ISIS Setup

• Overview
• Starting the observing system
• Physical setup: dichroic, dekker and gratings
• CCD rotation
• CCD tilt
• CCD focus
• Order sorting filters
• Re-focusing when changing dichroics or gratings
• Setup for spectropolarimetry
• Setup for imaging polarimetry
• Setup for fast and faint-object spectroscopy

Roughly one month before the observations take place you should have read the observer's proposal which is located at /home/ingpatt. The proposal should contain all the details of the required configuration (CCDs, dichroic, gratings, order sorting filters and central wavelengths). If this is not the case you should request a detailed instrument setup in the email that you send to the observer. You should be able to advice on a use of an order sorting filter (more details here). You should also check that the required configuration is met by the engineering schedules. If this is not the case contact the WHT manager. A detailed procedure how to start and shut down the observing system can be found here. At the control room check that no one is moving the telescope, the telescope power is off and the telescope is locked. You can now proceed to change the dichroic and dekker (note that even if the dichroic or dekker unit you need is the one on the mimic you should check that the dichroic and dekker units are physically in).

To make the change pull the dichroic out of the beam for easy access:

SYS@taurus> bfold 0

Then move the dekker out:

SYS@taurus> dekker 1

Then unlock the slit door:

SYS@taurus> slit_door open

You can now open the slit door located on top of the red cryostat. Inside you will have access to the dichroic, filters and dekker. Remove the filter slide (RFILTA) to protect filters from being accidentally damaged. Pull the pin that holds the dichroic and carefully slide the dichroic out. Put the dichroic back in its sealed box at the bottom of the WHT observing floor cabinet. Take the new dichroic out of its box and carefully slide it in and lock its pin holder.

Proceed in a similar way with the dekker unit. Just pull it out, put the dekker back in its box and introduce the other one in the unit. Be careful to arrive to the end and make sure that the dekker is locked.

Close the slit door securely by snapping the locks. Finally lock the slit door in the control window:

SYS@taurus> slit_door close

put the dekker in clear position:

SYS@taurus> dekker 8

and update the mimic:

SYS@taurus> setdichroic <dichroic name>

SYS@taurus> setdekkerset <dekker name>

The dekker name can be either observing, standard or polarisation. Here is an image of available dekkers. For the standard ISIS runs the observing dekker should be used. It has the same clear parts as the dekker standard but also eliminates ghosts in the blue arm caused by a dichroic.

If you have changed a dekker click on the "Update filters" button in both the "ISIS Observer" and "ISIS Eng." tabs of the Instrument Control Console so that the dekker drop-down menus are updated.

Now you can proceed to change the gratings, e.g.:

SYS@taurus> setgrating red R1200R

to change the red grating to R1200R. Then answer yes to access the grating door. The doors are then unlocked and can be opened manually. To release a grating hold it by its handles, push the "open" switch and gently pull it out from its holder. Place it back in its box and insert the new one.

You must ensure when you load a grating in the grating cell that the blaze arrow on the back of the grating points towards the left for the red arm, and towards the right for the blue arm, i.e. gratings should be inserted blaze-to-collimator. If the grating is inserted in the opposite sense, i.e. blaze-to-camera, then throughput is reduced by up to an order-of-magnitude.

Never touch the surface of a grating. Inspect visually all gratings that you put in and out and compare with a list of known scratches on the gratings. Report any unusual findings to the ISIS instrument specialist.

Once a grating is in place hold it by its handles and push the "close" button (place your fingers away from the three grating clamps!). Finally, securely close the grating doors by snapping the door locks.

Set the slit width to ~0.5-0.7 arcsec; don't close the slit more than this since the intrinsic line-width will not be fully sampled on the CCD.

SYS@taurus>  slitarc 0.7

Set the red and blue central wavelengths to the values requested by the observers, e.g.:

SYS@taurus> cenwave red 7500

Set the dichroic to the correct position:

SYS@taurus> bfold 2

to observe in both red and blue arms,

SYS@taurus> bfold 0

to observe in the red arm only or

SYS@taurus> bfold 1

to observe in the blue arm only.

