[marc4darkskies (Marcus)]

Originally Posted by ribuck

Someone on another forum suggested it could be as a result of tilt in the optical axis, so it might be worth me investing in a decent laser collimator such as the howie glatter, as my current hotech sca collimator might be giving me a poor reading when aligning the focuser on the optical axis

Rich.

It's almost certain to be a combination of collimation and misalignment / tilt of the mechanical axis with respect to the optical axis. I wouldn't rule out guiding issues either so when collimating it's best to take very short exposures (max 10 sec) on a dense star field to eliminate guiding as a source of misshapen stars. Moreover, it's very difficult to diagnose a specific single cause for collimation issues so my approach is always to start from scratch and eliminate things one by one. First, make sure the optical axis is coincident with the mechanical axis and this is coincident with the spot on the secondary - using a laser.

I use a Howie glatter quite effectively (1.25" / 2" version). http://www.bintel.com.au/Accessories...oductview.aspx

Originally Posted by ribuck

...
i literally just checked, and when i checked the laser position, then loosened and rotated 90 degree's and performed the check again the laser dot was not in the same place. so this suggests that it's either not well collimated or it's the self centering mechanism is reliable / repeatable.

Rich.

Yep, a compression ring eyepiece holder won't work reliably / repeatably. You need metal on metal contact.

I gave up trying to fix the laser to the scope and adopted a low tech approach. This only works for me because I have the 1.25" / 2" Glatter and a Tak collimation scope adaptor (screw fitted to the imaging train) that has an opening somewhat larger than a 1.25" eyepiece holder and a flat surface surrounding that (ie fortuitous):

1. hold the laser flat up against the adaptor for the tak collimation scope with the 1.25" tube of the laser sticking through the threaded opening of the adaptor and the 2" surface flat against the adaptor. I watch the dot on the secondary and physically move the laser in a circular motion. This assumes you have machined surfaces on the laser and adaptor that are flat and large enough to make good contact. This movement traces out a circle on the secondary
2. by eye, I note where the center of the laser circle is with respect to the spot on the secondary. (I did say this was low tech.)
3. I repeat this a number of times and rotate the laser between trials
4. I also determine what adjustment on the tip/tilt collimation ring on the scope is required and then make that adjustment
5. I repeat the steps above until the laser circle is centered on the secondary spot. If your laser is a bit skew then the above method will show a laser circle whose center orbits the dot as you rotate the laser between tracings

Regardless whether you can do exactly the same as I do above or not, being able to trace out a laser circle on the secondary is probably the most reliable way of using a laser to do the initial collimation.

After this then use a Tak collimation scope - an essential piece of kit IMO!

Originally Posted by glend

What does the TAK Scope show? Is the little back dot centred and is the secondary sticker centred around it?

See here.