Latest updates to the Meade Starfinder 12.5"

A while ago I discussed moving the "trunions" on my Meade Starfinder 12.5"
Dobsonian so that the focuser was pointing out the side rather than at the
factory setting of 45 degrees. The reason for the modification was to
improve the position of the eyepiece such that it was more readily
accessible while standing to the side of the tube. At my height of 5' 7",
the factory 45 degree angle rotation of the focuser required that I stand
with the outside of my left foot against the base when observing at angles
higher than say 60 degrees off the horizon (Jupiter for example). I also
needed to bend to one side at the waist in order to get to the eyepiece.
That was uncomfortable; very.

I had also previously updated the focuser to the 2" "all metal focuser" from
Orion. This altered the balance of the OTA requiring that I add 5 pounds to
the rear end (yuck), so I planned to drop the tube in the rocker box to try
to compensate, at the same time that I rotated the tube. I had three inches
to work with between the back of the mirror cell and the center bolt on the
base.

Today I took the plunge and moved them, and the result was even better than
I had hoped. I rotated and dropped the tube 2 inches. The balance is now
perfect with the Paracorr, Telrad and 9x50 RACI and any of my 1.25"
eyepieces. I stll need to add some weight when I pop in the 35mm Panoptic,
but in practice I will just remove the RACI. This is sufficient until I get
down close to the tree tops to my south. It takes an additional 2.5 pounds
to balance this out perfectly for full motion, so I might do something
simple for the big Panoptic.

All of that said, the best surprise about all of this is that not only did I
make the eyepiece position more accessible with respect to the base, but it
is now possible for me to sit in my Starbound observing chair and view all
the way to zenith! Way cool!!

All total, these simple little changes made this scope _much_ more
comfortable to use. But, I didn't stop there.

To improve the cooling of the mirror, I also drilled eight, one inch holes
in the sonotube centered at a distance from the back of the tube, equal to
the height of the mirror surface in its cell. When I did this same
modification on the XT10 I owned previously, I put the vent holes on the
base side, and the fan on the top side. I wasn't thrilled with this
arrangement. I figured that since heat rises, the fan is working against
nature, not with it. So this time I put the holes on the side away from the
base. Now, when I tilt the tube to the horizon, the heat flows directly out
the holes. Of the eight holes that I made, two are further south than 90
degrees when you look at the rear of the mirror cell face on (if you get my
meaning), and so there should be a draft effect. I intend to add three more
holes to fill out the base side ot the OTA and increase the natural
convection and draft effect. I'm thinking this will actually preclude the
need for a fan as a means to gently blow the boundary layer off the mirror.
If I do add a fan (or two), there is plenty of room between the OTA and the
front board on the rocker box, to accomodate pointing at zenith, provided
they are placed into the corners between the front board and the side
boards. So no worries about this arrangement at all. Ideally, convection
will take care of it. I'm really trying to keep this thing cheap and
unpowered.

I suppose I should also mention (I think again) that I gave this scope's
base the EbonyStar/Teflon treatment a while back, which was a major
improvement to its motions.

Stephen Paul
Shirley, MA

Great work! You may just be inspiring me to rotate my dob's tube and
reposition its side bearings---but in my case, I'm thinking of rotating
the tube to get to the 45 degree angle!

The convection idea sounds intriguing. Reminds me years ago of using a
product called MacChimney to cool a Mac. It was made of cardboard and
apparently worked well at lowering the temperature.

Do let us know how the holes work and if you tell the difference. It'd
be nice not to have mess with a fan and electronics of any sort.

Have you flocked the tube yet?

Larry Stedman
Vestal

"Larry Stedman" wrote in message
...

Have you flocked the tube yet?

Once I get everything else worked out down near the primary, that will be a
near future step. The 12.5" does reasonably well under mag 5.5 as it is. I
lose quite a bit of darkness heading south into the light dome of the local
prison, but I have a nice large area around zenith to work with from my back
yard (hence the pressing need to get this thing comfortable for the user,
when aimed there).

Thanks for the response,
Stephen