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5.2 Alignment and Focusing of CCD Dewar

The steps involved in preparing the echelle-CCD system for observing are:

  1. Selection of wavelength region and resolution.
  2. Selection of gratings and camera.
  3. Gratings/wavelength adjustment.
  4. Rotation of dewar to align columns (rows) parallel to spectra (or slit).
  5. Focusing of dewar to bring surface of CCD chip coincident with camera focus.
This procedure is not necessarily a one pass operation; rough focus may have to be achieved before rotational alignment or vice versa. The wavelength centering may have to be done as a final step, but it should be kept in mind that the rotational alignment varies from order to order. Thus when aligning orders parallel to rows or columns, one needs to be fairly well centered on the desired wavelength region before doing the final rotational alignment. Alignment of the slit parallel to rows or columns will not be affected by wavelength adjustments.

1. Selection of Wavelength Region and Resolution.
The needs of the science to be done tempered by the faintness of the objects determines the wavelength and resolution selection. Another factor is wavelength coverage. These decisions should be made at proposal time.
1. Selection of Gratings and Camera.
These decisions are driven by item 1.Table 4 may be of use to help determine the most appropriate optics. Prospective users are also encouraged to consult KPNO staff. These decisions should also be made at proposal time.
3. Gratings/Wavelength Adjustment.
Approximate encoder values for setting the gratings versus wavelength may be found in table 4 and figure 8. Spectra of the day sky in conjunction with a solar atlas may also be useful for wavelength positioning, especially in the far red where the Th-Ar is not as useful.
4. Rotational Alignment of the Dewar.
If sky subtraction is not anticipated, the orders can be aligned parallel to the chip columns (or rows). This alignment will not be constant with order number due to the effect of approximately constant cross dispersion on the more rapid variation of dispersion between successive echelle orders.

The rotational alignment is determined by obtaining quartz lamp spectra and imploting one end of the spectrum, expanding around a few orders, and overplotting the other end of the spectrum. The cursor can be used to measure the profile positions (use the C key). On the long focus camera CCD mount, the four dewar rotation clamp screws must be loosened before adjustments are made. With the UV camera, the four small rotation lock screws on the front collar must be loosened to allow rotation. Coarse rotations are made by loosening the large, knurled clamp on the back of the long focus mounting bracket, or the 3 screws on the back of the UV camera mount, and rotating the dewar using the gold electronics box for leverage. ( Do not exert large torques on this box!) Assuming the orders are parallel to columns (vertical), and if the upper end of the spectrum is shifted positively with respect to the lower end, then the following directions of motion apply:

When the orders are parallel to rows, the directions are opposite those given above. If sky subtraction is important, the CCD should be rotated so that comparison lines are parallel to rows or columns to insure the same wavelength for the telluric features along the whole entrance aperture. In this case, a long slit is used (use long decker) and cuts are made across each end of a comparison line to determine the rotational misalignment as above. The correction factor will have to be scaled to the full chip format.
5. Focusing the CCD Dewar.
1. Long focus camera. Since the focal plane of the camera is fixed with the collimator at its proper position, the dewar must be adjusted so that the surface of the CCD chip coincides with the camera focal plane.

With the the rail micrometer set at about 5.00 mm, the dewar mounting bracket should be clamped at its nominal position which has the dewar mounting surface at 45.6 mm from the front of the camera. Now with the lower clamp bolt nuts backed off and the focus rail clamp released, successive spectra of the Th-Ar lamp can be obtained while decreasing the micrometer position by 0.25 mm until a rough focus is achieved. The final focusing should be done using smaller micrometer decrements of 0.05 or 0.10 mm. A FWHM of about 1.2 - 2.0 pixels should be obtained using a narrow slit and the TI CCD. Finally the lower clamp bolt nuts should be brought up firmly against each side of the mounting bracket, using care not to tighten one nut more than the other. The focus rail clamp is also now applied.

2. UV camera. In this case the dewar is fixed and the camera itself is moved in or out with respect to the CCD. This is accomplished by turning the large castellated nut on the front of the camera. A pair of handle-mounted rods are kept in the UV camera storage box to help in rotating the focus nut. These rods are inserted into the holes in the nut for ease in rotation. If the spacing between the focus nut and the camera body has been set to the nominal value of 1.312 inch (TI CCD), the focus should be within a few thousandths. The change in focus between each raised section of the focus nut is 0.025 inch.

The UV camera may also be focused with the transducer attached to the camera as a method of reading out the focus position and setting it initially. With the Fluke meter, read and set the camera focus focus to the values given below making the final setting towards more positive volts. A typical focus interval might be 0.1 volt and the camera motion is 0.311 mmper volt. The total transducer range is +14 to -14 volts with +14 volts bringing the camera closest to the dewar window.

Table 10: Transducer Focus Voltages for UV Camera

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Next: Observing with the Echelle-CCD System
Previous: Dewar Design and Maintenance
Fri June 9 08:00:00 MST 1995