another collimation query

[mike.brightmore]

Although quite happy with the views I get through my Celestron, bright stars always appear fuzzy. I have never collimated the telescope because the out of focus star test doesn't appear to be lob-sided. I have taken images of the star test and an infocus shot of Vega (attached) using a webcam. In the star test I don't get a nice series of diffraction rings - only a few outer rings. Is this a collimation problem or something else?

[John]

Hi,

I think your Celestron needs a little adjustment to the collimation as the dark secondary shadow should be in the centre of the light "doughnut" in your top picture - it looks slightly off to the right. It's not far off but accurate collimation in SCT's is critical to getting the best performance out of them.

John

[Merlin66]

You could definately improve the collimation... needs a tweek!

[Brinders]

I have the C9.25, and yes I agree, your scope needs tweak - collimation is a little out.

I looked at Vega myself on Friday night and used it to adjust my collimation using a 8mm eyepiece. I am happy that collimation was spot on but Vega didn't seem as sharp or pin point as other dimmer stars. I noted when putting the scope away on Saturday morning that the corrector plate had picked up a little surface dirt (despite the dew-not heater being on max I got a little dew on the plate on Friday), whether this affected the view of Vega, I'm not sure.

Brinders

[Astronut]

Vega's a bit too bright to carry out a star test - try a second magnitude star. Also the defocused image in the top picture is defocused too much - all you're seeing there is the shadow of the secondary in the illuminated primary with no sign of diffraction rings. You only have to defocus slightly to see the diffraction rings and the diffraction ring disc will be alot smaller than the disc seen in that image. This is why you need a high power eyepiece.

Here's a good guide on star test collimation:

http://legault.club.fr/collim.html

sorry, I didn't read your post properly - you did state that the image was a defocused image of Vega (not a diffraction ring pattern).

[mike.brightmore]

You guys are great. I just wish the sky was clear so I could go out and do a star test properly, and tweak the collimation. Many thanks.