Upon Arriving
Initial setup is performed by the observatory staff. Starting computer programs, initializing the camera, and testing the detector are left to the observer. Upon arriving, confirm that the setup has been completed. Check that the dewar has adequate coolant and, in humid conditions, that the captive air cover has been installed. Consult the Cassegrain Logbook at the 3-meter, or the User's Logbook at the 1-meter for notes by recent users. Check Lirctop's electronic log for the latest updates.
Both vessels are filled from the top of the dewar using a funnel with two 90-
degree bends. Remove the tubes from the fill holes by unscrewing the knurled
couplings, or, if they're frozen solid,
carefully pull the hoses themselves out of
the couplings. Gently insert the funnel into the inner vessel's fill hole,
the one nearest the center of the dewar. The inner vessel generally
requires very little
nitrogen--minimize spillage by only pouring a small amount at a time into the
funnel. The vessel is full when the coolant overflows the fill hole.
Move the
funnel to the outer vessel's fill hole and proceed in the same manner.
The outer
vessel will typically require 2 to 3 litres every 12 hours. Avoid spilling
nitrogen
onto the dewar--loss of vacuum due to a frozen o-ring could end your observing
session and damage the camera.
Always replace the rubber hoses, they provide paths for the
coolant to flow harmlessly to the floor when the telescope is moved to large
hour angles. If they were removed at the couplings, simply screw
them back into
place. Turn the couplings several threads, but do not tighten.
If the hoses were
removed from the couplings, reinsert them after they have warmed up and
become pliable.
.
Should condensation form, call a member of the staff, do not attempt to
remove it yourself.
Turn on the NCD terminal and log in as `user' (the password is
posted on the keyboard). Start the data-taking program from an `xterm' window
by typing `lirc2', or use the pull-down menu. The data-taking program will
automatically create three windows: an xterm window displaying the data-taking
menu, an image window, and an image control window.
Figure 4 shows the x-terminal screen with the
image windows, and their parent data-taking window. (You may wish to
conserve screen space by reducing the size of the
font in the data-taking window. Put the cursor in the window,
hold the control key and the right-hand mouse button simultaneously and
select `tiny' from the pull-down menu.)
The data-taking window presents a menu for setting observing parameters
and controlling observations. The image window displays an image of the last
completed observation, or can be used to redisplay a previous frame, stored on
disk. The image control window provides options for a variety of measurements
and manipulations of the image. Additional windows, including quick row and
column plots and a variety of image statistics,
may be spawned by the data-taker.
Like the CCD data-taker, the LIRC-II version beeps when an exposure begins,
when it ends, and when the readout is complete. This can become tiresome
when making the many short integrations typical of near-infrared observing.
The `bell' may be turned off by typing `XSET B 0' from an xterm window.
Typing `XSET B 1' turns it on again.
Use of the coadd option affects the numbering of
stored images. Though the observation number increments each time a
frame is taken, only the coadded frame is stored, taking as its number
the number of the first frame in the coadded series.
If, for example, an observation is begun with
the observation number at 15, and the number of coadds set to 3,
the data-taker
will take three exposures--15,16, and 17--add them together, and write them to
disk as `d15.ccd'. If another image is made, frames 18, 19, and 20 will be
summed and written as `d18.ccd', and so on. Thus, gaps will appear in the
numbering of disk images, reflecting the number of coadds making up a
given image.
Keep in mind that when operating with the default condition of baseline
subtraction disabled, the baseline counts will appear in raw images as
approximately 2,000 additional DN that must be discounted when estimating
flux levels. The plots reproduced in this manual include the baseline.
Turn on the PC compatible computer and type `lircmc' at the DOS prompt.
On startup, the program checks its RS-232 connections with the
motor controller;
if a problem is detected an error message will appear. (If an error condition
occurs, you can check the cable connections at the computer and at the motor
controller, try cycling controller power, or call a staff member for help.)
After
successfully establishing communication with the controller, the program will
ask if you wish the wheels to be sent to their `home' positions. Typing `yes'
calibrates the positions of the three wheels. Doing so is recommended,
especially following an error condition. The controller program's top level
screen is illustrated in Figure 5a.
