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Diagnose my stars (M31)

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#1 raadoo

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Posted 27 November 2021 - 05:52 AM

I'm having quite the conundrum trying to figure out why my stars are looking like balls, rather than ... well, stars.

I shot M31 over two nights, first night doing 60s exposures, second night doing 120s exposures.

Both nights I shot Bias, Darks & Flats separately for each, making sure to adhere to best practices around camera temp, position, etc.

 

Equipment:

  • Scope: Askar FRA400 + f/3.9 Reducer
  • Focuser: ZWO EAF (star ratings were around 3.1 for both nights)
  • OAG: ZWO + ZWO 120MM Mini
  • Filter: Baader Neodymium IR-Cut
  • Camera: ZWO 183MC Pro (Gain 120, Temp -10°C)
  • Mount: AZ-GTi (RMS ~1.5" for both nights)
  • Control: ASIAir Pro

 

Integration over two nights:

  • Lights: 70x 60s + 50x 120s
  • Darks: 60x 60s + 30x 120s
  • Flats: 30x + 30x
  • Bias: 60x + 60x

 

I'm thinking it could be:

  • the filter (though someone on IG with the same filter got perfectly fine stars using it),
  • focus (it looked pretty well focused to me?)
  • some form of high altitude clouds / bad seeing (but what are the odds that would be the case on both nights?),
  • backfocus (although I did do the math properly, so I'm not so sure on this one)
  • some combination of the above / something I don't even know about?

 

Fully Stacked:

Used APP and its Multi-Session feature to combine the two nights' integration here:

diagnose-my-m31-big.jpg

 

1:1 Size, Stacked:

Here's the same as above, but at 1:1 pixel size so you can properly diagnose the issue.

diagnose-my-m31-100p.jpg

 

1:1 Size, Single Sub:

Single sub, stretched in Siril to reference against the stacked and calibrated from above.

Even in this single sub, it's weird that I can see tiny, about 9 pixel sized, stars clearly in focus and being more than a single pixel I doubt they're hot pixels. Even if they were hot pixels, surely Darks would've just taking those out, anyway. But no, it's a very similar affair to the stacked and calibrated version above.

diagnose-my-m31-big-singlesub-100p.jpg

 

So, in my best Seinfeld impression, what's the deal with these stars?

 

 



#2 terry59

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Posted 27 November 2021 - 05:58 AM

If darks don't remove hot or cold pixels you need to run some version of cosmetic correction



#3 Boeglewatcher

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Posted 27 November 2021 - 06:02 AM

Did you doublecheck focus w bahtinov mask? I had a similar issue w Auto Focus on my refractor and found out that autofocus gave good readings but was off a bit when double checking w bahtinov mask. When setting focus w bahtinov mask the stars got smaller and had no such halo anymore.

cs Joachim



#4 raadoo

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Posted 27 November 2021 - 06:03 AM

If darks don't remove hot or cold pixels you need to run some version of cosmetic correction

Comparing the stacked vs single sub above, I think darks did do their job. The comment about dark pixels was that those ~9px sized stars are actually stars, not hot pixels.

That, and I already applied cosmetic correction in Siril in the stacked version smile.gif even after APP told me that about 1000 or so dark pixels were fixed as part of stacking.


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#5 raadoo

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Posted 27 November 2021 - 06:11 AM

Did you doublecheck focus w bahtinov mask? I had a similar issue w Auto Focus on my refractor and found out that autofocus gave good readings but was off a bit when double checking w bahtinov mask. When setting focus w bahtinov mask the stars got smaller and had no such halo anymore.

cs Joachim

My process is that initial focus is done with a Bahtinov mask as I trust myself more than the EAF, but then every hour the EAF kicks in automatically to adjust for changes in temperature.

Just went through the first and last subs of one of the nights to compare if star shapes are any different. And sadly, they are not frown.gif  so it's probably not the EAF being a dingleberry.


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#6 terry59

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Posted 27 November 2021 - 06:20 AM

Comparing the stacked vs single sub above, I think darks did do their job. The comment about dark pixels was that those ~9px sized stars are actually stars, not hot pixels.

