Flats - what am I doing wrong?

[Starwiz]

19x 180s SII frames stacked from last night's session.

ASI1600mm-Pro @ -10 deg C.

Flats produced using light panel over white pillowcase.  I also tried an illuminated wall, but it turned out to be unevenly lit.

Three different light levels were tried with the panel, but although the exposure times were different, the results were similar.

Flats were also taken with the sensor at -10 deg C.  Same SII filter was used.

APT CCD Flats Aid was used to generate the image plan, with a target ADU of 20,000.

Subs were stacked in DSS, then opened in PS and auto-contrast applied to enable a comparison.

The first image was stacked without using the flats and the second image had 20x flats added.

It appears the flats have added a dark area into the middle of the image.  i.e. they've subtracted too much.

Any thoughts on what I'm doing wrong?

Thanks

John

P.S. My tracking wasn't good, so stars not great, but this obviously isn't relevant to the flats issue.

[-Joe_]

No help, but, I had the same problem on Weds. same camera and using a white panel to get flats. I threw those flats away in the end.

i suspected dew being reflected by the light panel, but far too much of a novice to be in any way certain... 

 

[Starwiz]

6 minutes ago, -Joe_ said:

No help, but, I had the same problem on Weds. same camera and using a white panel to get flats. I threw those flats away in the end.

i suspected dew being reflected by the light panel, but far too much of a novice to be in any way certain... 

 

At least I know my problem isn't related to dew.

34 degrees C out here in Malta when I took the flats, so well above the dew-point.

John

 

 

 

 

[bobro]

Looks almost as if the flats have been applied twice. To get feedback it helps if a single unprocessed sub is posted, plus the master flat.

Edited June 28, 2019 by bobro

[geordie85]

It looks like your flats are over correcting. Did you calibrate them with dark flats too? 

You could also try lowering the ADU to around 18000. That's what I use but with a qhy183m 

[-Joe_]

7 minutes ago, geordie85 said:

It looks like your flats are over correcting. Did you calibrate them with dark flats too? 

You could also try lowering the ADU to around 18000. That's what I use but with a qhy183m 

When you say “calibrate them with the dark flats”, do you mean using exactly the same settings apart from a lens cap, or something else? 

Edited June 28, 2019 by -Joe_

[geordie85]

1 hour ago, -Joe_ said:

When you say “calibrate them with the dark flats”, do you mean using exactly the same settings apart from a lens cap, or something else? 

Exactly that. They're the same as darks, but to match your flats instead of your lights 

[Starwiz]

12 hours ago, bobro said:

Looks almost as if the flats have been applied twice. To get feedback it helps if a single unprocessed sub is posted, plus the master flat.

Master flat and single sub uploaded.

Thanks

John

MasterFlat_ISO0.tif L_SII_2019-06-27_23-14-01_Bin1x1_180s__-10C.fit

[Starwiz]

11 hours ago, geordie85 said:

It looks like your flats are over correcting. Did you calibrate them with dark flats too? 

You could also try lowering the ADU to around 18000. That's what I use but with a qhy183m 

No, i didn't know I needed to do that, but I'll give it a go.

Thanks

John

[Tommohawk]

Not sure if this helps for your specific problem, but there's a useful thread on asi1600 calibrations here

I haven't used my asi1600 much lately but seem to recall dark flats were significant, though your issue seems different. Good luck! 

[wimvb]

Did you use darks to calibrate your light frames? If not, then that's the problem.

There is a simple test you can do to see if your master flat is doing its job.

1. Calibrate each flat sub with a dark flat master. (Darks can't be created by only using a plastic lens cap. These let too much light through)

2. Stack the calibrated flats to create a master flat.

3. Calibrate one raw flat frame with the master dark flat (subtraction) and master flat (division).

4. Stretch the calibrated flat frame and the master flat by the same amount.

The single flat frame should not show dust bunnies and no vignetting, while the master flat should.

