Active Optics and Encoder Guiding

[DaveS]

I may have asked this before, but if I did I've forgotten about it.

My DDM mount doesn't need guiding, it does it all with encoders and sky models, but I was wondering if there's any mileage in using an AO module to take out atmospheric disturbance? OK, I'm not expecting professional observatory levels of correction, but if it can get " OK seeing" down to "good seeing", or even "poor seeing" down to "OK seeing" that might be a bonus.

To be clear, there would be no connection between the AO unit and the mount, just a loop between the guide camera an the AO module.

[Tim]

Have a word with @MartinB Dave, he has used an AO unit. From what I can recall, you need an exceptionally bright star near to your target, that you can take very short exposures of to try and cycle the seeing out of the equation.

Personally, I think you'll achieve better, more consistent results with extra sub frames with critical rejection settings and deconvolution, and using your better FWHM frames as the reference, most of the seeing effects on poorer evening should be dealt with by the rejection routines. AO units have always seemed like just something else to go wrong and take up valuable sky time trying to fix!

Out of interest, does the DDM mount make allowance for atmospheric refraction too?

Cheers

Tim

[DaveS]

Thanks @Tim

That may be a better way, and I was thinking about the reference star and how short the exposures would have to be. Would probably need a Lodestar2 rather than my old QHY5-II.

The ASA mounts do two sky models. The first is a big whole-sky affair for GOTO and first-order errors, such as cone and atmospheric refraction, the second is a smaller model, only 5-10 points along the path of the imaging run that gets rid of any residual errors, getting down to the order of 0.1 sec RMS. The second model usually takes less time than PHD takes to do a calibration.

[MartinB]

Dave, as Tim says,  I have used an SX AO unit a fair bit and my experience is that it tightens stars and also enhances resolution.  This is when used with a 10" Meade ACF typically in conjunction with a focal reducer and working at a focal length of around 1800mm.  As a minimum I have found that you really need 4 guide corrections  and 10 corrections per second is better still.   With my lodestar 1 this requires a mag 8 star or brighter.  You set it up pretty much like any OAG but with additonal cables to the computer and, in my case, the mount.  It helps to plan in advance.  I use Sky 6 and have an FOV indicator showing the position of both chips which helps me work out the framing.  I use Maxim but presume there are drivers available for other capture software.  Other than the extra layer of complexity the main problem I have encountered is a bit of tilt as a result of the long image train.  I do think it is effective system even in good seeing but there are a lot of targets without a suitable guide star.

[DaveS]

Thanks Martin

Something more to think about.

What set me on this investigation was the 175mm f/8 LZOS on ABS at 1400mm fl. Using my Trius 694 puts the plate scale near the limit for OK seeing, and comparable with the Daws Limit for the OG. Using my ASI1600 gets me well under.

Or I could just save my money!

[Tim]

2 hours ago, DaveS said:

Using my Trius 694 puts the plate scale near the limit for OK seeing...

Theoretical limit   (That should keep the thread going )

I have found that longer focal lengths, with longer exposures, tend to naturally deal with the seeing, compared to faster wider setups which record every flare, skip, and wobble.

Waiting until the target is high overhead makes a difference too.

[DaveS]

Thanks again Tim.

If I do buy this 'scope(Still undecided) I'll start with only the cam, and think about the AO if the stars need it.