Re: ATM 10" Figuring progress: Job DONE

[astillma@nit.AirTouch.COM (Anthony Stillman)]

Jonathan Lavoie wrote:
...
To help me chosing my diagonal and eyepiece size, could someone explain
me the relationship between field of view, apperant f.o.v, vignetting,
and all that stuff.
...

Selecting eyepiece sizes

Most 10" instuments I have seen recently use 2" diameter eyepieces.  There 
is no doubt that these are nice, but they are also expensive.  If you go 
with a 2" draw tube, but want to save money, consider using an adapter 
(2" to 1 1/4") and then some used 1 1/4" eyepieces.  You may be able to pass
these on to others when you have a set of 2" oculars.

Apparent field

For eyepieces this is given in degrees and means that with your eye at the 
apprpriate position you will see a field which subtends the specified number
of degrees.  Apparent field is basically a function of design.  That is, 
eyepieces of a certain design type (Ramsden, Kellner, Erfle...) each have a
different apparent field, and don't vary too much with focal length and 
implementation.  Wide undistorted fields are nice.  Negler eyepieces are 
great for this, and alas rather expensive.  The closest I've come are a two
used super-plossels, with which I happily star test my telescopes, only, really 
ask anyone.

Unvignetted field

The field can be vignetted by a number of surfaces.  Starting from the top:
   the front end of the tube
   the secondary
   the draw tube for the ocular
   the ocular
   your iris
All of these must be thought out.   Make sure your tube is large enough, 
given its length, to accept the field you plan to have.
Make sure your secondary is large enough to accept the field you plan to have.
In simple terms this means the length of the light path from the secondary to
the focal plane devided by the focal ratio then add the size of the un-
vignetted field you want.
Example:  10" f/6,  12" from the secondary to the focal plane, 1" field
          12/6 = 2   2+1=3  
          a standard diagonal size is 3.1"
The draw tube, when fully inserted might enter the tube a 1/2" or more.  
Using the same steps as before:
Example   10" f/6   6" from end of the draw tube to focal plane, 1" field
          6"/(f/6) = 1    1+1=2
Now you see why 2" eyepieces are used on large telescopes
This same process can be used for the ocular.  It doesn't do any good to 
have a 1" field if the longest focal length eyepiece you plan on using has
a 1/2" field stop.   But then again you may want to take pictures, ah, of test
fields.

Lastly, the iris can only open so wide and this effects min. useful 
magnifacation.  Specifically, the magnifaction times the size of your iris must
be equal or larger than your aperature.
Example:  5mm iris (~0.2")
          magnifacation 50x    (10" f/6 60" fl => 1.2" (30mm) fl eyepiece)
          0.2"x50=10"
Using a lower power than 50x will result in an effectively smaller mirror than
the one you worked so hard on.
          0.2"x40=8" which is the amount of mirror you would be using with a
          ~40mm focal length eyepiece

I fear the wrath of the list, buuuuuuuut, a vignetted field is not that bad.  
The eye is hard pressed to notice a drop of even up to a 25% reduction in
field illumination.  And as its at the edge of the field, it's even less 
noticable.  ALL, and I mean ALL commertial camera lens fields are vignetted in
the corners and professional photographers don't complain.  If you allow for
a little vignetting you can get away with a smaller diagonal and smaller
diameter eyepieces, draw tube, ....  All of which costs less.

Jonathan, I hope I help you build your fire. To my winged friends, I'm still
properly chained to the rock and my liver has just about regenerated from last
week.   ;-)

Anthony