Sean,
Go to the Starmaster Telescopes website at:
http://www.starmastertelescopes.com/
Many of your questions are answered there (i.e., optical quality at f/4 vs.
f/5, mirror thinness, transportability). I have owned a few larger Dobs and
their sizes were all determined by my being able to manage the scope by
myself (one-person setup, operation, and breakdown) and getting it to a site
with my vehicle of the time.
I don't have any problem observing from my 6 ft. ladder while observing. I'm
tall and stand only on the 1st rung while comfortably resting against the
ladder while observing. This posture of resting against a ladder is more
comfortable than standing up for longer periods of time at the eyepiece when
you want to gaze at DSO's. A ladder also provides me with a platform to rest
my sketching pad while making my 15 minute pencil sketches. It is no of
consequence to tie down the ladder on top of my SUV for transportation since
it is too long for me to pack it inside along with the 20" Dob.
Truss length, mirror box size, and the other pieces will determine how large
a scope you can transport in your vehicle, so measure all pieces carefully
before placing an order for a larger Dob. Starmaster Dob's have the
advantage over all other larger Dob designs of having a removable primary
mirror in a steel frame. This feature lessens the weight of the mirror box
by 50%. Because of this, I can carry each piece out of my SUV as I set the
Dob up. This design achievement alone allows me to transport and enjoy a 20"
Dob by myself as easily as a 18" Obsession. A 20" Obsession would require me
to use wheel barrel handles and large ramps to roll the much heavier mirror
box assy. out of my SUV. This would make the transport of a 20" Dob with
additional wheel barrel handles and ramps more cumbersome and it would
require me to drive a larger truck.
There are many large aperture f/4 mirrors being made today that offer
diffraction-limited performance limited only by the seeing and your
collimating efforts. If you don't like taking a few moments every time you
set up to collimate a large f/4 (or f/5) Dob, then you won't get that
exceptional performance from it. With the Tectron collimation tools, there's
no difference in time between collimating a f/4 and f/5 Dob. It is only on
occasion that my 20" Starmaster with a Zabuto f/4.25 mirror will offer its
top performance due to the seeing, but the well figured and smooth Zambuto
mirror usually offers very nice views showing little visible coma and
aberrations. Regardless of the seeing, the 20" aperture always provides me
with brighter DSO's at the eyepiece. What you pay for in a primary mirror
is: great figure, superior surface smoothness, and ultimately a high
wavefront Strehl. So buy the best mirror that you can afford rather than
only one of larger aperture, and go with a name mirror maker.
Hope this helps,
Peter Natscher
Monterey
> From: "Sean McCauliff" <smccauli@No-Spam>
> Reply-To: The Astronomy Connection <sf-bay-tac@No-Spam>
> Date: Thu, 11 Dec 2003 10:32:26 -0800
> To: sf-bay-tac@No-Spam
> Subject: [TAC] Questions about Big Dobs
>
> After looking at several galaxies through Robert Armstrong's 20" obsession
> I decided this was a good scope to own. Eventually, when I get a garage,
> I would like to own something like that. So...
>
> What is the limiting factor on how big a telescope one can fit into a
> car (in my case this is a 98 Beetle)? Is it the ladder? The truss poles?
> Is there a big difference in optical quality between f/4 and f/5?
>
> At some of manufactures' web sites they mention 2" or 1.25" thickness
> mirrors. What is the difference between these choices? Durability?
> Does sweat pour down your face every time you need to move a thinner
> mirror?
>
> If this is OT then just point me in the right direction.
>
> Thanks,
> -Sean
>