Flats not fully helping in stacking using Hyperstar 6"

[Ehud]

Hi David,
I have stacking issue on Hyperstar 6". I did everything "by the book", still the result is terrible on stacking. I tried for few nights' nothing helped. Attached is a screenshot.
I upload everything (fits, processed, darks, flats, etc) to https://drive.google.com/drive/folders/ ... sp=sharing
I hope you can sort it out for me.
Thanks,
Ehud.

[Ehud]

I meant Hi Robin, don't know why I wrote David ...

Just to add, I tried linear and other types of Background removal, it haven't solved the issue, just the 'shape' of the artifact changed.

[admin]

Hi,

Unfortunately I can't fully reproduce the problem with the flat frame processing, as that would require me to have one or more saved 'raw' frames from the camera - that sort of thing needs the raw frames, not the stacked/processed images.

However, there is something you should start off by trying - making sure that the camera settings for the dark frame match those for the light frames. Looking at the two capture settings files, I can see the following differences:

Exposure : Dark Frames = 5s, Light Frames = 2s
Black Level : Dark Frames = 60, Light Frames = 50

Both of these are likely to cause issues, particularly the difference in black level, which means that the dark subtraction will incorrectly leave an offset in the subtracted image, which in turn gets corrected by the flat frame, leading to the flat correction working poorly or creating more problems than it causes.

I would also suggest leaving things like background subtraction and satellite trail removal turned off while you are testing flat correction - only turn those on when you are happy that you have flats working properly - that approach will help simplify things a bit.

cheers,

Robin

[Ehud]

Thanks Robin.
I tried all you said. No significant change.
But there are two strange things, maybe they are hints:
1. Yesterday I had pretty good results. But using same flats, darks & other exact setting today - wasn't so good. Is there a reason that you can see?
2. Single stack of 5 seconds is much brighter then just having an image of 5 seconds. What is the reason?

Is there a way for me to save the Raw frames and sent them to you?

Ehud.

[Menno555]

Hi Ehud

How do you make your flats? Do you use the "white t-shirt" methode, do you use a flat panel, or something else?
For flats is important to have a even lit source.

Menno

[turfpit]

Ehud

To progress this it would be helpful if you provided a single flat, a single dark and a single light, all in FITS format.

Dave

[turfpit]

Ehud

Might be worth a read viewtopic.php?t=2552

Dave

[admin]

Hi,

just to answer some of the questions above...

* When you activate live stacking, by default SharpCap adds a stretch to the image, so it will immediately look brighter (but noisier) than the single images - of course as more images are added to the stack, the noise level will drop.

* In order to compare this sort of issue sensibly from one session to the next, you have to take care to use the same level of stretching in the image - even really good flat correction will show flaws if the image is stretched heavily enough, whereas mediocre correction can look quite reasonable on an unstretched or lightly stretched image

* You can save raw frames by adjusting the 'Raw Frames' setting in the live stacking left hand panel - see https://docs.sharpcap.co.uk/4.1/#Left%20Panel - change the setting to 'Save Stacked' to save the raw frames that are added to the stack.

cheers,

Robin

[Ehud]

""How do you make your flats? Do you use the "white t-shirt" methode, do you use a flat panel, or something else?
For flats is important to have a even lit source.
Menno"

Thanks Menno. -I tried everything: Light panel (my Android tablet), white t-shirt on the afternoon, as well almost on dark to have ling-exposure flats' and direct sky on the afternoon and dome. It seems somehow, that direct blue sky on the afternoon, and grayscale flats (not color) settings, gave the less annoying result - but still not good enough.

[Ehud]

"Might be worth a read viewtopic.php?t=2552
Dave"

Thanks Dave' very nice document!
One thing - At the document, the Flats are taken with a black level. I thought that Flats should be taken with 0 black level, and this is what I read in few places.
Am I doing a mistake?