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| Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 17 | |||||
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9.3 Orbit Use Determination Examples
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 re-calculated to verify that the exposure times presented in the tables were achievable using the latest version of the planning software, APT 16.2.3, available at the time this handbook was updated for Cycle 17. Assuming the visibility duration specified in the text of each example, the total scientific exposure time reported by APT 16.2.3 for 4 of the 5 examples was equal to, or slightly better by <~2 minutes per orbit, that the results presented in the table associated with each example. Only for example 4 was the scientific exposure time lower (by ~ 1 minute per orbit) when recalculated using APT 16.2.3. The detailed overheads presented in Table 9.3 through Table 9.7 were not revised.
- Example 1 is a pattern-stepped series of long-slit CCD spectroscopic exposures mapping out the H
nebula in the center of the galaxy M86.
- Example 2 is a series of spectroscopic observations of the solar-analog, CVZ star P041-C, using all of the first-order, low-resolution CCD gratings.
- Example 3 is an imaging and spectroscopic program observing NGC 6543, the Cat's Eye Nebula.
- Example 4 is a set of long MAMA spectroscopic exposures of Sk -69° 215 using the
E230Hgrating through a narrow echelle slit, taken in the CVZ.- Example 5 is a faint CCD imaging program.
These examples represent fairly typical uses of STIS. The target acquisitions used in each example differ slightly as well:
- Example 1 uses a diffuse-source acquisition and no peakup.
- Example 2 uses a point source acquisition and peakup on the target.
- Example 3 uses a point source acquisition.
- Example 4 uses a point source acquisition and peakup on the target, and includes re-peakups during the course of the long observations.
- Example 5 uses no acquisition.
9.3.1 Sample Orbit Calculation 1:
Long-Slit Spectroscopy of the Galaxy M86This example is for an observation of the H
52X0.2slit and theG750Mgrating. 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:
- A
cr-split=2,~30 minute spectroscopic exposure1 withG750Mat a central wavelength of=6768 Å at location 1.
- A
cr-split=2, ~30 minute spectroscopic exposure withG750Mat a central wavelength of- A
cr-split=2, ~30 minute spectroscopic exposure withG750Mat a central wavelength ofWe need to include time for the guide-star acquisition at the start of the first orbit, followed by an acquisition exposure. No peakup will be done, since we are covering the nebula by stepping the slit and the slit is wider than 0."1. In this example, we assume a diffuse source acquisition, with a checkbox size of 25 pixels (roughly 1.25 arcseconds). The checkbox needs to be large as this galaxy has a very flat and dusty profile; see Figure 8.5.
The mean surface brightness of the galaxy within this region is ~2 × 10-15 ergs sec-1 cm-2 Å-1 arcsec-2, based on WFPC2 V-band images in the HST archive. Using the information in Section 6.4 or the
STIS Target Acquisition Exposure Time Calculatorwe determine that, using the CCD longpass filter,F28X50LP, for texp= 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.This is not a CVZ observation, so because more than 1 orbit is required we need to include time for the guide-star reacquisition at the beginning of each orbit. The individual exposures in this example are long enough that we do not need to include extra overhead for data management. We are satisfied with the automatic wavecal exposures which are taken for each spectroscopic observation at a new MSM position. We do not require fringe flats as we are observing at wavelengths shortward of 7500 Å.
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.
Sample Orbit Calculation 2:
Low Dispersion Spectroscopy of Solar Analog Star P041-CIn this example the scientific objective is to observe the solar-analog CVZ star P041-C from the near IR to the near UV with STIS' low-resolution, first-order gratings and the
52X0.5arcsecond slit. The series of exposures includes:
cr-split=2,~5 minute spectroscopic exposure withG750Lat the central wavelength ofcr-split=2, ~7 minute spectroscopic exposure withG430Lat the central wavelength ofcr-split=3, ~43 minute spectroscopic exposure withG230LBat the central wavelength ofWe 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
F28X50LPfor the target acquisition and the default overhead of 6.0 minutes from Table 9.1. 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, 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.
