Observatory Temperature Control for Optimum Performance

P.  Clay Sherrod (Dr. Clay) of Arkansas Sky Observatory, Petit Jean Mountain

Arkansas Sky Observatory is presently nearing its 40th orbit around the sun;
in that period of time, I have gained a considerable working knowledge -
including some rude awakenings - about the affects of temperatures and
humidity on telescopes and telescope equipment.

In those years, it becomes obvious that the temperature and atmospheric
conditions inside the observatory DURING USE is of very little importance,
if those conditions were ignored while NOT IN USE during daylight hours.
The maintenance of a good observatory is far more important during daylight
than it is at night.  Use the night time to enjoy God's Creation....use
daylight to protect your right to do so.

Here is a summary of my thoughts on observatory temperatures:

When telescopes that are going to be used for:
1) imaging
2) high resolution (i.e., planetary or double star) viewing or imaging
3) serious research applications

the ambient air temperature and humidity within the observatory confines
(the actual room in which the telescope is being used) must be as close to
that of the outside ambient air as possible.

Going one step farther:  the actual telescope itself, including the AIR
INSIDE the optical tube assembly as well as the primary optics (the main
lens or mirror) must also be within ONE DEGREE of the outside ambient air
for useful applications to be successful.

To use telescopes that are not so "acclimated" will result in differential
heat currents which are highly problematic from these sources:

1) air "boiling" inside the optical tube assembly, even if the OTA has one
end open; some form of ventilation can be used to allow faster cool-down
times; boiling air within the OTA will result in images that are soft, out
of focus and very low resolution, much as would be seen if the telescope
were looking through a thin layer of plastic sheeting.

2) surface temperature of the optics exceeds that of the outside air:  this
will result in "soft images" and no focusing efforts will improve upon this
until the mirror or lens is within less than one degree of the outside air.
Mirrors are more subject to this influence than are lenses.

3) mechanical contraction and expansion:  this is the number one cause of
the requirement to constantly focus through any given night, particularly as
the night air cools rapidly from a warm day OR in the case of a telescope
being confined inside of a closed observatory during daylight hours and
being forced to operate at a warm temperature when the observatory is first
opened.  This is a direct result of the metal components of any OTA, and
even the dovetail mounting brackets that are used.  Aluminum tubes are very
prone to differential contraction and this is the number one frustration
with users of SCT or Maksutov telescopes, since closed tubes require longer
to acclimate than open tube designs.  Modern telescopes are now being
equipped with Carbon Fiber or Kevlar tubes and fittings to minimize
expansion or contraction during nighttime hours.

4) differential air temperatures within an observatory environment and that
outside the environment; this is compared to looking down a hot highway on a
summer day and seeing "water" on the surface of a dry road.....this
phenomenon is particularly troublesome in summertime when a cool observatory
is opened to warm night air.

5) the one that we do not have any control over:  temperature inversion,
which is the cause of "star twinkling"; on nights when stars overhead are
twinkling, very little serious astronomy can be done by any observatory, no
matter how clear the night might be; this is air mixing in refractive
"zones", where warm air and cold air intermingle to change the refractive
values of the air, and thus create multiple "lenses" in our atmosphere which
change constantly; the result is blurring of your celestial target, or
constant changes in focus.  Many night with demonstrative star "twinkling"
will result in absolute frustration in a user's attempt to focus, with focus
good one minute, and terribly off the next.


For the purposes of this discussion, "observatory equipment" refers to:
1) Telescopes and mountings
2) Cameras and CCD equipment inside the observatory
3) Computers that remain inside closed observatories
4) Books, charts, wires, metal surfaces

It is far easier and safer to maintain instruments and observatory equipment
in cold conditions than in hot.  I strongly recommend insulation in all
observatory buildings.....not for comfort or to maintain temperature during
observing times, but to keep extreme temperatures and humidity from ruining
observatory equipment.  Condensation is far less likely to form inside your
observatory during daylight hours if the building is insulated.  In fact,
most damage from rust, mildew and electronic failure as a result of
condensation that I have seen has been a result of equipment being stored in
an un-heated and un-insulated observatory building.

