Hi Paul

Thank you for the tip. I found the below link in Richard response below. I’m attaching it as I found it useful

telescopemount.org/short-exposure-alt-az...o-get-great-results/


Thank you
Mohamed

I've imaged Altaz a long time ago, it does work with short exposures, less than 30 seconds usually. One thing that helps is to image objects as they are passing through east or west because in those directions the image rotation rate is smaller. Never tried guiding but the problem may be that if your guide star is not at the centre of rotation of your image the rotation will cause your guide star to move.

BTW, the focal length of an 8" F/10 scope is a bit long, even with a FR it has a focal length of 1260 mm. What I find is that for a budget imaging system an 80mm F/5 ED refractor is good. The shorter focal length reduces all you pointing issues and the light weight doesn't troube the mount. I have one on an AVX. I'm using an 80mm F/4 scope mounted side by side as a guider, that's a bit OTT but at least I never need to worry about finding a guide star.

Mohamed,

Generally with SCTs it becomes necessary to use an off-axis guider where the guide camera is using the main telescope. Reasons for this include mirror flop (which may be less of an issue for Alt-Az mounting) and the ability to keep a guide scope from moving differentially to the main scope.

If I've done the math right, 1 micron of movement of your guide scope (assumes 50mm f4 -- fl of 200mm) will equate to an angle of ~1 arc-second.

Using the same assumptions, your guide scale will be ~3.75 arc-seconds / pixel. With the reducer, your imaging scale will be about 0.6 arc-seconds / pixel. This may be OK if everything else is spot on.

[mention]RDBeck [/mention]
Thank you, Richard. Do you advise using OAG with the separate guide camera or use the main camera as a guide camera as well?


[mention]ChrisRowland [/mention]
I've read some posts about doing astrophotography with AltAz mount and I'm very interested in giving it a try before investing another $1k for the GEM


Thank you
Mohamed

Mohamed,

The OAG fits in your imaging train and uses a prism to direct light/stars to a guide camera which is 90 degrees to the main imaging train. I suggest you hold off on guiding completely in the short term and focus on shorter exposure photography (say, 30 seconds, just find the best ISO setting for this). I started with a Nikon D7000 on the SLT 90 and decided that would only work for me for very limited cases. When I moved to a 80mm refractor on a decent equatorial mount, I kept using the DSLR until I got bothered by so many shutter releases to focus the scope.

One other point, part of my issue with the SLT 90 was the single arm support which is what I think you also have. If you can lock up the mirror on your DSLR, it should reduce vibration (shutter still causes some).

Richard