9.3.2 Sample Orbit Calculation 3:
Imaging and Spectroscopy of the Cat's Eye
Planetary Nebula, NGC 6543In this example the scientific objectives are to obtain [O II] images of planetary nebula NGC 6543, as well as an optical spectrum at H
and an ultraviolet 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:
- A
CR-SPLIT=2,~5 minute exposure with theF28X50OIIfilter.- A
CR-SPLIT=2, ~30 minute exposure withG430Mat a central wavelength of52X0.1long slit.- A ~30 minute exposure with
G140Lat C IV and the52X0.1long slit.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 8.2, we conclude that the star is faint enough to not saturate the CCD in imaging mode with the longpass aperture
F28X50LP, and we therefore use it for the target acquisition. We use the overhead information in Table 9.1 and Table 9.2 to conclude that the target acquisition will take 6 minutes. We wish to perform a peakup exposure as well, to center the star in the 0.1 arcsecond wide slit. We consult Table 8.2 and conclude that the source is not bright enough to saturate the CCD if we perform an undispersed (white-light) peakup with the mirror. Again using the information in Table 9.1 and Table 9.2, we conclude the peakup will take 6 minutes.This is not a CVZ observation, so because more than 1 orbit is required we need to include time for the guide-star reacquisition at the beginning of each orbit. The individual exposures in this example are long enough that we do not need to include extra overhead for data management. We are satisfied with the automatic wavecal exposures which are taken for each spectroscopic observation at a new MSM position.
We assume a visibility period of 57 minutes per orbit, appropriate for a target at our source's declination of 66º (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
.9.3.3 Sample Orbit Calculation 4:
MAMA Echelle Spectroscopic Exposures in the CVZIn this example we wish to obtain a long total integration (~400 minutes) in the CVZ using
E230Hand the0.2X0.09slit. The exposure time calculations for this example are presented in Section 6.8.4.We choose to break the observation up into roughly identical 40 minute exposures. We acquire the target using a CCD point source acquisition and then peakup in dispersed light using the CCD and the same slit as intended for the scientific observations. The star is Sk -69° 215, an O5 star with a V magnitude of 11.6. Checking Table 8.2, we conclude that the source will not saturate the CCD if observed for 0.1 seconds in the longpass filter
F28X50LP, and we choose to perform the acquisition then on Sk -69° 215 with this filter as the aperture. We take the acquisition time as 6 minutes, from the overheads in Table 9.1.We then perform a dispersed-light peakup using the
G230LBgrating 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 for0.2x0.09(0.2X0.09 Aperture). Since we must attain 80,000 counts over the detector, we require roughly 9 seconds per dwell point of the peakup. The peakup overhead for this slit is 360 + 20 ×texp. We thus conclude that the peakup will require 360 + 20 × 9 = 540 seconds or ~9 minutes. In practice, using APT 16.2.3, the peak-ups ran slightly longer than 9 minutes, requiring 10 - 11 minutes each.Since this is a CVZ observation, we do not need to include time for reacquisitions. However, since it is a long observation and a narrow slit, we decide we will re-perform a peakup midway through the observation.
Additionally, since this is a long observation taken at a given grating position, we need to include time for the automatic wavecals which will be taken every 40 minutes of elapsed pointed time. Since the auto-wavecal exposures are short for this configuration, time must be allocated for a buffer dump prior to each auto-wavecal.
For CVZ targets, an orbit is 96 minutes. We conclude we require a total of 5 CVZ orbits to perform this program, as summarized in Table 9.6.
9.3.4 Sample Orbit Calculation 5:
Faint CCD ImagingIn 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 request
LOW-SKYas 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:We determine that we can execute this program in 1 orbit, as summarized in Table 9.7.
1Here and below, a '
CR-SPLIT=n, m minute' exposure implies there will be n exposures with a total of m minutes across the exposures. In this example there will be 2 exposures each of 15 minutes for a total of 30 minutes.
to slit|
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |
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