A built in dew heater for the DSI Pro / filterwheel

[kinetic]

I decided to make an internal dew heater on my DSI II Pro imager.
My DSI has a homemade peltier cooling (http://www.iceinspace.com.au/forum/showthread.php?t=52855) permanently attached.
It is also permanently fixed to my homebrewed filterwheel (http://www.iceinspace.com.au/forum/showthread.php?t=36956&highlight=homemade+filter+wheel).

Any dew heater added in the past has taken ages to be effective when
wrapped around the nosepiece (external) Even with Alfoil and
decent wattage thrown at the problem. Far too much metal is
heatsinking away the effectiveness.

So I decided to test a few ways to incorporate something internally to
what is my favourite camera.

I made two formers to hold resistors of various values in a circle, as close
as possible to the front window of the CCD. (PIC 1)
Firstly I tried an aluminium former, the idea being to wrap the risistors
on the outside of it in a shallow rebate and Araldite them in.
Conductive heat transfer was very minimal. Most of the heat was lost
by transferring straight to the backing mount of the filter-wheel and
then to the DSI casing metal and nosepiece.

So I tried a nylon former, the idea being to place the resistors INSIDE the
former and to radiate any heating effect directly into the CCD/ nosepiece
chamber (the chamber is sealed by the Baader filter in the end of
the nosepiece).

This worked extremely well. On a recent night with the CCD at 3.5C and
dewpoint at 11C , dew instantly formed on the chip.
When the resistors were switched on to my PWM 12v the dew was gone
in about 10 mins.
I then wound the PWM back to almost minimum to see if the dew started to reform.
No dew in 2 hours. This tells me i could probably feed in
6v to the PWM occasionally.

Steve

[peter_4059]

That should do the trick Steve - how many Watts are you putting into it to keep things clear?

[DavidU]

No end to your skill ! veeeeery nice work Stevo.

[rcheshire]

Steve. Fantastic. That is very similar to a project I have in mind. I can't fit resistors in the space and plan on glueing Nichrome wire to aluminium tape and sticking that to the glass, then covering with black tape.

I will PWM a MOSFET with a microcontroller to keep the glass 2C above dewpoint. I need to be careful with heating so as not to fracture the glass. What is the thickness of your filters?

Many thanks

Rowland.

[kinetic]

Pete, I can't say what will work from high humidity QLD, I can only
tell you what worked for my location.
I know you have a similar DSI casing metal cross sectional area so
this may work just as well for you...dunno...anyway:

I tried 4 x 120 ohm resistors (1/4 Watt) in series on 12v at 100% duty PWM.

I calculate 25mA and 75mW. Well under the 1/4 W rating of the
resistors. This made no difference on the dew on the night I tested it.
Gentle heat given off at 12V was almost undetectable.

I then tried 4 x 68 ohm (1/4W) in series, 12v / 100% duty cycle PWM.
I calculated and measured 45 mA and 130mW power.
I could feel this heat, both before Aralditing them and after.
This worked well.
Keep in mind, this was radiating heat into a chamber the volume of a 35mm
film canister roughly. (the DSI nosepiece with Baader IR filter at the end)




Rowland,

I'm not too sure on Nichrome, it sounds too much like taking a sledge hammer to the problem? :shrug:

This much power/ current draw might be necessary in high humidity areas
but it seems once dew is kept at bay before it forms, as you say, keeping
the area only a few degrees above the D.P. should be enough.
Unless of course it is very fine Nichrome...higher ohms /m:shrug:
Resistors seem to work fine if you keep under their power rating.

Dave, thanks mate:thumbsup:

Steve