Space Telescope Science Institute   6.2.7 Occulting Spot Motions  6.2.9 Choice of Filters for Coronagraphic Observations

6.2.8 Planning ACS Coronagraphic Observations


Exposure Time Estimation

The estimation of exposure time for coronagraphic observations is similar to direct-mode time calculations, except that the additional background contribution from the central source's PSF has to be accounted for. Generally, most coronagraphic observations are limited by the central source's PSF wings. The APT Exposure Time Calculator includes a coronagraphic mode for estimating exposure times. We will now demonstrate how exposure times for coronagraphic observations can be determined using the Web-based version of the ACS Exposure Time Calculator. The following steps are required:

Where:

In order to illustrate a calculation we shall consider the case where we are trying to determine the S/N achieved in detecting a M6 V star with a V magnitude of 20.5 at a distance of 4.25 arcseconds from a F0 V star with a V magnitude of 6, for an exposure time of 1000 seconds with the F435W filter. Using the ACS Exposure Time Calculator and considering the case for the 3.0arcseconds occulting mask:

Observing sequence for point source companions

The best way to detect faint stellar or substellar companions is to use roll subtraction to avoid color differences between the target and reference PSFs. This also provides duplicate observations that make it easier to verify true companions from noise. It is best to roll the telescope between visits and repeat the image sequence in a new orbit. This way, you can better match the breathing cycle of the telescope than if you rolled the telescope in the middle of an orbital visibility window. You can force this to happen by selecting both orientation and time-sequencing constraints in the visit special requirements. Remember that the coronagraphic field PSF is somewhat broader than the normal HRC PSF, which may influence your assumed signal-to-noise ratio. Off-spot PSF models can be generated with the Tiny Tim PSF software. You can estimate the residual background noise level using Figure 6.13.

Suggested point-source companion observing sequence:

  1. Obtain an acquisition image
  2. Execute image sequence.
  3. Request telescope roll offset (use ORIENT q1 TO q2 FROM n special requirement in visit).
  4. Obtain another acquisition.
  5. Repeat image sequence.
  6. Repeat 3 to 5 as necessary.

Observing sequence for extended sources (e.g. circumstellar disks and AGN host galaxies)

When imaging extended objects, the remaining scattered light must be subtracted using a reference star image, which should match the color of your target as closely as possible. To reduce the impact of noise in the subtracted images, it helps if the reference PSF is bright enough to provide higher signal-to-noise ratios in the wings, than that of the target source. If possible, select a reference star that is nearby (< 20°) and request that it be observed immediately before or after the target source. This reduces the chance that there will be large focus differences between the two visits. In order to better discriminate between subtraction artifacts and real structure, it may also help to obtain images of the target at two or more orientations of the telescope (there is no need to get reference PSF images at different rolls). You can estimate the residual background noise level using Figure 6.13.

Suggested extended source observing sequence:

  1. Obtain direct images of the science target in each filter to derive normalization factors
  2. Obtain an acquisition image of the science target.
  3. Take image sequence of science target.
  4. Request a new telescope orientation.
  5. Repeat steps 2 to 3.
  6. In a new visit immediately after the science observation, point at the reference star.
  7. Obtain an acquisition image of reference star.
  8. Take image sequence of reference star.
  9. Obtain direct images of the reference star in each filter to derive normalization factors.

Note that the order of the observations places direct imaging before or after coronagraphic imaging. This reduces cycling of the coronagraphic mechanism. Because the occulting spots are large, you may wish to image closer to the source using additional direct observations without the coronagraph. Multiple roll angles are necessary in this case because portions of the inner region will be affected by saturated columns and diffraction spikes. Direct observations of the reference star will be required as well to subtract both the diffracted and scattered light. Color and focus mismatches between the target and reference PSFs will be even more important in the direct imaging mode than with the coronagraph because the diffracted light is not suppressed. However, there are no mismatches caused by star-spot alignment to worry about.


 6.2.7 Occulting Spot Motions  6.2.9 Choice of Filters for Coronagraphic Observations
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