Using a long achromat telescope as a general long tele lens for 135 film (coverage, apparitions etc.)

[Helge]

I’ve long thought about this, but there are many gotchas.

My old beloved Asahi 1000mm scope inherited from my dad doesn’t cover a 36x24 frame.

Is there any reasonably priced achromatic refractors that do cover a full frame?
And by reasonable I mean around $1000.

Alternatively are there barlows/flatteners that extend a scope with a roughly 24mm coverage to full frame?

ED and apochromatic lenses are usually not much longer (if at all) than for example the terrific 400mm Rokkor I already own. But of cours you can use a Barlow or tele extender, but that brings us back to the question of coverage.

The lure is of course having a very long lens for relatively cheap. If you are already on a tripod and you are not shooting birds in low lightning then an f11 aperture is not a big concern.
Mirror scopes like S/C and Newtons are not my cup of tea. Usually the contrast is too low, they are finicky and hard to focus.

And maybe I should make it absolutely clear. It’s about film. APS sensors and whatnot is not of interest. Neither halfframe SLRs.

 

[xkaes]

There are lots of scope that cover 24x36mm. Knowing whether or not they do? I don't know how to figure that out.

Some are refractive and some are reflective. A lot depends on how long you want to go.

Every lens maker sold 800mm to 2000mm lenses from Asahi to Yashica. The Yashica (Tomioka) 800mm f8 (also sold as Soligor & Vivitar) is similar to the Nikon & Canon cannons since they break-down into two pieces for transport. They are big and heavy.

But past 500mm, most people prefer mirrors, due to size and weight. Like me.

Past 500mm, I now have a 800mm Vivtar CAT and a 1250mm Honeywell (Celestron C5) CAT, and a Tamron 20-60X (1000mm-3000mm) scope. That's a Tamron 500mm CAT with zoom teleconverter on the end.

The list of lenses is endless.

Most good tele/scopes -- like the Tamron and the C5 -- have T-mounts on the rear for photographic use.

 

[Helge]

There are lots of scope that cover 24x36mm. Knowing whether or not they do? I don't know how to figure that out.

Some are refractive and some are reflective. A lot depends on how long you want to go.

Every lens maker sold 800mm to 2000mm lenses from Asahi to Yashica. The Yashica (Tomioka) 800mm f8 (also sold as Soligor & Vivitar) is similar to the Nikon & Canon cannons since they break-down into two pieces for transport. They are big and heavy.

But past 500mm, most people prefer mirrors, due to size and weight. Like me.

Past 500mm, I now have a 800mm Vivtar CAT and a 1250mm Honeywell (Celestron C5) CAT, and a Tamron 20-60X (1000mm-3000mm) scope. That's a Tamron 500mm CAT with zoom teleconverter on the end.

The list of lenses is endless.

Most good tele/scopes -- like the Tamron and the C5 -- have T-mounts on the rear for photographic use.

Achromat refractors are usually long (ca. 1000mm or longer) and slow to combat chromatic apparition.
That’s what I like about them. They give you a lot of reach natively and they are high contrast and usually well corrected.

 

[pbromaghin]

I was all hot to try this with my Celestron reflector telescope. Then I had a long and enlightening conversation with the guy at the Denver telescope store. He convinced me I would be better off using my regular Minolta 500mm reflector lens with a doubler, both of which I already owned.

Mile High Astronomy - Denver, Colorado Telescope Store

Telescopes, Binoculars, Mounts, and fun and inspiring astronomy resources for Denver, Colorado, and the Rocky Mountains. Shop our online store for the best telescopes, astrophotography equipment, and more.

milehighastro.com

 

[Helge]

I was all hot to try this with my Celestron reflector telescope. Then I had a long and enlightening conversation with the guy at the Denver telescope store. He convinced me I would be better off using my regular Minolta 500mm reflector lens with a doubler, both of which I already owned.

