QHY8 Flat Field frame

[alistairsam]

Hi,
I just built a light box with some ultra white LED's and took some flats with a QHY8 OSC, but they've turned out green after debayering.
I can't imagine how this could be normal, so any idea what I could be doing wrong?
I captured it with maxim using an exposure length of 0.065s. that gave an average value of around 20,000.
I use DSS to stack frames. I've set the FITS file settings to QHY8 in DSS.
or is it green because the bayer matrix for the qhy8 is RGGB?

Thanks

[Shiraz (Ray)]

I doubt that anything is wrong. The QHY8 is most sensitive in green and the LED source may have a lot of green as well.

Its a while since I used flats (IRIS or StarTools can get rid of uneven backgrounds), but I think that the Nebulosity manual recommended using flats before deBayer.

If you apply a greenish flat (deBayered or not), it will bias the light frames to have reduced green, but you can correct for that easily enough - might even get rid of some skyglow. suggest that you use the greenish flat and see what it does.

edit: just found a flat I once took with my QHY5 OSC with sunlight as the source - looks similar

[alistairsam]

Thanks. I used these LED's with a colour temp of 6500k.
https://australia.rs-online.com/web/...e-led/6729063/
I might try the 9000K ones or just use the green flats.
I read in the qhy forums that the flats need to be used with the Bias. so might try that as well.
I've been trying out ccdstack and it seems to produce better results than DSS, but with software such as this, do I need to create a master flat or calibrate the flats with darks or bias? I believe I won't need bias frames if I use darks.

[Shiraz (Ray)]

I don't know either package, but my understanding is that you remove bias and dark current from both the flat and light frames before you can apply the flat calibration (which is a scaling by division, so you don't want any offsets). I guess that a dark frame incorporates both bias and dark current, so your statement is correct.

the Nebulosity manual is well worth a read on this topic - it's available online

[Osirisra (Ken)]

My QHY8L has green tinge to the flats as well.

[alistairsam]

Thanks
I've been reading up a bit and understand that flats and lights need to be bias corrected for flat calibration to work
I just want to know if software such as dss, maxim , ccd stack takes care of all that if you just point it to all the calibration frames or do we need to calibrate the flats first

[multiweb (Marc)]

Originally Posted by alistairsam

Thanks
I've been reading up a bit and understand that flats and lights need to be bias corrected for flat calibration to work
I just want to know if software such as dss, maxim , ccd stack takes care of all that if you just point it to all the calibration frames or do we need to calibrate the flats first

In CCD Stack you can choose to subtract the bias from the flat when you combine them and make the master flat or subtract the master bias from the master flat when doing the subs calibration. Both ways will work.