Ad Astra Observatory

Astrophotography has a lot of legends and rumors floating around as regards focal length of scope. Since the focal length of the scope, size of the CCD, weight of the rig, and carrying capacity of the mount are often intertwined, people try to take shortcuts when they describe things.

What I have found is that, combining scope and camera, if your rig can cover around 1° FOV, then most of the “famous” astrophoto targets will be in the scope’s useful range. From M42 (at ~1°) to M51 (at ~10′), lots of bright, well-known targets are out there, waiting.

There are a small handful (I count half-dozen) targets which are larger than this; if you can cover 5°, then you can grab all of those.

Once you’ve cleared those two hurdles (something that takes several years if you want to maintain family harmony and a day job), there is a plethora of targets which are *way* too small for either treatment. Of the largest 100 galaxies in the sky (as seen from Earth), only the top 20 are larger than 10′. Add to that most of the list of planetary nebulae (M57 is only 1′). This all points towards either a second camera with a smaller CCD, or towards a longer focal-length high-magnification telescope.

However, by the time you’re looking for that third scope, you’re fairly well hooked anyway, and know what you need.

So for most astropohotographers, a scope that will produce ~1° will perform nicely. For a DSLR, that turns out to be a scope of ~1000mm. For the smaller entry-level SBIG CCDs, a scope of ~600mm is better. Getting a shorter scope than you need will just mean that you run out of targets sooner.

The 13.1” f/4.5 has a focal length of 1500mm, right between Veronica and Kate. I look forward to the extra magnification on things like M27, but without the additional exposure time required by Kate. For me, this is a good compromise; I maintain a scope that can do well on M42 (Veronica), but I can get a little more needed magnification (and light grasp) on smaller targets like M51 and M27.

If all goes well, this scope will do a good job on me having to crop less and bigger output images.

Right now, the good part is that I can crop to get the best image. The bad part/good part is that there are always other interesting objects in the frame to look at or come out when I start stacking images.

I am sometimes horrified to see how much I’m cropping out of my images. Most objects only cover about 400 pixels in the middle of the camera.

In addition, I have found that longer integration times really help a poor image turn into a decent one. I have found that on galaxies, my integration times are better at 3 hours and marginally better at 4 hours.

I just “upgraded” my working-well-but-not-GOTO G-11 with a Gemini G-11. I like the GOTO, I like the computer integration, but I am still fighting some kind of RA drift demon.

This happens even when I’m autoguiding, either with MaxIM DL or ST-4 native. Both exhibit the problem.

I don’t know whether it’s a cable pulling, poor polar alignment, or something else.

I have used the polar alignment tool in the Gemini, but I’m still a little confused about the polar alignment tools and the A: E: display — when I pick the first few stars as I’m aligning, it says “off by 30+ arcmin”, then as I get more stars, it starts saying smaller and smaller errors until it says “off by 1 arcmin” at which point I stop. Does this mean that it’s correcting for a bad polar alignment, or does it mean that it thinks I have a good polar alignment? I’m confused.

Basically, I’m getting a steady drift in RA (I haven’t determined whether it’s RA+ or RA- yet). The drift is something like (if memory serves) 9 pixels in 5 minutes, which is pretty big. I can’t tell if I’m getting much image rotation; Max corrects for it.

After all of this, I’ve been really kind of focused on other stuff lately. I feel bad for not getting out there, but I am ready for summer nebulae. The baby is due any day now. I think it will probably be today, but it could be tomorrow…