Error creating a "Flat"

[Lloyd69]

Hello Robin,
Last week I tried to create and use a "Flat" through Sharpcap for the first time.
The creation worked well with the help of the instructions, my peak in the histogram for all colors was clean and evenly distributed over the 50%. But if I include the resulting flat in the live stacking, I no longer get any sensible images. There are no more stars to be seen and the whole picture is extremely falsified. Without the flat, I have the normal picture. I attach the created flat as a screenshot. I hope you can recognize my mistake and help me.
Thanks and greetings from Hungary
Lloyd69

11_59_23.fits.CameraSettings.txt

(880 Bytes) Downloaded 25 times

[admin]

Hi Lloyd,

well, the flat looks pretty much fine to me - sensible values for the exposure (perhaps you could have gone a bit higher on the histogram - up to 75-85% max, but that shouldn't be a big problem).

One thing I can see though is a couple of bright patches in the flat frame image - one just up and left of the center, and the other near the bottom left corner (about 10% of the way up and about 25% of the way across). If these patches are significantly brighter than the rest of the flat (say they have pixel values in the 50000-60000 range), they will be used as the reference brightness for flat correction, which means almost everything will get brightened by 50% or more. Basically any small bright area that is much brighter than the central bright area of the flat could cause problems.

If it's not that, it might be helpful to see a screenshot of what you get in live stacking both with and without the flat - maybe seeing the result and the live stacking/camera settings will help.

cheers,

Robin

[Lloyd69]

Hello Robin,
Thank you for your prompt reply. On the next clear night I will create a new "Flat" and test the old one again. The picture wasn't too bright, but rather everything was much too dark and only with extremely high gain did I see a red smeary spot in the center. You have to see that, I can't describe it. No stars, no real picture. That's why I thought of a gross mistake when creating the Flat. I also enlarged the light spots that you mentioned on the flat, that looks very unusual.

Screenshot_1.png (287.32 KiB) Viewed 309 times

[admin]

Hi,

did you get anywhere by adjusting the histogram stretch in the live stacking (and/or the one on the right in the mini histogram)? Usually a combination of those (preferring to keep the one on the right at 'default' and use the live stacking stretch) can deal with issues like darker than average images - in fact it could just be that the stretch settings were correct for 'without flat' and needed adjustment for 'with flat'.

There is definitely something odd with that patch of image. Do you see the same thing if you capture another flat with roughly the same settings? Do you see anything odd in that area of the image when capturing normal images?

cheers,

Robin

[Lloyd69]

I'll check all of this as soon as the weather allows. After the first attempts, I didn't try it for very long because I received my normal image without the flat and assumed that there was an error when creating it. The time is always just below clear skys ^^
I will also not clean the sensor now in order to be able to use the current flat again, even if there are enough donuts in the image plane.

[Lloyd69]

Here is an enlarged section of the picture which I took immediately afterwards without a flat screen (M1)
I can't find anything unusual there, but I'll also watch that better next time.

Screenshot_2.png (163.56 KiB) Viewed 306 times

Screenshot_3.png (247.44 KiB) Viewed 306 times

[Lloyd69]

Hello Robin,
Today I was able to try something again. But I couldn't reconstruct the problem. But I have other problems with SharpCap 4 that I didn't have with version 3.2, but I would like to write that in a different thread. For the last time I think that "Flat" was not responsible for my problem. The spots on the "Flat" are also exactly in the same place on the "Flats" created today, I suspect dirt directly on the sensor. I will clean all surfaces tomorrow.

Greetings from Hungary
Lloyd

[Lloyd69]

Hello Robin,
Today I was able to reconstruct the mistake. It only occurs when I create a "Flat" with the option "Capture and subtract Bias Frames" With a "Flat" without black level offset corrections I have a normal picture. Here are the screenshots.

without FLat, looks good

Screenshot_10.png (336.18 KiB) Viewed 249 times

with simple Flat, looks good

Screenshot_11.png (340.69 KiB) Viewed 249 times

with Flat with substract Bias Frames, whats this?

Screenshot_12.png (290.26 KiB) Viewed 249 times

and here the Flat with sustract Bias Frames at Fitswork. He looks like the other Flat

Screenshot_13.png (692.9 KiB) Viewed 249 times

Cheers Lloyd

[admin]

Hi Lloyd,

could you share the problem flat FITS file with me please? I would like to look at the pixel values in the problem area to see what is going on. It would also be interesting to know what exposure time you are using for flat frame capture? The bias frames are captured at 1ms exposure (because some cameras misbehave when set to minimum exposure, so we avoid that). We might have to dig in further to grab the individual flat and bias frames to look at those too in due course.

cheers,

Robin

[Lloyd69]

how can i send the file? Is bigger than 1M