Strange marks on luminance stack M101. Any ideas ?

[Craig123]

Hi all

Ive been looking forward to trying out PI and stacking some Luminance data I've had for a while. The first thing I noticed is some strange curving marks on the final image. Has anyone any idea what is causing this ? 

I went through each sub and applied STF removing any satellites and dud images but I couldn't spot anything like this. I didn't use calibration frames, just registration and integration.

Thanks

 

 

Edited October 4, 2019 by Craig123

[alacant]

Hi. Nice shot.

27 minutes ago, Craig123 said:

what is causing this

Hot pixels on the camera's sensor. You could use dark frames, dither between frames and stack using a clipping algorithm or -perhaps not the best- nail polar alignment.

HTH

[jjosefsen]

Yeah those are hot pixels..

You really need to properly calibrate your subs.

If you don't use darks then you can probably get the worst hot pixels by using the cosmetic correction process before stacking.

There is also a process for inspection all the frames without having to load and stf every single one, it's called blink.

[Craig123]

8 minutes ago, alacant said:

Hi. Nice shot.

Hot pixels on the camera's sensor. You could use dark frames, dither between frames and stack using a clipping algorithm or -perhaps not the best- nail polar alignment.

HTH

Thanks Alicant.Very helpful. So the marks indicate the alignment is off ?

I use sharpcap and try and get PA on to excellent each time and off I go thinking it's all good but maybe not. 

[Craig123]

7 minutes ago, jjosefsen said:

Yeah those are hot pixels..

You really need to properly calibrate your subs.

If you don't use darks then you can probably get the worst hot pixels by using the cosmetic correction process before stacking.

There is also a process for inspection all the frames without having to load and stf every single one, it's called blink.

Thanks. Is blink part of PI aswell? 

[alacant]

49 minutes ago, Craig123 said:

the alignment is off

Yes, but that's not a bad thing as it means DEC is guided constantly in one direction to counter the drift. Any closer to the pole however and I think you'd begin noticing banana stars; that would be the time to adjust polar alignment using alt-az drift.

HTH and clear skies.

Edited October 4, 2019 by alacant

[jjosefsen]

1 hour ago, Craig123 said:

Thanks. Is blink part of PI aswell? 

Yes it is.

 

[Craig123]

35 minutes ago, jjosefsen said:

Yes it is.

 

Thanks. Has to be better than spending an hour going through 5 min subs 

[Craig123]

1 hour ago, alacant said:

Yes, but that's not a bad thing as it means DEC is guided constantly in one direction to counter the drift. Any closer to the pole however and I think you'd begin noticing banana stars; that would be the time to adjust polar alignment using alt-az drift.

HTH and clear skies.

Thanks. A great help Alacant. Back on track with my astro day 👍