Spotting scope as prime-focus lens for astrophotography?

[johngwheeler]

Hi ,

I have a Celestron Regal 80mm ED spotting scope (f6) and am thinking of mounting it on my Celestron AVX EQ mount to try a bit of photography.

What would be the best way to attach a camera to the scope?

I have read quite a few posts on various bird-watching forums but still have a few questions.

I have a good quality micro 4/3 fixed lens camera, and a Canon DSLR (body only at the moment). I have tried "eyepiece projection" using a T-ring and T-adapter to couple the DSLR body to the spotting scope eyepiece, and used the fixed-lens camera just held up to the eyepiece. Here are my observations from terrestrial photos:

1) DSLR body only through scope eyepiece: the magnified image fills the frame, but is very distorted on the edges of the frame. I would say that only about the centre 50% of the frame is usable. The downside is that the camera without the lens can't use the LCD screen as a viewfinder (so I have to use the optical viewfinder which is much less convenient). It also doesn't seem to do any through-the-lens metering because the camera thinks the aperture is f0.0, so exposures take a lot of guesswork.

2) Fixed lens camera through scope eyepiece: the image is nicely focussed, but seems to suffer a lot of vignetting - the diameter of the image circle is only about 50% the width of the frame. I have tried changing the zoom of both the camera and the scope, but it doesn't make much difference. Maybe I need to hold the camera much further away from the eyepiece?

So, my questions:

1) Can I fit the DSLR body to the spotting scope *without* an eyepiece, and use it a telephoto lens, i.e. use the prime-focus method? 

I don't have the T-adapter for this yet, but my major concern is that the spotting scope won't have enough back-focus range to allow the image to focus on the camera's focal plane. Just holding up the camera body to the scope (without eyepiece) didn't get anywhere near focussed images. Are there any T-adapters that include optical elements to allow this? 

2) Will the quality of prime focus be any better than the afocal (eyepiece projection) method? If not, should I just a lens for the DSLR and use the afocal method?

3) Are there any couplings that allow a camera *with its lens* to be screwed to an eyepiece, rather than using a "non-contact" coupling platform that just holds the camera behind the eyepiece? Presumably such a coupling would need to fit to the filter thread of the camera lens - would this be strong enough to support the camera?

Thanks for any advice you can give me!

John

[kalasinman]

IMO you are inventing a host of problems. Good quality manual prime lenses (non zoom, non AF, ) can be picked up on Ebay for very cheap. I think you'll get to some problem free pictures much quicker with a made for purpose gear, rather than trying to re-invent the wheel. The Canon "nifty 50" is easy to find. Any longer than that in FL will require quite short exposures and high ISO to avoid star trails. BTW you are at the edge of a slippery slope!

[physicus]

I have some sympathy with you trying to do this. But while kalasinman's advice might sound a little defeatist, he may well be on the right tack.  I have a Swarovski Habicht AT80 spotting scope that cost a fortune new, and is actually a fantastic visual scope for astronomy - within its 20-60 mag range; so naturally the instinct is to put it into photographic service.  Problem is, the focus is buried somewhere behind the zoom eyepiece in the fantastically well made prism block, so not much choice other than eyepiece projection or holding your camera lens over the eyepiece (which is I think how most of the gadgets you can buy work: expensive gadgets in the case of Swarovski!).  I've seen some very cheap Chinese adapters that might help you - you'll just have to Google it - that just hold a standard camera off the eyepiece for about £10 (UK).  It's depressing when a whole lens costs less than your planned hack, and is probably better into the bargain, but that's sometimes the way of it!

Tim

[johngwheeler]

Thanks for the replies. As luck would have it, I've just remembered that a family member has an older Canon 35mm film camera, and a couple of Canon EF lenses (50mm f1.4 and 28-80mm f4.0-5.6), which appear to work in my DSLR. 

