dew device (with pictures)
- From: boo@xxxxxxxxxxxxxxxx
- Date: 11 Oct 2005 06:32:36 -0700
Went out to observe last night between midnight and 3am.
Very dank conditions we've been having. Cold air blowing over
the still warm great lakes has produced quite a few 'lake effect'
clouds, especially at night. Just a thin layer of cloudiness to
block out the starlight. Last night was finally clear, but the temp
was very close to dew point (about 43 F). A little fog, but
fortunately I was on top of a hill.
I just put together a small resistive heater for the scope, AKA
the dew device. I was able to test the contraption this morning.
Made a wrap for the finder, the objective dew cap, and a small
'plate' to keep the eyepieces warm.
Dew can be a big problem and annoyance, as anyone who has tried to
observe knows. A bad case a dew can leave behind a contrast destroying
residue, and for a refracter all element surfaces (inc. the ones that
cannot easily be cleaned) can be effected - not just the front lens
element. Those frisky water molecules need to be kept at bay.
As I set up shop I knew the dew was going to be a problem. All surfaces
- the grass, the picnic table, the plastic deck chair - were covered
with a thin sheen of dew. Almost immediately the aluminum tripod legs
began to collect DOO. I hastened to assemble the beta of my heater.
I basically wired about 15 resistors in parallel, for about 3.5 watts
of heat, half of this going to the objective heater, and the other
half is split between the finder and eyepiece heater.
Well, my device helped, but did not eliminate the dew. It slowed down
its formation and gave me 2-3 hours to observe before I was shut
down. Without the heater it would have been more like half and hour.
The finder heater worked perfectly - the finder 'objective' stayed
dry, but the finder 'eyepiece' (opposite, unheated end) got a little
wet. The eyepiece heater/korral was not warm enough and I needed
an enclosure for the plate, so I ended up keeping the eyepieces in
my breast pocket(s) to keep them warm.
The objective dew zapper helped, but I could have used a few more
watts. The middle of the OTA was covered in dew within an hour, but
the objective lens and dew shield made it over 2.5 hours before a
light sheen of dew began to form. When I got home I wired a few
more resisters into the objective heater, increasing the power
from 1.5 to 3 watts.
By the way, Mars was RAAAUGH! I got a good look at it when it was
high in the sky, after 2am. Huge! In moments of good seeing I
was seeing more detail than I can ever remember.
ALuminum foil, electric tape, copper speaker wire, and a radioshack
battery holder make up the dew device. Yeah it's a hack, but it's
functional and cost me next to nothing to make.
Got some pictures of it if you're bored.
Below - all the kit. I used aluminum foil to make the two heater
wraps, one for the OTA, and one for the finder. The circular 'plate'
is for the eyepieces (still needs work). Two coils of speaker wire,
stripped at one end, gator clips on the other. And the battery pack
(4 D cells, 6 volts), with more gator clips.
www.fractalfreak.com/astro/DD4.jpg
The setup.
www.fractalfreak.com/astro/DD1.jpg
>>From the finder zapper, first length of wire leads to objective
zapper (hair included free of charge)
www.fractalfreak.com/astro/DD2.jpg
The objective heater wrap. Resisters wired in parallel. I first
wrapped the resister leads around the wire using a needle nosed
pliers, then soldered them. Check out the three year old soldering
job. 8}. Second wire (gatered) leads to battery pack, after combining
circuit with finder heater.
www.fractalfreak.com/astro/DD3.jpg
In the tray, the eyepiece heater-plate/korral and battery pack.
Wire from finder and objective elements is combined with plate then
battery pack. Entire circuit wired in parallel.
At about 5 watts total heat, and assuming 'D' cells have a 8000 mah
capacity, I should get 8-10 hours of heat from the cells. Going to
switch to rechargable cells.
www.fractalfreak.com/astro/DD3_5.jpg
BTW, there's been this big screech oil that comes and stares and
screeches at me. When I first arrive at this observing spot I
assemble the scope under the starlight (no falshlight) and give
my eyes a chance to adjust. A couple nights ago this owl was
watching me (and screeching at me) the entire time I was there,
and while I was breaking things down the little beast was roosting
in a tree no more than 15 feet from me.
I call it the 'guard owl' as there's no way to enter this park
without this big old owl detecting you. They've got really good
ears, in addition to night vision.
-Eric B
.
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