The case of overcor...
 
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2022-05-29: APP 2.0.0-beta2 has been released !

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The case of overcorrecting flats, need help

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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 81
 

@vincent-mod 

Darkflats look fine to me, I have just ordered a larger flat panel one of the Pegasus Astro ones so that I can set light levels per filter as well, as I have 3 sheets of ND Filter in my Gerd Neumaan one which is no good for NB flats as the Ha exposure would be >1600S :-O

Here's the Master Dark Flat for Red:

image

And Blue:

image

Regards

Simon

 


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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4850
 

Not something very clear to see no. If you can't find out I can always have a look at the data of course.


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 81
 

@vincent-mod Let's wait for my new flatpanel to arrive and see if the issue goes away, it could be that the illumination circle on my GN Panel isn't enough, it's supposed to be for up to 8" but the illumination doesn't fill the whole apperute, hence why I have gone for a Flatpanel 250


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 81
 

@vincent-mod

So far there does not appear to be much different in flats between the GN Panel and the Pegasus Astro one, here they are side by side (GN Panel on left, Pegasus Astro on right)

image

However, I have not got all the required frames yet to produce the master flats to compare those.  But the Pegasus Astro panel makes lights so much easier as SGPro adjusts the intensity of the panel per filter so no more ND film 😀


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 81
 

Not completely there, but definitely much better

Left was original GN Flat panel and 29.5K ADU, right is new Pegasus Flat Panel again 29.5k ADU

image

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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4850
 

Nice, so would you say that solves it?


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(@andybooth)
White Dwarf Customer
Joined: 6 months ago
Posts: 22
 

@stastro just let me throw in that I had a similar problem way back when I changed from ccd to cmos camera. On my ccd, I only exposed flats to about 20-25k of the 65k adu saturation, to get get good calibration. when I moved to asi071mc cmos, I got same correction problem until I exposed to around 45k mean adu, with max adu up to 60k.  That may seem theoretically wrong,  but it has worked for me ever since. May be worth just upping the exposure in a test to see. 🤷‍♂️


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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
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That's not wrong at all Andy, as long as you don't clip any data on either side, it's better to get the histogram a bit more to the right, just to make absolutely sure the darkest parts are also well illuminated.


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 81
 

@vincent-mod 

That's really interesting as the general rule of thumb has always been 1/3 to 1/2 way up the histogram.  I might give that a try and see what happens, I made changes to the imaging train last night, and took some new flats as well as lights, so waiting for the calibration to complete


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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4850
 

Well, yes, that has been a "rule" people simply have told each other for years. Doesn't mean it's necessarily correct. 🙂 I think it has been said because it works for a lot of people and ensures you're never clipping. But with the high dynamic range of new sensors and some sensors in particular, they behave less well in the low illuminated area's (not very linear). It doesn't matter if you're at 50%, it matters you're not clipping and to make sure everything is well illuminated, shifting to the right is a good thing to do, but you have to test and check you're not clipping.


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(@andybooth)
White Dwarf Customer
Joined: 6 months ago
Posts: 22
 

@stastro yep, that was my teaching, 1/3 to 1/2 way up. 
now i do 2/3 - 3/4 , as long as max adu is no more than 60k, as Vincent said, no clipping at top end.

made a world of difference, on normal broadband, but especially when using LExtreme filter as well.

might just be my imaging train , who knows, but defo worth a test.


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 81
 

@andybooth I am acquiring 45K flats at the moment to see what happens, strange part about it is that the same camera on my previous scope which was a SharpStar 15028HNT worked well at 22.5k flats, but this 20032PNT clearly does not.

I presume that since you are using the L-eXtreme that you are using an OSC?  I moved back to Mono from the ASI6200MCPro (Which I still have sat here), so I am using the ASI6200MMPro with Baader CMOS Optimised filters, again worked fine on the 15028HNT at 22.5K ADU


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 81
 

Unfortunately all ADU value produce a very similar result, Left is 45K, middle is 29.5K and right is 22.5K

image

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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4850
 

Ok, stick with the 45K at least. 🙂 Maybe I missed it, but how are you taking the bias/darkflats, how fast are the exposures there?


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(@andybooth)
White Dwarf Customer
Joined: 6 months ago
Posts: 22
 

@vincent-mod good point on exposure.

i need at least 0.75 secs, and usually best for me is 1.3secs for flats.


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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4850
 

Yes, that's also because some new sensors don't behave linear in the very short exposure ranges. So we recommend at least 0.5s per bias sub and 1-2s for a flat at least..


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 81
 

L = 3.9s
R = 5.22s
G = 5.17s
B = 5.56s
Ha = 82.98s
OIII = 24.14s
SII = 133.05s

Dark flats are the same exposure, temp and Gain level

Yes I already new about the issues with sub second exposures for flats which is why I always aim for >2s, NB can't do anything with, that is with panel up at 100%, Luminance panel is at 1% and RGB around 40%


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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4850
 

Allright, then I think we need to see the data to be able to check for other things. If you want, please upload like 5-10 subs per filter and the flats for them, including 10 bias/darkflats, 10 darks.


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 81
 

@vincent-mod Dark will be masterdark anyway

I wil upload
1x Master Dark
1x BPM
10x Light frames for each filter
10x Flat frames for each filter
10x Dark flats for each filter

Do you want them in a zip / rar file?


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 81
 

All files uploaded, I created two subfolders, one for flats, the other for dark flats


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