View Full Version here: : Collimation

08-07-2017, 12:58 PM
I've been trying to get the collimation of my Schmidt Newt sorted but can't seem to get it just right. Today I've checked the focuser is square and then re-collimated with my Catseye tools. I think it look ok however the image through the Autocollimator does shift a bit if I rotate it. I'm also wondering if the position of the secondary is out.

I've taken some images through the Cheshire and the autocollimator.

Advice greatly appreciated.


08-07-2017, 01:03 PM
I found the autocollimator to be extra sensitive. Is it relatively tight and square in the focuser barrel when you rotate it?

Also make sure you make all your measurements where you assume your camera chip plane will be. Further or closer will be slightly out unless everything is 100% square which is usually not.

08-07-2017, 01:20 PM
Hi Marc. I've set the draw tube at the focal plane by focusing an image of some distant trees on a piece of grease proof paper held over the end of the draw tube. I'm not sure how else to determine this position as I'm using a paracor with the camera so the physical position of the image sensor is different. Not sure if this logic is correct?

I have the OTA sitting horizontal on a table with the focuser vertical so the autocollimator is sitting in the draw tube under its own weight so while not tight it should be sitting square. When I rotate the offset pupil I'm seeing a slight shift in the stacked hotspot images as shown in the last two photos.

08-07-2017, 01:40 PM
Ok, that's what I do to. Focuser vertical looking up the ceiling. That's the best way to have the auto collimator square by gravity with the focuser barrel. What I'm saying is that if the auto collimator diameter is slightly smaller than the focuser ID then it can move sideways when you rotate it and that would throw your alignment. I pack mine with some very thin alu shims. If that is not the case then the only other reason the center spots would move while rotating is that the mirror face of the auto collimator is not square to the light path.

09-07-2017, 09:34 AM
I made some alu shims from an old soft drink can and used 400 grade wet and dry to grind down the thickness required. Cheers!

09-07-2017, 12:57 PM
After lots of experimention yesterday I think the shift on rotation of the catseye was due to a small rotation error on the secondary. I've managed to get the colimation tools to show consistent images but was unable to star test last night due to clouds rolling in. Fingers crossed.

09-07-2017, 06:05 PM
Well it doesn't look like the weather is going to cooperate tonight either so I thought I'd post the one image I snapped last night. I did a screen grab of this because I thought it looked nice - perhaps what aurora would look like if you could see it from Brisbane!

In any case I'm quite pleased with how the stars are looking at this early stage :prey2:

09-07-2017, 06:12 PM
That looks really good.:thumbsup:

09-07-2017, 10:45 PM
Does look good Peter from that quick snap.
What were you unhappy with for your stars previously?

09-07-2017, 10:54 PM
Looks good Peter, fingers crossed you have nutted it out :thumbsup:


Don Pensack
11-07-2017, 10:24 AM
You might find this useful:

11-07-2017, 08:17 PM
Finally a chance to test the collimation tonight. I think it is looking better...

11-07-2017, 08:28 PM

Thanks for the link. I have seen that site before but it was good to refresh.