Help needed with flats

[Tony_]

G'day,

I often get this problem with images using flats flats - uneven circles etc.
Imaging scope: sct 9.25" @ f 6.3.
Camera CDS 60s cooled. (ISO 800, -14c).

The flats are taken with a good quality light box at the same configuration and time as the lights. Same camera position etc. I used the same ISO and reduced the exposure so the histogram of the flats matches the lights. I used 20 flats. The image was stacked in DSS. I seem to get this problem mostly when I use the reducer. The reducer was used for the lights and the flats.

Does anyone have any ideas what is happening - thanks for any advice.

Regards,
Tony.

[Atmos]

Tony,

Good flats can be difficult. Typically you want to try to bring the average ADU count of your histogram up to 50-75%. Although you want to have the flats taken in the same orientation, they don't need to be the same exposure length or exposure on the histogram.

When taking flats on my refractor I take 25x5s through each filter. Takes about 45-60s with narrowband though. With my DSLR I can do them at 1/100th second.

I have a feeling that it may be you not exposing the flats enough, 50-75% of saturation :)

[RobF]

Flats can be tough in longer focal ratio newts and compound scopes. I used to find issues under city skies (light pollution) shooting flats without my dew protector (that had been place while shooting lights) on my newt. Sky flats (possibly with T shirt) may be your best bet if the reducer keeps doing this to you. Not terribly convenient however, particularly without automation to keep varying exposure length.

If all else fails, fake it! Apply copious quantities of Pixinsight DBE! :D

[Tony_]

Thanks for your replies.
I have tried the longer exposure to get 50-75% on the histogram - that made it worse. I noticed that the flats I used seem to have a different visual profile than my lights - so I will need to trial exposures and ISOs a bit more.
Also the histogram of the colours is different - I don't know if that matters but the red is shifted in the flats. I may need to use a different white balance for the flats?

Also it may be associated with the dew cap being used for the lights but not the flats. I will try more options - I'll get it right eventually.

Tony.

[Atmos]

The colour isn't important in the flats and it is just for removing vignetting, pixel to pixel variation and dust bunnies.

[Tony_]

Final image (for now) isn't that special - but I have improved the gradient.

Needs more exposure. Lots of noise and background gradients.