Mile High Astronomy - Denver, Colorado Telescope Store

Telescopes, Binoculars, Mounts, and fun and inspiring astronomy resources for Denver, Colorado, and the Rocky Mountains. Shop our online store for the best telescopes, astrophotography equipment, and more.

milehighastro.com

And why was that? They are much the same technology.

 

[pbromaghin]

And why was that? They are much the same technology.

Most hobby-level telescopes are designed for viewing by eye via an attached eyepiece lens that focuses the image at eye-distance. My telescope is one of those. Without that focusing eyepiece, the image will focus at a distance well beyond the film plane of the camera. So attaching a camera to it would require a custom lens, and I don't think those exist. Camera lensed of course focus the image at the film plane. There are telescopes that work well with cameras, but I understand they are quite expensive.

The whole idea of using the telescope wasn't to do GREAT astrophotography, but to amuse myself, which can be done with other equipment.

 

[xkaes]

It depends on the telescope. As I mentioned, many (most) good ones can be, and are designed to be, attached to cameras. My Tamron and Celetron are examples. They both require a slight modification.

The Tamron 20-60X scope has a built-in T-mount around the eye piece. Once a T-mount adapter is attached, the rear of the scope is rotated and it extends to be used on a camera.

On my Celectron 1250mm, a short extension tube is placed on the back instead of an eye piece. It has a T-mount on the end that it screwed into a T-mount adapter for my camera.

I don't disagree with the shop employee completely. Your 500mm with a 2x converter will be fine -- especially if you use a good converter, like Minolta's 2X 300-L designed for long lenses. But if you want to get passed 1000mm, you might want to consider a longer telescope -- one designed for photography.

The "problem" with 2x converters is that they produce their best results when the lens is stopped down -- and CATS can't be stopped down. Refractive lenses can be stopped down, but that means LONG exposures. So pick your poison.

As I recall, I got my USED Honeywell Lumetar 1250mm f10 (stripped-down Celectron C5) for $75. With a 2X converter it's a 2500mm f20. On a sunny day that's 1/60s with a 2500mm lens!!!!!

I'll add that I've taken some decent pictures with my Tamron scope at 3000mm -- of Mountain Goats about a quarter mile away.

 

[Maris]

Three things that influence the usefulness of an astronomical refractor for full frame film photography.

1. The focusser needs to be a 2 inch or 3 inch diameter for full frame prime focus photography.
2. Eyepiece projection may be able to deliver a full frame image into a camera body if the camera lens mount diameter is big enough or the eyepiece magnification is high enough. Tight compromises.
3. The knife edge baffles in the refractor tube designed to minimise stray light may be optimised for visual work and could need to be removed for full frame photography. Telescope surgery.

An alternative I have done is to unmount the telescope objective and fit it to a camera with sufficient extension.

 

[xkaes]

An alternative I have done is to unmount the telescope objective and fit it to a camera with sufficient extension.

That's exactly what I do with my Honeywell Lumetar 1250mm f10 (stripped-down Celectron C5). It's easier than opening a peanut butter jar.

 

[Helge]

Most hobby-level telescopes are designed for viewing by eye via an attached eyepiece lens that focuses the image at eye-distance. My telescope is one of those. Without that focusing eyepiece, the image will focus at a distance well beyond the film plane of the camera. So attaching a camera to it would require a custom lens, and I don't think those exist. Camera lensed of course focus the image at the film plane. There are telescopes that work well with cameras, but I understand they are quite expensive.

The whole idea of using the telescope wasn't to do GREAT astrophotography, but to amuse myself, which can be done with other equipment.

That is some very bad advice.
A Celestron of course has a spacer available to mount a camera. It’s a very common piece of equipment.
A telescope has the advantage of being much faster for the same aperture. And much cheaper for a given aperture, over say 500mm, if it’s available at all.
And much better integration with the absolutely necessary tripod.

 

[Helge]

Three things that influence the usefulness of an astronomical refractor for full frame film photography.

