Wide Field and Planetary Camera 2 Instrument Handbook for Cycle 12
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| Wide Field and Planetary Camera 2 Instrument Handbook for Cycle 12 | ||||
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1.2 Which Instrument to Use: WFPC2, ACS, 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. These instruments include WFPC2 and STIS, as well as NICMOS, which was revived through the installation of the cryo-cooler, and the Advanced Camera for Surveys (ACS), which was installed during the HST Servicing Mission 3b1. Important imaging parameters for all instruments are summarized in Table 1.1 below.
Table 1.1: Comparison of WFPC2, ACS, NICMOS, and STIS Instrumental Imaging Parameters. Parameter WFPC2 ACS NICMOS STIS Wavelength range 1150Å - 11,000Å WFC: 3500 Å - 11000 Å HRC: 2000 Å - 11000 Å SBC: 1150 Å - 1700 Å 8000Å - 25,000Å FUV-MAMA: 1150Å - 1700Å NUV-MAMA: 1700Å - 3100Å CCD: 2000Å - 11,000Å Detector Si CCDs CCDs (WFC, HRC) MAMA (SBC) HgCdTe arrays CCD, MAMAs Image Format 4 x 800 x 800 WFC: 2 butted 2048x4096 HRC: 1024x1024 SBC: 1024x1024 3 x 256 x 256 1024 x 1024 Field-of-view and pixel size 150" x 150" @ 0.1" pix-1 34" x 34" @ 0.046" pix-1 (1) WFC: 202"x202" @0.049"/pix HRC: 29.1"x26.1" @0.028"x0.025"/pix SBC: 34.6"x30.8" @0.033"x0.030"/pix 1: 11" x 11" @ 0.043" pix-1 2: 19" x 19" @ 0.075" pix-1 3: 51" x 51" @ 0.2" pix-1 MAMAs: 25" x 25" @ 0.024" pix-1 CCD: 51" x 51" @ 0.05" pix-1 (2) Read noise 5 e- WFC, HRC: 4 e- SBC: 0 e- 30 e- MAMAs: 0 e- CCD: 4e- Dark current 0.004 e- s-1 WFC, HRC: 0.0027 e-/s SBC: 0.0001 e-/s <2 e- s-1 MAMAs: <0.0001 e- s-1 CCD: 0.004 e- s-1 Saturation 53,000 e- WFC: 80,000 e- HRC: 140,000 e- SBC: 100 counts/s/pix 200,000 e- MAMAs: 100 count s-1pix-1 CCD: 140,000 e-
1"L"-shaped field-of-view using 3 CCDs with 0.1" pixels, and one CCD with 0.046" pixels.
2Field-of-view is up to 51" x 51" if no filter is used, and down to 12" x 12" for some neutral density filters.1.2.1 Comparison of WFPC2 and ACS
Advantages of each instrument may be summarized as follows.
- Wider field of view in the UV - effective area of 134"x134" vs. 34.6"x30.8".
- Wider field of view in many narrow band filters - effective area of 134"x134" vs. up to 40"x70" (ACS LRFs).
- Proven performance.
- Wider field of view in broad band optical filters - effective area of 202"x202" vs. 134"x134".
- Factor of ~2 better sampling of the PSF.
- Higher detective efficiency (factor of 2-10 depending on wavelength). Table 1.2 compares the detective efficiency for WFPC2 and ACS filters with similar band passes.
- True solar blind imaging in the UV due to the MAMA detector.
- Coronographic capability.
For projects using optical broad band filters, ACS is better suited due to its wider field of view, better sampling of the PSF, and higher throughput.
For projects using UV and narrow band filters the choice may depend on source size. For relatively compact objects, ACS is better due to the better PSF sampling and higher throughput and solar blind performance. For larger objects, e.g., the large planets Jupiter and Saturn, and diffuse galactic nebula such as the Orion and Eagle Nebulae, the larger field of view of WFPC2 makes it competitive.
