Steps toward 8m honeycomb mirrors VIII

Design and demonstration of a system of thermal control

A. Y S Cheng, J Roger P Angel

Research output: Contribution to journalArticle

Abstract

If they are to achieve the best possible images, honeycomb borosilicate telescope mirrors must follow ambient temperature changes while maintaining low internal temperature gradients. This is best done by cooling or heating the internal, edge and back mirror surfaces at the same rate established by convection on the front surface. We propose to use directed jets of air at ambient temperature, arranged with more air flow on thicker sections so as to match cooling rates. A full scale glass thermal model of one honeycomb cell of an 8m mirror was built, along with a system to flow in air at controlled temperature. We find that the air jets realize high thermal coupling efficiency, and allow good control of internal gradients. With the air cooling steadily at 0.25°C/hour, typical of nighttime cooling at good sites, internal gradients were <0.1°C and the overall lag between air and glass temperature was 0.25°C. This performance, achieved with a flow rate of 6-10 liters/sec per cell, will ensure negligible image degradation from convection at the mirror surface (mirror seeing) or from thermal distortion of the mirror substrate.

Original languageEnglish (US)
Pages (from-to)536-544
Number of pages9
JournalProceedings of SPIE - The International Society for Optical Engineering
Volume454
DOIs
StatePublished - May 15 1984

Fingerprint

honeycomb mirrors
Thermal Control
Honeycomb
Mirror
Demonstrations
mirrors
Mirrors
Cooling
Air
Internal
Gradient
cooling
ambient temperature
Convection
air
convection
air cooling
air jets
Glass
gradients

ASJC Scopus subject areas

  • Applied Mathematics
  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics
  • Electrical and Electronic Engineering
  • Computer Science Applications

Cite this

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abstract = "If they are to achieve the best possible images, honeycomb borosilicate telescope mirrors must follow ambient temperature changes while maintaining low internal temperature gradients. This is best done by cooling or heating the internal, edge and back mirror surfaces at the same rate established by convection on the front surface. We propose to use directed jets of air at ambient temperature, arranged with more air flow on thicker sections so as to match cooling rates. A full scale glass thermal model of one honeycomb cell of an 8m mirror was built, along with a system to flow in air at controlled temperature. We find that the air jets realize high thermal coupling efficiency, and allow good control of internal gradients. With the air cooling steadily at 0.25°C/hour, typical of nighttime cooling at good sites, internal gradients were <0.1°C and the overall lag between air and glass temperature was 0.25°C. This performance, achieved with a flow rate of 6-10 liters/sec per cell, will ensure negligible image degradation from convection at the mirror surface (mirror seeing) or from thermal distortion of the mirror substrate.",
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