Removing the mirror flexure on an Orion Optics OMC 250

[Astrogeordie]

Note: I published this on UKAI already in July, but I find that maybe there are OMC 250/300/350 owners out there who may be interested in this tuning report.

Several months ago I acquired a 2nd hand OMC250, that sadly suffers from some engineering flaws. The main problem is that the mirror is mounted on a common bush with the baffle on one side and the focuser to the other. This bush rides on a thin back wall of the mirror cell that is perforated for ventilation.

Consequentially the whole contraption bends. When I focus outward, the image walks sideways as the primary tilts. No clue how this could hit the market ... however, I had to find a solution as for example on deep sky photography the different short exposures walked not in a straight line as expected by polar alignment errors, but in a curve determined by polar misalignment plus the gravity vector !

Now I thougth about a stiffening spider to tackle this fault. I want to describe it here as it may come in handy for other owners of the OMC250/300/350 with the same problem.

The mirror cell is held to the tube using four M4 bolts. To remove them, you can reach for the nuts through the ventilation holes. After removal, the whole back assembly can be removed, e.g. to clean the primary.

Here I started: Three of them bolts have been replaced by longer M4 bolts that carry a square aluminium profile. These profiles are equipped with M6x60 grub screws that I found in a well known online auction place. However, studding cut to size would have done the same trick.

I got a flange made (sadly it was too large for my lathe, 91mm ID), that has a slack fit on to the cylinder that holds the focuser. Three aluminium square tubes that got reinforced with hammered-in square solids at the ends were bolted to the flange. This forms a spider with the legs riding on the three M6 bolts outside.

This contraption gets fixed with M6 nuts and washers outside, and with grub screws on the cylinder at the inside.

I tested the spider on Dalby Starfest where at first I had to turn it around by 60 degrees as I blocked access to my fnder dovetail ! The primary is now stiff and there is no gravity-induced image shift any longer as this construction sucessfully holds the mirror in place. After the alignment I got rewarded with some breathtaking looks at Jupiter ...

Another hint: To fit the spider, the focuser has to be removed. It is held by three M3 grub screws. If you slacken them, the focuser SUDDENLY drops off ! The reason is that the cylindrical surface the grub bolts hold on to wears no safety groove. I re-did this on my lathe to avoid that one night all my post focal equipment crashes down.

A nice side effect: Now I can adjust the primary ! Within the range it flexed anyhow I now can move it into its optimum position before fixing it there.

With hint to the book in the background of the last picture: Did you like that ?

If any OMC 250/300/350 owner has the same problem and needs advice feel free to contact me.

[m.tweedy]

It is nice to see you posting on this forum sharing your great knowledge with us.

Cheers Jurgen.

Look out Kielder

[Astrogeordie]

Hi Michael,

thanks for your nice comment ... looking forward to Kielder now. Will be there twice - first for my holidays, then the star camp.