SYSTEM DESIGN NOTE
SDN0007 - Thermal Design Requirements
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Prepared by |
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Approved by |
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Jay Elias |
4/8/99 |
N. Gaughan |
4/9/99 |
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The thermal design of the instrument must meet requirements in several areas.
1. Cooldown
The instrument must reach an operational temperature in <6 days, with a goal of doing better.
A definition of “operational” temperature is needed, but basically it should be when
a) Mechanisms are functional.
b) Optics are functional.
c) Detector is functional.
d) Residual instrumental background permits useful operation [even if not yet
at final value].
Of these, (d) is probably the main driver.
The cooldown of refractive optics cannot be so rapid that they are over-stressed. (This might be a concern if some sort of rapid pre-cool were added.)
Final temperature to be ~65K (or less).
1a - Pre-cool considerations
A pre-cooling system is not currently under consideration. Concerns with such a system include weight and cost. Note that to be effective such a system must be capable of "processing" several hundred liters of liquid nitrogen in ~50 hours, which in turn implies a flow rate of, and efficient heat exchange from, several cubic meters of gas per hour. Venting and handling facilities must be provided both at NOAO and at Gemini if this capability is a permanent part of the instrument. Note also that the operating temperature of the instrument is significantly cooler than the temperature of liquid nitrogen, so that the design must include provisions to avoid turning the pre-cool system into a heat source during operations, among other considerations.
2. Warm-up
The instrument must reach ambient temperature in <24 hours. It is assumed that this is done using internal heating resistors. Back-filling with dry gas or other means of accelerating warm-up are not currently contemplated.
The warm-up of refractive optics cannot be so rapid that they are over-stressed. This may imply a need to regulate warm-up rate, since otherwise the warm-up curve is unlikely to be linear. As an example, consider data for Phoenix, which is warmed up using constant power applied to heater resistors (until a set-point is reached). Although complete warm-up requires slightly over 24 hours, going from ~60K to room temperature, the initial warm-up rate is nearly double the average, or ~20K/hour.
Warm-up must include adequate safeguards to avoid over-heating insides. This implies avoidance of control through external software.
3. Cryo-Pumping Concerns
Both warm-up and cool down must be managed so that material that is out-gassed and/or trapped in getters does not end up preferentially on optical surfaces or detectors. Design considerations must therefore include location of getters, presence or absence of heat switches, and operating sequences during warm up and cool down.
It should be assumed that the standard temperature control circuits and resistors on the detector mounts are not capable of elevating detector temperature substantially above normal operating temperatures.
4. Gradients and Stability
Gradients should be minimized. In particular, no part of the instrument after and including the filters can be >70K. Gradients along the optical path must be stable, and there must be adjustments and procedures that allow correction of the optical alignment for such gradients, if they are not negligible. Gradients will tend to be most critical in non-collimated light, over long focal lengths (e.g., cameras and collimator).
In addition, variations in overall temperature should, ideally, be minimized. Failing this, there must be adjustments and procedures that allow correction of the optical alignment for such variations. Temperature variations will affect refractive optics primarily (e.g. cameras and OIWFS). Since these variations may occur when the instrument is in use or between alignment in the lab and use on the telescope, the adjustments must be external and must be controllable remotely.
Note that temperature variations may arise from several causes potentially. These include (a) variations in ambient temperature (b) variations in cryocooler performance, whether due to orientation, aging, contamination, or other causes, and (c) varying heat inputs from internal mechanisms, heater resistors, etc.
Note that radiative coupling is roughly four times more sensitive to temperature variations than conductive coupling, so a fruitful approach to minimizing variations may be partial isolation of radiation shields from the instrument structure.
5. Mechanism Heat Input
The internal mechanisms will produce varying amounts of heat, depending on whether they are being moved or are stationary. Temperature control for detectors may have some effect as well, although it is likely to be less significant.
Overall, the effect of heat input from mechanisms must not produce significant variations in temperature gradients in the instrument. This can be accomplished in various ways, among them:
a - minimizing energy input to the mechanisms (avoiding high friction, large motors, etc.)
b - isolating heat sources (stand-offs, external motors)
c - directing heat flow away from critical areas (strapping internal motors, design of optical bench, etc.)
6. Cryocooler choice
Several factors influence choice of cryocooler heads:
a - commonality with other instruments/overall Gemini system
b - overall performance/stability
c - reliability/ease of maintenance
d – cost
7. Detailed Instrument Thermal Design
As the instrument design progresses, relevant design documents
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