SCT cooler

matt · #1 16-04-2006, 03:15 PM

Hi guys. Need some opinions/advice. I've recently bought a Lymax SCT cooler for my 9.25. Prob is, the dew in Canberra's as bad as the temp differentials. I've had a read of the instructions for the cooler and it says not to use the cooler in extremely humid conditions or when dewing is heavy. Well, that's evey night in winter in canberra! I'm guessing it's to avoid potentially blowing moist air into the scope tube, but it effectively rules out using my cooler. Humidity at night here starts around 50% and frequently climbs to 90%+ by 4 or 5am.... every night. What do you think? Still use the cooler? Your help much appreciated.

[1ponders] (Paul) · #2 16-04-2006, 03:47 PM

It's a hard call to make if your trying to get somewhere near ambient. I'd think very carefully about it matt. I have first hand experience of what can happen to your SCT in the presence of high humidity. Fungus fungus everywhere.

I know your trying to get the temp down, but I was given some advice the other day from someone who is in a position to know just about everything there is to know about fungus and scts and his advice was to seal the scope completely with a dust/humidity filter. Unfortunately that would preclude using the Cat cooler.

I know that doesn't help you but it's important info you should be aware of.

A quick question about how you use the cooler. Do you only need to use it at the start of the night or do you need to repeatedly use it to keep bringing the temp down?

matt · #3 16-04-2006, 03:57 PM

Thanks Paul. To your question. I think most use it early on to quickly bring the scope to ambient and then might give it another run a little later to help the mirror track any further temp change/s??? Anyone else care to comment?

matt · #4 16-04-2006, 04:04 PM

I guess the next question is: what level of humidity (%) is acceptable for Cat Cooler use???? Gee, if it wasn't tough enough with dew and heaters for the corrector plate etc!!!!!!!!!!!!!!!!!!!!

asimov (John) · #5 16-04-2006, 04:18 PM

Yes, I care to comment. I had a practical demonstration of what happens when using a cooler in heavy dew situations/conditions. Call me stupid (most do, lol) But I should have thought about this from the get-go when I built mine. Used it for about 15 mins the other night & It suddenly dawned on me that, ok, if this cooler is sucking in air from a dew laden atmosphere.......??..........I grab a torch & shine it down the OTA........DEW!! On the inside! On the corrector & Primary. Luckily, only a very light coat of dew (due to 15 mins use) but had I kept using it for 2 or 3 hours??????? I don't wanna think about it. I haven't used the cooler since.

Luckily it was the only very dewy night we've had here. No experience with high humidity as yet, but yep! Don't use it on very dewy nights!!

I'm now trying to think of a filtering system for dew/humidity. Damn!

matt · #6 16-04-2006, 04:33 PM

But where do you reckon the % humidity cutoff for cooler use might be, Asi?

asimov (John) · #7 16-04-2006, 04:47 PM

To figure that out, I'd have to find out the dew point for the...hell, not sure what night it was..thursday or friday. Is there a website for looking up this info I wonder?

matt · #8 16-04-2006, 04:53 PM

No mate. Keep it simple. What would you consider an acceptable humidity level for safe use of the SCT cooler?? Right now we've got 35% humidity in Canberra. That's fine I'd guess. But 60%+ ... maybe not

asimov (John) · #9 16-04-2006, 05:06 PM

Depends on the filter in the cooler, I'd imagine. It would have to filter out Xamount of moisture. (but then what happens when the filter gets wet from the intake of moisture.....)

I'd only be guessing here...45-50% perhaps?

asimov (John) · #10 16-04-2006, 05:08 PM

The other alternative is to regularly check for fogging of the optics. As soon as it starts, thats it. turn the bugga off!

matt · #11 16-04-2006, 05:10 PM

Yeah. Reckon you're right, and best to err on the side of caution. Super caution!!!!

[1ponders] (Paul) · #12 16-04-2006, 05:21 PM

There is a problem with working out when not to use the cooler depending on outside dewpoint. If you vent your sct with the cooler at 6:00 pm and the temperature is around 25 deg and relative humidity is only 50% you won't have a problem then. The problem starts to occur when the temperature drops. As temperature drops the relative humidity will go up, until you get to the dew point. Only natural.

But what is happening inside your SCT. You have put air in there at 25 deg 50% relative humidity, as the internal temp drops your internal RH will go up and you will get to a point (dew point) where the moisture that was suspended starts to precipitate out and settle on cooling surfaces, particularly glass surfaces.

asimov (John) · #13 16-04-2006, 05:21 PM

I'm thinking......Airconditioners. They not only cool the air, but filter out moisture/humidity/water vapour. Someone please correct me if I'm wrong here!?.......lol!! Ducted airconditioning up the spout of an SCT telescope! YEAH!

casstony · #14 16-04-2006, 05:38 PM

Matt,
have you considered exposing the ota internals to a desiccant when the scope is not in use? I use a film container with desiccant packs inside it inserted into the 1.25" eyepiece holder - the container has holes drilled in it. I think there are commercial solutions available too.
-Tony

matt · #15 16-04-2006, 05:53 PM

Thanks for the advice guys. Will be grabbing plenty of desiccant packs and keeping them close to the OTA during storage from now on. Will also be keeping a close eye on all the humidity/dew point indicators. Cheers

davidpretorius · #16 16-04-2006, 06:09 PM

cool it first, bird believes the peltiers dry the air as well.

[1ponders] (Paul) · #17 16-04-2006, 06:45 PM

Be careful with desiccants like silica gel. The advice I was given the other day came from Bintel. I thought of using silica gel to control my fungus problem. I was also advised to be careful using silica gel because of outgassing of the gel. This can coat the inside of the scope with a really fine powder that will stick to any surface.

I know this isn't help you with your problem matt but I'd rather be known as a doomsayer than see you suffer from something that is avoidable.

The air conditioning sounds like a great idea. Do you have an airconditioned room somewhere in the house? Where do you keep your scope when you are not using it?

[1ponders] (Paul) · #18 16-04-2006, 06:51 PM

What we need is someone like bird to develop a system similar to his mirror cooling

matt · #19 16-04-2006, 07:06 PM

Absolutely Paul. It's amazing it hasn't already been done! And don't worry about pointing out potential problems. Like you say, better to be forewarned than forever sorry. Much appreciated.

davidpretorius · #20 16-04-2006, 07:11 PM

i had given some ideas to asi and paul (rumples on this a few weeks ago)

take that peltier off the **** smith fridge, enclose it around the base of a cat cooler or asi's milled version. have an internal temp sensor inside and bobs your uncle