Cooling Off Period

[Yendor]

I have read many posts here referring to putting your scope outside for a period of time so that it has a chance to cool to the air temperature around it. Is this necessary with all kinds of scopes?

Another question. I gather, again from reading posts that my 127 Mak is prone to being affected by Dew. It's no problem to buy a shield, or make one for that matter, can I just make do with something like card or whatever to get me by till I have the proper thing. Does the Dew get inside of the scope? will this cause damage? I take it that if Dew does get inside, then do I leave the covers/caps off my scope when brought back into the warm so that it can air dry?

[Moonshane]

hi there

as a general rule I'd normally recommend approximately five minutes for each inch of aperture so for a five inch scope perhaps 25 mins. I have never used a Mak but I'd suggest from reading the posts of others that you can probably double this to 10 mins per inch for Maks and SCTs.

dew shields are useful for many scopes and Maks are certainly one of them. It can be made of anything that will hold it's shape and is quite light. I use the 'funky foam' 2mm sheets from hobbycraft. they are waterproof, easily sewn or held with velcro and work really well. for Maks I'd suggest 2x aperture in length so for yours a 10" shield would be ideal.

ensure it widens as you move away from the scope so that it does not get into the optical path of the scope.

[Kai]

Maks and SCT's require a lot longer as the tube is practically sealed , you can purchase "Cat Coolers" to speed up the process.

[Space Bat]

Does depend on the temperature differential.

So if you are carting your scope out from 20 degrees into a warm night say 10-15 degrees then my scope has cooled within 45-60 mins.

However doing this when air temp is down in the depth of winter it can take longer and sometimes up to 2 hours to be perfectly stable.

Yes a home made dew shield will help - but in reality just buys you a few mins before the inevitable...dew point at it's worse can quickly set everything wet...only solution is a dew heater.

These can be purchased - or you can make your own if fairly practical and there are lots of examples how to make one here and on the web.

[Yendor]

Thanks everyone, most informative. I think maybe I'll have a go at making one, and also I will investigate the dew heater too.

Can anyone let me know if dew is harmful to the scope?

[Space Bat]

Not at all

Just make sure if it gets real wet to allow it to air - don't be tempted to wipe anything off.

[todd8137]

hi there,mines normally out side ,well the reflector and refactor 2 hours before also think about you eye pieces i usally leave them out side as well they need time to cool people argue it makes no difference, i tend to disagree my 2" eps perform better when cold

thats my pennies worth

[John]

.. also think about you eye pieces i usally leave them out side as well they need time to cool people argue it makes no difference, i tend to disagree my 2" eps perform better when cold.......

I keep my eyepieces a little warmer than the outside temp. It stops them from misting from my body heat on cold nights.

[Klesk14]

The time you are referring to wait for temperature equalization is before use and after bringing it back in?... Or both?

Would the heat during the summer also need a long cool down bringing it in?

I use an 11" SCT, I guess I'd need 1-2 hours to let it sit alone before use and storage?

[MorningMajor]

I leave my 127 out for about an hour prior to viewing - the dew won't get inside it, but leave it pointing downwards whilst it cools and it's best to not to have any EP's in the diaganol to help speed up the cooling down

[Yendor]

All these responses are brilliant. Thank you all.

[Space Bat]

Here is my OCD routine

2 hours or more before plan on viewing - scope out all caps are on and the scope is pointing straight up to the sky for 30-45 mins Then I will point it down for same period..

Then I will wire up the heater and then take caps off and on with due shield -

Then cart all my gear from garage - EP's etc and will then setup lappy and align scope - then off for a stiff drink and a smoke...

Then off with all the house lights and sit and stare straight up - getting dark adapted...

Then am ready to observe

[Yendor]

OCD lol. Now now, you take all that time to make sure you get a steady clear image, then you spoil it by having a drink? I might just use your routine as a template for my own viewing habbits, then I can tell my mrs its your fault when I'm somewhat tipsy during my viewing session.

