Add and Average images

Image processing, astrometry, photometry, etc.
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VAastro
Posts: 19
Joined: 07 Jan 2019, 18:32

Add and Average images

Post by VAastro » 17 Apr 2020, 14:59

Hi,

When I either add or average images in AA6 SP4 I get a noticeable vertical variation in the background.
Why is this happening?

Thanks
Attachments
ADD.png
AVERAGE.png
SINGLE IMAGE.png

Dunmunro
Posts: 85
Joined: 10 Jan 2019, 01:11

Re: Add and Average images

Post by Dunmunro » 17 Apr 2020, 22:05

VAastro wrote:
17 Apr 2020, 14:59
Hi,

When I either add or average images in AA6 SP4 I get a noticeable vertical variation in the background.
Why is this happening?

Thanks
Hi. Are you using flat frames?

VAastro
Posts: 19
Joined: 07 Jan 2019, 18:32

Re: Add and Average images

Post by VAastro » 17 Apr 2020, 23:04

Yes, I am using flat fields, bias, flat dark and dark frames

Dunmunro
Posts: 85
Joined: 10 Jan 2019, 01:11

Re: Add and Average images

Post by Dunmunro » 19 Apr 2020, 18:40

VAastro wrote:
17 Apr 2020, 23:04
Yes, I am using flat fields, bias, flat dark and dark frames
Are you imaging from an area that has a lot of light pollution?

fabdev
Posts: 201
Joined: 03 Dec 2018, 21:43

Re: Add and Average images

Post by fabdev » 19 Apr 2020, 18:57

Maybe the defect is "inside" the flats/darks/bias : try the same preprocessing without any of those.

VAastro
Posts: 19
Joined: 07 Jan 2019, 18:32

Re: Add and Average images

Post by VAastro » 21 Apr 2020, 21:42

Dunmunro
I am imaging in a dark sky to the NCP region.
I am seeing this gradient all the time in add and avg images from all sky orientations and cooling to either 0 or -5 deg C.
Could it be "amp glow" in the camera? It is a 4 year old ATIK 428EX
The telescope is a GSO 250mm RC with a AP reducer. The scope is well baffled. Wanning moon.
I am doing photometry so I worry about this gradient.

Fabio
I am attaching images added and stacked without BDF

Thanks all
Attachments
ADD no BDF.png
AVG no BDF.png

fabdev
Posts: 201
Joined: 03 Dec 2018, 21:43

Re: Add and Average images

Post by fabdev » 21 Apr 2020, 22:22

Hello,
the problem is visible also on the single image, it's only masked by noise.
For example, in the last image of the first post, if you remove stars with the command "Ring Median" radius = 60 then the problem is visible.

You can verify if it's amplifier glow or charge accumulation if the problem is visible in dark frames. You could use Median or Gauss filtering to detect.
Greetings,
Fabio.

VAastro
Posts: 19
Joined: 07 Jan 2019, 18:32

Re: Add and Average images

Post by VAastro » 25 Apr 2020, 14:10

This is the response I received from Atik :

"What you are seeing is the build up of dark current while the image is in the vertical readout register. At the end of the exposure the image is moved from the photo diode (very low dark current) to the read out register (higher dark current). The first lines readout spend very little time in the register so pick up less dark current the last lines read spend more time so have higher dark current. It’s a small effect in a single image but becomes apparent in binned or stacked images. As its only relates to the readout phase it can be corrected with a bias frame.

Its origin and magnitude is different to glows seen in CMOS cameras."

fabdev
Posts: 201
Joined: 03 Dec 2018, 21:43

Re: Add and Average images

Post by fabdev » 25 Apr 2020, 20:29

I think that it means "... it can be corrected with a bias frame (if you don't use dark frames)".
So I suggest to try with Light Images and Dark Frames only (taken at the same temperature and same exposure time).

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