What went wrong?

Hi Michel,

It looks like flat calibration isn't working. You will need to subtract the bias offset from both light frames and flat frames.

So for your ISO400 light frames, you will need to create a Masterbias of ISO400. That will subtract the bias offset from the light frames.

The flats with ISO 100 need the ISO100 masterbias to have the bias pedestal/offset removed.

Then it should work perfectly 😉

So I would suggest that you shoot a batch if ISO400 bias frames to make an iso 400 masterbias and use that for calibration of the light frames.

APP internally, will match the 2 masterbias frames correctly to the flats and to the lights. It checks if the frames match for ISO value.

Let me know if it works 😉

Mabula

Hi Mabula, thanks again for your help! I did make new bias frames with ISO400 and it worked!

I still don't get the vignetting correction to work properly. Everytime i do that the picture looks like the one above with extreme white correction. So i skip that and did the Light pollution and calibrate background. Unfortunately the M31 is so big that even on my 150mm newton I had to cut the top and bottom part off due to stack artefacts.

Today i looked at the collamination and that i corrected as well so that should be better in next pictures. I do still see some big background circle in a more red tint, but my guess is that it is a sensor issue of my D750, since all astro pics show the same anoying circle.

This is the result:

thanks again for your help! Now i will look for a PS tutorial so i can improve the pictures even more. I love APP already but the Selective color tool is a bit too non-intuitive for me.

Mabula,

one more question regarding the error above. I read in '[Sticky] Data calibration priciples/rules - must read !" that light and flat frames can be calibrated only by the BPM and that this is the recommended way. Why did my picture at the start of this post resulted in the incorrect flat correction? Did i have to calibrate & save the flats seperately prior to calibrating the lights?

Cheers,

Michel 

Posted by: Michel

Hi Mabula, thanks again for your help! I did make new bias frames with ISO400 and it worked!

Excellent ;-). Try to keep in mind that for flat calibration to work properly, both the light frames and flat frames need to have the bias offset subtracted once (which can be done with either a masterbias, masterdark or a masterdarkflat). In future versions I will implement that APP warns the user if the needed calibration frames aren't loaded for accurate flat calibration.

I still don't get the vignetting correction to work properly. Everytime i do that the picture looks like the one above with extreme white correction. So i skip that and did the Light pollution and calibrate background. Unfortunately the M31 is so big that even on my 150mm newton I had to cut the top and bottom part off due to stack artefacts.

If you have applied flat frames already in calibration, then the vignetting correction function won't work, it will only work when flats weren't used at all. For any remaining vignetting to be corrected after having applied flats in calibration, just use the "remove light pollution tool". That works really well.

Today i looked at the collamination and that i corrected as well so that should be better in next pictures. I do still see some big background circle in a more red tint, but my guess is that it is a sensor issue of my D750, since all astro pics show the same anoying circle.

Yes, this circle that you mention, is in fact a known problem with some DSLR models, my Nikon D610  has it as well, even the D810a has it ;-( A solution seems to be to illuminate your flats for as long as you can without saturating the histogram on the right side. I got feedback from several photographers that the problem then dissapears or is much less present.

This is the result:

thanks again for your help! Now i will look for a PS tutorial so i can improve the pictures even more. I love APP already but the Selective color tool is a bit too non-intuitive for me.

Thank you for you feedback Michel. The Selective Color tool is up for a complete remake 😉 Will work on it next week. I will also integrate it into the left control panel instead of letting it have it's own window, the tool will be much easier then.

Posted by: Michel

Mabula,

one more question regarding the error above. I read in '[Sticky] Data calibration priciples/rules - must read !" that light and flat frames can be calibrated only by the BPM and that this is the recommended way. Why did my picture at the start of this post resulted in the incorrect flat correction? Did i have to calibrate & save the flats seperately prior to calibrating the lights?

Cheers,

Michel 

Hi Michel,

That is not entirely correct.

In the post

astronomical-data-calibration-priciples-must-read

it states for flat frames:

 4) Additionaly, Flat frames can always be calibrated by a Bad Pixel Map besides any of the above 3 methods, which I strongly recommend.

and for light frames:

 5) additionaly, LIGHT frames can always be calibrated by a Bad Pixel Map besides any of the above 4 methods, which I strongly recommend.

So that means that BPM can be applied on flats and darks besides the required calirbations.

The required calibrations ensure that for both flats and lights, the bias offset is subtracted once, and only once.

For flats and lights, you can read as well, that since you were using different ISO values for the lights and flats, you needed two separate masterbias frames 😉 to have both flats and lights properly calibrated.

Kind regards,

Mabula

Thanks! All clear now! 🙂