Flat Problem... wit...
 
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2022-09-02: APP 2.0.0-beta4 has been released !

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Flat Problem... without Flat Result much better..? please help...


(@brazzo229)
Brown Dwarf Customer
Joined: 1 year ago
Posts: 11
Topic starter  

I made a stack with and without flats, without it it looks much better, with flats I have a very strong brightening especially on the right and at the bottom of the edge.

What could be the reason for this?

The flats are exposed for 0.5s and are also nicely centred in the histogram.

The lights are 20x300s with gain 101 on an Askar Apo with reducer at F3.9 and Antlia Dual Narrowband Filter...

Camera ASI 533MC

 

without flat

Screenshot 2022 08 17 155752

with flats

Screenshot 2022 08 17 155844

Histogram seems ok

Screenshot 2022 08 17 155957

 

Screenshot 2022 08 17 160020

 

thanks for your help


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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1881
 

@brazzo229 Hi!

The histogram of the flats looks different from the 3rd to the 4th screenshot. What is the difference between them?

Also, I am sure that the answer is yes but the flats were shot with the filter as well, right? What other calibration data do you use?

Thanks, Wouter


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(@brazzo229)
Brown Dwarf Customer
Joined: 1 year ago
Posts: 11
Topic starter  

@wvreeven  4th screenshot is with neutralized Background

yes the flats are shot with the filter as well 

Integration , Calibration etc. is automatic

I use 20 Bias Frames and Darks as well,

the integration is completely identical except for the flats

This post was modified 1 month ago by Sascha Ebeler

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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1881
 
Posted by: @brazzo229

4th screenshot is with neutralized Background

Thanks! I failed to notice that.

Posted by: @brazzo229

The flats are exposed for 0.5s and are also nicely centred in the histogram. Camera ASI 533MC

The only thing that comes to mind is that 0.5 sec is very short for this camera. CMOS cameras like this one work better with flats of at least 1 sec because of the rolling shutter. This is an electronic way of mimicking a physical shutter and it takes a while to read out the sensor. During the read out, pixels that have not been read out yet keep on collecting light potentially leading to subtle illumination differences between the edges of the sensor. This could explain the red edge.


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(@brazzo229)
Brown Dwarf Customer
Joined: 1 year ago
Posts: 11
Topic starter  

@wvreeven ok thanks, i will try a longer exposure


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(@brazzo229)
Brown Dwarf Customer
Joined: 1 year ago
Posts: 11
Topic starter  

@wvreeven :

Now Flats with 3s exposure time... 

unfortunately no improvement

Screenshot 2022 08 17 183618

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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4987
 

Just as a thought; it looks like the flat is overcorrecting the right-hand and bottom side. It could be that the way you take the lights doesn't fully represent the vignetting of your setup, sometimes caused by an extra light leak.


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(@brazzo229)
Brown Dwarf Customer
Joined: 1 year ago
Posts: 11
Topic starter  

@vincent-mod

Hmm... I don't know where the light leak should come from, the camera etc. is screwed tight and the lights actually look normal.

Lightframe normal stretch

Screenshot 2022 08 17 190100

Lightframe over stretch

Screenshot 2022 08 17 190024

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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4987
 

Doesn't seem to be the case in the lights indeed, it's just a thought. Could also be a calibration issue perhaps with the bias/darkflats. But when you take the flats, you have no other light sources around? Could you specify how exactly your setup is?


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(@brazzo229)
Brown Dwarf Customer
Joined: 1 year ago
Posts: 11
Topic starter  

@vincent-mod 

the darks are not photographed with the filter maybe that is the reason ?

 

 


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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1881
 

@brazzo229 Darks do not require light so it doesn't matter if the filter, or even the telescope, was installed. It is perfectly fine to take darks with the camera completely disconnected.


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(@brazzo229)
Brown Dwarf Customer
Joined: 1 year ago
Posts: 11
Topic starter  

@wvreeven

i left out the darks and now it looks perfect...

I bought the camera used and the seller sent me the masterdarks, but apparently they don't fit somehow.

 

Screenshot 2022 08 17 195034

 


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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1881
 

@brazzo229 That's great! So it is a matter of shooting new darks and creating a master dark from it and you're set.


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