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(meteorobs) Rotating Shutters

In a message dated 96-06-14 12:50:47 EDT, you write:

>George, I know this is slightly off-topic for our list, but I'm curious how
>you 
>built and (especially) mounted your rotating shutter? Do you have a separate

>tripod for the rotating shutter, or is vibration not a problem? Also, do you

>have a variable RPM pot on it, or do you have it set at a fixed speed?
>(What?)
>
>Thanks in advance,
>Lew


Lew in regards to my rotating shutter...Well there are several designs.  The
major consideration is finding a motor that will have the right RPM's.  I
personally think the ideal RPM is in the neighborhood of about 1000.  Some
recommend about 750 ish.  Anyhow, finding an appropriate motor was my major
problem.  I ended up getting a 13 inch 3 speed fan.  I tore it apart and left
intact only the wiring, control switches and the fan motor and housing.  From
there I mounted it onto a 2 foot long piece of 2X4.  It's very hard to
describe, but in the end it is quite secured. I have it mounted onto a very
stable Tripod...A tripod that is built for use with a 4X5 camera.  For the
shutter, I cut  a 19 inch circle out of 1/8inch thick door skin.  Again, you
have to use your own imagination in securing it to the shaft of the motor.  I
screwed the control switches onto the 2X4 and secured any loose wiring. I
found that the slowest speed is the only speed that I can use and it usually
is in the neighborhood of about 2,100 RPM's.  A little fast, but it seems to
chop up the medium to very fast meteors quite nicely. I haven't caught any
slow meteors with it yet, so I don't know how to say it will look.  To
calibrate it's RPM, I have purchased an adjustable strobe light.  I check the
RPM's at least twice during the night...at the beginning and at the end.  If
I suspect that I caught something quite worthwhile, I will check the RPM's
again right after closing the camera's shutter and before turning the motor
off.  The heavy duty tripod helps to stabilize it immensely.  But it also
becomes necessary for me to suspend a cord under the tripod holding a 1
gallon bucket of sand. That helps control any wobble.  The most important
thing for stability is to make sure that you cut the wooden circle  very
accurate and evenly. It's sorta like a car tire...if any where along the
wheel there is a slight difference in weight, your wheel is gonna wobble.
 This is corrected with lead weights.  I've once corrected a wobbling shutter
wheel with a small bolt and nut attached to the end of one side of the
shutter.  It works...but I eventually cut out an accurate circle and that
solved everything. Since I usually operate a guided camera, it is necessary
for me to have an adjustable rotating shutter. I also don't want any of the
shutter assembly touching the camera mount to avoid any possible vibrations
to the camera. I have no problems at all during operation...only wished I had
about a 1000 rpm motor is my only real complaint.  I still have my fast one. 

Since I can't describe too well what I have, I do have on hand an article
that I wrote about constructing my shutter  in more details with some
sketches and photographs. One of the photos shows how I use it generally.
 Also in the back ground is a photo of my meteor observatory from which the
top I observe from.  If anyone is interested in getting a photocopy of the 4
pages in the article, just send a self addressed stamped envelope.  The
dimensions of the envelope should be about 9 1/2" X 4 1/2". Normally I would
just send it out upon request, but I never know when I will get swamped by
making that offer anymore. I think a self addressed stamped envelope is fair.

George Z.