Updating and including content on the PSF introduction #61
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| # Definition | |||
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| We can vaguely define the Point Spread Function as the response of imaging system to a point source. The concept of PSF is used throughout many imaging applications, some of them are astronomical imaging and medical imaging. Depending on the science application the definition and its sources may vary. However, they all include the [diffraction](https://en.wikipedia.org/wiki/Diffraction) phenomena. The fact of having a non-infinite lens sets a limit on the resolution of the imaging system. | |||
| We can vaguely define the Point Spread Function (PSF) as the response of imaging system to a point source. The concept of PSF is used throughout many imaging applications, some of them are astronomical imaging and medical imaging. Depending on the science application the definition and its sources may vary. However, they all include the [diffraction](https://en.wikipedia.org/wiki/Diffraction) phenomena. The fact of having a non-infinite lens sets a limit on the resolution of the imaging system. | |||
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- "We can vaguely define": there is probably a better way to say that.
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Just removing the "vaguely" may be enough.
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| - __Spatial__ | ||
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| The variations that change as a function of the focal plane positions. These are main type of variations targeted by the PSF models. They mainly depend on the telescope's optics and the spatial correlations introduced by the atmosphere. |
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Just a suggestion: "The PSF varie as a function of the position on the focal plane. These variations are the main...."
Also, it really depends of the kind of science you want to do. In some cases the temporal variations are the main ones (adaptive optics, people working on speckles,...). You should precise for "long" exposure time on large focal plane (if your focal plane is an optical fiber you don't really care about spatial variations).
Maybe precise "turbulence in the atmosphere". If the atmosphere is static it wouldn't be a big problem.
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| - __Spectral__ | ||
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| The PSF is a function of the wavelength. The PSF at a specific wavelength is usually called a monochromatic PSF. The observations of our telescope are integrated at all the wavelengths allowed by the filter's bandpass used. As a consequence, our observations are polychromatic. Depending if the filter's bandpass is narrow or large and the given error requirements, the modelling of the PSF spectral variations might be required (e.g. [Euclid](https://www.euclid-ec.org)) or can be neglected (e.g. [DES](https://www.darkenergysurvey.org)). |
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I think that I would rephrase a bot this paragraph. I see your point and I agree with it but I feel the explanation unclear. Maybe one sentence on narrow filter like DES where the spectral variation can be neglected. Then, an example of large band pass like Euclid where it has to be included in the PSF modeling. Also, you should define "narrow" and "large" filters.
You could also mention DCR (not 100% sure of the acronym). Basically it is the diffraction due to the atmosphere which also has a chromatic dependence. Normally it is not a problem with narrow filters if the target is high in the sky (less atmosphere to go through) but it can be an issue if the target is close to the horizon. This problem can rise even with filters considered as narrow (for example we will probably have it in UNIONS for observations around the north galactic cap since our r-band filter is not that "narrow")
| - __Temporal__ | ||
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| The PSF also varies temporally. The time we are integrating the PSF is given by the exposure time. This is a fundamental parameter that will determine several characteristics of the PSF. For example, for ground-based surveys, it changes the impact of the atmosphere on the PSF. | ||
| The instruments characteristics also change over time giving the PSF a low frequency variation. For space-based surveys where the atmosphere does not play a role, the characteristics of the instrument change over time following temperature changes for example. This makes unpractical the estimation of a single PSF model for all the exposures, as all the instrument's variability over time and its influence over the PSF should be modelled. It is common to estimate PSF models for each exposure (or several exposures) in order to tackle this low frequency temporal variations. |
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- "following temperature changes": suggestion "following temperature fluctuations". You can also say "(due to the orientation in regard to the Sun)" something like that. Those things are suppose to be "absorb" by the telescope and computed oil advance but nothing is perfect ;).
- "This makes unpractical the estimation of a single PSF model" I don't think we would ever do that. But I think what you mean is having a PSF determine in a lab under "perfect" conditions. Usually the model is parametrize and adjusted on observed stars for each exposures.
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| - _Size_ | ||
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| The size of the main mirror has a direct implicance to the size of the PSF. Following the diffraction theory, the larger the mirror for the lens, the smaller size the PSF should have. There are practical limitations on the maximal sizes of mirrors that are due to the manufacturing process. As an example, we can show the impact of the telescope's pupil diameter in the size of the PSF in the ideal Airy PSF we only take into account the diffraction phenomena. For this type of PSF we can write the Full Width Half Maximum (FWHM) as a function of the wavelength $\lambda$ and the pupil diameter $D$ as, |
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Some people could argue that we are building telescope with a diameter of 40m. It might interesting to talk about segmented mirrors since we are going to see more and more of them. But they are not a miracle solution since the PSF on those mirrors is kind of a mess..
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| ## Ground-based | |||
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| - __Seeing__ | |||
| - __Thermal variations__ | ||
| - The satellite experiences time where the sun is closer or were it is eclipsed by other object. This can occasionate high temperature changes in the space telescope. These temperature variations dilate and contract the optical system making it change of state as a function of temperature. It is sometimes to as _telescope's breathing_ for its repetitive pattern due to the orbits. See {cite}`nino2007` for an HST study of temperature variations. | ||
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| The satellite experiences time where the sun is closer or were it is eclipsed by other object. This can occasionate high temperature changes in the space telescope. These temperature variations dilate and contract the optical system making it change of state as a function of temperature. It is sometimes to as _telescope's breathing_ for its repetitive pattern due to the orbits. See {cite}`nino2007` for an HST study of temperature variations. |
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- "It is sometimes to as": I think there is some words missing here.
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| ## Space-based | |||
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I agree with the different point in this section but they are also true for ground base telescope. The guiding is more or less the same (probably easier on the ground) and the thermal variations are an issue for ground based telescope.
| - __Polarisation__ | ||
| - Starlight can be naturally polarised by galactic foreground dust. The different polarisations that the incoming light has can interact differently with refractive elements of the optical system. This occasionates variations that are polarisation dependent. This source is currently being studied. | ||
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| Starlight can be naturally polarised by galactic foreground dust. The different polarisations that the incoming light has can interact differently with refractive elements of the optical system. This occasionates variations that are polarisation dependent. This source is currently being studied. | ||
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| ## Detector level | ||
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| - __Undersampling and pixelation__ |
| - __Quantum efficiency__ | ||
| - This effect describes the ratio of created electrons from the number of incoming photons. It depends on the wavelength and on the detector's technology. | ||
| This effect describes the ratio of created electrons from the number of incoming photons. It depends on the wavelength and on the detector's technology. |
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Maybe mention that for current detectors this is very good.
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@aguinot Have you tried |
I did. But it didn't work. |
This PR updates and includes content of the PSF introduction.
Comments and suggestions are welcomed!