Can telescope get frostbight?

[iborg (Philip)]

Hi All

I was out the back at home until after 1am last night night.

When I was packing up, I noticed frost on the dew shield!

Philip

[xa-coupe (Jeff)]

I cant imagine the OTA getting frosty would do it any favours .. so well done on the insulation!

[leon]

Jeff the answer to your question is No, a bit of frost wont hurt a sealed unit, or any other for that matter, just let it dry out naturally and don't rush it inside to warm it up, it will be fine, I have seen worse case of Frost on Scopes.

Leon

[mental4astro (Alexander)]

Originally Posted by xa-coupe

I cant imagine the OTA getting frosty would do it any favours .. so well done on the insulation!

Jeff, like Leon said, frost will not cause the scope damage.

The reason for the insulation around Phillip's scope is a different one. It is to help mitigate tube currents inside the closed tube OTA after setting up so you can start using the scope straight away without needing to wait for the scope to cool down. You will find more info on this here:

SCT and Maksutov insulative dewshield

This insulation of the OTA will also extend the time the corrector plate of both SCT's and Maks stays free from dew, but will not prevent it if conditions are very heavy with dew. However, there is a way to stop dew forming on the corrector plate that does not require heat. Heat causes more problems than it solves with scopes. Instead, a fan is installed inside the dewshield section in from of the corrector and positioned in such a way that it generates a vortex of air that whirls around inside the dewshield. This keeps the corrector dry, and will dry off dew that has started forming. No heat to distort the glass, no infrared interference, and no loss of energy into the tube of the OTA which does nothing but drain your batteries and introduce heat into the OTA - something that you had sought to remove from the OTA from the very start!

You will find more info on using a fan for dew control here:

SCT & Mak total dew control without heat - a solution!

This fan approach is not limited to just SCT's and Maks. It can be used with refractors too:

Refractor dew control WITHOUT heat - it is possible!


As for drying off frost from optics, I cannot say for certain right now yes or no as I have not have had my scopes subjected to conditions that would see frost form since starting with using fans. But I do have my suspicions that it will after seeing frost form one night when the breeze stopped, and then sublime when the breeze started up again. Remember professional observatories do not use heat to keep their optics dry from dew or frost. They cannot afford to have their optics be subjected to heat and the mass of complications that heat creates with it.

Alex.

[Stonius (Markus)]

Interesting post Alex. I've often wondered about the wisdom of getting a scope to cool down only to heat it back up again.


Might have to play with this as I installing a boundary layer fan on my dob is an upcoming project. I know that's for a different issue, but while I'm playing with a soldering iron and fan control... :-)


I guess the question I have is does the high frequency vibration caused by the fan affect the image? I'll have to do an AB test, but I figure if you can feel the fan vibrating when you touch the scope, it can't be good?



Thanks for posting.

[iborg (Philip)]

Hi


I do have a dew heater inside the dew shield, only on low power. No sign of frost or dew inside the shield.


Plenty of dew everywhere else.


Philip

[mental4astro (Alexander)]

Philip, no dew for certain with your set up, it is still using a heat source. I would venture to say that your rig uses less power too because of the insulation, which is a good thing though.

Markus, no vibrations. Even at 560X, no vibrations. I have a PWM unit connected to my wee 40mm 12V fan, I have never seen vibrations at any fan speed setting.

I have been using fans with my scopes for many years now (dobs longest of all), and I have never experienced vibrations.

Heat does have a place, but it is less than we think is necessary.

Where we set up our scopes is also another factor for dew control. From home we may have no choice but to deal with dew. But if you are mobile, then site selection needs more careful consideration. The dark sky site I use, I've been using it for more than 12 years. In all this time I have seen dew form only five times. The biggest issue I have there is not dew but my breath which can fog up the exposed secondary of my dobs. I could just stop breathing...

[leon]

All Good Philip, rest easy, enjoy the crisp clear nights, nothing like a good frost under your feet when imaging and the grass crackles when you walk on it.

Leon

[iborg (Philip)]

Next time, I'll try with only 1/4 power output for the dew heater. I used ~1/2 this time, probably more than needed.


Philip

[erick (Eric)]

Below zero C temps are great! Stops dew running everywhere. It cannot when it is frozen solid. Just keep heating where it is needed and a heat pack in your pockets for your hands and your green laser pointer in your top pocket close to your (warm) skin. Hands and green laser pointers don't like the cold.