Quote:
Originally Posted by plasmodium
I have a question about GEM's and different OTA's - if I polar align say a small refractor on a GEM, then swap the OTA for say a large Newtonian - how much, if any, adjustment of polar alignment is needed.
My first thoughts say, not too much adjestment, but some..... is this right?
Does anyone switch OTA's with any frequency on a GEM???
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Hi Lee,
Depending on what your specific imaging goals are, performing a swap can make
a difference unless you take some care.
As has been discussed, the practicalities of accidentially bumping the mount are very real
as is a mount 'sinking' under additional weight.
However, there are some other practical considerations, which have a basis in theory
rather than be attributed to plain human error or soft ground.
In theory, it is the mount which is polar aligned. However, the fitting of the
OTA to the mount head is usually the weak link in the chain with regards
alignment and pointing issues.
For example, one has to take some care that when the OTA
is substituted, that the optical axis of the scope continues to be at right-angles
to the mount's declination axis.
This Dec to optical axis non-perpendicularity brings about a horizontal (east-west)
pointing shift known as Collimation in Hour Angle or CH for short.
The OTA may also point above or below the mount's nominal pointing axis.
This vertical component to mechanical collimation error can come into play
when you have then used one scope to polar align the mount and then swap
the OTA for another.
These effects can be very significant on some mount/OTA combinations,
perhaps as great as tens of arcminutes on some amateur mounts and scopes.
If the OTA is affixed using rings, these can sometimes be 'fiddly' in this regard
but we commonly also see SCT's fitted to dovetail plates provide plenty of
room for introducing error.
Therefore, before you perform the swap, center a bright star and set the RA
motor tracking and ensure the star continues to stay centered. Then perform the
swap and look through the new OTA. Is the star still centered? If not, one scope
has a different mechanical collimation error to the other when fitted.
The collimation of the OTA's own optical components can also play a part
in this regard. The scope's optical axis should coinicide with the mount's
nominal pointing axis.
As far as mechanical flexures go, most OTA's that are commonly used on GEM's
are sufficiently short in length that what is known as "tube flexure" (TF) is
not discernible. As you know, when you have the OTA on one side of the mount
and the counter-weight on the other, it acts like a big dumb-bell, what is termed
Declination Axis Flexure (DAF). Anecdotally, on many common amateur commercial
mount/OTA combinations, DAF is usually not significant to be of any impact.
However, what is not uncommon is unsystematic, sudden mirror flops on OTA's
with larger mirrors, particularly scopes like 14" SCT's. Also what is more common
in practice with regards flexures (i.e. a movement that is at least partially
a function of gravity) are the camera and whatever coupling is used if the
camera is mounted at the primary focus of the scope. Even attaching a different
camera or eyepiece can introduce a fixed mechanical collimation error, but
if the camera adapter, which is really just a short truss, or the coupling of the
camera into the adapter allows for any movement, then the error will
become dynamic as one moves the scope around.
Always keep in mind that rather than being some fixed point in the sky,
polar alignment is a somewhat rubbery concept. The Earth's atmosphere
and the changing refractive index, which is a function of how far from the
zenith you are pointing and the local temperature and pressure, shifts the ideal
pole. A good compromise is to always align with the refracted pole for your
locale.
The proof of the pudding, however, is always in the eating. The types of errors
discussed above exist in all mounts, to some degree or the other, irrespective
of cost. However, to what extent, if any, they affect your intended goal is probably
most expediently ascertained by performing some imaging. If your results
then fall short of your expectations, then consider some of the issues
highlighted above as possible candidates for the origin of the problem.
Hope the above is helpful.
Best Regards
Gary Kopff
Wildcard Innovations Pty. Ltd.
Mt. Kuring-Gai
