Astrophotography with a 12" dobsonian telescope?

Question

Can anyone give me as much advice on how to take nice pics. I know dobs arnt great for photo's but you still can try. I have A canon digital eos cam. i have the T-Ring and extender tube for the eyepeices. but when i take the shot as i focus, say on the moon, when focusing the center of moon is crystal clear, but outer parts blury, when i try refocusing and make the center blury the outersides of moon r clear. how do i remedy this???????????????

THANKS A BUNCH GUYS AND GALS

Answer 1

There seem to be about eight issues you have to deal with.  I'll start with the first four.

Focus, focus, focus, and focus.  Some people swear by motorized focusers, and software that finds the focus.  But here's another technique:

I don't use a motorized focuser. I do use a Stiletto focus aid to

nail the focus on all of my OTA's. I slew to a bright star and

replace the camera with the Stiletto. Focus until the Ronchi lines disappear, lock the focuser and you are done. Remove the Stiletto and replace it with the camera. I cannot get my head around all that image - adjust stuff!

I do find it a good practice to check focus if I move the scope to a new target a long way across the sky. I just stop at a bright star on the way and do a quick check. It really is quite simple. You need to come out my way some night and we can compare techniques. (or you could go to a local astronomy club.)

The other four problems are tracking, tracking, tracking and did i mention tracking?  Getting a really good tracking mount for a 12 inch is either difficult or expensive.

But before you spend gigabucks on a mount, try this:  Point your dob near Polaris and take very short images.  No tracking.  Your dob mount should be steady.  You can concentrate on focus.

Or, use a web cam instead.  Take huge numbers of short exposures, and stack them with software, like registax.

Some of the guys at the club have two scopes.  One for visual work, one for photography.  The visual scope is a light bucket, like your 12 inch.  The photography is often a very small APO refractor.  The idea is that a mount for a small scope is much, much cheaper.

At the high end, one guy has an 80 mm Tak, which he piggybacks on his 11 inch SCT.  He gets awesome narrow band nebulae images.  He uses the SCT for tracking.  He uses the 80 mm for imaging.  His filters let in only a little light, so he takes hour long exposures each for red, green and blue (which are H alpha, sulfur 2, and oxygen 3).  They look like Hubble Space Telescope images.  His setup can be had for just $11,000. Did i mention?  He's got a pair of SBIG astrocameras.

I've held a point and shoot digital camera to the eyepiece of my 10" dob.  I take 100+ pictures and pick the best.  Typically a few of the Moon shots will come out.  I've got a few of Saturn showing the rings and moons, Jupiter with moons, crescent Venus, transits of Venus and Mercury (using projection), the Sun (using a solar filter). I took Hale-Bopp comet pictures (from when it was really bright) with a web cam and a spotting scope.  While the Moon looks pretty good, most of these images are at least somewhat disappointing.  These cameras were not designed to do this, and it shows.

Answer 2

you don't do astrophotography with a dob.

you can try an equatorial table if you want. you'll need to sort out your optics (coma, field curvature, etc.). lose the eyepieces; eyepiece projection is a discredited technique for history books only. a barlow may be handy for both image scale and optical aberrations. you already know focusing is a pain.

there are books on the subject. look for astrophotography for the amateur and dslr astrophotography, both by michael covington.

please reconsider your plans. unless you know exactly what you're doing, you do not want to go there.

Answer 3

Mount it on a co-latitude wedge for tracking.  

Make a worm gear manual or motor drive.