YACQ (Yet Another Collimation Question)

[Joel Shepherd]

I have a Celestron Omni XLT 150: a basic Newtonian. In most ways it seems pretty well collimated: primary aligned with secondary, secondary centered in eye piece. However, I can only see two of the three clips holding the primary which -- if I understand correctly -- implies that a little of the light from the primary isn't making it to the eyepiece.

I'm not sure how to correct that. Looking down the OTA, the secondary holder has a phillips-head screw in the center, and three small allen bolts around it. Loosening the philips-head screw seems to simply loosen the post the secondary is affixed to, though I could rotate it. Try as I might, with the allen-head bolts I can't quite bring all three clips into view.

Any suggestions?

Thanks! -- Joel.

[Owmuchonomy]

Before you do anything else you should check the secondary is centred in the OTA by adjusting the spider vanes if necessary. Once you have done this you then use a Cheshire or homemade pierced cap to adjust your secondary. If you haven't done so already, check this link out. You may also need to put a 'milk bottle' plastic washer under your secondary adjusters.

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

[rwilkey]

Hiya Joel, my view is 'if it ain't broke, don't fix it'.  In other words, if you get a clear and accuarate view when you look through the eyepiece, then it is collimated.  The idea of seeing three clips is purely a guide for instruments that are way out of collimation.  But the proof is in the pudding.

[Joel Shepherd]

Thanks Chris and Robin - I'm fairly certain the secondary is centered with respect to the OTA. Hmm. How would I confirm that visually?

I don't think the scope is too far out of collimation: e.g. blurred stars appear as fairly symmetric doughnuts. I definitely understand leaving well enough alone. The main reason I'm pursuing it is that when I look at, say, M13, I feel there should be a little more detail or granularity, but maybe I'm overestimating the capabilities of the 'scope or the effect of light pollution and atmospheric conditions.

Thanks -- Joel.

[rwilkey]

For me, the secondary only has to look perfectly round when you look down the focuser.  On M13 you probably need a bit more aperture, good dark conditions and a good quality eyepiece to really tease out the detail.  I have never got what I call a great view of M13 except once, when I looked through somebody else's scope and they had a Pentax eyepiece in the focuser.

[Northern Soul man]

Hi Joel,a very warm welcome to the SGL mate,  as mentioned above - you seem to have collimation spot on - the comment about the blurred stars being fairly symmetrical doughnuts is the best test for collimation in my book - its only on a star test that you get a good idea of collimation - sounds spot on to me - the atmosphere and nights of poor seeing will degrade the image far more than a slightly out of collimation scope.

You mention granularity and detail, this comes with aperture - larger apertures give you better resolution and allow for higher magnifications to be used - so sounds about right - some nights will give you much better views than others - its down to the condition of the atmosphere - you can check this by selecting a star and moving in front and behind focus to give you a large doughnut - as you mentioned, then have a look at the "patterns" in the doughnut - if the seeing and atmosphere are poor - you will see this part of the doughnut "bubbling" and "boiling" away - like a really bad heat haze off a desert - on these nights its really hard to focus stars as what they really are - just pin points of light - poor seeing makes them "bloated" and very hard to get critical focus down to a single point of light.

Paul.

[Joel Shepherd]

With a little more twiddling, the secondary is presenting a bit better view of the primary ... and everything else seems aligned. I'm going to leave it at this point (agreeing that stars are the proof), and step away from it (the cheshire, not the scope!) for a few days before taking another pass at it. Not the kind of thing to do while feeling frustrated.

Never have noticed but have never really looked for the "boiling" around the edges of the blurred star: will start looking for that.

I reluctantly concede that more aperture would help. :-)  Oh well. My plan/goal is to stay with this scope for another 12-18 months while I'm learning the ropes (the constellations, how to aim, how to star-hop, how to tease out little blurries, etc.) and then invest in something larger: a 10" reflector of some sort seems like the largest I could haul in and out night after night.

Our local astronomy group is having a star party on October 3, so I'm crossing my fingers for a decent sky that night and hope to get some views to compare my own with. That would probably go a long way towards setting my own expectations appropriately. :-)

Thanks! -- Joel.

[Northern Soul man]

Hi Joel, nice 1 mate - it would be nice for you to take your scope if you can to the star party and let others enjoy this wonderful hobby - the boiling effect is more pronounced in the "airy disc" of the defocused star image in an SCT scope - I have been out many a night and checked collimation by moving focus in and out of the focused star and found the atmosphere to be really unsteady.

You have to move focus from a nice point of light (a focused star) to a large doughnut where you can see the image of the primary and secondary mirror with the spider vanes visible to see the problems the atmosphere plays on the quality of the seeing from one night to the next.

Enjoy the star party - and if you can - take your scope and let others enjoy the night sky.

Paul.