LRIS Red III: 2010 2x(2kx4k) LBNL mosaic and dewar
In comparison to LRIS Red I, these CCDs provide a
larger detector area, smaller
pixels (15 μm versus 24 μm in the Tektronix/SITE
2k×2k CCD), and a flatter focal plane. Because the area
per pixel is now 40% of what it used to be, observers are encouraged
to bin pixels during CCD readout, and both row and column
binning are supported.
These CCDs have
some traps, cosmetic defects, and other anomalies. These foibles
are described in
the accompanying section on cosmetics.
Quantum Efficiency
The quantum efficiency curves as measured in the lab at Lick are
presented below. As a result of operating the CCDs at a warmer
temperature than LRIS Red II (-100 vs -140 C), the QE is
slightly better in the far red. The QE curves below are
taken from the "LRIS-R Test Report:LBNL High Resistivity CCDs
19-2 and 19-3" supplied to Keck on 17 Aug 2010. The QE for 19-2
was not measured at the current operating temperature, and there
is a slight improvement in the QE at the longest wavelengths for
an operating temperature of -100C. The QE for 19-3 was measured
at the operating temperature.
QE for CCDs 19-2 (left) and 19-3 (right).
Gain & Readnoise
Under normal operations, observers
will use "normal" CCD speed with the gain set to "high" while
binning 1x1, 1x2, 2x1, or 2x2.
For graphs and files providing latest gain and readnoise
measurements see the gain and readnoise documentation.
Readout Times
The new red mosaic offers three readout speeds but only one is
used for observing.
- Normal. The default observing mode, it provides a compromise
between speed and readout noise. At "normal" readout speed, the
new mosaic takes longer to read out than LRIS Red I and II.
- Fast. Uncharacterized. But it does work and is available for use.
- Slow. Uncharacterized.
The following table gives the minimum
times required to acquire a full-frame LRIS exposure.
LRIS-R Mosaic Readout Times
| CCDSPEED |
Binning |
Readout Time (s)* |
| Normal |
1x1 |
127 |
| Normal |
1x2 (spectral) |
78 |
| Normal |
2x1 (spatial) |
83 |
| Normal |
2x2 |
58 |
*Elapsed time for 0 sec exposure, including erase, expose,
and readout phases.
Linearity
In contrast to LRIS Red I, the onset of
non-linearity occurs before the saturation of the ADC.
Observers should strive to keep the peak signal
level below 38,000 DN to ensure that they are operating in
the linear regime. Below is a plot that shows the linearity of the
four amplifiers.
Linearity fo rthe four readout regions.
The linearity was measured using dome flats. The dome flat
exposure times were 3 to 24 seconds.
The image below shows the layout of the four LRIS Red III video
amplifiers as the image is displayed in DS9 when
observing. The observational setup for DS9 rotates the images
by 270 degrees to place the bluest wavelengths on the left of
the image and the reddest on the right. The location of the
longslit spectrum (yellow line) lands on vid 4 when the object
is placed at the standard "slitb" pointing origin.
LRIS Red III amplifier layout
The number of cosmetic defects on CCDs 19-2 and 19-3 are
comparable to the to those of the previous detector. Instead of
the "tape effect" and low level traps, the new CCDs have hot
defects, traps, and glue voids to contend with. Below is an
image of a flat with overlays of the cosmetic defects. The
image is color coded:
- yellow lines - traps and hot pixels with tails
- red lines - column channel stop failures
- green plus - Clusters of hot pixels or traps
- Shaded green - Glue voids
Map of Cosmetic Defects
Hot Pixels and Traps
Below is a table that describes the locations of hot pixels and
traps. In some cases, the hot pixels and traps have a long tail
associated with it.
| CCD | DS9 Pane Coordinates | Notes |
|---|
| X pix | Y pix |
|---|
| 19-2 | 3906 | 3627 | Hot pixel |
| 19-2 | 3406 | 2878 | Trap cluster |
| 19-2 | 3375 | 2850 | Trap |
| 19-2 | 3796 | 3545 | Trap with tail |
| 19-3 | 1314 | 526 | Hot pixel with tail |
| 19-3 | 1188 | 1706 | Hot pixel with tail |
| 19-3 | 1065 | 3471 | Hot pixel with tail |
Location hot pixels and traps.
Magnified view of a hot pixel with a tail.
