WFC3IR Minimum Exposure Times and Subarrays

WFC3/IR is a multi-accum detector. There are many timing sequences, with the possibility of using 1 to 15 reads for each. The total integration time depends on the type of timing sequence, the number of reads (NSAMP), and the size of the selected array. Subarrays have shorter integration times than full arrays for the same timing sequence name. The integration times are shown in tables in the Phase II Proposal Instructions.

The read noise depends on the NSAMP. The ETC uses a value of read noise that assumes NSAMP=15. It underestimates the read noise for smaller NSAMP. The ETC accepts or computes only a total integration time, so observers should check the possible integration times and keep in mind the assumption of NSAMP=15 for the read noise.

WFC3 Filters and Dispersers

The following tables show the filter and disperser properties.

WFC3 UVIS Filters and Grism

Name [1]	Description [2]	Pivot (A) [3]	Width (A) [4]	Peak Transmission
UVIS Long-Pass (LP) and Extremely Wide (X) Filters
F200LP	Clear; grism reference	5686.9		0.98
F300X	Extremely wide UV	2829.8	753.0	0.53
F350LP	Long pass	6812.0		0.98
F475X	Extremely wide blue	4917.1	2199.6	0.94
F600LP	Long pass	8430.2		0.99
F850LP	SDSS z’	9756.4		0.96
UVIS Wide-Band(W) Filters
F218W	ISM feature	2183.0	351.7	0.23
F225W	UV wide	2341.0	547.3	0.32
F275W	UV wide	2715.3	480.8	0.46
F336W	U, Stromgren u	3361.1	553.8	0.75
F390W	Washington C	3904.6	953.0	0.96
F438W	WFPC2 B	4318.7	676.8	0.84
F475W	SDSS g’	4760.6	1488.9	0.92
F555W	WFPC2 V	5309.8	1595.1	0.95
F606W	WFPC2 Wide V	5907.0	2304.2	0.99
F625W	SDSS r’	6254.0	1575.4	0.95
F775W	SDSS i’	7733.6	1486.0	0.85
F814W	WFPC2 Wide I	8304.7	2543.3	0.97
UVIS Medium-Band (M) Filters
F390M	Ca II continuum	3893.8	210.5	0.88
F410M	Stromgren v	4107.0	182.8	0.98
FQ422M	Blue continuum	4217.7	113.3	0.69
F467M	Stromgren b	4680.7	218.0	0.98
F547M	Stromgren y	5447.0	714.0	0.88
F621M	11% passband	6216.7	631.0	0.99
F689M	11% passband	6886.0	708.6	0.94
F763M	11% passband	7636.3	798.6	0.97
F845M	11% passband	8468.9	886.7	0.96
UVIS Narrow-Band (N) Filters
FQ232N	C II] 2326	2326.9	32.2	0.12
FQ243N	[Ne IV] 2425	2420.6	34.8	0.15
F280N	Mg II 2795/2802	2796.8	22.9	0.27
F343N	[Ne V] 3426	3438.0	140.0	0.78
F373N	[O II] 3726/3728	3729.6	39.2	0.78
FQ378N	z ([O II] 3726)	3790.9	89.2	0.83
FQ387N	[Ne III] 3868	3873.0	23.1	0.72
F395N	Ca II 3933/3968	3953.7	72.9	0.86
FQ436N	H 4340 + [O III] 4363	4366.7	35.7	0.67
FQ437N	[O III] 4363	4370.6	24.6	0.70
F469N	He II 4686	4687.5	37.2	0.69
F487N	H 4861	4870.7	48.4	0.85
FQ492N	z (H)	4932.1	101.0	0.85
F502N	[O III] 5007	5009.0	57.8	0.87
FQ508N	z ([O III] 5007)	5089.7	117.9	0.87
FQ575N	[N II] 5754	5755.9	12.9	0.78
FQ619N	CH4 6194	6197.5	61.6	0.89
F631N	[O I] 6300	6303.0	43.1	0.86
FQ634N	6194 continuum	6347.5	66.2	0.88
F645N	Continuum	6451.6	85.0	0.86
F656N	H 6562	6561.1	13.9	0.86
F657N	Wide H + [N II]	6565.1	96.3	0.90
F658N	[N II] 6583	6585.2	23.6	0.92
F665N	z (H + [N II])	6654.4	109.0	0.90
FQ672N	[S II] 6717	6716.1	14.9	0.89
F673N	[S II] 6717/6731	6764.5	100.5	0.91
FQ674N	[S II] 6731	6729.5	10.0	0.68
F680N	z (H + [N II])	6878.6	323.6	0.95
FQ727N	CH4 7270	7274.7	64.8	0.89
FQ750N	7270 continuum	7500.6	68.8	0.85
FQ889N	CH4 25 km-agt [5]	8891.8	93.7	0.90
FQ906N	CH4 2.5 km-agt	9056.7	94.0	0.92
FQ924N	CH4 0.25 km-agt	9246.3	89.2	0.94
FQ937N	CH4 0.025 km-agt	9371.1	91.9	0.91
F953N	[S III] 9532	9529.7	84.6	0.90
UVIS Grism (G)
G280	UV Grism		1850	0.4

