Blow or suck?

[LewisM]

:eyepop::thumbsup::rofl::screwy:;)

Anyway, childish puns aside, what is the best wisdom regarding mirror cooling fans?

My newly acquired Orion Mak-Newt has a cover plate on the back that you can remove to install a fan (4 screw points provided). Would it be better to arrange the fan to evacuate (suck) the tube, hopefully removing tube currents and NOT inserting dust in from outside, or better to force flow onto the mirror. I am concerned about inducing tube currents and foreign particulate, given that this is a closed system more or less with the meniscus up front.

I have not, and MAY not remove the primary to check for any sealing around the primary cell (I have heard this may be a criticism of this design). I am daunted enough to have to remove the meniscus/secondary assembly to clean the rear of the meniscus, the secondary and blow off the primary (some particulate has entered the system unfortunately). It's all NICELY collimated now, so may leave well enough alone at this stage :thumbsup:

[AndrewJ]

Gday Lewis

Blow.
Either way you need an entry and exit point to allow for airflow
( and hence heat transfer ) out of the tube.
If you suck, then any openings become points where outside pollutants can be drawn in due to the vacuum created by sucking.
If you blow, you can create a mild positive pressure inside the OTA
and as such "stuff" wont get sucked in through any accidental openings.
It also means if you want to filter the input, you only need one filter,
and if you want to use chilling, you can chill the incoming air at one point also.

Andrew

[LewisM]

Thanks Andrew. Makes sense.

[clive milne]

Lewis,
another option would be to use a cooling back for the primary using peltiers. Anthony wesley and paul haese use this method to good effect.

[LewisM]

Now there's an idea! I have 2 spare good quality/efficacy peltiers and a spare heatsink with fan from a cooled DSLR project (unnecessary since going CCD).

Will give it a try.