WIRO DoublePrime (WIRO")
A wide-field prime focus imager for WIRO
Overview and sample data
WDP has a field of view of about 39' at a scale of
0.58"/pix. The ccd is a custom 4096x4096 UV-sensitive
array from the laboraotories of Mike Lesser. It was
integrated into a dewar by Astronomical Research Cameras
(Bob Leach) and uses ARC Generation 3 electronics. It was
received in the fall of 2014 at UW and commissioned through
October 2015. WDP was funded by a NASA EPSCOR grant to
Profs. Chip Kobulnicky and Adam Myers. It is used behind a
custom 4-element prime focus corrector designed by Prof.
Chip Kobulnicky and built and installed in 2006(?) by Prof.
Mike Pierce. The WDP filter wheel was designed and built in
the UW Arts and Sciences shops under the supervision of WIRO
engineer James Weger and Prof. Chip Kobulnicky. It was
commissioned in the summer of 2014 and has slots for five
3.5" filters. Smaller filters can be accomodated with insets
but the FOV is reduced. See the filter list.
When used in normal 4-amplifier readout mode the readout
time of WDP is 21 seconds. Normal operating array temperature is
-110 C or -100 C. The four overscan regions regions are
located near the middle of the raw image. The following
graphics show a typical bias (left), a typical bias with the
ccd window heater on (middle), and a typical i-band
normalized domeflat (right). Skyflats are recommended.
Because the window heater contributes non-negligibly to the
image, it is recommend to turn it off during observing. Bias
levels vary from the high 400's (lower right quadrant) to
high 700's (upper left quadrant). North is to the left left
and east is up in raw images. An 3W resistor located
between the shutter and the ccd window may be used to prevent
condensation on the window.
| typical raw bias
|| typical raw bias with window heater on
|| typical normalized domeflat in i' band
|| 60 s dark exposure with heater turned on; bias level has been removed
Notes on reducting data from the WIRO Double Prime (WIRO-DP) 4096x4096 imager.
Version 2015 October by Chip.
1. Overscan subtraction and Trimming
Overscan regions for each amplifier are in the middle of the raw image.
The image may be overscan subtracted and trimmed using the follwing IRAF script called
WDPzero.cl . Place this script in your
working directory (or somepleace IRAF can find it). Use this script as follows.
# define task in IRAF
# create a single column list of all raw images to be overscanned and trimmed
ls -1 *.fit > infiles.txt
# run script to use colbias to do overscan and trimming, producing 4096x4096 *_z.fits suffix files.
Dark current at -110 C is < 0.01 ADU/sec/pixel, although there does appear to be some pattern.
Below is a master dark exposure created from seven 300-sec darks.
Sky flats are recommended. Dome flats can work if the whole field of view is not required.
Below are typical normalized domeflats in each of the five 3.5" sloan filters with links to .fits images.
Below are typical skyflats
Below is the ratio of a domeflat to a skyflat in i band to give some indication of
how bad it might be to substitute domeflats for skyflats.
3a. Distortion correction (option 1)
The prime focus corrector introduces
pincushion distortion that is most noticable near the
edges of the 39' field. In order to stack and co-add images this distortion must first be corrected
so that the plate scale is the same at the edges of the field as at the center.
This can be accomplished with the IRAF task geotran using the coordinate transformation
described in WDPdistor.db which was created using geomap
with input being the raw data x-y reference and pixel positions in WDPdistor.txt
(by Chip, 2015 October 05 data). A description of the geometric correction is found in
WDPdistor_results.txt . Copy WDPdistor.db to your working directory.
# in IRAF use geotran
geotran @inputlist @outputlist WDPdistor.db WDPdistor.txt xscale=0.98 yscale=0.98 ncol=4096 nline=4096 bound=constant constant=0
3a. Distortion correction (option 2)
See Joe Findlay's guide to WDP stacking and astrometric correction.
4. Astrometric qualities
5. Sensitivity and color terms (from 2016Jan01 data by Joe)
|| Zero point (mag)
| u (Asahi)
| u' (Astrodon)
Color Transformations to Sloan mags(' denotes wiro mags)
u = u' - 0.008(u'-g')
g = g' + 0.039(g'-r')
r = r' - 0.031(r'-i')
i = i' - 0.086(r'-i')
z = z' + 0.004(i'-z')
The WDP ccd is linear to at least 60,000 ADU in all four quadrants.
7. Gain and Readnoise (Nov 2015 data by Joe Findlay)
|| Gain (e-/ADU)
8. Shutter timing effects
Exposure times of less than 2 sec are not recommended due to finite shutter open/close timing.
The following figures shows ratios of various exposures to an 8-sec exposure
on a dome flat lamp
9. Example Images