Last updated: 15 October 2002

Subject:	Prime Focus With Filters
Sent:	Friday, October 11, 2002 16:10:27
From: (Adrian Vergot)
I couldn't find a solution on your site for this problem, so I came up
with my own and decided to share it in the hopes that others might
benefit.  Thought this might be a good entry to the
"Astrophotography:Some Technical Tips:Prime Focus With Filters"

Prime Focus With Filters

I currently use my ETX70 for prime focus wide field photography, and
wanted to use my nebular filter.  I have a collection of 1.25" filters,
but I discovered that the internal thread on the ETX rear camera port
was too large (though close).  I was going to start looking for a
step-down adapter, but came up with the following solution using good
old "Fun-Tack" (though I think now adays they call it "sticky-tack"). 
It creates a rock solid temporary bond between the filter and the scope
without marring the metal, and which can easily be removed with a simple
twist of the filter.  Here is what I did.

Select your filter, and wad up a small ball of fun-tack as shown.

prime focus filters
Next, roll the ball into a rope about the size of the circumference of the filter.
prime focus filters
On the threading of the filter, snugly wrap the rope around it. Push the fun-tack slightly inward so that it sinks into the threads.
prime focus filters
Next, push the filter firmly into the back of the ETX rear port housing. Excess fun-tack will bulge out from the edges.
prime focus filters
Using your fingernail or small plastic spatula, trim off the excess fun-tack.
prime focus filters
Finally, Attach your T-Ring and camera, and you are ready to go!
prime focus filters

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