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Gary,

Your problems are inherent. The corrector cools faster than the surrounding
air and condenses moisture on the glass.

Step 1: Buy an Insulated Dewshield from Richard Just at
ricvic@...
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

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@...
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@...
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

Folks,

I once read a story about someone who woke up the morning after at a star
party and found a puddle of water at the bottom of his SCT tube! I know of
no other scopes who have such a problem to this degree. Perhaps the
manufacturers should put a drain valve at the bottom of the tube. ;-) My
personal strategy is use and location. I try for locations which resist the
formation of dew and avoid leaving my corrector uncovered all night long,
only during actual viewing. I do not leave it pointed up.

I do not think there is anything nicer to look at than a good telescope. I
love to look at them. That is the best part of public parties. However, I
put function ahead of form. I go with what works, and hope and try to make
it aesthetically pleasing too.

The corrector can be easily removed by anyone who has good coordination and
manual skills with delicate and precision work. I always recommend anyone
do the proper research and preparation on anything before hand, but put
simply, one must remove the peripheral screws on the retaining ring, these
thread right into the flange in the front casting--- there is no inside nut
on these. I advise the use of an electronic scribe or pick with a bent tip
to grab one of the screw holes to lift the retaining ring. I place the ota
in an upright position with the two rear handles on wood blocks (the floor
is good for this). Be careful to make sure you don't let a screw drop onto
the corrector. I trust myself to do this, but you must do things at
whatever level you are comfortable with. By taking the secondary out, you
now have a nice handle on lifting out the corrector.

First, after removing the outer ring, there is a mark on the edge, probably
around 3o'clock, which is the number for your optical set. This matches
with the primary. They are a pair. This is your mark. Make a reference
scratch on the casting to line that number up with. You must replace the
corrector in this position. There are cork shims and spacers. They must be
removed and replaced as you find them. They prevent lateral shifting of the
plate. The corrector will probably be stuck to the cork shims of the
backside of the glass (these cushion the pressure from the retaining ring
against the metal flange which holds the corrector. It might take
considerable pull before it lets loose, and when it does it can be sudden
and with no warning. The retaining screws must not be overtightened during
reassembly.

This will get you inside the tube. While I am not endorsing that everyone
reading this go out now and take their SCT apart, it can be done, if you
want to and take the necessary care. I have provided all of the salient
information to do this. However, I must also say that in attempting this,
there are many potential risks, and you are playing with the very life of
your scope. If you screw up or have an accident, you might trash your
optics set. You must be willing to accept this risk and trust your skills
before attempting any of this.

This is why many urge you not to do anything which might cause an issue
inside the scope, like blowing around a bunch of dirt. Dirt by nature finds
itself into the most remote and troublesome places given the time. However,
as an owner of an SCT and other cats, I found that I am much happier with
them knowing I am their master, and not that they are mine! I hate "black
boxes!" ;-)

The air purge is only a last resort when you find you have serious internal
moisture which cannot be cleared otherwise. The air must be warm and dry,
and the scope must be warm too, not chilled below ambient. The primary
baffle will direct the air 75% of the way to the corrector before exiting
the baffle, and it will tend to go on through and out through the hole where
the secondary normally is. Holding a dew zapper a few inches away from the
rear port should provide a very gentle stream enough for just a slow
exchange of air inside. It might take 30 minutes to dry out the interior.
Any cheesecloth or other filtering agent used to trap dust will extend the
time further. Personally, if I had this problem, and I have not, my first
measure would be to bring the ota inside and let it slowly warm up to
ambient, with the visual back off and rear port open, and the secondary
removed (being careful of the mirror surface), with the scope in a clean,
calm location. This might be all you need to get the interior dried out
overnight. I would then seal it back up, and put some dry dessicant (packs)
in the back of the rear port (maybe tied to a thread hanging out through the
flange) to further remove any moisture. This way, no one can move the tube
unknowingly and send the dessicant further into the tube where it shouldn't
go, and with the interior air sealed in mostly, it would be very unlikely
that you would get further dewing up inside, no matter what the outside
conditions, on your next viewing session.

It wouldn't hurt to look around inside regularly and assess the inside of
your scope for changes in the primary coating, dirt, flaked paint maybe, or
deteriorating tube stiffeners. I've had serious problems in the latter. No
matter what your scope, one should never touch the optical surface unless
absolutely necessary. People who obsess over dirt and clean their lenses or
mirrors regularly are not taking good care of their telescopes but are
harming them with microscopic scratches. Only when a foreign agent
threatens to attract and leave a permanent mark due to chemical reaction
with air-borne compounds do I attempt this. With great care, you might be
able to clean only once every 5 years or much longer.

Wayne E. Gondella
AFA Engineering Company
Pittsburgh, Pennsylvania