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Old 20-04-2016, 10:13 AM
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Shiraz (Ray)
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Join Date: Apr 2010
Location: ardrossan south australia
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Quote:
Originally Posted by alocky View Post
Hi Ray, it's good to challenge convention and especially in such a scientific manner. Do you get the same trend if you plot the ellipticity of the stars rather than fwhm? I'd be curious if the poorer results coincided with an elongation in RA.
Cheers
Andrew.
Hi Andrew. yes, there is elongation in RA - easiest to see in the attached image comparison where RA is aligned roughly diagonal from bottom left to top right.


Quote:
Originally Posted by gregbradley View Post
Interesting results Ray and as usual you have done a thought provoking post.

Did you know PDH2 has a guide assistant that actually measures the seeing and PE and recommends the settings from that? Its quite good.

I had some difficulty understanding how the assistant works, so have not used it.

I think its highly mount dependent

FWHM may also not be the best measure as seeing fluctuations could account for some of the results although there is no arguing there is a definite trend there in your results. PI has a measure of eccentricity of stars you could use if you have PI. If not you can download a trial of PI.
At the end of the day round stars is what you are after so it would be the better measure. Still affected by seeing though. Getting round stars at 10minute exposures has always been the starting point for any astrophotography. Its quite a hard target no matter how good your gear is!

my criteria for good guiding is for round and small stars - I can easily get just round stars by defocusing a little or in post processing, but the only way to get finest detail is with good guiding. FWHM is directly affected by seeing, so I ran the experiment to make sure that seeing was very close to the same for each set of comparison images.

Another possibility is that using a guide scope differential flexure may also favour shorter exposures as that occurs usually over longer time periods. It may not and its just a thought.

The only exposures that were changed were the guide exposures. The light exposures were all the same at 2 minutes - my system has no measurable flexure up to about 10 minutes.

Not really on topic but probably interesting to a lot is I measured the PE of both my AP1600 and PME recently using the same setup. The AP1600 was considerably better at around 1.5 arc secs error often less. The best I saw was .6 arc secs and .8 is common. The PME was more like 3.5. Both give round stars when properly setup though. The PME has been a reliable performer for me so I was surprised the AP guide graph was so much smoother and less peaky than the PME's (about a 10 year old PME which may have better PE than modern versions which state higher PE in their specs). This is with PEC turned on. Home position on the PME is a hell of a useful feature though.

interesting results. My criterion for mount quality is that it must be just good enough to do the job. Clearly your two mounts are at least that good.

Another point I would make is the guide star selected.

The guide stars for this test were chosen to be in the centre of the frame and the SNR was about 10 for all tests

Greg.
responses in italics Greg.

regards Ray
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