From: johno@sdd.hp.com (John Ongtooguk)
Newsgroups: sci.astro
Subject: Re: Purchasing a portable telescope
Date: 19 Dec 1992 00:50:35 GMT
Organization: Hewlett-Packard, San Diego Division
Distribution: world
NNTP-Posting-Host: hpsdlg10.sdd.hp.com

In article <1992Dec15.205703.2888@cs.wayne.edu>, mje@pookie.pass.wayne.edu (Michael J. Edelman) writes:
|> 
|> My 8" Coulter is not useable over 40x on
|> planets without masking down to 3"

[Mike suggested that I post this email response to the group.]

What follows is not a defense of Coulter rather it's an attempt
to help you get more than 40X out of it. The Coulter is my 1st
scope and I've only had it a year now.

From your responses it appears that most of your experience is
with smaller scopes, scopes that aren't as affected by seeing
conditions and scope temperature as a larger reflector. I have
an 8in Coulter and for low power work (to 80X) I just start
viewing but if I'm planning on some lunar or planetary viewing
I usually set it outside for at least an hour prior to observing,
even more if it's cold. Like Texerau says tube currents seem to
be the biggest problem with reflectors. 

Local seeing conditions affect my 8in a lot where heat rising
from parking lots and buildings can ruin an otherwise good sky.
If I view later in the night and avoid looking over heat sources
I can view from my 2nd floor balcony or over a parking lot as
everything has had time to cool down. I do notice when a car pulls
in though as the seeing will not be as good for 10 to 15 minutes,
sometimes chasing me across the street where I do my 'kind of
dark site' viewing in a school yard.

Atmospheric seeing is usually the limiting factor for me as I
know that I don't have a 1/8 wavefront mirror, it might be just
diffraction limited, and I need very good conditions to get a 
good 180X to 210X image (my current hardware limit) at the
eyepiece. Since the beginning of the summer when the bug bit me
hard I've been able to use 210X maybe a dozen times. A smaller
and/or higher quality scope will probably allow better use of
existing conditions but there's always a tradeoff. 

For me a Coulter is a starting point rather than a finished
product and due to it's cost it will probably always be so for
use at higher magnification. I've replaced the focuser with a
helical unit, opened up the slots in the secondary bracket to
be able to collimate it well, replaced the secondary, changed
the way the mirror is mounted while keeping the stock units, and
I usually use a Paracorr coma corrector with Plossl eyepieces.
The Paracorr helps but there is still a lot of astigmatism at
the edges due to the eyepieces working at f4.5. 

The primary is made of thin (7/8in) plate glass and it's
supported on three foam pads stapled to some particle board.
Duct tape is used to hold the mirror to the board with a large
hose clamp around the tape and mirror and the tape is stapled
to the board; a cobby affair that will work if the tape and
clamp aren't stressing the mirror and several foam pads are 
used (I use nine). A Novak nine point floating mount seems to
be the ticket and I'm thinking about getting one. In any case
if you don't like to diddle with stuff you're probably better
off with a refractor or some other better finished reflector.

What can I see ? I can resolve all of M13, on Saturn I can see
the C ring, the greying of the edge on the B ring, Cassini's
division as a sharp line, one belt, and a faint cap on the globe.
On the moon I can see the division in the central peak(s) of the
crater Clavius and the small 'craters' in the Huygens (?) rills.
On one night all of the faint stars in the field at 210X had in
focus diffraction rings around them which danced with the seeing.
The 'double double' is easy and although I've only tried other
doubles down to 1.5" they were no problem with decent seeing. 

Last night I finally got a good look at Mars as the scope was
cool, the seeing very good, and I had some time available. I
viewed from almost 11:30 to 1:00 (PST 16Dec) and sketched what I
could see before comparing it to the S&T and Astronomy Mars maps.
I started out at 180X, added a #21 orange filter and then moved
up to 210X with the filter. My sketches and views matched the
S&T map much better than the Astronomy map, with the exception
that the major dark features had more edge definition during
good seeing than the airbrushed effect (probably due to the 
relatively low magnification used). Syrtis Major and Minor, 
The Tyrrhenum and Cimmerium mares and Protonilus were prominent.
The Hellas, Ausonia Australis and Eridania formed what almost
looked like another polar cap (which was visible on the other
side of the planet). At times the Hadriacum Mare was visible, as
was the Hesperia (?) which almost seemed to split the mares.
I could also see some thin features next to the polar cap but
they aren't shown on the map. I glimpsed a dark spot smaller than 
the Protonilus which might have been the Propontis but my sketch
shows it a bit closer to Protonilus at about the same latitude
on the S&T map. Even more vague which I did not sketch were
some dark areas around the spot which were lighter than it. 
[Probably my error as it was my 1st attempt at sketching].  

I can probably see planetary detail as well as a decent 3in or
maybe 4in refractor, but I can see faint objects a bit better.
Anyway, if you've tried the Coulter after it has cooled down,
been collimated, and perhaps checked with the Ronchi screen that
Coulter provides to make sure that the mirror isn't stressed,
[and you still can't use more than 40X] you have a bad one.
Coulter seems to be good about replacing the mirror if it doesn't
provide straight lines with their screen.

Good luck.

John Ongtooguk (johno@sdd.hp.com)