Jump to content


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Observing through the Humidity

observing optics
  • Please log in to reply
5 replies to this topic

#1 ulek11


    Lift Off

  • -----
  • topic starter
  • Posts: 7
  • Joined: 19 May 2020

Posted 29 May 2020 - 11:41 AM

Summer is right around the corner, and living in the Southern US, we have already started to have some of those lovely humid nights we have missed all winter! I've been observing casually for a long time, but got more serious late last year, so this will be the first summer of active and consistent observing where I will be responsible for the viewing experience.  I have already had a couple nights ruined by dew settling in on the finder scope and am hoping to avoid that through the summer.


What are your best tips for handling humidity and high dew points? Do you have strategies for the different elements of your rig (eg: OTA vs Guide/finder scope vs eyepieces, etc)?  Also, what kind of care should you take after a viewing session to protect the equipment for the future?


My personal setup is a 8" Dob, with a RACI and then of course several EP, but I am interested in hearing tips for any kind of setup, or even general tips outside of the equipment (except for moving to a drier climate!).

#2 rustynpp


    Surveyor 1

  • *****
  • Posts: 1,764
  • Joined: 27 Aug 2004
  • Loc: NYC

Posted 29 May 2020 - 12:05 PM

Regarding eyepieces, keep them in a case with the lid closed when you aren't using them. If it's really bad you may want toss a hand warmer or some other heat source in the bag - doesn't need to be much.


When you're done for the evening, shut your EP case while you're still outside and don't open it for several hours when you get inside. You want it to acclimate slowly.

  • Jon Isaacs likes this

#3 tjschultz2011


    Mariner 2

  • *****
  • Posts: 212
  • Joined: 02 Feb 2019
  • Loc: Eldorado, WI

Posted 29 May 2020 - 12:22 PM

I just got a Kendrick dew heater system for my scope and it's been working great so far. You would probably only need their cheaper one and a couple of heater straps to go around your finder and your eyepiece. 

#4 Dean Norris

Dean Norris

    Surveyor 1

  • *****
  • Posts: 1,779
  • Joined: 05 Nov 2008

Posted 29 May 2020 - 05:42 PM

Welcome to CN ulek11,


On my 10" newtonian I made dew shields for the Ota as well as the 9x50 finder, from foams mats. For my Telrad finder a ski cap covers when not in use. 

I do have a fan blowing on the back of the primary mirror which helps as well.

  • Jon Isaacs likes this

#5 N3p



  • -----
  • Posts: 1,290
  • Joined: 25 Jun 2018

Posted 29 May 2020 - 06:59 PM

I use a dew strap to reheat my finder, which is 9x50 .. most of the time, that's the only thing I need to apply heat on with my 8" Newtonian, it happened a couple time that I had to reheat the secondary with a heating strap, mine is installed permanently around the support of the secondary, it works very well and the observation can continue.


Usually I let the gear dry for the whole night and the next day.. with the caps removed and a tissue over the openings then I close the telescope bag the next day with some silica bags inside, similar for EPS.


The key is the heating straps.. it takes 10 minutes to remove the dew from my finder, no touching the lens or anything funny, keeping the EPS in a closed container..


Usually when it's very humid outside.. i don't bother too much getting out.. over 80% RH.. with a ton of mosquitoes.. I miss winter in these conditions.

Edited by N3p, 29 May 2020 - 07:00 PM.

#6 astrohamp


    Vostok 1

  • -----
  • Posts: 144
  • Joined: 01 Jan 2020
  • Loc: Massachusetts

Posted 29 May 2020 - 07:12 PM

Dew heaters on ota, around EP diagonal, and on guide scope when I was using one.  My aluminum eyepiece case cover would tell me when the dew was starting, and how much throughout the night.  Heaters usually on before visual twilight, and humming by astronomical dark.


Eyepieces being used were taken out of their small Pelican cases and held for use inside the larger case enclosing the 4 small Pelican EP cases..  That way I had dry EPs (and some duplicates in their Pelicans) if/when one fogged or got exhaled on.  Opening the big case was much easier, microfiber bedding nested the precious, and those not used left untouched.  End of night the used EPs were left (out) inside the big case which could be sealed shut and all moved. 


In the morning all cases were opened to the sunlight and dryer air to bake off moisture.  Rinse and repeat the next night(s).

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Recent Topics

Also tagged with one or more of these keywords: observing, optics

Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics