Spectroscopy Planning for NIRC-2

Spectroscopy Planning for NIRC-2

 

Planning for NIRC-2 spectroscopy can be cumbersome. Typically observers will want to know:

The answers to some of these questions are easy; if you want an H-band spectrum you should use the H filter. In the K band there are two filter selections: K will give a broader passband that extends to longer wavelengths, while Kp extends to shorter wavelengths than K.

The grism/camera combination provides an interplay that also involves the slit width and spectral resolving power. The resolution will depend on the smaller of the slit width and the PSF.

If the resolving power you require is large enough that the spectrum will not fit entirely on the detector, you will either need to select a subset of the passband to observe, or observe at multiple slit positions in order to cover the entire passband. In the worst case (medres grism, narrow camera, H band) you can cover the entire passband in two exposures. However, each time you observe at a different slit position you will incur the overhead of recentering the target at the new slit position.

Wavelength calibration of spectra

The table below lists various useful parameters for NIRC-2 spectroscopy. The columns are:

  1. Camera
  2. Filter
  3. Order
  4. Central wavelength of the NIRC-2 filter (microns)
  5. Dispersion (Angstroms/pixel; remember that 1 Angstrom is 0.0001 microns)
  6. Wavelength range of the filter (microns)
  7. The number of pixels neede to cover the entire wavelength range. Bold red letters indicate that the entire filter passband will fit on the detector.
  8. The resolving power per pixel. If you use a three-pixel wide slit, e.g., your resolving power would be 1/3 that listed.
  9. The value of SLITMM required to center the bandpass.
  10. The column at which the slit should appear for the given SLITMM.

LOWRES GRISM

Camera
Band
Order
Central
wavelength
(microns)
Dispersion
(Ang.)
Wavelength
range of
filter
(microns)
# pixels
to cover
range
Resolving
power
per pixel
 SLITMM
to center		 SLITCOL
to center
Narrow
J
6
1.248
1.092
0.163
1490
11,430
    
J
5
1.248
1.308
0.163
1250
9,540
    
H
4
1.633
1.628
0.300
1840
10,030
    
K
3
2.196
2.144
0.336
1570
10,240
    
Kp
3
2.124
2.144
0.336
1640
9,910
    
Medium
J
6
1.248
2.190
0.163
740
5,700
    
J
5
1.248
2.616
0.163
620
4,770
    
H
4
1.633
3.255
0.300
920
5,020
    
K
3
2.196
4.113
0.336
820
5,340
    
Kp
3
2.124
4.113
0.336
850
5,160
    
Wide
J
6
1.248
4.373
0.163
370
2,850
    
J
5
1.248
5.236
0.163
310
2,380
    
H
4
1.633
6.519
0.300
460
2,500
    
K
3
2.196
8.226
0.336
410
2,670
    
Kp
3
2.124
8.226
0.336
430
2,580
    

MEDRES GRISM

Camera
Band
Order
Central
wavelength
(microns)
Dispersion
(Ang.)
Wavelength
range of
filter
(microns)
# pixels
to cover
range
Resolving
power
per pixel
 SLITMM
to center		 SLITCOL
to center
Narrow
J
6
1.248
0.644
0.163
2530
19,380
    
J
5
1.248
0.704
0.163
2320
17,730
    
H
4
1.633
0.929
0.300
3230
17,580
    
K
3
2.196
1.267
0.336
2650
17,330
    
Kp
3
2.124
1.267
0.336
2770
16,760
    
Medium
J
6
1.248
1.288
0.163
1270
9,690
    
J
5
1.248
1.407
0.163
1160
8,870
    
H
4
1.633
1.858
0.300
1610
8,790
    
K
3
2.196
2.534
0.336
1330
8,670
    
Kp
3
2.124
2.534
0.336
1390
8,380
    
Wide
J
6
1.248
2.576
0.163
630
4,840
    
J
5
1.248
2.797
0.163
580
4,460
    
H
4
1.633
3.420
0.300
880
4,770
    
K
3
2.196
5.078
0.336
660
4,320
    
Kp
3
2.124
5.078
0.336
690
4,180
    

 

Multiple Passbands in One Setting

In some situations it is possible to select a value of SLITMM that will allow you to get whole spectra in multiple filters. In general this is only possible for J(5), H, and K or Kp filters. [J(5) represents the fifth order J spectrum.] In particular: