another basic question...fans!

[snug]

Hi folks

I sit here pouring over telescopes and i think i get the gist on why dobs have fans ...but for the life of me i cant work out how they are powered. Is it batteries or mains power. I guess im wondering if i need to buy something else to power the scope i buy..Or do i run an extension cord to the house or a generator...batteries???

Sorry if my questions are bit basic but if i dont ask i'll never get it..

Ta Snug
Ps ta for your patience with L'plater

[astronut (John)]

Hi Snug,
The majority of fans run on 12V. Some people just wire up a 8AA battery holder for your twelve volts. If a fan is all you're running than this will do.
12V 7amp/hr sealed lead acid batteries, about half the size of a brick are useful if you are running something inconjunction with the fan like a dew heater. They will last a lot longer.

[snug]

Oh gees Astronut

Thats all chinese to me...When i buy the Dob will these things come with it...(and instructions)...???

Ta Snug

[Jupiter]

Mine runs off the solar power from the stars, nebula etc.

[tnbk00 (Daniel)]

I have a bintel 10" dob which came with a fan arrangement, and a batter pack that plugs straight in!

[astronut (John)]

I assume you're buying a new scope? The Bintel or Andrews dobs and really most other brands all come from China or Taiwan.
These will have fans mounted on the rear of the mirror cell. It will also have a socket to accept a plug that is wired to the battery holder (see image)

[Tannehill]

Dilithium crystals, of course. Antimatter reactor -small one- in the mirror cell.

Sorry, couldn't resist. I'm going thru Star Trek withdrawal since they (thank God) cancelled Enterprise. Hear a new movie is in teh works, tho....yikes....

Yours is an excellent question, for sure.

Most common solution: mount a battery pack somewhere on the tube. Most commonly this is at back end where it can act as a counterweight. A small 9V battery will run most fans for most or all of a night, really, depending on the length of time you observe. Can get expensive, that approach. But I think most of us that run fans who use batteries a lot use re-chargeable batteries. I like rechargeable 1.5 V AAs. I use a plastic thingie that holds 8 AAs. Came with my GSO but you can get at Dick Smith or comparable store. Velcro it to the corner of the tube…acts as a counterweight, too.

More complex solution: run 12 volt power connection to your tube from the rockerbox. This comes into play when you have a continuous fan and multiple dew heaters for your finder, secondary, focuser, etc. Most folks plop a battery pack or battery on the rocker. From there, to the tube, is the challenge. You'll quickly see the risk: that tube moves in ALT motion and any cords running to either end directly will risk tangling. Many have a system that works, actually, but I’ve not seen one yet that doesn’t risk tangling at some point. My solution is to run the power from the rocker to the tube across the one point with minimal translation between the rocker and tube: the ALT axis. Here, most tubes will accept – with minimal or no risk of tangling - some kind of system for bridging a power cord from rocker trunion to the tube with minimal or no risk of tangling. Could be as simple as duct-taping a cord with enough slack to avoid pulling free or tearing out. You could use semi-elastic style guitar cable, or just plain RCA cord (my favorite) and leave just enough slack so it the tube can move from zenith to horizon without it pulling free or tangling. Even more complicated, you could try to perforate the power in thru the exact alt axis line, maybe using the alt axis pivot bolt as one of the leads or something. Something like a slip-ring or brush plate, they are called, but that also is probably unnecessarily complicated.

Hope this helps.

Scott

Originally Posted by snug

Hi folks

I sit here pouring over telescopes and i think i get the gist on why dobs have fans ...but for the life of me i cant work out how they are powered. Is it batteries or mains power. I guess im wondering if i need to buy something else to power the scope i buy..Or do i run an extension cord to the house or a generator...batteries???

Sorry if my questions are bit basic but if i dont ask i'll never get it..

Ta Snug
Ps ta for your patience with L'plater

[snug]

Thanks everyone for your help. Esp 4 the photo Astronut. I have one of those but it plugs into the wall-so now i know what Im after.I will definitely keep in mind the advice on tangling once i get started. Tannnehill-Someone told me that Austar is having a Star Trek marathon.I dont watch tv so im not much help there.

Again ta...snug

[astro_nutt]

I found an old computer fan...fixed to the back of my 10" dob with a bit of 3-ply...wired it to a 9 volt battery and switch..lasts for about 2 hours...does the job.

[JethroB76 (Jeff)]

Originally Posted by astronut

12V 7amp/hr sealed lead acid batteries, about half the size of a brick are useful if you are running something inconjunction with the fan like a dew heater. They will last a lot longer.

John, how many hours do you get out of such a battery running say, a couple dew heaters and a fan?

[iceman (Mike)]

Originally Posted by snug

I sit here pouring over telescopes and i think i get the gist on why dobs have fans ...

Just to clarify, dobs have fans to try and bring the mirror temperature DOWN to ambient temperature.

In most cases, the ambient temperature is colder than the mirror temperature as the glass retains the heat built up during the day. This causes a boundary layer of air that sits on top of the mirror, and can also cause tube currents - as hot air tries to escape from your tube.

All this has the effect of distorting the light as it comes down your tube and hitting your mirror, giving the appearance of bad seeing and a generally poorer view.

The fan is acting as passive cooling - forcing ambient (cooler) air onto the back of the mirror in the attempt to try and cool the mirror down faster. It's not likely to ever catch up to ambient temperature (at least not in the short term), but having the fan is better than not having the fan. Sometimes it can take all night before the mirror is at ambient temperature, especially in winter months where ambient drops very quickly right after sunset.

Some serious planetary imagers have gone the route of active cooling - that is, using peltier cooling on the back of their scopes, in combination with fans, to force COLD air onto the back of the mirror - cooling the mirror down much faster than passive cooling (with a fan) or no fan at all. Using this method, the mirror can be cooled at the same rate as the dropping ambient.

When the mirror is at the same temperature as ambient, you eliminate the boudnary layer and any tube currents and maximise the potential to get good views (either visually or high-resolution imaging). You're still limited by the seeing, and then other factors like collimation etc, but you've eliminated one of the major factors of poor views.

Passive cooling (using a fan) is better than not doing anything, but depending on the arrangement of the fan you may need to turn it off while observing if it introduces vibrations etc.

Scott has written a nice article describing his fan arrangement on his 12" GSO dob.

[astronut (John)]

With the fan running all the time, plus the heaters for both ends of the finderscope and eyepiece, my 12v 7amp/hr battery gives me about 7 hours.
Now that's with the heaters running between 1/4 and 1/3 capacity.
If I have to crank it up, I lose between 1 to 2 hours.
This means that two batteries will see me through the longest winter nights without a problem.

Originally Posted by Isdonisgood

John, how many hours do you get out of such a battery running say, a couple dew heaters and a fan?

[JethroB76 (Jeff)]

Thanks John