CONTROLLING HEAT CURRENTS IN AND AROUND YOUR HIGH RESOLUTION TELESCOPEby Clay Sherrod
I have noticed many questionable heat-related issues that will affect your observations and imaging of very fine planetary detail; our main priority is rapid-fire patrol-type images, yet we too suffer from sever heating effects that appear to be non-atmospheric in nature.
There are essentially TWO types of bothersome convection currents that can be seen in all telescopes:
a) those caused by atmospheric effects as the nighttime air cools rapidly and heat is dissipating upward from the ground, roofs, pavement, etc.;
b) those created locally within your observing environment which may or may not be able to be controlled.
The 20" scope that we use for the nightly 200+ image shoots is in an observatory that actually has temperature controlled walls; the roof is an insulated fiberglass dome with a special thermal-resistant plastic resin material similar to automobile body fiber material.
1) I have noticed considerable heat currents directly over a very large DC inverter using that I run for powering dew preventing apparatus for the computer and the telescope/guidescope/imaging camera;
2) In the corner where our image monitor (a 17" CCTV with its own computer and image integrator) is "no-man's land" when it comes to imaging over that;
3) I open the roof at about 10:00 p.m. if I plan to image or observe a high magnification by midnight; this helps tremendously;
4) We have two power vents in the roof and one floor air vent and a "Florida-style" louvered window that we force air through DURING observing and imaging, which helps tremendously; I have also found that leaving the front (north) door open to allow additional circulation helps. Note that if you have a separate control room or office from which you control the telescope, do NOT leave the door open between the two rooms. The telecope room must be identical to the outside air; your office can be whatever makes you happy.
5) The dew strips (absolutely essential here in Arkansas on every night) cause considerable turbulence. Since we image throughout the night; the main scope dew strip (A large heated strap) is turned off 10 minutes prior to imaging; then it is LEFT ON, and set to low throughout the night.
These are some of the many things that we CAN control; however, just like you we had a tremendous cold front roar through here last night, dropping the temperature at 6:00 p.m. last night of 76 degrees to 27 degrees by 4:00 a.m. this morning. When the temperature drops so rapidly there is NOTHING that can be done to offset the tremendous amount of heat that is rapidly convecting upward from the ground, the rooftops around and the highway to our south and west.
Indeed in city or even suburban conditions, during "rush hour", although miles distant, rapidly deteriorating seeing will almost always occur with the increase in early morning traffic.
Telescopes should always be allowed to cool and equalize at least one hour prior to viewing for planetary OR for imaging certainly. Set out the telescope in a cool shady area at sunset and leave until after dusk (about 1.5 hour); the larger the telescope the more time to cool down. I have a trick that I highly recommend to those attempting very high resolution work, either visually or electronically with any closed-tube catadioptic:
1) turn the telescope where the front (lens end) is facing pretty much down to the ground;
2) remove the visual back or camera connectors from the rear of the telescope to reveal the opening going into the telescope and leave open;
3) cover this opening with a very fine mesh cloth to prevent debris from entering the telescope;
4) allow heat to convect upward from this hole for at least 1/2 hour like a chimney.
This will remove the internal heat three times faster than mere cool-down. Also, if there are many people, dew strips, and heated accessories close to the telescope, it is a good practice to exercise this routine about once every hour as well for about 8-10 minutes.
ONE FINAL note: The most steady air of any night begins about 3 hours before dawn. Those are the best hours of the entire night and your observing can be planned around that time window for best results in high resolution imaging or viewing.
Arkansas Sky Observatory