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Artificial star

Charon

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umadog

umadog

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Suiter discusses these extensively in his book. Their main use seems to be for star testing. I don't know how easy it would be use them for collimating. The source needs to be quite far from the scope (perhaps 50 or 100 m) and you need a still day.

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Charon

Charon

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Yes according to what little info on these I have found you need at least 100 feet for a 8" SCT. I am not buying one (just yet) as I will use Polaris (when I can see it of course!). I was just interested in reading anyones experience in using one.

Thanks again Umadog.

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26Left

26Left

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I use the Hubble torch as well for collimating my C11. It works extrememly well. Ideally, I prefer a real bright star, but the seeing needs to be really steady for any meaningful result.

I found a formula somewhere (maybe the Hubble star torch docs) that let you calculate the minimum required distance between the artificial star and your scope. As far as I remember, the distance gets much longer with a faster scope. I can reach the distance with my F/10 SCT, but it may be harder with a newt.

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Charon

Charon

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yes,

I use the Hubble Star torch version for checking the collimation of my SCT's (and other scopes)

Works very well and gives a choice of "star" size.

Which shop did you buy it from Ken / 26Left?

Cheers

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umadog

umadog

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There appear to be two factors. Star testing telescope quality

Firstly, the artificial star needs to be an approximate point source. Half the Airy disk diameter will work, which depends on aperture. Secondly, you don't want the "star" to induce spherical aberration by being too close. This probably isn't an issue for collimation testing but will be for a proper star test. It seems that the source needs to be further from a Newtonian than from a SCT, due to to the SCT's spherical primary. I don't understand why that's the case, though.

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ollypenrice

ollypenrice

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When I rang TeleVue once they suggested I glue a ballbearing onto a black card and illuminate it with a well focused torch. This gave one of the best star tests I've ever seen! One of our guests, a professional optical engineer, just looks for a gap in the distant trees through which a bit of bright sky is shining. That works as well.... for him, but he's a genius!

Olly

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Merlin66

Merlin66

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When used like an eyepieceyou see lines across an illuminated field, as you get closer to the focus the number of lines reduces until exactly at focus you only have a uniform grey background.

When there are only four or five lines visible the shape and uniformity of the lines indicates the quality of the image - distortions due to optics, seeing, collimation are readily visible.

Should be in every amateus tool kit.

Telescope Testing, Ronchi Tester, EASYTESTER®

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Telrad

Telrad

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When used like an eyepieceyou see lines across an illuminated field, as you get closer to the focus the number of lines reduces until exactly at focus you only have a uniform grey background.

When there are only four or five lines visible the shape and uniformity of the lines indicates the quality of the image - distortions due to optics, seeing, collimation are readily visible.

Should be in every amateus tool kit.

Telescope Testing, Ronchi Tester, EASYTESTER®

Looks interesting, thanks for the link Merlin! :)

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acey

acey

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In the 19th century, Webb advocated using an artifical star for daylight testing of telescope optics. His was a glint of sunlight reflected from a watch.

I have the Ronchi EasyTester which is nice, though really it's a one-use thing unless you're grinding your own mirror. The instructions say it's accurate enough to show if a mirror is a lemon, but won't enable you to say exactly how accurate the mirror is. Mine is not a lemon - though actually I knew that already from a star test. Still a nice thing to have, though.

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