WFPC2 Instrument Handbook for Cycle 10

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. As of this writing, WFPC2 and STIS have capabilities in this area¹. During the next HST servicing mission (SM3B) currently scheduled for mid 2001, the cryo-cooler should be installed for NICMOS, and the Advanced Camera for Surveys (ACS) should be installed. 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⁽¹⁾ 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 CCDs 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 (²)} 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: 28" x 50" @ 0.05" pix^{-1 (³)}

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 <0.1 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 NICMOS will not be available for observations until after the next servicing mission (SM3B, currently scheduled for 2001).
² "L"-shaped field-of-view using 3 CCDs with 0.1" pixels, and one CCD with 0.046" pixels.
³ Field-of-view is 50" x 50" if no filter is used, and 12" x 12" for some neutral density filters.

Comparison of WFPC2 and ACS

Advantages of each instrument may be summarized as follows.

WFPC2 advantages are:

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.

ACS advantages are:

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
WFPC2 ACS ACS / WFPC2 Wide-Field Imaging Effic'y¹

Filter FOV (arcsec)² Peak
Effic'y³ Filter Camera FOV (arcsec)⁴ Peak
Effic'y3

Broad Band

F160W

90" x 90"

0.06%

F140LP

SBC

26" x 29"

2% - 3%

4

F170W

134" x 134"

0.18%

F160LP

SBC

26" x 29"

1%

0.23

F185W

134" x 134"

0.19%

F165LP

SBC

26" x 29"

0.5%

0.11

F218W

134" x 134"

0.28%

F220M

HRC

26" x 29"

2.2%

0.33

F255W

134" x 134"

0.4%

F250M

HRC

26" x 29"

1.5%

0.16

F300W

134" x 134"

1.8%

-

-

-

-

-

F336W

134" x 134"

2.9%

F330W

HRC

26" x 29"

10%

0.14

F380W

134" x 134"

3.4%

F410W

WFC

200" x 204"

14%

9

F439W

134" x 134"

3.5%

F435W

WFC, HRC

200" x 204"

20%

13

F450W

134" x 134"

8%

F475W

WFC, HRC

200" x 204"

40%

11

F555W

134" x 134"

10%

F555W

WFC, HRC

200" x 204"

30%

7

F569W

134" x 134"

11%

-

-

-

-

-

F606W

134" x 134"

13%

F606W

WFC, HRC

200" x 204"

34%

6

F622W

134" x 134"

13%

F625W

WFC, HRC

200" x 204"

34%

6

F675W

134" x 134"

13%

-

-

-

-

-

F702W

134" x 134"

13%

-

-

-

-

-

F785LP

134" x 134"

4%

-

-

-

-

-

F791W

134" x 134"

9%

F775W

WFC, HRC

200" x 204"

36%

9

F814W

134" x 134"

10%

F814W

WFC, HRC

200" x 204"

42%

10

F850LP

134" x 134"

3.5%

F850LP

WFC, HRC

200" x 204"

17%

11

Medium Band

F122M

134" x 134"

0.12%

F122M

SBC

26" x 29"

5%

1.7

F410M

134" x 134"

3.6%

B. Ramp

WFC, HRC

22" x 60"

15%

0.31

F467M

134" x 134"

4.8%

B. Ramp

WFC

22" x 60"

29%

0.44

F547M

134" x 134"

11%

F550M

WFC, HRC

200" x 204"

33%

7

F1042M

134" x 134"

0.3%

B. Ramp

WFC

22" x 60"

4%

1.0

Narrow Band

F343N

134" x 134"

0.7%

F344N

HRC

26" x 29"

2.5%

0.15

F375N

134" x 134"

0.8%

OII Ramp

WFC, HRC

13" x 60"

4%

0.22

FQUVN
3767Å

60" x 60"

1.1%

OII Ramp

WFC, HRC

13" x 60"

6%

1.2

FQUVN
3831Å

67" x 67"

1.5%

OII Ramp

WFC, HRC

13" x 60"

8%

1.0

FQUVN
3915Å

67" x 67"

1.9%

OII Ramp

WFC, HRC

13" x 60"

10%

0.9

FQUVN
3993Å

67" x 67"

2.2%

OII Ramp

WFC, HRC

13" x 60"

10%

0.8

F390N

134" x 134"

1.9%

OII Ramp

WFC, HRC

13" x 60"

10%

0.23

F437N

134" x 134"

2.7%

OII Ramp

WFC

13" x 60"

10%

0.16

F469N

134" x 134"

3.3%

OII Ramp

WFC

13" x 60"

