Calibration failing for a single filter - offset necessary for flat - how to?

[rbotero]

Hello All

I calibrate all my frames manually.  I have created a Superbias frame out of 50 individual bias.  I have a master Dark out of 40 individual frames.  All my flats are within the linear spectrum of my Moravian G4-16000 (16803 chip).
Yet, when I calibrate my frames, the red frames (and sometimes the Ha frames) are over-corrected in the centre of the image where the flat intensity is greater.  I do not have this issue, or at least is not as noticeable, with L,G,B,OIII or SII frames.
If I use an offset for the flat to calibrate my reds I get nicely corrected frames.  I am using the following formula when I do this manually:

(($T-SuperBias1x1)-k*(Dark30m1x1_20C_-SuperBias1x1))*(mean(Flat_R1x1)+offset)/max(0.000002,Flat_R1x1+offset)

with k=0.121 (k0 obtained from calibrating using PI) and offset set at 0.2

How can I consistently add an offset to my frames so that I don't over calibrate them?

Thank you

Roberto

[rbotero]

Can the Output pedestal (DN) option in the ImageCalibration script be used for this purpose?

If my Dark base level is some 400-500, could I use a pedestal of say 100 as an offset?

Would this be used both in the lights and flats calibration or does it need to be used consistently for the darks and bias?

Thanks

Roberto

[jkmorse]

Roberto,

I saw this issue in the past as well.  A quick fix someone gave in another post that works beautifully for me is to open one of the lights and your flat master, then use Linear Fit to give the flat the same flux as the light frame.  Save it and then use it for calibration. 

Let me know if that fixes the issue.

Best,

Jim

[rbotero]

Jim
Many thanks!  I will try that out. I wish there was a way of consistently adding such an offset - either arbitrary or through the linear fit - within the ImageIntegration procedure say.
Roberto

[rbotero]

Jim

Got home and tried your suggestion.  Unfortunately the calibrated frames using a corrected flat were undercorrected and my final integration showed a clear hot spot in the centre where the illumination is maximum.

I can get very decent results using the offset formula I got from David Ault's post here: http://www.cloudynights.com/topic/503765-about-to-give-up-on-current-system/?p=6646438

I may try the pedestal route when integrating to see if I can solve the issue.

Roberto