1. The focusser needs to be a 2 inch or 3 inch diameter for full frame prime focus photography.
2. Eyepiece projection may be able to deliver a full frame image into a camera body if the camera lens mount diameter is big enough or the eyepiece magnification is high enough. Tight compromises.
3. The knife edge baffles in the refractor tube designed to minimise stray light may be optimised for visual work and could need to be removed for full frame photography. Telescope surgery.

An alternative I have done is to unmount the telescope objective and fit it to a camera with sufficient extension.

That sounds reasonable. Is that from empirical observation, an educated guess or from knowledge obtained first hand from building or selling scopes?
No value judgement, all three are good.

Since the frame is 43mm diagonal it would seem 45 mm (1.8inch) would be about OK?
But perhaps vignette sets in earlier because of the projection frustum and reflections from the unbaffled tube.

I’d prefer as straight a path to the lens as possible for obvious reasons. But optics that enlarges the image to a larger projection like some eyepieces do (with a tube meant for mounting a camera of course, not with an eye cup) would of course also be an option.
Anyone know the proper name for such a device?

 

[Mark J]

An normal achromat will have a focal surface of about 1.33x its focal length ( ie. the field curvature ). For 800mm EFL this means about 0.175mm out of focus in the corners, which is tolerable at slow f/numbers. The problem with achromats is that they will have significant secondary spectrum at that sort of focal length, so they will show some colour fringing ( longitudinal ) . They also have astigmatism off-axis which will be visible to some extent. A catadioptric approach is helpful for these problems because it can be corrected for both problems - however you have the inflexibility of one fixed f/number.
Leica made some simple telephotos around 560 and 800mm back in the 60's , they were however triplet Apochromats, and may have had reduced field curvature depending on the glass types.
Anyway, it's a worthwhile project and could be quite educational.

 

[Helge]

An normal achromat will have a focal surface of about 1.33x its focal length ( ie. the field curvature ). For 800mm EFL this means about 0.175mm out of focus in the corners, which is tolerable at slow f/numbers. The problem with achromats is that they will have significant secondary spectrum at that sort of focal length, so they will show some colour fringing ( longitudinal ) . They also have astigmatism off-axis which will be visible to some extent. A catadioptric approach is helpful for these problems because it can be corrected for both problems - however you have the inflexibility of one fixed f/number.
Leica made some simple telephotos around 560 and 800mm back in the 60's , they were however triplet Apochromats, and may have had reduced field curvature depending on the glass types.
Anyway, it's a worthwhile project and could be quite educational.

A CAT can be stopped down with an off center aperture mask.
It does have the usual disadvantages of mirrors though: Lowish contrast due to central obstruction and the mirror coating not being as efficient as optical glass. Also baffling is often not as good as with refractors.
And they are not necessarily better at all than comparable refractors WRT actual aperture, image circle and resolution.
Main selling point is length vs. girth.

I’d be willing to put in a field field flattener since much of the apparent lack of off-center sharpness of acromatic scopes is merely the projection not being flat. Something you can check by tinkering with a scope on terrestrial targets.

So, any recommendations for a full frame projection achro-scope around a $1000?

 

[Mark J]

I'm sorry I haven't got a specific recommendation for a commercial achromat. You're right though that a simple field flattener could be good, but you'd need to get it close to the film plane. Good point on the off-axis mask for a Cat ; and the lower contrast ( MTF ) from the central obscuration does then disappear. I think the stray light ( baffling ) would be very good for a Cat with an off-axis mask. Ps. I have an off-axis reflector ( Schiefspiegler ) which I made 30 years ago !

 

[xkaes]

But optics that enlarges the image to a larger projection like some eyepieces do (with a tube meant for mounting a camera of course, not with an eye cup) would of course also be an option.
Anyone know the proper name for such a device?

The Tamron 20-60X scope has a fixed eye piece with a rubber eye cup -- for normal viewing. For photographic use, a t-mount adapter is screwed in over the eye piece (the eye cup is not removed). Then the entire eye piece is rotated -- which extends the rear of the scope a bit -- with no vignetting on 24x36mm film. I assume other scopes use a similar approach.