13%

0.17

F487N

134" x 134"

4%

OIII Ramp

WFC, HRC

13" x 60"

18%

0.20

F502N

134" x 134"

5%

F502N

WFC, HRC

200" x 204"

20%

9

FQCH4
5433Å

30" x 30"

9%

OIII Ramp

WFC, HRC

13" x 60"

28%

2.7

F588N

134" x 134"

12%

OIII Ramp

WFC

13" x 60"

34%

0.12

FQCH4
6193Å

30" x 30"

11%

OIII Ramp

WFC

13" x 60"

29%

2.3

F631N

134" x 134"

12%

OIII Ramp

WFC

13" x 60"

31%

0.11

F656N

134" x 134"

10%

F658N

WFC, HRC

200" x 204"

28%

6

F658N

134" x 134"

11%

F658N

WFC, HRC

200" x 204"

28%

6

F673N

134" x 134"

11%

H Ramp

WFC, HRC

13" x 60"

28%

0.11

FQCH4
7274Å

30" x 30"

9%

H Ramp

WFC

13" x 60"

31%

3

FQCH4
8929Å

30" x 30"

3.3%

F892N

WFC, HRC

200" x 204"

17%

230

F953N

134" x 134"

1.7%

IR Ramp

WFC

13" x 60"

12%

0.31

¹ Relative 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.
² The 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.
³ Peak efficiency includes HST+instrument+filters. For ACS we have assumed use of "in hand" detectors.
⁴ For 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.

Comparison of WFPC2 and NICMOS²

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 or STIS 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, 15²

F1042M 10,443 611 0.20 18, 15²

LRF³ 8000 9000 9762 105 113 126 1.5 0.64 0.23 66 40 20

NICMOS F090M⁴ 8970 1885 2.0 90

F095N⁴ 9536 88 0.14 12

F097N⁴ 9715 94 0.18 16

F108N⁴ 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

¹ WFPC2 SNR assuming two 1800s exposures for cosmic ray removal. NICMOS SNR for central pixel of PSF.
² Values given for WFC (0.10" pixels) and PC (0.046" pixels).
³ LRF filter is continuously tunable from 3710Å to 9762Å. LRF field-of-view is 10"x10".
⁴ These NICMOS filters are available only on Camera 1 which has 11" x 11" field-of-view.

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. 0.0455" 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.

Table 1.4: Comparison of WFPC2 and STIS Detective Efficiencies.
Instrument Filter Mean
Wavelength
(Å) Bandpass
FWHM
(Å)¹ Peak 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%

¹ Note that definition of FWHM is different from "effective width" elsewhere herein.
² Includes instrument and OTA.
³ 5500Å long pass filter.

¹ The FOC also had UV imaging capability, but it is currently unsupported and will be physically replaced by ACS.
² NICMOS will not be available for observations until after the next servicing mission (SM3B, currently scheduled for 2001).

***Table 1.1: Comparison of WFPC2, ACS, NICMOS, and STIS Instrumental Imaging Parameters.***
Parameter	WFPC2	ACS	NICMOS⁽¹⁾	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 CCDs	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 (²)}	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: 28" x 50" @ 0.05" pix^{-1 (³)}
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	<0.1 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^-