Column channel stop failures
Columns in the CCD are defined by potential barriers that keep charge in one column from combining with neighbors, and the barriers are usually referred to as column stops. On CCD 19-3, there are three places where the column stops have appeared to break down, and charge has spilled into the neighboring columns. Below is a table which lists where the channel stop failures are located. These channel stops are mapped on the above image in red. Below the table, a figure shows the average row plots for the channel stop failure columns.
| CCD | DS9 Pane Coordinates |
|---|
| X pix | Y pix |
|---|
| 19-3 | 1979-1980 | 0-4096 |
| 19-3 | 1395-1396 | 0-4096 |
| 19-3 | 613-615 | 0-4096 |
Location of failed channel stops.
An example average row plot for a failed channel stop.
Glue Voids
The figures below show the magnified views of the "glue voids" that are present on both halves of the mosaic. At most wavelengths, observers will only detect the edges of the glue void which manifest themselves as a dark interior band a pixel or two wide and a lighter band exterior to the dark band. We suspect that the banding arises from a redistribution of charge, with some of the charge moving from the dark pixels into the lighter pixels.
So far, flat-fielding appears to correct for the glue voids at all wavelengths. However, because the edge of the void redistributes charge, objects or spectral features spanning the edge may be distorted. special care should be taken when measuring position or extracting spectra at the edges of the glue void.
Glue void for 19-2
Glue void for 19-3
At wavelengths longer than 900nm the CCD is becoming optically thin so that some light passes through the detector. Because the void is an air pocket, the response relative to the rest of the CCD is diminished. Below is a plot that characterizes the response relative to the rest of the CCD.
Relative response of the glue void relative to the rest of the detector.
Radiation events
The new CCDs are good cosmic ray event detectors, which is a nice way of saying that
observers can expect to see the same rate of cosmic ray events in
their images compared to LRIS Red II (higher than the detection rate for LRIS Red I).
The rate of cosmic ray and other
radiation events is such that in a 1200 second dark exposure,
between 0.7% and 1% of the pixels are contaminated at a level more
than 5σ above the background bias. The expected muon rate,
taken from work done by the LBL group, Stover and others (e.g., Smith et
al. 2002, Proc. SPIE, 4669, 172) in the process of
characterizing these fully-depleted, thick CCDs, is about 390
events per quadrant of the mosaic. That suggests an average size
for the trails of 100 pixels. The actual distribution of event
sizes in pixels is extremely broad, but 100 is reasonably close to
the mean. The majority of the tracks are straight, indicating that
the shielding is doing its job of keeping out local low-energy
electrons. An example of part of a frame is seen below.
Cosmic Rays in an LRIS-R Dark Image for LRIS Red II
The minimum FWHM at best focus for imaging and longslit mode is
comparable to that seen on the blue side. The 100 µm
gradient seen in the focus in the spatial direction in the LRIS Red I
is considerably less steep. A direct comparison will require
more work to distinguish changes to the focus software from
changes to the data. We will also analyze longslit focus data to
check the image quality in the new detector real estate. The
bottom line from the analysis so far is that image quality is, at
worst, unchanged.
Comparison to LRIS Red I
Steve Allen of UCO/Lick has kindly produced the following diagram
comparing the area of the new mosaic (shaded region) to the
previous red-side CCD (white outline). This is unchanged for both
upgrades.
LRIS-R Science Mosaic Geometry
Astrometry
Gap
The open cluster m67 was observed, and we measured the astrometry
on the two sides of the mosaic. CCD 19-3 is slightly tilted
relative to 19-2. The gap,
specifically the distance between the first column of the two
red CCDs, is at pixel Y=0 is 27.30 arcsec and at Y=4096
is 30.25 arcsec.
On 16 Dec 2010, observations were acquired with LRIS Red III during engineering time. These observations included imaging with the V & I filters and slitless spectroscopy of spectroscopic standard stars. In the two sections below, we describe the reduction and measured performance in both imaging and spectroscopic modes.
