Keeping 8inch Schmidt-Cassegrain dryin observatory

8 years 4 months ago #90479 by ezPZ64
On one of your boards I read that a 12inch Dobsonian would need some type of heating in the shed it is kept in when not in use.I built an observatory for my 8inch Schmidt-Cassegrain earlier this year. It is mounted on a concrete pier. It is very dry with good air circulation but I'm wondering now if I need to install some type of heater to keep the optics ok over the winter. Any thoughts anyone?

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8 years 4 months ago - 8 years 4 months ago #90485 by Calibos
Primary mirrors can dew up in the morning as they take longer to heat up than the shed itself. If this happens regularly in your location, what can happen is the daily dew cycle in the shed can cause mold to grow on the mirror and/or degrade the coatings. What people who run into this problem generally do is position a 15 watt light bulb under the primary on a timer to come on at say 4 or 5am and switch off at 10 or 11am. The bulb provides enough heat to keep the primary clear. So all you need in your observatory is power which I assume you already have. Then its just a case of a quick trip to your local hardware store to pick up one of those clip on light fittings, a 15 watt bulb and one of those plug timer thingies. Its basically a plug adapter with a timer that plugs into your power socket and your clip on bulb thing plugs into it in turn. Shouldn't cost more than 10 or 15 euros all in I would imagine.

BTW, IIRC this dew problem is actually more of a problem in the Summer rather than the winter. Dew forms on a surface that is colder than the dew point of the ambient air surrounding it. Your mirror is a big chunk of glass that cools down slowly but also heats up slowly. So in the Winter your mirror cools down slower than the air in your shed. Thus your mirror will generally be a little warmer than the ambient air temperature and dew cannot form on it. However in the Summer, the air in the shed heats up a lot quicker than the mirror and thus the mirror is generally cooler than the ambient temperature. If its below the dewpoint dew will form.

Hopefully someone else with experience of scopes in an Irish location will chime in because what I learned on the subject came from Cloudynights forum which is a US forum. Different climate/humidity conditions over there might mean my advice is not applicable to Ireland.

Keith D.

16" Meade Lightbridge Truss Dobsonian with Servocat Tracking/GOTO
Ethos 3.7sx,6,8,10,13,17,21mm
Nagler 31mm

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8 years 4 months ago #90487 by Seanie_Morris
Replied by Seanie_Morris on topic Keeping 8inch Schmidt-Cassegrain dryin observatory
Another little tip is to simply keep the air circulating. By doing so you give condensation very little chance to develop. A series of low power fans (not unlike what you'll find in a PC power supply or similar, only 12 volts) would do the trick. You would notice such a low power fan often affixed to the back of a primary mirror in any Dob/Newtonian telescope.

Seanie.

Midlands Astronomy Club.
Radio Presenter (Midlands 103), Space Enthusiast, Astronomy Outreach Co-ordinator.
Former IFAS Chairperson and Secretary.

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8 years 4 months ago #90491 by Calibos
Opp's :blush: I saw the word dob and ran with it. I now see that what you were saying was you have heard about dob/newt owners with this problem in observatories and are wondering if you need to worry about this with your SCT.

Well you don't need to worry about your primary dewing inside your SCT as long as you cap off the drawtube because its inside a reasonably airtight OTA. The corrector plate on the front can dew but I am not sure it is as much of a problem as Newt owners experience with their primaries because although it is made of glass like a mirror its nowhere near the same thickness as a mirror and thus heats up and cools down a lot quicker than a primary mirror. It can still be a problem but it probably takes a more rare confluence of temp/dewpoint/humidity conditions for it to happen. Dave Grennan or Carl O'Beirnes would be the men to anwser this one as they have SCT's in observatories.

Keith D.

16" Meade Lightbridge Truss Dobsonian with Servocat Tracking/GOTO
Ethos 3.7sx,6,8,10,13,17,21mm
Nagler 31mm

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8 years 4 months ago #90494 by ezPZ64
Thanks Calibos for your comments in this matter.Much appreciated. I think I might go with the idea of a bulb coming on on a timer, even though it is an SCT we are talking about. It would help keep the damp out of the air and certainly won't do any harm. Reassuring to know that SCTs haven't the same problem with dewing as Dobs. Thanks again.

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8 years 4 months ago #90495 by ezPZ64
Thanks Seanie for the advice. Great to have somewhere to go to ask questions.
I have a long list of them !
The following user(s) said Thank You: Seanie_Morris

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8 years 4 months ago #90510 by DaveGrennan
Replied by DaveGrennan on topic Keeping 8inch Schmidt-Cassegrain dryin observatory
The important thing is to keep the air inside the tube dry. My solution to this problem was to find a metal tube, about 1.5" wide. Take a well washed nylon sock (not new) shake the hell out of it to remove any loose fibres. Drill lots of holes in the tube and place it in the sock. Fill it with silica gel beads (you can buy a huge bag on ebay for a fiver). Pull the sock tight around the tube and seal the top with a cable tie. When the scope is not in use, leave this in the drawtube and close off the drawtube (I just leave my camera inplace).

This has the effect of drying the air inside the tube. It also gets over the problem of the SCT dewing up on the inside of the corrector plate. This sort of dew is much more difficult to remove when it happens.

The thing to keep in mind is when you take the silica tube out while using the scope keep it in an airtight ziplock bag whith most of the sir squeezed out to stop the silica gel getting saturated. You need to recharge the gel every few months. You can dry it out in your oven for a couple of houyrs at 150C but given gel beads arew so cheap I just replace. This method works a treat for my C14.

Regards and Clear Skies,

Dave.
J41 - Raheny Observatory.
www.webtreatz.com
Equipment List here

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