From: "Mike and Sara" 
To: 
Reply-To: "Mike and Sara" 


Hello!  I've designed a coude system, with the light cone coming out the
center of a side bearing on a dobsonian mount.  The focuser is on the side
bearing.  The center of the primary will be obstgructed by the tertiary
mirror.  I can align the tertiary with the focuser, then the secondary with
the system, much as a newtonian would be, with a black dot in the middle of
the secondary.  Then I can align the primary by tilt and positionally,
depending on the out of focus star test.

Alternately, I can glue the primary and try and align the other optics with
it, though field alignment in such a case would seem more difficult, since
I would not be able to see the center of the primary, and any assymetry or
apparent astigmatism in the star test pattern would mean that either the
secondary or the focuser would be off the center of the light cone, and
that would be more difficult to adjust rapidly before a night of observing.

Does anyone have any suggestions on aligning a primary when the tertiary
obstructs the center of the primary so that it cannot be used during
collimation?  Best regards.

Mike

----- Original Message -----
From: "J&S" 
To: 
Cc: 
Sent: Thursday, July 24, 2003 3:58 AM Subject: Re: ATM Cassegrain Primary
Mirror centering


> Hi Mike,
>
> >I am concerned about how accurately centered the mirror must
> >be...What are people's opinions regarding accuracy of centering a
primary?
>
>      Here's my 2 cents:
>      As long as three conditions are met, centering of the primary is not
a
> big issue (with reservations).
> Condition 1:   The primary can be accurately adjusted to aim squarely at
the
> secondary.
> Condition 2:   The secondary can be accurately adjusted to aim squarely at
> the primary.
> Condition 3:   The baffle tube protruding through the primary can be
> accurately aimed at the secondary, independent of the primary mirror (so
> that optical adjustments of the primary will not affect mechanical
> constraints of the optical path, ie, the baffle tube).
>      As long as those conditions are met, centering of the optics in the
> main tube are of no consequence - with the understanding that uncentered
> optics will usually result in the optical axis not being coincident with
the
> mechanical axis of the main tube, which is no big deal if you are not
going
> to use setting circles on your mount. (That's the reservation.)
>      But from an "ease-of-use" point of view, nicely centered stuff is
> always to easier to deal with when collimation time comes around.
>      For more on mechanical concerns and consequences regarding centering
> and mounting of the optics/baffle go here:
>
> http://home.att.net/~jsstars/cassbaff/cassbaff.html
>
>      Bear in mind that with a Cassegrain (especially with an F/3 primary!)
> good collimation is CRITICAL for best performance. Don't forget how bad
the
> images in an F/short Newtonian are when the collimation is out of whack. A
> baffle tube that is mounted directly to the primary is almost guaranteed
to
> make proper collimation a huge hassle, if not impossible. Been there done
> that many times...
>
> - Jim S.
>

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