Noisy images from QHY8L: why?

[Roberto Coleschi]

Dear englishmen and friends from anywhere, last night after a long long time of rain and moist, i have had a clear night for testing my QHY8L. So, I have started to take subframes in M63 (about 20, at 600sec, minus 20°C ccd cooled). I have used Ezcap, due the recent issues with Ascom driver ver 3.0 and 3.1, but all is has gone well, and after the calibration frames, this morning I have tried a really quick work about the processing. After a final stack, and a DDP stretching, I have noticed that the stack is very very noisy for coming from a cooled CCD. So, I'm thinking about my error about acquisition and stacking (Starting Ezcap, set cooling point at -20°C and waiting the cooling, starting focus, Phd and autoguiding, guided subframes, saving in Fits binning 1x1). The only thing that I haven't cannot doing was the "dithering between subexposures", a very useful method to clear the noise. This is my results: the sky wasn't really clear, I have used an IDAS LPS D1 filter, and this is the post processed image: calibration, stacking, stretching DDP, saturation and color correction, all with AA 5.0. Any suggestion will be welcome..

Thanks..

http://astrob.in/87568/0/

[Daniel-K]

to me there looks like alot of green noise in there probably mostly caused by poor conditions.

[russellhq]

What's the light pollution like from where you are?

Also, 20 x 600s is not a lot of data.

How many of each type of calibration frame did you take?

[Roberto Coleschi]

What's the light pollution like from where you are?

Also, 20 x 600s is not a lot of data.

How many of each type of calibration frame did you take?

The LP at North is not so bad, but yesterday in the sky there was a little "thin clouds". I have taken 20 + 20 + 20 bias - darks- flats for the calibration..

[Uranium235]

That might be your answer, if there is any thin cloud - it will ruin your photos (especially in RGB). Narrowband can cut through that to some degree, but its still no substitute for a properly clear sky.

Have a sort through your subframes and throw out the ones that have obvious cloud damage (sometimes less is more).

[Thalestris24]

Hi Roberto

There could be an issue with bias frames - something I need to check out myself whenever I might get some clear skies...

There is an old forum thread here: http://stargazerslounge.com/topic/116989-qhy8l-bias/

Please do post again if you find that longer bias frames make a difference to your noise problem!

Cheers

Louise

[Roberto Coleschi]

Thanks a lot, Louise, for the suggestions and for the interesting link. I have read, and while we are waiting a new driver release for the QHY8L (maybe the 3.2 ver.), I hope to have some clear nights for doing other tests in the sky..

[Thalestris24]

Thanks a lot, Louise, for the suggestions and for the interesting link. I have read, and while we are waiting a new driver release for the QHY8L (maybe the 3.2 ver.), I hope to have some clear nights for doing other tests in the sky..

Hi Roberto

I would make a set of 4s bias frames and redo your M63 image with them to see if that improves the noise problem. It's something to find out because the situation won't change with any future driver release - the short/long exposure readout situation seems to be a fundamental characteristic of the sensor chip. EZcap uses the native (not the Ascom) driver anyway and, as far as I know, there are no problems with it so I don't expect any new versions.

I've visually compared 0s bias frames with 4s ones and the latter are _much_ less noisy. So that suggests that 4s bias frames are better i.e. more representative of the true read noise. Of course, a 4s bias frame = a short exposure dark frame, but that's ok!

Regards

Louise

Regards

Louise

[r3i]

Hi Roberto

There could be an issue with bias frames - something I need to check out myself whenever I might get some clear skies...

There is an old forum thread here: http://stargazerslounge.com/topic/116989-qhy8l-bias/

Please do post again if you find that longer bias frames make a difference to your noise problem!

Cheers

Louise

Thanks Louise - will have to try this out.