1. UVIS Calibration Update
Greg Holsclaw
Bill McClintock
June 28, 2010

2. Outline Performance update Flat-Field modifiers
Recent calibration observations
An alternative approach to sensitivity updates
Cassini SPICE kernel finder
PDS updates
Calibration paper topics

3. Performance update
Total signal vs spatial position for the last three Spica calibration scans

4. Flat-field Modifiers At the last team meeting…
It was shown that there are significant declines in sensitivity at localized regions across the detector (the starburn, long-wave end of the clear aperture occ-slit in the FUV)
It was decided that a corrector be created based on each UVIS stellar observation and made available to the team in a timely fashion
Laura has made these flat-field modifiers available on the team website at:
The use of the correctors in the cube generator calibration pipeline has been implemented

5. Flat-field Modifiers
These correctors provide adjustments for changes in the relative sensitivity in each column with respect to the average of several “reference” rows which are located in the region obscured by the occultation slit lens holder
Corrector values are linearly interpolated for observations which occur between two calibrations. If an observation occurs after the last calibration, the last corrector is used

6. Date: 2009-315 Spatial average
Loss-of sensitivity around Lyman-alpha looks like the FUV occultation slit.
Spatial average
Spectral average
Reference rows
starburn
undercorrected
Lyman-alpha

7. Recent Observations
There have been three calibrations since the last meeting:
FUV2010_037_08_01_45_UVIS_126IC_ALPVIR001_PRIME
FUV2010_088_18_22_56_UVIS_129IC_ALPVIR001_PRIME
FUV2010_131_01_36_16_UVIS_131IC_ALPVIR001_PRIME
The first two scans were complete. For the second observation, a flat-field modifier (for both EUV and FUV channels) was produced and placed on the website in a short time.
The third observation exhibits some missing data in the middle of the slew. This complicates the automated processing, and I have not yet found a good solution to deal with this.

8. An alternative approach to sensitivity updates
The FUV detector experienced a significant increase in sensitivity (>3x at 185 nm) at long wavelengths early in the mission
This change in sensitivity as a function of wavelength and time was addressed by fitting an exponential function to the fractional change over time at each wavelength (in 21 pixel bins)
Perhaps a better approach:
Establish a reference spectrum of Spica based on SOLSTICE measurements (>130 nm) and early spectra of Spica from UVIS (<130nm)
Similar to the approach used for the flat-field modifiers, linearly interpolate the change in sensitivity between calibrations
Facilitates quick updates to the absolute response of the instrument

9. Absolute spectra of Spica over time
Ratio of UVIS to SOLSTICE
SOLSTICE
Overall sensitivity decline
The time-varying sensitivity is overestimating the adjustment for the most recent observations

10. SPICE Kernel Finder - Intro
Kris Larsen had provided a convenient online tool to identify SPICE kernels for a given date
This web-based tool allowed a single date to be input, and a list of kernel files were provided as links to their location at the NAIF site
A more versatile tool would allow the required and most current kernels not just to be identified, but also downloaded automatically, organized, and a text file listing created to facilitate their use with cube generator
This approach abstracts the user from the details of SPICE kernel naming conventions and time-consuming organization

11. SPICE Kernel Finder - Overview
Objective: Using IDL, identify and download SPICE kernels relevant for a specific date
Input: Year, day-of-year
Action: identify and download required kernels
Output: A text file listing the downloaded kernels which can then be used by cube generator with the “Load kernel batch” option

12. SPICE Kernel Finder - Function
Cassini SPICE kernel repository:
ftp://naif.jpl.nasa.gov/pub/naif/CASSINI/kernels/
A list of all files is obtained for each kernel type (ck, fk, ik, lsk, pck, sclk, spk)
For kernels which are updated incrementally (fk, ik, lsk, pck, sclk) the latest version is selected
For kernels which are relevant to a specific date range (ck, spk) the filenames are parsed and the appropriate file is selected based on the date
Multiple c-kernels can satisfy a specific date. Therefore, the latest version is selected in this order of preference: c-smithed, reconstructed, predict

13. SPICE Kernel Finder - Example
cassini_spice_kernel_list, 2005, 331, fkernel, local_kernel_path = 'C:\Greg_Holsclaw\Cassini\UVIS\SPICE\'
Local_kernel_path specifies where kernels will be downloaded and stored locally (contains subdirectories lsk, sclk, pck, spk, ck, ik, fk)
fkernel is an output string specifying the ASCII file containing the metakernel list for use with CG:
‘C:\Greg_Holsclaw\Cassini\UVIS\SPICE\2005_331.mk’
Contents of fkernel:
lsk\naif0009.tls
sclk\cas00144.tsc
spk\de403s.bsp
spk\060111R_SCPSE_05320_05348.bsp
fk\cas_rocks_v18.tf
fk\cas_status_v04.tf
fk\cas_v40.tf
ck\05305_05334ca_ISS.bc
pck\cpck20May2010.tpc
ik\cas_uvis_v06.ti

14. SPICE Kernel Finder - Features
year and doy can be vectors
Any Cassini instrument (or multiple instruments) can be specified using a keyword (instrument=‘uvis’)
If all identified kernels exist locally, no internet connection is required (the default)
A refresh of the NAIF kernel lists can be forced using a keyword (newlist=1)
All ftp functions are accomplished using native IDL routines (version 6.3 and above), no external code or libraries are required

15. PDS updates
The team-endorsed approach to calibrating the data forces evil pixels to be effectively deleted from consideration (replaced with NaNs)
For some heavily binned UVIS observations, the entire calibration matrix has a value of NaN
To address this issue, the unbinned calibration matrix is now included with the PDS data
Potential inclusion of the flat-field modifiers with the PDS calibration matrix

16. Too many calibration topics for one paper?
Radiometric calibration
At a minimum, the lab measurements of H2 emission
Instrument characterization and time dependent sensitivity
flat field, PSF, scattered light, backgrounds, polarization, linearity, etc
Spica absolute flux 91.2 – 190 nm (extension of SOLSTICE to EUV using H2 emissions)
Comparison with other investigators
Spica light curve in the UV
Data analysis approach
forward model versus inverse approach, I/F derivation, deconvolution, etc
Atlas of all stellar spectral irradiance measurements