Insert the order sorting filter GG495 if needed:

SYS@taurus> rfilta 3

Place a comparison mirror in and turn on calibration lamps:

SYS@taurus> agcomp

SYS@taurus> complamps cune+cuar

Set the correct CCD window:

SYS@taurus> window red 1 "[585:1550,1:4200]"

(should be OK for red arm + REDPLUS)

SYS@taurus> window blue 1 "[585:1550,1:4200]"

(should be OK for blue arm + EEV12)

SYS@taurus> window red 1 "[1:1072,440:739]"

(should be OK for red arm + QUCAM)

SYS@taurus> window blue 1 "[1:1072,440:739]"

(should be OK for blue arm + QUCAM)

Do not bin the CCD. Use fast readout speed to quicken the process:

SYS@taurus> rspeed red fast

SYS@taurus> rspeed blue fast

You are now ready to align the dispersion axis with the CCD columns, ensure that the CCD is not tilted with respect to the incident beam, focus the spectrograph, and finally check that the collimator position for the spectral focus also produces a good spatial focus, e.g. that there is no astigmatism. In order to check for the rotation, it is a good practice to first check that the CCD focus and CCD tilt are more or less OK. The reason for this is that lines used by setup script need to be reasonably and uniformly sharp across the CCD otherwise the rotation script may give a wrong value. If you are going to use a dichroic, check the rotation with the dichroic deployed since the dichroics can introduce some image rotation, especially in the blue arm.

Be aware that configuring the setup is an iterative process, and that there is "cross-talk" between e.g. rotating the cryostat and adjusting the cryostat tilt, and vice-versa. Therefore if you need to modify the tilt after setting the rotation, re-check the rotation again. After the final mechanical intervention for rotation and tilt, tighten bolts and capstan clamps, and make a final check that rotation and tilt remain acceptable. Finally, with the spectrograph anastigmatic (see below) fine-tune the focus by adjusting the collimator (this process does not involve mechanical intervention with the cryostat).

Load the isis package within IRAF,

ecl> ing

ecl> isis

Start with moving the collimators to expected correct values in order to avoid spectrograph astigmatism and checking that the CCD focus and tilt are more of less OK.

Take a test exposure:

SYS@taurus> glance red 3

SYS@taurus> glance blue 60

Check that you get reasonably intense arc lines. This is important in order to obtain good results when using the calibration scripts. Then take an exposure:

SYS@taurus> arc red 2 "Rotation test"

In IRAF, run the isis_rotation script with a mode 1=arc.

isis> isis_rotation r2345678

The isis_rotation.cl IRAF script extracts the spectra of an arc image in two windows separated spatially and cross-correlates them to find a pixel shift. The shift is then converted into a detector rotation offset in micrometer units. A common output of the rotation script is:

pixel shifts between top and bottom : 1.08

corresponding angle in degrees : 0.0935

corresponding micrometer offset in mm : -0.282

The pixel shifts are the important numbers; the other two numbers are estimates from a factor conversion and may be occasionally wrong (e.g. if the cryostat is rotated with respect to the nominal position).

For a well-aligned cryostat you should aim to have the pixel shifts between top and bottom as reported by the rotation script <0.5 pixel, preferably <0.25 pixel. Good alignment is not only necessary for extended sources but also for faint point sources as it helps to trace the spectrum without significant degradation of signal-to-noise.

If you need to change the rotation, loosen the cryostat rotation bolts, which attach the cryostat mounting ring to the spectrograph mounting ring, with an Allen key. Never loosen the two recessed bolts, which fix the spectrograph mounting ring to the spectrograph. Apply the recommended micrometer offset to the cryostat, clockwise (+) or anti-clockwise (-) as required, and re-tighten ~3 of these bolts which are the closest to the three capstances A, B and C (for anti-clockwise rotation of the cryostat, you should lower the value on the micrometer, similarly for clockwise rotation you should increase the value on the micrometer). Iterate this procedure until the rotation is within limit. Then re-tighten all rotation bolts with the Allen key. In order not to over-tighten them, engage the long end of the Allen key in each bolt, and rotate the short end. Finally, back-off the rotation micrometer so it is not in contact with the stop to ensure pick-up noise is not transmitted through it to the cryostat.

Bear in mind that the spectral and spatial directions are NOT perpendicular in ISIS. Therefore, rotation of the cryostat to set the arc lines horizontal (which is what most observers want to simplify sky subtraction), makes the spectrum of a star not precisely aligned with the CCD columns. The effect is best noticeable with the low dispersion gratings. If alignment of the dispersion with the columns is paramount, this can be checked by taking a tungsten lamp spectrum with a narrow dekker, e.g. 0.3", deployed and running the rotation script in a mode 2=tungsten only.

Note also that for 1200 gratings and H2400B grating, arc lines are slightly curved by approximately 2 pixels from centre-to-edge of the standard window.

Take two exposures, one with the left Hartmann shutter closed and one with the right Hartmann shutter closed. Do not move the Hartmann shutter until the frame has been read out. The order in which the exposures are taken is not important. Make sure that spectral lines are not saturated and that the collimator is in the anastigmatic range (see CCD focus below).

