Heating a dew shield

[icmoon]

Hi peeps,

What do you guys use (if anything) to heat your scope/dew shield?  I have a couple of working vivarium heaters which are set to about 74F and are 7 watts of heat over a square foot.  The scope is a Celestron Nextstar evo 9.25, which I have learned through your goodselves is a 'dew magnet'.  I realise this may actually be not enough heat rather than too much but is it better than nothing or should a buy a proper heater?

Haven't been too well lately & so missed the couple of nice clear skies we had but many thanks for all your help so far, I can operate the scope in my living room now but haven't seen any stars there! I just can't wait to actually get out and use the scope for real.  In the meantime am reading as much as I can, and yes, you guys are right, it is, a very steep learning curve!

Kindest regards

 

Simon (Sash)

[Mick J]

I don't know if a read of these pages may be useful for you,  also on Ian's site are suggestions for controllers to adjust the heat output.  You don't need much heat to keep the dew off.

[MarsG76]

I use a asrozap dew heater kit wrapped about the front of my 8SE just before the 1inch black corrector ring around the front and a dew shield over the top of it... normally I need it only at half way and it does the trick... never have dew problems.

74F degrees might be too much and cause tube currents.. I'd say get a dew heater controller for it, you dont really need too much heat to keep the dew off.. only about 1 degree above the dew point.

[cjdawson]

I made a dew heater for my telescope using a Pulse Width Modulator circuit (sounds impressing, basically a motor speed controller) you can pick these up already made a few pounds on e-bay.  The heater element is made from Nichrome wire which I poked through some heatshrink and put the whole lot into a copper tube.  I then bend the tube into a big hoop which I then press fitted inside the lip of the scope just in front of the corrector plate.   At full power the heater will pull about 3amps of power ish, which is lower than the max rating of the PWM circuit.  I tend to run the header at about 10-20% anyway.  So under normal conditions it pulls about 200-300mA of power.

 

My 8" LX-90 is also a dew magnet, but with the heater the main optics stay dry pretty much all the time.  The dew shield itself will dew up on the outside, it may also freeze on the inside if the weather is cold enough.  However, it will only freeze about 3/4 of the way down.  The heater puts out enough power to stop the optics and the immediate area from dewing up.  And remember I'm talking about when I'm running at about 10-20% power, so there's plenty in reserve should I need to ever turn things up.

[Physopto]

It is not the dew shield you need or want to heat. It is the front corrector glass that should receive the warmth from the dew heater. The idea is to raise the corrector just above the dew point so that dew cannot form. Heating the dew shield is a wasteful method of getting heat energy to the corrector plate. The extra heat energy  required to get heat across to the corrector plate will also tend to cause tube currents inside the dew shield. These currents will help degrade seeing. The dew heater should be fitted as close to the corrector plate as possible on the telescope body, not the dew shield. 

If you cannot do that and there is no other way then the dew shield may be the only method, but a poor second.

Derek

[PeterCPC]

+1 for astrozap heated dew shields.

Peter

[laudropb]

Astrozap dew shield works well for me

[Stub Mandrel]

Is there a rule of thumb that relates dew heater power to diameter or area of glass to be warmed?

I have a 50 watt 240V heater element that is along ribbon of suitable wire that I can unwind and chop up to make 12V dew heaters, but an idea of what sort of power would helpful.

[Mick J]

12 minutes ago, Stub Mandrel said:

Is there a rule of thumb that relates dew heater power to diameter or area of glass to be warmed?

I have a 50 watt 240V heater element that is along ribbon of suitable wire that I can unwind and chop up to make 12V dew heaters, but an idea of what sort of power would helpful.

There is a bit if theory and a spreadsheet in the Blackwatersky page linked above.

[cjdawson]

The heating elements of my own dew heaters is made with this stuff.

http://www.maplin.co.uk/p/28swg-resistance-wire-42-ohms-per-meter-bl64u

All I have is a single loop of the wire that I spoke about in my previous post.     The length of the element for my 8" is about 27" so produces about 2.8 ohm of resistance.  This can pull about 4.28Amps at full power.  Which equates to about 50 Watts.

Using the same stuff for your 9.25" scope...   so you'd use about 32" is wire (81.28 cm) is about 3.4 ohms.  So would work out to be 3.53 Amps at full power, which equates to about 42 Watts.  (The higher restance lowers the current) of power.

Even then, I think that you'd end up running the heater at 10-20% so it would use about 300mA.