From: "Dan Chaffee" 
To: 
Reply-To: "Dan Chaffee" 


John Sherman:

>>You mount the objective near the top (sky end) of the tube. You put a 5"
>>flat at the bottom of the tube. You put a 2" diagonal flat at the top of
the
>>tube (next to the objective), which reflects the light out sideways to the
>>eyepiece.

Mark Suchting :

>I think building a folded system partly defeats the advantages of the
>refractor in the sense that the poorer contrast of the same in a Newt is
>partly due to scattering and light loss in the aluminium coatings, an
>advantage that may be lost in a folded instrument.

I agree. Not only that, but you stand to lose 15-20% of the throughput with
adding two mirrors. One of the joys of  making/using a doublet is the
autonomy of its upkeep; never does it need to go anywhere for a coating
every few years.

You know folks, less than 50 years ago people were thrilled to look through
6-8" refractors, length,color and all; even without star diagonals and
making and having one was a dream for many. It's not like good newtonians
weren't around then; granted  large amateur instruments were few compared
to today's offerings.

The advent of reflectors on Dob/split ring/eq.platform mounts, fast apo
refractors, cats and deep pockets  has  redefined what
 is desireable--let alone acceptable regarding compactness in
scopes today. An f/15 8" doublet  is 120+ inches long and mounting it 
properly will be tantamount  to a right of passage. Personally I think it's
worth it (and mine is 126" long w/o dewshield). There can still be 
more to creative mount making than computer controled systems.
If you spend many hours at the eyepiece, you will almost certainly see a
difference; subtle, but there and not necessarily in the amount of detail
seen. More in the overall presentation. I've seen many handsome reflectors
on their respective mounts over the years, but I've yet to see anything as
elegent as a long focus refractor on a well made mount. Something rather
authoritative about it and inspiring about them. How about
http://www.deutsches-museum.de/ausstell/meister/e_fraun.htm
OK, enough subjective juice.

>A long focus Newt of slightly bigger
>aperture , with small secondary and good  baffling might be a lot easier
>and cheaper  to make than a folded refractor?

Advantages (in addition to the above): no secondary spectrum; Primary not
as prone to dewing over(sometimes a REAL problem, even w/dew shield);
figuring a paraboloid is more interesting than figuring a lens; you won't
sweat bullits wondering if the glass you ordered is really what the melt
data says it is:-)

Disadvantages (over folded refractor): much smaller fully illumated and
diffraction limited field, open to tube currents; spikes or glow from
spider on bright objects; more sensitve to collimation
-although there's less to collimate; mirrors get dirty faster from
exposure; you remain ignorant of lens work:-)

Dan Chaffee

--- BBBS/NT v4.00 MP