Mountain Instruments MI-250

It has been nearly two years since SkySlab was able to take any photos of the night sky.  Notwithstanding this, patience has it’s rewards.  Here’s a gallery of the Mountain Instruments MI-250 acquired in May 2017 for use in SkySlab.

I recently took delivery of a second-hand Mountain Instruments MI-250.  These mounts are famed for being made by one person (Larry Myers).  Not a factory, not a production work-line, but one man with high quality machining tools, painstakingly making and assembling every component (sourced through various suppliers of course) and producing a quality product with a periodic error of between 3 to 5 arc seconds before PE correction.  After PE it is meant to achieve sub arc-second accuracy…guess I’ll see how that works out.

Regardless of the PE outcome, it’s capable of holding 31kg of equipment.  What this means though is that a 12″ scope will be pushing the limit.

As a wise person commented to me on this mount, they prefer to drive their car below the red line, not close to it.  Good advice, which means I will be scaling down the aperture size of this mount to a 10″ scope.  I still hope for fantastic results using a 10″ aperture and an upgrade is something to aim for in the distant future, but I’m grateful that I can at least continue my hobby in the short term.

EDIT 2/7/2017

Turns out that buying a second hand mount is not easy.  Lesson learned: Insist on a video of the product operating before I purchase next time.

The DEC motors were making a lot of noise on testing.  I dismantled the mounts DEC axis and discovered that the bearings not only were noisy but that they were grinding something in them.  One of the bearings had lost a steel plate and was looking rusty, dry grease and was not up to the job.  You can see shots of this in the gallery.  I guess a 10 year old mount that has not been housed well will have these kinds of problems.

Here’s two short videos recording the grinding noise.

I’ve ordered some new bearings from a specialty shop in QLD which will make their way to me over the next few days.  I’ve disassembled the DEC worm drive per these instructions which are accurate and work well with the exception of the oldham-coupler.  For this mount, the coupler was attached to the worm drive by two small grub-screws, no green loctite at all.

EDIT 22/7/2017

Some things have progressed over the last 20 days.

I’ve successfully replaced the DEC bearings, re-greased it with the recommended grease (MP-50 low friction moly paste from Jet-Lube) and adjusted the worm block under full load so that there’s no backlash in the DEC axis at all.  Movement is smooth, quiet and hopefully very accurate (yet to test).  Here’s what it sounds like now (complete with frogs in the background).

The mount is now polar aligned to within 1 arc-second on both the DEC and RA axis according to the polar alignment report on TheSkyX.  However that may be taking into account flexure and mount mechanical errors so it will take a few more nights testing with PEMpro and a neat utility called Sharpcap.

After that it’s testing the periodic error with both TheSkyX and PEMpro to compare results, adjusting worm drives to perform optimally and then a final T-point run for pointing accuracy.

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