Previous: Signal-to-noise
Up: EXPOSURE TIME?
Next: Shutter Speed Limitations
Previous Page: Signal-to-noise
Next Page: Shutter Speed Limitations

Time limits

In any case there are real limits to the length of exposure.

The usual limit is saturation, as described in section 1b. The saturation limit is similar for GEC and RCA chips (Table 1), but the lower efficiency of the GEC chip means generally longer exposure times before saturation takes place.

The time to saturation can be calculated from the limits of Table 1, and the expression for signal, equation (2). In practice, some non-linearity begins to set in at about 0.7 saturation level; a rule of thumb is to keep counts/pixel below 40,000. Saturation is of course the principal factor in determining exposure time for bright stars, and experimentally determined curves are shown in Figs 8a and 8b for permissible length of exposure times, RCA and GEC chips. At dark of moon, the sky background takes 55 minutes and 170 minutes to produce signals of 40,000 counts in I for the RCA and GEC chips respectively for the INT. The JKT is f/15 and not f/3.3 so these figures should be multiplied by . Thus for all practical purposes sky background on the JKT CCD should not pose any problems.

With regard to bright stars, the shortest possible exposure times are set by shutter speed. For decent photometry, don't try exposures shorter than about 1 second. It is usually much better to defocus the telescope to use longer exposures. This reduces photon statistical noise and also noise introduced by imperfect calibration of the chip because a larger area of the chip is sampled. (See Section 3.3)

Cosmic-ray events may provide a limit to long integration times, if saturation doesn't. Event rates for the chips differ by a factor of 10 (Section 1); and are of course filter-independent. The events are readily recognized, and can be removed in the laundry - but too great a density will seriously damage any kind of photometry. Moreover if the experiment is to detect something at a given position, cosmic-ray events may confuse the issue to say the least. In detection observations, if not in others, it is highly advisable to add frames together to reach the required integration-time. This guards against the cosmic-ray at the position in question, and is equally efficient in terms of observing time as the single long integration, provided that the sub-integrations are are long enough to be sky limited. It is advisable to take several frames and from the median to eliminate cosmic ray events. If you offset the telescope by a few pixels between exposures you can eliminate chip defects as well.