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Focusing problems

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Hi all,

I've purchased a Skywatcher ST80 as a guidescope, and a QHY5 guidecam. All controled by PHD

I'm having problems with the focusing to get fine stars on the laptop screen.

When I turn the scope towards a bright star to focus, the smallest image it produces is about the size of a half pence peace on the screen, with difraction spikes.

(I've unsrewed the end of the barlow that came with the ST80, and i'm using that inbetween the ST80 and the Guidecam)

Looking at dimmer stars still only give small pixes, not a sharp image.

Is this correct, or am I expesting too much?

I was hopeing to see a laptop screen of the stars in pefect focus.

I should have taken a screenshot,but is was late! :)

Your help as allways is apreciated

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When I used an ST80 I needed an extension tube (~100mm ?) to get focus on my guidecam. Try focussing without the barlow, or with the complete barlow (or with a diagonal if you have one). You can use an eyepiece to judge approximately where the focal plane is (the focal plane will be approximately where the aperture stop hole is in the eyepiece, when you can see a focussed object in the eyepiece). I actually held my guidecam in 'air' watching the screen to see where the approx focus was. Not sure why you get diffraction spikes in a refractor.

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On the ST80 the distance between from last part of the main scope (the point at which the draw tube emerges) to the chip needs to about 150mm. I use the Barlow tube with my Atik 16ic guide cameras to get this distance. I can't be more precise because I don't know exactly where, in the camera, the chip lies.


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I use the barlow tube method with the QHY5 & ST80 and have to withdraw the tube slightly (3/4"?) to achieve focus. This still gives a degree of freedom in the focusing mechanism.


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