- New 20µm Filters Received and Installed
- In late October, 2003, the LWS cryostat was removed from the Forward Cassegrain
Module (FCM) in Keck I and transported to the Instrument lab at the Keck
summit facility. The dewar was opened up and two new filters, one centered
24.5 mm and one centered at 23.1 mm were installed. See the lws
filter page for the
manufacture’s band pass information. The delivery of these filters
completes a consortium procurement of Q and N band filters from the Univ.
of Reading (UK) Infrared Coating Lab. The two newest filters increase the
wavelength parameter space of the instruments capabilities and will create
new scientific opportunities for observers. The instrument was reinstalled
in the FCM in early November 2003 and testing occurred on a subsequent engineering
night. The intended sensitivity and image quality measurements of the new
filters were not well quantified due to very poor seeing and other problems.
However, the indications suggest they are of very good quality and the new
filters are now available to all LWS users.
- Choosing Guide Stars [March 2003]
A reminder about using SKY to find guide stars. The current guider annulus
in SKY is about a half width too small in radius. So you want to choose
stars that are close to the outer edge of the plotted annulus.
And in fact you can use guide stars that are just outside the annulus,
Keck I Emissivity Measurement [January 2003]
Keck 1 (F/25) Emissivity = 6.5±1.0%, Measured in July, 2002.
See: detailed analysis
Interactive Atmospheric Transmission Plot [September
Public access is now available to a user controlled plotting routine of
the Mauna Kea atmospheric transmission models via the Keck public web page
. Check it out by clicking here: atmospheric
New Filters! Aperture mechanism Working [April 2002]
Three new LWS filters have been installed
recently and are in the process of being characterized. Two of the filters
installed have standard star measurements but the third has not been tested
as of this writing. The preliminary results show that these
filters will improve the sensitivity of LWS.
The names of the three new filters installed in LWS are silicon carbide
or SiC, 17.65, and 18.75. The SiC filter is centered at 11.75 µm
and is 2.4 µm wide. After a few preliminary sensitivity measurements,
the SiC improvement of NEFD is about 40% better than the 11.7 filter
on the same star with the same integration time. The initial measurements
indicate the sensitivity is about 17mJy (SNR 1, 1sec of on source time)
For the 17.65 (0.9 µmBW), the preliminary sensitivity number is coming
in at 100 mJy, an improvement from 150 mJy sensitivity of the 17.9 filter.
Even though these numbers are preliminary, one thing seems true, the new
filters are considerably better in head to head competition with the existing
filters in LWS. Although the data are only preliminary for the characteristics
of these filters, results are encouraging thus far. There don't seem to
be any ghosts or other artifacts and they probably will become the most
popular LWS filters. The third filter installed in early April and not
yet tested is the 18.75 (0.9 µm BW). The 10.4 filter and the
old 18.7 filter were removed to make room for installation of the new filters.
The aperture mechanism is now working properly again after it suffered
repeated failures since the November 2001 service mission. The cause of
this problem was finally traced to an incorrect installation of a grid
of holes aperture mask intended for distortion testing. The grid of holes
are now properly installed and the aperture mechanism working fine. During
the April observing run that included spectroscopic observations
the mechanism was exercised extensively without problems.
Dewar Service Mission
An extensive dewar service mission was performed in late November 2001
with three objectives:
Optical alignment to fix vignetting
problem (noted below),
New filter installation,
Repair excessive backlash in
The optical alignment problem was found and repaired and two new
filters were installed. The filter backlash problem has also been eliminated.
However, during an engineering night to check the performance following
the dewar work, the aperture mechanism experienced a failure. The problem
was caused by broken rod made of a fiberglass materiel known as G10. The
problem turned out be relatively serious and LWS science observing was
impacted. The impact was minimized by rescheduling observers and providing
service observing for others. Currently [February 2002] LWS performance
is getting back to normal after a rough couple of months with mechanism
and cryogenic problems (not to mention particularly bad winter weather).
