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One for the collimation experts !


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A question :

Is it possible to have perfectly concentric rings either side of focus and nice Airy Rings at focus only to find that all your collimation tools tell you otherwise ??

Do you trust your eyes or go back and align everything again ?

Tools =

colly cap,

cheshire,

HoTech laser,

and Cats Eye AutoCollimator

PS the centre spot (triangle) has been triple checked to be centred !!

Karlo

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If I collimate as per anyone else and get everything spot on in the basement, then go out for the star test, I invariably need to tweak, when I come back in and recheck the appearance would indicate it's off !:)

I am leaning to trust my eyes and s&d the tools !

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When you say tweak, how much of a tweak.

I normally set up with the hotech at the beginning then later on will do a star test, it might need a very slight tweak maybe 1/4 of a turn of one or two of the primary knobs, but thats all.

Then during the night at say 60 minute intervals I'll reposition on polaris and do another star test sometimes another 1/4 of a turn is required as the scope reaches it's equalibrium temperature.

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Just rec'd this from Peter Wise which kinda confirms what I was already thinking,

"

Hi Karlo,

If you are getting perfectly concentric rings either side of focus, and a good Airy disc and diffraction rings at focus, then you need nothing more!

Collimation tools are only for getting near to good collimation; they are not perfect. Trust your eyes, and enjoy the telescope.

Best wishes,

Peter"

Karlo

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You may be having some mirror mounting effects. I borrowed a Hotech for my dob, to find that as I lowered the tube in altitude, the spot on the mirror moved about 1cm from top right to bottom left. I've not taken the 'scope to pieces to try and figure out what is causing this, but I suspect the mirror is moving slightly as the 'scope is tilted.

When I got it outside, it did need recollimating - which I did by eye and has stayed in collimation since.

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Unless the tools show it off o be a mile I'd trust my eyes.

If I were seeing good airey disks and the collimation tools showed way our of whack I'd be suspicious that something wasn't right somewhere in the tools. I'd go with the star test but I might investigate (ok I would investigate) why the tools are showing so much difference.

If the difference were slight I'd ignore it. My Cheshire always showed a slight difference between it and the Hotech I had on test. Not by much but a cats whisker of difference.

I'd go with the view that collimation tools only get you close - though I am surpised the Catseye would show a deviation - the real test is always a star and your eyeballs.

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I've had lengthy correspondence with Jim Fly et al at Cats eye who went off and made up images specifically for my set up ( I think they saw it as a challenge :) ), they were fab and really helpful but I can't help but think of this quote,

"....................To be fair, Peter did have a number of teething problems with outside optical component suppliers in the early days, particularly the small achromats forming the correcting system. These small lenses had to be very precisely set, any deviation had a marked effect on the focused image and is probably the root cause of collimation difficulties as regardless of the alignment of the main optics the correcting lenses could still introduce flaring and/or chromatic problems.

"

from this thread

http://stargazerslounge.com/equipment-discussion/64253-anyone-have-had-good-look-8ins-cape-newise-carbon-tube-2.html

So having wracked myself stupid reading/studying everything ever published about collimation and then trying to incorporate it into this scope, spent alot of dosh on kit, I find myself happy to to trust myself and my eyes and accept that the idiosynchrocies go on !

I had "classic" Airy rings etc as described and wonderful concentric rings either side of focus and thought, why am I trying to resolve what I see with the contrary "reports"of the tools.

So now I'm not going to worry anymore :icon_eek:

Perhaps like a SCT I ought to just make a note of where indicators sit at perfect collimation and be happy !

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