The easiest way to learn to compute total orbit time requests is to work through a few examples. The five examples provided below have been calculated using APT 20.2, the latest version available at the time this handbook was updated for Cycle 21. The examples are written as they would be displayed in
APT 20.2. and all times are rounded to the nearest tenth of a minute.
This example is for an observation of the Hα nebula in the center of the Virgo elliptical M86, using the CCD, the
52X0.2 slit and the
G750M grating. A series of exposures is taken, each stepped relative to the next by 0.2 arcsecond, in the direction perpendicular to the slit, in order to cover the inner 0.6 arcseconds of the galaxy completely. Based on the signal-to-noise ratio calculation presented in
Section 6.8.1, we require an integration time of ~30 minutes per position to obtain a signal-to-noise ratio of ~10. The scientific exposures for this proposal, therefore, comprise
all of the following:
The mean surface brightness of the galaxy within this region is ~2 × 10
-15 ergs/s/cm
2/Å
/arcsec
2, based on WFPC2 V-band images in the HST archive. Using the information in
Section 6.4 or the
STIS TA ETC we determine that, using the CCD longpass filter,
F28X50LP, for t
exp= 1 second, we achieve more than the required signal-to-noise ratio needed over the checkbox for the target acquisition. We use the formula in
Table 9.1, plug in CHECKBOX=25 and exptime=1.0, and determine that the acquisition will take roughly 8 minutes.
We assume a visibility period of 52 minutes per orbit, appropriate for a target at M86’s declination (see the Call for Proposals). Based on the reasoning presented in Table 9.3, below, we conclude that the observations can be squeezed into ~2 orbits with some loss of sensitivity. Alternately, one could choose to increase the signal-to-noise, and ask for 3 orbits.
APT provides the capability to select preset patterns with user-supplied options to facilitate planning. For this example, a mosaic of the inner 0.6 arcseconds of M86 can be obtained by specifying
Pattern Type=“STIS-PERP-TO-SLIT”,
Purpose=“Mosaic” (which will step the aperture on the target),
Number of Points=“3”,
Point Spacing=“0.2”,
Coordinate Frame=“POS-TARG” (which specifies pattern execution in the spacecraft frame) and click Center Pattern.
We need to include time for the guide-star acquisition at the start of the first orbit, followed by an acquisition exposure. This target is a bright point source. We will use the longpass filter F28X50LP for the target acquisition. Using the
STIS TA ETC we find that an exposure time of 0.1 seconds gives a SNR >100 without saturating the CCD. No peakup is needed as we are using the 0.5 arcsecond wide slit. This is a CVZ observation so each orbit is ~96 minutes. We need to include time for the CCD long-wavelength fringe flats (see
“Fringe Flat Fields” on page 221), and since this is a CVZ observation the fringe flat will not move into the occultation. As shown in
Table 9.4, we can easily perform this observation in a single orbit.
In this example the scientific objectives are to obtain [O II] images of planetary nebula NGC 6543, as well as an optical spectrum at Hβ and an ultraviolet (UV) spectrum at C IV
. The exposure time calculations for these observations are presented in
Section 6.8.3. The specific exposures in this series include:
We need to include time for the guide-star acquisition at the start of the first orbit, followed by an acquisition exposure. The central star of the Cat’s Eye Nebula is used as the acquisition target. It has a V magnitude ~11.5. Checking Table 8.3, we conclude that the star is faint enough to not saturate the CCD in imaging mode with the longpass aperture
F28X50LP and an exposure time of 0.1
seconds. We therefore use it for the target acquisition. We wish to perform a peakup exposure as well, to center the star in the 0.1 arcsecond wide slit. We consult
Table 8.3 and conclude that the source is not bright enough to saturate the CCD if we perform an undispersed (white-light) peakup with the mirror and an exposure time of 0.1 seconds.
We assume a visibility period of 57 minutes per orbit, appropriate for a target at our source’s declination of 66° (see the Call for Proposals). Based on the reasoning presented in
Table 9.5 below, we conclude that a total of 2 orbits is required to perform these observations. Note that the MAMA and CCD observations have been split into separate visits in accordance with the stated policy.
We then perform a dispersed-light peakup using the G230LB grating with the CCD detector. We can estimate the exposure time required by determining with the
Spectroscopic ETC the total counts over the detector in 1 second for the clear filter and scaling by 65% for the slit throughput for
0.2x0.09 (
“0.2X0.09 Aperture” on page 332). Since we must attain 80,000 counts over the detector, we require less than 1 seconds per dwell point of the peakup. We choose a 1 second exposure time. The peakup overhead for this slit is 360 + 20
× t
exp. We thus conclude that the peakup will require 360 + 20
× 1 = 380 seconds or ~6.3 minutes. In practice, using
APT 20.2, the peak-ups ran between 7.5 and 8.3 minutes.
In this program we wish to take deep images of a field to look for faint point sources, as described in Section 6.8.5. We look at the field around GGH2002 EIS J033259.26-273810.5, a K0V star with a V magnitude of 22.18. We request
LOW-SKY as this observation is background limited. At our declination, we find from the CP/HST Primer that there are 45 minutes of visibility per orbit. The observations consist of:
