Re: (ATM) Coma and Telescope Design

["Patrick Martin" <PAT@css.uoguelph.ca>]

I wrote:

>the secondary at 1/5 the diameter of the primary.  I think I will 
>be getting a JMI NGF-3 focuser (I believe it can lift 2.5 lbs - 
>will this be sufficient).  I have the 14mm and 6.7mm UW Meade 
>eyepieces ..>
>I'll probably be getting another eyepiece or two, perhaps 
>one of the panoptics or Meade super wides(?), to get a true field of 
>view of approximately 1.3 - 1.5 degrees that is 70% illuminated.>...

Chuck Grant wrote:

> Not with a 20% obstruction diagonal and NGF-3 focuser.

> Coma is not something you need worry about on your first telescope.
> The edge of field astigmatism of a Meade UW eyepiece is probably
> much worse than the coma of the primary.  I haven't actually
> ray traced that combination, so I don't know for sure, but I
> find it unlikely that the Meade UW is that good of an eyepiece
> at f/6.5 or faster.

    Thanks to all for the input.  I think I'll stick to the 8" f/6.5 
(unless I get a bigger trunk.)  Chuck's comment "Not with a 20% 
obstruction diagonal and NGF-3 focuser" with respect to my desire to 
achieve 70% illumnination of an approximately 1.4 degree true field  
has me a bit worried that I am not using the NEWT/Vignette/Diagonal 
software correctly.  I'm figuring on a 9.5" inside diameter tube 
that is .25" thick with a .25" velvet lining for controlling light 
reflection (barring input that says I need additional/different 
baffels).  I think I can put the focal plane 2" above the tube with 
the 2" diameter JMI focuser and still have a little in travel (no 
coma corrector to worry about anymore.)  Am I incorrect here?  With 
NEWT I get a 1.25 degree diameter field with 75% illumination with a 
1.6" diagonal, and with Vignette I get a 1.4 degree diameter field 
with 70% illumination.  Diagonal gives results that are consistent 
with these.  I'm working with a pupil diameter of 6mm which would 
allow me to get one of the 25-35 mm eyepieces with wide (~68 degree) 
apparent fields.  Any thoughts, or was Chuck referring to the 
infeasability of putting the heavy 14mm UWA on the focuser with the 
(now unneeded) coma corrector?  Or am I incorrect on the height of 
the racked in JMI focuser above the tube?  I thought I had read it 
was about 1.5" - 1.75".

    Thanks again for the help.
    
Patrick Martin