Reflector fan, does it really work?

[emadmoussa]

How much of the cool-down time does the  mirror fan take out?

I'm asking because it seems that the cool-down acceleration is minimal. Perhaps 5-10 minutes less than without a fan...

[nitram100]

I store my telescope in quite a warmish place in my house, so the 3 fans that I installed to the rear of the cell really do help. Reduces my cool down by approximately 20 minutes, which is quite a lot when observing time is minimal during the week. 

[John]

I have one on my 12" dob but I don't use it much too be honest. They say that a small fan mounted just above the primary and blowing across it is effective because it disperses the boundary layer of warmer air that forms over the top surface of the primary more quickly than a rear mounted fan.

I guess I'm using what they call "passive cooling" !

PS: This is another topic where you may get a multitude of differing views 

[emadmoussa]

I can see that. I might just go ahead and drop in a chunk of ice inside the tube

[Naemeth]

I think they are more useful with larger mirrors. A 10" doesn't really take that long to cool down anyway, and as long as you're using low power you can start straight away without any cooldown time.

[emadmoussa]

I think they are more useful with larger mirrors. A 10" doesn't really take that long to cool down anyway, and as long as you're using low power you can start straight away without any cooldown time.

Yep, this is what I usually do. Start off with a 25mm...then go down gradually as the telescope cools down. I kinda like observing on medium to high power.

[Naemeth]

Yep, this is what I usually do. Start off with a 25mm...then go down gradually as the telescope cools down. I kinda like observing on medium to high power.

I prefer low power observing, which is probably why I never bother cooling the scope down first .

[emadmoussa]

I prefer low power observing, which is probably why I never bother cooling the scope down first .

LOL...low power is tantalizing for me. I like the wide field it provides though.

[Optical Dob]

I think every little bit helps, depends on the weather, as you've  got one on your scope I would certainly give it a go. Very well specked Dob you have there. 

[AlexB67]

The SW 10 inch does not have a fan, and I gather the pyrex mirror cools quite quickly at that size anyway. Not sure what the GSO one is made off the top of my head. I am actually surprised it settles quite quickly and feel that a fan would not be something I feel I need with that 10 inch mirror so far.  If I want to view something like Jupiter or other high mag early on cooling is essential for at least 10 mins or so or the views are pretty noisy. 

I don't know whether it has to do with some locations I used in my garden, one of which is near wall so you get temperature gradients, but sometimes those pesky tube currents get in there and is a worse issue . Even in the small Heritage I had some at one stage I recall after 25 minutes and they can upset the views. Moved it about, it was gone in no time.

[Optical Dob]

Like all these things there are so many parameters that come into it. I've only used the larger Dob a few times and it seems to cool down quicker than the little heritage but this can't  be right can it     or it may be that the larger dob takes a little longer to set up.

If I was making a Dob to compete  with a Sky watcher I would put a fan on it, oh and a right angle finder..and a Telrad 

[emadmoussa]

The SW 10 inch does not have a fan, and I gather the pyrex mirror cools quite quickly at that size anyway. Not sure what the GSO one is made off the top of my head. I am actually surprised it settles quite quickly and feel that a fan would not be something I feel I need with that 10 inch mirror so far. If I want to view something like Jupiter or other high mag early on cooling is essential for at least 10 mins or so or the views are pretty noisy.

I don't know whether it has to do with some locations I used in my garden, one of which is near wall so you get temperature gradients, but sometimes those pesky tube currents get in there and is a worse issue . Even in the small Heritage I had some at one stage I recall after 25 minutes and they can upset the views. Moved it about, it was gone in no time.

it's BK4..I think Pyrex glass cools down slightly quicker, but the difference is only a fraction and can be detected on bigger mirrors.

[John]

I think all GSO mirrors (which the TS dobs use) are made from BK-7 glass.

The advantage with Pyrex is, I believe, that it retains its figure more accurately as it cools. I've never understood why Skywatcher use Pyrex for just the 10" models though

[emadmoussa]

I think all GSO mirrors (which the TS dobs use) are made from BK-7 glass.

The advantage with Pyrex is, I believe, that it retains its figure more accurately as it cools. I've never understood why Skywatcher use Pyrex for just the 10" models though

Lol..yes BK7...where did I get the 4 from!?

[John]

Lol..yes BK7...where did I get the 4 from!?

BK-4 is the stuff that half decent binocular prisms are made of. The optical properties of the glass that is used for mirrors is less important as its basically just forming a surface to apply the aluminised coating onto. Light never passes through it. Maintaining shape at differing temperatures is a good characteristic for the glass used for mirrors though, and Pyrex is good at that.

[cotterless45]

Used mine once. The noise was like a mini coffee grinder. Just the sort of grinding to put the teeth on edge and send shivers down the back....Yeeeeek !

Nick.

[ronin]

Cannot see one making much difference, the mirror itself is made of glass that is not a great conductor or heat so the internal flow is poor. So getting the heat from the centre to the outside for the air flow from the fan to remove it takes time. Installing 3 probably has about twice the effect if that but I would doubt that more would be of any use.

Would have thought that a gentle flow over the surface would have helped in dew reduction if it was a problem.