The three columns list the contents of the wheels under their respective
headings. Filters are not routinely changed, and users can be confident
that the
lists accurately reflect the contents. The currently selected position of
each wheel
is displayed at center right; commands are listed at the bottom of the screen.
Selecting `H' will cause the positions of all three wheels to be
reinitialized to their
home positions. This is a
useful recalibration procedure if there is any doubt as to their correct
positions.
To change the position of any wheel, type `F', `B', or `L' (Front, Back, or
Lens
wheel) followed by a carriage return. This will invoke a screen like the one
illustrated in Figure 5b, displaying the wheel's contents and a number
corresponding to each position. From this screen's menu the observer selects
the
desired filter by entering the code number followed by a carriage return,
initiating the movement and returning the program to the main menu. The
program automatically removes backlash while repositioning the wheel.
x
The data-taking system includes some tape
commands.
Tapes may also be initialized, positioned for appending, and written using
Vista's
`exabyte' and `totape' commands.
The observatory archives all data taken with facility instruments on the 3-
and 1-meter telescopes, including LIRC-II. Archival tapes
are intended as a backup in the event of the accidental loss of data. They
are not
meant to substitute for the observer's backup. Archiving is customarily done
at
the end of each observing run. However, if the number of stored frames begins
to task the available disk space, so that the disk must be cleared before the
end of
your run, you should notify a technician that an interim archival backup
should
be made. (The first indication that you're growing short of disk space is the
message, from the data-taker, that the data disk is full and that it has begun
writing to the vista disk.)
Observatory archiving has proven to be a worthwhile, but time consuming,
process. This is particularly true for the very large number of images
associated
with infrared runs. Please make our job easier by deleting unnecessary frames
before we archive, or informing the telescope technician if your data need
not be
saved, as, for example, calibration frames for a night during which no data
were
taken.
Some of LIRC-II's setup, such as focusing, is done with the data-taker
operating in a continuous loop. This can generate a great many frames which,
if
inadvertantly recorded, rapidly swallow disk space. Be sure to turn
recording off
at such times.
To run Lirctop, open a Vista window by selecting `Vista'
from the pull down menu which appears when pushing the left-hand mouse
button with the cursor in the gray area between windows. A blue Vista window
is created. (You may wish to conserve screen space by reducing the size of
the
font in the Vista window. Put the cursor in the window, hold the control key
and the right-hand mouse button simultaneously and select `tiny' from the pull-
down menu.) Type `rp lirctop' to load the program, type `go' to run it. See
the Lirctop Manual for a complete description
of its functions.
The boresite system does not require the introduction of a
plane diagonal
mirror, thus eliminating a significant source of thermal emission
which is present when guiding with the TUB camera.
Observing is easier with the boresite camera. It receives its light from the
visible portion of the telescope beam reflected from the dichroic beamsplitter
(refer to frontispiece or back cover), and is therefore centered on the same
patch
of sky as the infrared array. This simplifies observing by eliminating the
need to
offset the TV camera or telescope to allow the target to reach the infrared
array.
If you choose to use the boresite camera, especially if you are the first to
use it
following a run with TUB camera, or if you switch from one to the other,
changing TV cameras.
After acquiring a target with the TUB camera at the
1-meter, the telescope must be offset to allow light from the target to
reach the
detector. Offsets specific to LIRC-II are recorded in the Users' Log book.
Use the
most recent offsets recorded therein. Offsetting the telescope is described
in the
Nickel Telescope User's Manual, and is part of the regular training
for 1-meter
observers.
Thermal emission from the diagonal mirror increases background ten
to twenty percent at K-band, and affects flat-fielding in all bands. Thus,
flats for
images made when using the TUB camera must also be made with the diagonal
mirror in place (mirror position 3 at the 3-meter; `diagonal in' at the
1-meter).
If you do not plan to guide your exposures, you can
acquire targets with the TUB camera and remove the diagonal after offsetting
the
telescope and before beginning the exposure. You must, however, remember to
reinsert the diagonal for acquiring the next object, and remove it again for
observing.
If observing with the boresite camera, be sure that the mirror is removed.
