13.4 Spectrograph Design Parameters
13.4.1 FUV Channel
Table 13.12 presents design parameters of the FUV spectrograph and gratings. The FUV gratings are concave and have holographically generated grooves to provide dispersion and correct for astigmatism. The gratings have aspherical surfaces to correct for HST's spherical aberration. The FUV "M" gratings have been ion etched to produce triangular groove profiles for better efficiency. The G140L grating has grooves with a laminar profile. All FUV gratings are coated with MgF2 over aluminum.
The surface of the optic is a sphere of the quoted radius, but with a deviation of Δz = a4r 4 + a6r 6, where z is measured along the vertex normal. The quantities γ, δ, rc, and rd are the standard positions of the recording sources as defined in Noda, Namioka, and Seya (1974, J. Opt. Soc. Amer., 64, 1031).
Table 13.12: Design Parameters for the FUV Spectrograph and Gratings.
Dimension | G130M | G160M | G140L |
---|---|---|---|
secondary mirror vertex to aperture (z, mm) | 6414.4 | ||
V1 axis to aperture (mm) | 90.49 | ||
aperture to grating (mm) | 1626.57 | ||
α (degrees) | 20.1 | 20.1 | 7.40745 |
β (degrees) | 8.6466 | 8.6466 | −4.04595 |
α − β (degrees) | 11.4534 | ||
grating to detector (mm) | 1541.25 | ||
detector normal vs. central ray (degrees) | 9.04664 | ||
nominal groove density (lines mm−1) | 3800 | 3093.3 | 480 |
radius of curvature (mm) | 1652 | 1652 | 1613.87 |
a4 | 1.45789 × 10−9 | 1.45789 × 10−9 | 1.33939 × 10−9 |
a6 | −4.85338 × 10−15 | −4.85338 × 10−15 | 1.4885 × 10−13 |
γ (degrees) | −71.0 | −62.5 | 10.0 |
d (degrees) | 65.3512 | 38.5004 | 24.0722 |
rc (mm) | −4813.92 | −4363.6 | 3674.09 |
rd (mm) | 5238.29 | 4180.27 | 3305.19 |
recording wavelength (Å) | 4880 |
13.4.2 NUV Gratings
Table 13.13 presents design parameters of the NUV gratings. The NUV gratings are flat and were not constructed holographically. The NUV MAMA has low but measurable sensitivity at FUV wavelengths, and with some gratings second-order light could contaminate the spectrum. To minimize this effect, the coated optics are optimized for wavelengths above 1600 Å. Given the four reflections used in the NUV channel, wavelengths below 1600 Å, including geocoronal Lyman α, are effectively eliminated. In addition, gratings G230L and G285M have order-blocking filters mounted directly on them to block the second-order spectra below 1700 Å. Even with these filters, it is possible for second-order light to appear on the NUV MAMA when G230L is used, especially in the long-wavelength stripe.
Table 13.13: Design Parameters for the NUV Gratings.
Dimension | G185M | G225M | G285M | G230L |
---|---|---|---|---|
groove density (mm−1) | 4800 | 4800 | 4000 | 500 |
α (degrees) | 27.24 | 33.621 | 35.707 | 5.565 |
β (degrees) | 25.85 | 32.23 | 34.32 | 1.088 |
coating | Al + MgF2 | Al only | Al only | Al + MgF2 |
-
COS Instrument Handbook
- Acknowledgments
- Chapter 1: An Introduction to COS
-
Chapter 2: Special Considerations when Observing with COS
- • 2.1 COS FUV Detector Lifetime Positions
- • 2.2 Visit Length
- • 2.3 Central Wavelength Settings Added in Cycle 26
- • 2.4 ORIENT constraints for Extended Sources
- • 2.5 COS Observations Below 1150 Angstroms: Resolution and Wavelength Calibration Issues
- • 2.6 Time-Dependent Sensitivity Changes
- • 2.7 Spectroscopic Use of the Bright Object Aperture
- • 2.8 Non-Optimal Observing Scenarios
- • 2.9 NUV Spectroscopic Acquisitions
- • 2.10 SNAP, TOO, and Unpredictable Source Programs with COS
- • 2.11 Choosing between COS and STIS
- Chapter 3: Description and Performance of the COS Optics
- Chapter 4: Description and Performance of the COS Detectors
-
Chapter 5: Spectroscopy with COS
- • 5.1 The Capabilities of COS
- • 5.2 TIME-TAG vs. ACCUM Mode
- • 5.3 Valid Exposure Times
- • 5.4 Estimating the BUFFER-TIME in TIME-TAG Mode
- • 5.5 Spanning the Gap with Multiple CENWAVE Settings
- • 5.6 FUV Single-Segment Observations
- • 5.7 Internal Wavelength Calibration Exposures
- • 5.8 Fixed-Pattern Noise
- • 5.9 COS Spectroscopy of Extended Sources
- • 5.10 Wavelength Settings and Ranges
- • 5.11 Spectroscopy with Available but Unsupported Settings
- Chapter 6: Imaging with COS
- Chapter 7: Exposure-Time Calculator - ETC
-
Chapter 8: Target Acquisitions
- • 8.1 Introduction
- • 8.2 Target Acquisition Overview
- • 8.3 ACQ SEARCH Acquisition Mode
- • 8.4 ACQ IMAGE Acquisition Mode
- • 8.5 ACQ PEAKXD Acquisition Mode
- • 8.6 ACQ PEAKD Acquisition Mode
- • 8.7 Exposure Times
- • 8.8 Centering Accuracy and Data Quality
- • 8.9 Recommended Parameters for all COS TA Modes
- • 8.10 Special Cases
- Chapter 9: Scheduling Observations
- Chapter 10: Bright-Object Protection
- Chapter 11: Data Products and Data Reduction
-
Chapter 12: The COS Calibration Program
- • 12.1 Introduction
- • 12.2 Ground Testing and Calibration
- • 12.3 SMOV4 Testing and Calibration
- • 12.4 COS Monitoring Programs
- • 12.5 Cycle 17 Calibration Program
- • 12.6 Cycle 18 Calibration Program
- • 12.7 Cycle 19 Calibration Program
- • 12.8 Cycle 20 Calibration Program
- • 12.9 Cycle 21 Calibration Program
- • 12.10 Cycle 22 Calibration Program
- • 12.11 Cycle 23 Calibration Program
- • 12.12 Cycle 24 Calibration Program
- • 12.13 Cycle 25 Calibration Program
- • 12.14 Cycle 26 Calibration Program
- • 12.15 Cycle 27 Calibration Program
- • 12.16 Cycle 28 Calibration Program
- • 12.17 Cycle 29 Calibration Program
- Chapter 13: Spectroscopic Reference Material
- • Glossary