A fault which put RoboDIMM out of action for practically all of winter 2002/03
has now been repaired. The drive motor unit for the Right Ascension axis has
been entirely replaced using a spare sourced from the UK's Meade importer.
The newly repaired telescope was installed and aligned to first order by
TOs Carlos Martin and Juerg Rey in between carrying out their normal night
duties, between March 12 and 18.
The pointing accuracy of the Meade LX200 telescope started to go wrong
back in November 2002 and this got progressively worse, such that eventually
RoboDIMM stopped taking data in mid-December. Initially the difficulties
in pointing were disguised by cloudy weather and later, diagnosis was delayed
by intermittent wet periods and the vagaries of the Telescope Operator schedule. Much
of the delay in repairing this fault has been due to bad weather during this
period. However, it became clear that the fault was in to the RA motor,
as this became increasingly noisy and eventually it was observed to move
spontaneously without software command. Carlos Martin dismantled the telescope
in the first week of February and offline tests on the 'motherboard' and
components of the RA drive motor began soon afterwards.
Aperture Mask slimmed down: almost 1 kilo lost!
The fault in the RA motor described above may have been precipitated by
excessive loading on the telescope tube. The drive motor on our (old) LX200
is the same as on smaller 10" and 8" models so it is probably already overloaded.
On top of this load, we were using an aperture mask weighing about 1.5kg
plus optics of about 650g, plus counterweights and a CCD camera on the focus
end. The counterweights were added after about a month of operation of RoboDIMM
because we found that when the telescope was parked (pointing at the pole
star) during daytime, the telescope sometimes slipped the RA clamp, depending
on how tightly this had been applied.
While the telescope was dismantled, we took the opporunity to mill down
the existing aperture mask to a smaller thickness, and to cut holes between
the exisiting subapertures, in order to reduce weight. This removed almost
one whole kilogram, almost two-thirds of the weight of the mask. All this
weight has been lost wihout affecting the stiffness of the mask, important
for maintaining the baseline dimensions of the RoboDIMM constant. The extra
holes are now covered by light material.
On advice from our UK Meade supplier, the new RoboDIMM will be operated
without any more counterweight than that provided by the SBIG camera at the
focus end.
In the medium term, we also asked to have an even more lightweight aperture
mask designed and fabricated on site. The new mask would also incorporate
3-point clamps for more secure attachment and adjustment of the optics.
Mechanical Improvements on Wedge Mount
As well as reducing the weight of the aperture mask the ING mecahnical workshop,
again under supervision by Carlos Martin, very obligingly machined a metal
base upon which the wedge mount of the telescope can sit. This not only allows
the azimuthal adjustment bolts to be properly used, making for much easier
alignment of the telescope than previously, but also allows greater space
for cables when the telescope moves around. The design of the Meade wedge
mount was also "improved" in this repect, by cutting away square corners
on either side of the cable socket panel, thus reducing the likelihood that
cables might snag.
All in all, RoboDIMM has been so radically improved by these 3 steps that
we should consider renaming the instrument to RoboDIMM 2003 !