Mirror position #4 withdraws the mirror at the 3-meter. At the 1-meter, move
the TUB diagonal switch to the `out' position.
The TUB camera will have been set in advance to its nominal
focus position (7.6 at the 1-meter) by a technician, but may require slight
adjustment (see `folding flat mirrors' in the Nickel Telescope User's
Manual).
The bore-site camera may need to be focused by the observer at the 1-meter,
or, at the 3-meter, by the telescope operator at the observer's request. If,
after the
telescope has been focused on the infrared array, the bore-site image is not
well-
focused, correct it by rotating the focusing ring on the lens attached to the
bore-site camera.
Filling the Dewar
At the 3-meter the dewar is filled by observatory staff, but at the
1-meter it is the observer's responsibility.
The LIRC-II dewar is of the twin vessel type. Both
vessels use liquid nitrogen. The outer vessel requires refilling every twelve
hours; the inner vessel may be filled once a day.
Bear in mind, however, that there may be spillage when working at large
zenith distances, requiring more frequent refilling.
Note that the array may
exhibit some instability in the first hour after filling.
Captive Air Cover
Above about 70% relative humidity, the
dewar's input window can fog over. A transparent captive air cover prevents
fogging, with negligible light loss and image degradation.
The cover is placed
over the square, protruding collar of the input window, and its power supply
plugged into an AC outlet on the telescope. Use of the captive air
cover should
be standard operating procedure during the damp winter months, and in all but
the driest conditions throughout the year. Condensation on the window not
only ruins images, but can permanently damage the window's coating. Use
the captive air cover if humidity can be expected to reach 70% or higher
anytime during the night
The Data-Taking System
The LIRC-II data-taking program has much in common with the CCD data-
taking program on which it is based, and will appear familiar to most
Lick users.
Here we only discuss those aspects of the data-taker which are unique to the
LIRC-II version. A complete review of all data-taking commands is given in
Data-Taking System Guide.
Special LIRC-II Data-Taking Options
The three options `Number of erases', `Number of prereads', and `Number of
coadds' are unique to the LIRC-II version of the data-taker.
Note, also, that there
is no `fast readout' option, and that in the absence of a shutter,
the `dark' option
under `observation type' has no effect. Dark exposures are accomplished by
moving the filter wheels to their `dark'
positions.
Number of Erases
`Number of erases' sets the number of times the array will be erased
before the
actual exposure is begun. We recommend two to three erases, depending on the
brightness of the target, to insure that there is no residual charge
from a previous exposure.
Number of Prereads
`Number of prereads' sets the number of optional zero-second darks (bias
frames), used to reduce the effective readnoise in a single exposure.
The bias
frames are automatically added together, averaged, and held in a buffer until
readout, at which time they are automatically subtracted from the
target frame.
Using prereads lengthens the duty cycle by adding about 3 seconds per preread,
before the observation is begun. `Number of prereads' is set to
zero in most observing situations.
Number of Coadds
`Number of coadds' sets the number of frames to be coadded before being
displayed and saved. Coaddition of several consecutive images is a common
infrared observing practice. The data-taker's `coadd' option
automates the job by
adding the requested number of frames in memory, before the final image is
dispalyed and written to disk or tape. When using coadds, bear in mind that
only the summed image is saved. Sources of error which might otherwise be
recoverable, such as poor guiding or rapid changes in sky background
level, will be blurred. To take individual, non-coadded images,
set the number of coadds to one.
A/D Baseline (bias) Subtraction
In the CCD version of the data-taking system, automatic subtraction of an
array's baseline from raw images, is the default. The baseline is computed,
subtracted, and stored as the highest number column in an image.
The LIRC-II version disables baseline subtraction by default.
Baseline subtraction may be
enabled from the data-taker by typing `Z' and selecting option four.
The Motor Control Program
The motor control program allows the user to position LIRC-II's two internal
filter wheels and to select from three possible fields of view by
positioning the lens wheel.