That, and I already applied cosmetic correction in Siril in the stacked version smile.gif even after APP told me that about 1000 or so dark pixels were fixed as part of stacking.

I didn't see any hot pixels either but assumed you may not be aware of CC.

 

If your focus is good the stars do not look correct. Do single subs look like that too?



#7 raadoo

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Posted 27 November 2021 - 06:32 AM

I didn't see any hot pixels either but assumed you may not be aware of CC.

 

If your focus is good the stars do not look correct. Do single subs look like that too?

Yup. Here's a single sub, quick and dirty stretch:

 

Everything:

Light_M31_60.0s_Bin1_0101-big.jpg

 

1:1 Pixel size:

Light_M31_60.0s_Bin1_0101-100p.jpg

 

Obviously, in this single sub the noise is horrendous, but that's kind of the point I guess, that the calibration frames did to their job, and that whatever's going on with the blobby stars, it's probably not got something to do with calibration.



#8 terry59

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Posted 27 November 2021 - 06:35 AM

Yup. Here's a single sub, quick and dirty stretch:

 

Everything:

attachicon.gifLight_M31_60.0s_Bin1_0101-big.jpg

 

1:1 Pixel size:

attachicon.gifLight_M31_60.0s_Bin1_0101-100p.jpg

 

Obviously, in this single sub the noise is horrendous, but that's kind of the point I guess, that the calibration frames did to their job, and that whatever's going on with the blobby stars, it's probably not got something to do with calibration.

I agree that calibration is not a likely answer. I should have been more clear in my request. Can you make 1 raw untouched sub available to download?



#9 raadoo

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Posted 27 November 2021 - 06:47 AM

I agree that calibration is not a likely answer. I should have been more clear in my request. Can you make 1 raw untouched sub available to download?

Indeed I can: there you go. Thanks for taking the time to help!



#10 michael8554

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Posted 27 November 2021 - 07:03 AM

The problem goes back to the subs.

 

It's not guiding, hot pixels, darks, calibration, stacking or APP.

 

The stars are inherently in focus, but diffused.

 

So I'm thinking sensor frosting or dew.


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#11 raadoo

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Posted 27 November 2021 - 07:13 AM

The problem goes back to the subs.

 

It's not guiding, hot pixels, darks, calibration, stacking or APP.

 

The stars are inherently in focus, but diffused.

 

So I'm thinking sensor frosting or dew.

Had the dew band on, and did check to see if any dew was forming on the front element - everything was fine.

But I didn't take into account the possibility that the sensor got dew on it. I see ZWO have something to address this but it looks slightly flimsy to me. if this is the cause of my issues, do you have any alternative solutions I should be thinking about?



#12 Tapio

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Posted 27 November 2021 - 08:00 AM

That could well be frost.

Could you just test it with your own eyes if that seems to happen.

 

Maybe it's time to regenerate the desiccant tablets ?


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#13 raadoo

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Posted 27 November 2021 - 08:47 AM

That could well be frost.

Could you just test it with your own eyes if that seems to happen.

 

Maybe it's time to regenerate the desiccant tablets ?

Begs the question if I've even been using my desiccant tablets correctly. I mostly store my camera connected to the scope, in a cabinet with desiccant right next to it. Which I often check on and regenerate as needed. Should I be storing the camera not connected to the scope? Sorry if this all seems quite silly; best to seem silly and learn something than hope that I'm doing thing right as is.


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#14 raadoo

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Posted 27 November 2021 - 08:51 AM

That could well be frost.

Could you just test it with your own eyes if that seems to happen.

 

Maybe it's time to regenerate the desiccant tablets ?

... 5 minutes of searching here about the topic and I just learned that the camera itself has desiccant in it! Which is probably what you mean by regenerating the tablets.

Well I'll be, way to learn something new.



#15 DJL

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Posted 27 November 2021 - 08:51 AM

I recommend EAF rather than Bahtinov mask for best focus - it made everything I shot before look blobby.

