Coude Spectrograph vs Cassegrain?

In summary, flux throughput refers to the amount of light that is collected and sorted in a spectrograph. There can be a flux throughput advantage in using a coude spectrograph compared to a Cassegrain spectrograph due to factors such as lower stray light, improved resolution, and the use of multiple detectors. However, the design of a spectrograph involves a complex trade-off between resolution, sensitivity, and spectral range, and the dimensions of the instrument may also vary depending on telescope aperture and focal length.
  • #1
AlphaCrucis
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TL;DR Summary
Explain how there can be a flux throughput advantage in using a coude spectrograph compared to a Cassegrain spectrograph?
What is Flux Throughput? Explain how there can be a flux throughput advantage in using a coude spectrograph compared to a Cassegrain spectrograph?

I'm confused on what this actually means when comparing the two. Is it asking what are the seeing advantages in terms of resolving powers of a Coude vs a Cassegrain?
 
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I am not sure your question can be answered as it stands. Classical spectrograph design is a complex trade off between, resolution, sensitivity and spectral range. Its dimensions also scale with telescope aperture and focal length. Google spectroscopy design and look out for etendue which encapsulates throughput.

Regards Andrew
 
  • #3
If you don't know what flux throughput is, there is little change you would understand an answer about flux throughput advantage.

For any spectrograph (or any measurement), you have to be concerned about the ratio of the signal versus the noise. For a spectrograph, the signal is easy to understand. The more light you collect, the stronger the signal.

One source of noise is the electronics of your detector. For astronomical devices, the detector is usually cooled which significantly reduces noise from the electronics.

A spectrograph sorts photons by wavelength. You parse light into different buckets that cover a range of wavelengths. This isn't a perfect process and some photons don't get sorted properly. This is known as stray light - that is to say light that is scattered inside the instrument. A properly designed spectrograph will have low stray light. There are very efficient methods of correcting for stray light in a spectrograph see https://www.researchgate.net/publication/7254062_A_simple_stray-light_correction_method_for_array_spectroradiometers

You also have to consider the resolution. Detecting the signal in a 1nm bucket gives you a strong signal. Parse that light into 0.1 nm (1 angstrom) or 0.01 nm (0.1 angstrom) bins spreads that light out even more and the amount of light in each bin is lower. (It is also more difficult to sort the light so precisely).

Early spectrographs used a diffraction grating to sort light. A diffraction grating reflects (or refracts) light at different angles depending on its wavelength. A longer focal length increases the resolution, but may increase stray light. A longer focal length also increases the physical size of a spectrograph.

BTW: Cassegrain is a specific

Using a second diffraction grating (double monochromator) improves resolution since you are basically sorting the light twice.

A spectrograph can use a single detector and rotate the diffraction grating(s) to measure the signal vs wavelength. The slower the scan speed, the higher the resolution. This also means more time to collect a spectrum. Virtually all spectrographs these days use detector arrays which allows measurements of light at thousands of wavelengths simultaneously.

I hope this background helps you enough to let you find additional answers on your own.

BTW: Cassegrain is a specific type of optical design for a telescope. Coude (French for "elbow") refers to a focal point. The coude focus is stationary which makes it easier to install large or complex equipment.
 
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  • #4
Eric Bretschneider said:
If you don't know what flux throughput is, there is little change you would understand an answer about flux throughput advantage.

For any spectrograph (or any measurement), you have to be concerned about the ratio of the signal versus the noise. For a spectrograph, the signal is easy to understand. The more light you collect, the stronger the signal.

One source of noise is the electronics of your detector. For astronomical devices, the detector is usually cooled which significantly reduces noise from the electronics.

A spectrograph sorts photons by wavelength. You parse light into different buckets that cover a range of wavelengths. This isn't a perfect process and some photons don't get sorted properly. This is known as stray light - that is to say light that is scattered inside the instrument. A properly designed spectrograph will have low stray light. There are very efficient methods of correcting for stray light in a spectrograph see https://www.researchgate.net/publication/7254062_A_simple_stray-light_correction_method_for_array_spectroradiometers

You also have to consider the resolution. Detecting the signal in a 1nm bucket gives you a strong signal. Parse that light into 0.1 nm (1 angstrom) or 0.01 nm (0.1 angstrom) bins spreads that light out even more and the amount of light in each bin is lower. (It is also more difficult to sort the light so precisely).

Early spectrographs used a diffraction grating to sort light. A diffraction grating reflects (or refracts) light at different angles depending on its wavelength. A longer focal length increases the resolution, but may increase stray light. A longer focal length also increases the physical size of a spectrograph.

BTW: Cassegrain is a specific

Using a second diffraction grating (double monochromator) improves resolution since you are basically sorting the light twice.

A spectrograph can use a single detector and rotate the diffraction grating(s) to measure the signal vs wavelength. The slower the scan speed, the higher the resolution. This also means more time to collect a spectrum. Virtually all spectrographs these days use detector arrays which allows measurements of light at thousands of wavelengths simultaneously.

I hope this background helps you enough to let you find additional answers on your own.

BTW: Cassegrain is a specific type of optical design for a telescope. Coude (French for "elbow") refers to a focal point. The coude focus is stationary which makes it easier to install large or complex equipment.
Amazing answer! Thanks so much; it definitely cleared up a few things for me.
 

Related to Coude Spectrograph vs Cassegrain?

What is the difference between a Coude Spectrograph and a Cassegrain?

A Coude Spectrograph and a Cassegrain are both types of optical instruments used in astronomy. The main difference between them is the way they collect and analyze light. In a Coude Spectrograph, the light is collected by a primary mirror and then redirected to a separate instrument for analysis. In a Cassegrain, the light is collected by a secondary mirror and then redirected to a focal point for analysis.

Which type of instrument is better for spectroscopy?

The answer to this question depends on the specific needs of the researcher. A Coude Spectrograph is better for high-resolution spectroscopy, as the light path is longer and therefore allows for more precise measurements. However, a Cassegrain is better for wide-field spectroscopy, as it can collect more light from a larger area of the sky.

Can a Coude Spectrograph and a Cassegrain be used together?

Yes, it is possible to use a Coude Spectrograph and a Cassegrain together in a single instrument. This is known as a Coude-Cassegrain spectrograph and combines the advantages of both types of instruments. However, it is a complex and expensive setup, so it is not commonly used.

Which type of instrument is more commonly used in modern astronomy?

In modern astronomy, Cassegrain instruments are more commonly used due to their versatility and relatively simple design. They are also more cost-effective compared to Coude Spectrographs. However, for specific research purposes that require high-resolution spectroscopy, a Coude Spectrograph may still be preferred.

What are the advantages and disadvantages of each type of instrument?

The main advantage of a Coude Spectrograph is its ability to achieve high-resolution spectroscopy. However, it is a complex and expensive setup, and it also has a longer light path, which can result in some loss of light. On the other hand, a Cassegrain is a simpler and more cost-effective design, but it may not be suitable for high-resolution spectroscopy. Additionally, it may suffer from some optical aberrations due to the secondary mirror obstructing the light path.

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