Re: Other possibility for oblong stars
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Paul yes i think we're on the same page
The only thing i'd add here is just to skip the whole pixel comparison and just look at the RMS in arcseconds. That will tell me exactly how much error there is and I can translate to the imaging camera
On Sun, Aug 8, 2021 at 3:50 PM Paul Kanevsky < yh@...> wrote: That¡¯s what I meant, Brian. 1/10 of a guider pixel is possible with a bright, well focused star, but is lower with lower SNR. When pixel size and image scale are such that 1/10 of a guider pixel is still a large error on the main imager, PHD error number will indicate good guiding while the main imager nay have elongated stars.
On Sun, Aug 8, 2021 at 06:34 PM, Brian Valente wrote:
>>>PHD can¡¯t see the error that¡¯s below the resolution of the guider, and so will happily report low error numbers.
I am not sure exactly what you mean by this, but PHD has sub-pixel accuracy. I've seen it guide down to 0.10 pixel and even lower
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I think the more relevant spec is the image scale of the guider setup vs. the imaging. The general rule of thumb is 4:1 max image scale, where the guide camera image scale should not be more than 4x coarser than the imaging camera's image scale.
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On Sun, Aug 8, 2021 at 3:28 PM Paul Kanevsky < yh@...> wrote:
Not odd at all. Guider has to be able to detect guide star deflection from center. If the resolution/image scale of the guider is low, it will detect and correct errors too late, after they have become?already?sufficiently large to be visible on the main imager. PHD can¡¯t see the error that¡¯s below the resolution of the guider, and so will happily report low error numbers.
On Sun, Aug 8, 2021 at 06:01 PM, wa1vta01452 wrote:
Oddly, PHD2 showed nominal numbers but the images would still be smeared. I first started guiding with my ASI1600mm using my old ST4 autoguider in the ST80 400mm guider and had similar streaky results on stars till I realized I was guiding 3.8 micron pixel size with approx. 13 x 15 micron ST4 pixels. That's when I purchased an ASI120mm for guiding and all was fine.
Tom
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Brian?
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