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Benefits of SCT collimation - shadows and doubts?


Owmuchonomy

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As usual, all my forecasting apps were completely wrong.  Once the Owls had woken me up at 4am I thought I should glance outside.  Hang on, its incredibly clear.  So I headed out to the Obsy and sure enough Ursa Minor and its jewels told me it was at least a 5.5 sky.  Top of my To Do list was to collimate the 9.25" SCT.  It has never been done in 7 years but I noticed during my Monday Quasar chasing session that it was a bit out.  So armed with a marvellously simple schematic of which screw to turn and when I proceeded to twiddle.  I used Denebola as my star and my ES 14mm EP.  Three tweaks later and it was a close as I could get it to my eyes.  After a tour of famous globs in Bootes and Hercules I turned to Jupiter.  My last two outings had been very disappointing on this normally fruitful target and today started no different.  Bubbly and bouncy and two faint EBs shivering in and out of existence but then a moment of clear stuff.  The bands particularly the NEB appeared in all their glory but oh dear what is that sharp dark speck.  All my effort on collimation has upset something.  So I went inside and made tea with the desire to fix the damn scope later.  I came back out and the seeing was dramatically improved and sure enough the sharp dark speck was as clear as anything but it's moved, a lot?  Then it dawned on me to check Jupiter's moons motions.  I was observing right in the middle of an Io shadow transit and it was SO clear.  I have never seen such a sharp rendition of a shadow across the gas giants face.  As the culprit was Io, the transit movement was obvious too.  I felt quite chuffed.

Should I collimate? Oh yes, definitely.

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Nice one Chris!

you will be able to achieve even greater accuracy if you move to a higher mag and repeat the exercise. (Making it a 2 stage process).

in my c11, I usually collimate with a 10mm (x280) exactly as you did above.

then I slip in a 7mm (x400) and fine tune.

the higher power will show greater precision.

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It's great when those moments happen isn't it? Cooling and collimation are essentials for high power viewing and make a real difference.

I find that my refractor gives a lovely crisp view of these events more of the time, but using the C925 gives significantly higher resolution and more colourful views, but which are much more variable and susceptible to poor seeing.

Nice job on the collimation, glad it is giving you such great results now.

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3 minutes ago, Mr Spock said:

The C9.25 is one of the best scopes out there - when collimated properly. Without that last 'tweak' it can be quite ordinary.

I have a rather abused C925 which I nursed back to health. The collimation is nearly there, but definitely needs that last tweak which I will try to do next time I'm out.

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12 minutes ago, YKSE said:

Totally agree. Collimation for SCT are more vital than any other scopes. This is my reason to recommend Ed's guide for SCT collimation for those who haven't done it,

http://www.astromart.com/articles/article.asp?article_id=548

Not many guides (if any) puts so much emphases on the ill-effect of not-so-good collimation.

Thanks for that, really useful.

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My sincere 'Thank You!' to you, YKSE, for posting the collimation paper from Astromart, which abounds with other excellent articles and the like - aside from being a very reputable 'Used-Astro-Goodies' site in the US:

http://www.astromart.com/

Fair warning - to buy/sell any of the items they have in their classified-ads, they cost a bit of money to join and login, etc. But the articles are free. This being done to keep spammers & criminals at bay. Buyers & sellers are rated by other members accordingly to the history of prior encounters & sales, etc.

Enjoy! And thanks again -

Dave

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1 minute ago, michael.h.f.wilkinson said:

Oddly, my C8 seems to hold its collimation amazingly well. I have been able to get some detail on one of Jupiter's moon (imaging, not visually) so collimation cannot be off. I check collimation quite frequently, but it just seems to hold, and hold and hold.

Not sure if you have Bob's Knobs Michael, I'm assuming not? I don't with my C925 and just feel that keeping the original screws may be a little more fiddly to adjust, but when adjusted and tightened properly there seems no reason why the collimation should ever shift.

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1 minute ago, Stu said:

Not sure if you have Bob's Knobs Michael, I'm assuming not? I don't with my C925 and just feel that keeping the original screws may be a little more fiddly to adjust, but when adjusted and tightened properly there seems no reason why the collimation should ever shift.

I have Bob's Knobs, and they are still in their packaging, awaiting such moment that I feel I want to install them (i.e when collimation finally shifts). Until such time, the original screws will stay put

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