Collimating a Class Cass

[Stu1smartcookie]

Hi , i am a very happy owner of an 8" Classical Cassegrain , although its not been used in anger yet as the weather is , shall we say , unpredictable . 

I have a question about collimation . I have printed off the instructions that deal with collimation for this type of scope and understand them fully ( after reading them 6 times !! ) . Ok , so , apparently when one of these scopes needs to be collimated i take all extension tubes off the scope and just replace the focuser . Doesn't that mean that replacing the extension tubes after collimation actually mean that the scope would not appear to be collimated ? Surely the scope should be collimated with all the imaging/viewing train in place . It does seem counter productive if by collimating the scope with no extension pieces in place ( i need at least two of these to achieve focus) will , although correct for that set up will indeed not be correct when i use the scope . 

Any thoughts please 

Stu 

 

Edited March 9, 2022 by Stu1smartcookie

[johninderby]

The extension tubes have no real effect on collimation although always a good idea to do a star test to fine tune collimation afterwards.

[Stu1smartcookie]

thanks john.... i was waiting for that answer , so why does the blurb tell us to remove the extension tubes?

[johninderby]

So you can see the white ring around the primary when adjusting it. Remember you will only get the scope near collimated and will need to fine tune the secondary with a star test to get it spot on. 

As the scope comes collimation is usually fine for visual but a bit of fine tining is needed for imaging. 

[Stu1smartcookie]

Don't get me wrong , John ... i agree entirely with your explination . I had a sneak peak last night using a cheshire and it was ever so slightly out , certainly not enough to fret over for visual . It Yes a star test is a must   , Thanks 

 

Edited March 9, 2022 by Stu1smartcookie

[johninderby]

I find a simple coli cap works very well for collimation

The TS 2” LED collimator I bought from FLO went back as I found the coli cap did a better job with the CC.

[Clarkey]

On 09/03/2022 at 12:24, johninderby said:

The extension tubes have no real effect on collimation

Technically no, but.... As well as making the visual collimation a bit easier it also prevents any tilt between the main mirror and the focuser less of a problem. They should be perfectly alight but experience says otherwise - especially for the 'cheap' RC's