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Old 05-12-2011, 02:14 PM
stevous67 (Steve M)
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Join Date: Jul 2008
Location: Melbourne, Australia
Posts: 633
Quote:
Originally Posted by troypiggo View Post
I have some questions on your method, Steve. I'd like to try it out. I'm trying to refine by collimation and need all the help I can get.

When you mention the central stars should be perfect donuts, but the outer ones are elongated, shouldn't they all be donuts if you have a coma corrector?

When you say that upon refocusing, anything else unbalanced means there's a problem elsewhere, any tips on how to detect where to fix the problem?
Hi Troy,

Even with the CDK, which has a forgiving flat field, when you examine stars defocused, those in the outer of the FOV (especially with the STL) are not round and balanced in shape. This is what makes this alternative easy to define visually where collimtion is, as the pattern should be that central defocused stars are nice, round and balanced, whilst those in the outer reaches of the FOV will be elongated, the fat side (if i remeber correctly) pointing inwards evenly to the centre of the FOV. I am so confident in collimating my scope now that with the full moon approaching, I'll put my CDK badly out of collimation and take some images to define the views, from uncollimated to collimated.

I'll place these on the MPAS website as a reference for an alternative approach. Brendon is correct that collimation approaches vary between types of scopes, and the GSO RC's add an extra element of trickiness to the equation, as the focuser moves with the primary mirror. I also note my Rc10 had the incorrect mirror spacing which greatly effected it's flat field. The GSO factory finally told me the correct spacing to be 493mm, but I worked it out to 495mm using Ccdware's CCDInspector, but that is another story...

I can add too that the Tak collimating scope can help you get close to collimation, but it will be only a little more effective than the Cheshire.

Just out if interest, those flairs you mentioned are mostly associated with taking images of bright stars? If so, be careful not to over concentrate on these results. Once you get collimation as best as you can, take an image of a reasonably bright deep sky object, then have a look at it. Even when I have the CDK well collimated, with CCDInspector saying zero tilt, I still get offset reflections about super bright stars, eg: Canopus.

So something is obviously causing me this issue, but I'm not going to research it, as it is not noticeable in normal imaging of DSO's.

If you like, I'll send you a link to this alternate guide once I finish it? I also recommend making a graphic guide you can refer back to that assists you, or reminds you, which collimation screws you need to move to make corrections to your secondary, including which way a defocused star needs to be moved to correct it's shape.

BR

Steve
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