Resource pages for NIRC2

The purpose of this page is to provide an easy and interactive way for observers to share information on using the NIRC2 instrument, reducing and analyzing the data.

• Before reading anything in the current web page, you should be very familiar with the NIRC2 web page at Keck Observatory, where the essential and valuable information for NIRC2 is posted.
• The purpose of the present page is to complement the information available at Keck for the benefit of the Users and Keck community.

• NIRC2 (K. Matthews et al. 200x, in prep.) is installed behind the Keck II AO system. It is used in either NGS or LGS AO mode. Some quick facts for this instrument: it was built by Caltech and UCLA (PI: Keith Matthews). It has three cameras: narrow (~10 mas/pixel), medium (~20 mas/pixel) and wide (~40mas/pixel). The detector is a 1024x1024 Aladdin-3 array with 27 µm pixels. It is mostly used with its narrow and wide camera for J to K band imaging and spectroscopy. It is also used in the 3 to 5 micron window. Due to high background emission from the telescope and AO in the 3 to 5 micron range, observers can only use the narrow camera for that range.

Astrometry

NIRC2 astrometry has been documented in various occasions. Below is a table that summarizes some of the published work:

March 26, NIRC2 distortion meeting in Santa Cruz

We will have a meeting in ISB 221 from 11-am to 12noon on Monday March 26. Feel free to post any relevant material or link here.

Plate scale and orientation calibrations

From the preship review and commissioning data

The K2 AO rotator was "bumped" around June 2002. It was re-aligned, yet the telescope pupil on NIRC2 was consequently slightly misaligned (~30cm on the primary). This may have resulted in a slight change of plate scale and orientation as well from the commissioning time. The K mirrors were re-coated in 2004(?) and the rotator re-aligned as well. Here again, the plate scale and orientation may have shifted a little.

From Stan Metchev:

• See for now Stan's AO page, that includes NIRC2 calibrations for linearity, plate scale and orientation, geometric distortion, and coronagraph extinction.

From B. Cameron

From the work on the Galactic Center (Jessica Lu et al., 2007)

• From 2005 observations in the narrow camera: 9.961 (+/- 0.007) mas/pixel and PA of -0.015 (+/- 0.134) deg (cited in Konopacky et al., 2007)

Distortion map

From the preship review and commissioning data

• The distortion for the narrow, medium and wide camera has been documented in the 2001 pre-ship review.
• A. Bouchez used the pre-ship review information to create an IDL routine that corrects for the NIRC2 distortion: nirc2warp.pro. Note that there was evidence that the residuals were still very important in the wide camera (> 1 pixel), hence results for stitching large crowded observed areas together where not very satisfactory (A. Bouchez and J. Lu, priv. comm) .

From Stan Metchev:

• See for now Stan's AO page, that includes NIRC2 calibrations for linearity, plate scale and orientation, geometric distortion, and coronagraph extinction.

From the work on the Galactic Center (Jessica Lu et al., 2007)

From the work on pin-hole mask (B. Cameron et al, 2007)

• A new distortion solution for NIRC2 wide and narrow cameras with residuals < 0.1 pixels (in both cameras). This is work in progress, use with caution. distortion.pdf

Observing tricks and tips

Persistence effect

• Randy's paper on persistence effect

Dealing with background for 3 to 5 micron imaging

In the L and M band, the background comes mostly from the AO bench and the telescope. One of the main contributor to the background is the (de) rotator (a K mirror) installed at the entrance of the AO bench. The coating for the K mirror have been redone in 2003 and it is cleaned periodically.

During the observations on the science target, the K mirror rotates (to keep the science field at the Nasmith focus, fixed on the science array) and introduces a given background emission pattern. This emission pattern is quite non-uniform (spatial scale of the order of ~arcsec) and dominates any other emission in L and M band. It is important to choose an observing strategy that will allow to observe a different part of the sky with a similar rotator position.

The UCLA Galactic Center who is sharing its experience with others mentioned that it is important to reproduce the rotator position by ~5 degree or less. They also report that they were able to obtain good subtraction even by observing the background more than 30 minutes later (which is quite surprising for L and M band!), provided that one uses adequate dithering scripts for the science field (see example below).

One does not need to spend the same amount of time on the background (nor use the same dithering script) than spent on the science targets. Yet, make sure the integration time/coadd setup is reproduced and there is a significant number of frames (?) per setup.

The rotator physical position can be tracked by the rotator drive value displayed on the telescope FACSUM. Take note during the observations, or retrieve the value from the fits keyword header: OBRT (the AO rotator physical location, repported by the AO bench software) or ROTPPOSN (the physical position reported by the telescope control system and AO supervisory control). One way to reproduce that value is to offset the telescope in RA while keeping the elevation value from the observations.

Assuming a linearity of ~8000 adus/pixel, assuming average temperature conditions and sky transparency, one can anticipate the following setup:

Watch the overhead!: NIRC2 readout for each coadd is 0.18sec for 1024x1024 and 0.05 sec/coadd for 512x512. Any integration time below 0.18sec requires to sub-array the detector to 512x512.

A typical sequence on a crowded field or an extended object would be: bxy5 0.3 0.25 n=2 (a 5-position dither - or more- with offset of dra=0.3" and ddec=0.25" with 2 frames of n coadds per dither position).

For a single star observations (e.g, point source photometry), one can use bxy3 2.5 2.5 n=2, and does not need to observe a dedicated background later on.

Data Reduction and Analysis Tools

• See the Image Quality Tools page and find out what was developed to measure FWHM, Strehl, Encircled Energy, etc in NIRC2 images.

-- Updated - 21 Mar 2007