Flats on my 9.25 hd sct

[dasman]

I recently purchased a celestron Edge HD 9.25 SCT. I am having all sorts of issues with flats. I am getting concentric circles in my images. I use a light board and shoot for the white point of the histogram to be arround 50%. I have the gain as low as it goes, and the exposure is arround 400 to 500ms. Icany post any photos right now because Im not at home. I have used the light board with and without a tee shirt, no difference. The lights look like a normal sct light with vignetting and darher middle. The flats look like the vignetting is picced up and the center is bright. Dust motes are visible in the master

I will try to post images later if I can. Any help would be appreciated.

[admin]

Hi,

seeing an image or two will certainly help understand what is going on. One thing that occurs - what camera are you using? If you are using a camera based on the IMX294 or IMX492 sensors then there can be issues with flats not working correctly if taken at certain gain values.

cheers,

Robin

[dasman]

Here are a few shots. I have the flat and 2 of the shots it was applied to. The flat was made in Sarpcap and the histogram had the white peaking around 50% and the other colors were between 20 and 80%. this was 30 flats, and I had the 1st box checked for bias.

The camera I am using is the Mallincam DS26CTEC. I have made many successful flats in Sharpcap with the same camera on my William optics 130 FLT.

Something is different with this 9.25 HD Edge.

[ChrisR Oz]

Spooky, as I am also having flats issues on my HD 8 SCT using a flat panel and Flats generated in SC 4.1 (see my post today). In my case, the problem is also in the centre, but appears as a textured pattern.

Cheers, Chris.

[Menno555]

HI

I had issues like this in the past when I started. What helped to get good flats and calibrated stacks, was to use the same settings for the flats as the subs except the exposure time.
So for example when capturing my subs with Gain 120, Offset 10 , my flats have those same settings and I alter the exposure time.

Menno

[admin]

Hi folks,

looking at the bright ring shape in the middle of the flat frame, it looks to me very much like what you would see for an out-of-focus star on an SCT. When you captured the flat frames, was the SCT set up correctly so that it would be focused on infinity (ie if you took the flat panel away and pointed it at the sky, would it see focused stars?) . I'm suspecting maybe not, and given the huge focus range of an SCT, perhaps you are getting an out-of-focus but still recognizable image of the light panel (with dark spot in the middle from the secondary).

cheers,

Robin

[dasman]

The scope was already focused. This is very perplexing.

[admin]

Hi,

OK, scratch that idea then. Hopefully someone who uses an SCT regularly with flats will have some ideas to suggest. I haven't used an SCT for a long long time, and never for serious imaging, so am a bit stuck...

cheers,

Robin

[dasman]

Ok this is solved. It turns out it was mirror flop. My mirror locks were not engaged properly. So after taking flats at the zenith, then moving down to my target, the mirror would move ever so slightly (mirror flop in an SCT). That movement was enough to render the flats useless. With mirror locks now properly engaged, no issues with the flats.

Thank you to those that helped!

[admin]

Hi,

glad you sorted it, and thanks for posting the solution which might help others who encounter the same issue!

Robin