Observation Strategies

7.2 Observing Bright Targets


Saturation is the primary concern when observing bright targets. The analog-to-digital converter will run out of bit codes at ~28,000 e- pixel-1 in the ATD-GAIN=7 (default) setting, and at ~53,000 e- pixel-1 in the ATD-GAIN=15 setting. Count levels above these are merely reported as 4095 DN in the data. Hence ATD-GAIN=15 is recommended for targets approaching 28,000 e- pixel-1. The disadvantage of this setting is that the read noise is poorly sampled by this coarse digitization, and hence the read noise is slightly increased.

At count levels above ~90,000 e- pixel-1 charge will overflow the potential well of each pixel, and begin to bloom up and down the CCD columns. For example, this occurs in the F555W filter at about V=13.5 for a 10s exposure on the WFC, and at about V=13.0 on PC1.

At very high count levels, above ~10^8 e- per CCD column, the charge bloom will reach the top and bottom of the CCD and flow into the serial registers. CLOCKS=YES will dispose of this charge as it reaches the ends of the CCD, and thus prevent it from leaking back into adjacent CCD columns. This exposure level corresponds roughly to a 10s exposure of a V=7 star in F555W. Note that CLOCKS=YES offers no benefit unless the bloom reaches the ends of the CCD, and that it may slightly compromise the bias and dark calibration. Moreover, CLOCKS=YES will result in anomalous exposure times; exposure times are rounded to the nearest integral second, minus a delay time of up to 0.25s for the shutter to open. (See "Serial Clocks" on page 27 for further discussion of exposure time anomalies caused by CLOCKS=YES.)

Besides setting ATD-GAIN=15, the PC CCD can be used to reduce saturation effects for stellar objects. The peak of the PSF will be spread over more pixels on the PC (vs. WFC), so stars can be exposed about 50% longer on the PC before saturation sets in.

Note that the narrow band filters may be useful when observing very bright targets. For example, stars as bright as V~4.4 can be observed without saturation in F502N using the PC at ATD-GAIN=15 with a 0.11s exposure time.