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My First Star Test (1 Viewer)

Will K

Too well-known member
United Kingdom
Hi all,

I tried my first star test yesterday, following some of the comments made by Henry Link and others. I thought that I would share the results here so that I can learn from the experience and do my next one with a bit more acumen and awareness of what I’m looking for.

The setup for the test was as follows:

It was a cloudy night, so there were no real stars about. I covered a torch in tin-foil with a small hole and placed it at the back of my garden on a bench. I’m not sure of the exact distance, but it must have been at least 40 ft away from the optic I was testing. I aimed my scope – a Vortex Razor 85mm Gen II – at the light in the centre of the FOV and experimented by adjusting the focus from one extreme to the other.

I checked websites like this one which show the common results of star tests, including examples of well corrected and aberrant optics: How to star test a telescope - BBC Sky at Night Magazine

I captured the results on my phone, as well as assessing them by eye. Its impossible to accurately judge a set of optics when the results are photographed (and hence have passed through another optical system), but hopefully they will be indicative of what was going on with my experiment and the kind of things I was looking for and seeing. As some of you will be able to tell, the results are not exactly what is described in star test overviews: there were no rings (I was looking for that 3-5 ring optimal test position). I can confirm that the photographed images here are, to my eyes, identical to what I was seeing without a camera. The slight tracing in the high focus images is due to the vibration caused when I pushed the camera button on the phone.

High focus:




Although the results do not conform to the pattern I was expecting to see, I can see some purple glow around the light at high focus and greenish rings around the edge of the light at low focus, as well as some asymmetry. That might be expected, but I have no idea if this is significant, however, because I expect that I have overlooked something in my test setup. So if there's anything I can learn from this, please feel free to comment!

This is a breakaway thread from my hunt for a suitable spotting scope upgrade: Spotting Scope Research: Swarovski, Zeiss and Leica | BirdForum

This is the model I was testing...

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The "high" focus photos appear to show focused star points smeared into star trails from too much vibration and/or a shutter speed that is too low. The defocussed images are much too far out focus to show any diffraction rings. The most useful photos would be defocussed images with only about 3-5 visible diffraction rings.

The BBC article shows mostly images from reflectors with central obstructions, which look quite different from refractors like spotting scopes. Only the first image is from an unobstructed scope.

The thread on tests of the Kowa 99 has many photos that look like what you can expect from a spotting scope.

Keep it up!
This might not be the thread to which you are referring. Its 88 rather than 99, so maybe you have another one in mind, but it seems to contain some indicative images of a usable test result anyway.

I'll have a read and try again later...

Thinking back, it is curious that I didn't notice any rings growing as I slowly moved the light out of focus. That's what I was looking for. My eyes are probably not attuned to that kind of thing, but there was just that one moon-like circle growing and shrinking as I altered the focus. I'll have to go back and be more subtle, I guess.
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