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pvelez
20-09-2013, 04:48 PM
I'll start with a question and then give the background.

Has anyone used a Tak collimation scope to collimate a PW CDK?

I have been pottering around with my new CDK - T Point model, V Curves etc - while I wait for the moon to go away. CCD inspector tells me that my collimation could be touched up a bit. So I've been star testing and using CCDI to try and improve it.

So far I have found it a bit finicky - a small movement in the secondary screws makes a substantial change in collimation. But I believe that is normal.

Just to see how it looked, I slotted in my Tak collimation scope which I had bought to sort out the RC8. It looked quite close but could do with a touch up. So I adjusted the collimation so that the secondary spot was correctly centered. What I find interesting is that the Tak scope suggests that the primary is out a bit - not much mind. The Tak scope shows a rim around the secondary that should be symmetric. In my case, it isn't quite. This usually means that the primary needs adjustment. But the CDK has a fixed primary.

So I'm wondering - is this only relevant for other types of scopes and not the CDK? Or maybe the central spot of my second at is not quite centred. If so, I may think that I am properly collimated when I am not.

I plan on star testing it all this evening so there's no drama either way. But I was wondering if anyone else has undertaken this exercise.

Pete

gregbradley
20-09-2013, 04:56 PM
You'd be best off posting this on the Planewave Yahoo Group or Planewave direct. Especially if you think there is a problem with your mirror mounting.

My CDK shipped with special collimation tools. The ronchi grate for exact spacing.

I assume your Tak collimation scope was securely fitted and orthogonal?

Greg.

pvelez
20-09-2013, 07:36 PM
Greg

I might give PW a go if it proves a problem - though I'm not convinced of this as yet.

The Ronchi eyepiece gets the spacing right so there is no issue there.

The Tak scope is orthoganal and well mounted.

Playing with it this evening, stars that looked wonky had a low collimation error according to CCDI and vice versa. So I've collimated by eye based on defocused stars. Still nasty seeing as far as I can tell. Will take a few shots and see what happens

Pete

pvelez
20-09-2013, 08:48 PM
Collimating by eye seems to work best. I'm becoming an old fogey!

Ignored CCDI and played around with defocused stars - first image reduced the CCDI measured miscollimation from around 6 to 3 arcseconds. Leaving it there, I found the reported collimation shifted between 2.8 and 5.9 arcseconds. Looks like a better focused image yields a better result.

Perhaps I am being overly fussy?

Will play some more when the seeing is better.

One thing was clear - using the Tak collimating scope helped - but not a lot. I needed to play some more to improve it

Pete

Joshua Bunn
21-09-2013, 11:45 AM
HI Pete,

If, when looking through the Tak scope, you see the secondary dot off from the center or the tak scope dot, then, depending on how far off the dot is, you can tighten and loosen the opposing secondary spider vein supports to move the secondary mirror lateraly until the center of the secondary mirror and the tak dot line up. (be sure not to udjust the spider veins too much as to unduce a bend in them - but you should not have to do this)

Do this for both the horizontal and verticle spider veins.

If your unsure about the tak scope not being orthogonal to the mechanical axis of the scope, then i would suggest (in fact i would sugest to do this anyway) to lock in the tak scope and rotate the focuser. this is best done with the scope pointing down with the focuser retaining screws loose and just sitting in the back of the scope. in this case, the tak scope centre circle or dot should scribe a concentric circle around the secondary center dot.

the aim of this procedure is to get the optical axis of the scope aligned with the mechanical axis of the scope and focuser asembly (and therefor the primary aswell since the primary is bonded to the backplate and focuser).

Now, depending on how far out the collimation is with the collimation screws, this may be an iterative procedure between adjusting the collimation screws and adjusting the spider veins a little since they both have a similar effect.

I use a laser in the focuser tube, and if the laser is out a little or not mounted properly in the focuser, then rotating the focuser will make a concentric circle around the secondary center dot.

an attached picture shows what your trying to achieve when looking up the focuser on axis. concentric circles with the secondary dot centered (confirmed with the previous steps) and the short spider vein reflections in the light annulus ring lined up with the longer spider vein reflections. From here, fine collimation can be achieved under the stars.

If things are not concentric when looking up the focuser and collimation is done by using a defocused star in the center of the chip, then collimation will be off.

Josh

stevous67
14-10-2013, 09:34 PM
And I remember the comments at the beginning of how easy these scopes are to collimate.

Just another person fooled by Planewave! These scopes are notoriously difficult to get working right. Sure, simple on a small CCD, however full frame can be a looooong lasting nightmare, with the flimsy design and poor factory assembly.

If you can iron out the body flex, you will eventually get good images.

Sorry,

Steve

stevous67
14-10-2013, 09:44 PM
I'll confirm Peter, collimating the CDK12.5 by eye, that is, assessing full frame images in focus, and slightly out of focus is the way to go. If your scope was poorly assembled, like many of these seem to be, the tak scope will not work until it is assembled accurately. Most importantly, that your secondary is in perfect alignment to the focusers optical axis. That does not mean middle of the front ring.

Good luck, as once you eventually achieve collimation, you will be rewarded by excellent images.

Steve

gregbradley
14-10-2013, 10:06 PM
The truss designs must work better. I find my CDK17 fairly tolerant of collimation and when I have done it not really that hard (except for climbing up a small ladder back and forth!.

Greg.

Beni
18-09-2016, 07:59 PM
Hello Pete
I have the same problem with my CDK 12.5". Also fully collimated the rings a not concentric.
Could you solve the problem, and how have you done it?
Thanks
Regards from Switzerland
Ben