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Collimation star test - strange artifact


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Having got my new (to me) Mak I have of course been cleaning and tuning whilst I awaited a gap in the clouds. And that gap appear this evening, so even though it was very misty I could see stars, so out I shot to do some testing.

On the whole I think the collimation is quite close, but I saw a strange feature when doing a star test... Below is a dodgy mock up of the kind of thing I was seeing.

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So the pattern looked quite close to being right, but there was this weird wedge that was all messed up. Maybe I am just misreading it and that is just showing that the collimation is off in that direction, but it just seemed so sharp and localised, and the opposite side didn't appear to be more spaced out, which is what I'd expect if things were out of alignment.

Could there be some kind of obstruction or something? I can't see anything peering in through the front, or from the back via the focuser.

I swung around to Jupiter briefly, and even with the very poor conditions I could make out banding with a 12mm EP, so first light with "Mak" has been good, I'm just curious about the star test.

Any thoughts?

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Looks like a heat plume - I had something similar with my mak-newt this evening - it took over an hour to clear and then started to show slightly again as I stood alongside the scope to use it - my body heat was soaking through the tube I guess !.

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Awesome, thanks all :D Lots to learn about the new scope still! Yeah, it wasn't anything like properly cooled, the conditions were so poor and the gaps between clouds so few and far between I just wanted to have a quick go, I was quite prepared for the fact that I wouldn't get the best from it. I'm just hunting around for a bit and pieces to make a DIY cooler ;)

But as I said, even with the awful moisture filled skies, and a warm tube, I still got a pretty good view of Jupiter, so I can't wait till a good night! :)

Thanks again.

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There are three rubber plugs on the back plate of the mak that allow access to the collimation knobs. Remove these to get some circulation to the back of the mirror. If you can mock up a fan or buy one from maplins that you can insert in the diagonal to suck air through you'll notice a big difference. Even better if you can filter the intakes to stop dust getting into the sealed tube. Welcome to the world of Maks! Thankfully its not ever going to get much colder than recent nights!

Stephen

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Yup! I've already got a spare PC fan hooked up to a cigarette lighter plug, and I have some filter material left over from trying to keep the PC clean... The commercial product I've seen has a tube that goes right inside the OTA. Is that over kill? Will just blowing cold air into focuser do a good enough job?

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Yup! I've already got a spare PC fan hooked up to a cigarette lighter plug, and I have some filter material left over from trying to keep the PC clean... The commercial product I've seen has a tube that goes right inside the OTA. Is that over kill? Will just blowing cold air into focuser do a good enough job?

Suck... don't blow

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Oh really? The commercial product I saw seems to blow air in to the tube, hence the filter requirement... Lymax's Earth, Sky, & Astronomy This approach appeals as it would create positive pressure inside the OTA, so all the air going in is via a dust filter. Whereas sucking seems like air will get in through whatever (unfiltered) gaps are available.

Fors and againsts?

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I guess the filtration will make a difference but most cooling setups I've come across, from scopes to computers, have been based on drawing off the warm air. I always presumed it would be down to preventing blowing in dust as, at least when there is plenty of openings, the more diffuse ingress of cool air will be at a lower speed and less likely to pick up dust particles. I would have thought, too, that blowing could cause a more localised cooling effect within the tube?

Looking at your link wouldn't it be possible to make something like that but with some sort of heat exchanger so you don't even change the air in the tube?

(Tbh, I also just liked the double entendre :) )

Edited by haitch
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