When fall, winter and early spring days bring very cold temperatures,
equipment within the observatory is typically unaffected by such cold when
not in use.  When temperatures drop below 20 degrees F, it is best to turn
off computers however.  No special attention needs to be given to equipment
maintenance.  Part of the reason for this is that, as temperature drops very
low, dew points drop to a point where condensation inside of the building -
which will always be a slightly warmer than the outside air - is likely not
prone for moisture to form on any items in the building, even if

However in storage mode (daylight or nighttime), when humidity exceeds 75%,
an uninsulated building will result in condensation onto cool surfaces
quickly:  these surfaces include:  1) computer surfaces and screens; 2)
telescope optical tubes; 3) metal surfaces of telescope mountings; 4)
particularly a metal pier and its base; 5) inside walls and panels of your

This problem is FAR more common in summer than in winter.  In winter I
suggest a small 1000 watt or less space heater or 200 watt lightbulb being
left on during closed periods to minimize humidity.  If you are able to keep
the inside temperature to within ONE DEGREE of the dew point on all exposed
surfaces, you will prevent condensation from ever forming within your

** Without insulated walls and flooring, this is virtually impossible.
Insulation is imperative if you wish to protect your astronomical investment

Because most observatories are not ventilated, prolonged periods of exposure
to such condensation WILL eventually lead to not only rust, but dreaded
mildew which many times cannot be removed from metal surfaces and certainly
not from optics.  Like plant roots, mildew exudes tiny amounts of strong
acids as a means to soften surfaces on which to attach; these acids will eat
through the surface of your paint, anodizing, and optical coatings and will
ruin electronics in 3-4 days unattended.

If such condensation is found in your observatory after being closed for a
long period - either in summer or winter - do NOT turn on the power to the
instruments, but carefully dry by opening up the building, turning on a
space heater and using a hair dryer on exposed parts if possible.

More about humidity condensation is discussed in HOT CONDITIONS.

To Acclimate the Observatory Prior to Observing in Cold Conditions

In cold weather, always open the observatory up for an extended period prior
to your planned observing.  Do not open prior to sunset or you will be
introducing more warm air than releasing in many cases....if it is necessary
to open the building up (for example, if there is moisture on some
surfaces), open when the sun is at a very low angle OR if domed, rotate the
shutter opening AWAY from the sun for best results.

The observatory must be opened about 2 hours prior to observing schedule for
optimal results with equipment.

If blowing dust, pollen or debris can be a problem, leave the telescope
capped and covered with a cotton sheet, and cover computers and screens

A thermometer should be located in the observing room on a wall that is not
subject to daylight heating or direct sun (north, if in northern
hemisphere); a similar thermometer outside in shade should be located,
remote if possible.  When the two are within a few degrees of one-another,
your observatory is optimized for viewing.

Planning Your Winter Night's Observing Period

One tip that I always promote is to simply plan ahead a bit if possible on
wintertime observing.  If, for example, the moon is a first quarter and the
sky is going to be filled with moonlight until it sets at about midnight,
then plan to go out about 11 p.m. local time to begin your observing.  Check
the weather forecast and if all seems good for the night, simply open up the
observatory AT DUSK, cover your equipment with sheets (never plastics!), get
some sleep or watch television and leave the observatory OPEN until you go
out there at 11 p.m.  That way, all equipment is equalized to the outside
ambient air.

During some extremely cold nights, there may be periods where equipment
operates sluggishly and this may not be particularly good for your equipment
and pocketbook.

1)  if your telescope begins to slew difficultly, and if you can actually
hear the sound change as if laboring or you see it visibly moving slower
than normal, do NOT continue to operate the equipment.  This affect in some
telescopes will begin to get troublesome at about 15 degrees F., and if you
do not use all temperature low viscosity grease in the bearings and
gearworks, you are asking for a repair ticket on your mounting, electronic
focusing control and other moving parts.

2) if your CPU fan begins to hum louder than normal, this is a sign that the
computer may be struggling from excessive cold....this is rare and typically
the temperature would need to get into the single digits or lower to become

3) if your CCD camera continues to frost over, the night should be over.

4) if any of your electronic readouts (such as the hand control for the
telescope, a digital clock, computer screen characters, etc.) become jumbled
and unreadable, the temperature is beginning to affect them; this can occur
even as warm as 35 degrees F.

NOTE that I do not suggest using LCD computer screens nor laptop devices in
the cold observatory when the temperature drops below 10 degrees.

Unlike cold weather, in hot summer there is MUCH care and consideration
necessary for all telescope and related equipment inside your observatory.
Temperatures INSIDE a closed observatory with no insulation nor ventilation
WILL easily reach within 150 degrees for facilities located with 38 degrees
north and south latitudes.  When temperatures inside your building reach 95
degrees, you must begin to think of keeping it at least at that temperature
or below that mark for three reasons:

1) at about 102 degrees, some electronic components begin to "get soft" and
can become damaged or technically challenged once activated; electronics
will be absolutely ruined at high temperatures;

2) although the optics of your telescope will likely not become permanently
damaged from summer heat, they will retain this heat LONG into the night and
likely not be able to acclimate to outside air temperatures whatsoever
during the following night; this will result in deformed images and
inability to properly focus your telescope;

3) camera equipment will not perform properly, with a dramatic increase in
noise-to-signal interference; likewise, it is very easy to damage delicate
CCD cameras and digital cameras in excessive heat.

