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WFPC2 Instrument Handbook for Cycle 11

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Quantum Efficiency


The WFPC2 provides useful sensitivity from 1150 to 11000 in each detector. The overall spectral response of the system is shown in Figure 2.4 (not including filter transmissions). The curves represent the probability that a photon that enters the 2.4m diameter HST aperture at a field position near the center of one of the detectors will pass all the aperture obscurations, reflect from all the mirrors, and eventually be detected as an electron in the CCD. The throughput of the system combined with each filter is tabulated in Table 6.1 and also shown in Appendix 1.

Figure 2.4: WFPC2 + OTA System Throughput. This is the current SYNPHOT model (June 2001) as determined by on-orbit measurements.
 

The visible and red sensitivity of the WFPC2 is a property of the silicon from which the CCDs are fabricated. To achieve good ultraviolet response, each CCD is coated with a thin film of Lumogen, a phosphor. Lumogen converts photons with wavelengths less than 4800 into visible photons with wavelengths between 5100 and 5800, which the CCD detects with good sensitivity. Beyond 4800, the Lumogen becomes transparent and acts to some degree as an anti-reflection coating. Thus, the full wavelength response is determined by the MgF2 field flattener cutoff on the short-wavelength end and the silicon band-gap in the infrared at 1.1 eV (~11000).

With the WFPC2 CCD sensors, images may be obtained in any spectral region defined by the chosen filter with high photometric quality, wide dynamic range, and excellent spatial resolution. The bright end of the dynamic range is limited by the 0.11 seconds minimum exposure time, and by the saturation level of the analog-to-digital converter (ADC) at the chosen gain, which is roughly 53000 (gain=14, though called ATD-GAIN=15 in RPS2) or 27000e- (gain=7) per pixel. The maximum signal-to-noise ratio corresponding to a fully exposed pixel will be about 230. The faint end of the dynamic range is limited by photon noise, instrument read noise and, for the wide-band visible and infra-red filters, the sky background.

Table 2.2 gives characteristic values of the expected dynamic range in visual magnitudes for point sources. The minimum brightness is given for an integrated S/N ratio of 3, and the maximum corresponds to CCD ADC saturation (selected as 53000e-). The quoted values assume an effective bandwidth of 1000 at about 5600 (filter F569W). The planets and many other resolved sources are observable in this filter with short exposures even if their integrated brightness exceeds the 8.4 magnitude limit.
Table 2.2: WFPC2 Dynamic Range in a Single Exposure.
Configuration Exposure (seconds) Min. V Magnitude Max. V Magnitude
Wide Field
0.11
8.82
17.83
Wide Field
3000.
19.91
28.19
Planetary
0.11
8.40
17.47
Planetary
3000.
19.49
28.25

 


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