[1]

The spectral-element naming convention is as follows for both the UVIS and IR channels. All filter names begin with F, and grisms with G; if the filter is part of a four-element quad mosaic, a Q follows F. Then there is a three-digit number giving the nominal effective wavelength of the bandpass, in nm (UVIS channel) or nm/10 (IR channel). (For long-pass filters, the number is instead the nominal blue cut-off wavelength in nm.) Finally, for the filters, one or two letters indicate the bandpass width: X (extremely wide), LP (long pass), W (wide), M (medium), or N (narrow).

[2]

Filters intended for imaging in a red-shifted bandpass are given descriptions similar to the following: “z (H + [N II])”.

[3]

“Pivot wavelength” is a measure of the effective wavelength of a filter (see A. Tokunaga & W. Vacca 2006, PASP, 117, 421). It is calculated here based only on the filter transmission. Values are approximate for the long-pass filters. All wavelength measurements in this table were made in air.

[4]

Full width at 50% of peak transmission for wide and medium bands, and at 10% of peak transmission for narrow bands. For long-pass filters, the widths are approximate and include convolution with the detector QE.

[5]

km-agt (km-amagat) is a unit of vertical column density, equal to 2.69e24 molecules/cm^2

WFC3 IR Channel Filters and Grisms

Name [6]	Description	Pivot [7] (nm)	Width [8] (nm)	Peak Transmission
IR Wide-Band(W) Filters
F105W	Wide Y	1048.95	292.30	0.98
F110W	Wide YJ	1141.40	503.40	0.99
F125W	Wide J	1245.90	301.50	0.98
F140W	Wide JH gap; red grism reference	1392.10	399.00	0.99
F160W	WFC3 H	1540.52	287.88	0.98
IR Medium-Band(M) Filters
F098M	Blue grism reference	982.93	169.48	0.97
F127M	H2O/CH4 continuum	1273.64	68.79	0.98
F139M	H2O/CH4 line	1383.80	64.58	0.98
F153M	H2O and NH3	1533.31	68.78	0.98
IR Narrow-Band(N) Filters
F126N	[Fe II]	1258.26	11.83	0.90
F128N	Paschen	1283.30	13.54	0.94
F130N	Paschen continuum	1300.62	13.28	0.96
F132N	Paschen (redshifted)	1319.04	13.07	0.91
F164N	[Fe II]	1645.13	17.48	0.93
F167N	[Fe II] continuum	1667.26	17.16	0.93
IR Grisms (G)
G102	“Blue” high-resolution grism		250
G141	“Red” low-resolution grism		600

[6]

See Footnote 1 of UVIS table for naming conventions.

[7]

“Pivot wavelength” is defined as in Table 6.2. All wavelength measurements in this table were made in air.

[8]

Full width at 50% of peak transmission.

WFC3 Scan Mode

Spatial scan mode is available for IR imaging observations, IR spectroscopic observations, and UVIS imaging observations. The target is moved along the detector at a fixed rate during the exposure time. The ETC requires a pair of input parameters to describe the scan: either scan rate (arcsec per sec) and scan length (arcsec) or exposure time (sec) and scan length (arcsec). For the imaging modes, the height and width of the region that will be used for SNR calculations is selected. For the IR spectroscopic mode, only the height of the region is selected. For that mode, the width will be 1 pixel for computing the count rate and one spectral resolution element (2 pixels) for computing counts.

The scan rate can be any number between 0.0 and 7.84 arcseconds/sec. Rates higher than 5 arcseconds/sec are performed under gyro control. The scan length or exposure time should be chosen to fit the array or subarray used for the exposure.