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Sub-frame rejection rate, exposure length and ISO

20 May, 2020 03:09
I only starting this amazing hobby one lunation ago - and joined this forum at the same time.  I am pretty pleased with my photos to date, although very much in awe of what many are able to do on this site.  In that vein, I hope the answers to these technical questions will help me improve.

My photos have primarily taken with Canon 6D MkII, 200mm lens at f/4 on Sky Adventurer mount 1hour total exposures with 45-60sec subs at 3200ISO.  15xdark & 15xflat  every night .

1) I am rejecting about 20-30% of my lights due to poor tracking (elliptical images, double images, random walk images).  Is this normal?  I have tried to track down possible sources (wooden deck v concrete base), non-snagging leads from intervalometer, anti-dewer, tightening clutches etc.  But with no appreciable luck.

2) My tracking appears to be otherwise good, successive 1min exposures can show minimal sub-pixel shifts when reduced with Pix insight, but then there is the inevitable 10-20pix jump. [Note: PixInsight does give me a typically best value of 0.7 for my ellipticity, but the stars look round to me - I wonder if that reflects the slight coma towards the sensor edge]

3) With these jumps I am reluctant to go too much longer, in case all frames are rejected.  I am running at 3200 ISO, which gives me also 7stops of DR.  I worry that this too much for the Canon 6k II - although it is supposed to have good performance at high ISO.  I also wonder if this is why some of my targets (e.g. Rosetta, Eta Car and Trafid look a little washed out in colour.  Although it could be because I have done anything after applying the standard STF colour curves to my background and colour corrected images in PixInsight.

Many thanks in advance to anyone who might be able to provide me some pointers for my next lunation.

20 May, 2020 11:07
I admittedly do not have too much more experience than you (coming up on 1 year of this). I do have a Star Adventurer (SA) mount, but have only used it for very wide angle, short focal length (<50mm) shots which are quite forgiving with regard to tracking accuracy. I have the impression that guiding is likely important for using the SA at longer focal lengths. Marc Agostini is the Astrobin user I've found so far who seems to have pushed the SA mount to its limit and had (in my opinion) great success.

Is the tracking jump issue possibly a periodic-error (PE) of the mount? Roughly occurring at some consistent interval(s)?

And may I assume you are using the large bracket with the counterweight to get the weight of the camera/lens balanced (rather than the simpler ball-head attachment)?

I can suggest in general that you might consider trying eventually a small guide scope on the SA, especially if you want to press for longer exposure times and/or longer focal lengths. There are a couple 30mm aperture guide scopes that are well sized for the SA mount - I have one on order right now to try to make my SA into an eventual travel rig  (hoping it arrives next week). Obviously guiding does require using a computer and so complicates setting up somewhat, but I have found at least personally that PHD2 "just works" with my equatorial mount and I am assuming it will do the same for the SA in RA tracking. PHD2 does have a lot of diagnostic features too and could also help you understand the intermittent problem(s) better.

Your images are looking great by the way. I just looked through them and am very impressed. I'm always fascinated by the LMC and SMC and hope to get to see them someday.
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