NICMOS utilizes three grisms in Camera 3 to achieve multiple object spectroscopy in the spectral range between 0.8 and 2.5 microns. The spatial resolution of the spectroscopy is similar to the spatial resolution of Camera 3. The NICMOS grisms do not have an input slit or aperture. A grism creates a dispersed spectrum centered on the location of the object in the camera field of view.
It is necessary to obtain an image of the field before each grism observation is taken. This will make it much easier to identify the target of interest in the grism images. Dithering is highly recommended to optimize the flux calibration. Ideally, the dithering should have non-integer pixel spacing.
For the following example, a spectrum is obtained in each quadrant of the detector, and an image of the field is obtained at each dither position before the spectrum is obtained. Executing a pattern twice, once for images and a second time for the grism observations, is not recommended. In addition, a pattern orient of 0.11 deg is specified to achieve a 1/4 shift over 128.5 pixels. The NIC3 pixel scale is ~0.203"/pixel.
Note: For emission line point sources, one should dither in both directions, along the dispersion and cross-dispersion directions, to improve the line flux and wavelength measurement.
Pattern_Number: 1 Primary_Pattern: Pattern_Type: NIC-SPIRAL-DITH Pattern_Purpose: DITHER Number_Of_Points: 4 Point_Spacing: 26.09 Coordinate_Frame: POS-TARG Pattern_Orient: 0.11 Center_Pattern: NO
Exposure_Number: 10 Target_Name: NGC6681 Config: NIC3 Opmode: MULTIACCUM Aperture: NIC3-FIX Sp_Element: F175W Wavelength: 1.75 Optional_Parameters: SAMP-SEQ=SCAMRR,NSAMP=9 Number_of_Iterations: 2 Time_Per_Exposure: DEF Special_Requirements: POS TARG -12.99,-12.99 PATTERN 1 10-20 Comments: exp=1.827 sec
Exposure_Number: 20 Target_Name: NGC6681 Config: NIC3 Opmode: MULTIACCUM Aperture: NIC3-FIX Sp_Element: G206 Wavelength: 2.06 Optional_Parameters: SAMP-SEQ=STEP8,NSAMP=8 Number_of_Iterations: 2 Time_Per_Exposure: DEF Special_Requirements: POS TARG -12.99,-12.99 Comments: exp=23.969 sec