Vignetting of Field [30 September 2001]
Something within the optical train of LWS has been creeping into the light
path. This is causing vignetting of the top of the LWS images. Adjusting
the positions of the aperture and the filter do not affect the vignetting.
Currently (September 30) the vignetting is about 20% of the field. A mission
to open the dewar and readjust the alignment is being planned.
Filter Notice [30 September 2001]
Due to excess backlash in the filter wheels, observers should set the filters
using the "filname" command. For example, to set to the 11.7 micron filter,
type "filname 11.7". This will move the filter wheel in such a
way to remove the larger backlash.
New Features in QuickView 1.8
The newly-released version 1.8 of the LWS QuickView
widget includes several notable new features:
LWS Dewar Sevice Mission Improves Noise and Filters
Peak Meter giving the current maximum pixel value (known hot pixels
Spectroscopy Mode for extraction of spectra;
Radial Profile Mode for inspection of object shapes and sizes (a
Growth Curve Mode for analyzing enclosed light;
Static Mode not includes ability to change between "chop diff" and
"raw DC frame"
Engineering efforts completed 17 Mar 2000 in the LWS dewar have resulted
in several improvements. See the Full Report
LWS IDL Tool Released
IDL users can now use the new lwscoadd
program developed at Keck to to sum images taken in either stare, chop,
or chop-node mode. The program requires the installation of the IDL
Astronomy Library routines.
Using NIRC as an LWS Acquisition Camera
Starting in semester 2000B, LWS observers will be able to request the use
of NIRC as an acqusition camera for LWS observing. These
instructions describe the procedure for acquiring targets with NIRC.
CARA will support this mode of observing as long as the intent to use the
NIRC/LWS combination is clearly indicated in the proposal coversheet. (Note
that because the instruments view different parts of the telescope focal
plane, it isn't possible to observe the same target simultaneously with
NIRC and LWS). For more efficient observing, we also strongly encourage
(but do not require) having two observers when using NIRC only for acquisition.
See the Keck Available Instruments
Page for details about opportunities and requirements.
Interactive Atmospheric Transmission Plots
Observers can now plot the Mauna Kea atmospheric transparency across any
desired mid-IR wavelength range using a new
interactive Web tool developed by Randy Campbell of WMKO. This new
feature combines previously published data with the power of RSI's ION
(IDL on the 'Net) technology.
WMKOLWS Package Released
WMKOLWS, a custom IRAF package for LWS, is now
available for general distribution from the Keck
FTP site. The package currently contains the lwscoadd
task (program), which combines the various dimensions of the FITS files
produced by LWS to yield a properly summed image taken in either stare,
chop, or chop-node mode.
Filter List Revised
We recently learned that the filter we have been calling "10.3" is actually
centered at 10.7 µm with a passband of 10.0-11.4 µm. Documentation
will continue to refer to this filter as the "10.3" filter until the LWS
control software is updated to reflect the changes. Note that this filter
covers a relatively favorable atmospheric window and is competitive with
the 11.7 µm filter as a deep imaging option.
QuickView 1.2 Released
The new version of the LWS real-time data inspection and analysis tool,
is now released for use with LWS. New features in this version include
image statistics and circular aperture photometry.
Atmospheric Transmission Plots Released
We have used Alan Tokunaga's model for the absorption spectrum of water
vapor to generate plots of the atmospheric transmission as a function of
LWS Update #5
Discussion of LWS commissioning status, including solution of the nod-delay
LWS Update #4
LWS has begun its on-sky commissioning! View results of the first night
of LWS on-sky tests.
LWS Update #3
LWS passed its final pre-commissioning hurdle in flying colors with a successful
LWS Update #2
Review of LWS progress for summer of 1998.
LWS Update #1
Review of LWS progress as of June 1998.
Go to: LWS Home Page - Instruments
Home Page - Keck Home Page
Last modified: Thu Jun 8 10:06:43 HST 2000