Secondary mirror and collimation

[shane.mcneil]

Greetings.

In my continuing quest for collimation perfection, I have a question about the secondary mirror. I can't quiet get it centered under the focuser. I have adjusted the centre screw and wound it up tight against the spider, but it still seems to need to come up some more.

Is there an adjustment I'm missing? Can the mirror itself be moved on the mount? Does it matter anyway?

The focuser is at right angles to the tube.

Thanks for any advice you may have.

Regards,

Shane

[Robh]

This might seem obvious but have you loosened (wound up) all the collimation screws to allow the central screw holding the secondary mirror cradle to wind up as far as it needs to go?

Regards, Rob

[shane.mcneil]

Well I'll check but I am sure I did wind it right up before adjusting the collimating screws. There may be a bit more room to move. But I doubt there is the room I need.

[bmitchell82]

if your after collimation perfection what tools are you using?

[shane.mcneil]

Well, when I say perfection I am just being silly. I have the Orion tool (not the laser). I am actually happy with the view but it bugs me that I can't get the secondary right. Would it make that much of a difference?

[bmitchell82]

Are you talking about a cheshire eye piece? As for visual i couldn't see it being such a issue as your only chasing a fully illuminated spot about 5-10mm diameter. If you where talking AP, your looking at more 20-50mm full illumination in which case your chasing the outer extremes!

[shane.mcneil]

I've got this one (http://www.amazon.com/Orion-Collimating-Telescope-Eyepiece/dp/B0000XMUDM/ref=sr_1_3?ie=UTF8&qid=1321870198&sr=8-3). Now to sound really dumb, what is AP?

[Robh]

The Orion collimating eyepiece is a type of Cheshire eyepiece, which was what Brendan was referring to.
AP I think is Astro-Photography.

I am surprised that you can't get the secondary mirror high enough. When you collimate with the Cheshire, can you see the whole primary mirror with clips and all. If so, it should be fine. You might have to pull the cheshire out a bit to see the whole mirror.

Regards, Rob

[bmitchell82]

AP is Astro photography where things become extremely sensitive to collimation issues.

I wouldn't be too worried about not seeing the mirror clips on your dob as long as its not along way from the FOV then your only chasing a small area of illumination.

When i used the cheshire on my newt back when it was still in a un adulterated state I was never able to see all the mirror clips at once! Even now i have issues seeing the mirror clip surrounds but thats a whole different issue and configuration :)

As long as you have the secondary centered within your focuser, and the dots line up with your cheshire that will be fine enough for what you want to do. A laser collimator wont do it any better and perhaps possibly worse and the next step up would be a barlow laser setup or auto collimators for higher accuracy.

[shane.mcneil]

Yeah I can see the entire mirror including clips and I can see all of the secondary mirror too. The secondary just isn't quite in the centre of the focuser. If I pull on the spider vane I can get it there almost. Like I said, it isn't out by much.

I'll go over it again and see if I can sneak some more by loosening off the 3 collimating screws. Thank you both for your help too.

[bmitchell82]

A little trick that i do to help with centering the secondary is get a piece of card that blocks the view of the primary mirror thus eliminating a distraction and sits behind the secondary basically making a Equal Angle behind. then you can stand right back and line up your eye as best as you can with the focuser this will help you approximately center secondary.

Failing that a Cats eye sight tube does the same job but with alot more precision.

Brendan

[shane.mcneil]

Just in case anyone wants to know how the story ended, I came up with a solution.

I very carefully took hold of the secondary mirror mount. I then gave it a good hard tug.

It is now centred under the focuser. :thumbsup:

Shane

[brian nordstrom]

:thumbsup: good news Shane , just be really , really certain that you aint broken any thing :( , the last thing you need is the whole secondary assembly falling onto the primary :eyepop:, in the middle of the night . OUCH!
Ive seen this happen once , its not nice .
Brian.

[shane.mcneil]

I checked it out and all is good. Thanks for the reminder. It all lines up nicely now.