RCC: M31 and bloated stars

Pleiades Astrophoto PixInsight ZWO ASI533MC Pro CMOS Guiding Request for feedback
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Casey Offord avatar
Casey Offord avatar
Hello everyone!  This is my first real effort with my new ASI533MC Pro.  This was also my first fill process in PixInsight.  I'm fairly happy with the image in general (post processing can and will be improved next time), but many of the stars are very large.  Is this due to guide error, ineffective IR cut filter, or some other issue?  I did shoot this on a night of marginal seeing and the actual and RMS error moved around a lot, especially after dithering.  I increased the delay after dither and that greatly improved the subs.  Perhaps I just need more delay.  RMS error was around 1.1", but would spike above 2 occasionally after dithering.

Thanks,
Casey

https://astrob.in/twxozy/0/
Well written Engaging
dkamen avatar
dkamen avatar
Hi Casey,

This could indeed be just the seeing. Some nights stars will look blurry and large even if you have your best focus optically.

You might also be overexposed (too high gain or too much exposure for your given gain), since the stars look whiter than I would be expecting, especially the small ones.

Cheers,
Dimitris
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Nejc Ucman avatar
Nejc Ucman avatar
The fainter stars are pretty sharp, I don't think it is seeing. My bet would be on post processing.
Alessio Pariani avatar
Alessio Pariani avatar
Casey I think we need more data to help you:
- does your camera have IR/UV blocking filter in the sensor window?
- if not, did you use any filter that has a IR/UV cut feature?
- how did you set exposure settings (gain, offset)?
- how did you processed the image?

RMS peaks after dithering are absolutely normal, since during dithering the system needs much more correction to return in place. RMS is a mean value so it gets affected every time
Helpful
Casey Offord avatar
Casey Offord avatar
UV/IR cut filter was installed.  Gain 135, offset 20.  Processed in PixInsight following the Light Vortex Astronomy M31 tutorial.

Here's an untouched sub (with increased delay after dithering):  https://drive.google.com/file/d/1x7ye71HwOu74ZHqx7n4u-sN8pKXtE0rm/view?usp=sharing

Thanks,
Casey
dkamen avatar
dkamen avatar
All right, it's not overexposure (except perhaps in post processing).

I can see the small tracking error and I think the star shapes are slightly more deformed at the NW corner vs the SW which could indicate a small alignment/spacing problem in the whole telescope-flattener-camera train.

But assuming you focused as sharply as possible, I still lean towards seeing as the most probable explanation.

Cheers,
Dimitris
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