Setting up the DBS ready for observations

Important Notes:
The DBS must be at particular park positions before removing or changing the grating, dichroic etc. If the telescope control software is already running, this may be done using the ROTATOR command. More commonly you will use the rotator handset located in the dome next to the DBS.

Changing Gratings

 When removing gratings, the spectrograph must be in the horizontal position. (See rotator handset controls.)

A suite of nine interchangeable gratings plus two mirrors in grating cells is stored in a cabinet on the accessories trolley. Hatches to access the currently installed gratings are found on the ends of the DBS. i.e., facing the front and back of the dome building.
  • To access the grating, the grating angle setting must first be moved to `Change grating'. The grating angle control dials are located on top of and beneath the DBS and can be awkward to reach. If so, use the control handset to rotate the spectrograph into a diagonal position. Release the locking nut and rotate the large setting wheel to `Change grating'. Re-secure the locking nut. Return the DBS to the horizontal park position.
  • Gratings are removed by first firmly attaching a protective cover which also incorporates a carrying handle. Take an unused grating cover from the storage cabinet located on the accessories trolley and blow it free of dust with the air gun that hangs on the near end of the accessories trolley.
  • Open the grating access door. Two thumb screws hold the grating hatches in place and may be undone by hand. If an alarm sounds close the door and correctly park the spectrograph in the horizontal park position.
  • Insert the grating cover into the slot in the grating cell and attach the grating cover handle to the grating cell with the two captive thumb-screws.
  • Pull the grating release lever. This lever actuates an over centre lock and the lever will rotate through 90° and protrude through the access door aperture when fully open.

    Do not under any circumstances pull the grating release lever if the spectrograph is not in the horizontal park position as the unprotected gratings can slide out of the spectrograph and fall to the floor.

  • Carefully withdraw the grating cell by pulling on the grating cover handle, ramps will guide the cell sideways as it is withdrawn, support the cell with both hands as it clears the spectrograph body.
  • Replace the cell in the storage cabinet and select the next grating.
  • Insert the next grating with the dovetail claws of the cell facing towards the telescope. Ramps will guide the cell sideways to engage the dovetail claws. Push firmly on the cell handle and while maintaining this force close the grating lock lever. Pull gently on the cell handle to ensure that the cell is locked. Then undo the two thumb-screws, withdraw the grating cover and close the grating access door.
  • Store the grating cover in the storage cabinet and if necessary change the grating in the other arm of the spectrograph. Return the spectrograph to the diagonal position and set the grating angles. Very approximate grating angles are engraved on the setting dial.

    Changing Dichroics

     The properties of the dichroics are described in the DBS capabilities section.

    When accessing the beam-splitter, the spectrograph must be in the vertical position. (See rotator handset controls.)

  • At the spectrograph, unscrew all five grey lock levers on the beam-splitter door about two turns and rotate all five black knobs by one quarter of a turn until the they are in the open position. Using the black handle on the door gently swing it open. Use care as the beam-splitter is cantilevered from the door's inside face.
  • Push the door gently against the cable boom until a latch fixed to the cable boom locks the door in the open position.
  • There are three handles protruding from the beam-splitter unit. To change the dichroic, locate the dichroic handle and release the four thumb screws securing the dichroic cell, grip the dichroic handle and gently slide the dichroic cell out of the beam-splitter unit. Take careful note of the dichroic cell's orientation so you know which way round to insert the replacement! Do not touch the dichroic or bump or shake the cell as the glass is only lightly constrained. The four thumb-screws are captive in the cell.
  • Immediately store the cell in the appropriate box in the storage drawer on the accessories trolley.
  • Select the next dichroic and insert it into the beam-splitter. The handle of the dichroic cell should be downward when facing the beam-splitter. Ensure that the four thumb-screws securing the cell are properly positioned or they will foul the slithead turret.

    If the beam-splitter door is to be left open for some time, cover the beam-splitter with the black velvet bag that hangs on the near end of the accessories trolley.

    In addition, a multilayer coated mirror and multilayer coated window are available for use when using the spectrograph as a single armed red or blue instrument.

    Setting the Central Wavelength / Grating Angle

     The grating angle setting assembly consists of three dials. The small locking knob should be released before moving the grating and re-set once the movement is complete. There is a large dial which gives the approximate grating angle, and an engraved scale down to the arc-minute level on the smaller dial with a handle. The blaze angle settings are engraved on the setting wheel. To observe away from the blaze, angles may be read off this plot. They give a good starting point. Take an arc and tweak the angle as necessary.

    Alternatively there is an accurate online grating angle calculator.

    Collimator Focus
  • Before moving the focus setting dial, you must first release the small adjacent locking knob. Gently, but securely, re-set the locking knob into place once the focus is set.
  • The easiest way to focus is take an arc exposure and use the tablet menu to make a horizontal cut through the image. Then you have to shuttle up and down stairs, moving the focus, taking an exposure, making a plot and back down to the dome to try a new focus. It's in focus when you have the maximum counts in the arc lines and they are at their narrowest.
  • Note that changing the focus will move the image on the detector in the spatial direction. Bear this is mind when focussing and choosing the read-out window.
  • The focus should be around about 0mm, but frequently is not. It seems to drift off over time. I have had it vary by ±50mm from observing run to observing run. A minimum change of about 3mm is required to have any noticeable effect on the spectra. Taking a sequence of exposures, stepping the focus by 5mm at a time seems to be adequately fine to get a good focus. Larger steps may be necessary initially to find a rough focus.
  • Non-negligible astigmatism (principally from the spherical collimator) causes the best spectral and spatial foci to differ. For the blue arm, they are separated by 8mm and the red arm, by 12mm. The extra astigmatism in the red arm is from the beam-splitter. For some applications (e.g., aperture plates or slitless spectroscopy), it may be preferable to use a compromise focus between the two optimal limits.
    Last modified: Tue May 30 13:11:59 EST 2000