L I R C 2 M O T O R C O N T R O L L E R
---------------------------------------------
BACK WHEEL FRONT WHEEL LENS WHEEL
K-band (1.98-2.42) Open Small Field
J-band (1.10-1.44) ND2 2% trasm. Medium Field
Open .37inc. SP Filter Wide Field
OII (1.237) Open Adaptive Opt. Lens
FeII (1.644)
H2 S(1) 1-0 (2.122)
BrG (2.166)
H2 S(1) 2-1 (2.248) Front Wheel Posit.: Open
CO (2.295) Back Wheel Posit. : K-prime
K-prime (1.85-2.35) Lens Wheel Posit. : Medium Field
H-band (1.50-1.82)
D - Go to Dark position F - Change filter on front wheel
K - Go to K-prime & Medium Field Lens B - Change filter on back wheel
L - Change Lens wheel
Q - Quit the program
L I R C 2 M O T O R C O N T R O L L E R
---------------------------------------------
Filter Enter Code
H2 S(1) 1-0 (2.122) ________ 1
BrG (2.166) ________ 2
H2 S(1) 2-1 (2.248) _____ 3
CO (2.295) _____ 4
K-prime (1.85-2.35) ____ 5
H-band (1.50-1.82) ____ 6
K-band (1.98-2.42)_____ 7
J-band (1.10-1.44) ____ 8
Open .37inc. _______________ 9
OII (1.237) ________ 10
FeII (1.644) ________ 11
Current Back wheel position is: K-prime
Enter the new position _
Recording Data
The data-taking system allows images to be recorded on disk, 8-mm tape,
both, or neither. Most observers choose to record only to disk during the
course
of the night, thus improving the observing duty cycle slightly by saving the
time
needed to write each incoming image to tape. The data are then typically
written
to tape in the morning, and sometimes FTPed to a remote site, as well. Data
quality casette tapes may be purchased on the mountain.
Lirctop
Lirctop is a top-level, menu-driven program which calls a variety of Vista
procedures for performing routine tasks such as making calibrations, taking
repetitive exposures, dithering, mosaicing, and performing tests. Lirctop is
largely self-explanatory and includes online help.
Choosing a TV Camera
For acquisition and guiding, the observer may opt for either the standard
cassegrain TUB TV camera or LIRC-II's boresite TV camera. Both share the same
controller and are nominally the same type of camera. The boresite system is
two to three magnitudes less sensitive, and has a somewhat narrower field of
view than the TUB system. Nevertheless, we recommend using the boresite
system whenever possible. The TV cameras are compared below.
The Boresite Camera
The boresite camera can reach stars of
approximately 17th to 18th visual magnitude at the 3-meter, and 15th to 16th
at
the 1-meter, under the best conditions. Diffuse objects are more difficult.
Moonlight can dramatically reduce the camera's ability to reach
faint objects. Beyond these limits, it may be necessary to resort to the TUB
camera, which, on a clear, dark night, can see objects at the limit of the old
Palomar Sky Survey (about 19th magnitude) at the 1-meter. However, if the
boresite is adequte to acquire a target, it is the preferred camera.
The TUB Camera
Despite its disadvantages, the TUB camera may be necessary for acquiring
faint sources. The 3- and 1-meter TUB cameras guide on patches of sky
adjacent
to the target. At both telescopes, a perforated diagonal mirror must be
inserted to
allow light from the target to pass through to the infrared camera.
This mirror
represents a significant source of thermal emission.
The TUB Diagonal Mirror
As noted above, an additional mirror is required to use the TUB camera at
either telescope. The backside of this mirror faces LIRC-II and is seen by
the
infrared array as a source of thermal emission. At the 3-meter, the diagonal
is in
place when mirror position #3 is selected. At the 1-meter, the mirror is
inserted
by moving the switch behind the sliding door in the rack below the TV camera
controller to the `in' position.
TV Camera Fields
Figure 6 shows the fields of view and orientations for both TV cameras, at
both telescopes, with the camera controller in binning mode 2. Note that
while
the TUB and boresite plate scales are comparable, vignetting narrows the
boresite's field of view. Also note that
with the TUB camera at the 1-meter, the cardinal directions are rotated 30°,
due
to TUB rotation. The directions on the array itself remain vertical and
horizontal for all setups (see
Figure 2).
