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Guiding - How round is round?

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Got PHD working last night for the first time on NGC1931 and M36. I was so surprised when it worked I sat and watched the cross hairs for 25 minutes going 'Woah!' and 'Cool' every time it moved the mount. Mrs M37 was well pleased after over half an hour of that.

This is the stacked (and very quickly processed) product of 5 x five-minute subs through the old ST102, are the stars what would be considered round for this length of sub? They look OK ish to me but I don't really know what to compare them to. Also I've been staring at them for so long I can't tell any more. :)


full image:


zoom in the centre:


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TTBOMK the stars can never look round even in the highest of magnifications. They're just too far off to show as anything more than points of light. Any apparent 'roundness' must be caused by the same atmospheric fluctuations which make them appear to have points and also 'twinkle'. Since the processed pictures are a recording of an average of the photon arrivals at the receptors over a period of time this can only make them fuzzy in the short term and appear to be more 'solid' as the period of observation gets longer. Since this is something that the human eye cannot do we don't see this phenomenon directly, but only perceive it as the star moving around in the field of view. It's the same kind of thing that happens when looking at something through a heat haze where the air 'shimmers' distorting the shape of things on the other side of it. Not that we get that kind of weather here in Blighty very often, but I have experienced it on a very few occasions.

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Your guiding looks OK to me. Round stars don't prove good guiding, though. If the errors are fairly random they'll produce larger round stars than they should.

You have something else going on, though. The blue haloes, invitable in a fast achromat, are not centred on their progenitor stars. They are fan shaped with all the fans on the top side of the stars. This is probably tilt. The ST scopes don't have imaging quality focusers but getting everything tight might help. Watch the three screws that hold in the focuser to the tube as well. If not tilt it could easily be collimation. One trick is to stand the tube vertically on the floor and while away a quarter of an hour by tapping it with your fingers to settle the lens elements. No, I'm not crazy, I got this idea from Ed Ting!


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I second Olly's comments.

That looks spot on. So I wouldn't worry about it. If you connect to the Starklabs website. I think you can download a very good user manual for PHD and- Youtube has some nice videos.

PHD is a great piece of software isn't it  :smiley:

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Cheers guys, very helpful ideas.

Yup, PHD is amazing. Can't believe it's free! Must donate something.

Interesting about the violet halos. I upgraded the focuser (saving long-term for an APO) so I need to work on focus more and I will try that tapping thing. It sounds slightly made-up but if you say it's worth a go I will believe you ;) Didn't know refractors could require collimation. Think I will go and read up about this.

thanks all for taking the time to respond .


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