Image cuts-off at top while Live Stacking

[snubem]

I have an 8" SCT on a CGEMII. Live stacking, 5sec/frame with Flats and Darks applied. I roughly polar align the mount (due to living downtown Chicago and very few stars) and GoTo aligned with StarSense.
In the attached pic you can see the image cutting off at the top during LiveStacking and even more as time goes by.
I'm new to the CGEM II (I had the Evolution 8) and hoping you can help me stop the cut-off from occurring.
Thank you,
Greg

[BlackWikkett]

Was this live stacked? If so were you guiding and dithering? This looks a lot like dithering with setting very high that caused a very large dithering offset. Could also be simple drift and stacking is compensating with the result you see.

[snubem]

Live Stacking. I do not guide. I don't know what dithering is?
Is it because I'm not polar aligned?
I'm only doing 5secs/frame. Would increasing that time help?
What can I do to stop the "simply drift effect"?

Thanks
Greg

[turfpit]

Greg

No guiding, no dithering leaves alignment. The only way you will get away with a rough polar align is if you guide.

I have a Celestron AVX mount - my process is check mount level using a pipe layer's spirit level (not the one in the mount), polar align via polar scope, 2 alignment + 4 calibration stars, All Star Polar Align (ASPA). ASPA allows polar alignment without sight of NCP. https://www.celestron.com/pages/all-sta ... -alignment. If your mount is new or new to you, it is advisable to check that the polar scope is parallel with the mount https://www.cloudynights.com/topic/5601 ... tructions/.

If none of the above is right then disappointment is guaranteed - been there.

With my AVX I can track unguided for best part of 2 hours. See my image here viewtopic.php?f=16&t=1724 and look at the drift after 1h 40m.

Dave

[admin]

Hi,

If you turned off live stacking and just watch the image on screen you would see that the stars are drifting up at the rate at which the black is appearing at the top of your stacked image. To counteract this, the alignment code in the live stacking moves each image down by the right amount to make the stars line up again, but this leaves a growing area at the top of the image that is not receiving any new data for each new frame because by now it is outside the area being imaged.

This is basically a sign that your mount is tracking very poorly – seeing a little bit of this appearing very close to the edges isn't unusual but seeing it on this scale is very unusual. You will need to make your polar alignment much more accurate and ensure that your mount is tracking at the right rate to correct this problem.

Cheers, Robin

[turfpit]

Good point Robin.

Greg - check that the tracking rate in the handset is set to Sidereal.

Dave

[snubem]

Thank you Robin and Dave!

I will PA tonight with SC PA tool.
Greg

[turfpit]

Greg

Another one to check which I fell foul of last night. If I am imaging to the east then I balance the mount slighty telescope heavy, if imaging to the west then the balance is slightly counterweight heavy. It helps the scope 'sit' on the gears and reduces play. I found this in the Celestron manuals and in forums like CN.

What happened to me last night was I was going after M16 with 60s exposures. It was just past the meridian so I balanced counterweight heavy. Just as I started, a cloud bank crept in. I decided to go after an object in the east as the skies were clear. Imaging the Western Veil with 60s exposures, I could see the image moving with every completed frame, always in the same direction. Session trashed. That could be the issue you are having. Best way to test is to do a conventional test of repeated images with the reticule turned on. Centre the image and capture a run of frames - something will come out of that.

Dave

[snubem]

Thanks Dave,
I do try to slightly weight to the correct imaging side.
I will do more testing on it as you suggested.

G