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mick pinner
10-01-2010, 10:25 AM
is there anyone who lives near me with a good understanding of collimation procedures that may be willing to come around and try to sort a problem l am having?
l have an f/5 newt that l am having trouble with, l am collimating with a Howie Glatter laser collimator, l can get perfect alignment according to the instructions but still have elongated stars, tweak the primary and the elongation gets better but by no means perfect, and this is with a Baader MPCC.
when the primary is tweaked this obviously throws out the laser but the stars get better so that is the puzzle, could anyone spare some time to give me a hand with this problem. thanks.

DavidU
10-01-2010, 10:30 AM
It may be because your scope is setup with an offset secondary. Short f/l scopes should be setup like this.
http://www.bpccs.com/lcas/Articles/offset_of_secondary_mirror.jpg

mick pinner
10-01-2010, 10:50 AM
l've tried moving the secondary up and down in relation to the primary but how do you tell when it's in the correct position?

DavidU
10-01-2010, 11:31 AM
It's a little difficult to explain but the secondary should be a few mm both toward the primary and a few mm away from the focuser.
I have looked at my Skywatcher 12" and it looks factory offset, ie: the center shaft support for the secondary is not attached to the physical center of the mirror but slightly towards the upper edge. So naturally the secondary mount is dead center of the OTA but because of the off set mounting it is away from the focuser.
A bit of a read but great inpho.....
http://www.atmsite.org/contrib/Carlin/collimation/

astro_nutt
10-01-2010, 01:11 PM
Hi Mick.
Check to see if the focuser is squared in relation to the tube..test this by placing the laser in the focuser then focusing fully in and out whilst viewing the laserdot on the primary mirror...if the laserdot moves then the focuser needs to be squared.
Cheers!

mick pinner
10-01-2010, 04:43 PM
checked the focuser and it seems perfect focuser racked in and out and beam does not move of centre. l've attached a couple of enlarged pics one from the centre of field and one from the edge. baader mpcc used with 400d.

DavidU
10-01-2010, 04:54 PM
Mick, can you take an out of focus image of a bright star?
The diffraction ring should show any problems.

mick pinner
10-01-2010, 05:17 PM
l have this one taken last night

DavidU
10-01-2010, 06:24 PM
A bit hard to say but the collimation is out. Can you do a bright star dead center of the frame with the out of focus disc covering 1/2 to 1/3 of the frame?
What is the specs of the scope?

mick pinner
10-01-2010, 06:39 PM
1000mm f/5, shall try and get out tonight.

mick pinner
10-01-2010, 08:42 PM
photo of out of focus star, any suggestions?

DavidU
10-01-2010, 08:56 PM
Yep she's out. Use a 3-5mm eyepiece while on the star and de-focus it and collimate the primary until the center shadow is perfectly centred in the disk. the diffraction rings around the disk should be perfectly concentric and evenly spaced.

DavidU
12-01-2010, 12:49 PM
How's it going with collimation Mick?

mick pinner
12-01-2010, 01:06 PM
l'll have a crack next clear night David and let you know.

gbeal
12-01-2010, 01:08 PM
Mick, I've held off making any suggestions as you stipulated at the outset "good understanding", and I'm basic at best.
The latest image does indeed look slightly off.
Couple of things to ponder. I sometimes use the laser to get the secondary pointed at the primary, but normally use the cheshire and star to finish the primary, not the laser.
In my case (and possibly yours, you image and don't observe as much, correct? If so, using an eyepiece to collimate may result in the result looking good, but when you put the camera, CFW, extension tubes, etc, etc, on maybe this changes?? Another option is to collimate with the webcam in place, although this will be lighter than the imaging unit I suspect.
Good luck,
Gary

mick pinner
12-01-2010, 01:16 PM
thanks Gary, my initial problem was when collimating with the laser (Glatter Laser, supposedly one of the best), it came up spot on but when imaging you get the result in the pic.
l will try a couple of methods when it clears and let you know how l get on, thanks for the reply.