Download Word 2001.

Good Day,
I enjoyed participating in the HET June 12-13 Seeing Review. In response to John Booth’s notes
released June 14, 2001 of our meetings and discussions, I have the following comments that I
hope you find helpful.
Post Meeting Comments on HET Seeing:
Cavedoni
6/21/01
1.
Quantify Existing Heat Loads:
Your initial round of decisions on how to move forward are relatively easy because the problems
are big and the money is plentiful. I expect the next round of decisions dealing with the residual
problems to be much more difficult due to diminishing perceived seeing improvements and
diminishing funds. To assist you in these future, more difficult decisions such as should I
Actively Cool the Dome Floor, Insulate the Pier, Ventilate the M1, …; I emphasis again the
importance of completing ASAP a campaign to quantify existing steady state and transient heat
loads by day and night and than extrapolate for both winter and summer conditions. With a
couple $k of funds and 100+ hrs of effort one can gain much needed knowledge.
Take a look at a new product from Omega called HHM290 Super Meter for ~$400.
2.
Exhausting Hot Air Out the top of the Dome Roof Vents:
If you need to exhaust hot air during the day out the vents in the dome roof, the building
AC/Mixing System is not working adequately! At no time in preparation for observing should
one have a significant temperature gradient in the dome requiring exhausting. It appears that the
AC is plentiful, the mixing poor and the heat load from the non-insulated single-skin dome
excessive! I suggest improving the Mixing and Insulating the Dome ASAP. After insulating the
dome, the temperature stratification, the heat load and the electric bill will decrease dramatically
however I expect you will still need to thoroughly mix the dome air volume to maintain at the
start of the night better than 1 C uniformity.
Do adequately mix the air pooling in the Pier Area directly below the mirror. This may help
Stacking.
3.
Actively Exhaust Heat from In and Around the Observatory:
I recommend investing in a building Exhaust Tunnel to discharge both internal and adjacent
external heat safely away from the observatory. To not do so, may seriously hurt you on lowwind nights - another one of those difficult choices.
4.
Top End Seeing Control Problem:
Most of my post meeting consideration has gone into how best to deal with your “top end”
seeing. I believe this is a significant source of current seeing. By installing the DVS, Insulating
the Dome, and by properly AC/Mixing the dome air, the problem should significantly improve
but not be eliminated. I think you must still address the nighttime 1 C/hr temperature drop - not
to mention the significant dust problem to SAC optics.
For mirror seeing dependencies people routinely claim Mirror Seeing = 0.4” per 1 C difference
assuming an upward looking mirror and the mirror warmer than the adjacent air. I suspect
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currently on a majority of HET nights in the early hours, top end structure and optics is a couple
to 10 C too warm.
For steel structures with an 8.3-mm effective thickness (half the full thickness of a 16.6-mm
structure with both surfaces exposed to air) and an 8 W/m-C convection coefficient, I calculate a
time constant of 60 minutes. If you double the thickness you double the time constant.
Assuming an initial temperature difference of 5 C, a 60-minute time constant, and a 1 C/hr
temperature drop; I calculate it takes 5 to 6 hours to reach a minimum temperature difference of
0.6 C.
If you start the night with a 1 C temperature difference, that same structure achieves a minimum
0.6 C temperature difference in 2 hrs. The above two calculations ignores the effects of radiation.
You can either blow/scrub away that residual (0.6 C x 0.4”/C) seeing per mirror with dusty dome
air or you can actively control the temperature of the SAC using an insulated bag enclosure and a
liquid-to-clean-air heat exchanger.
I believe one can actively maintain a maximum temperature difference between the interior SAC
volume and the dome ambient to a few times the accuracy you can measure the temperature
difference or better than 0.5 C. In addition, I suggest the temperature of the circulating internal
air could be uniform better than 0.05 C.
Assuming a 0.5 C temperature difference between the inside and the outside of a cone shaped (1m long x 1-m dia x 0.7-m dia) airtight enclosure covered with 25.4 mm of 0.039 W/m-C
insulation and a 8 W/m2-C convection coefficient; I calculate a heat loss of 5 W, an amount
easily blown away with a DVS.
5.
Tracker and Payload Upgrades:
Based on the above reasoning, I repeat the recommendation to actively cool the SAC optics in an
airtight, insulated thermal enclosure using a liquid-to-air heat exchanger and the existing top end
glycol cooling system. This arrangement should significantly if not completely eliminate the dust
problem and the local mirror seeing. Unabated, the dust problem will only worsen (as well as M1
wind buffeting) with the future DVS.
With regards to the electronic heat sources above the SAC, I also repeat the recommendation to
thermally enclose and actively cool them with a liquid-to air heat exchanger similar to the SAC.
Thank you for your time and thoughts.
Aloha
Chas
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