MOIRCS Detector Information

!! All Information below are for the previous detectors (for data before June 2015) !!

Appearance of the data

There exists one-pixel-width non-data rows/columns between quadrants for both detector. The unreadable regions are [1:1024,1025:1025], [1025:2048,1024:1024], [1024:1024,1:1024], and [1025:1025,1025:2048]. The detector for channel-1 has a large (~100-pixel radius), circular dead pixel island, as well as a few bad-data lines and some bad (high-dark) regions. There are no dead pixel islands with the size larger than 1.5 arcseconds on the detector for channel-2, though several scratches or small holes do exist. Channel-2 detector has a picture-frame-like high-dark region around the edge. The linearity is worse for the region due to the complex behavior of the dark current. Examples of raw images can be downloaded from here (CHIP 1 [gif, fits] / CHIP 2 [gif ,fits] / preimage format [fits]).

The detector for channel 1, which we installed in July 2008, has on average 27% and 5% higher sensitivity in Y and J bands than channel-2 detector, respectively. In H band both channel is almost similar. In Ks-band the channel-2 detector is on average ~5% higher than channel-1 detector. The upper-half of the channel-2 detector has ~20% higher sensitivity than lower half (see the example of flatfield data below as a reference of the sensitivity map).

Though our Hawaii-2 detectors have good linearity, any NIR detector is not perfect. Suzuki et al.(2008) reported that the non-linearity of our system is less than 3 % up to ~30000 ADU level. However it is not straightforward to compare their value with the actual science data taken by the Correlated Double-Sampling (CDS) Method, as they took the data using the Single Readout Method. Motivated by some discussions with an open-use observer^(*1), we have re-measured the non-linearity level again in 2012 following the method described by Vacca et al. (2004, PASP, 116, 352).

Our new measurement has shown that the detector readout is linear by 1%, 2% and 3% level at the signal level of each 18300 ADU, 26700 ADU, and 33100 ADU, respectively (based on the dome data with NB119 filter). Again, the values are NOT for the detector readout by CDS method but for the Single Readout method: namely the values are for the charge accumulated on the pixels after reset, and so it include the "pedestal level" which is subtracted when the CDS data is made. We then calculated the relation between the non-linearity level and the detector counts under the CDS readout. The table below is the guideline of the exposure for these non-linearity level for various input flux level.

The above images are the result of the detector linearity measurements for the chip 1 (red) and the chip 2 (blue). The lower panel shows the level of the non-linearity with the signal accumulated on the detector after the reset (i.e. not the CDS readout counts). We assume that at low counts (< 10000 ADU) the response of the detector is linear and the non-linearity become more evident at higher counts (see Vacca et al. 2004). We note that the measurement is based on the dome data with the NB119 filter. We saw slight wavelength dependence. At redder filter (> 1.5um) the linearity seems to become better than the NB119 data by 20-30%. We will look into the possible wavelength dependence in the future.

The relation between the measured detector signal (the count after reset: not the detector readout value in the CDS), x, and the corresponding signal by the perfect linear detector, s, is fitted as below.

l = x [1 + 0.00370292 (x/32767) + 0.0255509 (x/32767)²]
(for NB119 data taken in July 2012)

We can correct the non-linearity fairly reliably up to ~4% level. The software for the non-linearity correction is now being tested. If you want to use the software, please contact the SA. Note that it is the shared-risk distribution.

The table below is the guideline for the maximum exposure for 1%, 2%, and 3% non-linearity level (NB119 case) for the NDUMMYREAD=2 CDS readout case. In the table, "Maximum Exposure (least)" indicates the non-linearity at the line that are read out in the end (i.e. the line that have shallowest well depth). On the contrary, "Maximum Exposure (most) [sec]" is for the line that are read out at first after the detector reset (i.e. the line that have deepest well depth). In other words, the full well of the pixel for CDS readout depends on the position on the detector: the pixels that read out first after the detector reset has the deepest well and the last-readout pixel has shallowest well.

Our new analysis shows that at 22600 ADU (in CDS) the pixels in the last scan line (=shallowest well depth) starts saturation, and at 31500 ADU the first scan line reaches saturation; namely, all pixels are virtually saturated by the exposure indicated. Keeping the peak counts of the targets of your interest always less than 20000 ADU (or <15000 ADU for 1% level) is important for keeping the affection of the non-linearity negligible.

(*1) We appreciate Dr. Ian Crossfield for his great contribution on the MOIRCS non-linearity analysis.

Latent Image

The "latent" or "residual image" is the common problematic characteristics in HAWAII and (pre-RG) HAWAII-2. It will appear on the position where the strong light is illuminated. For example, the latent from very bright stars will put its "footprints" on the data during the dithered imaging observation. For spectroscopic mode, the latent from the slit images, bright stars during the alignments, or sometimes the spectra from the alignment hole from the previous observation may appear after long exposure. Enough care should be paid for the possibility of the spurious objects by latents on the reduced image.

The typical amplitude of the latent in the next image is about 0.1% (under near-saturated case). The e-folding time this case is roughly 200 sec (or 4-6 shortest exposures). If fully saturated, the latent could be as strong as 0.5% level. If the saturation is really severe, the affection may remain for a few hours.

Here is a report of the 2015 characterization summary report about latent image (in Japanese only).

Reset Anomaly (Bias Tilt)

Reset anomaly (Bias tilt) is also another problematic character seen in some HgCdTe FPAs such as HAWAII and the old 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 usually use the minimum exposure of 13.0 sec for NDUMMYREAD=0 and 20.5 sec for NDUMMYREAD=2 for full readout case.

MOIRCS bad pixel map (2011-04-16: md5)

Miscellaneous:

1. Wrong Informations on the Fits Header Keywords

Some fits header keywords in the data taken during Aug 2008 to early Apr 2010 were wrong.

The keyword "DETECTOR" for channel-1 data was "HAWAII-2 040 ENG", while it should be "HAWAII-2 096 SCI". The values "GAIN" were 2.95 and 3.24 for channel 1 and 2, respectively. But they should be 3.5 and 3.3. Note that the true value of gain have some uncertainly due to the uncertainty in measurement.

The "WEATHER" and "SEEING" keywords are always unreliable. Do not believe them.

It has been known that some fits header informations (e.g. OBSERVER, RA, DEC, OBS_MOD and so on) occasionally become wrong due to the communication error. The frequency of the occurence is 1-10 frames per 2500 exposures during 2009. It will not occur when the GEN2 new summit operation system is to be used.





2. 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.





3. (Obsolete) The evaluation page for new detector for channel-1 (#96) during late 2008

The page is kept just for the record. Please visit the link to the page.

Updated 2014-10-13