 

[Nodda Duma]

Check out scopes such as Orion, Sky-watcher brands, as well as William Optics telescopes. Most models cover full frame (such as my Sky-watcher 80ED) Many of them will cover full frame but you may find yourself needing a field flattener to maintain high quality imagery across the image since telescope refractors suffer from uncorrected field curvature. These are all used for astrophotography. William Optics refractors seem to be really popular. You’ll have to dig through specs to find the right one. Remember, telescopes are identified by aperture diameter and f/#, not focal length and f/# like in photography.

Cloudy Nights forum classifieds are worth watching to save money since amateur astronomers go through gear faster than most any other hobby.

 

[Helge]

Check out scopes such as Orion, Sky-watcher brands, as well as William Optics telescopes. Most models cover full frame (such as my Sky-watcher 80ED) Many of them will cover full frame but you may find yourself needing a field flattener to maintain high quality imagery across the image since telescope refractors suffer from uncorrected field curvature. These are all used for astrophotography. William Optics refractors seem to be really popular. You’ll have to dig through specs to find the right one. Remember, telescopes are identified by aperture diameter and f/#, not focal length and f/# like in photography.

Cloudy Nights forum classifieds are worth watching to save money since amateur astronomers go through gear faster than most any other hobby.

Yeah, the entrance diameter stuff threw me off for a minute too. Makes sense though since an astro scopes use as a light bucket is important.
No marketing teams to sell lenses with f stops, with the actual speed playing second fiddle at best.
Every photon matters.

Right now I’m considering this one.

Vixen A80Mf Porta II Telescope Set | X002519

Buy Vixen A80Mf Porta II Telescope Set directly from the manufacturer!

www.bresser.de

Vixen-A80Mf

Seems a well regarded scope well within the budget, with a serviceable mount and tripod.
I don’t need an equatorial mount for my needs right now so the included one is fine.

Problem is again, back focal distance and image circle coverage is impossible to glean clearly from any of the data sheets.

 

[Mark J]

I think you'll run into trouble with this. Lens and f/number are fine, but it is clearly built around 1 1/4" eyepieces, diagonal and focuser, at the rear. The biggest image circle these eyepieces take, is just under 1" diameter. You may find that the tube baffling is only just enough to pass this limited field, and also the internal sizing of the focuser assembly.
Bear in mind that the exit pupil of a simple lens like this is up front, at the back of the objective lens - hence the ray bundles just progress in straight lines from the lens to your desired image circle, they do not converge and then expand again like in a telephoto lens.
You minimum image circle for 35mm full-frame is 43.2mm.
What you need is a lens of approx this spec that is built for a 2" focuser and 2" eyepieces.

 

[Nodda Duma]

Yeah, the entrance diameter stuff threw me off for a minute too. Makes sense though since an astro scopes use a as a light bucket is important.
No marketing teams to sell lenses with f stops, with the actual speed playing second fiddle at best.
Every photon matters.

Right now I’m considering this one.

Vixen A80Mf Porta II Telescope Set | X002519

Buy Vixen A80Mf Porta II Telescope Set directly from the manufacturer!

www.bresser.de

Vixen-A80Mf

Seems a well regarded scope well within the budget, with a serviceable mount and tripod.
I don’t need an altazimuth mount for my needs right now so the included one is fine.

Problem is again, back focal distance and image circle coverage is impossible to glean clearly from any of the data sheets.

The nice thing about refractor telescopes is that the image circle is inherently much larger than the telescope tube, so vignetting is due to the focusing tube. You’ll want to upgrade to a 2” focuser, then to mount your camera get a 2” -to - T-mount adapter and then T-mount to your bayonet camera mount, or 2” to bayonet mount.

The scope is a classic refractor type. the Mf optics are made by Synta, so they are ok. Seems like the optical quality is better than in the past. Mechanical quality is not as good as the versions made in Japan but I’m guessing they are ok for the task.

There’s some threads in Cloudy Nights discussing the scope.

