Preparing a spectrograph setup

Mounting and focusing spectrum cameras on ports A and B

1. Choose your camera. Megaplus 1.6 is probably the best choice because of its higher frame rate. If a longer spatial or spectral coverage is really needed, a 4.2 could be used. Note that these cameras show surprisingly different fringing properties. Notably bad are the 1.6 cameras XII, XIII, and XIV which show a strong cross-like pattern, see Fig. 4. Avoid these cameras.

   Figure 4: Bad fringes from camera XIV (June 2004).

   
   

2. Select your filter and mount it in front of the camera. Sometimes putting tension on the filter by taping it or tilting it to a certain angle may cause fringing.

3. Place the camera on a mount and slide it onto the track. Start it in focus mode.

4. Turn the grating to the approximate wavelength of your target line. For most angles, you will see no spectrum because the filter is blocking the light.

5. Focus the camera roughly by sliding it along the track while looking at the spectral lines. By now it is most convenient to have the computer monitor that is connected to the switch moved to a place where it can be seen from besides the spectrograph.

6. Adjust the height and the tilt of the camera. Illuminate the slit with sunlight. Carefully slide the focusing device (found in a marked container on the slit box - the device is shown in Fig. 5) over these rods and make sure that it touches the slit plate. The focusing device has an air slit of 100 $\mu$m width. Thus the spectrum camera will show a very narrow spectrum. Adjust the height of the spectrum camera to get that centred. Use shims, etc, as for an imaging setup. Adjust the tilt at the same time so that the dispersion is as parallel as possible with the CCD rows. Be careful to do this adjustment with the camera close to focus. The outgoing beam has an upward tilt and the mirrors are not perfectly mounted so the lightbeam is not exactly parallel to the track.

7. The spectrum camera is focused using a method similar to that used for focusing imaging camera in that it involves finding the focus position as the mean of two well-defined out-of-focus positions. Instead of measuring intensities, eye estimates of sharpness are used. Instead of moving the camera between two positions straddling the focus, the Littrow lens is moved between to preset positions.

   Figure 5: Both sides of the focusing device.

   
   

   The slit should be illuminated. Around the slit there are three rods. Carefully slide the focusing device in place and make sure that it touches the slit plate and that it does not get stuck on the rods. Focus on the narrow spectrum by sliding the camera along the track which should have a ruler taped to it. For each position the Littrow lens is moved between its '+' and '-' positions. note which of the two positions that give the sharpest spectrum edges. Use the Zoom display option and set the contrast ``high'' limit to about 20 counts. The spectrum will then look wider in the vertical direction when out of foucs. When the focus is reached, the '+' and the '-' positions give equally sharp spectral edges (not spectral lines). With the settings recommended here it i easy to look for the spectrum being equally wide at the '+' and '-' positions. The method is surprisingly fast and efficient and allows the focus to be determined with 1/2 mm precision.

   Do not forget to bring the Littrow lens back to its focus position by pressing the Focus button on the spectrograph GUI! The button will turn green.

   Being two observers, one in the spectrograph room and one in the observing room, with an intercom connection makes focusing somewhat faster.

   Note that is not possible to focus on the slit-jaw edges! They are situated several mm behind the slit.

8. Once the camera is securely fastened to its track, it should be straylight protected by building a 'house' of black paper over it.

9. If needed, the slit should be turned to have the spectral lines close to vertical on the CCD (they are slightly curved so this cannot be exact): Loosen the screws of the clamps holding the slit plate. Then turn the screw that rotates the slit until it reaches the desired orientation.

10. Adjust the slit jaws. There should be no more light than necessary being let into the spectrograph. It may be useful to shade the upper and/or lower part of the spectrum CCD to have some control on straylight levels.

11. The camera is in place and everything can now be controlled via computer. Finetune the grating. Then turn off the Newport control unit.

If a new wavelength is chosen, the camera must be refocused.

Mounting and focusing a spectrum camera on port C

The C port on top of the spectrograph has a custom-made camera holder. This only allows the small Megaplus II cameras. Once the camera is fastened to the holder, the filters can be switch by removing the plate beneath the camera (fastened by two screws). Withdraw the plate and put the filter in place, then put the plate back.

NOW: The Megaplus II cameras have a dark level problem. To reduce the impact of this, loosen the filterplate and slide in a strip of shim plastic so that the field of view of the camera is reduced slightly on one edge. The edge to be covered is the one opposite to the side where the cable contact in the camera house is situated. When mounted on port C this side is the one closest to the wall.

Adjust the camera holder so that the dispersion direction is horizontal, etc. Then fasten it with four screws.

The focusing is then easy because there is a screw to move the camera vertically and a scale to read its position.

Focusing the telescope on the spectrum camera

This procedure is equal to cofocusing the wavefront sensor with the slit position. This is usually done once when the spectrograph has been set up. It is not necessary otherwise unless there has been some changes of the setup that affects the light path to the wavefront sensor or the wavefront sensor itself has been moved. Follow the first part of this procedure to check the focus, but do not start moving the wavefront sensor until having consulted the SST staff.

1. Do the normal wavefront sensor and control matrix calibration for the AO system. Then put in the clear-glass ND filters used when focusing the CCD cameras (if they are not already there).

2. By adjusting X-offset voltages, move the 40 $\mu$m (AO-) pinhole exactly to the middle of the slit sideways (vertical position is irrelevant).

3. Enable (unclick "disable") tip-tilt correction on the AO GUI such that the pinhole will not move on the slit when closing the loop.

