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WFPC2 STAN

S T A N / W F P C 2 - Number 24, March 1997

CONTENTS:

Preliminary Results of WFPC2 Post Service Mission 97 Check-out:

by J. Biretta

The Second HST Service Mission (SM97) was successfully completed on February 19, 1997. During the weeks that followed, WFPC2 was run through an extensive series of calibrations whose purpose was to protect WFPC2 from contamination and verify that its performance had not been affected by HST servicing. Nearly one-thousand calibration images were obtained and analyzed. So far the performance appears nearly identical to that before the service mission. The only significant change has been a small and temporary increase in the rate at which contamination grows on the CCD windows, and a corresponding increase in the amplitude of far-UV throughput variations. Herein we summarize the preliminary results of the post-SM97 WFPC2 calibrations.

We note that a description of the post-SM97 calibration plan can be found in WFPC2 Instrument Science Report 97-03 "Summary of WFPC2 SM97 Plans".

  • Contamination control/thermal management:
  • One of the most important goals of the post-SM97 calibration, has been contamination control. During normal on-orbit operation of WFPC2, molecular contaminants slowly grow on the cold (-88C) CCD windows, and are removed during a monthly "decontamination" where the CCD windows are warmed to +22C. The primary effect of these contaminants is to reduce the far-UV throughput. The presence of the Space Shuttle and astronauts in the vicinity of WFPC2 was expected to increase the rate at which contaminants collect on the cold windows. In an effort to avoid possible permanent contamination, the far-UV throughput has been extensively monitored (proposal 7016), and the thermal conditions of WFPC2 carefully managed.

    During servicing the CCD windows were kept at a temperature of -10C. Several days after HST release the windows were cooled to -43C, and the far-UV throughput was measured. After several more days they were cooled to -88C with far-UV (F170W filter) throughput measurements every few hours. The initial data at -88C showed that contamination was growing at roughly twice the pre-SM97 rate. Nonetheless, a 12 hour decontamination at +22C on 4 March demonstrated that the extra contaminants could be successfully removed, and full UV throughput restored. A plan was then formulated to resume normal science observations with bi-weekly decontaminations in order to keep the accumulated contaminants within known levels. Normal science observations resumed on 6 March. As of this writing, approximately one thousand WFPC2 science images have been successfully obtained since the service mission.

  • UV Throughput Variations:
  • A consequence of the increased contamination growth rate is that WFPC2 UV observers will need to make larger throughput corrections when performing photometric calibration of data taken within a month or two after the servicing mission. The calculation is the same as for pre-SM97 data, but the constants are larger, since the growth is more rapid. (See WFPC2 Instrument Handbook, v. 4, Section 6.9.) The table below shows the throughput loss in percent per day for the F170W filter in the different CCDs. The first set of numbers shows the normal pre-SM97 growth rates. The second set gives the measured rate after the 4 March 13h UT decon., and the third set gives rates measured after the 21 March 04h UT decon. Each decon effectively resets the throughput loss to zero, with the constants giving the decline per day following the decon.

    Dates  (UT)               CCD      F170W throughput
                                     loss / day in percent
    --------------------     -----   ---------------------
    pre-SM97 (<2/13/97)       PC1        0.53 +/- 0.04
                              WF2        0.95 +/- 0.02
                              WF3        0.95 +/- 0.02
                              WF4        0.77 +/- 0.02
        
    4 Mar 13h to 20 Mar 22h   PC1        0.84 +/- 0.05
                              WF2        1.32 +/- 0.05
                              WF3        1.21 +/- 0.05
                              WF4        1.06 +/- 0.05
                            
    21 Mar 4h to now          PC1        0.46 +/- 0.10
                              WF2        0.95 +/- 0.10
                              WF3        0.85 +/- 0.10
                              WF4        1.14 +/- 0.10
    

    The results after the 21 March decon (except for WF4) are generally consistent with pre-SM97 rates, and show that contamination levels are returning to pre-SM97 levels. Updated numbers for the period after 21 March will be posted as they become available.

    Throughput corrections for other filters can be roughly estimated by comparing the pre-SMOV corrections for them against F170W (see WFPC2 Instrument Handbook v.4 Table 6.10). Below we give ratios of (filter / F170W) throughput corrections for various filters. For example, using these two tables, we would predict the F336W throughput loss in WF3 on 15 March to be (11 days)(1.21% per day)(0.15)=2.0%.

    Filter          Ratio of throughput corrections
    ------         ---------------------------------
    F218W                F218W / F170W = 0.88
    F255W                F255W / F170W = 0.47
    F336W                F336W / F170W = 0.15
    F555W                F555W / F170W = 0.07
    

    These corrections will probably be updated in the coming weeks, though we do not expect any large changes.

