Primary mirror cooling fan question

[GrahamL]

Yes just a standard 10" dob , open ota mike ,with the fan as it came .
I thought it was mounted the wrong way at first .
Good luck , looks like a fantastic scope

If you want a headache try and google
"Thermal dynamics and primary mirror cooling in small telescopes"
or something similar

[Photon]

A very good example of what the mirror feels is to hold your mouth 1" (25mm for those into metrics) or so from the back of your hand and blow firmly, then reverse this and suck hard. It's very obvious what you would do to cool things. For those that don't have air-conditioning at home, remember we usually stand in front of a cooling fan, not behind it.
Regards IanG

[strongmanmike (Michael)]

If the primary mirror end of your tube is open Graham wouldn't it be better to have the fan blowing air on to the mirror rather than sucking it away? I assume the fan is baffled ie mounted with an extended flat flange around it of some sort?

To suck or to blow, that is the question...

Mike

[strongmanmike (Michael)]

Originally Posted by Photon

A very good example of what the mirror feels is to hold your mouth 1" (25mm for those into metrics) or so from the back of your hand and blow firmly, then reverse this and suck hard. It's very obvious what you would do to cool things. For those that don't have air-conditioning at home, remember we usually stand in front of a cooling fan, not behind it.
Regards IanG

Is that a good comparison though? Your hand is damp and thus you feel the cooling effect due to evaporation.

Besides if you cup your other hand into a cylinder then hold it just off your other palm and blow through that, then compare it to sucking the differece is much less obvious. The cupped hand is rather like the back of my scope as the air, if sucked, would enter around the perifery of the back plate and move across the mirror and out through the fan (my mouth)...

It's also hard to compare becasue your breath is warmer than the surrounding air so doing the cupped thing means that blowing delivers warm breath air onto my palm while sucking pulls cooler air across my palm...?

Still no answer....

Mike

See? still confusing

Mike

[RB (Andrew)]

Mike, whether you blow or suck on a mirror, one thing's for sure;

You don't need that old 6" refractor anymore so I'll take it off your hands.
Cheap of course.

[Photon]

Hi Mike,
Yes the moisture and chilling factor will certainly be a factor but not a lot of moisture on my hand in Tas compared to Newcastle perhaps.
The back of a main mirror has a rough surface which actually increases the surface area compared to a smooth polished surface due to the undulations etc. (In electronics a heatsink has more thermal efficiency if it has ribs milled into the fins compared to a smooth flat surface, as it effectively increase the surface area). Blowing air onto the rougher surface on the back of the mirror aids the cooling process compared to drawing it around the sides of the mirror edge and exhausting it through the fan where the partial vaccuum created around the middle of the back of the mirror probably doesnt have as much air circulation where it is needed, compared to blasting air directly onto the rear surface. I can picture in my mind from my electronics career but not sure whether I have explained it very well. IMO blowing would be superior to sucking but of course both options will acheive the same end.
Regards IanG

[strongmanmike (Michael)]

Originally Posted by Rocket Boy

Mike, whether you blow or suck on a mirror, one thing's for sure;

You don't need that old 6" refractor anymore so I'll take it off your hands.
Cheap of course.

Hmm? Let's see? A 4" focuser, 4" field flattener, one of only two of its kind in the world ever made, and probably near on a 10 year waiting list to get a new one?...?...Ooooh? I'd say 20K might secure it for ya?

Just kidding, I wouldn't sell it for all the tea in China it's a collectors item and one fine scope ideed

The only reason the ASA is really any better is its 12" of aperture of course but it's also much faster (F3.8 compared to F 7.5) and has a more temp stable carbon fiber tube (they both look very sexxy). The Starfire may even prove to give sharper images technically due to having no secondary..?

Mike

[strongmanmike (Michael)]

Yep unless ASA give me some compelling explanation I'm with ya Ian :-).

[strongmanmike (Michael)]

I just heard back from ASA and they indicate that the fan is supposed to be wired to "suck" air out from between the mirror and back plate but gave no indicationas as to why (they must have missed that part of my email? ) so I have asked them again, why in this case they feel sucking is better than blowing

Mike

[rmcpb (Rob)]

Sounds like they are just moving air and like I said it works either way. I happen to think it works better in the opoosite way they have yours set up. Besides then it helps with dewing problems

[iceman (Mike)]

Originally Posted by rmcpb

Besides then it helps with dewing problems