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3.6 CCD Dewar on Spectrograph Camera 6

Camera 6 may be used with the CCD when resolution higher then ~0.20 Å is needed. The echelle/grism combination on camera 5 may be more advantageous however, due to the higher demagnification.

An image slicer is available for use on camera 6 and may be useful for certain types of observing (see §3.1 for dimensions). Each of the 10 output slices (0.04 mm x 0.50 mm) will have the dimensions given in Table 17.


 

Table 17: Projected image slice dimensions on camera 6


While the slicer may improve the total throughput, the image will be spread up to about 15 times as wide on the chip perpendicular to the dispersion. On-chip summing can be used to decrease the number of rows or columns involved, but ``cosmic ray'' hits will be increased by a factor of 15 also. This may not be serious for short exposures of brighter objects where high S/N or high time resolution is needed. Since typically 2 or 3 spurious events occur per spectrum per hour, 30 - 45 may occur during the same time with the image slicer. Please discuss these matters with your instrument assistant to decide upon a suitable mode of on-chip summing with the image slicer.

Preparations for use of the image slicer are the same as described for the slit below. After alignment is finished, the slicer is carefully inserted in the slots above the slit and the slit opened to at least 500. The camera focus should now be decreased by 2.0 mm (small collimator) from its value with the slit, and checked with a comparison exposure. Note that the acquisition TV front lens will also have to be refocused to compensate for the slicer height.

3.6.1 Installation of CCD dewar on Camera 6

Since the dewar is mounted on camera 6 looking upwards, it must be attached to the dewar mounting plate before being inserted into the camera 6 dewar mounting bracket. The dewar should be attached to the mounting plate with the electronics package pointed opposite the rotation micrometers, as on camera 5. Be sure to use the long, stainless, 10-32 screws to attach the dewar to the mounting plate.

The next step is best accomplished with two people as the combined weight of the dewar and mounting plate is ~75 lbs. The dewar and mounting plate must be rotated so that the dewar is looking up and the electronics package facing south towards the number 6 camera mirror. The mounting plate can now be slid into the guides on the camera 6 mounting bracket and clamped with the 4 large, captive screws in the corners of the mounting plate.

The interface chassis can now be mounted on the stand that sits on the floor grate next to camera 6, oriented to allow the short cables to reach between the dewar and the interface chassis. Always be certain that the interface power supply and controller main switchs (under protective covers on the controller rack) are off before attaching or removing cables from the interface box or dewar. Cable up the interface box and check the voltages as in section 3.5.1. After switching the power off, cable the dewar to the interface chassis.

3.6.2 Alignment of the CCD Camera Head on Camera 6

Alignment and focusing of the CCD on Camera 6 is basically the same as on camera 5, with the added complication of the flat pick-off mirror which deflects the beam downward into the dewar.

  1. Set the flat mirror vertical height to 30.00.

  2. Set the dewar mount vertical height micrometer and camera focus to their nominal values: recent values may be found on a card attached to the camera or in the coude spectrograph CCD logbook. The small flat pick-off mirror should not be tampered with as it has been preset and clamped in place.

  3. The dewar rotational alignment and vertical height can be adjusted as in §3.5.2, steps 2 -- 3.

  4. The final focus can now be determined using a narrow slit (<50 microns), and a comparison lamp suitable for the wavelength region of interest. The focus procedure discussed in §3.5.2, step 4 may be used. A typical focus step size for moving the dewar mounting assembly is 0.20 mm as read on the dial indicator. After running through the focus and selecting the setting that minimizes the value of FWHM, set the camera focus and recheck. Note that grating D will not give line widths 3 pixels (TI CCD) since its theoretical resolution is only 0.02Å. The convolution of this resolution with the dispersion (0.03Å per pixel, red), and camera resolution (~0.04Å) cannot yield an image smaller than ~3.4 pixels (TI CCD). The echelle grating also will not yield line widths less than about 3 pixels (TI) due to the long camera focal length actually resolving the argon and thorium lines. In fact the argon lines may be noticeably broader than the thorium lines due to the smaller mass of the argon atoms.


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