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Star test questions

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#1 tomhole

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Posted 28 May 2003 - 11:16 PM

First question: Is it OK to post this on both groups until we decide which one we are going to switch to?

Next issue. I did some star testing tonight with the binos to look at collimation and stuff. First off, in cyclops with the 9mm in the 2x barlow (267x) I get a pretty good image for collimation checks and it looks dead on. There are some signs of overcorrection error, but I'm not certain in the below average seeing (and I'm certainly not a star test expert). The outside focus image has very distinct and clear diffraction rings, while the inside focus rings are a bit blurrier. It looked like this image below, only the difference between inside and outside focus was not nearly that prominent. Also, it wasn't as turbulent as the top pictures, but not as calm as the bottom one.

http://home.earthlin...e/StarTest2.jpg

But the collimation looked spot on.

I put the binos in with the 2.5x powermate as the OCS and the 9mm's for the same magnification. The star test showed miscollimation. The rings are circles, but the center hole is not centered in them. Also, the rings were not as distinct as in cyclops mode and the phenomenon that I described earlier for the cyclops test that I called overcorrection was much worse (I'm just calling it overcorrection for convenience, I really don't know what it is). The inside focus rings were MUCH fuzzier than the outside focus ones. It looked more like the top set of pictures in the above link.

Any thoughts? At least it's clear tonight and I get to look at something. I hope to do some Mars viewing this morning.

Clear skies,

Tom

#2 Guest_**DONOTDELETE**_*

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Posted 29 May 2003 - 10:20 AM

Tom,

I have an 8" LX200GPS UHTC along with the Denk Standards. I also have the Starsweeper .5 focal reducer. I noticed the other day that with the Starsweeper installed I would get a nice star test with the star in the center of rhe FOV but if I moved it off center I would get the same effect you mentioned...round rings and off center center hole. I was wondering what that meant also. Maybe someone has the answer and can help us out.

Bob Veazey

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Posted 30 May 2003 - 02:31 AM

Hi there Tom, regards the collimation I find (since we have a similar set up) depends on getting the bino to sit squarley against the focusser, the two small screws on my GS (which presumable are much the same on yours) are not the most effective method to ensure that once the bino is sitting squarely it stays so. Although it does work I just need to be careful getting it right and tight. I'm waitng on your fix for this! This is also to say that getting the bino to sit flush/square onto the focusser appears to retain the cyclops collimation.
re the star test, I read that a light cone passing through a flat (ie perpindicular) glass plane produces overcorrection, remember I mentioned to you before that Mr Roalnd Christen said that his binoviewers' 'inbuilt' corrector (?1.2X)is corrected for this aberation produced by the prisms and that Mr Nagler says it's not significant aberation with high focal ratios. So if it is overcorrection you see this may have something to do with it.
If Al' says over F10 it can be neglected, and with Al being a kinda particualar with aberations we might think it is at most (at >f10) a 1/6 (less than 1/4 wave anyhow) aberation and yet it can need 'correcting' at some lower F ratio according to Mr Christen then it may be that without 'correction' (at lower than F10 maybe) it is an apparent aberation ie it is at least able to be detected in the star test. (if perhaps not often in practical use)
Scott.
It would be interesting to hear of your star test impressions in this regard with your various focal extenders.
Although you might want to loan or buy in any case read 'star testing astronomical telescopes' at least once and then see what you think.
S.

#4 tomhole

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Posted 30 May 2003 - 06:47 AM

Scott,

Excellent reply, as usual.

Re collimation, I was using the binos with the powermate as the OCS. This results in some droop of the binos and may have casued the slight mis-collimation indications. I will try this again and move the binos around with the same configuration. I will also try a star test with the OCS only.

I think you have nailed it with respect to overcorrection. I was using the powermate as the OCS and this certainly will not help that aberration. The intra and axtra focus images certainly were indications of overcorrection. I talked to David Nagler yesterday and asked him about the TV 2x corrector. He said that the TV corrector was designed to fix overcorrection and false color with the TV binos and should work with the Denks as well. I'm going to end up with all 4 correctors (Denk 1.25 and 2", Siebert 1.3x OCA and the TV 2x corrector) and pick which one I like. I am shying away from using the powermates and barlow as an OCS. Too many unknown variables that I can't account for.

Then I'll order a pair of Denk II's :)

Again, spot on reply. Very helpful.

Clear skies,

Tom


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Posted 30 May 2003 - 07:18 PM

Hi Tom,
Technically speaking, slight mis-collimation of a Newtonian
scope shows itself as something that looks like astigmatism
just inside and outside of focus.If you collimate your scope without the weight of the bino/ep/powermate.Then the scope is correctly collimated.When the extra weight and length of your more complex optical train results in misalignment you haven't actually disturbed the telescopes
collimation.The problem is more accurately described by saying your eye is no longer correctly positioned with respect to the optical axis.I have successfully used Scotch Brand Magic Transparent mending tape as shim material on the various shafts that have to mate with one another.It isn't perfect but it does minimise the problem. Suiter
discusses collimation errors in Newtionians in Chapter 6
page 104.Re-aligning the the optics to compensate for the
crooked path is probably a mistake.Suiter does not discuss
the problems of actually having a wobbly stack after the focuser.As far as your telescopes optics are concerned I would wait for the best seeing conditions with regards to turbulence before making a judgement on over or under correction of your mirror. Good luck fixing your alignment problem.I hope the shim idea does some good.
Scotty :D




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