OSIRIS has three spectroscopic modes: low-resolution (R~1200) and
high-resolution (R~3000) long-slit modes plus a low-resolution
cross-dispersed mode (R~1200) with a short slit. Separate slits are provided
for each mode as described below.
OSIRIS is put into spectroscopic mode by retracting the prefilter wheel and
pupil mask, and then flipping the grating into the beam in place of the
imaging flat. This operation requires approximately 2 minutes to complete.
The imaging flat may be flipped back into the beam rapidly (3 seconds) to
provide a quick-look Acquisition Mode to
center the object on the position of the slit (either by taking an image of the
field or viewing through the slit).
This section describes:
Spectroscopic observing strategies and techniques (including target
acquisition and calibration issues) are discussed in the section on Spectroscopic Observing.
OSIRIS has three spectroscopic modes:
- Low-Res
- R~1200 single-order long-slit spectroscopy
covering all of the J, H, or K bands in a single spectrum.
The Low-Res long slit covers the entire width of the field
of view of the f/2.8 camera at a fixed width.
The grating tilt should be set at 950
in low-res mode.
- High-Res
- R~3000 single-order long-slit spectroscopy
covering most of the J, H, or K bands. The wavelength region
adjusted by tilting the grating.
The High-Res long slit covers approximately 80 arcseconds by 0.48
arcseconds.
- X-Disp
- Low-Resolution (R~1200) multi-order
cross-dispersed spectroscopy
with a short slit and the f/2.8 camera that covers all of J, H, and K
bands simultaneously in adjacent orders. The cross-dispersed slit is
only ~15% the height of the low-res long slit at a fixed width.
All modes use a single diffraction grating blazed at 6.6-microns in first
order, which places the K-band in 3rd order, H-band in 4th order, and
J-band in 5th order. A grism and JHK filter (integrated into the short
slit) are used to cross-disperse orders 3,4, and 5 (and part of order 6)
across the detector.
The High- and Low-resolution modes are selected by changing the camera and
slit, and using one of the filters as an order sorter. Cross-dispersed
("X-Disp") mode is selected by introducing the cross-dispering grism (in the
filter wheel) and short slit into the beam. This slit has an
integrated JHK filter to provide blocking outside of the bands of interest.
The table below summarizes the camera, slit, and filter
configuration used by each of the OSIRIS spectroscopic modes.
Table 1: OSIRIS Spectroscopic Configurations
| Mode |
Camera |
Slit |
Filter |
| Low-Res |
f/2.8 |
0.34mm x 63mm
(LoRes Long) |
J, H, K or LPK |
| High-Res |
f/7 |
0.14mm x 24mm
(HiRes Long) |
J, H, K or LPK |
| X-Disp |
f/2.8 |
0.34mm x 8.6mm
(XDisp Slit) |
Grisma |
The order separation filters are as follows
- J Filter for 5th order (center ~1.25-microns).
- H Filter for 4th order (center ~1.65-microns).
- K Filter for 3rd order (center ~2.2-microns).
- Using the Long-Pass K (LPK) Filter in 3rd order extends the red coverage
a little past 2.4-microns (until the thermal background and
atmosphere clobber you).
The following table summarizes the basic properties
of the OSIRIS spectroscopic modes. The properties of the slits depend
on the telescope, and are summarized in below.
Table 2: Spectroscopic Parameters
| Mode |
Filter
(order) |
Center
Wavelength (Å) |
Linear Dispersion
(Å/pix) |
Wavelength Range
(Å) |
Resolution
(2 pix) |
| Low-Resa |
J (5) |
12500 |
5.525 |
2500 |
1200 |
| |
H (4) |
16500 |
6.906 |
3000 |
1200 |
| |
K (3) |
22000 |
9.208 |
3800 |
1200 |
| High-Res |
J (5) |
selectb |
2.208 |
1435 |
3000 |
| |
H (4) |
selectb |
2.762 |
1795 |
3000 |
| |
K (3) |
selectb |
3.683 |
2394 |
3000 |
| X-Disp |
JHKc (5-3) |
n/a |
(low-res)d |
10000-24000
(3.5 orders) |
(low-res)d |
Table Notes:
- a) Low-Res mode central wavelengths are given for the
optimal grating tilts (see Table 4) that
center the entire band within the limits of the filter
transmissions.
- b) Center wavelength is selected by tilting the grating
(see below), within limits prescribed by
the filter transmissions and the physical limits of the grating
travel.
- c) The "JHK" filter used in X-Disp mode is integrated into
the cross-dispersed slit, and not in the filter wheel proper (which
is occupied by the cross-dispersing grism).
- d) Dispersion and resolution depend on the order, and are the
same as for the individual low-resolution orders given in the table.
- e) Because of vignetting by the f/7 camera, the true coverage
for extracted spectra should be restricted to pixels between about 250 and 900
even though there will
be dispersed light outside this range.
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Slit widths and heights are fixed (pre-cut masks) as described in
Table 3 below, and depend on which telescope you are
using.
Table 3: Slit Properties
| |
Physical |
CTIO 4-m |
CTIO 1.5-m |
| Slit |
Width |
Height |
Width |
Height |
Width |
Height |
Low-Res Long Slit |
0.34mm |
63.0mm |
1.2" |
216" |
3.6" |
666" |
X-Disp Slit |
0.34mm |
8.64mm |
1.2" |
30" |
3.6" |
90" |
High-Res Long Slit |
0.14mm |
24.0mm |
0.5" |
81" |
1.5" |
254" |
The Low-Res and High-Res long slit heights span the entire unvignetted
imaging fields of the f/2.8 and f/7 cameras, respectively.
The X-Disp slit is only 14% the
length of the long slit so that the different orders can be stacked on the
array without overlap.
