Relative Spectral Response and Flat Fields with Internal Calibration Lamps Luisa M. Lara IAA-CSIC Granada (SPAIN)

WAC Relative Spectral Response – Data from Sept WAC Relative Spectral Response – Data from Sept. ’01, warm CCD at T=292 K NAC Relative Spectral Response – Data from Aug. ’01 and July ’03 with cold CCD (~190-200 K). Relative (and/or absolute) detector measurements not available for the NAC Delta Calibration. WAC “flats” (should be called “lamp images”) with Internal Calibration Lamps (main and redundant ones) – Data from ’01 calibration with warm CCD at T=292 K. NAC “flats” with Internal Calibration Lamps (main and redundant) – Data from NAC Delta Calibration in July ’03 (some inconsistency because I did not have proper flats).

The available measurements for the Wide Angle Camera were obtained during Sept.’01 with warm CCD at 292 K.. The monochromator produced monochrome light every 0.5 nm from 240 to 1100 nm and the intensity of such light was measured by a relative detector placed in front of the beam. The data reduction is the classical with any astronomical data, i.e. subtraction of dark current and bias. Blue line: relative spectral response of the CCD #42 (WAC FM) at T=295 K as obtained from measurements (DNs/sec/RelDet) with the camera in the vacuum tank. Green line: the same curve after correction by the transmission of the 3 cm width suprasil window and the non-absorbing plate in the vacuum tank, M1 and M2, and the suprasil filter. Red line: measurements in the laboratory of the quantum efficiency vs wavelength of the CCD #242 at T=295K.

There are two set of measurements for the Narrow Angle Camera: set 1) acquired during August 2001, and set 2) obtained in July 2003, that is, during the NAC Delta Calibration. The set-up was completely similar to that established for the WAC. In both runs, the CCD was cooled down to ~195-196 K. The monochromator produced monochrome light at desired wavelengths from 240 to 1050 nm, for set 1 and from 260 to 1100 nm for set 2. For the images in set 1, we used (FFP-Vis,FFP-UV) for 240    400 nm and (FFP-Vis,FFP-IR) at longer wavelengths, whereas for images in set 2 we considered (FFP-Vis,FFP-IR) at 350    1050 nm, (FFP-UV,NearUV) when 310    360 nm and (FFP-UV,FarUV) in the spectral range between 270 and 300 nm. During the Delta Calibration occurring after the replacement of the CCD, we did not take note of the light intensity as seen by the Relative Detector, and thus no consistent results can be obtained when considering those measurements made in July ´03, and the measurements by the relative detector noted in Aug. ´01.

Transmission of the 3 mirrors and the Anti Reflecting Plate (ARP).

Transmission curves of the different NAC optical components Transmission curves of the different NAC optical components. NAC Relative Spectral Response as computed from the measurements (i.e. DNs sec-1 / Relative Detector Meas.). The unexpected dip at 520 nm is removed when considering the transmission curve of the filter FFP-IR.

Relative spectral response of the CCD #241 (NAC Pre-FM) as obtained from measurements with the camera in the vacuum tank after correction of all optical elements (blue line) compared with that obtained from laboratory measurements (pink line). Normalization is done at 600 nm.

From measurements during NAC Delta Calibration in July ’03

Flat Fields with Internal Calibration Lamps: NAC and WAC OSIRIS “internal flat fields” were also obtained by illuminating the CCDs (NAC´s and WAC´s) with the Internal Calibration Lamps, both the main and the redundant ones. The internal lamps do not provide homogeneous illumination (especially for the WAC) on the CCDs as it will be shown later. Flat field images of the main and redundant calibration lamps are created by median combining 5 raw images of the lamps obtained with the same filter combination. Criteria for the selection were the CCD shutter performance and the absence of known problems with the image. For the NAC, we have considered the lamp images acquired during the Delta Calibration in July 2003. Regarding the WAC, the images were obtained in August, 2001. The selected images are full frame (2048 x 2048 pixels) images which are read out with amplifier A or B, making necessary to identify the pre-clocked pixels in each image before working with it. Bias and dark images were obtained before each session. The lamp images were acquired with warm CCD in the case of WAC (T=292 K) and with cold (T~ 195-200 K) CCD for NAC.

Available Data for Main and Redundant Lamps Empty Red640 Green535 Vis610 Na589 NH2 571 OI 630

Main Main Redundant Redundant Green Empty Empty OI 630 Green Empty

Main Main Redundant Redundant Empty Red640 Empty NH2 570 Empty Red640

Available Data Main Redundant ffp-vis, orange645 ffp-vis, green535 ffp-vis, blue480 ffp-vis, red740 ffp-vis, ffp-ir Ortho800, ffp-ir Near-ir, ffp-ir ffp-vis, hydra700 Fe2O3, ffp-ir

Processed images of the main (top) and redundant (bottom) internal calibration lamps acquired with filter combinations (ffp-vis,blue480), (ffp-vis,red740) and (ffp-vis,ffp-ir) from left to right. The dynamic range of the look-up is +/-0.2 times the value of the average in columns, allowing for a clear identification of any feature.

Average in columns (solid line) and in rows (dotted line) of a normalized image of the main and internal calibration lamps acquired with filter combination (ffp-vis,red740). Note that the redundant lamps illuminate the CCD “more evenly” than the main ones.

Conclusions There is a noticeable difference between the CCD spectral response measured in the lab and in the OSIRIS Calibration setup at wavelengths shorter than ~350 nm, either for the WAC FM and the NAC Pre-FM. For both OSIRIS cameras, the CCD illumination by the internal lamps is uneven, that is, the “bright” area is displaced to different positions depending on the set of lamps: in the case of NAC, the main lamps illuminate the top-left area, whereas the redundant ones the right-bottom area. Regarding the WAC, the main lamps illuminate a region going from left to right, approximately, whereas the redundant ones illuminate a diagonal region going from upper-left to bottom-right.