Flats not being sub...
 
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Flats not being subtracted


(@mikisj)
Molecular Cloud Customer
Joined: 1 year ago
Posts: 6
Topic starter  

(I did a forum search and did not find any topic relevant.)

I am a very new user to APP (3/21/21) and like the quickness of processing through 'integration'. Immediately prior to the imaging session I took 20 'sky flats' and already had 20 'darks' with a similar exposure/temp as the 'lights/ and 20 'bias' frames.

I ran through the APP sequence and it appeared the 'flats' were not subtracted out and in fact, looking at the images below, it appears the 'flats' were add to the light image. I used the default settings for each of the processes.

The image on the left is the result of using flats and the image on the right is without flats.

M97   wFlats reduced
M97   woFlats reduced

There was a 'Master Flat' and a 'Bad Pixel Map' created and applied per the file window.

What did I do wrong?

My imaging equipment is an ASI294MC-Pro taken using SGPro 4.0.0.661.

This topic was modified 1 year ago 2 times by Michael Perata

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(@mikisj)
Molecular Cloud Customer
Joined: 1 year ago
Posts: 6
Topic starter  

I inverted the 'Flats' in PixInsight and re-ran the process with typical values and the image came out as I would have originally expected. Here is the image with the 'Flats' inverted:

M97   wFlats Inverted reduced

Will I have to invert my 'Flats' to use APP?


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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1645
 

@mikisj Hi Michael. If inverting the flats gives the result that you expect then that probably means that your flats are much too dim and need to be a lot brighter. Can you post a screenshot of APP with a flat loaded in it showing the histogram with the "stretch" checkbox unchecked? In any case, you should not need to invert the flats to be able to use them with APP.


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(@mikisj)
Molecular Cloud Customer
Joined: 1 year ago
Posts: 6
Topic starter  

I am a new user, so be gentle - how do I load a single 'Flat' into APP?

I am using Dusk Sky Flats and the ADU is ~23,000 for a well depth of 63,000. The Flats should be bright enough.

This post was modified 1 year ago by Michael Perata

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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4727
 

When you load in the flats, you double-click on one in the list and it is shown in the preview. Would be nice to see a screenshot of that and also, when you look at the right hand side where there is a stretch preset (it says 15% BG standard), select "no stretch" there. That will show the actual histogram of the flat without stretching and that peak should be clear of the left side of the histogram (and from the right), but doesn't necessarily need to be in the middle.

It may also be interesting to see what happens when you use bias frames here I think. The bias signal needs to be corrected from the flats for them to work well, APP does this automatically when you have bias or darkflats.


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(@mikisj)
Molecular Cloud Customer
Joined: 1 year ago
Posts: 6
Topic starter  

Here it is:

2021 02 25 20210225 18 00 t0.6sec 3C gain 125

 I don't understand the double peak, but then there is a lot I don't understand about processing images - sigh!


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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1645
 

@mikisj Thanks very much Michael and sorry for jumping deeply into technicalities straight away. Your camera is a color camera and APP should be able to turn the black and white images into color images automatically. To do so, go to tab 1 and select RGGB in the pattern drop down and then enable "force Bayer/X-Trans CFA". Then go back to tab 1 and double click a flat again as Vincent explained. You should see that the peak to the right is for the green color channel and the other peak for blue and red. However, I suspect that that may not be the case for you and that the left peak is for green. Can you follow these steps please and post a new screenshot?


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(@mikisj)
Molecular Cloud Customer
Joined: 1 year ago
Posts: 6
Topic starter  

Here is the RGGB screenshot:

2021 02 25 20210225 18 00 t0.6sec 3C gain 125   RGGB

I appreciate your responses!


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(@mabula-admin)
Quasar Admin
Joined: 5 years ago
Posts: 3115
 

Hi Michael @mikisj & @wvreeven,

First of all, flats are never subtracted/added, the light frames are divided by flats. Vignetting is a multiplicative process on the incoming light. So to correct that, you need to have a model of that vignetting (our flats) and to correct that problem, you need to divide the original with the model to remove the problem.

I have never come across a situation where you would need to invert flats to make them work, so that is not very logical at all to me, I suspect something else is the problem here.

So, your camera is a OSC camera and you obviously need to enable Force CFA to have APP interpret the data correctly, right? This is because relevant metadata is not in your fits frames apparently, this is caused by the capture software most likely.

Now, what happens when you process your data with the NOT-inverted flats with this setting enabled? So make new masters now and integrate all data. Is then everything okay?

If not, I think we should have a closer look at your data 😉

Mabula


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(@mikisj)
Molecular Cloud Customer
Joined: 1 year ago
Posts: 6
Topic starter  

With apologies. I had another AP'er look at my raw images (lights, Flats, Darks) and it was determined I had really bad date. 

Thank you for taking the time to answer my query.


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