I recently purchased a Starlight Xpress AO-LF active optics unit and thought it would be interesting to see how the results of using it compared to auto-guiding. So, I took advantage of clear(ish) brightly moonlit skies last night to do an experiment...
Equipment:
- Mount: Astro-Physics AP900
- Scope: GSO RC10, FL 1984mm (according to Pinpoint plate solve)
- Imaging train: Atlas focuser, SX AO-LF, SX Filterwheel, OAG/Lodestar, SXVR-H18 camera (0.56 arcsec/pixel image scale)
The scope was in an exposed position and there were strong gusty winds, so I was unable to make long guided exposures without wind gusts ruining every one of the subs. I ended up using 60 second exposures.
The experiment: I ran a series of 20 auto-guided and 20 AO-guided Luminance subs, each 60 seconds long. I actually interleaved groups of 5 subs alternately auto-guiding and AO-guiding to try to keep conditions approximately the same for both in case variations in temperature or seeing affected the results. Auto-guiding was done using 5 seconds subs and AO-guiding was done at a 4Hz rate (approximately).
The results:
- the big difference was how well wind gusts were handled. Of the 20 auto-guided subs, 4 were badly ruined by wind. Only one of the 20 AO-guided subs was noticeably affected (in that case the wind gust was strong enough to knock the guide star out of the tracking box completely).
- AO-guiding produced consistently rounder stars. I threw out the 4 bad auto-guiding subs and the 1 bad AO-guiding sub and then measured the remaining ones with CCDInspector. The average aspect ratio for the auto-guided subs was 13% (standard deviation of ~4.6) compared to 6.5% (standard deviation ~1.72) for the AO-guided subs (lower percentages are better). After integrating the auto-guided and AO-guided subs separately I also did a DynamicPSF analysis in PI which gave a roundness of 0.926 for the auto-guided image and 0.952 for the AO-guided image.
- Auto-guiding produced stars with a FWHM a few percent smaller than AO-guiding but the variation in FWHM between subs was bigger with auto-guiding. Once again considering the good quality subs only, CCDInspector gave an average FWHM of 2.22 arcsec/pixel (standard deviation ~0.115) for auto-guiding compared to 2.34 (standard deviation ~0.084) for AO-guiding. The FWHM for the integrated images was 2.29 for auto-guiding and 2.37 for AO-guiding which is a little closer.
- visual inspection of the integrated subs showed that the AO-guided image contains some very dim stars that the auto-guided image does not, but the effect was very subtle.
I don't understand why auto-guiding produced better FWHMs than AO-guiding. It certainly wasn't what I expected. I will probably do some further experiments on a still night when I can make much longer guided subs to see if I get the same result.
I was using a very good mount and had good polar alignment. I expect that AO-guiding would show greater benefits on a poorer mount!
I did cheat a little bit and didn't remove the AO unit from the imaging train to do the auto-guided subs - it was still present but not operational. The extra optical element may have had some effect on the auto-guided subs, but I don't think it would have been significant.
Overall, I am very pleased with the performance of the AO unit. Last night after running these tests I was able to take 15 minute Ha subs despite the wind. This would have been completely impossible with auto-guiding. If only the AO unit was also able to deal with the clouds that appeared around 11pm...
Cheers,
Rick.