1. UVIS Calibration Update
June 11, 2007
Greg Holsclaw, Bill McClintock

2. Outline New (March 2007) FUV calibration routine
Recent changes in the FUV sensitivity?
EUV Mesa mitigation
EUV occultation port contamination
Evil pixel response experiment
Spica paper
Continuing issues

3. FUV Calibration Update
An update to the FUV calibration was released in March, and integrated into cube generator
Calibration steps in Get_FUV_07_lab_calibration
Read in 1999 laboratory sensitivity (1-D vector, 1024x1)
Apply correction for the change in sensitivity over time (derived from Spica observations)
Replicate the 1-D vector into a full image 2-D detector array (1024x64)
Apply the flat field (includes color variations for each row)
if no flat field applied as set by keyword ‘noff’, set all evil pixels to NaN, as the calibration is now scaled for ‘good’ pixels only
Window and rebin the data as specified by the input parameters
Invert the sensitivity to create a calibration multiplier
The calibration matrix must then be applied to the data
Following this, the NaN values in the result must be interplolated using interpolate_nans2.pro

4. Recent FUV sensitivity changes
Average Spica signal
Average Spica irradiance
Two Spica raster scans, separated in time by 265 days, were used to determine if the senstivity has changed recently.
Of particular concern is that the calibration, derived from a set of Spica observations over time, must necessarily extrapolate to future times and therefore may produce erroneous results.

5. Recent FUV sensitivity changes
The sensitivity may have decreased by about 4% over this time period, as seen in this ratio
Ideally, the ratio of the calibrated curves should be one
Apparently, the current calibration does not correct the absolute calibration to this level, though this may improve if the more recent Spica observations are incorporated in the sensitivity time variation
However, minimal structure (~1.5%) is introduced through the calibration process

6. EUV Mesa experiments with the UVIS engineering model
The UVIS engineering model was employed to better understand the source of the EUV mesa feature
The grating assembly was removed to view the inside of the instrument cavity
A red laboratory laser provided a visible ray path through the occultation port

7. EUV Mesa experiments with the UVIS engineering model
View from the grating
Occultation door is closed
The laser is aimed through the HSP telescope, and then through the occultation port
The beam strikes the top of the occultation door, scatters to the ceiling, then onto the detector
Occultation port
Second-surface scatter
Door mechanism
Detector
Entrance slit
Telescope tube

8. EUV Mesa experiments with the UVIS engineering model
The distribution of stray light on the engineering model detector approximates that seen in the mesa
It was hypothesized that leaving the occultation port door in the open position would mitigate this stray light
Therefore an observation was planned to observe the LISM with and with out the door open
Engineering model
In-flight EUV mesa image EUV2007_106_02_22_57_UVIS_042SW_IPHSURVEY009_RIDER

9. EUV Door Test Door closed
EUV2007_106_02_22_57_UVIS_042SW_IPHSURVEY009_RIDER
Door open
EUV2007_106_00_04_37_UVIS_042SW_IPHSURVEY009_RIDER

10. EUV Door Test
With the door open, the row average is comparable in magnitude to the RTG background

11. EUV Door Test
Row 60 shows the worst case mesa remnant, peaking at about 2x the RTG level
The remaining stray light is reduced in magnitude and affects the top ~1/3 of the detector

12. EUV Mesa Subtraction from Titan data
Although an operating mode has been found which greatly reduces the stray light, much data exists with the mesa feature
The mesa measurement from the door experiment can be used to estimate the stray light in the data

13. EUV Mesa Subtraction from Titan data
Titan average
EUV2004_348_03_43_55_UVIS_00BTI_EUFUV002_PRIME
Mesa measurement
EUV2007_106_02_22_57_UVIS_042SW_IPHSURVEY009_RIDER
Difference (after RTG subtraction and scaling the mesa)

14. EUV Mesa Subtraction from Titan data
Titan and mesa spectrum, row average
Difference
A constant RTG background is assumed, and the mesa is scaled visually
Subtraction does a good job, with residuals below 95nm on the order of 20% of the RTG background
However, the Lyman-alpha wing has not been estimated well, and a different scaling above 105nm would help this

15. New Titan data with occultation door open
Titan was observed at a large phase angle on with the occultation door open
The nitrogen emission in this observation is seen to be significantly lower than that from
The mesa remnant is seen to be quite large, likely due to a bright source near the open occultation port FOV (the Sun?)

16. Estimation of the mesa remnant from a LISM scan?
This shows the row average for the Titan data and the door-open LISM scan.
The RTG is assumed to be constant, and the LISM spectrum is scaled upward by a factor of 4, but this does not provide a good background estimate.

17. Estimation of the mesa remnant from a LISM scan?
Mesa remnant distribution is different, perhaps due to an off-axis source
Titan average
EUV2007_132_11_15_00_UVIS_044TI_EUVFUV001_PRIME
Door open
EUV2007_106_00_04_37_UVIS_042SW_IPHSURVEY009_RIDER

18. EUV occultation port contamination
EUV2007_130_21_23_47_UVIS_044SA_LIMBSKIM001_PRIME
EUV2007_147_05_52_21_UVIS_045MI_ICYLON003_PRIME

19. EUV occultation port contamination
The feature appears to be localized to the lower half of the detector, centered around 78nm, and a few nm wide
This is likely due to a non-dispersed stray light source scattering off the occultation port door mechanism
Further investigation into the observation geometry should reveal some pointing constraints
However, if this feature is somewhat persistant, this still seems an improvement over the mesa

20. Evil pixel characterization
It remains unknown if the low-response ‘evil’ pixels present any useful information
One test is to see if they respond linearly to a change in incident flux
We have many observations of several UV bright stars
However, to do this experiment properly, the star should be slewed along the slit, as in a Spica raster scan, thereby illuminating each column uniformly

21. Evil pixel characterization
These plots show the spectra of all the stars observed thus far
The comparison star should fill most of the detector, be of a significantly different flux level than Spica, but not be too dim

22. Evil pixel characterization
Plot of the total count rate from each star, normalized and sorted
Choose three candidate stars, ~4x lower in irradiance than Spica
Alp Pav
Del Sco
Eta UMa
Scheduled for observation this summer
Comparison with absolute irradiance measurements by SORCE-SOLSTICE will provide an additional path to validating the calibration

23. Spica paper
A paper is planned to report the UVIS-measured absolute spectral irradiance of Spica in the EUV and FUV
The EUV ground calibration methods enable a unique observational dataset
Provides documentation of UVIS calibration techniques
This will also include an intercomparison with previous observations from: rockets, Voyager, IUE, EURD, and SORCE-SOLSTICE
A subsequent paper will provide an atlas of all stellar sources observed with UVIS

24. Continuing Issues A flat field for all targets
Two-dimensional PSF for scattered light removal
Two-dimensional EUV/FUV sensitivity changes?
Sensitivity change at Lyman-alpha?
Agreement between EUV and FUV calibration?