Dew heater design

[gajjer]

Ok. This is the idea. I have a Skywatcher 200PDS and I want to fit a dew heater. From what I understand the main problem is the secondary mirror.

Certainly that seems to be my problem. My solution is to make a ring that clamps around the stem joining the secondary mirror to the spider assembly.

The ring will be about 15mm long, 60mm OD and 30mm ID. The ring will be drilled to take two 3W heater resistors fitted with thermal paste and/or araldite.

The wires to the resistors will be brought out via two of the spider limbs, then off to a PWM controller.

The ring will be split so that it can clamp on to the stem.

Can anyone see a problem with this or have an alternative suggestions/improvements.

cheers

gaj

[Islander]

It might be better to make a matrix of smaller resistors to spread the heat more evenly across the back of the mirror. There are some design ideas on this site:

http://www.dewbuster.com/heaters-330ohm_resistors.html

[gajjer]

Hi Islander

if you look at the dimensions of the 'ring' you will see that there will be no problem with heat distribution. It's quite a substantial bit of aluminium.

cheers

gaj

[Islander]

If you're happy that you'll get even heat distribution then it should be fine. It only needs to be a few degrees above the dew point to work

[simonmc]

Have a look at post "has any used these.. ". His prices seem good, but also post the instructions up. You could do something similar or buy one of these. No connection by the way.

Simon

[Tinker1947]

Just don't get it to warm, there will be thermals floating out the end of your tube.........

[simonmc]

Just don't get it to warm, there will be thermals floating out the end of your tube.........

Thats true, you dont want other people warming there hands on your secondary..

[gajjer]

I gave the heater a trial run the other night. It appears to work ok, but as yet I haven't got the PWM controller built. As it stands its pumping out about 12Watts. And it gets hot!!

I have been trialing it by turning it on for about 3min then turning it off for 5min ish. I'm hoping to get the PWM sorted over the next couple of days. One time I had a senior moment and left it on much longer.

It got very hot and I was concerned that the glue on the mirror might melt. It didn't, but I'm not risking that again. It also seems to take a long time for the heat to get into the mirror. Not sure if that is because of the thickness of the glass or the insulating properties of the glue holding the mirror in place. I was wondering about putting some thermal 'putty' round the join between the mirror and the bar. Any suggestions would be welcome.

Anyhow. I think it is going to work ok. I'll let you know how I get on when the PWM controller is fitted.

cheers

gaj

[Tinker1947]

You could stick a dew shield on the scope that keeps dew at bay a lot longer so your dew heater can be turned right down...