Re: ATM focuser placement

[Robert Duvall <rduvall@jlc.net>]

At 11:45 AM -0700 6/12/98, Cho, Wilson wrote:
>Hello:
>
>  Having followed the emails on the focuser placement,
>and how critical the "xtra travel" has to be figured for
>eyepiece focusing, I have a question that maybe someone can answer.

>  How much "xtra in travel"
>should I put into the measurement for
>
>1) a regular length Barlow;
Well, I've got one, but skies won't let me measure it, sorry. No clearing
expected for days ;-(I know it's more than my shorty (see below)

>2) if I use a Shorty Barlow instead.
I have found that the shorty on mt two f/5 systems needs only abot 1/8 to
1/4" in focus travel.

>3) What are the advantages of the two types of Barlows?
>
As far as I know, the shorty is just shorter. It may cost more for if it
uses more 'exotic' glasses to allow less chromatic abberation due to
shortness, but I don't know.

>
>  I don't dare to drill any holes or place the mirror
>box until maybe the experts can possibly provide some
>direction!

Always allow room to move the mirror cell in your design. Then you can
drill the focuser hole, which never gets redrilled- if you err, you adjsut
where you place the mirror in it's box(or at it's end of the tube).

If you have 1/2" in travel, I'd think that should be plenty. If it proves
not to be at some point, changing 3-4 screws/bolts that hold the cell to be
1/4 to 1/2 inch lower in the box should be no big deal.

BTW, if you ever want to do prime focus photography, you need to move the
mirror _up_ the tube (towards the ep) - usually by about 2 inches, the
depth of the camera body.



Robert Duvall
Force5 Telescope Making Resources

Software Engineer, Amateur Astronomer (TN),
and General Scientist (according to my son)
---------------------------------------------------------------
http://www.jlc.net/~force5/    <- Visit Our ATM Pages
http://www.system.missouri.edu/atm/    <- the ATM list archives