1.2.1 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. 35"x31".
• Wider field of view in many narrow band filters - effective area of 134"x134" vs. up to 40"x70" (ACS LRFs).

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.
• Coronagraphic 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	Approx Peak Effic'y3	Filter	Camera	FOV (arcsec)4	Approx Peak Effic'y 3
Broad Band
F160W	90" x 90"	0.07%	F150LP	SBC	31" x 35"	3%	5.74
F170W	134" x 134"	0.17%	F165LP	SBC	31" x 35"	0.9%	0.32
F185W	134" x 134"	0.19%	-	-	-	-	-
F218W	134" x 134"	0.28%	F220W	HRC	26" x 29"	5%	0.75
F255W	134" x 134"	0.45%	F250W	HRC	26" x 29"	6.1%	0.56
F300W	134" x 134"	1.9%	-	-	-	-	-
F336W	134" x 134"	3.5%	F330W	HRC	26" x 29"	10.5%	0.13
F380W	134" x 134"	3.7%	-	-	-	-	-
F439W	134" x 134"	3.9%	F435W	WFC HRC	202" x 202" 26" x 29"	33% 22%	19.2 0.24
F450W	134" x 134"	8.5%	F475W	WFC HRC	202" x 202" 26" x 29"	36% 24%	9.62 0.12
F555W	134" x 134"	11%	F555W	WFC HRC	202" x 202" 26" x 29"	37% 23%	7.64 0.09
F569W	134" x 134"	11%	-	-	-	-	-
F606W	134" x 134"	14%	F606W	WFC HRC	202" x 202" 26" x 29"	44% 27%	7.14 0.08
F622W	134" x 134"	14%	F625W	WFC HRC	202" x 202" 26" x 29"	43% 26%	6.98 0.08
F675W	134" x 134"	14%	-	-	-	-	-
F702W	134" x 134"	14%	-	-	-	-	-
F785LP	134" x 134"	5%	-	-	-	-	-
F791W	134" x 134"	9%	F775W	WFC HRC	202" x 202" 26" x 29"	42% 22%	10.6 0.10
F814W	134" x 134"	10%	F814W	WFC HRC	202" x 202" 26" x 29"	42% 22%	9.54 0.09
F850LP	134" x 134"	3.9%	F850LP	WFC HRC	202" x 202" 26" x 29"	25% 13%	14.6 0.14
Medium Band
F122M	134" x 134"	0.11%	F122M	SBC	31" x 35"	0.9%	0.49
F410M	134" x 134"	4%	-	-	-	-	-
F467M	134" x 134"	5.5%	B. Ramp (FR459M)	WFC HRC	65" x 100" 26" x 29"	29%	0.44
F547M	134" x 134"	11%	F550M	WFC HRC	202" x 202" 26" x 29"	40% 25%	8.26 0.10
F1042M	134" x 134"	0.5%	B. Ramp (FR914M)	WFC HRC	65" x 100" 26" x 29"	4%	1.0
Narrow Band
F343N	134" x 134"	0.39%	F344N	HRC	26" x 29"	10%	1.08
F375N	134" x 134"	0.9%	OII Ramp (FR388N)	WFC HRC	65" x 100" 26" x 29"	4%	0.22
FQUVN 3767Å	60" x 60"	1.3%	OII Ramp (FR388N)	WFC HRC	65" x 100" 26" x 29"	6%	1.2
FQUVN 3831Å	67" x 67"	1.5%	OII Ramp (FR388N)	WFC HRC	65" x 100" 26" x 29"	8%	1.0
FQUVN 3915Å	67" x 67"	1.9%	OII Ramp (FR388N)	WFC HRC	65" x 100" 26" x 29"	10%	0.9
FQUVN 3993Å	67" x 67"	2.3%	OII Ramp (FR388N)	WFC HRC	65" x 100" 26" x 29"	10%	0.8
F390N	134" x 134"	1.9%	OII Ramp (FR388N)	WFC HRC	65" x 100" 26" x 29"	10%	0.23
F437N	134" x 134"	3%	OII Ramp (FR423N)	WFC	45" x 85"	10%	0.16
F469N	134" x 134"	3.7%	OII Ramp (FR462N)	WFC	60" x 85"	13%	0.17
F487N	134" x 134"	4.8%	OIII Ramp (FR505N)	WFC HRC	65" x 100" 26" x 29"	18%	0.20
F502N	134" x 134"	5.8%	F502N	WFC HRC	202" x 202" 26" x 29"	28% 19%	11.0 0.14
FQCH4 5435Å	30" x 30"	9.5%	OIII Ramp (FR551N)	WFC	45" x 85"	28%	2.7
F588N	134" x 134"	13%	OIII Ramp (FR551N)	WFC	45" x 85"	34%	0.12
FQCH4 6199Å	30" x 30"	12%	OIII Ramp (FR601N)	WFC	60" x 85"	29%	2.3
F631N	134" x 134"	13%	OIII Ramp (FR601N)	WFC	60" x 85"	31%	0.11
F656N	134" x 134"	11%	H Ramp (FR656N)	WFC HRC	65" x 100" 26" x 29"
F658N	134" x 134"	11%	F658N	WFC HRC	202" x 202" 26" x 29"	44% 26%	9.09 0.10
F673N	134" x 134"	12%	H Ramp (FR656N)	WFC HRC	65" x 100" 26" x 29"	28%	0.11
FQCH4 7278Å	30" x 30"	10%	H Ramp (FR716N)	WFC	45" x 85"	31%	3
FQCH4 8930Å	30" x 30"	2.9%	F892N	WFC HRC	202" x 202" 26" x 29"	12%	3.47
F953N	134" x 134"	2.2%	IR Ramp (FR931N)	WFC	60" x 85"	12%	0.31

1Relative efficiency for ACS vs. WFPC2 for wide-field imaging. Defined as [(ACS FOV area)x(ACS efficiency)] / [(WFPC2 FOV area) x (WFPC2 efficiency)]. For WFPC2 we have reduced FOV for the missing "L" shaped region around PC1. 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. 4For ACS the full WFC FOV is 202"x202", the full HRC FOV is 26"x29", and the full SBC FOV is 31" x 35". When using the narrow band ramp filters the larger WFC FOV gets reduced, depending on the FOV location. There are three possible locations: inner region (45"x85"), middle region (65"x100"), and outer region (60"x85").