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[APML] Re: [ap-ug] Dew problems in obs? ...(long)
Marc;
I had your exact problem except year around in my observatory here in
W.Central Fla. I too tried running my Kendrick full volume until
0800-0900hrs on my 6" A-P to prevent dew without success. When I went to
the DewBuster my problems went away.
What's happening is your lens glass is cold, when the 'air' warms up
moisture condenses out of it on to the lense like a glass of iced tea.
Kinda reverse of nighttime dewing where the objective cools below the air
temp. I run my DewBuster at +5deg all night long, shutdown the obs and
leave the dust caps off my 6" A-P and 80mm Guide Scope. The DewBuster, set
at +5dge with its temp probe keeps the glass above dew point without
trouble. Yes you can run the DewBuster all day but I turn mine off about
0800hrs and cap my lenses.
Make sure you have sufficient VDC to run until the morning, check your
lights on the DewBuster in the morning. Leave off your Dust Cover, often
Dew Shields have flocking which absorbs moisture which will condense on your
lens if the Dust Cover is on place.
I see now this is on the Instructions now. Be sure to place your heat
strips 'behind' the lense/corrector cell. This heats the tube allowing heat
to wash the entire surface of the lense/corrector. If you put your heat
strips on the cell the whole cell has to be heated, then the heat travels
through the glass towards the center sometimes leaving a central region
below Dew Point temp, dew condenses there.
Since you mentioned excessive moisture you might consider water seeping
past the lens into the OTA itself. This can cause other problems if you
don't dry it out. Dewing inside your lens or grundge growing on the inside
of the lense. I wouldn't suggest a hair dryer, they sometimes spew oil
/grease. Open the eyepiece drawtube and stretch a 'thin' tee-shirt across
it to keep out dust. Let it sit all day in the sun and wind.
Another trick would to build yourself a 'Telescope Tampon' as I did. I
used some left over Cotton Velevtine used to flock another dew shield. I
sowed up a tampon, ~1" in diameter by 5-4inch long leaving one end open.
Turn the velevtine side inside of the bag to conseal the seams and to allow
the velvetine fingers act like little filters over the cloth's weave. Fill
the tampon with ~1/4oz. of "SiliaGell Crystals" and sow the tampon closed
with a 6-7" piece of string inserted into the final seam. You want the
'crystal' style SilicaGel instead of the bead type. Crystals have a larger
surface area. Stuff the Telescope Tampon into the eyepiece drawtube, loop
the string over the focuser knob and cap the draw tube.
Indicating SiliaGell Crystals from McMaster-Carr
Bottom of page, pg# 592, ~$14 p/lb.
http://www.mcmaster.com/ctlg/DisplCtlgPage.asp?ReqTyp=CATALOG&CtlgPgNbr=592&CtlgEdition=&sesnextrep=426441947004937&ScreenWidth=1024&McMMainWidth=812
One word of caution, when you recharge the SilicaGel DO NOT over cook
it. If you do SilicaGel will loose its abosrbancy and creates dust...joe
:)
>From http://www.silicagel.net/, (wholesale only), slightly edited:
Recharging Silica Gel:
Dependent upon humidity and moisture conditions, recharge as required at
250F for 4hrs.
NOTE: Use of temperatures higher than 300F will cause Silica Gel to
decompose loosing its absorption properties and create dust.
Details and Specifications:
Indicating (Blue) Type: Amorphous silica, synthetic amorphous silica, and
amorphous silicon dioxide, Silica Gel. Indicative Silica Gel particles are
impregnated with Cobalt Chloride indicator. When free from moisture the
particles are blue. As they are put to use, the particles take up moisture,
Silica Gel gradually turns pink and gives a ready indication that it has
done its work and requires replacement or regeneration. The product is free
from foreign impurities such as Chlorides, Sulfates and organic matter and
has high absorption efficiency.
Absorption efficiency:
: Silica Gel can absorb up to 40% of its own weight.
: Above 25% under 50% R.H. and 68F temperature
: Above 65% under 100% R.H. and 68F temperature
Recharging
: Maximum 250F, 4hrs. (Higher than 300F Silica Gel decomposes,
looses absorption properties, creates dust.)
Friability and Dust : Less than 0.05%
Chloride (as NaCl) : Less than 0.05%
Sulfate (as Na2 So4) : Less than 0.25%
Organic Matter (as NH3) : Less than 0.001%
PH : Between 6 to 8
Cobalt Chloride : 1 to 1.5% (in Indicating type Blue)
Average purity : 99.8 or more % Silica (SiO2) on dry basis
Pore Volume : 1.2 to 1.3 cc/gm
Pore Diameter : Average 140 to 150 Angstrom Units.
Surface Area : 500 to 700 square meters per gm
Average Porosity : 60 to 65%
Thermal Conductivity : 1 BTU/sq.ft /hr/F/in
Sp Heat : 0.2 to 0.22 BTU/lb/F
Specific Gravity : 2.1 to 2.3
Hardness : 5 on Moh's Mineralogical Scale
Oil absorption : 1.6 g/g
Percentage of Fe/ other impurities : Nil
Capacity to absorb:
Silica Gel can absorb up to 40% of its own weight.
Moisture from air, any condensable vapors, oil.
Acetylene and other hydrocarbons from liquid oxygen
"May You Go Among The Imperishable Stars"
Joe Mize: jmize@svic.net
StarFields Observatory http://www.cav-sfo.com
Chiefland, FL 29:24'33.4"N 82:51'37.7"W
Moon Phase: http://tycho.usno.navy.mil/cgi-bin/phase.gif
----- Original Message -----
From: "yooody" <yooody@yahoo.com>
To: <ap-ug@yahoogroups.com>
Sent: Tuesday, October 12, 2004 5:28 PM
Subject: [ap-ug] Dew problems in an observatory?
>
>
> Hi,
>
>
> I recently put my Tec 200 apo. on the AP 1200 mount. Two days
> later i found the lens covered with water. It air dried but left
> water marks. Very upsetting. I am not sure how it happened. I had
> left the Kendrick controller on after I closed up for the night. I
> did leave the dew shield extended.
> Since then I have put in a de-humidifier, but this will only work
> down to 44 degrees. I also purchased the Dew buster. The manual
> suiggests leaving it on all day as the observattory warms up it will
> prevent dew.
> I have had several suggestions. How are people handling a
> refractor in an observatory. I live in N.J. so dewing is only a
> problem a few months a year. However it seems capping a scope after
> observing and letting the wrmers on the Dew buster run can only
> cause some water condensation.
>
>
> Thanks,
>
> Marc zukoff
>
>
>
>
>
>
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