1. Marco Sirianni TIPS 12/18/2003 Anomalous scattered light in ACS/WFC Data Marco Sirianni D. Van Orsow, A. Welty, T. Wheeler R. Gilliland, R. van der Marel ACS/WFPC2 group INS Division

2. Marco Sirianni TIPS 12/18/2003 On Dec 03 two PIs reported an anomaly on their ACS/WFC data WFC-1 WFC-2 B A C D Anomalous scattered light in ACS/WFC Data

3. Marco Sirianni TIPS 12/18/2003 Initial clues: Both programs used F814W and were executed on Nov 11 2003 The shape of the pattern is the same The count-rate is similar in different images (~ 0.18 e-/sec) The photometry of the objects in the contaminated region is consistent with an additive background. SCATTERED LIGHT ? First investigations on all ACS data between Nov 10 and Nov 12: Not all F814W images were contaminated. 14 contaminated images ( only F814W and F606W, no contamination in F555W ). F606W images show a similar pattern but a much lower count-rate Some images show a lower count rate than others taken with the same filter LAMP CONTAMINATION ? VERY RED LAMP ? Anomalous scattered light in ACS/WFC Data

4. Marco Sirianni TIPS 12/18/2003 Analysis of telemetry/commanding: No Anomaly in ACS Lamps No Lamp Activity in STIS or NICMOS WFPC2 was taking a series of 1800 sec internal flat fields (Tungsten lamps + F390N filter) in those days. The comparison of the ON-OFF cycles of the WFPC2 INTFLAT lamps and the timing of the ACS anomaly gives a perfect match: when ACS/WFC images show the bright pattern the WFPC2 lamps are ON when the count rate is lower (images with the same filter) the lamp has been turned on during the exposure (the count rate is uniform if the real exposure to the lamp is taken into account). The telemetry data allowed us to find more contaminated images CONTAMINATION FROM WFPC2: INTFLAT+F390N Anomalous scattered light in ACS/WFC Data

5. Marco Sirianni TIPS 12/18/2003 Pick-off mirror From OTA Shutter A B SOFA Pyramid mirror Fold mirror To Cassegrain + CCD Lamps Pick-off mirror Shutter A B SOFA Pyramid mirror Fold mirror To Cassegrain + CCD Lamps WFPC2 optical configuration: Brightness and shape of the pattern are consistent with the following scenario: TO ACS Light leakage from WFCP2 shutter (  1 %) to WFPC2 POM to ACS any other light path (OTA secondary or glint near ACS entrance aperture) would produce much fainter illumination Anomalous scattered light in ACS/WFC Data

6. Marco Sirianni TIPS 12/18/2003 For a direct path from WFPC2 a simple ray tracing model predicts a bright image of ACS WFC IM1 mirror shifted diagonally toward WFPC2 Edges of IM1 Mirror Edges of ACS entrance aperture ? Multiple images due to WFC CCD windows reflections Anomalous scattered light in ACS/WFC Data

7. Marco Sirianni TIPS 12/18/2003 Immediate actions: - All WFPC2 INTFLAT programs on hold - More detailed investigation of the problem Occurrence WFPC2 filter dependence Are other channel/instrument contaminated? WFPC2 VISFLAT potential problem? Anomalous scattered light in ACS/WFC Data

8. Marco Sirianni TIPS 12/18/2003 Approximate average count rate e-/sec Preliminary results: Problem since SM3B, but it is quite rare circumstance (~ 40 images so far, mostly parallel, 4 GOs programs) No contamination in ACS/HRC STIS or NICMOS No correlation with the WFPC2 Filter ( -> direct leakage) Clear Correlation with WFPC2 Shutter Blade: Anomalous scattered light in ACS/WFC Data

9. Marco Sirianni TIPS 12/18/2003 Anomalous scattered light in ACS/WFC Data Summary / Conclusion: WFPC2 shutter leakage contaminates ACS/WFC data during INTFLATs The level of contamination depends on: WFPC2 TUNGSTEN LAMP CYCLE WFPC2 SHUTTER BLADE ACS/WFC FILTER ACS/WFC EXPOSURE TIME The PI of the programs with corrupted data will be contacted. The problem is fully predictable and therefore can be avoided with an appropriate scheduling strategy WF3 should be a better neighbor (we’ll plan a specific SMOV test for SM4)