Retrofitting a DSLR camera to a super-cheap Tasco reflector telescope

Question

I was given a 1995 vintage Tasco "302003" Newtonian telescope recently and have had some exciting success with it. I have managed to view the Moon in beautiful detail and also had my first glimpse of Jupiter (with Enceladus Ganymede & Europa in the same shot!).

I was able (with very considerable difficulty) to photograph both Moon and Jupiter scenes by holding my camera up to the eyepiece. Far from ideal but it has really fired my imagination about doing more astronomical photography.

Here's a picture of my telescope and it's accessories. Despite spending the last 15 years in the back room of a charity shop, it's in brand new condition and all the parts are present including the tripod and all fixings!...

(Blue arrows indicate places where eyepiece lenses can be inserted).

My camera is a Nikon D40 (APS-C format, DX lens ring system).

Is it possible to modify this telescope's eyepiece (a rack-and-pinion tube-length adjustable type) so that I can attach my camera in order to take photographs of astronomical subjects? Photographs of the Moon would be my primary goal.

I am happy to make severe modifications to the telescope if necessary. But I realise that certain measurements would be absolutely critical to making good photographs possible. I think I can make an adaptor that goes between the eyepiece socket and the camera. But I don't know all of the parameters I will have to know to design such an adaptor.

Problems I believe I need to solve...

• I think that holding the camera's central axis perfectly concentric with the eyepiece's axis is critical.
• I need some sort of Relay Lens between the secondary mirror and the camera body.
• Keeping the rack-and-pinion tube adjustment (actually a fairly sturdy mechanism) is important and would allow better focusing of the picture as the camera is not likely to have enough auto-focus range to cope with the 700mm F length.

I do have a small Milling machine and most of the basic machine tools. I have a ready supply of aluminium stock and industrial plastic stock (RG1000) that I can build an adaptor from.

Is this project feasible?

I'm not looking for perfect photos, but rather something that is better and more convenient than holding my cellphone camera close to the eyepiece and holding my breath! :)

My astronomy/optics knowledge is practically zero but I'm a fairly savvy mechanical engineer and can build things to a specification.

Thank you for reading, I look forward to your responses.

Answer 1

The problem you have with many newtonian reflectors - especially smaller ones - is that to use them for prime focus photography (where you're effectively using the telescope like a big telephoto lens) , you need to set things up so that the camera sensor ends up at the focal plane of the telescope. And in a DSLR, the sensor is some way inside the camera body (behind the shutter, which is behind the mirror.

With a newtonian scope, the focal plane is usually somewhere inside the focuser tube, and many newts don't have enough focuser inwards travel to let you get the focal plane the 40mm or so beyond the end of the focuser that you need to get it to reach a DSLR sensor.

(With other types of scope that use a separate star diagonal, things are usually easier - you can leave the star diagonal off, which has the same effect as a big chunk of inward focus).

With a newtonian, if you can't reach prime focus, there are three basic options - either take photos through the eyepiece (afocal photography) as you already tried,

or move the main mirror further up the tube (which moves the focal plane further out, giving you more in focus). It also means the light cone is fatter when it reaches the secondary mirror, so you may lose some light as the outer part of the light cone misses the secondary.

or you can try using a barlow lens - that replaces the last bit of the light cone with a longer, narrower angle version (which increases the magnification and moves the focal plane further out). That MAY or MAY NOT let you reach infinity focus; it's not guarenteed, but may help. You can get barlows that have a T-Thread on the output end, so you can attach a T ring directly to the barlow (yours doesn't look like one of this type) and then hook the camera up that way. (as an added benefit, since you end up with a closed tube setup (the barlow lens closes off the access to the camera, it also keeps spiders and dust out of the camera :) ).

If you have a look at the bottom bit of the barlow (with the lens) you may find that you can unscrew it. You might be able to make a short top section with a T thread for the T ring at the back, and a suitable thread for the lens at the front; you'd probably need to make it the same diameter as the eyepiece barrel (to fit in the focuser) with a fatter bit at the back for the T thread and to stop it sliding all the way into the focuser. I've used a commercial 1.25" barrel T threaded barlow (somewhat shorter than yours) for lunar and solar (WITH A SUITABLE FRONT OF SCOPE FILTER - DON'T TRY LOOKING AT THE SUN WITHOUT ONE) imaging with an APS sensor camera.

(You may need to check your scope - older ones often used a .965" barrel rather than the current 1.25" (or 2") barrel size used by most modern astro scopes.)

Answer 2

Another problem I found, is that the tasco rack and pinion focus system, was not robust enough to deal with the weight of a DSLR, as you try to obtain focus, the tube would get stuck! You may need to replace it with one that can handle the weight!