Keeping it cool...

[Mr. Subatomic]

Hi,

I am curious as to how long I should run the cooling fan of my 10" Dob prior to observing. Should I run it whilst observing, and is it ideal to take all dust caps of the OTA, finderscope and eyepieces to cool all of the equipment?

Thanks for your help.

:)

[Tannehill]

Good question. This comes up a bit.

There are two excellent references that you should be able to access on the internet.

Bryan Greer’s website
http://www.fpi-protostar.com/bgreer (http://www.fpi-protostar.com/bgreer)

S&T article by Alan Adler
http://www.pha.jhu.edu/~atolea/WAS/thermal_management_newtonians.pdf (http://www.pha.jhu.edu/~atolea/WAS/thermal_management_newtonians.pdf)

Both detail the impact and issues of hot mirrors….both are a great read and very helpful in appreciating the phenomenon, and are fairly convincing that a newtonian user should consider using a fan.

A search of IIS and other internet forums - including the various Yahoo groups and Cloudy Nights - will also deluge you with information. PM me and I can list some for you that relate specifically to a commercial dob (is that what you have?)

If you are not keen to tinker, or are not terribly interested in going the extra mile to fine tune the quality of every last photon you can, then run the fan until you start observing. Odds are – if you have a stock commercial scope – the fan is directly mounted to the cell, and will cause vibrations at the eyepiece, and defeat the goal of improving image quality. Better still, blow a larger/stronger fan (a room box fan, for example) at the back of the mirror (with dust cap off to permit air movement thru the tube) before you observe.

I’d leave the finder and EPs covered and warm. It will delay them dewing up, and their temperature has little or no impact on image quality, at least compared to a hot primary mirror.

If you are a tinkerer and fussy, there are many variations on how to install and use fans on Newtonians. My GSO Dob Fan article in IIS presents my version, which evolves out of others work…nothing original here. There are high tech and low tech approaches. I know many who just remove the fan, and instead attach it to the cell with bungee-type cord, with foam weather-stripping tape between the fan and cell to insulate the vibration transmission to the cell. Takes 15 minutes, costs about $5-10. (but you should check there is no vibration at a star test testing with fan off, then on...) Then there are the more sophisticated techniques to redirect airflow across the mirror, baffle the air flow to force it up the tube and not just bouncing off the back of the mirror. Et cetera.

Good luck.

[Mr. Subatomic]

Wow, thanks for the info Tannehill! :)