 

However this looks to me like it could be dew. Do you have dew heaters? A giant beach umbrella has also been reported by another member as keeping dew off their optics.

 

In APP, you can see the FWHM for each sub. See if it is increasing over the session, which would indicate dew forming gradually. Look at the subs with the lowest (best) FWHM and worst (highest) FWHM. 2-3 is good (where I live in Bortle 5-6), over 5 is not so good.


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#16 IonClad

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Posted 27 November 2021 - 09:34 AM

Is this your first use of the telescope, or did you see well-shaped stars with prior use of it?

 

It looks like very under-corrected spherical aberration, which would show as a tight center point surrounded by a larger halo that cannot be focused further. Unfortunately that would be due to a problem with the optics during manufacture and likely not fixable. You might try it without the reducer just to make sure that's not the culprit.

 

For comparison, here's what I and several others saw in defective versions of the Askar FRA600 https://www.cloudyni...6#entry10961460. This effect looks identical and was not due to anything else in the image trains, nor dew/frost.


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#17 raadoo

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Posted 27 November 2021 - 09:45 AM

I recommend EAF rather than Bahtinov mask for best focus - it made everything I shot before look blobby.

 

However this looks to me like it could be dew. Do you have dew heaters? A giant beach umbrella has also been reported by another member as keeping dew off their optics.

 

In APP, you can see the FWHM for each sub. See if it is increasing over the session, which would indicate dew forming gradually. Look at the subs with the lowest (best) FWHM and worst (highest) FWHM. 2-3 is good (where I live in Bortle 5-6), over 5 is not so good.

FWHM was a consistent ~3.2 throughout both nights. So it's likely the culprit was too fast a cooling of the camera combined with desiccant that needed refreshing and probably needing a dew band for the camera itself (the scope didn't dew over as I had the dew band on it and checked that everything was fine throughout both nights).



#18 raadoo

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Posted 27 November 2021 - 09:50 AM

Is this your first use of the telescope, or did you see well-shaped stars with prior use of it?

 

It looks like very under-corrected spherical aberration, which would show as a tight center point surrounded by a larger halo that cannot be focused further. Unfortunately that would be due to a problem with the optics during manufacture and likely not fixable. You might try it without the reducer just to make sure that's not the culprit.

 

For comparison, here's what I and several others saw in defective versions of the Askar FRA600 https://www.cloudyni...6#entry10961460. This effect looks identical and was not due to anything else in the image trains, nor dew/frost.

Had the scope since summer and have used it extensively since, without issues.

Granted, previous uses of the scope were without the reducer and looking at your examples it does seem to present similar blobs as my subs.

Perhaps it's the reducer that's the problem?

 

It's definitely getting me thinking about running tests as soon as the skies clear a bit and check with and without the reducer before going down the frost/dew/overcooling route.



#19 terry59

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Posted 27 November 2021 - 10:11 AM

I am glad to see the conversation. Looking at the sub it shows me the same thing. I see this in places with lots of humidity


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#20 DivisionByZero

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Posted 28 November 2021 - 01:13 AM

You can try to isolate dew on the sensor by not running the camera cooler at all.

 

If you have a spare dew band, you can temporarily strap it around the reducer and see if that helps eliminate it as well.


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#21 raadoo

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Posted 04 January 2022 - 02:10 PM

It only took 37 days until I actually had skies clear enough to attempt to image anything lol.gif

 

Between the anti-dew heater strip I got for the ASI183MC, the new desiccant pills I popped into the camera or the new dew bands (that actually feel warm now) something is definitely working well now. I managed 1.5h of M31 between clouds last night and the seeing was quite poor as well. But golly geez, would'ya look at what I got in the end: https://www.astrobin.com/fuca9r/

 

Huge thanks to all of you for your help. I am one happy camper. Now if only the sky would clear up ...


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#22 Boeglewatcher

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Posted 04 January 2022 - 02:29 PM

great, thanks for the feedback and congrats to the result!

cs Joachim


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