4. Close the AO loop on the pinhole. (The pinhole will of course not be exactly at the centre of the subimages.) Repeat the previous steps if necessary in order to keep the pinhole very well centered on the slit.

5. With the AO loop closed, check the focus of the pinhole on the SPECTRUM camera in the vertical direction. Again use the "- Focus" and "+ Focus" positions. When the height of the spectrum is equally large in these two positions, then the wavefront sensor is in the right position. If this is the case, you do not have to do anything. (This is more difficult than the spectrum-camera focusing procedure.)

6. If the wavefront sensor is not in the right position, consult the staff first. If given a go for refocusing, you have to move the wavefront sensor in order to improve focus. Open the AO loop, move the wavefront sensor a few mm in one direction, close the AO loop again and see if the focus of the spectrum camera improved or became worse (such that you know in what direction to move depending on which of the two "- Focus" and "+ Focus" positions look best. This is because we did not yet have the possibility to check which way the wavefront sensor should be moved when e.g the "+ Focus" position gives the sharpest image). Repeat the procedure with smaller movements until you feel confident that the wavefront sensor is within 0.5 mm of the correct focus.

7. Check the wavefront-sensor optical alignment (also pupil illumination), adjust if necessary, repeat the AO calibration and lock up again on the pinhole. Verify that the focus on the pinhole is still OK. If not, adjust.

Chromatic effects

The reimaging triplet (on the AO platform) has a focus curve that is slightly wavelength dependent. This is a problem for spectroscopy because here we need to have the image formed on the slit. In imaging every camera is focused individually. Thus a slit-WFS cofocus found at one wavelength will not be exactly valid at another wavelength. Figure 6 shows a plot of the theoretical focus curve.

Figure 6: Theoretical focus curve of the reimaging triplet lens.

Problems can be expected when co-observing lines in the blue and in the red, and when going into the IR.

Slit-jaw camera

Figure 7: A slit-jaw setup with two cameras. In this setup, the reflected beam is split by a cube just behind the 240-mm reimaging lens (in the black holder).

The slit-jaw camera is used to check the position of the slit. It should have a filter close in wavelength to the spectrum wavelength - one reason for this is the differential atmospheric dispersion. One slit-jaw approach is to run the camera at the same exposure time as the spectrum camera in order to give exactly the same view. It must then be precisely synched with the spectrum camera. Alternatively the exposure time can be short for optimal image quality and possibly image reconstruction from several exposures. Then the stored timing of the exposures has to be used to approximately pair spectrum frames with slit-jaw images.

We use a 240-mm Rodenstock lens for imaging the slit. Typically one wants to demagnify the image somewhat which means that the distance from the slit to the lens should be slightly more than twice the focal length. Make sure that the slit-jaw view covers all of the spatial view of the spectrum camera. This lens is not perfectly centred in its holder. Experts may want to take this into account in the setup.

The slitplate can be rotated around a vertical axis which coincides with the slit. Use this to direct the beam. Loosen the two protruding screws on both sides of the slit holder. Then just turn the plate and fasten the screws.

The smile distortion causes the spectral lines in the cameras to be tilted (and curved) relative to the vertical. This causes the slit to deviate from the vertical when it is rotated to get lines and dispersion as orthogonal as possible in the spectrum cameras. It does not matter very much for the spectra, but anyone who wants the slit to be nicely vertical in the slit-jaw camera field of view will have to tilt that camera.

Choose a filter with a CWL close to the wavelength of the spectrum. If the same exposure time is to be used, a lot of neutral blocking is needed (unless the filter is very narrow). Set the slit-jaw camera to the lowest available gain to minimise the blocking requirement. It is convenient to have rotating polaroids so that exposure levels can be finetuned with the spectrum camera. But ND filters and polaroids introduce reflections and straylight which can be quite problematic.

Otherwise follow the instructions for mounting and focusing imaging cameras. This means that the camera should be focused using a pinhole while the AO is running. Do not focus on the slit. The slit will be sharp if the slit-jaw wavelength has a focus that is close to that of the wavelength where slit-WFS cofocusing was made (see Fig.6). Otherwise the slit may be noticably out of focus. Note that if the setup is demagnifying, the sharpness depth will be decreased and the situation may look worse than it really is. Still the aim must be to get the sharpest possible images of the sun, not of the slit.

Figure 7 shows an example of a slit-jaw setup.

Simultaneous observations of two wavelengths

TRIPPEL allows observations of three different wavelengths simultaneously. The grating rotation sets what wavelengths are possible to observe in the different ports. Obviously not all combinations are possible. There is some stretch allowed in that the cameras can be moved sideways.

There are also other problems:

• Currently a master camera can only have one slave. Thus only two cameras can be synched. This is a definite problem when scanning, because only one camera communicates with the tracker. The solution is to run the third and fourth cameras with external trigger enable. This will synchronise exposures and if all cameras run save all mode this will ensure that frame selection does not mix images up. Refer to the camera manual for more information.

• Differential refraction causes the slit to sample different parts on the sun at different wavelengths. The SST can in principle correct for this by tilting its field mirror, but there is not yet a system for doing this.

• Some wavelength combinations will suffer from longitudinal colour in the reimaging lens that causes different focus positions.

Parallel imaging

The observer is free to do anything with the slit-jaw beam, but that will always show the slit (and dust specks that can be hard to remove). There is a possibility of putting a dichroic beamsplitter in front of the beam to give access to blue light. It is in principle possible to put a cube just before the WFS. This requires refocusing of the WFS and changing its filter combinations, however.

In principle, though, all available photons should go to the spectrograph.