The Echelle and Peakups etc

There allways seems to be confusion about the Echelle grating and the need to do 2 row peakups etc. Hopefully, this text outlines the definative proceedure.

There are two complications with the Echelle that make this more complex than with the low-res gratings, firstly the curvature of the slit, and secondly, the fact that the pixel size varies significantly with echelle grating angle.

Here's my suggested proceedure, assuming that you're nodding along the slit.:

Initially, set up your ORAC Science Program with the default offsets.

Slew to the target, and ask the TSS to do a 2 row peakup, using the SAVEPEAK and SAVE_PA commands. You will need to tell them which two rows you would like them to peak up on. I suggest 140 and 160, the same as we use for the low res gratings. It will help the TSS if you can tell them the approximate offset in arc-secs between these two rows. To do this, look up the grating angle on one of the VAX terminal screens once you have set to your config, then go to the table in the "Pixel scales" section of this document and estimate the pixel scale for the grating angle that you're at.

The SAVE_PA command the TSS uses on the TCS tells the TCS the correction between the demanded slit angle in your config and the actual angle on the sky of the point joining the position of the star when placed on the slit at the two points intersecting the peakup rows. This means that you can simply ask ORAC to "offset along the slit" and it will know the exact angle to go to hit the slit on your two peakup rows.

Note that if you change to a different slit angle, there should be no need to repeat the two row peakup. Simply do a normal single row peak up on row 140 - the correction to the angle will be exactly the same as before, assuming that you're still offseting the same distance and that you haven't changed the wavelength or order settings of the grating. If these conditions (ie same configuration, same size offset) do not apply, then you will have to repeat the 2 row peakup in your new configuration.

It is advantageous to your signal-to-noise ratio if you have the spectra centred on pixel rows on the array. Peakup also helps you do this. After doing the 2 row peakup, the TSS will be able to give you the exact size of the offset between your two peakup rows. You should go into the offset iterator in your sequence and replace the default offset size (usually 11.74) with this number. Again - there is no need to enter an offset component perpendicualr to the slit unless you really do want to offset perpendicular to, and hence off, the slit. This offset size will apply to all observations in the same CGS4 config, no matter what the slit position angle.