|WFC3 Instrument Handbook for Cycle 24|
10.4 The easiest way to learn to estimate total orbit time requests is to work through a few examples. Below we provide five different examples:These examples represent fairly typical usage scenarios of WFC3. However, it should be noted that in several of the examples we have used un-dithered images. In most actual cases, observers are advised to use dithering. Furthermore, although observers can use the shadow or low-sky target visibility restrictions, the examples below are all for the standard (i.e., unrestricted) target visibility (see the HST Primer, Section 6.3, for further discussion).Consider a target to be imaged with UVIS in a given filter in one orbit. Let us suppose that, by using the Exposure Time Calculator (ETC) (see Chapter 9), we find that we need a total exposure time of 2400 s (40 minutes) to reach the desired S/N. Given that we desire the observation to be split into two exposures for cosmic-ray removal (using the default CR-SPLIT=2), we map the overheads and the science exposure times onto the orbit as follows:Table 10.3: Orbit Calculation for Example 1
Thus, with a total time of nearly 51 minutes, this set of observations would fit into all unrestricted HST orbits. The exposure time could, if needed, be adjusted so as to fill the actual target visibility interval (which depends on several factors, including the date and target location in the sky, as described in Chapter 6 of the HST Primer). The time needed to dump the buffer following the second sub-exposure incurs no overhead in this example, because it can be performed during target occultation.It should be noted that this simple sequence of two fairly long, non-dithered exposures would produce an image with a gap between the two CCD chips (see Section 6.3), and that cosmic-ray removal might not be optimal (see Section 5.4.10).This example illustrates the impact of short exposures on the useful time in the orbit. Suppose we intend to use one orbit to observe a target with UVIS in two filters, F606W and F814W. The observation consists of two sequences, each one with two identical CR-SPLIT exposures, for a total of four individual sub-exposures. Suppose that the ETC shows that the exposure time must be 540 seconds for each of the filters, so each of the CR-SPLIT sub-exposures must be at least 270 seconds long. For the target declination, which in this example is −35°, we find that the unrestricted visibility time is 55 minutes. The time budget for the orbit is as follows:Table 10.4: Orbit Calculation for Example 2
2 × 2.6 = 5.2 2 × 2.1 = 4.2 2 × 5.8 = 11.6 4 × 4.5 = 18.0 Compared with Example 1, we see that the efficiency is very low due to the large overheads associated with buffer dumps. We have achieved only 18 minutes of exposure time during 45 minutes of target visibility, whereas in Example 1 we obtained 40 minutes of exposure time during 51 minutes of visibility. Of course, this is caused by the short exposures of this example, versus the long exposures of Example 2, where “short” and “long” are relative to the time to dump the buffer, 339 seconds. This time that is “lost” to dumping the buffer can be recovered by sufficiently increasing the exposure time. For example, if the 540-second exposure time is required to obtain a minimum S/N (and not to avoid saturation), then increasing the exposure times to 720 s will improve the S/N and require the same amount of target visibility time, 45 min.Alternatively, if compatible with the scientific goals, a subarray could have been used to read out only a fraction of the detector area, allowing more frames to be stored in the buffer before requiring a dump. In this example, using UVIS 2k×2k subarrays for 4 short (<339 seconds) exposures would save about 8 minutes of readout time and 12 minutes of dump time.The third example demonstrates the orbit calculation for a simple IR observation. We want to obtain full-frame images of a target in two filters, F110W and F160W. Suppose that the ETC has shown that the exposure times adequate for our scientific goals are 10 minutes in F110W and 20 minutes in F160W. These times can be achieved with the up-the-ramp MULTIACCUM sequences SPARS50 (11.7 min) and SPARS100 (23.4 min), respectively. From the orbit visibility table (see Chapter 6 of the HST Primer), suppose that we find that at the target declination (here assumed to be 0°) the unrestricted target visibility time is 54 minutes. The orbit calculation goes like this:Table 10.5: Orbit Calculation for Example 3
2 × 1.0 = 2.0 The total time used in the orbit shows that our target can indeed be imaged in the selected filters within one orbit. Furthermore, the first exposure can be dumped from the buffer during the second exposure. The ~9 minutes of unused time could be used for an additional exposure, during which the second exposure would be dumped.This example illustrates the orbit calculation for a UVIS observation with a WFC3 UVIS box dithering pattern, which implements imaging at four pointings. The goal of the observation is to obtain a dithered image of a field in such a way that would allow us to bridge the ~1.2 arcsec inter-chip gap between the UVIS CCDs in the combined image. As indicated in Table 10.1, for a 2-arcsec offset maneuver, the three dithers will take 0.5 minutes each. Suppose we have determined that the exposure time necessary to reach the desired S/N ratio is 80 minutes, and that the visibility time at our target declination, assumed to be +53°, is 58 minutes. Furthermore, we will use the cosmic-ray removal provided by the dither data-reduction package, and therefore set CR-SPLIT=1. As a result, the orbit calculation will involve a sequence of four exposures of 20-minutes duration (i.e., one exposure at each of the four dither pointings). These observations will be distributed across two HST orbits, as shown in the following Table 10.6.Table 10.6: Orbit Calculation for Example 4.
2 × 20 = 40.0 2 × 2.1 = 4.2 2 × 0.5 = 1.0 2 × 20 = 40.0 No overhead is incurred to dump the exposures, because they are all longer than 339 seconds. Thus the desired exposures can be accomplished within the two orbits, and in fact there are ~7-8 minutes of unused visibility time per orbit that could be used to increase the exposure times.This example illustrates the orbit calculation for an IR G102 grism spectroscopic observation. We will use the full-frame, up-the-ramp MULTIACCUM sequence SPARS200 with NSAMP=13, requiring 40 minutes to expose. We will also obtain undispersed images to calibrate target positions and wavelengths, using a SPARS10 (2.4-minute) exposure before and after the grism exposure. The overhead calculations are presented in Table 10.7.Table 10.7: Orbit Calculation for Example 5.
3 × 1.0 = 3.0 2 × 2.4 = 4.8 The buffer dumps incur no overhead because the first undispersed exposure can be dumped during the long grism exposures, and the last two can be dumped during the subsequent target occultation. Thus, since at least 54 minutes of target visibility are available at any target’s declination, this set of observations can be obtained in one orbit.