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LX - 50 Collimation and Bob Knobs Question

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#1 raydar

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Posted 09 October 2005 - 12:18 AM

Hi People,

I have a stange problem that I can't seem to solve. When I am trying to collimate my LX-50, I seem to get it looking okay, but then after a few minutes, it seems to be off again.

I also noticed that when I use a Meade 40 or 26mm plossl eyepeice to collimate, the thinest part of the defocused star image changes location as I slew (not adjust collimation). I then tried a 15mm LV eyepeice, and this seemed to keep a true representation of the defocused star no matter where I slewed the star in the eyepeice.

The only thing I can think of, as to why my collimation would change from good to bad in a few minutes, is that Bob's Knobs are plastic and the flexibility in them is causing this loss of alignment.

Can anyone shed any light on this please?

Incidently, is there a prefferred direction to defocus a star image (anti clockwise or clockwise), because the different directions cause the thinnest part of the defocused star image to change sides?

Thanks in Advance

Ray

#2 garyc11

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Posted 09 October 2005 - 03:40 PM

are you sure the scope is cooled down? i normally wait a good 2 hours before checking collimation. i wait for a night when seeing conditions are good. as far as what direction to use for the defocused star i use the one that looks the best. i dont think the bob's knobs are the problem it's probably the seeing conditions or thermal problems.

#3 sixela

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Posted 09 October 2005 - 05:34 PM

I also noticed that when I use a Meade 40 or 26mm plossl eyepeice to collimate, the thinest part of the defocused star image changes location as I slew (not adjust collimation).


That's why you keep collimation stars centred - off-axis, there *are* aberrations - notably, coma - and even if they're corrected by the optics, the out of focus star images aren't as in the text books.

I'm assuming you mean that the image changes when you slew and decenter a given object, not when you center a different object by slewing.

#4 raydar

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Posted 11 October 2005 - 12:03 AM

Thanks guys.

Yes Sixela, that is right, it is when I decenter the image that it gets worse.


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