Useful Number of Dark Frames for Integration

[Farzad_k]

Hello.

I have heard the more calibration data we integrate the more effective the generated master calibration file will be.

I have experimented with 50 and 100 dark calibration frames and the resulting statistic data of the masters aren't different by a lot. Is the small difference in min and max important enough to take the time and create twice as many masters?

Thanks

Farzad

[TriplaneDave]

Looking at Warren's book- he suggested 16-25 darks.  There isn't much further explanation of why that number (other than to have 2x the number of lights), but let me offer this.

When doing image integration to create your master dark, the master dark represents the noise (plus bias) at your chosen exposure and temperature.  Hot/cold/stuck pixels are going to show up here.  They're not likely to change much at all over the course of 25-100 frames.  You'll probably catch them just fine.  The noise however, you'll want to model that as accurately as possible so you can remove that from your light masters.  So, rather than looking at the range, you might try doing an ImageIntegration with different threshholds and see what you get for pixel rejections.  Take a look here: http://www.astrosurf.com/jordigallego/articles.html and go to the section: "Image integration techniques: Increasing SNR and outlier rejection with PixInsight"  - There is a good PPTX walkthrough of ImageIntegration techniques that let you tease out what's rejected. 

Bottom line, if you find that you get the same rejected pixels from 50 as you do 100 frames, there's no value in doing 100 frames.

[Farzad_k]

Thanks

[sharpie78]

I have a Library of Dark Masters ranging from 30s-600s. Each master is 5hrs worth of Darks..... 600x30s, 300x60s, 150x120s etc.
I created them like this as I aim for as many light frames as is practical, usually 2-3hrs. I then have a MasterDark for whichever exposure time I'm using which depends on sky conditions, Lunar cycle etc. Each of these MasterDarks is obviously then comprised of roughly double the amount of Darks as I have lights.
It works for me and produces pretty good results imho.

[Farzad_k]

I agree that we need to get as many Lights as possible because if nothing else, we can be more picky when it comes to selecting frames that are the best. The return on investment dwindles, I believe, at 20 or so - according to legends.

Farzad

[sharpie78]

I spent ages experimenting when I used my old DSLR but since getting a dedicated cmos astro camera I've just plumped for old "more data is better" way of thinking. I'll probably do some more research into Dark frame characteristics for my camera in the summer when the nights are too short and light for really effective data capture.