 

[reddesert]

The reasons why telescopes are specified in aperture diameter versus photographic lenses in f/stops are also rooted in the difference between sensitivity for point sources (unresolved stars) versus sensitivity for extended sources (terrestrial objects, diffuse objects such as nebulae). For the former, sensitivity depends on aperture; for the latter, it depends on the f-number of the system. IOW it isn't the marketing department, there are good reasons why it has been done that way.

 

[Helge]

I think you'll run into trouble with this. Lens and f/number are fine, but it is clearly built around 1 1/4" eyepieces, diagonal and focuser, at the rear. The biggest image circle these eyepieces take, is just under 1" diameter. You may find that the tube baffling is only just enough to pass this limited field, and also the internal sizing of the focuser assembly.
Bear in mind that the exit pupil of a simple lens like this is up front, at the back of the objective lens - hence the ray bundles just progress in straight lines from the lens to your desired image circle, they do not converge and then expand again like in a telephoto lens.
You minimum image circle for 35mm full-frame is 43.2mm.
What you need is a lens of approx this spec that is built for a 2" focuser and 2" eyepieces.

Interesting. That goes against what many diagrams shows though.

The nice thing about refractor telescopes is that the image circle is inherently much larger than the telescope tube, so vignetting is due to the focusing tube. You’ll want to upgrade to a 2” focuser, then to mount your camera get a 2” -to - T-mount adapter and then T-mount to your bayonet camera mount, or 2” to bayonet mount.

The scope is a classic refractor type. the Mf optics are made by Synta, so they are ok. Seems like the optical quality is better than in the past. Mechanical quality is not as good as the versions made in Japan but I’m guessing they are ok for the task.

There’s some threads in Cloudy Nights discussing the scope.

I know CN and have tried asking some questions over there. It’s seems they are not really interested in anything involving “less” than ED glass and they are very happy with suggesting APS sensors even in the film section.

As said earlier I want to shoot really long terrestrially. And for that, most Apo scopes that are roughly within $1000 are simply too short.
Using a tele extender on a shorter scope would be an option, but it’s not elegant or ideal from an optical standpoint.

Getting the A80M (sans f) would be an option too. Actually thought it was out of stock.
Would it be simple and economically sane (as opposed to just getting a scope with the right one) to change the focuser?

The reasons why telescopes are specified in aperture diameter versus photographic lenses in f/stops are also rooted in the difference between sensitivity for point sources (unresolved stars) versus sensitivity for extended sources (terrestrial objects, diffuse objects such as nebulae). For the former, sensitivity depends on aperture; for the latter, it depends on the f-number of the system. IOW it isn't the marketing department, there are good reasons why it has been done that way.

That sounds like a half truth to me.
F number and aperture is much the same thing.
It just doesn’t make sense to talk about aperture stops with a scope, because there is seldom a reason to stop down.
Both are about getting the most amount of light through.

T stops with cine lenses for instance, is there because f stops with still lenses are academic in nature. They are calculated from a rough model of the lens, as opposed to what really gets through.

A long scope is just more selective of the light getting through. The extreme example being the aerial scopes of the 17th century.

Aperture is also the arbiter of resolution with terrestrial photo lenses. It’s just that DoF gets much less with earth distances, this masks much of the advantage and good short focal length lenses are harder to make.

 

[Nodda Duma]

Helge it’s not difficult to swap out the focuser. It’s bolted on with a handful of screws, either hex or Philips. Most folks end up doing that even for higher-end scopes. You do have to do your research to make sure you get one that correctly fits your scope. Vixen should have one available.

As for getting the M.. the QC is better as well as the upgraded components, so I think perhaps it would be worth considering. I’d be cognizant of the risk investing money for a higher chance of poor optics (which will show up in terrestrial imaging) with the Mf, although as mentioned the QC on the Synta-made Mf version has significantly improved.

As for CN: I never ask questions there directly, because as you found it’s kind if worthless to do so. Like here on Photrio, googling the archives reveals a wealth of relevant information built up over the years for whatever topic I’m looking up.

Hope this helps and good luck!

