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

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

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

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.

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

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

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

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

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

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

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

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

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)

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

New Titan data with occultation door open Titan was observed at a large phase angle on 2007-132 with the occultation door open The nitrogen emission in this observation is seen to be significantly lower than that from 2004-348 The mesa remnant is seen to be quite large, likely due to a bright source near the open occultation port FOV (the Sun?)

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.

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

EUV occultation port contamination EUV2007_130_21_23_47_UVIS_044SA_LIMBSKIM001_PRIME EUV2007_147_05_52_21_UVIS_045MI_ICYLON003_PRIME

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

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

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

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

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

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?