MOIRCS Information for Imaging Mode

How To Make an OPE File for Imaging Observation

Manual and sample OPE files (for imaging observation):

• Description of an OPE file: Guide to Observing with MOIRCS [ Word ] [ PDF ] (Last update: 2006-04-11)
  
• Sample OPE file after the chip-1 replacement on Nov 2007: download here. (UNIX or Linux only!! -- Last update: 2008-04-20)
  
  The appearance will become curious under Windows browser -- it looks like there is no line break. It should look fine in unix editor.
  

Optical Distortion

The distortion of the MOIRCS optics is expressed well by the third-order polynomial as a function of the distance from the optcal center (xc=858, yc=1034 for chip 1, xc=1178, yc=1012 for chip 2). The distortion coefficients will change every time when we execute the warm-up / cool-down process for engineering or when we move the internal focus adjustment system. The date of occurence after the open use is as follows.

15 December, 2005
14 Feb, 2006
24 July, 2006
13 October, 2006
13 January, 2007
09 April, 2007
29 June, 2007
03 Oct, 2007 (chip-1 replaced!)

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

A Photometric Offset in Partial-Read Mode

A systematic offset of the photometry between the whole-read and a partial-read data is suspected.

Shown in the table below is the change of counts in the K-band lamp-off dome image with the PRD_SIZE. The data was taken during the stable condition in cloudy night. Exposure time is fixed to 21 seconds with three different (512x512, 1024x1024, 2048x2048) read-out size. Three images were taken for each setting and the last image in each setting were used for statistics. Darks with the same setting were subtracted. For statistics we use the IRAF imstatistics with nclip=3.

Table: The Difference of K band Dome Level

PRD_SIZE	MIDPT (CHIP1)	MIDPT (CHIP2)
2048 (1st)	14501.41	17715.98
1024	14854.86	18209.54
512	14959.48	18353.05
2048 (2nd)	14568.63	17813.49

There is a systematic discrepancy between the mid-point counts by whole read and by partial reads. Though each count shoud be close with each other, partial-readout data always show a higher counts by about 2-3% level. As the darks are already subtracted, the change of dark level cannot be the cause. Also, the change of the counts in whole-read (2048) mode shows only small rise, the change of the dome temperature cannot explain the result. The cause of this ~3% discrepancy is not yet known.

We also checked the result of the photometry between the data taken by the whole-read and th partial-read modes. We took the whole-read data with 21 sec exposure, the 21-sec exposure data with 1024x1024 partial-read, the 6-sec exposure data with 1024x1024 partial-read, the 6-sec exposure data with 512x512 partial-read, and the 3-sec exposure data with 512x512 partial-read mode. Four images were taken for each settings, and the data is reduced by the standard manner (flat-fielded, sky-subtracted, and combined for final iamge). All data is registered to the whole-read image, and a photometery is performed using the IRAF phot. The seeing size was 3pixel with fairly stable condition. The photometry for ~40 stars in the field with enough high S/N is done using the 4xFWHM (12-pixel) apperture. The resulting magnitude of same stars are compared to the result by the whole-read mode.

The figure shows that the photometry by the partial-read mode tends to go brighter (dmag goes to positive) slightly. Note that the level of the systematics seen here may also be easily explained by the natural cause. However, the systematics seen here and the one seen in the dome counts shown above is consistent with each other.

Considering that the systematics seen in the photometry is caused by the same reason as seen in dome count data, we applied a correction (+0.028mag for 1024x1024 data, +0.035mag for 512x512) to the photometric results. Below is the result.

The data after the correction seems to show better matches with each other, especially at the brigher side. At the fainter side it is difficult to say about the improvement due to the photometric scatter. The result implies that applying the correction to the photometric result by the partial-read mode may get a better result, though the correction is rather small.

In conclusion, we recommend to apply the correction shown here to your photometric data taken by the partial-read option. The level of offset by different PRD_SIZE or EXPOSURE will be checked further in the near future.

Stray Light from Nearby Bright Stars

If your observing field is close to very bright (JHK < 1 mag) stars, the data may suffer from a significant contamination by that stars, like the examples below. It is caused by the interferred stellar lights that come through the side of the secondary mirror. As the secondary mirror does not have a blocking buffle to suppress it (or, we cannot put it as it emits strong thermal emission), it is currently very difficult to eliminate it. The light tends to contaminate the field when the bright star is located between 2.5 to 0.5 degrees from the target. As the pattern is localized in the focal plane, it is not always happen even if the star is located in that range.

We strongly recommend to check whether there are such bright stars or not before the observation. If there is such a star within 2.5-0.5 degree from your target, please prepare the backup targets beforehand.

The Website below (VizieR) may be useful for the check.

• VizieR -- "2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)"
  Put the target coordinate, and set 2.5 degree Radius to "Target Dimension" tab. You can add the magnitude constraint to JHK < 2.

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. Clearly, the rise of sky level is negligible until ~15 degrees. At 10 degree a clear affection is observed. We also saw a scattered pattern in background at <10 degree.

As the AutoGuide is operated in R-band wavelength, the affection of the moon is serious. Autoguider failed automatic sky estimation at 15 degrees (in some cases, the affection occurs even at 25 degrees away). If the observation uses the Autoguider, <20 degree from the moon may be with high risk of AG failure.

In conclusion, the affection of the moon may be negligible if your targets are away from the moon by ~20 degrees or more.

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.

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.

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