Brightness Calibration of Optical Spectrographs

Transcription

1 Brightness Calibration of Optical Spectrographs D. Pallamraju 1, J. Baumgardner 1, and R. Doe 2 1 Center for Space Physics, Boston University, MA 2 SRI International, Menlo Park, CA CEDAR, Santa Fe NM. June 23, 2006

2 Schematic of a typical optical system Aperture Collimator Band pass, interference filter, etc. Filter Dispersing element Fabry Perot, Grating, Grism, etc. Reimaging Lens PMT, IPD, CCD, ICCD, etc. Detector Not all systems may need all these components

3 Filter Transmission (i) For small bandwidth reduction in transmission from low angles. (ii) For large bandwidth contribution from other wavelengths not avoidable. AT 20 o INCIDENCE 61% AT 0 o INCIDENCE 10% (a) Be careful with all-sky measurements with narrow bandwidth filters.

4 Angular/Geometric Calibration The relation between the view angles/sky position and pixels on the detector. Laboratory calibration. Imaging the markers on the outside dome. Sky H H tanθ θ CCD

5 High Resolution Imaging Spectrograph using Echelle grating: (HIRISE) Twilighttime Daytime Oxygen Red Line Several Interesting results have been obtained using HIRISE Ring Effect: Pallamraju et al., 2000 (GRL) Daytime auroral arcs:pallamraju et al., 2001 (JGR) HIRISE instr.: Pallamraju et al., 2002 (JASTP) Dayglow from Chile: Pallamraju et al., 2004 (AG) Daytime Cusps: Pallamraju et al., 2004 (GRL) Daytime aurora over Boston: Pallamraju & Chakrabarti, 2005 (GRL) Review of Daytime emissions: Pallamraju & Chakrabarti, 2006 (JASTP)

6 Flat field correction: (i) Vignetting: The decrease in illumination away from the optical axis in an optical system. (ii) Structures or dust within the instrument, hot pixels on the detector, etc. I( x, y)! I ( x, y) Before I corrected ( x, y) = ( x, y)! I ( x, Correcting the image with a structure-less white light image. I flat dark dark y) K After DN s -1

7 Brightness Calibration: Method 1: Theoretical Estimate For a brightness B in Rayleighs, no of photons N passing through the instrument is: N = BSt Where, t is integration time, S is the sensitivity of the instrument (DN s -1 R -1 ). At any given pixel, sensitivity per Angstrom is:

8 Pallamraju et al., 2002; JASTP Here, Q(λ) is overall eff. Of CCD; q(λ)/g τ is the optical efficiency of the instrument, A is the area of the slit, Ω is the solid angle of the sky that the slit sees d is the dispersion in Å pixel -1 n rows are the no. of rows that have been co-added in the spatial dimension. S pix = 4.6 x 10-4 (DN Å R -1 s -1 ) - Dividing the observations by S pix we obtain RÅ -1 - Integrating in x-direction we obtain brightness in Rayleighs.

9 Brightness Calibration: Method 2: Empirical Estimate Using a calibration lamp Keo Lamp Source Calibration Curve Calibrated Lamp Aperture Collimator nm 45.5kR/A Dispersing element Reimaging Lens Brightness of Keo source cross-calibrated with the Stanford source at HAARP Detector

10 PROCEDURE - Place the calibrated lamp in the front of the fore-optics. The spectral image recorded on the detector is in DN s -1 - Subtract dark counts from this image. - Divide such dark subtracted spectral image with the calibrated brightness of the lamp (in RÅ -1 ) at the required wavelength. Now the units are: DN Å R -1 s -1 - Divide the observational data (in DN s -1 ) obtained by this calibrated image (DN Å R -1 s -1 ). Yields an image in RÅ -1.

11 Brightness Calibration: HIRISE and ASI calibrated using a Keo calibration source. This source has been successfully cross-calibrated with Stanford source at HAARP. Planned a special 9-hour long ISR run for cross-calibrating HIRISE, ISR/GLOW and ASI measurements. Remarkable agreement between HIRISE emissions and ISR/GLOW Excellent agreement between HIRISE emissions and ASI emissions Daytime Nighttime GLOW model s performance: The latest version of GLOW shows good reproduction of ISR measured ionization in the E- and lower F-region [Doe et al., 2005]

12 Comparison of HIRISE measurements with ISR/GLOW Estimates Emissions along Up B Magnetic quiet day Sep08, 2005 Magnetic disturbed day Daytime Soft Energy (F-region) Arcs seen in HIRISE ~ UT Sep12, 2005 Daytime

13 SUMMARY - A brief overview on parameters to be considered for calibrating optical spectrographs has been presented. - Daytime OI 630.0nm emissions have been carried out using HIRISE from ISR radar facility at Sondrestromfjord. - Calibrated emissions from HIRISE show excellent agreement with ISR/GLOW emissions and ASI data, especially during magnetic quiet periods. - Magnetically disturbed 630.0nm daytime data by HIRISE shows emission enhancements due to soft energy precipitation.