I haven't seen any system that allows eyepiece project *with* the DSLR lens fitted, so I may try one of the digiscoping brackets, which would also work with my compact camera. I haven't found any option for connecting eyepiece to camera lens via coupling adapters, and this may not be possible due to strength limitations of the threads - hanging the entire DSLR + lens from the filter thread on the lens sounds risky. Putting all of this on the eyepiece thread sounds worse! 

I fear that as suggested, the 'scope's focal point is deep inside the telescope, so there's no hope of using it as prime-focus lens without some kind additional optics, such as a Barlow. I know that Celestron does make a 1.25" T-adapter that includes a 2X Barlow, so this might be worth experimenting with. 

Swarkovski make something similar for their modular scopes, but it costs over $550! At that price, I might just as well buy a dedicated small astro refractor, without the focussing problems.

As you say, I could be on a slippery slope, but as Macbeth said "I am in blood stepped in so far that should I wade no more, Returning were as tedious as go o’er" 

[The Admiral]

John, I understand where you are coming from on this. I have an Opticron spotting scope and now that I have some sort of tracking mount I can put it on, I thought it'd be fun seeing what I can record photographically. This is not a serious foray into astrophotography, which I would neither be able to afford nor have sufficiently dark skies (or patience) for, just trying to get the most out of what I already have. After all, birders do use their 'scopes for photographing wild-life quite successfully, though I recognise there will be some limitations. I stress though, that I have yet to try it for real!

From what I understand, and from what others have said, I don't think you'll be able to use prime focus. The options will be to photograph through the eyepiece, with or without a camera lens, or use a subsidiary attachment which replaces the eyepiece and contains a lens to project a field-flattened image directly onto the sensor. The latter is supposed to give better quality. Fortunately, Opticron make attachments to do either of these (http://www.opticron.co.uk/Pages/telephoto_slr.htm), and they are not that expensive, though I realise makes like Swarovski charge a fortune for the like. Can the same be obtained for your Celestron?

At the end of the day it will be your choice as to how much money and time you spend trying it, or whether it'd be worth buying an astro 'scope instead, or just accepting defeat .

Good luck!

Ian

[kalasinman]

The 50mm 1.4 sounds great, the iother might give some good results too.

[RichM63]

Hmmm, I tend to try just about anything even when there are more obvious and better solutions.

Here you will see my 700D mounted with, a T2/EOS adapter, an empty skywatcher barlow tube, the short supplied  T2 adapter, on the back of my ST80 telescope, all mounted with a sliding front tube ring and fixed both ends to a long dovetail bar on my EQ3-2 mount. 

The empty Barlow is required to have enough focus travel.

Focusing is manual using the ST80 knobs.

The camera is set to M mode and exposure controled via the histogram.

There is no camera lens in the mix.

The rig can be used for day time out and about photography without the EQ3 but it requires either a very steady hand or a cushion on the car roof.

Have fun.

[johngwheeler]

Hmmm, I tend to try just about anything even when there are more obvious and better solutions.

Here you will see my 700D mounted with, a T2/EOS adapter, an empty skywatcher barlow tube, the short supplied  T2 adapter, on the back of my ST80 telescope, all mounted with a sliding front tube ring and fixed both ends to a long dovetail bar on my EQ3-2 mount. 

The empty Barlow is required to have enough focus travel.

Focusing is manual using the ST80 knobs.

The camera is set to M mode and exposure controled via the histogram.

There is no camera lens in the mix.

The rig can be used for day time out and about photography without the EQ3 but it requires either a very steady hand or a cushion on the car roof.

Have fun.

IMGP1452_1.jpg

Nice setup! I like the look of the long dovetail on which both the scope and camera are attached. This looks like a really solid setup - far better than hanging a ton of adapters and cameras off the end of the eyepiece!

Where did you get the dovetail from?

[RichM63]

The dovetail is a stock item from an astronomy shop, they can be bought in different lengths, and yes, it is solid with everything being supported.

Best of luck,

Rich