What primary mirror heater do you use?

stephenb (Stephen) · #1 08-08-2010, 08:06 PM

I have rarely had to resort to dew heaters for my primary mirrors in my scopes over the years, probably due to a number of factors (limiting myself to locations and extreme cold conditions), but I am considering a primary mirror heater shortly for my next scope.

My choices are, the common heater strap, and the circular "flat-type" heater, which fits on the rear surface of the mirror.

Has anyone had any experience with either dew heater? Kendricks website state their circular "flat-type" heater is more effective than a side strap.

Also, what about "mixing" brands of dew systems. i.e. a Kendrick dew heaters plugged into a Thousand Oaks Dew Controller. Is the matching of controllers to heaters easy?

mbaddah (Mo) · #2 08-08-2010, 08:13 PM

Hi Stephen,

I was considering at one point getting a dew heater for my primary but was warned that introducing heat to the primary may bring back what you initially tried to remove when taking your scope out to cool in the first place! i.e. thermal currents and ruining your view again...

Is the shroud not proving enough?

pgc hunter · #3 09-08-2010, 10:48 AM

Quote:

Originally Posted by mbaddah

Hi Stephen,

I was considering at one point getting a dew heater for my primary but was warned that introducing heat to the primary may bring back what you initially tried to remove when taking your scope out to cool in the first place! i.e. thermal currents and ruining your view again...

Is the shroud not proving enough?

That's exactly my worry when it comes to heating the primary. It'll warm up the mirror above ambient ruing the views. The heater itself will be giving off its own heat waves aswell. I've never had my primary dew up with a shroud on, even when the rest of the scope is literally dripping. So I can't see it being necessary. It might happen in cold climates like North America or Europe winter, but here unlikely.

astronut (John) · #4 09-08-2010, 03:22 PM

I concur, stay away from primary heaters, unless you going to observe in the snow.A fan against the primary should be enough, if not add a bigger fan or multiple fans.

ausastronomer (John Bambury) · #5 13-08-2010, 05:29 PM

I can also add that in practise I have found that by the time the primary mirror has become dewed up on any of my scopes (both tube and truss), the dew heater controls on all other parts of the scope had long prior ceased being effective. Consequently, any dew controls on the primary would have been purely academic.

Cheers,
John B

Paddy (Patrick) · #6 13-08-2010, 08:15 PM

I agree about a fan doing the job. The fan on my 16" fortuitously directs air across the face of the primary. The primary never dews. At Snake Valley last November on an incredibly dewy night, a 16" lightbridge with shroud but a fan that only blew across the back of the primary dewed up totally, whereas mine with no shroud stayed clear. And it was a very dewy night. Peter modified his lightbridge to direct air across the face of the mirror (see http://www.iceinspace.com.au/63-400-0-0-1-0.html ) and has had no more problems.

Insomniac · #7 14-08-2010, 10:27 PM

Hi Patrick,
What's special about your rear fan arrangement that allows it to blow across the front of your mirror? Have you got some sort of baffle in place to re-direct air currents from the fan?
Chris

Paddy (Patrick) · #8 15-08-2010, 08:46 PM

Hi Chris,

Its really just the shape of the rocker box, the face of which directs the airflow form the rear mounted fan across the face of the mirror. If you look at Tim's article on making the scope, you might get an idea of how it works http://www.iceinspace.com.au/63-324-0-0-1-0.html . I did use an internal baffle and neoprene sheet as per Scott Tannehill's article http://www.iceinspace.com.au/63-400-0-0-1-0.html on my erstwhile GSO 12" dob with very good effect. I didn't have to remount the fan as he suggests as the fan didn't seem to cause much vibration anyway.

stringscope (Ian) · #9 18-08-2010, 09:19 AM

Hello Stephen,

I would be interested in understanding why you feel you need a primary mirror heater.

What type of scope do you have in mind?

On group observing nights in heavy dew conditions I have regularly seen steel tube newts with closed bottom ends dewing up their primaries. My experience with wooden/sonotube scopes is the same and as John B's, if your primary is dewing up, then the conditions are probably so bad you might as well give up anyway. I regularly do (or at least try to) all nighter sessions in conditions of heavy dew and/or frost with wooden truss tube dobs of various sizes and I haven't had a dewed up primary for many years.

Regards,