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 The five examples provided below have been calculated using APT 20.2, which will be somewhat out-of-date. The results for the latest version of APT, which should be used by proposers, will be similar but slight differences might appear. All times are rounded to the nearest tenth of a minute. Also note the slight differences in the target acquisitions.

• Example 1 is a pattern-stepped series of long-slit CCD spectroscopic exposures mapping out the Hα nebula in the center of the galaxy M86. This uses a diffuse-source acquisition and no peakup.
• 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. This example uses a point source acquisition and no peakup on the target.
• Example 3 is an imaging and spectroscopic program observing NGC 6543, the Cat's Eye Nebula. This uses a point source acquisition.
• Example 4 is a set of long MAMA spectroscopic exposures of Sk –69° 215 using the E230H grating through a narrow echelle slit, taken in the CVZ. This uses a point source acquisition and peakup on the target, and includes re-peakups during the course of the long observations.
• Example 5 is a faint CCD imaging program of point sources around the star GGH202 EIS J033259.26-273810.5. This uses no acquisition.

9.3.1 Sample Orbit Calculation 1: Long-Slit Spectroscopy of the Galaxy M86

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:

• A CR-SPLIT=2, ~30 minute spectroscopic exposure¹ with G750M at a central wavelength of λ=6768 Å at location 1.
• A CR-SPLIT=2, ~30 minute spectroscopic exposure with G750M at a central wavelength of λ=6768 Å at location 2.
• A CR-SPLIT=2, ~30 minute spectroscopic exposure with G750M at a central wavelength of λ=6768 Å at location 3.

We 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 arcsecond. 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/s/cm²/Å/arcsec², 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.

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 54 minutes per orbit, appropriate for a target at M86's declination of +13° (see the HST Primer). 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.

Table 9.3: Orbit Calculation for Example 1.

9.3.2 Sample Orbit Calculation 2: Low Dispersion Spectroscopy of Solar Analog Star P041-C

In this example the scientific objective is to observe the solar-analog CVZ star P041-C from the near-infrared (NIR) to the near-ultraviolet (NUV) with STIS' low-resolution, first-order gratings and the 52X0.5 arcsecond slit. The series of exposures includes:

• CR-SPLIT=2, ~7 minute spectroscopic exposure with G430L at the central wavelength of λ = 4300 Å.
• CR-SPLIT=3, ~43 minute spectroscopic exposure with G230LB at the central wavelength of λ = 2375 Å.
• CR-SPLIT=2, ~5 minute spectroscopic exposure with G750L at the central wavelength of λ = 7751 Å.

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 Section 11.2.3), 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.

Table 9.4: Orbit Calculation for Example 2.

9.3.3 Sample Orbit Calculation 3: Imaging and Spectroscopy of the Cat's Eye Planetary Nebula, NGC 6543

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:

• A CR-SPLIT=2, ~5 minute exposure with the F28X50OII filter.
• A CR-SPLIT=2, ~30 minute exposure with G430M at a central wavelength of λ = 4961 Å using the 52X0.1 long slit.
• A ~30 minute exposure with G140L at C IV and the 52X0.1 long 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 8.3, 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.

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 that are taken for each spectroscopic observation at a new MSM position.

We assume a visibility period of 59 minutes per orbit, appropriate for a target at our source's declination of 66° (see the HST Primer). 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 (See Section 2.7).

Table 9.5: Orbit Calculation for Example 3.

9.3.4 Sample Orbit Calculation 4: MAMA Echelle Spectroscopic Exposures in the CVZ

In this example we wish to obtain a long total integration (~400 minutes) in the CVZ using E230H and the 0.2X0.09 slit. 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 8.3, 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 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 (see 0.2X0.09 Aperture). Since we must attain 80,000 counts over the detector, we require less than 1 second 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.

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.

Table 9.6: Orbit Calculation for Example 4. 

9.3.5 Sample Orbit Calculation 5: Faint CCD Imaging

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 HST Primer that there are 45 minutes of visibility per orbit. The observations consist of:

• A single CR-SPLIT=4, ~28 minute exposure using the 50CCD clear aperture with the CCD.

We determine that we can execute this program in 1 orbit, as summarized in Table 9.7.

Table 9.7: Orbit Calculation for Example 5.

¹ Here and below, 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.