Rich,
The C11 is a darned nice scope. It's not so big that you couldn't do
exactly as you say. The OTA by itself is very manageable.
Michelle
> -----Original Message-----
> From: sf-bay-tac-bounces@No-Spam
> [mailto:sf-bay-tac-bounces@No-Spam] On Behalf Of rnapo
> Sent: Monday, March 14, 2005 7:03 PM
> To: The Astronomy Connection
> Subject: Re: [TAC] SCT OTAs
>
> Thanks very much folks!
>
> For a long time I've thought a 10" f/5 Dob would be a great
> quick setup all around scope. Now I tend to find it not so
> easy to use a Telrad type finder on a Dob so low to the
> ground. Guess I could set it on a box.
>
> Since I seem to have my AP900 mount with me much of the time,
> if I wanted more aperture than my refractors will give me I
> could pop on something like a C11 OTA. I would love to pop
> on an AP 10" MC but I don't think Roland is likely to make
> more any time soon.
>
> A C11 on the 900 would give me tracking and let me use my
> favorite finders and be relatively comfortable. Heck, I
> could even learn how to use the GOTO on the 900. This
> wouldn't be a quick setup scope but once together it would be
> nice to use.
>
> Rich
>
>
> > Thanks Phil. You know what I think that it really boils
> down to for
> > the manufacturers? They are thinking "cool factor". "What feature
> > can we throw in there to get one over on the competition?" Since
> > there's not too many things that can easily be changed, a
> CF tube is a
> > quick shot. Potential customers drool at the mouth with new
> > technology whether it makes a whole lot of sense... right?
> ;) Maybe, maybe not. I'm not hooked on the pitch.
> >
> > Michelle
> >
> > > -----Original Message-----
> > > From: sf-bay-tac-bounces@No-Spam
> > > [mailto:sf-bay-tac-bounces@No-Spam] On Behalf Of P T Chambers
> > > Sent: Monday, March 14, 2005 12:35 PM
> > > To: The Astronomy Connection
> > > Subject: Re: [TAC] SCT OTAs
> > >
> > > Hi Rich
> > >
> > > I think this is going to be the subject of debate for
> quite a while.
> > > I know of no "real" science that has been used here.
> > > Probably because it takes two identical scopes with the
> exact same
> > > exposure and measuring instruments like thermocouples
> inside and out
> > > of each to do it right.
> > >
> > > However, the differences are slight if any IMO. The aluminum
> > > casting on the front and rear and the corrector area are pretty
> > > large by weight and area and they did not change.
> > >
> > > The benefit of the composite tube is that it does not change its
> > > length with temperature so the focus is more stable over time.
> > >
> > > The overall weight might be slightly different but again, this is
> > > overwhelmed by all the stuff that didnt change such as
> the primary
> > > and the rear casting.
> > >
> > > There is another interesting thing to be noted. SCTs are
> > > notorious dew
> > > collectors on the corrector plate. Of course, as we all
> know, dew
> > > collects because the temp of the object goes lower than the
> > > surrounding air and the air against the surface gets
> colder. Colder
> > > air cannot hold
> > > as much moisture so dew forms. One of the ways to counteract dew
> > > formation is the "dew shield". That prevents the corrector from
> > > "seeing"
> > > so much sky which, because of the black body phenom, keeps the
> > > corrector from cooling off so quickly.
> > >
> > > In any case, insulating the case has the same effect. Black
> > > or dark blue
> > > radiates heat moreso than a light color and aluminum more than
> > > graphite
> > > composite. So the composite case, once reaching ambient,
> > > wont cool off
> > > the air as fast inside which should help prevent dew formation.
> > >
> > > The effect should not be great but should be more than
> the cool down
> > > time issue since keeping the air warmer inside is a lot
> easier than
> > > the amount of heat required for cool down.
> > > (one has the thermal content of the primary and one
> doesnt and also,
> > > the thermal differential (the driving
> > > pressure) is greater for the cool down issue).
> > >
> > > This could easily be the subject of a doctorial thesis on
> > > thermodynamics.
> > > All we need is a Phd interested in thermodynamics and needing a
> > > subject.
> > >
> > > LOL
> > >
> > > There are several folks convinced that the graphite case
> cools down
> > > slower
> > > but in general, I dont believe you can tell the difference.
> > > There is
> > > more variation caused by where you set the scope up
> (which side of
> > > the MB parking lot, for instance) than there is in the case
> > > material.
> > >
> > > It is not worthy of consideration in buying a scope. The
> graphite
> > > ones should be more dimensionally stable.
> > >
> > > One thing I have wondered about is the length change with
> temp. In
> > > terms of - ok, you put a losmandy aluminum track on the top and
> > > bottom (top for a refractor guide scope, bottom for a
> weight set) -.
> > > Now you have a situation where the case doesnt want to expand and
> > > contract much with temp differential but the tracks expand and
> > > contract a whole bunch. There are some stresses that have to go
> > > somewhere.
> > > With the aluminium case, since thermal expansion is two
> dimensional,
> > > the expansion is the same for the tracks and the case and
> the only
> > > stress is longitudinal in the bolts that hold it together.
> > >
> > >
> > >
> > > ---------
> > > Phil Chambers [ptchamb@No-Spam] (S.F. Bay Area - Calif. USA)
> > >
> > > On Mon, 14 Mar 2005, rnapo wrote:
> > >
> > > > Hi folks,
> > > >
> > > > Celestron makes SCT OTAs with aluminum tubes and carbon
> fiber tubes.
> > > > The carbon fiber tubes look great. I've heard cool
> down times are
> > > > better with the aluminum tubes.
> > > >
> > > > What do you folks think? What are the pros and cons of the
> > > aluminum vs.
> > > > carbon fiber tubes?
> > > >
> > > > Thanks very much,
> > > > Rich
> > > >
> > > >
> > > >
> > > >
> > > >
> > >
> >
>
>
Received on Tue Mar 15 10:37:29 2005