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3.4 Filters and the Shutter

Filters can be used in two places in the spectrograph:

• The upper filter wheel, located above the slit, contains neutral density filters (2.5, 5.0, 7.5 magnitudes of attenuation) and a clear position. If these filters are used for absolute or relative fluxes, they should be independently calibrated using standard stars. The filters are Inconel on a quartz substrate.
• The lower filter slide, located beneath the slit, contains the order separation filters. The filter dimensions are : 1.75 inches diameter and up to 8mm in thickness.

We have two interchangeable slides of order separating filters. Each of these slides has a positive lens in place for knife-edge focusing of the telescope with the rear slit viewing optics. Table 2 contains the current filter combinations available and a list of available filters.

PLEASE NOTE THAT THE CRYSTAL COPPER SULFATE FILTER IS VERY SENSITIVE TO SUDDEN TEMPERATURE CHANGES AND SHOULD NEVER BE TAKEN FROM, SAY, THE COLD DOME INTO THE WARM CONTROL ROOM. The large difference in thermal expansion between the sulfate and the quartz cover plates can cause a shear fracture. Accordingly, the entire collection of order separation filters is kept in the old plate loading room in the dome. When you change bolts/filters please change the marking tape on the spectrograph so the filter positions are identified. Figure 6 is a transmission plot of the crystal CuSO4 filter.

The motorized shutter on the spectrograph is a modification of the original manually operated design and requires a noticeable time to open and close. The shortest exposure time possible with the mechanical shutter is 2 seconds (don't try 1 second - it leaves the shutter open requiring a manual reset). minimum of 20 seconds is recommended if an absolute measurement of better than 1% is desired.