Grism-based Spectrograph Modeling¶
Version: | 0.6 of 16/09/27, 15:24 |
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Author: | Yannick Copin <y.copin@ipnl.in2p3.fr> |
Abstract: | This module implements the spectrograph optical model used for the SuperNova Integral Field Spectrograph [Lantz04] data-reduction pipeline, and presented in [Copin00]. It provides a flexible chromatic mapping between the input focal plane and the output detector plane, based on an effective simplified ray-tracing model of the key optical elements defining the spectrograph (collimator, prism, grating, camera), described by a restricted number of physically-motivated distortion parameters. |
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Optical modeling¶
The model only handles the geometrical part of the light propagation through the spectrograph, i.e. provides a mapping between a wavelength and a position in the entrance focal plane to a position in the detector plane (at the output focal plane). It is therefore assumed all the PSF and scattered light effects (related to focus, axial chromatic aberrations, diffusion, etc.) are modeled independently.
Optical elements¶
The spectrogrism.Spectrograph
model includes optical elements between
the input focal plane and the output detector plane:
- an optional
spectrogrism.Telescope
, which converts a direction in the sky into a position in the focal plane; - a
spectrogrism.Collimator
, which converts a position (in the focal plane) into a direction; - a
spectrogrism.Grism
(an association of aspectrogrism.Prism
and aspectrogrism.Grating
), which converts a direction into another direction depending on wavelength and dispersion order; - a
spectrogrism.Camera
, which converts a direction into a position (in the detector plane); - a
spectrogrism.Detector
, which actually project positions onto the translated and rotated detector.
Both spectrogrism.Collimator
and spectrogrism.Camera
include
geometric quadratic radial distortion and lateral colors (transverse chromatic
aberrations). spectrogrism.Prism
includes 3D-tilts.
Positions and directions¶
- A 2D-position \((x, y)\) is encoded in complex number \(x + jy = r\,\exp(j\varphi)\).
- A 2D-direction \((\theta, \phi)\) – where \(\theta\) is the angle between the optical axis \(Oz\) and the propagation direction, and \(\phi\) the phase in the transverse plane \((Oxy)\) – is encoded in complex number \(\tan(\theta)\,\exp(j\phi)\).
- A 3D-direction is encoded by the coordinates of the unit-vector \((x, y, z)\).
Near Infrared Spectrometer and Photometer (NISP)¶
The spectrogrism
optical model is used to simulate the Near Infrared
Spectrometer and Photometer (NISP) of EUCLID, mimicking full sky-to-detector
Zemax 1st-order simulations for a nominal NISP configuration (courtesy
J. Zoubian).
Questions regarding the current Zemax simulation:
- input y-coordinates are offset by +0.85 deg
- output y-coordinates are not centered: (dx, dy) = (+0.7, 179.8) mm, corresponding to (+0.7, -4.2) mm for centered y-coordinates
- input position (+0.4, +1.25) is missing the 1.85 µm wavelength
References¶
SNIFS optical parametrization
(SNfactory Technical Documentation)Redefinition of the blue grating
(SNfactory Technical Documentation)- Euclid Mission Database
- NISP Technical Description (EUCL-LAM-OTH-7-001)
[Copin00] | 2000PhDT.........6C |
[Lantz04] | 2004SPIE.5249..146L |