5 replies to this topic

[laserdan]

I came across an SBIG 7xe CCD in my lab the other day and was interested to find out it has a second chip to use for simultaneous autoguiding. It got me thinking… has anyone ever used their DSLR to make periodic tracking adjustments by taking short exposures in between their long exposures instead of using a secondary guide scope?

I know it wouldn’t be great, but I’d imagine it could slightly improve images for small setups on something like the skyguider pro.

Edited by laserdan, 06 December 2021 - 11:52 PM.

[radon199]

To be honest my Star Adventurer drifts so little that if I didn’t dither the framing would change very little in an evening and it remains bang on target until I need to do a meridian flip.

The problem is movement within a single frame, for which taking a second exposure wouldn’t solve anything. I am surprised that with todays combined cameras and filter wheels nobody has thrown a small chip offside the main sensor, but I guess that would be affected by the current filter.

A modular approach seems best all around.

[the Elf]

That does not improve the situation at all. You want the mount to track well _during_ the exposure. The main purpose is to take longer subs. You can go that path and implement dither. That is intentionally moving the mount in a random pattern. After star alignment and stacking the hot pixels are no longer in the same place and can be sorted out. This needs both axes to be guided. A skyguider pro is a single axis device. So probably no use at all.

[laserdan]

Thanks. I really didn’t think that one through. Of course autoguiding is only relevant during  the exposure. The only benefit I could see is to keep the same image in frame so it doesn’t crop during stacking. 

[radon199]

To be honest you want the frame to not be perfect the whole set of subs. That is why people recommended to dither.

If you kept perfect framing the whole run then the fixed pattern noise would not average out. If you move the camera a little every couple of frames and the star align then that fixed noise is spread over a larger area and can be better rejected.

Of course calibration frames are supposed to deal with that kind of noise but nothing is ever perfect and dithering has other benefits such as supersampling during integration.

[Alex McConahay]

>>>>>>The only benefit I could see is to keep the same image in frame so it doesn’t crop during stacking.

 

 

If your image is shifting that much, you need to address whatever is causing the improper tracking. If in an hour worth of exposure you are having framing issues, you really need to direct the attention to the source rather than looking to guiding as a solution. Never hope that correcting after the fact will be preferable to not making the mistake in the first place. 

 

Alex