Space Telescope Science Institute  Chapter 5:  Imaging  5.2 Important Considerations for ACS Imaging

5.1 Imaging Overview


ACS can be used to obtain images through a variety of optical and ultraviolet filters. When the selected ACS camera is the WFC or the HRC, the appropriate filter in one of the two filter wheels is rotated into position, and a clear aperture is automatically selected on the other filter wheel. For SBC imaging the single filter wheel is rotated to the required position. A number of apertures are defined for each ACS camera. In general, these refer to different target positions on the detector.

Table 5.1, Table 5.2, and Table 5.3 provide a complete summary of the filters available for imaging with each detector. Figures 5.1 through 5.6 show the filter transmission curves, and Figure 5.7 shows the integrated system throughputs.

The CCD filter wheels contain filters with two different sizes. Some filters (F435W, F475W, F502N, F550M, F555W, F606W, F625W, F658N, F660N, F775W, F814W, F850LP, and, G800L) are full-sized filters that can be used with both WFC and HRC. Others (F220W, F250W, F330W, F344N, F892N, POL0UV, POL60UV, POL120UV, POL0V, POL60V, POL120V, and PR200L) are smaller, giving a full unvignetted field of view when used with the HRC, but a vignetted field of view of only 72×72 arcseconds when used with the WFC. Use of the small UV filters is not supported with the WFC due to the unpredictable behavior of the silver coatings shortward of 4000 Å.

The ramp filters are designed to allow narrow or medium band imaging centered at an arbitrary wavelength. Each ramp filter is divided into three segments, of which only the middle segment may be used with the HRC. More information is available at Section 7.7.2.

Note that although the CLEAR filters are specified in the filter wheel tables, users do not need to specify these filters in their HST proposals; they are added automatically in conjunction with the desired filter in the complementary wheel. In the SBC filter wheel, every third slot (#1, 4, 7, 10) is blocked off, so that in the case of a bright object limit violation, it is only necessary to rotate the filter wheel to an adjacent slot to block the incoming light.

With either the WFC or HRC, it is possible to select a filterless observation by specifying CLEAR (this is an "available-but-unsupported" filter) as the filter name, although the image will be of degraded quality. Rough wavelengths and widths, when used with the WFC or HRC, are listed in Table 5.1 under CLEAR entries. Use of CLEAR will provide slightly degraded PSFs with the HRC and seriously degraded PSFs for the WFC. More details on PSFs with use of CLEAR are provided in ACS ISR 2003-03. Applications are expected to be rare, but a valid use could be astrometry of extremely faint targets with the HRC when color information is not required.


Table 5.1: ACS WFC/HRC filters in Filter Wheel #1.
Filter name
Central wavelength (Å)
Width (Å)
Description
Camera
CLEAR
6200
5200
Clear aperture
WFC/HRC
F555W
5346
1193
Johnson V
WFC/HRC
F775W
7764
1528
SDSS i
WFC/HRC
F625W
6318
1442
SDSS r
WFC/HRC
F550M
5580
547
Narrow V
WFC/HRC
F850LP
9445
1229
SDSS z
WFC/HRC
POL0UV
2000 to 6000
-
0° UV polarizer
HRC[/WFC]
POL60UV
2000 to 6000
-
60° UV polarizer
HRC[/WFC]
POL120UV
2000 to 6000
-
120° UV polarizer
HRC[/WFC]
F892N
8917
154
Methane (2%)
HRC/[WFC]
F606W
5907
2342
Broad V
WFC/HRC
F502N
5022
57
[OIII] (1%)
WFC/HRC
G800L
5800 to 11,000
-
Grism (R~100)
WFC/HRC
F658N
6584
78
H (1%)
WFC/HRC
F475W
4760
1458
SDSS g
WFC/HRC

[/WFC] indicates that polarizer filters, designed for the HRC field of view, induces vignetting when used with the WFC, producing a 72 by 72arcsecond field of view.