Being serious though....... It does seem to make sense :-))

[jimmyjamjoejoe]

OCD lol. Now now, you take all that time to make sure you get a steady clear image, then you spoil it by having a drink? I might just use your routine as a template for my own viewing habbits, then I can tell my mrs its your fault when I'm somewhat tipsy during my viewing session.

Being serious though....... It does seem to make sense :-))

If you drink juuuuust the right amount, your iffy vision cancels out the atmospheric wobbling, and you have this zen moment of perfect stillness. An unimaginable sense of sentience and clarity washes over you as you journey through the heavens in viewing perfection...

Then you wake up in a puddle on your lawn with a banging headache, having forgotten the entire experience.

[MorningMajor]

If you drink juuuuust the right amount...

More tricky to get that bit right than polar alignment!

[Virtual Cold]

Hi all, a bit late to this thread but I have a question which I don't think has been addressed yet (forgive me if it has, I've skimmed this!).

When allowing the scope to dry off after a dewy session, in what position is it best to leave it?

Many thanks!

[wiseman]

And can someone please explain the physics between what seems to me to be a contradiction.

1. leave your scope outside before observing so the optics cool down.

2. If you have a dew problem ( I have dew on the dew), then heat both mirrors.

???

So If I have dew, can I just wham on the heaters and start observing straight away?

PS, how to build you own dew heaters here

Heaters

I've got the resistors to have ago at the rigel quick finder one. But f tonights viewing is anything to go by, I will be making some heated gloves as well!!

Cheers

Wise

[Moonshane]

your argument/query is understandable but one thing's for sure, with misted optics you'll see less than with not properly cool optics.

[John]

And can someone please explain the physics between what seems to me to be a contradiction.

1. leave your scope outside before observing so the optics cool down.

You are not just cooling the optics - it's the equalisation of the outside air temp and the air within the tube and the dissipation associated air currents that is more important.

[RikM]

Hi all, a bit late to this thread but I have a question which I don't think has been addressed yet (forgive me if it has, I've skimmed this!).

When allowing the scope to dry off after a dewy session, in what position is it best to leave it?

Many thanks!

I leave mine pointing downwards with the dust caps off. That way dust shouldn't fall and stick to the wet mirrors.

[rory]

well ive lean't somthing thismorning reading this thread,thanks all.

i was leaving my refractor for 10 min's . sometimes even jumping straight in.

[Moonshane]

at lower power you can even use quite large apertures surprisingly quickly.

When I first built it, I could not wait and looked through my 16" dob after about 10 minutes. the view was actually pretty sharp and it was only when the power was increased that I saw the effects of not 'cooling'.

that said, you need to recheck collimation after the scope is cooled of course.

[John]

well ive lean't somthing thismorning reading this thread,thanks all.

i was leaving my refractor for 10 min's . sometimes even jumping straight in.

Refractors tend to cool much quicker, your 90mm will work pretty well straight out of the house - my 102mm does

An SCT or Mak can deliver quite poor images until it's properly cooled though. A newtoniain (as an open tube) cools faster but not as fast as a refractor.

[wiseman]

you need to recheck collimation after the scope is cooled of course.

What?

Nobody told me that!!!!

How is it with this hobby that, each week, I discover that I still need to do something else to get it all right??

So my routine should now be,....

1. switch on heaters on 10% power, switch on fan (blowing over primary from behind)

2. allow to cool down so air inside is equal temp. ( say 40 mins for 8 inch dob)

3. Collimate with laser ( check laser with jig first)

4. Star alignment

5. Start looking?

Cheers

Wise

[Virtual Cold]

I leave mine pointing downwards with the dust caps off. That way dust shouldn't fall and stick to the wet mirrors.

Cheers, that's what I thought. Who better to confirm than someone who has the same scope as me, and lives in roughly the same area?