Re: [ATM] Looking for information on a telescope design

["Scott Milligan" <starzkey@charter.net>]

Ok Clyde, I think I understand that you may have evolved the design since I
first saw it at Stellafane.  I'm thinking that the reason you can't use
eyepieces longer than 26 mm is that the shadow caused by the central
obstruction becomes too large in relation to the useful exit pupil, which
for most of us has an upper bound of 6-7 mm diameter.  

Still, the Mersenne does get the eyepiece down at a comfortable location,
and the fact that each element in the system is well-corrected for
aberrations will make the ensemble much less sensitive to alignment errors.
The all-spherical relay cass solves the pupil imaging problem quite nicely,
but the large spherical aberration present in the airspace between the
primary and secondary mirrors means that both mirrors are quite sensitive to
position errors (decenters and tilts), so it would seem that indeed there is
no "free lunch" to be had in the world of telescope design.


Scott Milligan
 
 

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Clyde Bone
Sent: Thursday, August 30, 2007 4:40 PM
To: atm@atmlist.net
Subject: Re: [ATM] Looking for information on a telescope design

Scott:

 

If the Mersenne design is constructed so that the front element of the
imaging instrument is as close as possible to the secondary, the finder
field is easily masked out.  I use a circle of black posterboard behind the
secondary.  This is impossible with the Gregorian configuration due to the
extra distance.

 

I welcome pros & cons discussions when they contribute to improvement.  The
Mersenne has some problems, i.e. I cannot use an eyepiece longer than 26 mm
focal length.  Longer eyepieces start to make a dark hole in the center of
view. My favorite is a 20mm Nagler II.  Finding an object is difficult for
me if the telescope is pointed very near the zenith using the Sky Commander
because the mirror arrangement creates direction confusion.

 

If designed properly, the Mersenne has no aberrations to third order and the
field is flat.

The problem that the imaging element has to handle wide angles projected
from the secondary mirror is easy to solve-it does not exist.

 

I do not know where " This means that the imaging element in the Mersenne
arrangement will have four times more coma and sixteen times more
astigmatism than it would have if used alone" comes from, but it is totally
in error.  There is no coma or astigmatism to the edge of the power field.

 

Clyde
_______________________________________________
ATM mailing list http://www.atmlist.net/

_______________________________________________
ATM mailing list http://www.atmlist.net/