  • Photometric Calibration Verification:
  • Following the service mission the standard star GRW+70D582 was observed in filters F160BW, F170W, F218W, F255W, F336W, F439W, F555W, F675W, and F814W in all four CCDs (proposal 7020). In all cases the throughputs agree with pre-SM97 values to well within the uncertainties (typically 1% to 2%). For all filter and CCD combinations the mean (post-SM97 / pre-SM97) throughput ratio was 0.997 +/- 0.002.

  • Extreme-UV Throughput Verification:
  • Observations were made before and after servicing in the F122M and F160BW filters of a UV-bright star (proposals 7018, 7029, and 7211). After corrections for red-leak, we find the pre- and post-SM97 throughputs are consistent at about the 10% to 15% level in these filters. This indicates little or no contamination of the WFPC2 pick-off mirror occurred during servicing. Further work is underway to reduce the uncertainties in this difficult measurement.

  • Flatfield Verification:
  • A large number of F502N Earthflats (proposal 7019) are being taken and compared with pre-SM97 flatfields. So far we are not able to see any changes attributable to servicing at about the 0.5% level. This accuracy will improve as we gather more Earthflats.

  • Focus and PSF Verification:
  • Immediately after the service mission, a large number of stellar images were observed on CCD PC1 to test the focus of the optical telescope and WFPC2 (proposal 7017). A very small offset was found (approx. +4 microns in the position of the telescope secondary mirror), and a small correction (-2.4 microns) was made on 18 March. This offset may be related to a slowing of the long-term shrinkage of the telescope truss, rather than the servicing mission. (See separate article below on the secondary mirror move.)

    The point spread function was tested by observing a star field in Omega Centauri using position dithers (proposal 7021), and then constructing a critically sampled PSFs from this data. Comparison of such PSFs from before and after the service mission show no significant changes, other than those easily attributed to the focus variations.

  • Dark and Bias Calibration:
  • The decontamination procedures also have a beneficial side effect, in that they anneal and remove hot pixels caused by radiation damage to the CCD. Observers requiring accurate dark current calibration will need to be mindful of the increased frequency of decontaminations following the service mission. Numerous dark calibration frames have been taken (proposals 7022 and 6903) following the service mission, and reference files will be created to track the changing dark current in the normal manner. Besides this change in the frequency of decons, we have not seen any appreciable change in the dark current performance. Nor do we see any significant change in the bias calibration (proposal 7022).

  • Early Release Observations:
  • Images of the "butterfly" planetary nebula NGC2346 and the pair of interacting galaxies NGC454 were obtained on 6 March to illustrate the post-SM97 performance of WFPC2. The former image is available on the STScI WWW page "picture & news release" area, and both data sets are available from the archive. We also point-out the beautiful color images of Mars obtained on 10 March with the linear ramp filters by Crisp, et al.

  • Future:
  • The current plan is to perform the next decontamination on 5 April. If the F170W UV monitoring continues to show a decline in contamination growth rates, we will probably wait 3 weeks before performing the following decontamination (approx 26 April), and possibly return to the pre-SM97 schedule of monthly decons after that date.

    We will be releasing an Instrument Science Report detailing these and other results in the near future.

  • Acknowledgements:
  • Many individuals contributed to the success of the WFPC2 SMOV program: Sylvia Baggett, Shireen Gonzaga, Max Mutchler, Massimo Stiavelli, John Biretta (UV monitor); Stefano Casertano, Matt Lallo, Anatoly Suchkov (focus); Sylvia Baggett, Chris O'Dea (extreme UV check); Michael S. Wiggs, John Biretta (flat fields); Brad Whitmore, Inge Heyer, Shireen Gonzaga (photometry); Andy Fruchter, Matt McMaster (PSF); Massimo Stiavelli, Max Mutchler (internal monitors); Merle Reinhart (scheduling); Dorothy Fraquelli and OPUS team (real time processing); John Biretta (planning and coordination).

    Secondary Mirror Movement on Mar. 18th:

    by S. Casertano, M. Lallo, and A. Suchkov

    Detailed measurements of the OTA focus position were obtained from a series of WFPC2 images of standard stars after the Servicing Mission. On the basis of these measurements, it was determined that the secondary mirror was at a focus position of +4 micron with respect to its optimal position. This is probably a consequence of the slowing down of the OTA desorption trend, which thus far had necessitated a compensatory focus move roughly twice a year.

    In order to optimize WFPC2 observations and orbital verification activities for STIS and NICMOS, the secondary mirror was moved on March 18 by 2.4 micron (secondary moved towards the primary); the time of the move was chosen so as to minimize its impact on science observations and new instrument activities. Early results indicate that the desired focus position was achieved. No other focus moves are expected for the next 6-8 months.

    As in previous instances, the consequence of the focus move is to change slightly the shape and the sharpness of the PSF in WFPC2; this change is mostly noticeable in point sources observed in the PC. A recent ISR (WFPC2 97-01) details the consequences of focus changes on aperture photometry, which are only significant when using very small apertures (< 5 pixels) in the PC. The amount of the move is comparable to the fluctuations in focus position (+/- 2 micron) expected because of OTA breathing in a typical orbit; the current focus position reduces the impact of breathing to less than 2% in aperture photometry with radius as small as 2 pixels.

