Refractor collimation part two!

[Moonshane]

Some of you may recall that I had a bit of a mare with an ED triplet and the collimation being out of whack. I messed it up and managed to put clamshell chips in the edge of one of the lenses due to bad design and me not checking the mechanism.

I bought an Astro-tech 102mm f7 ED off a fellow SGLer recently and initially was very happy with it. the recent cold snap though seemed to create slightly odd flares on stars and I suspected the contraction of the cell might be the cause. The star test view was broadly like the badly done image I created below with dimples on one side of focus and 'diffraction spikes' on the other side once the scope was cooled properly (temps about -1c or so). I looked for foil spacers of which there were none and in any case there are usually three.

The only thing with four bits/elements were the collimation screws which are a bit like the ones shown in the image below - coming in laterally like on the scope I damaged - oh dear I thought!

 

I looked at the collimation without changing anything and it looked pretty good so with the scope pointed at Polaris, I defocused and rather gingerly loosened as little as possible one of the side collimation grubs, literally until it 'snapped' loose. I was delighted that instantly the apparent pressure on the glass was released and I now get a 'classic' star pattern which is actually better than my 120ED! The stars also definitely tightened up with more even shape and good airy patterns.

I have posted this in the hope that it might help folk with the same sort of issue as I could not really find my problem online.

I do though wish that scope makers would use the more user friendly collimatable cells like this one

or alternatively the non collimatable cells like the Skywatcher ED scopes which seem to hold collimation very well indeed.

Cheers

Shane

 

 

 

[CraigT82]

Good stuff... you're braver than me!  It sounds like the cold is causing the cell to contract and put pressure on the lens like you say.  Wonder if the cell is made from an alloy that's more susceptible to thermal changes?

Anyway... I thought fracs were supposed to be trouble/maintenance/collimation free?!

[spaceman_spiff]

Nice report, Moonshane!

I'm just wondering did you only loosen one screw or did you alter more. I would have thought that if only one screw is loosened, the lens will be slightly offset from the main axis of the telescope. Would it be worth tightening that screw a small amount and loosening the screw on the other side?

Thanks again

Dan 

[Moonshane]

Cheers guys. Good / fair point Dan although I literally loosened it by no more than 1/20th of a turn so I'd be surprised if it's had an effect on collimation. The start lest looked spot on and I'll be checking the collimation later too although as the test and stars look better I am much happier. Even before the adjustment the views of the sun in white light with binoviewers (one of the main reasons I bought the scope) were stunning.

[xtreemchaos]

I had to the same with my AR127L when I first got it over 10 years ago, it was shipped well out at both ends, but in the 10 years ive had the scope its never had to be touched again. well done for sorting it. charl.

[spaceman_spiff]

8 minutes ago, Moonshane said:

Cheers guys. Good / fair point Dan although I literally loosened it by no more than 1/20th of a turn so I'd be surprised if it's had an effect on collimation. The start lest looked spot on and I'll be checking the collimation later too although as the test and stars look better I am much happier. Even before the adjustment the views of the sun in white light with binoviewers (one of the main reasons I bought the scope) were stunning.

Ok I see. That's a very small movement, no point messing around without good reason, you don't want to risk any more clamshells!

Thanks 

[Moonshane]

40 minutes ago, CraigT82 said:

Good stuff... you're braver than me!  It sounds like the cold is causing the cell to contract and put pressure on the lens like you say.  Wonder if the cell is made from an alloy that's more susceptible to thermal changes?

Anyway... I thought fracs were supposed to be trouble/maintenance/collimation free?!

Yeah, me too Craig! I am hopeful it won't need doing again for a while.