Best method of collimating my SCT?

[NightCal]

I’ve been using my 8” LX200 classic for 4 years now. Its main use has been for lunar and planetary imaging with colour and mono-chipped Toucam Pro IIs. My scope set up has a permanently mounted 33cm long dew shield attached to scope. This makes any system that involves looking through an eyepiece to adjust the collimation somewhat problematic as I don’t have 2 metre long double-jointed arms. To adjust the collimation I need to be standing at side of or in front of the scope.

I have always given the collimation a quick check before imaging by lining the telescope on a brightish star then popping the webcam in the back. I then defocus the star and compare the resulting doughnut with the red concentric circles reticle on K3CCDTools. If it’s out, I adjust the collimation knobs so that the shadow of the secondary is dead centre of the reticle on the computer screen.

I’m always keen to improve my images and felt I was getting close to the limit of what I can do with my scope and camera set up. So I did a bit of reading around and realised that my collimation assessment method may not be anything like sensitive enough. So here are my questions.

1 - Is this collimation checking technique sensitive enough for planetary and planetary imaging? (My guess is no).
2 - Is using a laser collimator sensitive enough?
3 - I’ve tried using Metaguide software ( http://www.astrogeeks.com/Bliss/MetaGuide/ (http://www.astrogeeks.com/Bliss/MetaGuide/) ) for checking the collimation. On paper, it seems to provide the ideal combination of sensitive tuning and autocentreing between adjustments. However, in reality, I’ve found the autocentreing hasn’t worked and I’ve found it confusing and difficult to use. In fact, I’ve only ever seemed to make the collimation worse, not better! I’m sure this just down to me being a klutz. Should I persist and learn how to use it properly, or will a laser collimator give me just as good or better results?
4 – Has anyone got any other suggestions for hardware, software or techniques that might help me quickly and easily fine tune the collimation?

Thanks

[multiweb]

I would remove the dewshield while collimating. If the aperture is oblong it migh affect the shape or your diffraction rings during a star test and give you the wrong information. Besides as you've pointed out it's not practical to access the secondary collimation screws.

Aligning an SCT is tricky. In a newt you can iterate through a process that will give you a true coincidental optical axis between primary and focuser. On an SCT you can end up 'collimated' in a position which is a compromise.

I use the ACT from Hotechusa. It's a very good tool to get you accurately aligned (on a bench) prior to do a start test. It is a cross laser and three dots that simulate the parallel light path from a star.

I also use a cardboard with a pinhole, a cross, and two concentric circles. One is the diameter of the primary, the other one the diameter of the secondary. Looking through the pinhole in front of the scope while matching the big circle to the edge of your primary and matching the smaller circle to the secondary and aligning all the straight lines will place your pupil exactly on the optical axis defined by the corrector/secondary/primary. Having a quick look will then show you what's inline and what's offset at a glance. That gets you in the ballpark.



The fine tuning is still done on a star. That's the final test IMHO.

[NightCal]

I spent a couple of hours today getting to grips with Metaguide. The author has given it several nice little teweaks since I last tried it and it makes quite a difference. I particularly like the false colour gradient on Airy disc image - nice touch.

After about 20 minutes of tweaking I have a much more symmetrical set of Airy rings around star images. This software gives me a usable technique for fine tuning my collimation that is not dependent upon having brilliant seeing. :)

[OICURMT]

:question: How "permanent" is permanent?

[NightCal]

Undo 5 hex screws, de-mount the piggy-backed Orion 80ED + one of its mounting rings.

[Barrykgerdes]

Hi

I had a 12" LX200 permanently mounted that needed collimating. I had "bobs knobs" but it was still physically impossible for me to make adjustments from the eypiece.

I have a video camera that can be attached (a toucam should be OK) so I set up a video screen that I could watch while I made the adjustments on the star test. This made it very much easier to make the necessary adjustments to get nice concentric and even airy rings.

Barry