Flat problem

[mmirot]

I get two bad results the left is with PI Calibration using master made a Vincent suggests. The right is with MaxIM DL. PI over corrects and MaxIM DL under corrects the field.
Help

Max

[vicent_peris]

Hi Max,

could you upload the master and light frames to evaluate the problem, please??
And could you please tell more data about the frames? Exp. time, temp., etc.


V

[vicent_peris]

Your master flat is already bias and dark subtracted before the target frame calibration? If so, please  disable the "calibrate" checkbox of the Master flat section. I think that's what happens. You're subtracting two times the bias (and possibly the dark) pedestal to the flat; this causes the over correction of vignetting (bright corners).


V.

[mmirot]

That must be it. You should add a reference note to your master frame article.

Max

[Niall Saunders]

add a reference note to your master frame article

I have now read the article about twenty times (trying to get my head round this new workflow ) and the reference that you have requested is (I believe) already there, just above the section titled

Dark Frame Scaling Considerations

I do think that the article stops 'prematurely' - it would have been nice to see just one more section taking the calibration process to completion, showing the 'raw' Lights being fully calibrated, ready for StarAlignment and subsequent ImageIntegration.

It would also be nice to see the tutorial eventually get expanded to cover the workflow needed to calibrate OSC images (from imagers that use a Bayered CFA).

Otherwise, it seems fine 'on paper' !!

Cheers,

[vicent_peris]

I do think that the articel stope 'prematurely' - it would have been nice to see just one more section taking the calibration process to completion, showing the 'raw' Lights being fully calibrated, ready for StarAlignment and subsequent ImageIntegration.

I prefer to speak about these topics separately. I think I will first speak about light frame calibration, and then about superflats. I have more observational techniques we have been developing at Calar Alto... there's much to write on!


V.

[vicent_peris]

I have now read the article about twenty times (trying to get my head round this new workflow ) and the reference that you have requested is (I believe) already there, just above the section titled

Dark Frame Scaling Considerations

Yes, this is the text:

Now we have our master flat frame and will use it in the IC tool to calibrate the light frames. Just remember to disable the Calibrate checkbox in the Master Flat section, as we already have calibrated our master flat frame.

[mmirot]

Your master flat is already bias and dark subtracted before the target frame calibration? If so, please  disable the "calibrate" checkbox of the Master flat section. I think that's what happens. You're subtracting two times the bias (and possibly the dark) pedestal to the flat; this causes the over correction of vignetting (bright corners).


V.

Vincent,

Why did you do pixel  math subtraction  in the tutorial?
The Check box works to calibration works the same.

Max

[Niall Saunders]

Sorry Max - you have 'lost me', where was the PixelMath process that you refer to?

Cheers,

[vicent_peris]

Sorry, I'm lost too!

V.

[mmirot]

Sorry I mean Calibration step I

Max

[vicent_peris]

Vincent,

Why did you do pixel  math subtraction  in the tutorial?
The Check box works to calibration works the same.

Max

Before creating the master flat, you must subtract the bias and thermal noise to each flat. Any additive to the flatfields must be removed. This is esential to make a good illumination fit between all the flats.


Vicent.