Re: [ATM] Collimating, the eccentric way

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

 Mike Byorick wrote about collimation with a string between spider and
mirror cell:

> At this point, you have a properly positioned spider (relative to the
> mirror's optical axis and to the focuser), and a focuser that is
> 'automatically' aligned in one plane relative to the optical axis, since
> the string/line has established the 'left/right' focuser alignment by
> default, leaving only the 'up/down' alignment (with respect to the
> longitudinal axis of the OTA).

This should work, but as an alternative (possibly easier and even more
accurate) is to do a full collimation, then check the actual position of the
optical axis relative to the OTA opening. Just take a ruler, hold it over
the edge, moving the end until you see, in a sight tube, its reflection
enter the mirror's - measure the distance here from the edge at 4 places.
Now you see if the optical axis is close enough to where you want it - if
not, re-position the diagonal/spider and/or focuser by the required amount
and repeat.


> The 'up/down' focuser alignment can be
> carried out simply by putting a square to the string and aligning the
> focuser accordingly. You are now 'in the ballpark', as David Harbour
> would say.

> I think the above 'mirror-centric' technique may avoid a result where
> you have a perfectly collimated system, but where the 'focuser-centric'
> method has allowed the focuser to remain 'tilted' relative to the
> tube/truss,

The importance of the exact 90 deg flection has been repeatedly claimed and
also debated here - my view is that since the diagonal is flat, this is
fairly unimportant, and I have yet to hear what harm a moderate error could
do to the image. Any takers? There has been made telescopes with 45 deg
between incident and reflected axes, with AFAIK no ill effect (round
diagonal - lower eyepiece position, but trickier baffling and larger
secondary-to-EP distance).



> and subsequent collimation relative to the focuser has
> caused the mirror's optical axis to be skewed relative to the
> tube/truss, such that vignetting (etc.?) may occur.

The check above will also show the margins to vignetting - anyway, if any,
it is easily seen if you look for it into the focuser, as a reflection in
the primary of the outer part of the tube.


regards,

Nils Olof

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