Re: ATM help with refractor

[Anthony Stillman <atmer@flash.net>]

>Two refractor questions:
>
>1) How do I figure out the best placement for the baffles?
>
>2) With an achromatic doublet with a stated fl of 1000mm, should I measure
>from the front of the lens assembly, the center, or the rear when figuring
>the focal plane position?
>Scott Berfield


Scott,

To answer your questions I consulted the sage tome "all about TELESCOPES"
by Sam Brown and avalible through Edmund Scientific.  Its three pages
covering refractors note that a 3" f/15 may differ from its intended focal
length of 45" by as much as 1/2" and therefore the focal length should be
measured.  The drawing shows that focal length is to be measured from the
lens center.  That is , halfway between the front and back surface of the
lens.  Though this is commonly how the focal length is given, it may also
be given from the back center or the back edge.  In general its a good idea
to go ahead and measure the focal length.  I do this by mounting the lens
on a long measuring stick.  Finding a distant object (the Moon) and sliding
a card until the image is in focus.  I then repeat this a few times and
average the results.

As for baffle positions, again, "all about TELESCOPES" shows a drawing in
which a truncated cone (the light cone) with its base size determined by
the objective clear aperature and top size determined by the desired field,
which for a 1/2 degree field with a 45" f.l. is about 3/4".  The horizontal
scale is compressed to fit nicely on the page.  The baffles are placed in
the tube every 8" and jut inward just up to the previously described light
cone.  Next, a line is drawn from the edge of the field to the edge of each
baffel on the apposing side of the tube.  If that line intersects the next
baffel and not the tube wall, there will be no glare off the tube between
thoses two baffels.  If that line intersects the tube wall a "glare spot"
will be visible off the wall at that location, and one or more baffels
should be repositioned.

It is also noted that the eyepiece holding tube may truncated the light
cone if care is not taken when sizing it.

Hope this helps.

Anthony