What is best RH PERCENTAGE for a Humidistat controlled - TARP Dew prevention

Joe Zeglinski

Hi Rolando,
    With the recent discussions about eliminating excessive dew formation on the (CP3 to CP5) controllers, and everything else under a scope tarp, I thought I would (finally) ask THE EXPERT,  master optical craftsman, just ...
what should be the “ideal Relative Humidity Level” for Optics and Electronics, while covered  in outdoor storage (tarp or unheated observatory) ???
    As you recall, I designed my own heater control, based around a Humidistat, with its  “dialled-in PERCENTAGE humidity level sensing”. So, I have total control on how much and how long,  the telescope sits soaking wet, when not in use.
    Ref. the Forum FILES section:

“An - Un-Dewer - Keeping Dome and Scope Dry.pdf “.

A home-made $60 device to Control dew formation inside the Dome or under the telescope Tarp

    For almost a decade now, I have kept the BROAN humidistat, around which  I designed my environmental dew control, set at an  ... Arbitrary 60% RH.
  The cycled 900 watt heater, (I chose),  keeps everything, mirrors, lenses, and electronics, and my (previous AP-900 and now  AP-1200)  mount itself, perfectly dry under the nylon tarp’s air bubble, in all seasons, and any storms.  The BROAN controller cycles ANY chosen AC-powered heating device, within its  deadband operation of about plus/minus 10% RH, so that the heater gets some “hourly respite” (sometimes shorter on frigid  –20 deg.C nights). from otherwise continual heating .
    No need to keep the backyard cozy,  when it is under a foot of snow !   :-)
UN-DEWER and CAR WARMER under TARP TEST--IMG_6305--IMG_6334--
    It  quickly dries up a soaking wet telescope system after “tarping” everything, then restarting my Un-Dewer. Of course, I switch it off during actual telescope use,  since I have an LED bulb on its controlled AC-power section, lighting up the tarp to monitor it from the house, when it is actually heat-cycling.
    Even though it rarely, if ever,  turns on during the day, and cycles itself on for about 20 minutes of an hour under worst weather & outdoor temperature conditions ...
     I wonder WHAT the IDEAL & SAFE Relative Humidity Level should be for optical mirror and lens coatings ???
    Perhaps keeping the tarped scope at an average 60% RH (peaking upto about 70% to 72%, before cycling itself  off), may even be overkill.
So knowing the best RH setting, would maintain the quality of optics and electronics, and still save more energy from being needlessly wasted, year round.
    Thanks for your thoughts on this topic, Rolando,
Joe Z.