Calibration Frames for 130GTX

Nick Iversen

I have been taking long exposure dark frames with my 130GTX covered at each end to prevent light leakage. But I just realised that I haven't been covering the gap between the tube and the dew cap. That gap is almost airtight. It doesn't leak light does it? I will cover it from now on however.

Speaking of calibration I have seen it said that for flats you don't have to cool the camera because exposure is too short. That surely can't be right for CMOS can it? I always cool my camera for flats.

Chris White


I have found the seam between the tube and dew shield to be 100% effective at blocking light.   The dew shield threads onto the back of the lens cell so there is a couple of inches after those threads where it's a pretty tight fit as well.  I've taken 20 min darks in the daytime and do not register any more ADU than when I take darks in the dark.  So you are good and don't need to worry about this.  (I assume you are locking the dew shield in the extended position and using the dewshield end cap)

As far as flats go, flats are used to even out the light fall-off towards the corners as well as disruptions from dust motes, etc in your image train.  That said, flats should be bias calibrated.  I'm not sure if it truly matters or not to take flats while cooled, but the finicky nature of CMOS ADC as well as that some cameras do not behave in a linear fashion I think it's a good idea to take all calibration frames (darks, flats, bias) at the same temperature as your light frames.  It's also important to make sure you use the same gain and offset as well.  While the impact of mismatched settings may be more or less insignificant (depending on the type of frame) I'd hate to think I've taken such care with my guiding, my scope and everything else only to not calibrate optimally and have the data suffer, especially since there is really no additional work required to get all the settings the same between calibration and light frames.  (I could think of the scenario where the next day its too warm to cool enough, but in that case maybe cool less for your light frames.  There is very little thermal noise difference whether at 0C or -20C).

One more thing to think about, is that thermal noise is very low on the latest breed of CMOS cameras (like IMX571 or IMX455) and it might not make sense to bias calibrate your darks.  I'm experimenting with this right now.  The ADU is so similar between a 0s Bias and a 1200s dark that there is a tremendous aount of clipping when dark frames are calibrated with a Master Bias.  So instead of the traditional (Bias Calibrated Flats) + (Bias Calibrated Darks) + (Bias) to calibrate my Light frames, I'm starting to use (Bias Calibrated Flats) + (Uncalibrated Darks) to calibrated Light frames.

This may not apply to cameras that have excessive amp glow like the 294, or 183 chips.  In this case you might need to calibrate the Flats with a dark flat. 

Anyway, more info that you were asking about, but hopefully helpful. 

Nick Iversen

That's a pretty good reply Chris. I think "finicky" is the keyword here and I'm going to continue to cool when I take flats.

It doesn't make sense to me to bias calibrate darks (for CMOS). The formula is Calibrated image =  (L-D)/(F-DF) where L = light, D = dark, F = flat. Nowhere in there do you subtract bias from dark. It's always subtract dark from light. This formula wouldn't lead to clipping. Your formula has plus signs in it. I would suggest that for CMOS you don't use bias frames at all regardless of the quality of the camera. It's easy enough to take dark flats instead.

Chris White

My plus signs were not a formula, they represented the word "and".

With ccd you bias calibrate flats, darks and lights. With cmos if you bias calibrate darks it leads to clipping so they can be omitted. 

You must bias calibrate flats regardless. 

So the more simple version of calibration With cmos that behaves in a linear fashion: bias calibrate the flats, and then calibrate the lights with calibrated flats and uncalibrated darks. 

Sorry for the confusion.