Cooling fans on a Skywatcher 200P

[Wbarkingmad]

Has anybody tried mounting cooling fans onto the back of a skywatcher 200P or similar OTA? The evidence seems to be in favour of fans being a good thing but do I need to modify the back end of the tube to fit them or is it sufficient just to blow air onto the existing end cap assembly? I'm thinking along the lines of 3 CPU fans or similar.

[Moonshane]

I put one on my 12" dob and built this into the end cap (see http://stargazerslounge.com/diy-astronomer/109577-latest-dob-mods-fan-balancing-weight.html). I also have a side mounted fan to blow air across the primary. They certainly help with cooling (I keep both end caps on and blow air out of the focuser) and I believe also the views at high power (I leave the fans on when observing but as slow as possible).

I am contemplating adding a single small fan to my 6" dob too.

I don't think you'd need more than a single fan on your 8".

see also:

http://stargazerslounge.com/equipment-discussion/104657-cooling-fans-do-more-than-just-cool.html

there are lots of ways to do this and much online about adding fans to newts generally.

[dweller25]

I made a plate out of an old clock face to which I attached the fan. Then I attached the plate to the back of the 200P - it works VERY well.

[Wbarkingmad]

Thanks for the advice, it looks like the next modification Dweller25, are you using the three apparently spare holes (not the collimating screws) in the ring on the back of the tube? They look to be around M3 in size.

[dweller25]

Yes

[TopHouse]

Dweller, what difference have you noticed? is it only faster cooldown times or is there more?

[dweller25]

I keep the fan running whilst at high power on the planets and the difference is dramatic, much steadier and sharper image. The best £5 I ever spent.

[Rusty Strings]

I fitted a fan to my 200p. I made a replacement back panel from black plexiglass and fitted the fan to that and it does make a difference to cool down times. Must try leaving it running whilst on high mag.

[TopHouse]

Dweller, how are you powering it? battery or mains adapter or powertank or what?

[dweller25]

Powertank

[twotter]

Does the fan not vibrate at all affecting the image???

[dweller25]

Not at x343.

[Moonshane]

I don't find that I get vibration either although I turn the variable transformer down to 4.5v to reduce the speed when observing.

[irishbloke]

i mounted a fan on the back of my 250px by attaching it to a neoprene circle which is velcro'd in place. The thinking was the neoprene would help lessen any vibrations. Mines a 80mm pc case fan attached to a 4 AA battery pack.

I totally agee about the steadying effect of fan while observing.

Brian

[TopHouse]

So, when you do this, are you also trying to seal the fan/baffle arrangement at the primary end so that any pressure created by the fan is pushed up and out of the holey end (technical term) or is that not a factor? ie replacing as much air 'inside' the tube as is reasonably possible, also, does this have any effect on reducing dew?

[dweller25]

Yes, the idea is to push air up the tube, that gets rid of tube currents.

The one down side is that the mirrors will dew up if you run the fan too long on a damp night.

[Wbarkingmad]

Isn't there also the effect of cooling the main mirror to ambient temperature much quicker? Or is this not so much of an advantage.

[dweller25]

Yes, in fact the fan will do three things :-

Accelerate the cooling of the primary to ambient.

Remove the thermal boundary layer from the front of the main mirror.

Remove the thermal gradient between the inside of the OTA and the outside.

HTH

[TopHouse]

Now see what you've done! I'm now wondering where to source a nice round ally plate for my 200p

[Beulah]

Yes, in fact the fan will do three things :-

Accelerate the cooling of the primary to ambient.

Remove the thermal boundary layer from the front of the main mirror.

Remove the thermal gradient between the inside of the OTA and the outside.

HTH

Uh-oh, looks like I have landed myself another mod job...

[gfletcher]

So let me get this right.

Is the idea to blow the air up the tube ?

I am sure i have read some threads that say the best method is to "suck air" out of the tube .

Graham

[dweller25]

So let me get this right.

Is the idea to blow the air up the tube ?

I am sure i have read some threads that say the best method is to "suck air" out of the tube .

Graham

Same difference ???

[gfletcher]

No i mean to suck air out of the bottom of the tube

Graham

[Moonshane]

I use my fans with both end caps on and so that the air is blown out of the focuser.

One good thing about this is that you can feel with your cheek how cool the air is and when you are ready to observe (roughly.)

[TopHouse]

I'm thinking a nice Papst 90mm fan would be nice for this, whisper quiet and unquestionable quality the papst.