    User Input to Cycle 7 Calibration Plan Solicited:

    by S. Casertano

    The WFPC2 group is in the process of designing the calibration plan to support Cycle 7 observations, and would like to solicit user input and suggestions.

    Our calibration effort will generally follow the lines of the Cycle 6 calibration plan, given in the Instrument Science Report WFPC2 96-08 and summarized in the Instrument Handbook (both available through our web site, or request a copy by e-mailing help@stsci.edu). However, it is likely that some programs will be redesigned or augmented, while others may be scaled back or even discontinued. Specific comments on the relative importance of the various programs would be especially welcome.

    RECENT PREPRINTS:

    We draw your attention to these papers, based on WF/PC and WFPC2 data, that will appear in the next few months. This list includes all preprints received by the STScI Library not yet published in the journals. Please remember to include our Library in your preprint distribution list.

    BUIE, M.W.; THOLEN, D.J.; WASSERMAN, L.H.  "Separate
    lightcurves of Pluto and Charon"  Icarus accepted
    
    DE VRIES, W.H.; O'DEA, C.P.; BAUM, S.A.; SPARKS, W.B.;
    BIRETTA, J.; DE KOFF, S.; GOLOMBEK, D.; MACCHETTO, F.;
    MCCARTHY, P.; MILEY, G.K.  "Hubble Space Telescope imaging
    of compact steep spectrum radio sources"  ApJS 4-97
    
    HUNTER, D.A.; VACCA, W.D.; MASSEY, P.; LYNDS, R.; O'NEIL,
    E.J.  "Ultraviolet photometry of stars in the compact
    cluster in the Large Magellanic Cloud"  AJ accepted
    
    BAUM, S.A.; O'DEA, C.P.; GIOVANNINI, G.; BIRETTA, J.;
    COTTON, W.B.; DE KOFF, S.; FERETTI, L.; GOLOMBEK, D.; LARA,
    L.; MACCHETTO, F.D.; MILEY, G.K.; SPARKS, W.B.; VENTURI,
    T.; KOMISSAROV, S.S.  "HST and MERLIN observations of 3C
    264 - a laboratory for jet physics and unified schemes"
    ApJ 6-20-97
    
    DE MARCHI, G.; CLAMPIN, M.; GREGGIO, L.; LEITHERER, C.;
    NOTA, A.; TOSI, M.  "The structure of the super star
    clusters in NGC 1569 from HST WFPC2 images"  ApJ 4-10-97
     
    DE MARCHI, G.; PARESCE, F.  "The intermediate mass function
    of low-mass stars in globular clusters"  ApJ 476: L19-L22,
    1997
    
    HAN, M.; HOESSEL, J.G.; GALLAGHER, J.S. III; HOLTZMAN, J.;
    STETSON, P.B.; TRAUGER, J.; BALLESTER, G.E.; BURROWS, C.;
    CLARKE, J.T.; CRISP, D.; GRIFFITHS, R.; GRILLMAIN, C.;
    HESTER, J.; KRIST, J.; MOULD, J.R.; SCOWEN, P.;
    STAPELFELDT, K.; WATSON, A.; WESTPHAL, J.  "Stellar
    populations in the dwarf elliptical galaxy NGC 147"  AJ 3-97
     
    BAUM, W.A.; HAMMERGREN, M.; THOMSEN, B.; GROTH, E.J.;
    FABER, S.M.; GRILLMAIR, C.J.; AJHAR, E.A.  "Distance to the
    Coma cluster and a value for H0 inferred from globular
    clusters in IC 4051"  AJ accepted
    
    CANALIZO, G.; STOCKTON, A.  "Spectroscopy of close
    companions to QSOs and the ages of interaction-induced
    starbursts"  ApJ accepted
    
    COLE, A.A.; GALLAGHER, J.S. III; FREEDMAN, W.L.; PHELPS, R.
    "Ultraviolet color-magnitude diagram studies of
    intermediate age Large Magellanic Cloud star clusters. I.
    NGC 1783"  AJ 5-97
    
    HOOPER, E.J.; IMPEY, C.D.; FOLTZ, C.B.  "HST imaging of Z >
    0.4 quasar host galaxies selected by quasar radio and
    optical properties"  ApJ accepted
    
    SURACE, J.A.; SANDERS, D.B.  "HST images of warm
    ultraluminous infrared galaxies: QSO host progenitors"
    Quasar Hosts
    
    VOGT, N.P.; PHILLIPS, A.C.; FABER, S.M.; GALLEGO, J.;
    GRONWALL, C.; GUZMAN, R.; ILLINGWORTH, G.D.; KOO, D.C.;
    LOWENTHAL, J.D.  "Optical rotation curves of distant field
    galaxies: sub-L[star] systems"  ApJ accepted
    

    APPENDIX: WFPC2 Contacts:

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