|
|
AUSTRALIAN
NATIONAL UNIVERSITY System Design Note 5.15 Created: 4 April 2000 Last modified: 5 April 2000 |
NIFS OPTICAL THROUGHPUT
Peter J. McGregor
Research School of Astronomy
and Astrophysics
Institute of Advanced
Studies
Australian National
University
Revision History
|
Revision No. |
Author & Date |
Approval & Date |
Description |
|
Revision 1 |
Peter J. McGregor 19 August 1999 |
Jan van Harmelen 23 August 1999 |
Original document. |
|
|
|
|
|
Contents
5 NIFS Optical
Throughput Summary
1 Purpose
This document describes the optical throughput budget for the Near-infrared Integral-Field Spectrograph and the assumptions on which it is based.
2 Applicable Documents
|
Document ID |
Source |
Title |
|
IGPO |
NIFS Concept Study Work Scope |
|
|
RSAA |
NIFS Optical Design Specification |
|
|
RSAA |
NIFS Diffraction Analysis |
|
|
RSAA |
NIFS Optical Coatings |
|
|
|
|
|
3 Introduction
NIFS is required to have an optical throughput of > 15% in the range 1-2.5 mm for the complete optical train, including, but not limited to, the telescope, gratings, filters and the detector, but not including the adaptive optics train. The assumptions used in modeling the NIFS optical throughput are described in this document. The result is an optical throughput budget for the instrument that will be used in assessing performance. Optical throughput is calculated for the nifs_ful_40.zmx optical design assuming 4 mm high pupil mirrors and 90 mm long gratings. This design uses a Maksutov spectrograph collimator and paraxial camera. The paraxial camera is substituted with the spectrograph camera from nifs_ful_19.zmx (the design in the original proposal) for the purpose of calculating throughput.
4 Modeling Assumptions
The optical throughput has been modeled by considering the reflection losses at each optical surface. All mirrors are assumed to be coated with over-coated silver. All lenses are assumed to be anti-reflection coated with a single layer of MgF2 with a design wavelength of 1.50 mm. Reflection losses for these surfaces are modeled based on the wavelength-dependent refractive indices of the coating and lens materials using single layer anti-reflection coating theory (Melles Griot Optics Guide 5). The order blocking filters are required to have optical throughputs > 80% over their spectral bands. We adopt a value of 80%, independent of wavelength. Diffraction losses at the NIFS pupil mirrors and grating are treated in a separate System Design Note. These depend on wavelength and field position across each slitlet; they are typically <3%, and always less than 10% for 90 mm long gratings.
The diffraction grating efficiencies are a major uncertainty in the throughput budget. Caution dictates using a grating efficiency of ~ 50%. However, efficiency curves are presented in Richardson Grating Lab. literature for the 600 l/mm 17.5° blaze angle grating proposed for the J1 and J2 bands. The efficiency of this grating is ~ 80% over the wavelength range of interest for NIFS. We therefore adopt a grating efficiency of 65±15% for the present purpose. Actual grating efficiencies should be determined at a later date.
We adopt the detector quantum efficiency function for a HAWAII-1 array published on the Rockwell Science Center FPA web pages (Figure 1). The HAWAII-2 array to be used in NIFS is expected to have similar quantum efficiency. Rockwell arrays based on CdZnTe technology may have better quantum efficiency.
Figure 1: Adopted detector quantum efficiency function based on a Rockwell HAWAII-1 array.
We assume that the bulk absorption properties of the transmissive component are negligible.
No allowance is made for absorption or scattering in the Earth’s atmosphere.
Emissivities are calculated for each surface based on their absorption properties.
5 NIFS Optical Throughput Summary
The NIFS optical throughput and emissivity budgets are summarized in Table 1. The wavelength dependent throughput for the complete instrument including telescope, filter, grating, and detector is shown in Figure 2. The optical throughput for NIFS alone, excluding the telescope and detector is shown in Figure 3.
Table 1 NIFS Optical Throughput Budget
|
Component |
Coating |
Transmission |
Emissivity |
||||
|
|
|
1.00mm |
1.65mm |
2.20mm |
|
2.20mm |
|
|
|
|
|
|
|
|
|
|
|
Telescope Primary |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
Telescope Secondary |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
ISS Fold |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
Cryostat Window |
CaF2/MgF2 |
0.949 |
0.960 |
0.955 |
|
0.045 |
|
|
Science Fold Mirror |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
Offner Primary |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
Offner Secondary |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
Offner Primary |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
Filter |
|
0.80 |
0.80 |
0.80 |
|
0.20 |
|
|
Fold Mirror |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
Focal Ratio Converter 1 |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
Focal Ratio Converter 2 |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
Image Slicer: Reflection |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
Image Slicer: Diffraction |
|
0.99 |
0.98 |
0.97 |
|
… |
|
|
Pupil Mirrors |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
Field Mirrors |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
Collimator Fold |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
Collimator |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
Collimator Corrector |
CaF2/MgF2 |
0.949 |
0.960 |
0.955 |
|
0.045 |
|
|
Grating |
Gold? |
0.65 |
0.65 |
0.65 |
|
0.02 |
|
|
Camera 1 |
Silica/MgF2 |
0.950 |
0.951 |
0.950 |
|
0.050 |
|
|
Camera 2 |
CaF2/MgF2 |
0.949 |
0.960 |
0.955 |
|
0.045 |
|
|
Camera 3 |
BaF/MgF2 |
0.948 |
0.966 |
0.958 |
|
0.042 |
|
|
Camera 4 |
C9763/MgF2 |
0.934 |
0.987 |
0.964 |
|
0.036 |
|
|
Camera fold |
O/C Silver |
0.979 |
0.986 |
0.987 |
|
0.013 |
|
|
Detector QE |
|
0.518 |
0.583 |
0.623 |
|
… |
|
|
|
|
|
|
|
|
|
|
|
TOTAL (without AO) |
|
0.137 |
0.190 |
0.195 |
|
|
|
|
ALTAIR |
|
0.773 |
0.825 |
0.843 |
|
|
|
|
TOTAL (with AO) |
|
0.106 |
0.157 |
0.164 |
|
|
|
Figure 2: Optical throughput for the complete NIFS instrument, including telescope, filter, grating, and detector, but excluding ALTAIR adaptive optics system. The upper curve includes single layer MgF2 AR coating on all lenses. The lower curve is for uncoated lenses.
Figure 3: Optical throughput for NIFS excluding the telescope and detector, and excluding ALTAIR. The upper curve is for AR coated lenses. The lower curve is for uncoated lenses.
Appendix A: List of Figures
|
Figure 1 |
trans_det.gif |
|
Figure 2 |
trans_full.gif |
|
Figure 3 |
trans_nifs.gif |
|
|
|