Last updated: 9 February 2009
Subject:	Observatory Temperature Control for Optimum Performance
Sent:	Sunday, February 8, 2009 17:22:29
From:	P. Clay Sherrod (drclay@tcworks.net)
With the risk of going overboard, I finished this today and thought it
might be something that the group can post in the FILES section.

Observatory Temperature Control for Optimum Performance

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

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

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 uninsulated.

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 observatory.

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 observatory.

** 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

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 problematic.

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

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 opened....in 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 building.

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

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. 

Dr. Clay
February, 2009

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