MOIRCS Information for Imaging Mode

Optical Distortion and Mosaicking

The distortion of the MOIRCS optics is expressed well by the third-order polynomial as a function of the distance from the optical center (xc=858, yc=1034 for chip 1, xc=1178, yc=1012 for chip 2). The distortion coefficients can change every time when we execute the warm-up / cool-down process for engineering or when we move the internal focus adjustment system. Listed below are the dates of the engineering thermal cycle (since 2006).

15 December, 2005
14 Feb, 2006
24 July, 2006
13 October, 2006
13 January, 2007
09 April, 2007
29 June, 2007
03 Oct, 2007 (Engineering chip)
17 July, 2008 (Scientific chip)
10 Aug, 2009
Early Sep, 2010
Late Dec, 2010
31 Jan, 2011
Late Jan, 2012
Feb~early March, 2013
mid July, 2013
mid Oct, 2014

If you need the geotran database file (for IRAF) for correcting distortion, please contact to the SS.

As MOIRCS is the twin-optics system, the affection of the gravitational flexure differs for each channel. Therefore, the mosaicking rule (position/magnification of channel-2 image relative to channel-1 image) may also vary with the position (elevation and rotator angle) of the instrument when the data were taken. The measured shift of the relative ch1-ch2 position from the canonical value (e.g., the one provided by MCSRED) is typically ~2 pixels at 30 degree elevation. It will also be affected by the differential atomospheric dispersion when the image are taken at low elevation (<30 deg).

Please note that the mosaicking rule may also change by the thermal cycle.

The replace of broken detectors that occured on Oct 2007 and July 2008 has also changed the mosaicking rule significantly.

A Photometric Offset in Partial-Read Mode

Stray Light from Nearby Bright Stars (updated in Feb 2012)

Affection of Moon for Imaging Mode

The affection of th Moon to the background level in near infrared is generally smaller than in optical wavelength. But the sky tend to be brighter than dark night a bit, especially in bluer band. The affection is large when there is a cirrus in the sky.

On bright night we checked how the background level go up with the distance from the Moon. The figure below is the result of our experiment. In the figure, plus marks at >30deg is the reference sky magnitude that are measured far away from the moon. The rise of sky level is almost negligible at >15 degrees, while at 10 degree a clear affection is observed in all bands measured. We also saw some pattern by the scattered light in background at <10 degree.

As the autoguider (AG) is operated in R-band wavelength, the affection of the moon is serious. The AG software will fail the automatic sky estimation roughly at <15 degrees (in some cases, the affection occurs even at 25 degrees away). If the observation uses the autoguider, <30 degree from the moon may be with high risk of AG failure. Of course the affection of the scattered moonlight will be much stronger if the condition of the sky is not clear. Especially the halo from the moon (at ~22deg) will affect the AG background level much.

In conclusion, the affection of the moon on the MOIRCS data may be negligible if your targets are away from the moon by >20 degrees (on clear condition). If you plan to use the AG (for Y, NB, and the spectroscopic observation), the separation must be at least >30 degrees. You should check the separation between the moon and your targets when submitting the proposals, and should explisitly state the dates you want to avoid the assignment on the "Scheduling Requirement" section of the proposal.

Notes on the Autoguider Users

You may require the use of the Autoguider (AG) for more accurate tracking if the single exposure is longer than 3 minutes. The users of Narroubands or Y-band filters will have to take it into account. We strongly recommend the AG users to check the availability of the suitable stars for Autoguider before you finalize the observing coordinates. You can find the more detailed description about the Autoguider Issue below.

Flat Fielding

The dome flat in J band shows a tilt along x direction with a level of ~6%. In H-band a slimilar level of tilt is also suspected. These tilt pattern should be removed from the raw dome data using the sky flat. On the other hand, the sky flat generally contain some level (< a few %) of affection by the fringe pattern caused by filter substrate.

Unfortunately, the dome flat in Ks-band is usually very difficult to take because the thermal-background light (without dome lamp on) from the surface of the dome screen is already too bright and variable. Currently we have never succeeded to take high-quality dome flats in Ks band.

The sky under the narrowband imaging observation is usually not "flat". This is because the central wavelength is the function of the angle of incidence to the filter, or approximately, the distance from the pointing center. The use of dome flats for NB data is recommended, especially the observation is aimed the high-accuracy photometry.

If the sky during an exposure varies rapidly due to cloulds etc, you will see a tilt pattern at each detector quadrant on the image. It is the artificial pattern caused by the CDS readout method. If such pattern appears frequently and strongly on your data, the accuracy of the sky flat by these dataset will become poor, though it depends on the level of tilt patterns on your dataset (low-level pattern is usually seen). We recommend to use the domeflat as well as the sky flat if the sky changes significantly during your observation.

Information about Troubled Data

The periods with any kind of troubled data are summarized in the Spectroscopy Information page.

Data Reduction Packages