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Old 05-08-2014, 08:55 PM
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Shiraz (Ray)
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Join Date: Apr 2010
Location: ardrossan south australia
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initial test of an SX AO at f4 - calm conditions - windy conditions added

Peter (Poita) very generously let me use his SX AO unit to see if it would work OK for hi res imaging with an f4 Newtonian imager.

Initial testing, designed to study the effects of AO with minimal mount errors, yielded the following:
1. There is slight chromatic aberration from the 13mm corrector plate at f4, but it is not enough to worry about in normal imaging conditions.
2. When active, the AO generally reduced the peak brightness of stars (by up to about 30%) and increased their sizes (by up to about 25% in FWHM) when compared to results with the AO off. This was on a good mount (EQ8) in windless conditions and with average seeing of about 2.5 arc sec FWHM (see attached image for typical un-stretched results)

Discussion:
- An AO would be expected to increase star sizes by a factor of up to 1.4 outside of the isokinetic patch (eg with off axis guide camera). The AO did better than this, so it was presumably also correcting some mount error. However, the AO still degraded the imagery by a noticeable amount.
- The difference between AO and non-AO imagery is detectable at the sampling of 0.93 arcsec/pixel, but with the data resampled to 1.86 arcsec, there is no discernable difference with/without AO.

Opinion:
1. On a good mount under reasonably good seeing, with no wind and with near-Nyquist sampling, there was a perceptible overall drop in resolution, so AO does not appear to be beneficial under these circumstances.
2. However, if seeing is really high quality, it is possible that the beneficial effect of the AO in correcting mount errors may outweigh the loss in resolution. The 2.5 arcsec seeing was nowhere near good enough for this possibility to be tested, so this is conjecture.
3. If the mount is of lower quality and/or the wind is significant, the mount correction by an AO may provide a nett benefit in many other seeing conditions. This is also conjecture, but others have reported that this is the case.
4. If the imaging is undersampled (eg ~2 arcsec is widely used), the effect of an AO may only be noticeable if it corrects significant mount errors.

The next stage will be to test the system on an EQ6 and under windy conditions. For now though, the answers to two of the basic questions would seem to be:
"can an SX AO be used for hi res imaging at f4 ?" - yes
“does an AO increase resolution in average seeing ?” - probably not, likely depends on wind and mount quality, but you may lose resolution in calm conditions

Thanks for reading. Would be very interested to know if these results tally with the experience of others. And once again, thank you Peter for letting me use the AO. Regards ray

Test configuration: SX AO with an off axis Lodestar guide camera at 2x2 binning and 0.1 sec/frame under PHD2. 200f4 Newtonian scope, on an EQ8, with no coma corrector and with a QHY5 L2 as the imaging camera (at 10 second frames to minimise the mount error while still allowing the effects of AO to be assessed in isolation). Imaging sampling was 0.93 arcsec. Test sequences were 10 frames with AO active, followed immediately by 10 frames with it off.
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Last edited by Shiraz; 13-08-2014 at 06:04 PM.
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