Two OAGs on a dual rig?

[tomato]

I bet this has been tried before but I was wondering could you get around the drawbacks of using an OAG on a dual rig by having one set up on each scope and swapping the guiding over when you need to refocus? There are problems with doing this as your master/slave scope designations would keep changing, but that wouldn’t be a big deal if you stay on the same target for the entire session which is now the norm for me.

Obviously all differential flexture between the scopes would need to be eliminated.

I know a separate guide scope is a far simpler set up but I cannot achieve the same guiding performance with a separate scope compared to the OAG.

[ollypenrice]

How would you time the takeover and how long would it take for the guiding to settle? It sounds like a bit of a nightmare to me. Could you not synchronize the captures by having a reasonably long 'down time' between filter changes? You'd have to use the same sub length on both scopes but that wouldn't be the end of the world, would it? Then you could stick with a single OAG.

Olly

[Laurin Dave]

Hi Tomato

What problem are you trying to solve?  Are your slave subs bad when the master scope focuses?    if so I'd suspect your polar alignment or maybe the RA rate on the Mesu is slightly out   … my system is Esprit 150 SX46 with piggybacked Esprit 100  ASI1600..  I run an OAG on the Esprit 150..  when that focusses I lose guiding....  but the Mesu is so good (4" peak to peak over about 30 minutes) that over 120 seconds you just don't notice on the Esprit 100 subs (with your longer focal length you may need to use 60seconds) ..  I dither and lose those but its maybe 10% of the frames at most..  last night I reduced the dither scale and lost only 10 out of 200 !

I know this isn't the answer to your  question but based on my experience I think you'd make you're life much simpler by investing in an ASI1600mm (same size sensor as a 683) or similar and running colour on that at 60 or 120sec exposure and put one of your 683 chip cameras with an OAG on the other scope and run luminance though it at 10 or 20 minutes .. 

You'll also get back that lovely fov and at 1" pp you won't lose any real resolution ..  you could also bin the ASI1600 2x2 ..  and scope alignment will be less demanding..  with the ASI1600 bright stars can be a bit messy..  Why not experiment by replacing one of your 178s with a 683/OAG as the master and running short binned subs on the 178. 

And what a week of imaging … 7 out of the last 8 nights clear all night..  its been like being at your place Olly

Dave

Edited March 28, 2020 by Laurin Dave

[tomato]

Thanks for the replies guys, excellent  food for thought.

There are some sizeable deviations when the auto focuser routine runs on the OAG scope as you’d expect, and at my current imaging scale I like to refocus quite frequently. It’s not a big deal in the grand scheme of things, but the second OAG set up is available, so I might try it just as an experiment.

I do have access to a 1600mm courtesy of @Tomatobro, so it could be time for a camera change. I don’t think him bringing it over complies with the essential travel restrictions right now though...

Yes, a record number of consecutive clear nights for me, even though a couple were below par due to thin high cloud (can’t be con trails), my M101 project is now at 15 hrs+.☺️

[Laurin Dave]

How far is the guide star drifting, and how long does the autofocus./re-start guiding take? Also how often do you autofocus and are you using updated SGPro with quadratic fit? I do it every 1 deg C..  I've just checked Friday nights autofocus runs and the temperature went from 8 to 4 deg C  over the first 4 hours then stayed there...  looking at the graphs SGPro outputs the focus movement was all within the flat bit of the curve ( ie the three bottom points) from which I conclude that refocussing made very little difference..  worth looking at yours I'd suggest 

As for the ASI1600 perhaps Tomatobro could send it over.. 

[tomato]

I do need to sort out the temperature compensation on the auto focus. I’m currently refocusing between filter changes on the slave scope (every hour) and doing a corresponding refocus on the Lum. The whole operation takes about 5 mins for both scopes, if it works as it should.

I have started to log the shifts in focus but don’t have too much data, but the Lum scope definitely drifted more in the final hour on the last session, which is a bit counter intuitive. The scopes were pointing straight up, but there is not much potential for the focuser to wind out under gravity, the camera load is not that excessive