Edited June 29, 2019 by wimvb

[Starwiz]

2 hours ago, Tommohawk said:

Not sure if this helps for your specific problem, but there's a useful thread on asi1600 calibrations here

I haven't used my asi1600 much lately but seem to recall dark flats were significant, though your issue seems different. Good luck! 

Thanks for the info.

John

[Starwiz]

1 hour ago, wimvb said:

Did you use darks to calibrate your light frames? If not, then that's the problem.

There is a simple test you can do to see if your master flat is doing its job.

1. Calibrate each flat sub with a dark flat master. (Darks can't be created by only using a plastic lens cap. These let too much light through)

2. Stack the calibrated flats to create a master flat.

3. Calibrate one raw flat frame with the master dark flat (subtraction) and master flat (division).

4. Stretch the calibrated flat frame and the master flat by the same amount.

The single flat frame should not show dust bunnies and no vignetting, while the master flat should.

No, I haven't taken darks yet as I usually do these at the end when I have all the light subs I need.  I've left the camera on the telescope so the orientation is the same when I come to take Ha, OIII and some more SII.  I usually remove the camera to take darks so I can be sure there's no light getting in.

I didn't know that it's vital to include the darks.  I've seen previous posts where people say they don't use darks, but these may have been with different cameras.

I'll have a go at the test you suggest when I've got the data.

Thanks

John

[bobro]

10 hours ago, Starwiz said:

Master flat and single sub uploaded.

Thanks - I get the same result as you.

As @wimvb says, the lack of darks will be a problem, especially as a narrowband image of 180sec subs will need a good deal of stretching. For a quick test, below are 2 images from the session I did last night. The filter used was just a red filter with 120sec subs, so the amount of stretching required is less than for narrowband. Nevertheless, the sensor amp-glow (IMX183 sensor) is clear in the first image without darks (2nd image has darks added).

The ASI1600 sensor has a different and less distinctive amp-glow pattern. This link has details : https://astronomy-imaging-camera.com/tutorials/what-is-amp-glow.html

 

Edited June 29, 2019 by bobro

[wimvb]

9 hours ago, Starwiz said:

No, I haven't taken darks yet as I usually do these at the end when I have all the light subs I need.  I've left the camera on the telescope so the orientation is the same when I come to take Ha, OIII and some more SII.  I usually remove the camera to take darks so I can be sure there's no light getting in.

I didn't know that it's vital to include the darks.  I've seen previous posts where people say they don't use darks, but these may have been with different cameras.

I'll have a go at the test you suggest when I've got the data.

Thanks

John

If you standardise the cooling, gain and exposure time, you can reuse darks. The exact settings depend on your setup and sky conditions. Start with a fixed temperature (I use -20 C) and gain, then experiment with time untill you find an exposure time that works. You may need one for L, one for rgb, and one for narrowband.

CCD cameras may not need darks, and neither can uncooled cameras, but for cooled cmos they are essential.

[Starwiz]

20 hours ago, bobro said:

Thanks - I get the same result as you.

As @wimvb says, the lack of darks will be a problem, especially as a narrowband image of 180sec subs will need a good deal of stretching. For a quick test, below are 2 images from the session I did last night. The filter used was just a red filter with 120sec subs, so the amount of stretching required is less than for narrowband. Nevertheless, the sensor amp-glow (IMX183 sensor) is clear in the first image without darks (2nd image has darks added).

The ASI1600 sensor has a different and less distinctive amp-glow pattern. This link has details : https://astronomy-imaging-camera.com/tutorials/what-is-amp-glow.html

 

Thanks for the info.  I'll definitely be using darks when I've got all the lights data.

John

[Starwiz]

17 hours ago, wimvb said:

If you standardise the cooling, gain and exposure time, you can reuse darks. The exact settings depend on your setup and sky conditions. Start with a fixed temperature (I use -20 C) and gain, then experiment with time untill you find an exposure time that works. You may need one for L, one for rgb, and one for narrowband.

CCD cameras may not need darks, and neither can uncooled cameras, but for cooled cmos they are essential.

Thanks

John