HOMEMADE DEW HEATER Last updated: 19 September 2007

Subject:	DIY Dew Heaters.
Sent:	Tuesday, September 18, 2007 13:03:51
From:	PETER SMITH (psmithuk@btinternet.com)
Latest project is some DIY Dew Heaters for the ETX105 and the SLR lenses
I use with the DSI.

I sourced some 30SWG Nichrome wire which has a resistance of 14.4 Ohms
per metre. running from a nominal 13-13V DC supply this 1 meter length
gives out about 14W. The heater wire has been insulated with heatshrink
(to shrink it down jsut run 24-28V throught the wire after feeding it
through the heatshrink). It was then folded in half and stiched to the
inside of an AE flexible dewshield which has then been covered with
black duct tape. The outside heated end has been covered with red tape
so I remeber which way round to mount it to the OTA. A second strip of
velcro "hook tape" alongside the on already fitted to the AE dew shield
allows for the slight increase in circumference because of the heater
wires.

I run the 1m length throught a PWM heater controller which gives a range
of 30-95% power and this keeps the front element of the ETX 105 between
about 3 and 6 degrees Centigrade above the ambient in the dew shield
tube - haven't tried it in anger yet but will as soon as I get a chance.

Buit a dual channel controller with the second channel being split into
2 outputs - so I can run 3 heaters the main tube on independently
controlled and 2 others of the second channel.

I  have also fitted a 1m wire heater to the 135mm lens I use on the DSI
the smaller thermal mass sees temperature rises between 4-12 C on its
front element.

Hopefully will not have a repeat of the last nights DSI/SLR imaging
session where "DEW" stopped play after a few hours just as things were
starting toi get interesting ! (Well what I mean is I was getting the
hang of the DSI/SLR setup and Envisage)
  
Best Regards,
 
Peter Smith (UK)

And:

Its been pouring rain since I made it so still no real world feedback
about how effective it is . "Bench" thermal testing looks interesting
though.
  
Haing had my first trip to a darker site with the DSI and SLR lenses
brought to an early finish becasue of the onset of heavy dew, I decide
to search the web for designs for Dew Heaters.

I found this information
http://www.deep-sky.co.uk/telescopemaking/tm16.htm ( check out the home
made scopes on the same site - awesome!)

Having found a cheap supply of Nichrome wire on E-Bay  and the PWM
controllers available in the local electronics store....I decide to
spend a few hours putting together a multi-channel dew heater system for
my ETX 105 the SLR lenses and anything else I could find!

The Main OTA/Corrector heater adds a heating band to the AE flexible Dew
Shield - I used 30 SWG Nichromne Wire which is 14.4 Ohms/m. Iwasn't sure
how much "heat" I needed to put into the OTA/Corrector plate area so
decide to initially try 1m of the wire - from a nominal 12V DC supply
this gives out about 12W rising to just over 13W at 13.8V.

The Nichrome wire was covered in 1.5mm heatshrink which was shrunk down
by heating the wire - this needed about 28V to get the wire hot enough
to shrink down neatly onto the nichrome wire.

An additional row of holes were made alongside the stitching at one end
of the flexible dew shield and the now insulated heater wire was folded
in half and laid along this edge - initially it has been stiched in
every inch or so incase the lenght or number of wires needs to be
increased after field trials.

Luckily the AE flexible shield for the ETX105 is slightly oversize so
the addition of another strip of velcro hooks accomodated the increase
in diametr caused by adding the heater wire to the shield.

The wirew was fixed in place using a piece of black duct tape folded
around the edge and RED insulating tape was used on the outside to
indicate dthe heated end of the dew shield.

The controller uses a pair of PWM motor speed controllers used for 12V
DC mini drills. First build had problems becasue I had grounded the
negative output sides of both controllers through the metal case. When
any one controller was on on its own it was fine but switching both on
added together the output of the individual controlls. A quick look at
what was going on showed that the controllers were switching the
negative supply so the outputs had to be floated rather than commoned -
could of used a plastic box but seeing as I was committed to the metal
one Imade some fibre glass insulating bushes instead - The controller
now has fully independent control of both channels. (sorry no pics of
the internals its much too untidy wires everywhere!)

As you can see I like "FUSES". The input fuses protect the PWM
controllers and are fused for the maximum current that the controllers
can safely handle. The ouput fuses are "sized" based on the heaters that
are going to be used...for example the input fuses are 3A and the output
ones are typically 1-2A depending onthe heater "tape" rating

The PWM controllers allow 30-95% range . Bench testing with
Thermocouples attached has shown that the OTA tape can hold the
corrector plate cell between 2.5 and 6C above ambient in still air. On
the 135mm lens a similar 1m tape gives an increase of 4 to 12C above
ambient.

To decrease the output of a heater using Nichrome wire you lengthen the
wire to increase it you either shorten it or use multiple loops in
parallel.

All I need now is a break in the weather to test it outdoors - at the
moment its 100% humidity - its pouring down!
 
Regards,
 
Peter Smith (UK)

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