Dealing with dew/frost on eyepiece and front objective

[KirkD]

Last week a rare clear night was in the forecast so I took my Celestron 9.25 Evolution outdoors a few hours ahead of time to cool down, as well as my eyepieces. The temperature was several degrees below freezing. A few hours later, after dark, the first thing to do was to use the app to point to three different stars so it could track. But my eyepiece immediately started to frost over (Brandon 48mm) such that by the time I had the first star centered and entered, the eyepiece was already so heavily frosted over it was unusable. I also had the celestron dew shield on my telescope, but by the time I had just zeroed the first of three stars, the front glass element of the Celestron also had a light coating of frost despite the dew shield, though not near as bad as the eyepiece.

 

Question: What do I need to do? I'm thinking an eyepiece heater might help. What about the front lens on the Celestron? Do I need a heater there too? Would heating the front element of the Celestron cause optical issues after I've gone to the trouble of cooling it down?

Edited February 20, 2023 by KirkD

[TerryMcK]

Hi Kirk. A eyepiece heater and a front element dew band are both required. The very slight heating of the front element and eyepiece do not affect the optics one bit as you are only raising the temperature to just above the dew point.

I recently saw Astro YouTuber James Lamb experimenting with how warm the air in front of the glass gets (differentially) and it is only about 5 Celsius different. But that is enough.

Have a look at his video 

 

 

He is an Astrophotographer but the experimentation he did in this video applies to visual too.

[KirkD]

A big thank you, Terry, for that very helpful information. I'll be ordering some warmers today so that my next night out with my new SCT is not a bust. Thanks for taking the time to answer my quetion and supplying that excellent video. It is much appreciated!

[Moonlit]

On 2/20/2023 at 3:36 AM, KirkD said:

Question: What do I need to do? I'm thinking an eyepiece heater might help. What about the front lens on the Celestron? Do I need a heater there too? Would heating the front element of the Celestron cause optical issues after I've gone to the trouble of cooling it down?

Not everyone will agree with me on this, but having had the same problem I deduced that:
a) Frost or dew forms when a surface has a temperature below the dew point

b) The dew point is usually below the ambient temperature, and the higher the moisture level in the air the closer the dew point is to the ambient temperature
c) Telescopes and equipment that is exposed to open sky WILL lose a lot of heat through emissive radiation, determined by the "emissivity" of the material. Painted aluminium is very emissive.

So it seems to me that the reason we get dew/frost is that the telescope/eyepieces get to be even colder than the ambient air. When it gets gold enough that it reaches the dew temperature, moisture will settle and cause dewing/frosting.

One way of combating this it to actively heat up the EPs/telescope, but I figured it can be done passively too.

So I bought a roll of "Low-E" (low emissivity) insulator material in two thicknesses, 6.5mm and 3mm, along with some low-emissivity tape to hold it together, and I wrapped pretty much all of my telescope, diagonal and eyepieces in it.
The material is coated in durable aluminium foil on both sides, and is filled with air bubbles in plastic which provide "traditional" insulation from the cold.
In theory this should prevent the telescope/EP from losing heat so that it takes much longer for the equipment to reach the dewpoint, which means less dew/frost. 

I've used my wrapped telescope/EPs for a few nights and have had no dew problems since I wrapped it, but then the humidity has not been that high so I can't prove that it works.

Here is a link to a forum thread I made about this on Cloudynights, with pictures of my wrapped scope/EPs:
https://www.cloudynights.com/topic/850571-tube-outer-covering-to-combat-radiative-heat-loss-and-dripping/?p=12374111