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2022-08-17: APP 2.0.0-beta3 has been released !

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Problem with Master Flats versus Single Flat

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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 91
Topic starter  

After solving the issue with stray light, I am now encountering a totally different issue with my master flats, they seem to be doing something to my image stack, so as a process of elimination, I designated a single flat for each channel as a "Master Flat" and re-processed the image stack and the image no longer has the artifact and looks how I would expect it to look.

All my flats are taken with a flat panel, 101 frames for each channel, and 101 dark flats to match.  If you compare the Master flat and Single flat side by side, the master flat seems to have some stepped gradient changes rather than a smooth transition like the single flat frame, here is the comparison of the Master Flat (Left) and single Flat (Right)

image

And then the resulting image stack, Master Flat applied to left, single flat applied to right:

image

You can see a light "Curve" on either side, fully post processed this is very evident:

image

All of my Master Flats and Master Dark Flats were created during the same process using APP, so not sure what is going on here, any help is much appreciated.

Simon


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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1816
 

@stastro Simon, what version of APP are you using? Can you check and verify that "32 bit masters" is enabled at the bottom of tab 2?


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 91
Topic starter  
Posted by: @wvreeven

@stastro Simon, what version of APP are you using? Can you check and verify that "32 bit masters" is enabled at the bottom of tab 2?

Morning Wouter

I am using 2.0.0-Beta1 and yes 32 bit masters is checked

image

Regards

Simon


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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1816
 

@stastro Good morning Simon! Could you upload, say, 30 flats and dark flats using


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 91
Topic starter  

@wvreeven 

I'll do that as soon as I am finished presenting, I'll create two subfolders, one for flats, one for darks, and I will also upload my current master flat and master dark flat too for the same filter


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 91
Topic starter  

Hi Wouter

Files all uploaded now

Regards

Simon


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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1816
 

@stastro Thanks for the upload Simon. I loaded the 30 flats and 30 dark flats and processed them in APP by going to tab 2 and clicking "create Masters & assign to Lights" without changing anything else. The resulting MasterFlat looks like this:

Screenshot 2022 06 29 at 18.52.59

which looks fine to me. Is this not what you expected and, if it isn't, what did you expect?


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 91
Topic starter  

@wvreeven Look carefully at the flat that was created, it contains gradients that are not gradual, and you can also see boundary lines in the pattern of a tree stump grain, is this normal?


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 91
Topic starter  

So I wonder why a master flat is having such a negative impact on the image stack, despite the fact a single flat file seems to work pretty good


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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1816
 
Posted by: @stastro

Look carefully at the flat that was created, it contains gradients that are not gradual, and you can also see boundary lines in the pattern of a tree stump grain, is this normal?

It maybe just a presentation issue. Have you actually zoomed in on the MF to see if the boundary lines remain?

I can investigate further if you want. Can you upload Lights and Darks (and if necessary Bias) please?


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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1816
 

@stastro When I zoom in, I see this:

Screenshot 2022 06 30 at 01.33.14

As you can see, the lines disappear and the gradient becomes smooth.


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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1816
 

@stastro Zoomed in even further:

Screenshot 2022 06 30 at 01.36.54

The lines that are visible now are due to the screenshot I took on my MacBook. They weren't visible on my screen.


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(@mestutters)
Red Giant Customer
Joined: 5 years ago
Posts: 144
 

Hi,

I may be missing something by looking at your screen-grabs but I think the streaking effect you are seeing is likely a result of limitations of screen display technology than of APP MF production.  

Basically you are displaying a 32-bit resolution MF on a screen that is not capable of display to the same level of granularity. To deal with this the source 32 bit values must be rounded to a lower-range  grayscale value for display purposes (aka posterization).  Because the shading of your MF is so gradual the step points at which the 32 bit values have been rounded to a lower level (?256 steps) are just starting to become visible.  The effect is probably also more pronounced than it actually is as a result of a screen stretch.

If one zooms in sufficiently all sorts of pixel level imperfections start to become visible.

I conjecture that your single frame MF is less 'smooth' so the step-points are less apparent but none the less must still be there. 

I have to say that I cannot see the effect in your final images.

 

 

Regards

 

 

This post was modified 2 months ago 6 times by mestutters

(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1816
 

@stastro I completely agree with @mestutters, who has explained the display effect much better than me in my previous comments. The effect is called posterization. 

Thank you for chiming in @mestutters!


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 91
Topic starter  

I think the crux of my problem is the severe vignetting on the SharpStar 20032PNT with the ASI6200MM Pro, which is strange because the scope is sold to be excellent for full frame, I had a 15028HNT before this that did not produce vignetting so bad.

