Re: ATM A Cage as Secondary Holder

["KELLY, A" <akelly@ghg.net>]

atm-digest wrote:
> One way to do this is to
> collimate the primary/secondary initially and then do the OA
> orthogonality calibration. Any subsequent collimation would
> be done by moving the entire secondary cage normal to the
> axis of the primary (rather than tilting the primary). If
> the primary mounting is sufficiently rigid the collimation
> tweaking should result in the OA being returned to its original
> location.

Hi Folks,

        Having built several large scopes with Andy and being the frequent
beneficiary of his excellent designs, I need to throw my rare two cents
in on this issue.  I believe that Andy's existing designs work even
better than he thinks they do.  There is no doubt in my mind that the
primary design drivers are to 1) assure that collimated OA orthogonality
(with the OTA mechanical axes) is "built in" to the system to the
maximum extent possible by mechanically requiring the  primary to be
centered within the OTA and offsetting the secondary spider assembly the
correct amount for the calculated secondary offset and to 2) build all
necessary adjustments into the mount so that the optically collimated
OTA can be used to assure the mount's mechanical orthogonality. 
Obviously, if the scope is made to be frequently portable, then parts
construction must be very concise and rigid, so that collimation is
extremely repeatable in terms of OTA and mount orthogonality.  The key
here is, I believe, rigid adherence to repeatable collimation parameters
and techniques.
        The great enemies of good unguided CCD imaging with such systems, if
they are built as described above, are vibrations (wind, any amplified
stepper vibrations, etc.), bearing slippage, and periodic error.  IMHO,
bringing these to heel are more important than the precision
orthogonality issue, although I certainly think it has to be dealt
with.  My 17.5" f4.5 Newtonian that Andy and I originally built in 1982
was modified (by Andy and I) two or three years ago to an alt-az-fp
(focal plane) drive system.  I use it as my large portable CCD imaging
system.  Its basic OTA construction is very rigid and its collimation is
very repeatable.  I find that its use as an effective unguided imaging
system is top-notch.  I have occasionally gone as long as 4 minutes
unguided with no greater than two arcseconds of tracking error, although
I usually stay with 15-to-60-second exposures and stack as many of those
as is required for the desired result.  The limiting factors with this
system are wind and any bearing slippage which may have crept in
(usually due to my own lack of proper system maintenance!)

Al Kelly
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