Optical design of a multi-channel narrow-band imager for Lyman ultraviolet observation of diffuse sources

Zheng Lou; Li Ji; Sen Wang

doi:10.1364/AO.58.001016

Applied Optics
Vol. 58,
Issue 4,
pp. 1016-1024
(2019)
•https://doi.org/10.1364/AO.58.001016

Optical design of a multi-channel narrow-band imager for Lyman ultraviolet observation of diffuse sources

Zheng Lou, Li Ji, and Sen Wang

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Zheng Lou, Li Ji, and Sen Wang, "Optical design of a multi-channel narrow-band imager for Lyman ultraviolet observation of diffuse sources," Appl. Opt. 58, 1016-1024 (2019)

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Abstract

This paper describes a novel optical design for a multi-channel narrow-band imager observing at the Lyman ultraviolet wavelengths. As one of the key instruments onboard the Census of Warm-Hot Intergalactic Medium, Accretion, and Feedback Explorer space mission, the imager is designed to map the neutral hydrogen Lyman-α line emissions from the intergalactic medium and nearby galaxies. The optical design features an instantaneous field of view (FOV) of 20 arc min in diameter and a moderate and adjustable spectral resolution of 500–2000. A highly dispersive monochromator system is used to split the incident light into several wavelength channels, each with a full-field imaging capability and adjustable bandwidth, and the ensuing imaging optics form the individual channel images on different areas of the same microchannel plate detector. Channel wavelengths are tunable and a continuous wavelength scanning across the entire band of interest is proposed, which yields spectrally resolved 3D images with moderate spectral resolutions.

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Parameter	Specification
Wavelength coverage	124.9–127.7 nm
Number of wavelength channels	3 channels, separated by 1 nm
Spectral resolution	500/1000/2000
FOV	$20^{'}$ diameter
Spatial resolution	$< 30^{''}$
Sensitivity	500 PU ( $3 σ$ , $10^{4} s$ , $R = 500$ )
Payload dimension constraint	$< 1000 \times 500 \times 500 mm$
MCP size and resolution	$30 \times 40 mm$ , 40 μm

Telescope Parameters	Primary aperture	$500 \times 50 mm$	Secondary aperture	$28 \times 6 mm$
	Primary focal length	613.43 mm	Secondary magnification	16.9
	Primary off-axis distance	120 mm	Final focal length	1700 mm
Monochromator Parameters	Incident arm length	731 mm	Exit arm length	731 mm
Monochromator Parameters	Incident angle	36.2°	Exit angles	42.0°/42.7°/43.5°

		80% energy encircle diameter (arc sec)
		$R = 500$			$R = 2000$
x-field (deg)	y-field (deg)	125.3 nm	126.3 nm	127.3 nm	125.3 nm	126.3 nm	127.3 nm
0.0	0.0	15.7	16.0	14.9	13.0	13.7	11.8
0.115	0.0	15.1	16.3	17.4	9.5	10.6	11.5
0.0	0.115	12.8	12.7	12.8	5.6	4.8	5.5
0.0	−0.115	15.3	14.3	13.2	12.1	10.8	9.0
0.163	0.0	17.3	19.4	21.0	11.4	14.1	17.1
0.0	0.163	18.1	16.3	16.9	15.9	13.0	14.2
0.0	−0.163	11.4	10.3	9.8	7.7	6.8	7.4
0.115	0.115	17.4	17.7	19.6	12.8	12.7	15.2
0.115	−0.115	12.5	13.1	13.8	7.5	7.3	8.8

Symbol	Physical Meaning	Value	Unit	Notes
$A_{0}$	Geometrical collecting area	250	${cm}^{2}$	$500 mm \times 50 mm$
$Ω$	Source solid angle	$7.6 \times 10^{- 7}$	sr	$3 arc min \times 3 arc min$
$Δ$	Image spot size	0.022	${cm}^{2}$	With plate scale of 8.3 μm/arc sec
$R$	Reflectivity	0.75		With $Al + {MgF}_{2}$ coating
$η$	Grating efficiency	0.22		According to HORIBA Jobin Yvon
$c$	Contamination loss	0.1		Estimation according to FUSE
$h$	Detector QE	43%		MCP specification
$L$	Source radiance	500	PU	From science requirement
$p$	Detector background	0.035	$cnt / {cm}^{2} / s$	MCP specification
$W$	Sky background	75	PU/Å	According to Leinert, 1998 [26]

Spectral Mode		$R = 500$	$R = 1000$	$R = 2000$
Ray tracing	FWHM bandwidth (nm)	0.252	0.128	0.064
Ray tracing	Spectral resolution	501	988	1962
Ray tracing, effect of finite footprint included	FWHM bandwidth (nm)	0.262	0.138	0.074
Ray tracing, effect of finite footprint included	Spectral resolution	482	918	1704

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Applied Optics