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:

• Wider field-of-view, effective area of 134" x 134" vs. 50" x 50" or less.
• Greater selection of filters, including polarizers.
• Bright Targets: WFPC2 has no bright target safety issues, and can give useful data on faint targets near very bright objects. STIS MAMAs can be damaged by bright objects.

STIS advantages are:

• Much higher UV throughput.
• True solar blind imaging in UV due to MAMA detectors. WFPC2 CCDs are very sensitive to filter red-leak.
• PSF sampling: STIS offers 0.024" pixels vs. 0.046" on WFPC2.
• High time resolution is possible ( ~125µs) with the MAMA detectors. Also the STIS CCD may be cycled on ~20s time scale using a sub-array.

In general, WFPC2 has a much greater selection of filters and wider field-of-view than STIS, but STIS has greater detective efficiency in the UV and for its long-pass and unfiltered modes. Table 1.4 compares the detective efficiency for WFPC2 and STIS filters with similar bandpasses. For UV imaging STIS is 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 is 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 have higher efficiency than WFPC2, though with reduced field-of-view.

Table 1.4: Comparison of WFPC2 and STIS Detector Efficiencies.

Instrument	Filter	Mean Wavelength (Å)	Bandpass FWHM (Å)1	Peak QE2
WFPC2	F122M	1420	100	0.11%
STIS	F25LYA	1216	85	0.32%
WFPC2	F160BW	1492	500	0.07%
STIS	25MAMA (FUV)	1370	320	4.5%
WFPC2	F255W	2586	393	0.5%
STIS	25MAMA (NUV)	2220	1200	3.1%
WFPC2	F375N	3738	42	0.9%
STIS	F28X50OII	3740	80	3.7%
WFPC2	F502N	5013	37	5.8%
STIS	F28X50OIII	5007	5	11%
WFPC2	F606W	5935	2200	14%
STIS	F28X50LP	~73003	2720	12%
STIS	50CCD	~5800	4410	15%

1Note that definition of FWHM is different from "effective width" elsewhere herein. 2Includes instrument and OTA. 35500Å long pass filter.