Darks affecting stacking

[Oberon510]

Hi,

I have built a library of darks for my ASI colour camera but I found using them has a huge affect on the histogram graph in live stacking. Using the same Gain, Exposure, Brightness the histogram graph of the colour bands will appear 30% along the X axis but with the dark enabled the graph will come out of middle of the Y axis (e.g no peak curve) (Log box enabled in histogram).

The darks don't show any light (amp glow etc) so I am only worried about managing any pixel issues.

I have increased the exposure times to improve the stack when using darks but wondered if there is anything else I should be doing to mitigate/improve this.

Thanks,
Ken

[admin]

Hi Ken,

the best thing to do is to check the brightness levels of the dark and light frames - if they are in FITS format then you can load them into 'FitsLiberator' to show the average pixel value of the images. Alternatively you could load them into SharpCap's 'Deep Sky Test Camera' and look at the histogram which shows the average pixel value.

You would normally expect the average pixel value of the dark frame to be lower than that for the raw light frames - the darks contain the bias level and thermal noise, but the lights contain the bias level, thermal noise *and* sky glow. It sounds like that's not the case for your darks/lights.

Things to check out that could affect the level (and should be the same between darks and lights) are

* gain
* exposure
* brightness (black level/offset on some cameras)
* white balance control settings

You should have saved 'capturesettings' files from both the darks and the lights that you can re-check to see if these settings were accidentally changed between the two.

cheers,

Robin

[Oberon510]

Thanks Robin,

Exp, Gain and Offset are the same for the Darks and Lights but I double checked the brightness/offset on the histogram and it may be the cause. I usually have a brightness of 15 but it was at 5 and that didn't bring the peak of the histogram away from the left hand side.

I went back and revisited the Offset setting and most recommendations are for 50 (ASIAir default) but 25 seems to be good based on the SharpCap histogram method. Is there any issue with using 50 over 25?

Thanks,
Ken

[Oberon510]

One other thing is the histogram brain recommendation for exposure, gain, offset - it recommended 10 as the Brightness offset. Is that a more accurate assessment than eyeballing the histogram peak movement from the left side?

Thanks,
Ken

[Gfischer]

Ken,
I live in the desert near Phoenix and I really have to change darks a lot because of temperature changes. However I’m not using a cooled camera.

Just an idea

[admin]

Hi,

generally there are only two real problems with a wrong brightness/offset level

1) Different values between lights and darks
2) Too low so the histogram touches the left hand side

Setting it a bit too high so that there is extra space between the histogram and the left hand side (ie 50 instead of 25) is a really minor thing - you lose a tiny bit of full well depth, but the practical significance is virtually zero unless you leave a *massive* gap to the left of the histogram peak.

cheers,

Robin

[Oberon510]

Thanks Robin and Gfischer

The journey continues

[Gfischer]

Ken,
The fun is in the journey