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3.5 CCD Camera Dewar on Spectrograph Camera 5

Camera 5 is perhaps the most effective and versatile location for the CCD dewar on the coudé spectrograph, as resolutions from 1.8 - 0.04 Å are possible. The large demagnification (6.3, small collimator; 9.4, large collimator) enables the use of a wider slit and yields narrower spectra on the chip than camera 6. This vertical demagnification is important since the fewer columns the spectrum falls on, the less likely the chance for a cosmic ray hit and the lower the final readout noise per resolution element. If on-chip summing is used, then the later limitation need not apply. Table 5 and table 11 give the various dispersions and resolutions obtainable with camera 5.

3.5.1 CCD Installation on Camera 5

The installation of the dewar is best accomplished with two people as the dewar is moderately heavy (~45 lbs.). The dewar may be mounted to the removable mounting plate either with the plate on the camera bracket or off. In either case the screw-on shutter cap must be removed. The use of CCD dewars other than TI5 will require removal of the shutter assembly and installation of a special modified shutter mount. This allows unvignetted use of large CCDs, but requires a normal shutter be mounted just below the slit to control integrations. The orientation of the dewar on the mounting plate is with the electronics box up (spectrum parallel to rows, F3KB). If it is desired to have the spectrum parallel to rows with the TI5 CCD, the dewer mount rotation ring must be rotated counter-clockwise until it hits the other stop before mounting the dewar. Secure the dewar with the long stainless, 10-32 screws provided in the plastic bag always kept with the dewar shipping box. If the mounting plate is removed from the bracket, it must next be mounted back on the bracket and secured with the large captive screw on each corner.

The interface chassis should be mounted on its stand behind the dewar. Always make sure that the interface power supply and controller main switches (under protective covers on controller rack) are off before attaching or removing cables from the interface box or dewar. Cable up the interface box, but not the dewar. Turn on the interface power supply and check all of the voltages displayed on the interface chassis. If they do not agree to within a few tenths of a volt to the nominal values, electronics support personnel should be called to make adjustments. Cable up the dewar to the interface chassis. Again, be sure that all of the voltages are proper before starting to cool the dewar. Section 3.3 may be consulted for instructions on filling the dewar with LN2.

3.5.2 Alignment of CCD Camera Dewar on Camera 5

Once the CCD dewar has been installed, the following procedures are used to insure proper alignment, rotation, and focus of the spectrum on the chip.

  1. Set nominal height and focus. Set the dewar mount vertical height and camera focus adjustments to their nominal values: Approximate values may be found on a card on the camera or check the observing log for recent values. The vertical height clamp must be released before setting. This is a large socket head bolt just below the vertical height readout.

  2. Check vertical height. Using a small stellar decker and the quartz lamp, take a spectrum using the command tests to check the vertical height of the dewar. The position of the spectrum on the chip can be determined by using the implot task to plot a cross-section of the spectrum. The difference between the spectrum position and the desired postion can be noted and the dewer moved vertically with the aid of the vertical height dial indicator (1 division = 0.01mm). When the spectrum is oriented along rows the height adjustment works as follows: For F3KB and TI5, if the spectrum is too low on the chip, the dewar should be raised (increasing numbers on dial indicator). The best areas cosmetically might be determined from the CCD Characteristics Manual kept at each telescope. When using dewars other than these, consult with your instrument assistant for proper placement of the spectrum on the CCD.

  3. Adjust dewar rotation. Once the spectrum is falling more or less in the desired region of the chip, it may be useful to reformat the CCD to be a narrow strip of about 50 - 100 rows or columns centered on the spectrum. This will reduce disk usage and readout time. Even with the two-dimensional echelle format it may be possible to reduce the width of the chip perpendicular to the dispersion somewhat. Type detpars to edit the readout parameters.

    The rotational alignment may be checked by sploting one end of the spectrum, expanding around the profile, and using the ``k" key on each side of the profile baseline to measure the position. Then overplot the other end of the spectrum and measure again. If the alignment is off by more than few tenths of a pixel, the dewar can be rotated using the rotation micrometer on the dewar mounting bracket.

    First, loosen the rotation clamping screws ~1/2 turn. Assuming the order(s) are parallel to rows, if the upper end of the spectrum is shifted positively with respect to the lower end, then the dewar must be rotated with the rotation micrometer:

    F3KB:
    counter-clockwise 0.05 mm/pixel
    TI5:
    clockwise 0.18 mm/pixel

    1 mm on micrometer = 2 revolutions. Note that if the micrometer is turned to larger values, the dewar will have to be manually rotated to bring the rotation stop against the micrometer.

  4. Adjust focus. The last step in preparing the spectrograph for observing is the final focus. Using a narrow slit ( 100 microns, camera 5; 50 microns, camera 6), the same decker used for the rotational alignment, and a comparison lamp suitable for the wavelength region of interest, obtain a series of spectra which bracket the position of best focus. A typical focus step size for moving the number 5 camera mirror (sphere) is 0.05 mm as read on the dial indicator. The comparison spectra may be plotted using the splot task with the line or column number given from the display window. If the spectrum is parallel to columns, the plot can be obtained by specifying the dispaxis parameter as ``2'' in the kpnocoude or echelle package parameters. Now one can expand around suitable lines and use the ``k'' key to fit gaussians to the line profile to obtain the FWHM. More than one comparison line should be used since the undersampling may cause some fluctuation in apparent line width.

    Another useful tool for focusing is the SPECFOCUS task. EPAR SPECFOCUS and set the number of spectral regions desired and the upper and lower slit boundaries. Now type SPECFOCUS image name and plots will be generated showing the FWHM of the autocorrelation profiles for the regions designated. The task can also be run on a series of focus images. Some skewing of the results might occur if strong cosmic rays or hot columns are present.

    The spectrograph camera focus is somewhat sensitive to wavelength; observers working in the red and blue may wish to check the focus at different spectral regions. It has been found that with the red corrector, the focus at 4500Å is about 0.12 mm higher than the focus value at 6500Å. A focus change may also be required if filters of different thickness are used at different wavelengths. If the copper sulfate filter is used for example, the camera focus will shift approximately -0.25 mm, compared to the focus with one of the 2 mm thick order separation filters.

    After running through the focus and selecting the setting that minimizes the value of FWHM, set the camera focus and recheck. The focus setting and FWHM values should be recorded by the observer for reference as subsequent plots will erase the screen. The FWHM of a comparison line should be 2.0 - 3.0 pixels for the Ford CCD and 1.5 - 2.0 pixels for the TI CCD. The Ford chip will yield narrower line profiles in the red than the blue.


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Next: 3.6 CCD Dewar on Spectrograph Camera 6
Previous: 3.4 CCD Control System