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Aligning echelle grisms, slits and cross dispersers

As the Echelle orders appear very curved on the chip, and as the Echelle slits are very short, there is no real reason for aligning any of these.

However, it is important to define the center of the slitlet in the slitpos.def file.

This procedure is used for aligning slits, grisms and grisms acting as cross dispersers when these are to be used for echelle spectroscopy. It requires the ALFOSC user interface (UIF) and BIAS to be running on sandra and ALFOSC to be mounted on the telescope. All the described BIAS batch files are in: sandra:~obs1/alfosc/

Notice, that if the ALFOSC camera has been seperated from the instrument, you re-mount it and position it according to two corresponding marks. Hereafter the instrument is aligned after the camera.

When aligning a slit, grism or cross disperser, the first to do is to obtain an image in which the currrent state of alignment can be determined: Aligning echelle slits


Aligning echelle slits

This is a temporary procedure only and will improve

  1. In the UIF, select the slit to be aligned.
  2. In the UIF, select the B filter to reduce the light.
  3. Make sure that all other positions are free or open.
  4. Move the calibration lamp mirror to the IN position.
  5. Turn on the Halogen calibration lamp.
  6. In BIAS, type run ec_salign.run.
  7. The addition in steps to the current wheel position you will have to guess at. If the deviation from a vertical slit image is barely visible, try adjusting with some 100 steps in one direction.
  8. Edit the setup file in question to reflect the new position using the GUI editor. For unforeseen cases, it is possible to manually edit the setup file . This is described in the document called How to Edit The Definition Files
  9. Repeat this procedure untill the slit is perfectly vertical.

After aligning slits, The pixel X and Y positions must be measured by making a slit image with the CCD camera. The CCD positions of the different slits are written in the slitpos.def files (see How to Edit The Definition Files ) Aligning echelle grisms


Aligning echelle grisms

  1. Turn off all phosporus lamps in dome - they have strong emission lines.
  2. In the UIF, select the grism to be aligned.
  3. In the UIF, select the pinhole from the aperture wheel.
  4. Make sure that all other positions are free or open.
  5. Move the calibration lamp mirror to the IN position.
  6. Turn on the Halogen calibration lamp.
  7. In BIAS, type run ec_galign.run.
  8. The image should show a perfect horizontal line. If not you will have to add to the current wheel position: To turn the tilted line clockwise, increase the step count. To turn the tilt counterclockwise, decrease the stepcount.
  9. A difference of 10 pixels from one end of the image to the other corresponds roughly to 200 step counts.
  10. Edit the setup file in question to reflect the new position using the GUI editor.
  11. Repeat this procedure untill the image shows a perfectly horizontal line.

Aligning cross dispersers


Aligning cross dispersers

  1. Turn off all phosporus lamps in dome - the have strong emission lines.
  2. In the UIF, select the cross disperser (grism placed in the filter wheel) to be aligned.
  3. In the UIF, select the pinhole from the aperture wheel.
  4. Make sure that all other positions are free or open.
  5. Move the calibration lamp mirror to the IN position.
  6. Turn on the Halogen calibration lamp.
  7. In BIAS, type run ec_cdalign.run.
  8. The image should show a perfect vertical line. If not you will have to add to the current wheel position: To turn the tilted line clockwise, decrease the step count. To turn the tilt counterclockwise, increase the stepcount.
  9. A difference of 10 pixels from top to bottom of the image corresponds roughly to 200 step counts.
  10. Edit the setup file in question to reflect the new position using the GUI editor.
  11. Repeat this procedure untill the image shows a perfectly vertical line.

In echelle spectroscopy much of the CCD will likely have no spectra or will have overlapping orders and it is thus desirable to window the CCD. With the current readout direction this even means shorter readout time. For the combination grism #9 + CD#11, there is a window defined in ~/alfosc/spec_ec_9_11.run on sandra.

Document initiated by Thomas Dall, 15. September 2000