Dark frames are not correctly subtracted

[Bobinius]

It seems that you're doing the right thing. What exposure times do the flats have? How do you create the master flat? Do you use filters when imaging? If yes, do you take flats for those as well?

Lots of questions, I know

I am asking myself the same questions : ). I use a LPS filter UHC-S from Baader which stays on all the time.
The exposure is the one needed to bring the histogram close to the middle, usually between 1-3s.
The master flat is created by the batchpreprocessing script. Or I created by respecting the protocol from Warren's book, Inside Pixinsight, multiplicative normalization when integrating, calibrating before with the bias and master and enabling optimization.

For my latest preprocessing image I made the masters manually Bias, Dark and Flat and integrated using batchpreprocessing without obtaining the Starburst.

Why would optimizing the darks for the flat calibration fail to subtract the Starburst?

I imagine that the capturing program writes the duration of  the exposition in the fits file. Could it be a problem at this level? Can I check somehow the parameters in the fits file?

[Bobinius]

The settings that seem to work best are when I shoot them in RGB

You should be shooting all your frames the same way you appear to ve shooting your Lights - i.e. in RGB mode. You should not deBayer until you have fully calibrated all of your (still in grey-scale) Lights - just before you Align and Combine the Calibrated Lights. You should only expect to see the beginnings of a colour image if you unlock the channels duriing STF.

Hope  this helps.

Thanx Niall, that's what I'm trying to do (and since I use the batch it does it on its own). SharpCap tells you however to shoot the flats in Mono because RGB flats could alter the white balance. I was wondering if that really holds.And their Flat script is creating a mono flat.  I am having the impression that my images with the Altair Astro 294C have less blue in the galaxies compared to other images. Or could the Baader filter have an influence? There's something fishy about these flats...

[wvanreeven]

Baader filter? Do you use the filter also when shooting flats? If not then do. The flats should have the same color balance as the lights.

[Bobinius]

Yeah I leave the filter on. I'll have to experiment with the settings and some flat darks to see how it goes.

What do you guys use as target for the flat histogram? I saw multiple recommandations on the web of around 20-25 k ADU, for a 16 bit image (not my camera) that's lower than the 50% for 65 k (which would be around 32 k). So should the intensity be lower than half? APT has a nice flat aid where you introduce your ADU target, I intend to use APT for acquisition (works really well for DSLR).

[wvanreeven]

In my experience targeting for 50% is not necessary at all. Somewhere between 25 and 75% is good enough. I usually inspect my flats visually. If they have a pale pink hue (I use a modded Canon DSLR) then I am happy. So far I haven't had any traces of vignetting in my corrected subs nor in my stacked images. I use an Astronomik L-3 luminance filter because my ED refractor introduces horrible blue halos and the filter reduces them somewhat. I take the flats with the filter on.


HTH, Wouter

[Niall Saunders]

If you are using a filter when acquiring Lights then it should remain in place, and be untouched before, during and after acquiring Flats.

If you are matching temperatures and exposure times between Lights and Darks, then you don't need to worry about LightBiases. The same argument applies for Flats and FlatDarks. In other words, the case can be made against any need for aquiring or use of any kind of Biases at all.

If your imager and capture software can save images that are RAW (i.e. they would need to be deBayered before any colour was visible, the RAW images being the actual 'shaes of grey' produced by the imaging chip itself, with no in-camera processing or white-balancing whatsoever) then it is those RAW images that you want to save and subsequently process. All of the RAW images that you save and process should have the same bit-depth.

Hope this helps.

[Bobinius]

Thanx guys. I'll try to shoot this weekend if the weather alows and test the settings.

Niall, I gues the bit difference explains the mono subtraction problem for the final integration image. I try to work with RAW fits. Good point about the bias frames.

Do you use the same gain when taking flats or the minimal/unitary gain when shooting with CCD cameras? I fail to see the point of shooting with minimal gain (advocated by some) since the spot/vignetting on the light frame are produced when shooting with another gain.

[wvanreeven]

I shoot bias, dark, flat and light all at the same gain and offset with my ASI1600MM.

[Niall Saunders]

Thanx guys. I'll try to shoot this weekend if the weather alows and test the settings.

Niall, I gues the bit difference explains the mono subtraction problem for the final integration image. I try to work with RAW fits. Good point about the bias frames.

Do you use the same gain when taking flats or the minimal/unitary gain when shooting with CCD cameras? I fail to see the point of shooting with minimal gain (advocated by some) since the spot/vignetting on the light frame are produced when shooting with another gain.

I use the gain and offset adjustment tutorial provided by Crak Stark. It has always worked for me, and I never change the settings frim one year's end to the next (I just work through the tutorial to confirm that all is well - which is the same reasun that I take Biases).

Similarly, I never change focus because my 3" fouser is rock-solid, I don't change imager and my OTA is carbon-fibre and is therefore more or less unaffected by any environmental conditions.

[Bobinius]

Ok, thanks!