Your computer equipment located in the hot building in summer months should
NOT be left running; the CPU fan cannot keep up with the temperature of the
CPU and the increased outside temperature of a closed observatory on hot
summer days.  If you must, for whatever reason, leave the computer running,
I suggest always using a small desktop fan directed at the back of a PC
unit, or at the keyboard of a laptop at all times during daylight hours.

If the temperature inside your observatory reaches 110 degrees, you MUST
either open the roof, door, or both.  Ideally some type of ventilation
should be provided at all times.

Using an Air Conditioner and Condensation

For extremely hot days, the problems with condensation are typically not an
issue since the air temperature is far above the dew point and moisture will
not form on surfaces.  However, if you are running a small cooling unit in
the observatory during the day, you MUST be very careful prior to opening
the observatory AND taking the caps off the telescope; if your air cooling
unit has gotten the inside temperature and the surface temperature of the
equipment down TO the dew point, then you will immediately have "fog"
(condensation) form on the surfaces of all cool equipment.  Make sure to
turn OFF your cooling unit at least ONE HOUR prior to opening your
observatory for an evening's session.

Also note that on very humid summer days, such as when rain is expected or
if the dew point exceeds 50 degrees, you may likely produce condensation if
your cooling unit is working "too well."  Always make a habit to check the
surfaces frequently (I always use the section of the pier very near the
floor level) for moisture....if you have some developing, it is time to turn
on a small space heater or lightbulb (see COLD, above) similar to winter
months, simply to lower the dew point INSIDE your observatory environment.

To Acclimate Your Observatory for a Summer Observing Session

If proper observatory maintenance is maintained as described above AND if
proper ventilation and insulation is in place, it is typically easier to
acclimate your observatory equipment for a night summer session than it
would be in winter.  Once again, wait until the sun sets until the
observatory is summer, your twilight will last up to 1 hour or
slightly more in some latitudes, so this will provide plenty of time for
acclimation to take place.

Remember, that - if you have properly ventilated and insulated your
observatory - the inside temperature should be about 12-15 degrees F. cooler
than the air outside while the sun is up; this means that the air outside
will begin dropping quickly after sunset and at some point be at the same
temperature as your observatory inside.  The trick here is to not keep your
observatory SO COOL during daylight (if using an air conditioner) that the
outside air will never reach the temperature of your equipment and thus the
equipment must actually acclimate UPWARDS to match the warmer air outside.

Warning about Warm, Humid Days

Warm and humid days can occur in summer, winter, spring or fall.  Humidity
WILL get inside your observatory and the only way to control it is either
through the use of a small space heater, a de-humidifier, or a 200-watt
light bulb left burning continuously.  If you own an observatory, you also
without a doubt have invest a LOT of money in your equipment that is inside
that observatory.

You are the keeper of that equipment and only you can prevent mold and
mildew and direct water damage through constant maintenance.  Keeping an
observatory is much like keeping a prized dog:  you love them and they love
you....but they only love you if you feed and pet them.  Pet your

Watch the local forecasts regularly and look for high dew points and high
humidity.  Fog and condensation, and even rain, will form when the air
temperature reaches the dew point temperature.  This can happen inside and
outside of buildings, and will happen in your observatory on a regular basis
if precautions and care are not exercised.

When you see that there are going to be prolonged periods of high humidity,
rain, high dew points and general cloudiness, there are several precautions
that you - as a responsible observatory owner - must take always:

1) turn off and unplug all electronic equipment, including your telescope
and dome control if you have it;
2) provide for some way of warming the interior of the room as described
3) cover your telescope and other equipment with a soft thin cotton sheet -
NEVER cover with plastic, as the plastic traps moisture, assuring you of
condensation, damage and certainly mildew.

Remember that you have invested in your observatory as a TOOL to your
pursuits of astronomy as they increase your knowledge of the night sky.  The
observatory is not a panacea of all things irritating to your time at the
telescope and away from the comforts of your warm bed.  It is only a
tool.....and a good one, provided that you are a good maintainer of that

If YOU would be uncomfortable living inside your observatory, day or night,
when it is NOT being used in your explorations of the sky.....then something
is not right with things internal; it might be improper ventilation,
improper temperature control, too much humidity, too many bugs, or a cold
draft coming from a wall or floor with no insulation.

My acid test of a good observatory room is simple:  If you can take your
favorite astronomy book inside during each of three specific days:

1) a hot sunlit summer miserably oppressive day;
2) a gloomy rainy and foggy day; and,
3) a cold, bitter, wind-blowing day.....

....and sit down on your observatory stool and actually READ your book
without distraction.....

....then you are a good maintainer of your astronomical pursuits.  Enjoy.

Dr. Clay
February, 2009
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