The properties of NIRI in imaging mode are summarized in the following sections. Three selectable fields of view and pixel scales are available. With adaptive optics NIRI is available at f/32 for J-L' imaging (but see caveats for L'), and at f/14 for imaging out to 2.5 microns due to the increased thermal background. Imaging at L' is impossible at f/6 and only sometimes possible at f/14. The narrow band L filters (e.g., the PAH filter, Br alpha) can be observed at f/32, f/14, and f/6. M' imaging is only possible at f/32.
NIRI occasionally suffers from vertical striping and shifts in DC levels which can both vary from quadrant to quadrant. Details can be found here.
First Frame Issue: When starting exposures after changing the detector configuration (well-depth, read mode, exposure time, etc) the background or dark current level is different. Thus The first exposure of every new sequence will show poor background subtraction and should be rejected (see the detector array section for more detail). We recommend sequences include an extra step (either at the beginning, or repeat the first position at the end, or simply include N+1 different offsets). Short exposures that are not background limited (e.g., standard star measurements) are usually fine without an extra exposure. These bad first images ARE included in the distributed data, so care should be taken to review and reject them from all sequences (science, flats and darks) as necessary.
- Pixel scales and Field of View
- Field Orientation
- Imaging filters
- Exposure times
- Zero points
- Observing Strategies
- Detector array
Pixel Scales1 and Field of View2
|Camera||Pixel dimension (arc sec)||Field of View (arc sec)|
|f/6||0.1171||119.9 x 119.9|
|f/14||0.0499||51.1 x 51.1|
|f/32||0.0219||22.4 x 22.4|
|f/32 + Altair Field Lens||0.0214||21.9 x 21.9|
1NIRI with Altair has been measured to have radial barrel distortion at f/32. This is most prominent for programs performing large offsets. The formula for removing the distortion is r' = r + k*r*r, where k = (1.32 ± 0.02) * 10^-5 . r is the uncorrected distance from the field center in pixels and r' is the corrected distance from the center in pixels. Thus, at a distance of 500 pixels from center, an object will appear about k*500*500 = 3 pixels closer to the center of the array than reality. This correction is available in an IDL program, and PIs interested in high-precision astrometry are encouraged to contact either the NIRI or Altair instrument scientists to discuss this issue. Note: This solution is valid for data taken with f/32 before August 2018. A new solution is being developed after the f/32 mirror shift.
2For full (1024 x 1024) array
The NIRI field orientation is determined by which telescope port (bottom or side) NIRI is installed on, and if on the bottom port, whether or not Altair is used. For most cases, at a PA = 0, North is up and East is to the left, however, when installed on the bottom port and used without Altair, East is to the right:
|Side (3 or 5)|
A zero point is defined as the magnitude of an object that would yield 1 ADU/sec at an airmass of 1. They depend on instrument and telescope transmittances as well as detector electronics. Zero points tend to be stable at the 3% level over many months. Measured values need to be corrected for variation of atmospheric extinction with airmass. The following table contains zero points for NIRI with its f/6 camera and broad band filters. Extinction values are taken from Tokunaga, Simons & Vacca (2002 PASP 114, 180) for 2mm of precipitable water vapor. See their Table 2 for extinction values for other amounts of precipitable water.
NIRI Zero points
For more detailed information on the NIRI zero points please see the Instrument Performance Monitoring project web page.
|Zero point magnitude
(for 1 ADU/s)
|J||1.25||23.97||15 to 16||0.015|
|H||1.64||24.04||14 to 14.5||0.015|
|Kprime||2.12||23.68||13.7 to 14.2||0.059|
|Kshort||2.15||23.40||13.6 to 14.1||0.043|
|K||2.20||23.43||13.5 to 14.0||0.033|