In the near-infrared the OH lines from the sky are strong and variable,
and in the K-band thermal emission is seen from the telescope and
sky. There are also a significant number of bad pixels in the array.
As such it would be a good idea to take spectra at different
positions along the slit.
can also be some flexure (~0.5″ / 1 pix) when going from vertical
to horizontal with the spectrograph, so even if the OH lines didn't
vary you may see poor subtraction of the lines.
can also get a quick look at your data by taking the difference of
two different positions along the slit (with the provisos above).
For short integrations (~30 seconds) a simple difference is likely
to work well.
Above is a snap shot of the spectrograph control GUI, a DS9 session,
and an IRAF window, as they typically appear on one of the dual
displays on the workstation running TSPEC in the 200-inch control
room. For this document it will be assumed that the observer knows
about DS9 and IRAF. If you need help with DS9 or IRAF, talk to the
instrument support engineer in the afternoon when you get to
Spectrograph control GUI.
The window in the upper left is the camera control GUI. This
GUI is a LabVIEW + C code based architecture we call ARCVIEW, that
runs many of the Palomar public instruments.
The lighter colored upper half of the GUI, titled Status, is
purely informational, and not controllable.
The darker lower half of the GUI, titled Control, is
what the observer uses to control the spectrograph camera. The
following bullets in this section detail the Control section
features from top to bottom.
The image path is set automatically when the GUI is started, or when
the user pushes the change date button next to the image path.
This automatically creates a new directory in /rdata/TSPEC/ for the
current UT date.
The exptime window is of course the exposure time of the next
image in seconds. Below exptime is the window which selects
the number of consecutive images that will be taken at the selected
exposure time when GO EXPOSE is pressed.
The write to disk selector should always be selected. The observer must save the image to disk
in order to
eventually see it on DS9 with the display command in the IRAF
The number of coadds and Fowlers can be selected just below the
write to disk selector. If you are unfamiliar with coadds or
Fowlers, talk to instrument support.
The next section deals with more exposure information. The pull down
that currently reads none selects image type information
(object, dark, flat, etc.) which goes in the header. basename
is the image file prefix. use type as basename selects
the image type from the image type pull down and makes that the
image prefix. Do not use add pref. to basename, since it
can crash the GUI.
comment adds comment information into the FITS header.
The Plug-Ins pull down selects some sub-vi windows
(temp, ROI, header, etc.) that don’t have too much application to
QUIT is a very important button. Pressing QUIT
brings up the hidden QUIT button. QUIT is the nice
way to shut down the camera control GUI. Use QUIT if the GUI
seems hung, and when the control GUI disappears, re-start with the
TSPEC_SPEC icon found in the TSPEC folder on the VNC desktop.
Instrument support will show you how to do this.
The last three buttons at the bottom of the control GUI are GO
EXPOSE which starts an exposure (or exposure sequence), stop
after current image which cleanly stops a sequence of images when
the current image is finished reading out, and stop now which
stops an exposure (and exposure sequence) by reading out the chip.
These two stop buttons are preferred to put a clean halt
Above is a screen shot of the guider VNC window as it will appear on
one of the monitors on the workstation on which you will be
Most of your time observing will be spent in this window. The guider
GUIs controls the dithering sequence, the guiding, the images the
guider saves, AND IT REMOTELY CONTROLS THE SPECTROGRAPH (taking
images during the dithering sequence).
Guider control GUI.
In the upper right is a green window that looks very much like the
spectrograph camera control GUI. This is in fact the LabVIEW/ARCview
camera control GUI for the guider camera. It is laid out just like
the spectrograph GUI. Status information in the top half, and camera
controls on the bottom half.
As with the spectrograph, the status indicators are self explanatory.
Starting from the top of the control half, there are the object
and comment windows in which the observers can enter
information into the IRAF headers.
The exptime and num. of images windows are just like those
on the spectrograph.
The cont_read control is for the guider in continuous read
mode. This mode is usually used for guiding. Clicking on this
control deselects continuous read, and the write to disk
selection appears. When write to disk is selected, the
bottom of the control GUI expands to include the following: image path,
observer (FITS header entry), image base name, image
number, and the obs. type pull down menu that selects the
image type for the FITS header. This write to disk section
acts just like the same windows in the spectrograph GUI, except this
information is applied to the saved guider images.
Below the the write to disk selector is a write last
button. Press this button and the GUI will save the image currently
in the Guider buffer even though write to disk is not
Just below write last is the small QUIT button that
brings up the QUIT button. QUIT cleanly
brings down the camera control GUI, which is useful if the camera
(PCI card) locks up.
The Select PI menu has two selections; one is headers that brings up a FITS
header vi, in which the observer can add FITS header information.
