Hi Wayne, Paul et al.
Thanks for your response. It seemed to me that moisture inside
is unavoidable but I'd not read of this happening to others, so I
wanted to check before I try to improve the situation. Paul's
comments about Forida humidity and his battle with it reminded me
that things could be worse - much worse. It seems to be a common
human weakness - to feel better when one realizes that there are
those who are even more afflicted.
Wayne recommends an insulated dewshield, and as it happens I have
one, a laminated aluminized mylar, fiber glass batt, vinyl blanket
with velcro attachments. I've not been using it because it's huge,
floppy, and I'd need to really cut it up to fit around my Telrad,
mount, and GuideStar guidescope mounting. However, I suppose I could
put some effort during daylight into it and, with a bit of judicious
velco application to the OTA, make it work OK. I must admit that, to
my shame, I seem to be one who looks at the scope almost as much as
throught the scope - and the insulated cover is ugly! Fortunately,
the is less of an impediment in the dark.
Wayne also observed that the corrector plate can be removed and, if
done carefully, even the collimation might be preserved on replacing
it. I'd not considered doing this before, but will now. It seemd
that I've read comments on the group to the effect that the nuts for
the frount housing are on the inside, complicating its removal. I
must look more closely.
What I think I'll try first is a gentle dry air purge, using a pvc
pipe to insert the air well above the primary and pulling the air
with a small fan trough a filter and over some dry silica-gel. The
suggestion of parking the scope with some dry dessicant in the
eyepiece port is a good one, and I'll try that also. Because of the
low roof in my observatory, I can't park it vertically, but the dryer
doesn't actually have to enter the OTA, a screw on cup at the eyepice
should be OK.
So, I've got a few things to try. I do like the possibility that I
could shut off the Kendricks heater - applying heat at the corrector
can't help the optical performance - but cooking the thing with a
heat gun every half an hour doesn't lead to good images either.
Thanks again and Best Regards,
Gary
--- In C14@y..., "W. Gondella" <gondella@s...> wrote:
The fact is that the Kendricks takes a lot of current and battery
power and
causes convective currents at the level needed to heat a C14 as you
indicated. The corrector will be warmer at the edge and colder in
the
center, setting up differential stresses in the glass. Such
heaters are a
last resort when nothing else works. You have all of that to hook
up, wires
and all, and if your battery dies you are sunk. None of this is
needed with
the dewshield I indicated because it covers the entire tube down to
the
finder and out the front by about a foot or more. It is a special,
custom
made shield, fitted to your scope, and of special materials, that
attaches
with velcro and rolls up, and is light. It eliminates the dewing
problem
without all the clutter and power requirements and expense.
Lastly, why
would one want an aluminum dewshield? Not only would it be heavy,
but
aluminum is a great conductor of heat, just the very thing you
DON'T want in
a dewshield. The problem here is the corrector cooling down too
fast! Next
time you dew up, look where the dew is: on all of your aluminum
surfaces
first.
As far as kicking up debris in the tube, I didn't say to stick a
sweeper
hose attachment to the thing! First, the OTA is sealed. Where did
all of
that dirt come from. I've taken many C14s apart and they are always
laboratory clean inside (they are assembled in a clean room). I
said to
apply a gentle heated air, like a dew zapper into the rear port.
Don't stick
it right again the scope, hold it back a couple inches. It will
mostly
blow straight on through and out the front hole with the removed
secondary.
Due to the "venturi" effect, known to anyone with a basic knowlege
of
physics, when the air moves from the constrained area of the baffle
into the
large cavity of the interior of the tube, the speed and pressure of
the air
will dramatically drop. In no way will that "kick up" dirt and
debris in
the tube. It will cause a gentle and slow circulation and
exchange, the
drier air for the moister air. We are not talking blowing a wind
through
the tube. The condensation left in place on the corrector and
secondary
will do far more damage to the optical surfaces than a little dry
air.