Slit Orientation
The slit is oriented E-W on the 4m in the standard
installation. By default,the instrument rotator = 270 degrees when OSIRIS
is mounted. We have measured the true E-W position to be 271.5 degrees. The
mounting of OSIRIS seems to reproduce this value to within about 0.5 degrees.
Therefore, you should ask the Night Assistant to adjust the rotator to this
angle at the start of your run. You can then check the accuracy by doing a
slitscan in Acquisition mode, measuring the stellar Y pixel position.
The instrument may be rotated to larger angles,
but this should be arranged prior to an
observing run.
[ Top of Page | Contents ]
The wavelength region is selected by tilting the diffraction grating.
How much you need to tilt for a given configuration is described
for each mode below.
Since all of the given band is on the detector at once in low-resolution
long-slit mode, the optimal grating tilts to center each band have been
measured in advance and are summarized in Table 4.
In general, it is not necessary to set the grating tilt to other than
these settings.
The "center" here is defined as the wavelength at row 575 (the
center of the f/2.8 imaging mask) in a vertical cut made along column 515
(the center of the low-resolution long slit). The slit is curved as seen
on the detector such that the central wavelength of an object shifts
to the RED as its position moves toward the end
of the slit from the center.
The shift is about 10 pixels from center to end.
If adjustments are required, the general rules of thumb for tilting
the grating in low-res mode are as follows:
- POSITIVE STEPS make the central wavelength
BLUER, and move lines
toward LOWER pixel values (on the IRAF display) by approximately
1 PIXEL/STEP
.
- NEGATIVE STEPS make the central wavelength
REDDER, and move lines
toward HIGHER pixel values (on the IRAF display) by approximately
1 PIXEL/STEP.
Cross-Dispersed Mode
Since all of the principal orders (J through K) are on the detector at once
in X-Disp mode, the optimal grating tilt for this mode has been measured in
advance. We do not recommend using a different grating tilt in X-Disp mode
unless there have been serious mechanical changes in the instrument (at
which point the support scientist should be immediately informed of
this).
- X-Disp Grating Tilt: 950
- Prospero Command: XDSPEC
In general, the optimal tilt for X-Disp mode is selected so as to best
center 4th order (H-band) on the array. The adjacent orders (J and K,
respectively), sort themselves out across the detector with minimal losses
off the edges of the field of view (most of these regions are in bad
atmospheric windows, anyway).
See the Cross-Dispersed Mode Order Map for details
of how the orders are arranged across the array and descriptions of some of
the other stuff you might see.
It may be necessary to tilt the grating in order to cover the alternate ends
of the JHK bands in
high-resolution long-slit mode.
The "center" for high-res mode is defined approximately
as the pixel at row 560
(center of the f/7 camera imaging mask) in a cut along column 500 (the
center of the high-res long slit). Since the image of the slit is slightly
curved, the central wavelength of an object will get REDDER (the wavelength at
constant y pixel gets bluer) as you move out from the center to the
end by a little over 4 pixels (8-12Å, depending on the order).
The grating tilt can generally place the wavelength
of interest within ~1 pixel of the center (as defined above).
In general, the rules of thumb for adjusting the grating in high-res mode are
as follows:
- POSITIVE STEPS make the central wavelength
BLUER, and move lines
toward LOWER pixel values (on the IRAF display) by approximately
2.5 PIXELS/STEP
- NEGATIVE STEPS make the central wavelength
REDDER, and move lines
toward HIGHER pixel values (on the IRAF display) by approximately
2.5 PIXELS/STEP
[ Top of Page | Contents ]
Sensitivity
Accurate sensitivity numbers have not yet been measured. Proposers
can make rough estimates
for signal to noise calculations by using the IR spectroscopy
Exposure Calculator.
OSIRIS was designed to work at f/17, hence the collimated beam overfills the
grating at f/14.5 resulting in a loss of
throughput (presently estimated up to factor of ~2). It is hoped that
for many programs, the combination of increased wavelength coverage at higher
spectral resolution (relative to the IRS) and increased
visibility between the OH sky lines for the XD mode
will make up for this.
Grating Stability & Repeatability
Tests show that once a grating tilt has been selected it will be stable to
within 0.05 pixels RMS if not moved. If you are only going to be using one
spectroscopic setting for a night (e.g., only taking cross-dispersed
spectra), it is best to set the grating once and then leave it alone,
unless you have evidence that the grating has been moved.
If you change the grating tilt, even setting it to the same tilt as it was
before, this operation involves some mechanical imprecision in the
repeatability of the encoders. Typically, if you ask the instrument to
reset the grating to its home position and then tilt it to a desired
setting, the grating will be driven to within ~1 pixel RMS in high-res
mode, and ~0.4 pixels RMS in the low-res and X-disp modes. The
non-repeatability in tilt is due to the discreteness of the tilt encoders
(recall that 1 encoder step moves a line by 2.5 pixels in high-res mode,
and 1 pixel in low-res mode). You will probably have to take new standards
and wavelength calibration frames if you move the grating and restore it to
the same position. New flats will probably not be necessary.
There are occasional mechanical glitches when flipping between
spectroscopic and acquisition modes that can introduce approximately 0.3
pixels of shift in the high-res mode, and 0.1 pixels in the low-res
mode. It is not repeatable, and appears as a slight jump in the spectral
lines on close inspection. If you see a "large" jump of 2-3 pixels, you
should reset the flip by typing ACQMODE then SPMODE to "cycle" the
grating/mirror flip mechanism. These glitches are more likely to occur if
the instrument has not yet reached equilibrium after being filled following
warm up, or if it has run out of cryogen and is starting to warm up.
Flexure
To be added.
[ Top of Page | Contents ]
Updated: 1999 February 16
