Collimation woes?

Hope everybody can give me some pointers

Haven't used the Traveller for a few months and took it out to do some Ha imaging but when I fine adjust the focus instead of a round star I get an enlongated star even on axis. I can't imagine that the scope has been knocked out of collimation since it lives in a padded hard case in the garage and although its cold in Toronto, its only about -10 C. Any ideas? These are also very short exposures on the order of 1 s so its not a mount drift issue and I'm focusing via the CCD and observing the star shape on my laptop. I initially thought that maybe the CCD wasn't orthogonal to the tube and adjusted the compression ring so it was better centered but no difference.

It sounds like the elements have shifted and not a collimation problem. Martin

The lens is permanently collimated and there are no collimation adjustments of any kind. In fact, even if you try, you cannot knock it out of collimation, it's simply impossible. It is also impossible for the elements to shift with respect to each other since there is no room inside the lens cell for that to happen.

Now, the reason for the elongated stars - what temperature was it when you went out to shoot? Did you bring the scope from a warm place out into a freezing night? If so, you probably had severe tube currents (tube currents = warm air rising and cold air falling inside the tube). This kind of thing will produce plumes which show up as elongated stars. Did you try looking through the scope with an eyepiece? If so, what did you see?

You can always tell tube currents by the fact that the plumes or elongation is always in the vertical direction. Why? because warm air rises. It can take an hour or two for tube currents to subside if the night is especially cold. Nothing you can do about it except to pre-coll your scope before using. Leave it in an unheated garage before taking it out.

Rolando

Just to let everyone know that Roland as usual was right on. I used the Traveler last night in much milder conditions well above freezing where cool down was much less an issue and the stars looked perfect!

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