 

[reddesert]

That sounds like a half truth to me.
F number and aperture is much the same thing.
It just doesn’t make sense to talk about aperture stops with a scope, because there is seldom a reason to stop down.
Both are about getting the most amount of light through.

T stops with cine lenses for instance, is there because f stops with still lenses are academic in nature. They are calculated from a rough model of the lens, as opposed to what really gets through.

A long scope is just more selective of the light getting through. The extreme example being the aerial scopes of the 17th century.

Aperture is also the arbiter of resolution with terrestrial photo lenses. It’s just that DoF gets much less with earth distances, this masks much of the advantage and good short focal length lenses are harder to make.

Do you have a rationale or numerical argument for saying this is a half truth?

Suppose that we have a 100/2.8 lens and a 50/1.4 lens (using this pair because these are common lenses a photographer might have access to). Both of these have entrance pupils (aperture diameters) of approx 36mm when wide open.

For the photography of an ordinary scene, say indoor lighting, if your meter says the room is 1/60th sec at f/2.8, you use the 100/2.8 lens at f/2.8 (wide open), and the 50mm lens at f/2.8, stopped down to an 18mm entrance aperture. Why is that? Basically, it's because the 50mm lens is collecting light from a larger field of view (2x larger linear, 4x in area) and putting that light onto the same area of film or detector. So it needs to use a smaller entrance aperture.

Another way of making the same argument is that the 100mm lens has greater magnification, so it spreads the light from any individual patch of the subject onto a larger area of film, thus it needs to collect more light for the same exposure. This is why meters can read in f-numbers without needing to know the focal length of your lens.

Now consider the astronomical case of an unresolved star. Both the 50/1.4 and the 100/2.8, when wide open, are essentially 36mm diameter telescope objectives. Both collect the same amount of light from the star. If you put the same eyepiece behind each, you would get a higher magnification from the 100mm, but the star is unresolved - magnifying its image doesn't make it larger, unlike a terrestrial subject. So it turns out that the star brightness you can see is sensitive to the aperture diameter, not the f-ratio. Stopping the 50mm lens down to f/2.8, you'd see fewer faint stars than the 100/2.8.

So there's a good reason why photographers use f-numbers and telescopes use aperture diameter. However, it turns out that astrophotographers do care about f-ratio, because if you want to take pictures of diffuse objects such as nebulae, the Milky Way, etc, you are back to the case where magnification and f-ratio matter. Amateur astrophotographers are capable of spending outrageous amounts of money on fast optics. Professional astronomers are too, but with large telescopes one rarely sees final f-ratios faster than f/5 or so.

Not to be completely useless, if the main goal is a telephoto lens, I wonder if you actually need the telescope, focuser, mount etc that comes with the typical entry level refractor. If you just want an achromatic objective, those are available by themselves, for example Surplus Shed: https://www.surplusshed.com/category/Objective_Lenses They have an 80mm diam, 900mm fl achromatic objective, coated and mounted, for USD 42. I have no idea of the optical quality, but if you're going to replace the focuser on a commercial telescope right away anyway, maybe getting an achromat, a plastic tube, and the focuser you want is a way to start.

 

[Dan Fromm]

Not to be completely useless, if the main goal is a telephoto lens, I wonder if you actually need the telescope, focuser, mount etc that comes with the typical entry level refractor. If you just want an achromatic objective, those are available by themselves, for example Surplus Shed: https://www.surplusshed.com/category/Objective_Lenses They have an 80mm diam, 900mm fl achromatic objective, coated and mounted, for USD 42. I have no idea of the optical quality, but if you're going to replace the focuser on a commercial telescope right away anyway, maybe getting an achromat, a plastic tube, and the focuser you want is a way to start.

Diaphragm?

 

[redbandit]

hate to hijack, but i have to ask.

This is about actual, commercially sold TELESCOPES....

Vivitar sells the funky t mount telescope CAMERA lenses... i have one.... its not actually a fixed aperture... minimum lenght is X f stop... but every 100mm of focal length gives you a smaller and less useful f stop to use.