Reply To: Suck or blow

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Martin Lewis

I suck and blow!

My 18″ mirror is in a square mirror box and I have a soft surround to the mirror with 4 small fans one in each corner to pull air down the tube during cool down prior to imaging. Meanwhile I blast the middle of the mirror rear with a 100mm fan which has a large baffle around it so the air get funnelled raidally across the mirror. This cools the primary at the maximum rate and faster than if I were sucking air past the back. I can control the small fans separately from the main primary fan and both sets are speed controlled. The idea was that I could switch off the main fan at the start of an imaging session but still have the small fans sucking down the tube. Trials showed little benefit in doing this and generally I give everything an hour’s blast and switch them all off during imaging. Blowing during hi-res planetray imaging is definitely is a bad idea from my trials.

If the scope or mirror is below the dew point, condensation will form on it, so you either have to warm the scope and mirror or decrease the dew point of the surrounding air. Don’t think there are any other options. You could bag the whole scope and put sachets of dessicant in the bag to drop the moisture in the surrounding air? You’ll get through a lot of dessicant, though it is rechargeable through heating. Or put an electric dehumidifier unit in the vicinity.

Good luck