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Can Catseye XLKP auto collimator be used on RC scope ?

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AFAIK the autocollimator is of no use when collimating an RC telescope and having once owned an 8" RC I never found any material supporting or describing the use of an autocollimator for collimating an RC.

The classic autocollimator is designed for use with a single parabolic primary mirror and a flat secondary where the primary alone can form at it's focus point a perfect image of an object at infinity, and this inherent property means the bouncing of an image of the primary centre spot back and forward between primary and autocollimator can be used to adjust for perfect axial alignment of primary, secondary and focuser.

In an RC the primary and secondary are both hyperbolic and a hyperbolic mirror alone can not form a perfect image at it's focus point of an object at infinity, both hyperbolic primary and hyperbolic secondary have to work as a pair, it is possible to position the two mirrors relative to each other so that their principle axis do not correspond but even so an image can be reflected between the primary and the autocollimator making it look as though the telescope is collimated even though it is not.

The weapon of choice for collimating a RC professionally is the collimating telescope and I had the use of an Arriflex version at one time for use with light distributors in medical imaging systems, sadly I had to give it back when I retired and once I sold on the 8" RC I never really needed to buy one of my own: https://www.optcorp.com/takahashi-collimating-telescope-for-refractors-cassegrains-ritchey-chretiens.html

In many ways there is a cross over between the collimating telescope and the autocollimator but in the collimating telescope you only look for the reflection of the collimating telescope illuminated target-face reflecting back off the secondary and reflections back are of no use for aligning the primary, also the use of the collimating telescope requires that the secondary is accurately optically centre spotted and most of the mass market RC secondaries are not even mechanically centre marked.

To get a definitive answer you could try sending a PM to Peter Drew who is a member here and has a lot of experience with collimating all kinds of telescopes, there is also quite a good publication linked below from a manufacturer of RC telescopes. Though written around their own products it does contain much useful information regarding RC collimation in general and describes how to collimate an RC accurately without using any special tools at all and is pretty much how I used to collimate mine although I did use a Ronchi eyepiece to set the distance between primary and secondary correctly: http://www.deepskyinstruments.com/truerc/docs/DSI_Collimation_Procedure_Ver_1.0.pdf


Edited by Oddsocks
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Hi oddsocks, many thanks for your detailed response and clarifying the usage of the auto collimator.

in regards to the Tak scope, I have been less than impressed with it, as it sight tube has a sliding component to focus the sight, and unfortunately the sliding component has an awful lot of slop / play in it, which results in the centre dot being used for collimation process moving about due to the slop.

i also have the howie glitter, so will probably strip down my Rc and rebuild it to see if I can resolve my ongoing colimation issues.



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