Off axis or Guide scope?

Eben Gay — Fri, 29 Jan 2021 16:44:35 +0000

I have recently purchased a used 8" EdgeHD Celestron on a Nexstar mount. I built a wedge for it so I can do astrophotography. My experience with it is mixed - sometimes I get it aligned just right and it tracks like a rock for 3 min exposures and sometimes it's iffy at 60 seconds. As I learn the gear I'm finding that the mount itself has some play in it, there are fundamental scope-to-mount alignment issues (that I'm working on), and my expectations are going up (from "so cool!" to "wait, this could be a lot better").

Of course I have a limited budget so I'm looking at what would be the best next step to improve the stability of long exposures.

I'm thinking that an auto-guider could work around issues in the mount and be significantly less of an investment than a really good mount (which is on my "someday" list).

Assuming that's reasonable, I've found diametrically opposed recommendations for a 2000mm scope. Either an off-axis guider is the only way to go because of the scope's long focal length or a separate guide scope is the only way to go because of the scope's limited field of view.

I'd love to hear your experience and opinons.

Thanks!

Eben

Planetary Imaging (Mars)

Terri Zittritsch — Sun, 01 Nov 2020 13:22:19 +0000

Some things I've learned since embarking on planetary imaging with the opposition of Mars this past October.

I am using an 11" Celestron EDGE HD SCT for imaging. I am also using a 2X powermate putting the scope at F20 and 5600mm focal length.

First thing you need to do is polar align, align your finder scope (really well), and align coordinates to mount because Mars is small and if you want to find Mars in the small camera frame you need good alignment.

The best free capture software I've tried is called FireCapture. It's free and works with most major camera. I am using a ASI224MC with 3.75um pixels.

For Mars I'm using a fairly fast exposure of 2.5ms at 230 frames/sec. I exposed for approximately 80% histogram, but I've found some areas are overexposed. I think it should be in the 60-70% range. I am capturing 4 minute videos on Mars.

I've captured at both 16bits and 8 bits. 16bit files are enormous I'm going to be going with 8 bits until I determine it's not sufficient. Chris Go uses 8bit files exclusively, which should be good enough for me.

Debayer should be turned off for capture. Debayer, while capturing, will result in 3X image sizes.

Gamma should be turned off while capturing. Gamma will effect imaging capture contrast to no value other than ending up with overly contrasty images.

I believe I should be using a UV-cut filter with a color camera. Next imaging session I will be using a UV-CUT

I should be using an atmospheric dispersion corrector, I have the ZWO version.

Will update as I refine my process.

Imaging - vtastro.org Forum

Off axis or Guide scope?

Planetary Imaging (Mars)