NICMOS Instrument Handbook for Cycle 11

Filters & Optical Elements

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Each camera has 20 filter positions on a single filter wheel: 19 filters and one blank. As a result, not all filters are available in all cameras. Moreover, the specialized optical elements, such as the polarizers and grisms, cannot be crossed with other filters, and can only be used in fixed bands. In general the filters have been located in a way which best utilizes the characteristics of NICMOS, thus at shorter wavelengths the most important narrow band filters are located in NIC1 so that the diffraction limited performance can be maintained wherever possible, while those in NIC2 have been selected to work primarily in the longer wavelength range where it will also deliver diffraction limited imaging.

Nomenclature

The name of each optical element starts with a letter or group of letters identifying what kind of element it is: filters start with an "F", grisms with a "G", and polarizers with "POL". Following the initial letter(s) is a number which in the case of filters identifies its approximate central wavelength in microns, e.g., F095N implies a central wavelength of 0.95 microns. A trailing letter identifies the filter width, with "W" for wide, "M" for medium and "N" for narrow. In the case of grisms, the initial "G" is followed by a number which gives the center of the free-spectral range of the element, e.g., G206. For the polarizers, a somewhat different notation is used, with the initial "POL" being followed by a number which gives the PA of the principal axis of the polarizer in degrees, and a trailing letter identifying the wavelength range it can be used in, which is either "S" for short (0.8-1.3 microns) or "L" for long (1.9-2.1 microns).

Tables 4.1 through 4.3 list the available filters and provide an initial general description of each, starting with NIC1 and working down in spatial resolution to NIC3. Figures 4.1 through 4.3 show the effective throughput curves of all of the NICMOS filters for cameras NIC1, NIC2, and NIC3, respectively, which includes the filter transmission convolved with the OTA, NICMOS foreoptics, and detector response. Appendix 1 provides further details and the individual filter throughput curves.

Table 4.1: NIC 1 Filters

Name	Central Wavelength (µm)	Wavelength range (µm)	Comment
Blank	N/A	N/A	blank
F110W	1.025	0.8-1.4
F140W	1.3	0.8-1.8	Broad Band
F160W	1.55	1.4-1.8
F090M	0.9	0.8-1.0
F110M	1.1	1.0-1.2
F145M	1.45	1.35-1.55	Water
F165M	1.6	1.54-1.74
F170M	1.7	1.6-1.8
F095N	0.953	1%	[S III]
F097N	0.97	1%	[S III] continuum
F108N	1.083	1%	He I
F113N	1.13	1%	He I continuum
F164N	1.644	1%	[Fe II]
F166N	1.66	1%	[Fe II] continuum
F187N	1.87	1%	Paschen
F190N	1.90	1%	Paschen continuum
POL0S	1.1	0.8-1.3	Short Polarizer
POL120S	1.1	0.8-1.3	Short Polarizer
POL240S	1.1	0.8-1.3	Short Polarizer

 

Figure 4.1: Filters for NIC 1 (at 78 K)
 

Table 4.2: NIC 2 Filters

Name	Central Wavelength (µm)	Wavelength range (µm)	Comment
Blank	N/A	N/A	blank
F110W	1.1	0.8-1.4
F160W	1.6	1.4-1.8	Minimum background
F187W	1.875	1.75-2.0	Broad
F205W	1.9	1.75-2.35	Broad Band
F165M	1.7	1.54-1.75	Planetary continuum
F171M	1.715	1.68-1.76	HCO2 and C2 continuum
F180M	1.80	1.76-1.83	HCO2 and C2 bands
F204M	2.04	1.98-2.08	Methane imaging
F207M	2.1	2.01-2.16
F222M	2.3	2.15-2.29	CO continuum
F237M	2.375	2.29-2.44	CO
F187N	1.87	1%	Paschen
F190N	1.9	1%	Paschen continuum
F212N	2.121	1%	H2
F215N	2.15	1%	H2 and Br continuum
F216N	2.165	1%	Brackett
POL0L	2.05	1.89-2.1	Long polarizer
POL120L	2.05	1.89-2.1	Long polarizer
POL240L	2.05	1.89-2.1	Long polarizer

 

Figure 4.2: Filters for NIC 2 (at 78 K)
 

Table 4.3: NIC 3 Filters

Name	Central Wavelength (µm)	Wavelength range (µm)	Comment
Blank	N/A	N/A	blank
F110W	1.1	0.8-1.4
F150W	1.5	1.1-1.9	Grism B continuum
F160W	1.6	1.4-1.8	Minimum background
F175W	1.75	1.2-2.3
F222M	2.3	2.15-2.28	CO continuum
F240M	2.4	2.3-2.5	CO band
F108N	1.0830	1%	He I
F113N	1.13	1%	He I continuum
F164N	1.644	1%	[Fe II]
F166N	1.66	1%	[Fe II] continuum
F187N	1.875	1%	Paschen
F190N	1.9	1%	Paschen continuum
F196N	1.962	1%	[Si VI]
F200N	2.0	1%	[Si VI] continuum
F212N	2.121	1%	H2
F215N	2.15	1%	H2 continuum
G096	0.9673	0.8-1.2	GRISM A
G141	1.414	1.1-1.9	GRISM B
G206	2.067	1.4-2.5	GRISM C

 

Figure 4.3: Filters for NIC 3 (at 78 K)
 

Out-of-Band Leaks in NICMOS Filters

In order to make use of the high spatial resolution of HST, many observers will wish to use NICMOS to observe very red objects (e.g., protostars) at relatively short wavelengths. By their very nature, these very red objects have very low effective color temperatures. Thus if we observe such an object at a wavelength of 1.0 microns, we can expect that its flux at 2.5 microns will be orders of magnitude larger than its flux at the desired wavelength. In such a case, exceptionally good out-of-band blocking is required from the filter. We have therefore investigated whether the measured filter transmissions would allow any sources with extreme colors to yield erroneous photometry due to out-of-band leaks.

For observations of objects with extreme colors (such as highly reddened sources, protostars, etc.), out-of-band filter leaks could potentially have a detrimental impact on photometry. Early estimates based on pre-flight filter transmission measurements suggested that there might be red leaks for several filters, and the original edition of the Instrument Handbook recommended caution for the filters F090M, F095M, F097N, F108N, F110M, F110W, F113N, F187N, and F190M. A limited set of in-flight tests for red leaks have been made. Actual red leaks were insignificant or non-existent.