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| Joint Astronomy Centre |
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NOTES ON USING THE ECHELLE GRATING IN CGS4.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 you 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). 2. CurvatureThe slit is slightly curved, so atmospheric and arc lines are not quite perfectly aligned on one column of the array. In addition the dispersion 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 that if an arc/atmospheric/astronomical line is straight along a column of the array in the middle of the array, lines at the edge of the array will not be so well aligned. In the dispersion direction the curvature amounts to approx 0.5pixels across the 256 pixels. In the spatial direction the misaligment 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 nodded along about 30 rows the effects are very small and it is still reasonable to combine the two nodded spectra to cancel sky lines. There is software available, for example in Figaro, which was designed to remove these sorts of distortion for optical spectrographs. There is information about using these in the CGS4 data reduction notes in correcting curvature in CGS4 spectra . Using these techniques a residual distortion 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 that the effects cannot be fully removed. Slit alignmentThe curvature of the slits means that even if the postion angle is nominally N-S on the sky there will be a small (0.3 - 1 arcsec) E-W offset when you 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 CONFIGS3. Optimum OrdersWhen using the Echelle it is normal to select ECHELLE_AUTO_ORDER as the grating when defining a config. If this is selected then the CGS4 software will automatically select the optimum order for your wavelength. The optimum order gives a higher throughput than other orders. The optimum ordering software has recently been improved and now works very well for all wavelenghts. You may wish to use another order, e.g. to get lower or higher spectral resolution or to increase the spectral coverage. To use a different order than the optimum one selected automatically, choose ECHELLE as the grating when defining your config and then explicitely enter the order that you want. You can calculate the optimum order for any wavelength as follows. The blaze angle of the echelle is 64.6 degrees. This corresponds to the product of wavelength and order (n x lambda) being about 55.0 microns. (e.g., we 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 is best to fudge this somewhat, as the efficiency drops off more rapidly at higher angles than at lower angles. The CGS4 software uses a lookup up table calculated according to the above, with appropriate adjustments at the longer wavlengths, to select the order for the echelle. 4. Order sortingOrder 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 gradientsThere is a slight gradient in the transmission of the CVFs along the slit. The CVF calibration has been set for the middle of the illuminated area, row 134. If you want to use rows towards the edge of the illuminated region or nod more than about 30 pixels you may wish to check the CVF calibration for the rows you will be using. First of all define an astronomical config 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 menu allows you to define an intermediate configuration. The menu items diplayed 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 decreases. Once you know what wavelength you want to set the CVF to calculate the difference between the grating wavelength and the desired CVF wavelength. You can then use UKIRT_PREP to save an astronomical config with this CVF offset. Be very careful if you decide to make a change like this - it is possible to get "lost" in order on the CVFs because at 1-2.5um the Echelle orders are very close together. For example 2.2um in 25th order is at the 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 order.6. CVF fringesParticularly in the thermal IR a ripple is seen in echelle spectra which is caused by fringing from the CVF. This ripple can be difficult to remove 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 amount, or try puttting your source slightly out of focus. I think the latter helps because it makes the source and the background fill the slit to the same degree. Also take oversampled flats in preference to the usual undersampled one.7. Wavelength CalibrationBecause of the narrow wavelength range covered by the array when the echelle is used, there are wavelengths where it is impossible to find lamp lines 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 the 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 example, you want to observe at 3.00um in 16th order, where there is no line, but 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) setting the echelle to the wavelength you wish to observe (ie in this example 3.00um) 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 observing your source. This arc CVF wavelength will only be used when you take arcs.At some other wavelengths you will see lamp lines that were not expected - these are strong lines in a different order and wavelength being transmitted through the wings of the CVF transmission profile. At wavelengths beyond the K window, observable lamp lines are generally few and far between. For calibration with the echelle, one often must use 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. |