[Eden (Brett)]

ONAG with AO-- The ultimate combination?

Finally, with the help of OpenPHD, I added the SXV-AO-LF to the ONAG and put the two to the test.

It should be made clear: depending on what you're expecting to get from your AO, this combination may be better suited for larger aperture telescopes given that longer exposure times are sometimes needed in the near-infrared and shorter exposure times are often preferred for AO operation. Larger aperture telescopes have the luxury of being less discriminant when it comes to guide star availability and will obviously be able to extract a higher SNR from fainter M-Class targets at shorter exposure times.

Attempting to run the SXV-AO on my system at a rate of 5-10Hz required a reasonably bright guide star and in the tests I have conducted so far, even with the improved guide star stability in the near-infrared, resulting image quality was degraded because I was "chasing the seeing". Increasing the AO update interval (and ultimately the guide camera exposure time) to 500ms and then to 1s and 2s improved image quality and increased the range of available guide stars with suitably high SNR.

I eventually settled on 2s exposure time and later tested on another target using 3s and 4s. In spite of the AO making less frequent corrections, the adjustments made by the tip/tilt element as opposed to physically moving the mount resulted in slightly better image quality through tighter stars, as would be expected from an AO system.

In the interest of factual integrity, I have opted not to post "ONAG vs ONAG with AO" image comparisons in this review, as I feel that more testing is required, proper FWHM analysis is needed and other factors including transient changes in seeing needed to be ruled out before being able to accurately gauge an improvement over ONAG guiding alone, if any.

Anyone who is interested in seeing preliminary results can send me a PM and I will forward the comparison images to you.

Considerations


-- Because of insufficient inward focuser travel on my system, I am not able to use a focal reducer in conjunction with the ONAG and certainly not with the ONAG and SXV-AO-LF. Although my OTA could be modified to support a focal reducer, on most telescopes this would not be a problem.

-- Without modification, some Newtonian telescopes lack the sufficient backfocus required for the ONAG to operate.

-- Due to the increased load (~800 grams) on the imaging train, a good quality aftermarket focuser may be required and is definitely recommended on low and mid-ranged telescopes. A motorized focuser is obviously required to take advantage of SharpLock auto-focus.

-- Because of the orientation of the imaging camera (vertical, as opposed to the typically horizontal), effective cable management is important to ensure that the telescope can slew without cables getting tangled up or caught on any equipment.

-- ONAG requires 66mm backfocus to the imaging camera and 90mm backfocus to the guiding camera. The ONAG XT requires 68mm and 92mm respectively.

-- The ONAG XT is the better value investment; I purchased the standard ONAG and found that my CCD sensor (28.4mm diagonal effective area) only just fits into the illumination area. This is only a minor inconvenience but worth noting if your APS-C sized sensor is approaching the ceiling of this format size.


Conclusion

Would I recommend the ONAG? Absolutely. The benefits of near-infrared guiding are not simply theoretical, you can see them right there in your guide camera images and in the results that you take home at dawn. Compatible with the majority of systems out there, the reasons in favor of switching to ONAG far outweigh those against it.

I'm glad to be rid of my guide scope!

Acknowledgements

Dr. Gaston Baudat from Innovations Foresight for spending a great deal of time answering my many questions and for kindly assessing my results. His dedication, service and diligence is second to none.

Terry and the team at Starlight Xpress for their work on the SXV-AO, their friendly service and for promptly supplying me with the parts that were missing from my SXV-AO kit at purchase.

Theo from Gama Electronics for his fast turnaround on supplying the QHY IMG0H and for assisting me with the QHY12.

Everyone at IIS for making it such a great resource and a fantastic community to be a part of!

Clear Skies!
Brett