MOIRCS Instrument Parameters

MOS
Detectors	HAWAII-2 2048x2048 x 2
Pixel size	18 um
Pixel scale⁽¹⁾	0.117''
Field of view	3.94' x 6.90' ⁽²⁾
Gain⁽³⁾	(TBD for Channel 1) 3.24 e^-/ADU @ Channel 2 (Sci #64)
Dark current	< 0.1 e^-/s
Saturation level	146 000 e^- (chip 2) ⁽⁴⁾
Read noise	(TBD for Channel 1) 33 e^- rms @ Channel 2 ⁽⁵⁾
Number of amplifiers per detector	4
Minimum Exposure⁽⁶⁾⁽⁷⁾	11.5 s (NDUMMYREAD=0) 19.7 s (NDUMMYREAD=2)
Filter exchange time	15~60 sec
Number of available mask slots	up to 21 (15 for users during S09A)
Number of slits per mask	30~50⁽⁸⁾
Days needed for cooling mask	> 2 whole days
MOS aquisition time	25 to 40 min.⁽⁹⁾

(1) Pixel scale for each detector has slightly different value. And also, the values in raw image also changes with the distance from the optical center due to the optical distortion.
(2) About 0.4 arcmin² is vignetted in the field of view of 4' x 7' (Please see the FOV page).
(3) New estimate by photon-transfer method in Dec 2007. The values shown here before (3.15 e-/ADU for old chip1 #87, 2.86 e-/ADU for chip 2 #64) may have been affected by the read-level anomaly in partial-read mode.
(4) Newly measured value as of Dec 2007.
(5) Directly measured value using a set of 21-sec dark frames in Dec 2007. The readout noise estimated by the photon-transfer method in Dec 2007 showed a similar values.
(6) These values are for the read of the entire array. We recommend to use about +1.5 sec longer than those values as the "minimum" for actual science data to suppress the reset anomaly (see the Detector Information below).
(7) To achieve shorter exposures than those listed values, a partical read mode is also available (Please see the Exposures page).
(8) Assuming the typical length of slitlets of 10-20 arcseconds.
(9) Please also see the MOS page. The acquisition time for Long slits should be similar.

Detector Information

We replaced the channel-1 detector to the scientific-grade chip on July 2008. The basic characteristics is now under evaluation.

Appearance of the data

There exists one-pixel-width non-data rows/columns between each quadrant. The unreadable regions are [1:1024,1025:1025], [1025:2048,1024:1024], [1024:1024,1:1024], and [1025:1025,1025:2048]. There are no dead pixel islands with the size larger than 1.5 arcseconds on the channel-2 detector, though several scratches or small holes do exist. Examples of raw images can be downloaded from here (CHIP 1 [gif, fits] / CHIP 2 [gif ,fits]).

Linearity

The figures below are the result of the detector linearity measurements executed in Dec 2007 (i.e. the channel-1 detector is old one!). The linearity is maintained within 1% up to 85000 e^- and 110000 e^- for (old) chip 1 and chip 2, respectively. Note that under the CDS readout the full well will vary depending on the position on the chip and the input flux density (above values were measured near the center of detectors: i.e. deepest well is expected). And also, the measurement for chip 1 was done using region that are relatively clean. For high-dark "ring" region on chip 1 the linearity may be poor.
We recommend to keep the sky count under 20000 ADU for science data, because the detector characteristics starts to change.

Figure 11a.

Figure 11b. Figure 11c.

Figure 11: The result of the detector linearity measurements for old chip 1 (left: Figure 11b) and chip 2 (right: Figure 11c). Red plus symbols refer to the residual from the linear fit assuming that the count is propotional to the exposure (usual case). Blue plus symbols are for residual from the linear fit with y-intersection. In any case the linearity is very good. Measurements were done under very stable dome environment during cloudy midnight using the whole read mode with NDUMMYREAD=2. K-continuum filter was used (measured by I. Tanaka in Dec 2007).

Latent Image

A latent image appears at the level of ~0.06-0.6%. The amplitude seems to depend on the background level (under investigation). In imaging mode the latent seems generally weak ( <0.3% ) and in J it is a bit prominent. In spectroscopy mode or during the narrow-band observation the latent sometimes goes up close to 1% due to the low sky level. Though time is the major factor for eliminating the affection of the latents by saturation, taking several short-exposure dark frames may also work.

Low-level signals by latents may occasionally remain even in your fully-reduced data. A special care must be paid about the reality of faint objects that are close to bright stars. If the magnitude or appearance of the object in question largely changes between final-combined images by average and by median, it can be a spurious. Also, the objects in question around a bright star traces the same pattern as the dither, it may be spurious.

Reset Anomaly (Bias Tilt)

Reset anomaly (Bias tilt) is a specific effect seen in some HgCdTe FPAs such as HAWAII and HAWAII-2. It can be reduced by operating an array continuously and sampling by the CDS (Correlated Double Sampling) method. The current channel-2 detector shows only a small level (a few %) of the reset anamaly. Currently there is no data for new channel-1 detector. Taking data with dummy-read option (NDUMMYREAD=2) will suppress the residual reset anomaly much.

The instrumental minimum exposure times are listed in the table at the top of the page. In the minimum exposure data we see relatively strong reset anomaly. This can be suppressed when we set a bit (+1 to +1.5sec) longer exposure times than the true minimum. So we tend to use the minimum exposure of 13.0 sec for NDUMMYREAD=0 and 20.5 sec for NDUMMYREAD=2 for full readout case.

Multi-Sampling Data Acquisition

Mutil-sampling readout is the effective way to reduce the detector readout noise. Usually 8 multi-sampling data acquisition is applied during the spectroscopic observation. For usual imaging observation it is not necessary because the background noise dominates the data. The overhead by multisampling readout is 8.5 sec times the number of sampling, so too many multisampling is not much effecient. Note that the data is stored after averaging on the memory.

Engineering detector used for channel 1 during October 2007 to June 2008

Due to a technical problem in the detector on channel-1, we operated MOIRCS with an Engineering-grade chip during October 2007 to June 2008. The channel-1 data taken during the period should be handled with enough caution for scientific use. See the old information webpage for more detail.

Please note that all data on these pages are subject to change as the evaluation of the performance of MOIRCS progresses.

Created by Yuka Katsuno Uchimoto (Feb 2006)
Revised by Ichi Tanaka (2008-07-30).