Chopping (astronomy)
A technique to cancel background heat in infrared telescopes.
Chopping is a method used in infrared astronomy to reduce interference from heat sources, both inside the telescope and in the surrounding environment. A device called a chopper rapidly rocks a mirror—usually the secondary mirror—back and forth. This fast motion is necessary because background radiation can change too quickly to be measured otherwise. By comparing images taken at slightly different angles, astronomers can separate the heat coming from the target object from the background heat, revealing details that visible-light or ultraviolet telescopes cannot detect.
However, chopping has two main drawbacks. First, the background radiation can vary as the telescope shifts between beams, which may produce unusual-looking results. Second, the technique only works for objects that are relatively close; if the target is too far away, it falls outside the telescope’s effective range.
The Gemini Observatory consists of two infrared telescopes, one in Hawaii and one in Chile. Both use chopping to eliminate background radiation, rocking their secondary mirrors at a rate of 3 to 5 Hz. They also employ tilt-tip correction to keep images sharp, and the University of Florida helped improve their chopping mechanisms.
The SOFIA Telescope was an infrared telescope mounted on a Boeing 747. It relied on its Second Mirror Assembly (SMA) primarily for chopping, and partially for small corrections. The signals SOFIA sought could be tens of thousands of times fainter than the background emission, so it chopped at an average frequency of 2 to 5 Hz, though it could reach 20 Hz. The chopping motion followed a square wave pattern.
The James Webb Space Telescope, despite being an infrared telescope, does not use chopping. Instead, it relies on gold-coated mirrors that reflect nearly all heat. It also avoids chopping because it is designed to observe very distant objects, and the required angle would need constant refocusing depending on the target’s distance.
- Chopping frequency gemini
- 3-5 Hz
- Chopping frequency sofia average
- 2-5 Hz
- Chopping frequency sofia maximum
- 20 Hz
- Chopping waveform sofia
- square wave
- Telescopes using chopping
- Gemini Observatory (two telescopes, one in Hawaii and one in Chile), SOFIA Telescope
- Telescope not using chopping
- James Webb Space Telescope
Lore & Background
Chopping is employed by infrared telescopes to eliminate background radiation by taking pictures at slightly different angles. By comparing separate images, it becomes much easier to tell what the background heat is and what is coming from the object in question. This allows it to be used to gain information about the object that a visible light telescope or a UV telescope cannot see.
The Gemini Observatory, a set of two telescopes with one in Hawaii and the other in Chile, both use chopping to eliminate background radiation. Their telescopes chop their secondary mirrors at a rate of 3-5 Hz, and both observatories utilize tilt-tip correction to ensure crisp image quality. The observatories also received help from the University of Florida to improve their chopping mechanisms.
The SOFIA Telescope, an infrared telescope integrated into a Boeing 747, utilized its Second Mirror Assembly (SMA) primarily for chopping and also partially for small corrections. The signals it searched for could be tens of thousands of times less significant than the background emission, so the telescope had an average chopping frequency of 2-5 Hz, but could reach rates of 20 Hz. Its chopping action followed a square wave pattern.
Reader's Guide
Chopping is significant because it allows infrared telescopes to observe objects that would otherwise be invisible due to overwhelming background radiation from internal or external heat. The technique is used by major observatories such as the Gemini Observatory and the SOFIA Telescope, enabling them to capture data that visible or UV telescopes cannot. However, chopping has two major drawbacks: variance of background radiation can cause strange appearances when the telescope shifts between beams, and the method can only be used when the object is not too far away, otherwise it would be out of the telescope's range. Notably, the James Webb Space Telescope, though an infrared telescope, does not use chopping; instead it uses gold-coated mirrors to reflect almost all heat, and because it is meant to bore into deep space, the angle would have to be refocused depending on how far away the object is. The legacy of chopping lies in its role as a practical solution for ground-based and airborne infrared astronomy, despite its limitations.
Did You Know?
- The SOFIA Telescope could reach chopping rates of 20 Hz, following a square wave pattern.
- The James Webb Space Telescope does not use chopping; it uses gold-coated mirrors to reflect heat.
- The University of Florida helped improve the chopping mechanism of the Gemini Observatory.
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