The photometric standards in m67 (Yadav, et al. 2008, A&A, 484, 609) were used to measure the zero points for the V and I filters. The cluster was observed at the LRIS pointing origin with central coordinates of 08:51:24.0 +11:48:28.0 J2000 with a PA of 90 degrees. 20s exposures were acquired, and images were divided by a flat field. Standard IRAF routines were used to identify stars, measure the flux from the source, and subtract a background. In the table below, the number of stars identified and used to calculate the average zero point is presented in the first column, followed by the mean, median, and 1 sigma uncertainty. Observations were obtained at an airmass of 1.1. At V and I bands, the measured zero points are ~0.4 magnitudes fainter than those measures with the LRIS Red I.
| Filter | N-stars | ZP-mean | ZP-median | 1 sigma |
|---|
| I | 16 | 27.80 | 27.81 | 0.09 |
| V | 40 | 27.86 | 27.86 | 0.02 |
Measured Zero Points
Spectroscopic Throughput
Slitless spectroscopic observations were acquired of Feige 110 and G191B2B standard stars. Feige 110 was observed with the 831/8200 line grating using the 1.0 arcsec longslit and in a slitless mode. For G191B2B, spectra were acquired in slitless mode using the 400/8500, 600/10000, and 831/8200. Arcs and flats for these observations were acquired during a morning when LRIS was not in the telescope. Brad Holden and Jason Prochaska reduced and analyzed the spectroscopic data to measure the throughputs. Please see Spectroscopic Throughputs for results on throughput.
Unlike the blue CCD mosaic, the red CCD mosaic may be set to read
out any desired rectangular region. Windowing is now accomplished
using the PANE keyword. Observers can modify the PANE keyword
directly either by updating the keyword via the command line or
using the window selection on the Xpose
GUI. Windowing the CCD in the spatial direction will not save
readout time. Windowing the CCD in the spectral direction will
save time, but will reduce wavelength coverage and thus is not
typically something the observer wants to do.
On the CMD... menu available on the red Xpose GUI, there are options for windowing
the CCD:
- Window full frame is used to readout the entire
detector.
- Window longslit is used to read out only the long
slit region. As noted above, this is not any faster than
fullframe readouts but will save disk space.
Observers should bin the detector in the spatial and/or spectral
direction for two reasons:
- First, to be sky noise dominated in the same amount of time
at 7000 Å as the LRIS Red I, it is necessary to
bin by two in either axis.
- Second, binning decreases the readout time (see readout times).
Four commonly used binning modes are available on the
CMD... menu available from the Xpose GUI. These modes are 1x1, 1x2, 2x1,
and 2x2. Selecting these modes does not change how the CCD is
windowed. Note that these scripts will set the CCD readout speed
to normal.
If binning is not set to one of these standard modes, then the
amplifier offsets and pixel delay clock will need to be re-tuned
to minimize pattern noise for the desired configuration. This is
a time consuming process and should be avoided until we have an
opportunity to calibrate additional readout modes.
Observers should never modify binning using the
WIN... option on the Xpose
GUI, because doing so will not adjust the pixel delay clock
and hence pattern noise will occur in the images. Instead,
binning should only be changed using the options available on the
CMD... menu.
With LRIS Red I, the FITS files written for both the red and blue sides
were written as simple FITS format with all data assembled into a
single 2-D array. To comply with accepted standards for storage
of CCD mosaic image data, LRIS red and blue images are now storage
as multi-HDU (header data unit) FITS files. As with HIRES and
DEIMOS, data from each CCD amplifier are stored in a separate
image extension. Each extension includes both data and a
header describing the content of that extension. Furthermore, a
primary HDU contains information common to all extensions (e.g.,
LRIS mechanism keywords).
The simplest way to display LRIS multi-HDU images is to read them
into a FITS viewer which understands how to assemble mosaic data
into an image. The ds9 image display
tool is one such program. To view an LRIS image in ds9, simply
open the image using the File > Open Other > Open
Mosaic IRAF.. function. Note that you must set the ds9 flip
and rotation parameters as follows in order to achieve the
customary view which puts the spectral direction increasing to the
right: [TBD]
[TBD: note old vd. new ds9 difference]
IRAF users may use the lrisdisplay task in the keck.lris to view images on the IRAF
display tool.
IDL users can use the readmhdufits routine to read the
image into a 2-D array, then display it using the atv image
display tool.
WMKO has made available two software tools which will generate a
monolithic single-HDU image (i.e., one that combines the pixels
from all amplifier regions into a single-HDU FITS file) from a
multi-HDU LRIS mosaic image:
- readmhdufits is an IDL
routine which reads in the multi-HDU LRIS images and assembles
them into a single 2-D array, with optional bias subtraction
based on the overscan region.
- multi2simple is a task distributed as part of
the keck.lris package to read in
multi=HDU images and output a simple FITS image.
- splitmos is a utility that will convert a
multi-HDU FITS file into a set of a single-HDU format files,
with one file per readout amplifier region. This program is
installed on the LRIS host computer.
- afternoon calibrations
- science acquisition