The other selection is sguide which is the slow guider display
window to the left of the camera control GUI. Since this window
comes up at start up, the observer should never need to select sguide.
The window to the left of the Control GUI is the imager, the
dithering tool, the remote spectrograph control, and the launching
pad for many pop up GUIs.
The image seen in the snap shot above shows the guider
reflective surface. At the top can be seen the 1″ × 30″ slit, which
in this snap shot has a small yellow square sitting on the right
side of the slit, and a small red square on the left. In the bottom,
left, and right, appear other slits. In the Palomar version of
Tspec, these three other slits are not used.
Above the main image is a zoomed-in image of the slit. This is where
the observer places the slit dither marks. Various controls for the
placing of marks are located below the zoomed-in slit image.
To the right of the zoomed-in slit imager are the dithering sequence
and remote spectrograph setup and controls.
Above the zoomed-in imager and dithering controls, is the guider
control and telescope status indicators. Guiding is relatively easy;
just put the yellow guide box on a star and toggle off to
on. As the target is dithered across the slit, the software
moves the guide box the same distance to keep the guide star
centered in the guide box.
tcs_rotation refers to the Cass ring rotation. The actual
rotation is the chip orientation on sky. The Cass ring
and the chip orientation are offset. More on ths below.
To the right of the main image display is a smaller display that
shows the image inside the guider box. There are some contrast
controls below this display. On the screen shot above there is a
sub-vi to the right of the guider display that is a plot of the
intensity profile of the pixels inside the guider box. The actual
guider box in the image can be moved and resized with mouse drags.
Click and drag to move the box, and click and drag the lower left
corner of the box to resize it.
An important selector in the guider display area is the MouseMode
selector. With this you can measure distances, add fiducial
marks, and manipulate the guider box.
Above the guider display area are some controls for the fiducial
markers, and offset controls.
And finally there is a line of pull down menus at the bottom of the
main image display window. These selections lead to a vast number of
controls and displays. There is an auto contrast sub-vi, an
automated focus sub-vi, there are zoom displays for fiducial marks,
various one-D plotting tools, image subtraction, and more. See Instrument
Support for a tour of these features when you arrive.
Slit and Cass Ring Rotation
The slit and cass ring indicators on the top of the slow guider window can cause some
confusion with regards to actual slit orientation on sky.
The left indicator shows the guider chip orientation on sky.
The right indicator shows the tcs ring angle.
When the ring angle is at 139.32, the Tspec guider is at 0, where 0
is North up and East left.
Moving to a desired slit angle is confusing because the slit software rotates in
an opposite sense to the TCS rotation. The following is a table which shows the values for
the four cardinal directions.
Tspec on sky actual rotation
tcs ring angle tcs_rotation
North & East
N-up & E-left
N-left & E-down
N-down & E-right
N-right & E-up
The slit is East/West when Tspec on sky is North-up & East-left.
This dialog box is called by pressing the
Settings button on the Guider area (bottom right corner) of the
Guiding Threshold: Sets the threshold
required for the guider to assume there is enough signal to guide. If 5 % of
the pixels inside the box area have at least a _threshold_ % value of the peak,
then the correction will be sent.Otherwise it will be rejected and an error message will appear on the
messages area of the main window ("-34 NO SIGNAL"). Also the
"last" LED will be turned off.
Seeing Tolerance: Any correction with a value < Seeing_Tolerance will
NOT be sent, assuming they are variation due to seeing and not real corrections.
Gain: This is the "attenuation factor" applied to the calculated
correction.This number must be adjusted to avoid
overshoot or oscillation on the telescope.
Min Correction time: Sets the minimum rate for the corrections, in seconds.
Any correction calculated before _min_ seconds will not be sent.
Telescope Move Rate: Sets the speed at which the telescope will move when
a correction is sent (in arcsecs/sec).
Algorithm: Defines which algorithm will be used for the guiding process:
CMASS (Center of Mass) or QUAD (four quadrant weighting). CMASS is the algorithm used by default.
Xcenter/Ycenter: Sets a specific position (pixel-precision) for the center of the
guide box. When the box is moved with the mouse this value gets updated and vice/versa.
Xoff/Yoff: Sends an offset for the guide box. The offset can be specified in
pixels or in arcsecs. If arcsecs are used, Xoff and Yoff become RA and DEC.
Xsize: Sets the size of the guidebox at pixel-precision. This can be specified in arcsecs
The shortest exposure time is about 4 seconds. There are three possible
limits to the maximum exposure time: source saturation, saturation in
the K-band and saturation of OH lines. The plot below shows the
background measured by TripleSpec near zenith on 16-Feb-2008 (ambient
temperature ~ 0° C). Using a saturation level of 28,000 DN, the
maximum integration time is approximately 2400 or 1200 seconds for
saturation to start occurring in the H and K-band respectively.