What do you think people with newtonians and dobs do with open
trusses out
in the weather? The way you talk, after one night, the optics
would be
done! I doubt even blowing a strong wind through the tube would
cause the
problem you describe. The inside tubes are clean. If any dust is
present,
it will be near the near port where you attach things. Take out the
secondary, and place a gentle vacuum at the rear port if you want
to be
sure. You can also place some cheesecloth over the opening where
the air is
drawn or blown in, if you are unsure. Nearly all the air will go
right out
the hole in the corrector. If you think your insides are THAT
dirty,
everytime you handle the tube and carry it, that dirt will fall and
move
around and end up on the corrector, etc. anyway!
It is a simple matter to remove the corrector and clean and dust the
interior anyway without doing any harm, if you are at all
competent. A
typical newt will see more dirt on its optics in a week than your
C14 will
see in 10 years. But given your typical dirty optics, this dirt
contributes
a small fraction of 1% light loss and scatter to the optic. Taken
together,
it would amount to an area the size of the head of a larger pin.
Look at
the stars, planets or whatever and place a pin in front of the
scope. Can
you see it? Put your finger there? Can you see the effect in the
image?
Try your fist. It will take about that much to seriously alter the
diffraction pattern enough to make a visible effect. That is equal
to a LOT
of dirt!
Think about it.
PS: To they guy looking to choose a C14 over a 12" Meade, if you
are reading
this, buy the C14. It will kill the Meade on the galaxies, and may
have
better images as well. You also get the option of the Fastar
feature. But
don't buy the CM1400. Get a better mount, at least a G11 from
Losmandy, or
an AP 900 from Astro-Physics.
Cheers,
Wayne E. Gondella
AFA Engineering Company
Pittsburgh, Pennsylvania
Date: Fri, 17 May 2002 08:41:57 EDT
From: paulatkinson22@a...
Subject: Re: Question re: Moisture in OTA
Gary,
I found that when I only had one Kendrick heater strip up
front running that the OTA would literally begin to sweat the
farther back
you went towards the EP, like a car left out on a humid night.
Here is
the
solution I found. Remember that the C14 has a huge OTA and is
33" long.
That is a lot of scope. It is very easy to have the air near the
corrector
above ambient, but have the air in the rear of the OTA be below.
This
means
the scope is battling itself to equalize and if it is humid
enough one
Kendrick strip is not enough especially if the scope is point up
towards
the
Zenith. I put a Kendrick heater strip up front around the
corrector plate
of
the OTA and then I have a second heater strip that I place about
the
middle
to last third of the tube. I found that by running it this way
that it
kept
the entire OTA above ambient and didn't allow certain areas to
cool more
than
others. I also found I did not have to run my heater as hard.
Lastly, If
you can use a dew shield do it. Astrozap (no affiliation) now
makes a
really
nice aluminum dewshield for the 14. I believe they are the only
ones
making
one in aluminum.
Hope this helps.
Paul
One last thing, never, and I mean never blow air into the OTA
through the
rear cell! This will kick up and dust or other debris that may
have
managed
to get inside. If any part of the inside is moist, i.e., the
back of the
corrector that is exactly where is is going to stick. Even if it
is not
moist this dust will end up coating your optics. Avoid this at
ALL cost.
Paul
Step 1: Buy an Insulated Dewshield from Richard Just at
ricvic@d...
Step 2: Put away your Kendricks, it can't keep a 14" glass plate
dry.
Step 3: Allow the tube to breath after humid weather and keep
desicant in
the rear port (scope looking up) when not in use.
Step 4: If in doubt, unscrew the secondary retainer ring, remove
secondary.
Be careful of the mirror surface. Blow warm, dry air in from the
eyepiece
port end. This should dry out the tube prior to use. Also, do
the same
if
it condenses up inside again. The seconday is keyed and can only
be
replaced in the correct orientation. If you are careful, you
won't even
need to recollimate.
Hope this helps,
Wayne E. Gondella
AFA Engineering Company
Pittsburgh, Pennsylvania
