Notes on using the Echelle Grating
NOTES ON USING THE ECHELLE GRATING IN CGS4.
NB. Optimum echelle orders are listed here.
When using the echelle grating there are more details that need to
be checked than with the low resolution gratings. These are summarised
in this note.
1. Pixel and Slit Sizes.
Due to anamorphic demagnification, CGS4's square pixels do not view
a square patch of sky. Although the solid angle seen by a pixel is
constant, the shape is a function of grating angle. The difference
from a square field of view is very small at the low angles of the
first order gratings, but is quite prominent at echelle angles. For
example, at the nominal blaze angle of 64 degrees pixels are more
elongated in the spatial direction than the dispersion direction by a
factor of ~1.6. With the long camera at angles close to optimum the
pixel size is 0.91 arcsec (spatially) X 0.40 arcsec (spectrally). The
nominally 1-pixel wide slit is in fact about 1.3 pixels wide. The
2-pixel wide slit is 2-pixels wide (ie. 2arcsec spectrally).
The change in shape of the pixels as a function of grating angle is
very rapid at large angles. As a consequence, when using the echelle
should always measure the size of the pixels by peaking up in two rows.
Edit your sequence to put in the correct offset between the rows
being used when nodding (see also the comment on slit alignment below).
The slit is slightly curved, so atmospheric and arc lines are not
perfectly aligned on one column of the array. In addition the
axis is also slightly curved, so that the spectrum of a point source is
not perfectly along one row of the detector. Both of these effects are
present with the low resolution gratings, but they are more significant
with the echelle. The curvature of the dispersion direction means that
the dispersion has slight dependance on row on the array. This means
if an arc/atmospheric/astronomical line is straight along a column of
array in the middle of the array, lines at the edge of the array will
be so well aligned. In the dispersion direction the curvature amounts
approx 0.5pixels across the 256 pixels. In the spatial direction the
with the columns due to curvature is about 0.1-0.5 pixels close to the
centre of the array, depending on grating angle. For point sources
along about 30 rows the effects are very small and it is still
to combine the two nodded spectra to cancel sky lines.
There is software available, for example in Figaro, which was
to remove these sorts of distortion for optical spectrographs. There is
information about using these in the CGS4 data reduction notes in
curvature in CGS4 spectra . Using these techniques a residual
of less than 1% can be obtained. However, in conditions of good seeing,
CGS4 data are undersampled in the spatial dimension and this may mean
the effects cannot be fully removed.
The curvature of the slits means that even if the postion angle is
N-S on the sky there will be a small (0.3 - 1 arcsec) E-W offset when
nod along the slit. Since this offset depends on the grating angle and
how far along the slit that you choose to nod, it means that you should
measure it for each echelle wavelength/order that you observe at. After
you have measured the offset between the two rows you need to update
your sequence accordingly.
INFORMATION FOR CREATING CONFIGS
3. Optimum Orders
When using the Echelle it is normal to select ECHELLE_AUTO_ORDER as
grating when defining a config. If this is selected then the CGS4
will automatically select the optimum order for your wavelength. The
order gives a higher throughput than other orders. The optimum
software has recently been improved and now works very well for all
You may wish to use another order, e.g. to get lower or higher
resolution or to increase the spectral coverage. To use a different
than the optimum one selected automatically, choose ECHELLE as the
when defining your config and then explicitely enter the order that you
You can calculate the optimum order for any wavelength as follows.
blaze angle of the echelle is 64.6 degrees. This corresponds to the
of wavelength and order (n x lambda) being about 55.0 microns. (e.g.,
find that the transmission at 2.12um in 26th order (n x lambda = 55.1)
is 10X higher than in 25th order (n x lambda = 53.0) In general for any
wavelength, the order for which n x lambda is closest to 55.0 gives the
highest efficiency. However, at lower orders (longer wavelengths), it
best to fudge this somewhat, as the efficiency drops off more rapidly
higher angles than at lower angles. The CGS4 software uses a lookup up
table calculated according to the above, with appropriate adjustments
the longer wavlengths, to select the order for the echelle.
4. Order sorting
Order sorting for all wavelengths is now achieved with the use of CVFs.
We no longer use narrow band filters for the shorter wavelengths.
5. CVF gradients
There is a slight gradient in the transmission of the CVFs along the
The CVF calibration has been set for the middle of the illuminated
row 134. If you want to use rows towards the edge of the illuminated
or nod more than about 30 pixels you may wish to check the CVF
for the rows you will be using. First of all define an astronomical
for the echelle in the normal way and set to this config. Peakup a star
on the desired row or look at a lamp line and then run MOVIE. Now while
MOVIE is running go into the menu called DIRECT_MOTOR_CONTROL . This
allows you to define an intermediate configuration. The menu items
represent where the CGS4 motors are currently positioned. To check the
CVF calibration try changing the CVF wavelength by a very small amount
and check whether the signal on the MOVIE display increases or
Once you know what wavelength you want to set the CVF to calculate the
difference between the grating wavelength and the desired CVF
You can then use UKIRT_PREP to save an astronomical config with this
offset. Be very careful if you decide to make a change like this - it
possible to get "lost" in order on the CVFs because at 1-2.5um the
orders are very close together. For example 2.2um in 25th order is at
same grating angle as 2.11um in 26th order - so if you move the CVF by
as much as 0.09um you could be looking at the wrong wavelength and
6. CVF fringes
Particularly in the thermal IR a ripple is seen in echelle spectra
is caused by fringing from the CVF. This ripple can be difficult to
if there are amplitude variations between your source and your star. If
you observe very strong ripples, try moving the CVF by a very small
or try puttting your source slightly out of focus. I think the latter
because it makes the source and the background fill the slit to the
degree. Also take oversampled flats in preference to the usual
7. Wavelength Calibration
Because of the narrow wavelength range covered by the array when the
is used, there are wavelengths where it is impossible to find lamp
that fall on the array. In this case you may be able to find lamp lines
at different wavelengths that are present at higher or lower order at
same echelle setting. I.e., for such a line of wavelength lambda there
is an n that gives the same n*lambda as your observing setup, For
you want to observe at 3.00um in 16th order, where there is no line,
notice that there is an Argon line at 2.40um which in 20th order would
appear at the same echelle angle. Such lamp lines can be found by (1)
the echelle to the wavelength you wish to observe (ie in this example
with 16th order selected and (2) changing the arc CVF wavelength to the
wavelength of the calibration line. The arc section of a config allows
you to enter a different CVF wavelength for observing arcs than for
your source. This arc CVF wavelength will only be used when you take
At some other wavelengths you will see lamp lines that were not
- these are strong lines in a different order and wavelength being
through the wings of the CVF transmission profile.
At wavelengths beyond the K window, observable lamp lines are
few and far between. For calibration with the echelle, one often must
the above technique for finding arc lines in different orders, telluric
absorption/emission lines (atlases are available in the control room at
HP, and at JAC), or observations of astronomical line sources.