Last updated: 5 February 2001

Star Testing Your Telescope

From: sherrodc@ipa.net (Clay Sherrod) This was in response to an inquiry about star testing and what the results might mean:

Your scope sounds wonderful; don't worry about your optics! From what you have said, you really could not ask for more. Sometimes when things are going so well, we tend to "look for" problems that we think "should" exist....the old "too good to be true," nature of human-folk.

Let's talk about your star test for a second, though. First, in focus, there is probably a condition that may arise with "too many diffraction rings" from an Airy pattern around a bright star, but I would like that luxury. With my ETX 125 on a very clear night and, say Arcturus or Vega, I can see up to seven rings plus the central star "disk." My ETX 90 reveals as many as five on a good night. Diffraction rings are good....if you were NOT getting them, or if they met/combined/mingled then you would have a problem.

The Airy pattern is one of the most stringent of all star tests.

Okay, let's talk about your out-fo-focus star disk test. Intra-focus and extra focus images on a very clear night (always choose a bright star nearly overhead on a very steady night) will reverse themselves as you move in and out of focus. It is common for one to appear "sharper" than the other. Use about 150x per inch for the test. If the out-of-focus star image (a disk) appears to wiggle or oscillate, forget the test,....the seeing is too poor.

With a bright star at high magnification in your scope you should see very, very fine and close concentric rings within the out-of-focus star image, with the donut hole of the secondary right in the center of the disk; an offset secondary shadow indicates a collimation problem....OR it indicates that your scope is NOT CENTERED on the star! You will note that the "hole" appears to shift to one side as you move the star closer to the edge of the field.

The tiny little rings that you should see should be essentially un-disrupted from being cleanly separated from the others; if they appear to merge periodically, that is okay. However, if you see major distortions on the disk, like a splash in a puddle or such, it is an indication of some degree of optical flaw. I doubt you will see this.

Always make sure your telescope has "equalized" to the outside air for AT LEAST two hours before doing any star test.

Important note: small telescopes have a distinct advantage (see my upcoming dialogue on Mike's site regarding seeing and transparency which I am now writing) over large ones in average-to-poor seeing. In short, the greater the aperture the greater the resolving power; this means the larger scope can resolve AIR TURBULENCE even better as well, not just a festoon on Jupiter! This is why small scopes like the ETX show considerably more detail on nights of poor seeing than large ones at the same star party!

Also, the small the scope, the LARGER is the Airy disk (the central image of the in-focus bright star), appearing more like an actual planetary disk than with larger scopes. This, obviously is simply a result of the greater resolving potential of larger scopes.

You've go a good scope....hide it. There are other enthusiasts out there who would die for it!

Good luck and continued excellence in your observing!

P. Clay Sherrod

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Copyright ©2001 Michael L. Weasner / etx@me.com
Submittals Copyright © 2001 by the Submitter
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