"vic20owner" wrote in message
...
Stephen Paul wrote:
Well, now I've done it. I put too many ventillation holes in the mirror
cell
end of my Starfinder Dob, and the star test now shows a poorly supported
mirror. Not to worry though, I'll fix it.

I find it hard to believe that a weak tube could deform that thick Meade
particle board cell, but I suppose it could happen. I know these are
obvious questions, but it doesnt hurt to ask: Did you let the mirror
stabilize before the star test? Have you removed the mirror from the
cell lately? It's good practice to remove and reglue the mirror every so
often because the wood will eventually warp, pinching the mirror. I had
this problem twice with my old starfinder 10" dob. Dont forget to use
stacks of dimes or something else to create a sufficient air gap when
gluing the mirror to the cell.

Thanks for the feedback Tom.

I keep the scope in the garage which is considerably cooler than the outside
temps these days, so I suppose it could be that the mirror was too cold.
I've had the same results twice now though, and once persistently over a
period of more than a few hours wth the fan running full speed. So, although
I could certainly be wrong, I'm not convinced that it's thermal issues. The
star test looks pretty much exactly like that in Suiter's book, Figure 2.5
on page 27.

In the past, tube currents would have the classic plume on one side, where
this is definitely a triangular shaped pattern the closer you get to focus.
If you go far enough out of focus, the Fresnel rings begin to appear
circular. I first noticed this effect after drilling the three inch hole and
installing the cooling fan. Before the fan, I had no such issues, even
though, and unlike Alan Adler's mod, I've had eight 1 inch holes, about two
and a half inches apart, drilled in the tube for quite a while.

That said, I _am_ basing my conclusion solely on the difference in results
after adding the fan, and fan on or off makes no difference.

Of all that you have suggested, the mirror mounting itself being deformed
seem the most reasonable. Certainly, looking at the sonotube, it doesn't
_look_ like there are too many holes in it. Well, anyway, I ordered a UO
mirror cell today, so this weekend's project will be ridding myself of the
enclosed particle board setup. If for no other reason, than to get better
convection.

Stephen Paul