***Table 1.2: Comparison of WFPC2 and ACS Filters***
WFPC2	ACS	ACS / WFPC2 Wide-Field Imaging Effic'y¹
Filter	FOV (arcsec)²	Peak Effic'y³	Filter	Camera	FOV (arcsec)⁴	Peak Effic'y3
Broad Band
F160W	90" x 90"	0.06%	F140LP	SBC	26" x 29"	2% - 3%	4
F170W	134" x 134"	0.18%	F160LP	SBC	26" x 29"	1%	0.23
F185W	134" x 134"	0.19%	F165LP	SBC	26" x 29"	0.5%	0.11
F218W	134" x 134"	0.28%	F220M	HRC	26" x 29"	2.2%	0.33
F255W	134" x 134"	0.4%	F250M	HRC	26" x 29"	1.5%	0.16
F300W	134" x 134"	1.8%	-	-	-	-	-
F336W	134" x 134"	2.9%	F330W	HRC	26" x 29"	10%	0.14
F380W	134" x 134"	3.4%	F410W	WFC	200" x 204"	14%	9
F439W	134" x 134"	3.5%	F435W	WFC, HRC	200" x 204"	20%	13
F450W	134" x 134"	8%	F475W	WFC, HRC	200" x 204"	40%	11
F555W	134" x 134"	10%	F555W	WFC, HRC	200" x 204"	30%	7
F569W	134" x 134"	11%	-	-	-	-	-
F606W	134" x 134"	13%	F606W	WFC, HRC	200" x 204"	34%	6
F622W	134" x 134"	13%	F625W	WFC, HRC	200" x 204"	34%	6
F675W	134" x 134"	13%	-	-	-	-	-
F702W	134" x 134"	13%	-	-	-	-	-
F785LP	134" x 134"	4%	-	-	-	-	-
F791W	134" x 134"	9%	F775W	WFC, HRC	200" x 204"	36%	9
F814W	134" x 134"	10%	F814W	WFC, HRC	200" x 204"	42%	10
F850LP	134" x 134"	3.5%	F850LP	WFC, HRC	200" x 204"	17%	11
Medium Band
F122M	134" x 134"	0.12%	F122M	SBC	26" x 29"	5%	1.7
F410M	134" x 134"	3.6%	B. Ramp	WFC, HRC	22" x 60"	15%	0.31
F467M	134" x 134"	4.8%	B. Ramp	WFC	22" x 60"	29%	0.44
F547M	134" x 134"	11%	F550M	WFC, HRC	200" x 204"	33%	7
F1042M	134" x 134"	0.3%	B. Ramp	WFC	22" x 60"	4%	1.0
Narrow Band
F343N	134" x 134"	0.7%	F344N	HRC	26" x 29"	2.5%	0.15
F375N	134" x 134"	0.8%	OII Ramp	WFC, HRC	13" x 60"	4%	0.22
FQUVN 3767Å	60" x 60"	1.1%	OII Ramp	WFC, HRC	13" x 60"	6%	1.2
FQUVN 3831Å	67" x 67"	1.5%	OII Ramp	WFC, HRC	13" x 60"	8%	1.0
FQUVN 3915Å	67" x 67"	1.9%	OII Ramp	WFC, HRC	13" x 60"	10%	0.9
FQUVN 3993Å	67" x 67"	2.2%	OII Ramp	WFC, HRC	13" x 60"	10%	0.8
F390N	134" x 134"	1.9%	OII Ramp	WFC, HRC	13" x 60"	10%	0.23
F437N	134" x 134"	2.7%	OII Ramp	WFC	13" x 60"	10%	0.16
F469N	134" x 134"	3.3%	OII Ramp	WFC	13" x 60"	13%	0.17
F487N	134" x 134"	4%	OIII Ramp	WFC, HRC	13" x 60"	18%	0.20
F502N	134" x 134"	5%	F502N	WFC, HRC	200" x 204"	20%	9
FQCH4 5433Å	30" x 30"	9%	OIII Ramp	WFC, HRC	13" x 60"	28%	2.7
F588N	134" x 134"	12%	OIII Ramp	WFC	13" x 60"	34%	0.12
FQCH4 6193Å	30" x 30"	11%	OIII Ramp	WFC	13" x 60"	29%	2.3
F631N	134" x 134"	12%	OIII Ramp	WFC	13" x 60"	31%	0.11
F656N	134" x 134"	10%	F658N	WFC, HRC	200" x 204"	28%	6
F658N	134" x 134"	11%	F658N	WFC, HRC	200" x 204"	28%	6
F673N	134" x 134"	11%	H Ramp	WFC, HRC	13" x 60"	28%	0.11
FQCH4 7274Å	30" x 30"	9%	H Ramp	WFC	13" x 60"	31%	3
FQCH4 8929Å	30" x 30"	3.3%	F892N	WFC, HRC	200" x 204"	17%	230
F953N	134" x 134"	1.7%	IR Ramp	WFC	13" x 60"	12%	0.31

***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, 15²
	F1042M	10,443	611	0.20	18, 15²
	LRF³	8000 9000 9762	105 113 126	1.5 0.64 0.23	66 40 20
NICMOS	F090M⁴	8970	1885	2.0	90
	F095N⁴	9536	88	0.14	12
	F097N⁴	9715	94	0.18	16
	F108N⁴	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

***Table 1.4: Comparison of WFPC2 and STIS Detective Efficiencies.***
Instrument	Filter	Mean Wavelength (Å)	Bandpass FWHM (Å)¹	Peak 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%

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WFPC2 Instrument Handbook for Cycle 10

Which Instrument to Use: WFPC2, ACS, NICMOS, or STIS?

Comparison of WFPC2 and ACS

Comparison of WFPC2 and NICMOS2

Comparison of WFPC2 and STIS

Comparison of WFPC2 and NICMOS²