Table 5.2: ACS WFC/HRC filters in Filter Wheel #2.
Filter
name
Central
wavelength (Å)
Width
(Å)
Description
Camera
CLEAR
6000
5200
Clear aperture
WFC/HRC
F660N
6602
40
[NII] (1%)
WFC/HRC
F814W
8333
2511
Broad I
WFC/HRC
FR388N
3710 to 4050
2%
[OII] Ramp-middle segment
WFC/HRC
FR423N
4050 to 4420
2%
[OII] Ramp-inner segment
WFC
FR462N
4420 to 4820
2%
[OII] Ramp-outer segment
WFC
F435W
4297
1038
Johnson B
WFC/HRC
FR656N
6270 to 6850
2%
H Ramp-middle segment
WFC/HRC
FR716N
6850 to 7470
2%
H Ramp-inner segment
WFC
FR782N
7470 to 8160
2%
H Ramp-outer segment
WFC
POL0V
4000 to 8000
-
0° Visible Polarizer
HRC[/WFC]
F330W
3354
588
HRC U
HRC
POL60V
4000 to 8000
-
60° Visible Polarizer
HRC[/WFC]
F250W
2696
549
Near-UV broadband
HRC
POL120V
4000 to 8000
-
120° Visible Polarizer
HRC[/WFC]
PR200L
2000 to 4000
-
NUV Prism (R~100 @ 200 nm)
HRC
F344N
3434
60
Ne V (2%)
HRC
F220W
2228
485
Near-UV broadband
HRC
FR914M
7570 to 10,710
9%
Broad Ramp-middle segment
WFC/HRC
FR853N
8160 to 8910
2%
IR Ramp-inner segment
WFC
FR931N
8910 to 9720
2%
IR Ramp-outer segment
WFC
FR459M
3810 to 5370
9%
Broad Ramp-middle segment
WFC/HRC
FR647M
5370 to 7570
9%
Broad Ramp-inner segment
WFC
FR1016N
9720 to 10,610
2%
IR Ramp-outer segment
WFC
FR505N
4820 to 5270
2%
[OIII] Ramp-middle segment
WFC/HRC
FR551N
5270 to 5750
2%
[OIII] Ramp-inner segment
WFC
FR601N
5750 to 6270
2%
[OIII] Ramp-outer segment
WFC

[/WFC] indicates that polarizer filters, designed for the HRC field of view, induces vignetting when used with the WFC, producing a 72 by 72arcsecond field of view.

Table 5.3: ACS SBC filter complement.
Filter name
Description
F115LP
MgF2 (1150 Å longpass)
F125LP
CaF2 (1250 Å longpass)
F140LP
BaF2 (1400 Å longpass)
F150LP
Crystal quartz (1500 Å longpass)
F165LP
Fused Silica (1650 Å longpass)
F122M
Ly- ( = 1200Å, = 60 Å)
PR110L
LiF Prism (R~100)
PR130L
CaF2 Prism (R~100)
Figure 5.1: ACS broad-band filters.

 
Figure 5.2: ACS SDSS filters.

 
Figure 5.3: ACS UV and medium-band filters.

 
Figure 5.4: ACS narrow-band filters.

 
Figure 5.5: ACS SBC filters.

 
Figure 5.6: Comparison between the ACS and WFPC2 ramp filters.

 
The crosses and open circles are for the ACS narrow and medium band ramps. The open squares are for the 4 WFPC2 ramps. For each of the ACS ramps the peak throughput that was calculated for eleven central wavelength values is plotted. For the WFPC2 ramps, the peak throughput calculated every 100 Å within the field of view of any of the 4 chips, and a 0º filter rotation angle (as mapped in Figures 3.4 and 3.5 of the WFPC2 Instrument Handbook, version 3.0), are plotted.
 

Chapter 5:  Imaging  5.2 Important Considerations for ACS Imaging
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