To reject solar radiation photons at the front aperture for large telescopes, a mosaic of large transmission mode filters is placed in front of the telescope or at the aperture of the dome. Filtering options for effective rejection of sunlight include a smaller filter down-path near the focus of the telescope, and a large-diameter filter located in the front of the main aperture. Two types of large filters are viable: reflectance mode and transmittance mode.
In the case of reflectance mode, a dielectric coating on a suitable substrate (e.g. a low-thermal-expansion glass) is arranged to reflect only a single, narrow wavelength and to efficiently transmit all other wavelengths. These coatings are commonly referred to as notch filter. In this case, the large mirror located in front of the telescope aperture reflects the received (signal and background) light into the telescope. In the case of transmittance mode, a dielectric coating on a suitable substrate (glass, sapphire, clear plastic, membrane, and the like) is arranged to transmit only a single wavelength and to reject all other wavelengths (visible and near IR) of light. The substrate of the large filter will determine its mass. At first glance, a large optical filter with a diameter of up to 10 m, located in front of the main aperture, would require a significant thickness to avoid sagging. However, a segmented filter supported by a structurally rugged grid can support smaller filters.
The obscuration introduced by the grid is minimal because the total area can be made insignificant. This configuration can be detrimental to a diffraction- limited telescope due to diffraction effects at the edges of each sub-panel. However, no discernable degradation would result for a 20× diffraction- limit telescope (a photon bucket). Even the small amount of sagging in each sub-panel should have minimal effect in the performance of a non-diffraction limited telescope because the part has no appreciable optical power.
If the front aperture filter is integrated with the telescope dome, it will reject heat from the dome and will significantly reduce dome temperature regulation requirements and costs. Also, the filter will protect the telescope optics from dust and other contaminants in the atmosphere. It will be simpler to clean or replace this filter than the telescope primary mirror. It may be necessary to paint the support grid with a highly reflective material to avoid overheating.
This work was done by Hamid Hemmati and James Lesh of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. NPO- 40421
This Brief includes a Technical Support Package (TSP).
Solar Rejection Filter for Large Telescopes
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Overview
The document titled "Solar Rejection Filter for Large Telescopes" discusses the development and specifications of filters designed to enhance the performance of optical communication receiving stations, particularly those with large apertures (around 1 meter or more). These telescopes must operate effectively during both daytime and nighttime, necessitating the rejection of solar radiation to prevent overheating and misalignment of the receiver.
To address these challenges, the document outlines two primary filtering options: small filters located near the telescope's focus and large filters positioned in front of the main aperture. The small filter option, which includes a "cold" reflecting pre-filter followed by a bandpass filter, is less effective due to significant heat entering the telescope. In contrast, large filters are more viable and can be categorized into two types: reflectance mode and transmittance mode.
In reflectance mode, a dielectric coating on a suitable substrate reflects a narrow wavelength while transmitting others, functioning as a notch filter. In transmittance mode, the coating allows only a specific wavelength to pass while rejecting all others. The document emphasizes that a large optical filter, potentially up to 10 meters in diameter, would require significant thickness to prevent sagging. However, a segmented filter supported by a robust grid can mitigate this issue, allowing for smaller filters with minimal obscuration.
The document also specifies the top-level requirements for the filter, including parameters such as transmission rates, out-of-band rejection, wavefront quality, and surface quality. For optimal performance, the filter should have a transmission greater than 90% at the center wavelength (1064 nm) and an out-of-band rejection of 1E-5 from 200 to 1500 nm. The anticipated thermal loading and the need for a low thermal expansion substrate are also discussed.
Overall, the document highlights the importance of these solar rejection filters in ensuring the effective operation of large telescopes in varying solar conditions, thereby enhancing their utility in optical communication and other applications. The insights provided are part of NASA's broader efforts to develop technologies with potential applications beyond aerospace, contributing to advancements in various scientific and commercial fields.