Table 1.2: Comparison of WFPC2 and ACS Filters.
Effic'y3
Effic'yc
1Relative efficiency for ACS vs. WFPC2 for wide-field imaging. Defined as (ACS FOV area)x(ACS efficiency) / (WFPC2 FOV area) / (WFPC2 efficiency). For WFPC2 we have reduced FOV for the missing "L" shaped region around PC1. For ACS we assume WFC if available.
2The full WFPC2 FOV is a 150" x 150" L-shaped region, with area equivalent to a 134" x 134" square, which we use for comparisons to ACS.
3Efficiency near filter pivot wavelength; includes HST+instrument+filters. For ACS we have assumed use of "in hand" detectors.
4For ACS narrow band ramp filters we have assumed a FOV of 13" x 60", which we believe to be the region suitable for photometric work based on WFPC2 ramp filter experience. Similarly ACS broad ramp FOV is estimated to be 22" x 60". When a filter can be used with two ACS cameras, we give the larger format.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 WFPC2, STIS, or ACS must be used. Table 1.3 compares the detective efficiency of WFPC2 and NICMOS in the wavelength region where both instruments overlap in capabilities. 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 We note that the ACS WFC is optimized for the far red and has polarimetric capability.
Table 1.3: Comparison of WFPC2 and NICMOS Count Rates for a V=20 A0 Star. Instrument Filter Mean
Wavelength
(Å)Effective
Width (Å)Count Rate (e- s-1) SNR in 1 hour1 WFPC2 F785LP 9366 2095 14 215 F791W 8006 1304 30 314 F814W 8269 1758 33 333 F850LP 9703 1670 7.1 150 FQCH4N (Quad D) 8929 64 0.47 34, 292 F953N 9546 52 0.21 19, 15b F1042M 10,443 611 0.20 18, 15b LRF3 8000 9000 9762 105 113 126 1.5 0.64 0.23 66 40 20 NICMOS F090M4 8970 1885 2.0 90 F095Nd 9536 88 0.14 12 F097Nd 9715 94 0.18 16 F108Nd 10,816 94 0.17 15 F110W (Camera 1) 11,022 5920 6.6 160 F110W (Camera 2) 11,035 5915 14 260 F110W (Camera 3) 11,035 5915 26 350
1WFPC2 SNR assuming two 1800s exposures for cosmic ray removal. NICMOS SNR for central pixel of PSF.
2Values given for WFC (0.10" pixels) and PC (0.046" pixels).
3LRF filter is continuously tunable from 3710Å to 9762Å. LRF field-of-view is 10"x10".
4These NICMOS filters are available only on Camera 1 which has 11"x11" field-of-view.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.
- 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.
- 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. 0.0455" on WFPC2.
- High time resolution is possible (
~125µs) with the MAMA detectors. Also the STIS CCD may be cycled on ~20s timescale using a sub-array.
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 1.4 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.
1The FOC also had UV imaging capability, but it has been physically replaced by ACS.
Table 1.4: Comparison of WFPC2 and STIS Detective Efficiencies. Instrument Filter Mean
Wavelength
(Å)Bandpass
FWHM
(Å)1Peak QE2 WFPC2 F122M 1420 100 0.12% STIS F25LYA 1216 85 4.0% WFPC2 F160BW 1492 500 0.065% STIS FUV-MAMA ~1300 300 4.5% WFPC2 F255W 2586 393 0.45% STIS NUV-MAMA ~2400 1300 3.4% WFPC2 F375N 3738 42 0.8% STIS F28X50OII 3740 80 5% WFPC2 F502N 5013 37 5% STIS F28X50OIII 5007 5 15% WFPC2 F606W 5935 2200 13% STIS F28X50LP ~73003 2600 21% STIS F50CCD ~5800 4500 22%
1Note that definition of FWHM is different from "effective width" elsewhere herein.
2Includes instrument and OTA.
35500Å long pass filter.
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