Introduction

1.2 Which Instrument to Use: WFPC2, FOC, NICMOS, or STIS?


In this section we compare briefly the performance of HST instruments with imaging capability in the UV to near-IR spectral range. As of this writing, both WFPC2 and FOC have capabilities in this area. Two new instruments, the Near-Infrared Camera and Multi-Object Spectrograph (NICMOS) and the Space Telescope Imaging Spectrograph (STIS) should be installed in HST during the second service mission in Early 1997. Important imaging parameters for all instruments are summarized in Table 1.1
below.

Table 1.1: Comparison of WFPC2, FOC, NICMOS, and STIS Instrumental Imaging Parameters.

1.2.1 Comparison of WFPC2 and FOC

Advantages of each instrument may be summarized as follows.

WFPC2 advantages are:

FOC advantages are:

The choice of instrument will be largely based on wavelength, required field-of-view, and need for good PSF sampling at short wavelengths. We note also that the FOC polarization data should be easier to calibrate, since there are fewer large-angle reflections than WFPC2, and since all polarization angles are available at all positions in the field-of-view.

Table 1.2: Comparison of WFPC2 and FOC Detective Efficiencies.

1.2.2 Comparison of WFPC2 and NICMOS

Both WFPC2 and NICMOS are capable of imaging at wavelengths between ~8000Å and ~11,000Å. At longer wavelengths NICMOS must be used; at shorter wavelengths either WFPC2, FOC, or STIS must be used. Table 1.3 below compares the detective efficiency of WFPC2 and NICMOS in the wavelength region where both instruments are viable. Count rates for a V=20 star of spectral class A0 are given for all filters at common wavelengths; the signal-to-noise (S/N) is also given for a 1 hour exposure of this same star. For bright continuum sources WFPC2 and NICMOS offer similar efficiency over the spectral range from 8800Å to 10,500Å; the choice of instrument will likely depend on other factors such as field size and details of the passband shape. However, for very faint sources, the lower read noise of WFPC2 (5e- for WFPC2 vs. 30e- for NICMOS) should prove advantageous.

Both instruments have a polarimetry capability, but the WFPC2 polarizers are not viable above 8000Å; above this wavelength NICMOS must be used for polarimetry.


Table 1.3: Comparison of WFPC2 and NICMOS Count Rates for V=20 A0 Star.

1.2.3 Comparison of WFPC2 and STIS

Both WFPC2 and STIS are capable of imaging over the same wavelength ranges between ~1150Å and ~11000Å. At much longer wavelengths NICMOS must be used.

Advantages of each instrument may be summarized as follows.

WFPC2 advantages are:

STIS advantages are:

In general, WFPC2 has a much greater selection of filters and wider field-of-view than STIS, but STIS will have greater detective efficiency in the UV and for its long-pass and unfiltered modes. Table 1.2
below compares the detective efficiency for WFPC2 and STIS filters with similar bandpasses. For UV imaging STIS will be greatly superior due to higher throughput and insensitivity to filter red-leak; only if some detail of a WFPC2 filter bandpass were needed, would it be a viable choice.

For both [OII] 3727Å and [OIII] 5007Å imaging STIS has much higher QE and will be preferred, unless the larger WFPC2 field-of-view is an important factor. The WFPC2 [OIII] filter is wider than its STIS counter-part, which may also be useful for redshifted lines. For broad-band imaging the unfiltered and 5500Å long-pass modes of STIS again will have higher efficiency than WFPC2, though with reduced field-of-view.

Table 1.4: Comparison of WFPC2 and STIS Detective Efficiencies.

1.2.1 - Comparison of WFPC2 and FOC
1.2.2 - Comparison of WFPC2 and NICMOS
1.2.3 - Comparison of WFPC2 and STIS