DIY Dew Heater Resistor or Nichrome

[Itabot]

Hi, living on the coast dew stops play frequently, getting a really pain. Sorted my LED controller bits thanks to another topic, now wanting to make a heater band to go round to the outside of the tube on my SW Skyliner 200p then fit at a camping mat dew shield over the band.

Can't decided which to go for Nichrome wire, anybody made one successfully for a SW Skyliner 200p and can advise what length and gauge of wire to use, or a 75cm ladder using 47x 330R 1/2W resistors, what sort to use carbon film or metal film etc?

Many Thanks

[Islander]

Why would you want to heat the outside of the tube? You're just going to introduce the thermal currents you've allowed cooling time to eliminate. You shouldn't really be seeing the primary dew up - being at the bottom of the tube offers it a lot of protection. A grid of resistors wired in parallel place on the back of the secondary is all that's normally required or a dew shield made from a foam camping mat around 1.5 times as long as the diameter of the tube.

[leelee970]

I myself used Nichrome wire from RS - works a real treat on EP's, scopes etc keeps dew away all night. Simply sandwhiched between Gaffa tape with leads off to 12V heater unit which i made myself.

24SWG 80/20 nichrome wire 0.20kg

P/N 714 1735

[Itabot]

Why would you want to heat the outside of the tube? You're just going to introduce the thermal currents you've allowed cooling time to eliminate. You shouldn't really be seeing the primary dew up - being at the bottom of the tube offers it a lot of protection. A grid of resistors wired in parallel place on the back of the secondary is all that's normally required or a dew shield made from a foam camping mat around 1.5 times as long as the diameter of the tube.

Hi Islander, bit of newby, thought I would make a Heater Band and Dew Sheild similar to AstoZap's band and sheild or combined, am I getting it wrong.

I myself used Nichrome wire from RS - works a real treat on EP's, scopes etc keeps dew away all night. Simply sandwhiched between Gaffa tape with leads off to 12V heater unit which i made myself.

24SWG 80/20 nichrome wire 0.20kg

P/N 714 1735

Thanks leelee970

[JAS]

I have used nichrome wire for my 200pds and have a dew shield in place. I have still had the primary dew up a couple of times, when I havnt set the heater correctly.

I will have a look tonight, I may still have written down what wire I used.

Jason.

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[leelee970]

Islander is right - you only need (in worse case) to have the secondary wired to remove dew.

I myself have a foam shield on the top half of my Reflector. My Refractor is the one with the wire dew strip on the front lens.

It is still a good idea to have a dew strip for the Ep’s though.

[Islander]

The bands are usually used around the object cell of a refractor or the corrector plate cell of a catadioptric (Maksutov types) or occasionally round an eyepiece tube. With a Newtonian reflector, the primary is well protected by its position in the tube and really shouldn't dew up whereas the secondary is at the top of the tube and more vulnerable to dew. There are a couple of ways of dealing with this. The simplest is to make a dew shield - that may well sort the problem out. Failing that, you can use a dew heater but not around the tube itself - otherwise the heat doesn't reach the secondary itself. Secondary dew heaters are placed on the back of the secondary and the power lead is usually taped to the top of one of the spider arms.

This website gives you an idea of how to make your own:

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

[Islander]

I have used nichrome wire for my 200pds and have a dew shield in place. I have still had the primary dew up a couple of times, when I havnt set the heater correctly.

I will have a look tonight, I may still have written down what wire I used.

Jason.

Sent from my LT15i using Tapatalk 2

Do you mean the secondary rather than the primary, Jason?

[JAS]

No, the primary!!!

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[Itabot]

Thanks Islander..... thought all scopes were treated the same, the penny's dropped, that's why I couldn't find any made for a SW 200p. I'll see how I get on with camping mat dew shield first.

If I do need some heat am I right in thinking a small heater band nichrome or resistor is used on the eyepiece & finder on a Newtonian reflector?

Thanks

[Islander]

Yes, you can buy (or make) small bands specifically for that purpose.

[Islander]

No, the primary!!!

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Blimey!

I've never had a primary dew up even living where I do (right next to the sea and with a typical RH of 70-80%).

[JAS]

I have had the primary mirror dew up the last 2 times I have been out!

I currently have a 13" dew shield with the nichrome wire paced inside it (the dew shield) about 1" from the end of the OTA.

But to be honest I was imaging and didn't adjust the heater as I should have, the primary still dewed up tho even with the dew shield.

I have used nichrome wire for all 3 of my dew heaters, I worked out what I needed by measuring the circumference of the tube and the resistance of the variouse gauges of wire availible and aimed for a resitance of 9.6 ohms.

For example 36swg wire = 19 ohms per meter, so 0.5m would give you 9.5 ohms = 15 wats.

You just have to compare the guages of wire, so that you wrap it around a fixed number of times, not 1.5 times it needs to be once, twice ect.

Hope this helps.

Jason.

[Itabot]

Thanks JAS