Space Telescope Science Institute  Chapter 4:  Imaging  4.2 Photometry

4.1 Filters & Optical Elements


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. Therefore 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.

4.1.1 Nomenclature

Following the traditional HST naming convention, 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 A 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)

 

4.1.2 Out-of-Band Leaks in NICMOS Filters

In order to make use of the high spatial resolution of HST, many observers expect to use NICMOS to observe very red objects (e.g., protostars) at relatively short wavelengths. These objects have very low effective color temperatures. Thus, the flux of such objects at 2.5 microns is expected to be orders of magnitude larger than their flux at desired wavelengths. In such a case, exceptionally good out-of-band blocking is required from the filter since out-of-band filter leaks could potentially have a detrimental impact on photometry. We have, therefore, investigated whether any of the NICMOS filters show evidence for out-of-band levels. The results indicate that actual red leaks were insignificant or non-existent.


Chapter 4:  Imaging  4.2 Photometry
Space Telescope Science Institute
http://www.stsci.edu
Voice: (410) 338-1082
help@stsci.edu