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Mottled Flat Frames

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#1 ks__observer

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Posted 23 January 2021 - 01:59 PM

I recently purchased an ASI294mm.

I was running flats with my 200mm (8in) Newt.

I am getting a mottled pattern in flats.

Below is a 10sec Ha.

But I get it with my 1sec Lum.

It does not stack out.

It does not seem to be present with SV70 flats.

 

Any thoughts?

 

gallery_263201_9741_16932.jpg

 



#2 Tapio

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Posted 23 January 2021 - 02:04 PM

How is your flat setup like ?



#3 imtl

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Posted 23 January 2021 - 02:10 PM

https://www.cloudyni...exposures-bin1/



#4 ks__observer

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Posted 23 January 2021 - 02:32 PM

I have a t-shirt over the front, no light box, just aimed at my skylight, and my ADU is about 1/3 the max.

This is not a banding issue.



#5 imtl

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Posted 23 January 2021 - 02:59 PM

I have a t-shirt over the front, no light box, just aimed at my skylight, and my ADU is about 1/3 the max.

This is not a banding issue.

Yes sorry I was thinking about the banding issue reported.

 

A few questions/comments:

 

1. Do you get this with all your filters?

2. Do you get it in your lights?

3. Are these showing up in master (calibrated) flats? Or individual ones?

4. If it shows up in the master flats, how are you calibrating your flats? (bias/DF)

5. What gain/offset did you use for this?



#6 ks__observer

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Posted 23 January 2021 - 03:11 PM

I tested Lum and Ha and present in both.

I looked a light I shot yesterday but not see any mottling, but the very little ADU compared to a flat.

It shows up in master flats.

I bias subtract with a flat-dark.

I have to check if gain makes a difference.

 

Thanks.



#7 imtl

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Posted 23 January 2021 - 03:13 PM

What did you use to stretch the flats to show this? Are you using PI?



#8 StevenBellavia

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Posted 23 January 2021 - 03:31 PM

Do a dozen flats, then rotate the camera and filter by 90 degrees and do another dozen.

If the stacked images are identical, it is in the camera-filter.  If not , it is in your light source, tee-shirt, etc.  Much easier to remedy.

See if you can rotate the filter only.

My guess is that you are revealing imperfections in the filter(s).  They are difficult to make, and I doubt they can be "perfect".  My Ha flats always look terrible, but as long as that is the same as was used in my light frames, it calibrates out.

But I am surprised the lum flats look the same.


Edited by StevenBellavia, 23 January 2021 - 03:32 PM.

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#9 StevenBellavia

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Posted 23 January 2021 - 06:24 PM

I took a look again and the flat you posted looks like FPN - exactly the reason to do flat frames.

It looks like one of the examples here:

 

https://www.dpreview.../thread/3806574



#10 ks__observer

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Posted 23 January 2021 - 08:12 PM

I really appreciate everyone's input and help.

I re-ran my flats later in the day and for some reason the problem seemed to subside.

The OP has an Ha flat.

My latest Ha flat still has a little mottle.

It is not the t-shirt because the latest Ha, which still has a little mottle, was taken with no t-shirt and scope pointed to the sky through a skylight.

Earlier in the day my work area was very lit up from sun -- I don't know if that had anything to do with it -- I don't know what I did different the second time.

Hopefully the new flats work.

 

Thanks again!


Edited by ks__observer, 23 January 2021 - 08:13 PM.


#11 ks__observer

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Posted 24 January 2021 - 11:55 AM

There is a discussion here of the mottle problem, which the person said was cured with higher gain:

https://forums.sharp...opic.php?t=1531




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