G2V Calibration

[AstronomyUkraine]

There are a number of members using the G2V calibration method to determine the correct number of exposures/ length of exposures for RGB images. I have just tried G2V calibration on an image of M101, using eXcalibrator. Does this software give out reliable results? Also there seems to be two school of thoughts how to use the results. My results come out as follows:

Red 1.00

Green 1.17

Blue 1.07

 

With these results I could take 100 Red images, 117 Green images 107 Blue images to obtain the correct balance. These are 60 sec images I used to calibrate. So the other method to use these results is by increasing the exposure time of the Green and blue, resulting in 70.2 sec for Green, and 64.2 sec for Blue. Which is the best method, increased exposure time, or increased number of frames?

 

Do I need to calibrate each target, or can these results be used on any RGB target?

 

Brian

 

[MarkAR]

No idea about most of your queries but seeing as you're calibrating your filters these figures should hold for all RGB targets.

[paul]

From what I've read its more shots beats longer exposure for noise with a CMOS camera whereas CCD you need longer shots to overcome the readout noise. How complicated a dark frame library do you want grow to🙂?

 

All else being equal I would have thought increasing the number of shots rounded up to the nearest one would be a good strategy.

Edited April 25, 2021 by paul

[AstronomyUkraine]

11 minutes ago, MarkAR said:

No idea about most of your queries but seeing as you're calibrating your filters these figures should hold for all RGB targets.

That's my thoughts too. One G2V star is the same as the next one, I would have thought.

[AstronomyUkraine]

8 minutes ago, paul said:

From what I've read its more shots beats longer exposure for noise with a CMOS camera whereas CCD you need longer shots to overcome the readout noise. How complicated a dark frame library do you want grow to🙂?

 

All else being equal I would have thought increasing the number of shots rounded up to the nearest one would be a good strategy.

I was thinking about the dark library. I would need another hard drive for all the dark frames. 😁

[Jkulin]

Hi Brian, it was @peter shah who introduced me to G2V and we both work on changing the duration of the subs not increasing/decreasing  the number of subs.

 

The basic concept of G2V is that you capture the correct exposures that closely match or own visual perception, I would have thought that increasing/decreasing the number of subs will not have the same affect, but I remain to be corrected.

 

I can remember Peter banging on to me that you don't waste time capturing more of a channel than is needed.

 

I used it exclusively on my 10" RC and now I have figures for my 10" Newt then I will use the calculations with that.

 

I have still to do it with my other rigs, but currently they are running OSC.

 

Ideally you should run the calculation on a regular basis and ideally on each part of the sky you are capturing from, personally I just get the calcs and use them and haven't updated since the beginning.

[MarkAR]

16 hours ago, AstronomyUkraine said:

That's my thoughts too. One G2V star is the same as the next one, I would have thought.

You can always try a couple of other G2V stars, should have the same result.

[peter shah]

Yes G2V ratios will save you time....My point is if you alter your RGB weights you can then process each channel equally with a recorded curve either in Photoshop or Pixinsight. It makes colour balancing so much easier too.

 

The idea is that you will save several mins on lets say green and a little in blue (as on most chips are very green sensitive).  What is the point of capturing those extra precious mins when you are going to supress that signal when doing your non linear stretches during processing to get your colour balance. Those few mins can add up to hours.

I must stress that this only works on RGB and not Narrow band as NB you cannot treat the same as RGB as the object will dictate what exposure is needed in SII and OIII. In most H-alpha is the prominent signal and the SII and OIII will vary from object to object.  

To get a correct G2V weight you actually need to do measurements on stars that are in the part of the sky at that time you are imaging and to be honest that is a pain to do (that's only if you really want to be scientific about it) as sky conditions and varying atmospheric conditions will effect the calibration. So I just took measurements on a good night at the meridian and kept mine very general. It gets me close enough and still save me bags of time. 

 

To answer the last part of your question I've found there to be very little difference in the sub length to over all exposure length. The difficulty is if you alter your sub length you will need a separate set of calibration frames. Having said that I always altered my sub length only because I was never guaranteed several clear nights in a row...so if the good weather ended early then I would still have a balanced image if I fell short of what I was intended to get. 

Edited April 26, 2021 by peter shah