Thanks Michelle.
This all started with a few of us going out now and then to try and
see an occultation of a star by an asteroid. We have tried this three times.
It seems the stars being occulted are around mag 10. I was having a hard
time seeing these mag 10 stars. Or, I was looking in the wrong place. I've
been thinking more aperture wouldn't hurt. I could use my Obsession 15 but
I would have to unload all my other stuff to make room for the Obsession.
The C11 would also be nice for other DSO observing.
Rich
> 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
>
>> >
> > 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 11:11:44 2005