Quote:
Originally Posted by gregbradley
Everything is magnified including the tracking errors.
Greg.
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thanks Greg. I understood that OK, but did not quite see how magnifying the distorted stars would make for a worse outcome - they would still be the same shape. As pointed out in other posts, its a perception issue - the stars are no different in shape for given tracking error, regardless of fl, but if they are bigger you notice distortion more.
Quote:
Originally Posted by alistairsam
Hi,
Is there a sweet spot or optimum range for image scales in a guiding system that provide the best autoguiding?
eg. many use the finder guider where at a 200mm FL and a camera like the qhy5, the image scale is 5.4 odd arcsec/pixel.
would it be beneficial to use a longer FL guidescope with the same camera and if so are there any guidelines or numbers that help determine the optimum guidescope FL?
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Craig Stark (phd guiding) suggests that his software is capable of at least 1/5 pixel resolution and much finer for high SNR. On that basis, if the angular pixel scale of the guider is less than about 2.5x that of the imager, you should be OK.
http://www.stark-labs.com/help/blog/...-ask-craig.php (about the fourth article).
I use a QHY5 at 200mm (about 5.5 arc sec/pixel) to guide a QHY8 on an 800mm fl scope (about 2 arc sec/pixel). This is a track/image scale ratio of about 2.7 and it works fairly well if the seeing is OK. If any use to you, attached is a 10% crop cut from a lagoon image at native resolution - the tracking is definitely not perfect, but it is as good as I would expect from a 12kg package on an EQ6 - sub pixel tracking works pretty well. Have tried up to a 900mm fl guidescope - no benefit over 200mm and hard to find guide stars.
You indirectly make the point that we should be talking in terms of pixel scale, not focal length, when dealing with these issues - strongly agree.
regards Ray