Notes for applying for CIAO time

Choice of wavelength

For most objects, K(Ks), H, and J bands are useful for coronagraph. Note that the AO corrections become poorer at shorter wavelengths. For L' and M' bands, the background is too high for the coronagraph to be useful for most objects and coronagraphy at these wavelengths is not recommended.

Choice of mask size

A wide range of masks is currently available. The choice of mask depends on your scientific targets and the seeing conditions. If the seeing is poor and AO correction is not sufficient, small masks are useless. Also, if the targets are too bright, the halo just outside the mask could be saturated. Thus, do not plan your observations depending on only a small mask. If your faint targets are far from the central source, it is recommended to use the largest possible masks for effective coronagraphy. Please consult with the support astronomer at the time of observations for the best mask size under particular seeing conditions. It is recommended to prepare backup programs (with a larger mask) in case of bad seeing.

Choice of lyot stop

You have two choices for the lyot stop. One has a circular aperture whose diameter is 90% of the pupil image. The other has the same aperture size and has an additional blocking mask, a cross spider with a disk at the center of the circular aperture. The mask suppresses the light diffracted by the secondary mirror and the four spider arms. A comparison of stellar images taken with the two stops is shown here.

With the latter lyot stop, the detection limit is deeper by about 0.7 mag where the spider pattern is seen. However, it is shallower by about 0.2 mag in the other area because the effective area is 22% smaller than the circular lyot stop.

Choice of point spread function (PSF) stars

CIAO observations need two kinds of "reference stars". One is the visible "AO reference" star for AO correction. In CIAO's case, this is often the central source itself. See the AO pages for the choice of the AO reference star.

Another is the "PSF reference" star for PSF subtraction. It is recommended to observe one or more PSF reference stars just before and after your target. Select single, nearby (within a few degrees) stars whose magnitudes are comparable to that of the AO reference star. The difference of the R magnitude between the object and the PSF reference star may be less than 0.3 mag. Otherwise, the PSF could be significantly different, making the PSF subtraction difficult. If only brighter PSF reference stars are available near the target, it is possible to adjust the R magnitude by using the AO density filters. If only fainter PSF reference stars are available near the target, PSF subtraction will be difficult.

Choice of pixel scale

CIAO's pixel scale (and pupil imaging mode) can be swiftly selected by rotating the camera optics.

  • 22 mas/pixel is suitable for most cases (in all bands)
  • 12 mas/pixel would be suitable for J and H if the seeing is good.

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