The longest integration times we have done on sky thus far are 600
seconds. With a dark current of 0.085 e−/sec and a read noise of 3.5
e− (w/ Fowler sampling), then the noise due to the dark current will
equal the read noise in ~ 144 seconds. The inter-OH continuum is
difficult to measure but appears to be ~0.1-0.8 e−/s
over the J and H-band. This implies that 300 seconds should be
sufficient to overcome the read noise. (Note: this result is
Plot of sky + telescope emission for TripleSpec in units of
DN/sec taken on February 16, 2008.
Plot of sky DN/sec per column for a 10th mag A0V star.
S/N ratio per column for an m = 15 (A0V type) source. Coadding across
a resolution element (3 pixels) will decrease the number of coadds by
a factor of 3 or (for fixed number of coadds increase the S/N by a
factor of √3. No differencing noise is included.
Same as above but for m = 17.
Same as above but for m = 18.
Same as above but for m = 19
The response (integrated across a column) to at 10th magnitude star is
given in the second plot. This can be used for scaling to other magnitudes.
These are a subset of the Elias standards (A-stars). Typical integration times
will be 30 seconds. They will be saturated in the guider field. A
K-star (or model profile) will be needed to remove the H
recombination lines. Additional calibrators can be found via the
usual searches on-line.
Name RA (1950) Dec (1950) J H K
HD 225023 00 00 11.8 35 32 14 7.065 6.985 6.96
HD 1160 00 13 23.1 03 58 24 7.055 7.045 7.04
HD 3029 00 31 02.3 20 09 30 7.25 7.12 7.09
HD 18881 03 00 20.5 38 12 53 7.125 7.13 7.14
HD 22686 03 36 18.7 02 36 07 7.195 7.19 7.185
HD 40335 05 55 37.6 01 51 09 6.54 6.47 6.45
HD 44612 06 21 09.7 43 34 35 7.06 7.035 7.04
HD 77281 08 59 05.4 -01 16 45 7.105 7.05 7.03
HD 84800 09 45 35.9 43 53 56 7.56 7.53 7.53
HD 105601 12 06 56.1 38 54 39 6.81 6.715 6.685
HD 106965 12 15 24.0 01 51 10 7.375 7.335 7.315
HD 129653 14 40 38.2 36 58 07 6.98 6.94 6.92
HD 129655 14 41 11.0 -02 17 38 6.815 6.72 6.69
HD 136754 15 19 24.3 24 31 19 7.15 7.13 7.135
HD 161903 17 45 43.3 -01 47 34 7.17 7.055 7.02
HD 162208 17 46 20.7 39 59 40 7.215 7.145 7.11
HD 201941 21 10 13.6 02 26 12 6.7 6.64 6.625
HD 203856 21 21 37.1 39 48 12 6.925 6.88 6.86
The figures on the right are plots of estimated S/N ratio for an "A0V" star
at various magnitudes. These are computed per column so that
averaging across a resolution element improves the signal-to-noise
ratio by a factor of √3.
The estimates are based on calibrator
and sky measurements taken in February 2008. The ambient air
temperature was ~ 0°C.
Note: An increase in noise due to subtraction of two spectra is NOT included.
Whether this is done or not depends on the final reduction technique.
If a simple difference of spectra taken at two positions in the slit
to remove background and bad pixels then the signal-to-noise ratio
will decrease by a factor of √2.
Read Noise 3.5 e− (16 samples)
No. Pixels in extraction: 4
Fraction of flux in extraction: 0.7
Spectra not subtracted: no √2
loss due to differencing
Spectra are not averaged no √3
improvement in S/N or over
a resolution element factor of 3 reduction in time (or coadds)
The plot label gives the magnitude, exposure time for each image and the
number of coadds (ncoadds). Obviously, the S/N will improve by
The spectrograph will flex on average about 1/2 pixel. The engineering team did an experiment where
they spun the cass cage at 45° zenith angle, and the average
flex was 1/2 pixel. They estimate the maximum as 1.5 pixels,
considering that some observers will push the telescope over to 60+°
Wavelength Ranges for each Order (µm)
2.4644498 to 1.8760142 ± 0.00014177590
1.8501315 to 1.4103792 ± 0.00010556104
1.4835071 to 1.1297587 ± 0.000084883749
1.2379470 to 0.94296033 ± 0.0000654381
1.0629230 to 0.80778160 ± 0.000060547528
Where the ± is the wavelength amount of 1/2 pixels.
You can multiply up for 1.5, if that's the better number.
Note that the sensitivity of TSPEC drops off significantly short of 1 µm, so
the observers shouldn't get the impression they can get 0.8 µm
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TripleSpec Cookbook / v 2.1.1
Last updated: 31 March 2019 CMH/ACM