Cooling a reflector telescope - caps on or off

[Becchiooo]

How about when you come in on cold nights like we are having now, into a warm house. Is pointing the scope downwards with the cap off ok ?

[Moonshane]

I point mine at about 45 degrees with the cap off but the same principle, yes.

[knobby]

Now 45 degrees we can't argue about... As Kermit used to say its halfway...

[Becchiooo]

I'm more of a 47 degree man myself

[8kids]

My reflectors cool , I've just brought it in and it's got a thick covering of ice on it.

[Special K]

No, horizontal to the ground. Pointing up, there is no easy way for cool air to replace the warm air rising out of the tube and point down, the warmer air would rise up to the primary end and take longer to cool unless it can escape from vent holes. By being horizontal, warm air can easily escape out the top edge of the tube's end while cooler air can be sucked in to replace the exiting warm air at the bottom of the tube end edge

aka The Smoking Gun effect! This is my current method too, though I've debated whether to leave the focus cap on. I'm wondering about the air currents hitting the secondary as cooling takes place. As the primary takes longer to cool I should keep the secondary warm for longer. Now I wonder if it makes a difference which way I rotate the scope to horizontal position? One way has the secondary mirror facing up and the other way is facing down. If facing up it might be better as it is facing warm air leaving the tube rather than the cold air entering at the bottom?? Last night the secondary was dewed within 3 hours, but I did a quick cool down by setting the scope to 45 degrees, instead of the Smoking Gun.

[ollypenrice]

I leave mine out for 30 mins or so with the caps on, but it's just a strange habit that I have. I think that the above advice is sound; start with the Moon, Jupiter etc, and move out as it cools down.

Well, like Steve I don't agree with this advice and think it should be the other way round. The moon and Jupiter need high powers and good resolution. It's resolution, not light grasp, which is affected by turbulence so I begin on the kind of faint objects on which there's nothing to resolve at the apertures I have. Only later do I go to high powers. Besides, the seeing usually improves during the night so I would always wait for that before going to high powers, unless the object were well past the meridian.

Olly

[brantuk]

Pop a fan on the back of the mirror cell so it blows cool air into the tube from below - the mirror will soon clear and warm air currents will flow out of the end of the tube stabilising the air inside. Point 45 degs upwards to assist - caps off while you do this - of course - after 30 mins you'll be ready to observe. It works for me.

When you bring it in - point downwards 45 degs (caps off) so any condensation drips out of the tube and not down onto the primary. Once dry - caps on - cover on - wait for next clear night and then repeat all the above. Simples eek ....

[ollypenrice]

Ah, now if you want to go down the fans route my optical engineer friend (whose 40th wave primary is watercooled...) insists that you should create a breeze across the top of the primary to break the boundary layer on the optical surface. He says that one side should have a blowing fan and the other a sucking fan. In blowing from below the mirror itself protects the boundary layer which he says causes all the bother.

I just keep the scope at ambient and rarely notice any effects, I must say.

Olly

[LeeB]

I've also tried many ways but for me the fastest way is horizontal with the 250 newt. i also found a focuser fan helped but not much. the other thing to consider is its normally cooling in the early evening and living quite near an airport if you leave straight up it gets what looks like aviation fuel deposits on the mirror until the fights slow down later in thye night. Its a beggar to remove too.

[brantuk]

I was reading up on fan cooled newts recently - it's all very technical with lots of heat transfer formulae, mirror cooling dynamics, and air flow graphs. It all went way above my head so I just though "blow it" and slapped an old pc case fan on the back. lol

[keeplookingup]

this is something of a fuss for me too. I thought that down on an angle with caps off would help but what about dust that exists in the house?

Beijing is quite dusty and although its not so bad inside(i am serious) I think that it could be more than most places.

I just got my 150mm in and noticed the condensation on the primary and came online right away to see what was to do.

i read several places and got an array of answers. some where even saying leaving the caps on.

well i am unsure what's best. I dont like the idea of using a hair drier as someone said but a cool fan may be ok.

caps off, down at 45 degree angle is what I will try tonight.

tomorrow. who knows?

[Steve Ward]

I leave mine just off horizontal , mirror down , caps off , with the primary end facing any prevailing breeze / wind .

Let nature save you the hassle of fitting a fan . . . .

I would leave viewing bright things such as Jupiter or the Moon until the end of the mixed session to save the precious dark adaption .

Steve.

[keeplookingup]

i meant to say that the caps were suggested to be on while others said to leave them off.

i kept mine on and kept it horizontal until it was dry. will keep reading on later.

[Redbird48oo]

Thinking here, wonder if put a fine mess net....wifes panty hoss come to mine over open end of tube, this would let air move about while at same time keeping leaves, bird dropings, etc off mirror, Just a thought.

[keeplookingup]

i keep the scope horizontal, caps off. getting warmer now so haven't had serious condensation after viewing.