Tonight I had a go at star testing my XT10i telescope to check the collimation and optics.
There was a full moon tonight, transparency was not particularly good and only the brighter stars were properly visible, so there wasn’t much point in trying to see much else, and instead I figured better use of my time would be familiarising myself with how to do a star test and see what it looks like. I have tried in the past but think I just didn’t understand what I was supposed to be looking for. This was prompted by an article in November issue of Sky at Night, which included some good photos of what to expect.
I also wanted to see if I could get it captured on webcam to show on my website some footage of what is seen while focusing inside and outside of focus. Unfortunately the view was not bright enough to show clearly what I was seeing, so I gave up on that.
The S@N article suggests that the eyepiece should give a magnification of 1.6x the aperture of your telescope. For my XT10i this is 254mm (10 inches) x 1.6 = 406x. So divide the focal length of my XT10i which is 1200mm by 406 = 2.95mm eyepiece. I don’t have an eyepiece this small, so I needed to use my 2xBarlow lens to help double the magnification of my Hyperion eyepiece. This Hyperion EP has fine tuning rings which increase the mag, and with both added they increase the 13mm to be 8.1mm, then with the Barlow it effectively becomes 4.05mm (296x), which is the closest I can get to 2.95mm.
With a dobsonian mount, and no motorised tracking, you must select a star which won’t move much, and the obvious choice is Polaris in Canis Minor. However, I found that with this much magnification the pole star, which was already quite faint tonight, was made even dimmer, and it helps to have a fairly bright star when star testing. Also the high magnification made lining up on the star a bit troublesome.
After a while, as an experiment, I replaced the Hyperion with my 10mm Plossl + the Barlow and found that this gave a perfectly adequate view of the star when defocused. It was a bit brighter, and easier to centre in the view too.
Tube air currents were a right pain in the ass, making the unfocused star image shift constantly, and so at first making it very difficult to see the star test properly. So I got a desktop fan and set that up at the base of the telescope tube blowing the cold night air into the bottom of the tube in an effort to cool the tube better (although I do have a small computer fan mounted to the underside of the primary mirror cell, this cools the mirror, but doesn’t move air in the tube very well). The faster air current helped break up and move the slowly shifting thermal distortions, and also cooled the primary mirror much faster.
After a while the tube currents were broken up sufficiently, and the mirror must have cooled a good deal more, because I started to identify the concentric rings that are the key indicator used in a star test for the state of the optics.
After comparing the concentric rings both inside and outside focus, I was pleased to discover the collimation of my scope was pretty well spot on. I found the trick to being able to see the concentric rings was not to defocus too far, and to try to ignore and “see through” any thermal distortions which cause “bleeding”, a kind of flickering flame effect that spreads out from the central part of the defocused star image. I found that the rings were quite hard to see at first, but I think this may have been due to Polaris being so dim tonight.
Maybe better results next time, but at least I know now what constitutes a star test. Anyway I finished off by spending some time viewing the Moon.
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