Mottled background

cuiv

In between typhoons hitting Japan, I've had my first light with my ASI294MC-Cool. I am in Tokyo, so heavy light pollution. Tried 400x20s exposures at gain 300 and offset 10 (from SGP), temperature -10 (ouside temperature around 10-15, not humid), dithering every 10 frames. Lens is 350mm f4. While I'm very impressed to see that the galaxy I was pointing at (M33) is clearly visible, the background appears very mottled. It's invisible on individual subs, but it's very obvious on the stacked result (I also tried to stack 100 frames only, same thing). I tried with and without flat frames. The Master Dark looks fine. Any ideas?

The stacked master frame is here: https://drive.google.com/open?id=1E5UWv9ZLelL9Ms1E8zy42iKFAy4TwfO5

cuiv

Well. I'm trying with flats, and I'm getting the same thing, no matter my gain (tried 120, 150, 200, 300). My process is:

- Take flats

- Stack flats

- Apply ABE to make any irregularities obvious

And the exact same mottling pattern is visible on each final flat image... Regardless of gain, temperature of sensor, etc. Changing the angle of the camera doesn't change anything, so it's not the lens. Dark frames do not show any mottling.

So I have no idea what it can be, and I'm very confused - the moment light hits the sensor, things go haywire with the mottling. If anyone has any idea what this could be, please let me know!

Support@ZWO

Hi there

I think Gain300, offset 10, offset is too low, might cut the data on the left in histogram, I suggest to turn up offset to 110.

cuiv

Thank you! The SGP native driver only allows offsets up to 80, so 110 was not possible - also I think the Sony sensor offset needs to be multiplied by 5, but I may be mistaken. At any rate, I tried with the Ascom driver instead, and this time, no problem at all, as attached. (This is from Tokyo, super white zone, just 1 hour integration!)

Thanks again!

Support@ZWO

cuiv wrote:
Thank you! The SGP native driver only allows offsets up to 80, so 110 was not possible - also I think the Sony sensor offset needs to be multiplied by 5, but I may be mistaken. At any rate, I tried with the Ascom driver instead, and this time, no problem at all, as attached. (This is from Tokyo, super white zone, just 1 hour integration!)

Thanks again!

That's great, in Ascom we set a big value of offset, so there won't have any problem.