter K]

GPS, thanks for your correction. I've started with the assumption an, often used, eyepiece with 17mm eye relief. Together with a "normal" camera lens - no Tele or WA - in use. With such a lens one has some degree of freedom. And no vignetting if the camera lens aperture is wide open.

Peter

[Joseph O'Neil]

Attaching a DSLR to any telescope (1.25" or 2 inch focuser) is extremely easy. All you need is a T-ring, (about $20), then either 1.25" or 2" adaptor (another $15 to $40) and in some cases, and extention tube ($30 to $60). When possible, 2" adpators are best for full coverage.

For wider coverage , Takahashi Petvals can cover 120 film (seen it in person first hand) and I suspect you can try them on 4x5 sheet film with spome decent luck. You might not have full coverage, you you could get a nice image.

Most of the time, I do not see actual 4x5 or other large format cameras attached tot he back of a telescope, rather, you see 4x5 backs attached to telescopes - all sizes of telescope.

In fact, two weeks ago I was touring the 72 inch David Dunlop observatory north of Toronto, and it used ot use 4x5" glass plates. Kodak stopped producing thier specialty films for astronomical use - oh, around the mid 1990s, if memory serves more correct. The plates and films had special spectral response,a nd had names like 103a, etc.

The camera at the DDO north of Toronto used liquid air (nitrogen I think - but I stand ot be corrected) for extreme cooling during exposures, but the other way to do it was bake the glass plates, and in some cases, gas hypering. I still have my gas hyerping setup, while tech pan was always used gas hypered, it works on all films.

so it isn't all that hard, your major issue, IMO, is lack of image coverage. If you are happy with a small image circle on the centre of a sheet of film, it can be done. One last thought, to hold the film steady for long term or time exposures, Linhof holders were always prized.

joe

[adrian tyler]

thanks for all the interesting replies, it woud seem a shame to spend a lot of money on a tesescope and no be able to do astronomical work with it so it looks like i have to do a bit of reading...

"GPS" well both 35mm & LF are of interest but LF was certainly not in mind when i made the post, so yes, smack bottom and banished to the lounge.

Dan: thanks for the tip to the Televue refractors. Struan: thanks for the oriental "porn" make me sweat looking at the price list...

adrian

[Tim Povlick]

A good APO telescope makes a nice camera lens, although a very small FOV. If shooting with camera attached to scope directly looking at the objective then one needs a field flattener (FF) to cover 35mm to 6x7 size. Attached are two images taken with two different APOs. The smaller sign (right) was taken with a 480mm f/l scope at f/6, but no FF. The field curvature in the full size image is very apparent (this is a tiny crop). The sign post is about 3 inches tall and about 120 feet distant.

First (left) image is with a 152mm APO (1200mm f/l) with FF. Both scopes and the FF were designed by the late Thomas Back, optical genius. Yours truly did the mechanical design (with great help from "my" machinist Ross) of the FF.
The sign is about 200 feet distant and taken with a 6x7 PentaxII. The FF made full FOV in perfect focus. The biggest problem to really sharp images is the daytime turbulence. Had I taken this image about 2 hours earlier it would be sharper.

Tim

[adrian tyler]

A good APO telescope makes a nice camera lens, although a very small FOV. If shooting with camera attached to scope directly looking at the objective then one needs a field flattener (FF) to cover 35mm to 6x7 size. Attached are two images taken with two different APOs. The smaller sign (right) was taken with a 480mm f/l scope at f/6, but no FF. The field curvature in the full size image is very apparent (this is a tiny crop). The sign post is about 3 inches tall and about 120 feet distant.

First (left) image is with a 152mm APO (1200mm f/l) with FF. Both scopes and the FF were designed by the late Thomas Back, optical genius. Yours truly did the mechanical design (with great help from "my" machinist Ross) of the FF.
The sign is about 200 feet distant and taken with a 6x7 PentaxII. The FF made full FOV in perfect focus. The biggest problem to really sharp images is the daytime turbulence. Had I taken this image about 2 hours earlier it would be sharper.

Tim

hi tim, they both look very sharp which is encoraging, what is a "field flattener?"

thanks

[Tim Povlick]

hi tim, they both look very sharp which is encoraging, what is a "field flattener?"

thanks

Hi Adrian,

A field flattener is a multi-element (2 to 4) optic placed close to the focus plane (placement is typically critical +/- 1mm). It flattens the curved focus plane of a typical refractor telescope. The diameter must be large enough to not vignette; for the 6x7 this required a 93mm lens followed by an 88mm lens. When focusing the FF must move with the camera, hence the focuser has to deal with the weight of the camera and FF.

If you check the bottom of this page:

http://www.ccdware.com/products/ccdi...r/examples.cfm

you can see the effect of without / with the FF. The images are mapping the Petzval surface (plane of focus). I believe some of the old Schmidt cameras (telescopes) had film holders that bent the film to match the curvature of the focus plane.

Best Regards,
Tim