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Focuser Position for Boston and Cooldown/Boundary Layer Mitigation Best Practice


I just gave myself a one-year astronomy fix by putting down deposits for a JP Astrocraft "Sweet 16" scope as well as a Lockwood 1.1-thick Cer-Vit mirror. The speed will probably be about f/3.7 to allow a very comfortable seated position even at zenith, where the eyepiece will be at ~60". The scope and mirror probably won't be ready until next summer at the earliest, but that does give me time to nail down the details.

I've got two kinda stupid questions, "stupid" in the sense that despite doing research I still haven't come to a concrete conclusion!

1. In terms of determining whether I should put the focuser on the left or right side of the scope (when viewed from the back), this second half on body temperature in this Daniel Mounsey article (as well as his numerous other CN posts) has been very helpful.

I live and observe in Boston. Is there a prevalent (marine layer) nighttime wind direction in Boston? Logan airport seems to report that the wind is NW-ish most of the year but day-to-day I seem to observe a more expected wind from the east at night. For some strange reason, despite being left-eyed, I greatly prefer having the focuser on the left side which would be work well with a wind from the W to NW.

2. I have precious little time to observe nowadays. We're talking less than half an hour, with a hoped-for setup time less than 5 minutes -- including wheeling the scope in and out. I also don't have the bandwidth to set up a scope beforehand: with a growing family I will inevitably forget that I had set up the scope and it will rain overnight and the sun will burn my house down the next day! So not only is cooldown important but high quality views *while* the scope cools down is of utmost importance.

The thin Cer-Vit mirror should hold its figure and cool down rapidly. That leaves the boundary layer to deal with in those precious first few minutes: what’s the latest thought on removing its effect?

FYI: this wondrous thread has been my guide on boundary layer mitigation, but it is from 2005. Daniel Mounsey concluded at the time that it is sufficient to have a single rear fan blowing on the primary as well as a bank of venting fans on the top side of the mirror box to pull the boundary layer off, kitchen hood-style.

At the same time, Mike Lockwood just posted an interesting article on a customer's method of merely "stirring" the boundary layer with four internal fans with zero venting to the outside other than what goes out the top of the scope.

Perhaps the best combination would be a couple "stirring" fans at the bottom of the mirror box, pointed directly at the mirror at very low speed to assist in stirring the boundary layer (mitigating its effect while it exists), plus a bank of fans Daniel Mounsey-style at the top of the box to pull out the (now-stirred) boundary layer? Or is the bank of pull fans sufficient (or even better) by themselves?


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