Star Test - IN focus & OUT focus

[multiweb]

What does it mean when the disks on an out of focus star on an SCT are different when the focus is In or Out. In focus I have concentric rings but Out of focus on the other site I get some dark shaped spots into the star rings. What could cause the picture to be different from the In Focus? Thanks for any tips. :shrug:

[casstony]

Most importantly, the scope needs to be perfectly cooled before considering a star test.

The first link shows a decent sct, with no greatly brighter/dimmer inner or outer rings and just a moderate difference in the size of the central shadow on either side of focus: http://aberrator.astronomy.net/scopetest/html/cassegrain250_1.html

The second link shows a poorer quality sct with obvious differences in brightness across the rings, a bright inner ring on one side of focus and a bright outer ring on the other side, and a bit larger difference in the size of the central shadow on either side of focus: http://aberrator.astronomy.net/scopetest/html/cassegrain_200_4.html

I don't know that much about star testing and it's easy to misinterpret the star test. I can say from experience that a crappy sct will still show more detail than a good refractor half its aperture simply by virtue of the greater resolving ability of the bigger aperture - it's easy to get overly worried about optical quality(worth worrying about in the warranty period though).

'Star Testing Astronomical Telescopes' by Suiter is a good book if you want to learn more. Hope that helps.

[Dennis]

Another useful read is this write up by Roland Christen of AP.

http://geogdata.csun.edu/~voltaire/roland/startest2.html

Cheers

Dennis

[multiweb]

Thanks for the links guys, got some reading to do, starting with suiter's book that seems to be the "bible" in that field. :thumbsup:

[Satchmo]

You certainly need to ensure that the instrument has fully cooled before making judgeement in a star test. Strong light and dark rings and uneven secondary shadow can all indicate zones and spherical aberation, although the star test for zones is incredibly sensitive: a star test showing zoning may well be within a 1/10 wavefront tolerance. An easier test for a beginner to interpret is a Ronchi grating in place of the eyepiece. At F10, it is fairly sensitive and can show at a glance what the problems are and if they are significant.




The larger aperture will only give better resolving power if the light is constrained in or near it theoretical airy disc diameter. This also ensures that the visual limiting stellar magnitude delivered by the scope will be appropriate for its size.

Larger apertures , even if not diffraction limited can impress on extended nebulous objects. Stellar limiting magnitude will suffer though. Considering just pure spherical abberration, a largere telescope of 2/3 wavefront error will show the same faintest stars as a diffraction limited one of half the aperture.

Mark

[Merlin66]

Marc,
If you can set up your webcam to take an image of each disk, then WINRODDIER can analyse it for you...
www.astrosurf.com/tests/Roddier.htm (http://www.astrosurf.com/tests/Roddier.htm)

It's shareware and there's an english version on the Yahoo support group.

[multiweb]

Thanks Mate! :thumbsup: It's even written in French! Whoohoo! :) I'll have a read at it too.

[Barrykgerdes]

Hi Marc
Here is another site you may like to look at. It also has a french version. Thierry had a lot on collimation etc a few years ago. I have it all in MS word form but I only saw his bit on collimation in the reference

http://www.astrophoto.fr/

Yipee! It also has a french version.

Barry