This is from the 20032PNT, Luminance with STF applied in PixInsight

image

And here is a Master Flat from the same camera, same filterwheel but using the 15028HNT, again STF Applied

image

Now the 15028HNT Had an OAG in there as well, but that should not make much difference.

Now the interesting part is, according to the light cone for a 44mm diagonal sensor, at the 55mm distance from the sensor, the light cone would be 61.1875mm, which is a larger diameter than the M54 imaging train (distance from sensor / focal ratio) + sensor diagonal, (55/3.2)+44, however the bit that throws me out there is on the 15028HNT I was using M48 imaging train, and 55mm from the sensor would have had a light cone of (55/2.8)+44 = 63.64286mm, which is significantly higher than the 20032PNT requirement but I had so much less severe vignetting with that scope using the same camera and filters. 

So the investigation continues!

 


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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1816
 

@stastro Interesting analysis. I hope you manage to get to the bottom of this. I am facing a similar challenge using an ASI6200MM on a RedCat 71.

Out of curiosity, what filters do you use? And what light source to take flats? I have used filters before that had an IR leak combined with an iPad for taking flats and that introduced all kinds of weird artifacts in the integrations. 


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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 91
Topic starter  
Posted by: @wvreeven

@stastro Interesting analysis. I hope you manage to get to the bottom of this. I am facing a similar challenge using an ASI6200MM on a RedCat 71.

Out of curiosity, what filters do you use? And what light source to take flats? I have used filters before that had an IR leak combined with an iPad for taking flats and that introduced all kinds of weird artifacts in the integrations. 

Hi Wouter

For the flat light source I am using the Pegasus Astro Flat Panel, as it has full control over the brightness, so for example, I can have it on 1 for Lum, 40 for RGB and 100% for NB filters.  I used to use a GerdNeumaan FlatPanel, but I wanted to automate flat creation as much as possible.

My filters are Baader CMOS Optimised 50.4 unmounted, and the Narrowband ones are the Ultra Fast NB filters from Baader also, they are sat in my ZWO EFW, so the filters are around 18mm from the sensor, so the filters are bigger than the light cone at 18mm distance.

I have just discovered that the backfocus distance from the M68 to the sensor is 61mm, as the M68 to M54 is 6mm in thickness, so at 61mm, the light cone width would be 63.0625mm which is lower than the M68, so I might go from M54 to M68 to see if that helps.  I have reached out to a couple of people with regards to them using full frame sensors on the 20032pnt to find out what their imaging train is, and also to get a view on their flats, just to see if what I am getting is Normal and if my flats are flawed in some way shape or form.

Regards

Simon

 

This post was modified 2 months ago by Simon Todd

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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 91
Topic starter  

As a test I am also going to try and increase the exposure time of my flats, so for RGB lower the flat panel intensity by 50% which should in theory double the exposure time to see if that helps, not sure how I am going to get luminance longer as the flat panel is already at the lowest value of 1 for flats at 22500ADU, I could also opt for higher ADU since it makes no difference whatsoever to flats whether the ADU is 22500, 29500 or 45000, that should get around the problem with Luminance

This post was modified 2 months ago by Simon Todd

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(@stastro)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 91
Topic starter  

I'm getting there, here's the current processed image with new flats

image

Far less of an issue with the above, so what did I do?  Well after doing soem research, CMOS cameras do not produce good flats with short exposures, so I opted to push the exposures up, rather than change the intensity of the flat panel, I just upped the ADU to 49.5K from 22.5K (22.5K is what I always shot Flats at on the 15028HNT), comparing flats between the ADUs is starkly noticable (both have had Auto STF applied), 22.5K on left, 49.5K on right:

image

So whilst vignetting is still an issue, it is starting to be corrected successfully, now the difference in exposure times are like this:

22.5K ADU

L - 1.31S
R - 2.1S
G - 2.08S
B - 2.15S

49.5K ADU

L - 4.32S
R - 5.87S
G - 5.94S
B - 6.0S

I am not even close to clipping the histogram on the right either

image

So I am starting to get there, but maybe you should try some of these with your Redcat setup Wouter

Regards

Simon

This post was modified 2 months ago by Simon Todd

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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
Posts: 1816
 
Posted by: @stastro

maybe you should try some of these with your Redcat setup Wouter

Thanks for the tip! I generally take flats of between 1 and 5 seconds and make sure that the peak is as far to the right as possible without clipping. This works for Ha but not for OIII and I don't know why. Light leak or internal reflections (possibly in IR) are likely candidates.

Anyway, I am glad that you're managing to solve your flat issues.


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