Help with Astrophotography through a Telescope

petrochemist · Apr 7, 2015

My DSLR can't get close enough to the mirror on my Newtonian to reach infinity focus, but my mirrorless cameras can.
With the DSLR adding a barlow (as suggested by others above) solves the problem.
It also works OK projecting through the eyepiece & using a lens on the camera giving more magnification.
There are multiple ways of connecting camera & scope, investigate prime focus, afocal, positive projection & negative projection for the full range of options.
IMO Prime focus tends to give the best results, if your hardware permits it, but perhaps thats just because my eyepieces are cheap.

astroNikon · Apr 7, 2015

I missed the part of a Newton scope.

The Prime focus that Tim mentions .. this is an example for a Meade 125 ETX, is the small middle front silver piece in the photo

The black part attaches to a T-Mount and the silver part slides in as if it is a eyepiece. I don't think your Newton has the features for the "tube" rear port camera mount.

The rest of the items you see allows you to use a eyepiece as a magnifier. It's a PITA to adjust focus though. The eyepiece goes into the contraption. And you loosen/tighten the screws and slide it up and down to obtain focus. Just the one piece for Prime Focus works great though.

On a Meade 125 Scope it replaces the eyepiece, such as this

With just the Prime Focus part it's right up to the eyepiece holder part of the scope - unlike the above photo which used the entire contraption lol

The "tube" aspect goes right out the middle back of the scope. This is preferred in a way as the eyepiece part requires the mirror to redirect the image up. The tube part does not use the mirror and is flipped out of the way by a switch on the scope.

On smaller scopes with plastic drive gears and small motors the prime is better as it balances the scope better front to rear. With it sticking out the back my 125 could not slew as it threw the weight balance off, which required balancing weights which the small motor couldn't really handle anyways.

Because of the weight problem, what you see above is I took the OTA off the forks and just plopped it on my heavy duty tripod. Since then I bought a much bigger scope.

TCampbell · Apr 7, 2015

With a Newtonian reflector the eyepiece is near the front of the optical tube (on the side). Since the scope is intended for visual use and the eyepiece is already near the front, there's no need of a 90º diagonal. The position of the focuser is designed to focus without one.

When you attach an SLR or DSLR to a newtonian, the focus plane is a bit farther back because it needs room for the reflex mirror. This positions the sensor beyond the distance where the telescope achieves focus. You end up racking the focuser all the way "in" and often just as you see the image is starting to come to focus, the focuser runs out of travel and you can't achieve sharp focus. When you have such a problem you can usually use a "Barlow" to help the scope achieve focus. But using a barlow changing the focal ratio of the scope (by the factor of the barlow... a 2x barlow doubles the focal ratio.)

But as I re-read the thread... the OP doesn't have a Newtonian reflector... they have a refractor. Refractors have the opposite issue.

On a refractor, the eyepiece is at the back of the scope. This means that to use the scope visually, one might have to crouch down low and try to look "upward" into the eyepiece... that's not comfortable for observing for very long. Hence nearly all refractors include a 90º diagonal so that you can observe from a more comfortable position. But when using a camera, the diagonal is usually removed. The diagonal adds about 2" worth of distance in the focal length of the scope and without those 2" the scope may not come to focus. In this case you end up racking the focus all the way "out" and run out of travel (the opposite of what happens with a Newtonian reflector). When this happens, the problem is much easier to solve... a simple 2" extension tube adds that 2" back into the path and the scope will be able to give the camera a focused image.

SCTs (Schmidt Cassegrain Telescopes) and Maks (Maksutov Cassegrain scopes) don't have either problem because the scope is focused by moving the primary mirror (not an eyepiece drawtube). Every 1mm of travel of the primary mirror will shift the focus of the scope by 2mm, and usually the travel on the mirror is generous. I've never encountered a Mak or SCT that couldn't focus with a camera without having to add anything else (no extensions are needed, no barlow is needed, etc.) These scopes tend to have such long focal lengths that sometimes the imager uses a "focal reducer" (the opposite of a barlow or teleconverter) to shorten the effective focal length, reduce the focal ratio, and provide a wider area of sky in the image.

My TeleVue NP101is is an apochromatic refractor designed specifically for imaging. It has a generous enough focus travel that it needs no accessories to help it focus (it shouldn't... it was designed with the intent that a camera would be attached.) But I also have an inexpensive Orion ST-80 achromatic refractor that I bought to use as a guide-scope. When a camera is attached to that particular scope, it will not come to focus unless I add a 2" extension (and you want these extensions to have a good fit and be rather snug because any "wobble" of the extension will create blur in long-exposure images.)