You can use /home/whtobs/hart script on taurus or one of the following scripts:
1- this script to take the necessary images in both arms
2- this script to take the necessary images with the red arm only
3- this script to take the necessary images with the blue arm only

This is basically what the scripts do:

SYS@taurus> rhart l

(left Hartmann shutter closed)

SYS@taurus> arc red 4 "test hart l"

SYS@taurus> rhart r

(right Hartmann shutter closed)

SYS@taurus> arc red 4 "test hart r"

SYS@taurus> rhart 0

(Both Hartmann shutters open)

Now analyze the data with IRAF. First display one of the spectra taken and note the y-coordinates of three well-spaced spectral lines, i.e. near the bottom, centre and top of the detector. The positions need to be accurate only to within 10 pixels. Try to choose lines that are strong, that are not doublets or blends, that are not too close to other lines and that are not located in the vignetted part of the CCD. Run the isis_tilt task, e.g.

isis> isis_tilt r2345678 r2345679 y1 y2 y3

where y1, y2 and y3 are the y-coordinates of the three fiducial lines, e.g. 600, 2000 and 3400, respectively. Apply the recommended capstan turns, if any, in order A, B, C. The capstan micrometers should be backed-off when turning the capstans. You should aim to have the top-bottom and left-right tilts restricted to <0.25 pixel and each value (overall, top-bottom and left-right tilt for A, B and C) should be <0.1 pixel. To adjust capstan A, loosen its clamp, and loosen slightly the clamps of capstans B and C (to prevent any buildup of mechanical tension when applying the adjustments to capstan A), while supporting the cryostat with your other hand. Then proceed similarly with capstans B and C (see also sa notes). When adjustments are finished, ensure all three capstan clamps are tight, but don't over-tighten to avoid introducing further tilt in the system. It's a good practice to take and analyse a further set of spectra to ensure the tilt remains within acceptable limits after final tightening.

Never completely loosen all three capstan clamps simultaneously.

Proceed similarly for the blue arm.

When you are happy with the capstan settings, back-off their associated micrometers again if you have used them to measure the capstan positions, to avoid transmission of pick-up noise to the cryostat.

The nominal collimator values with no extra refractive components (dichroics, filters, polarisation module) between the slit and the collimators are 5100μ for the blue arm, and 9300μ for the red arm. The spectrograph should be focused with the collimators within ± 1500μ of these nominal values, otherwise it will be astigmatic due to de-collimation of the beam incident on the grating. If the spectrograph were astigmatic the best focus on a spectral line would result in a degradation of the spatial resolution along the slit, and hence of signal-to-noise in the extracted spectrum.

If the optimum spectral focus set by moving the collimator would move the collimator beyond this nominal range, then instead the spectrograph should be focused by moving the detector using the capstans so that the collimator remains within the anastigmatic range. So, conceptually the focus procedure involves setting the red and blue collimators to 9300 and 5100 respectively with no additional optics in the beam, reducing the Hartmann shifts as reported by the focus script to <1 pixel by adjusting the respective cryostat capstans, and then fine-tuning the focus to its optimum value by adjusting the respective collimator values.

If you subsequently deploy a dichroic or filter then the appropriate focus offset must be applied manually by adjusting the collimator. The recommended values can be found here for D5300 and GG495 and here for remaining dichroics and filters.

Therefore, the focus procedure is:

SYS@taurus> rcoll 9300

SYS@taurus> bcoll 5100

Take two exposures, one with the left Hartmann shutter closed and one with the right Hartmann shutter closed. Use the same procedure as for CCD tilt above.
Now run the isis_focus task

isis> isis_focus r2345678 r2345679

If the Hartmann shift reported is >1 pixel turn each cryostat capstan by the recommended amount (see sa_notes). Alternatively, if the Hartmann shift reported is <1 pixel apply the recommended collimator shift, i.e.

SYS@taurus> rcoll "value"

SYS@taurus> bcoll "value"

where value=original_value+shift.

Repeat this sequence until the recommended collimator shift is 200μ or less and the collimator value is in the anastigmatic range.

Please, don't forget to note the A, B, C, & D micrometer values down in the CCD capstan settings folder.

Finally, you can confirm a good spectrograph setup by measuring the position and FWHM of three lines (top, centre and bottom of CCD) in three different positions (left, centre and right). The individual FWHM should be in within 0.5 pixels and positions should have no apparent rotation across the CCD.

Check that the final central wavelength is correct, comparing with an atlas of cuar+cune lines.

Update the ISIS set-up database when you are finished.