Kast Double Spectrograph User's Manual: Observing Hints

Observing Hints

Direct Imaging

Direct images may be taken on both sides.

Use setting 350 (or "open") for a wide open decker, and 2250 (or "open") for a wide open slit. The unvignetted field of view is about 145 arcsec square (185 pixels at ~0.78 arcsec/pixel).

A suggested blue-side window using the direct mirror is number of rows = 210, number of columns = 200; start row =0, start column = 530. For the red side, with the tilted flat mirror in the grating tray at grating tilt 19000, use window 200, 200, 25, 450. Check direct windows with the top lights, not the tub lights.

An obvious diffculty is that most of our filters are 2" square and must go in the user filter wheel. Their use instead in the red or blue side filter holders where the beam is 3.5" in diameter will reduce the effective aperture to about a meter; you may as well use the 40" and avoid the wrath of Practically Everybody. A more reasonable course is to put the 2" filters in the user filter wheel, which will utilize the full beam, and just use one side for the observation.

Flat Fields

In general, you will need a number of flats well exposed at the red end in order to get a reasonable total number of counts at the blue end. When you are calculating the statistcal accuracy of your flats, remember to convert from DNs to electrons.

When doing low resolution dispersed flats, in order to keep the red end of a spectrum from saturating so soon, you may wish to use the filter stack called BG 14++ in position 1 of the lower filter wheel.

Arc Lamp Calibrations

There are a large number of arc lamps available for calibrating Kast spectra. The following combinations of lamps are often used:

Blue Side: He, Hg-Cd
Red Side: Neon, Hg-Ar, He

The Hg-Cd lamp does need a couple minutes to warm up to get all the lines.

See Calibration Lamp Spectra for sample Kast spectra.

Acquisition and Guiding

A very sensitive CCD camera is used for object acquisition and guiding. It is mounted on a remotely controlled x-y stage for offset guiding. The field of view is about 2 arcminutes with the diagonal mirror in position 2. Once the field is identified, the object will be positioned on the slit (either directly or with blind offsetting). If possible, the night assistant will guide off the science target on the slit. Otherwise, the NA will look for an off-axis guide star.

The night assistant will operate the camera for you. If you wish to see the guide camera image, you may start the guidercopy program on either gorgon or karnak by clicking on its icon. More information on the autoguider software, refer to the Lick Autoguider Manual.

The autoguider produces a reticle of on the guide camera image, which the night assistant will position over a guide star. The reticle is divided into four quadrants. The autoguider senses what fraction of the light from the guide star falls into each of the quadrants, and then guides the telescope so as to maintain that balance. The autoguider is a pretty good device, but not infallible, so you should watch if from time to time to make sure it's doing the job. Fall asleep at your own risk. Ask the night assistant to explain the autoguider history display to you; it's a useful check on performance.

There is a reticle which may be projected onto a pellicle and thereby mixed into the TV image. The pellicle is a highly stable with respect to the slit when mirror positions 2 or 3 are used, but in position 4 the periscope may cause some wander of the apparent slit image relative to the reticle. This is usually used by the telescope technicians to align the guide TV x-y stage and not during regular observations.

The most worry free situation is if your object is bright enough to see directly on the slit. If you can guide on that portion of the light that does not make it down through the slit, then at least you are assured of where the rest of the light is going. In many cases your object will be too faint to guie on the lost light, in which case you may be able to guide on some nearby object which happens to fall onto the slit jaws, or you may have to resort to offset guiding.

Offset guiding is done in mirror position three, which enables the camera to focus on the solid portion of the diagonal mirror, while an on-axis hole in the mirror passes the light from the object on down to the spectrograph.

The night assistant will operate the x-y stage for the camera for you, and help you find a guide star. Usually one just scans around randomly until one finds a suitable star, but in rare instances it may be helpful to know which way to look for a likely candidate which apears on your finding chart. The useful area of the mirror is approximately as shown below.

Centering on very faint objects (blind offsetting)

If the object is too faint to visually center it on the slit, then a major advantage of this spectrograph design becomes apparent. In almost any case one might imagine, you can dead reckon the object to within an arcminute or so of the slit center. Then, take a direct image of the object while offset guiding to prevent drift, identify your object (down to 23rd mag is not unusual), and use the telescpe offset routine to move the telescope so as to center the objet in the slit for a spectroscopic observation. The offset routine is under Z-5. You type in the row and column numbers, and then the offset routine offers you the choice of moving from position 1 to positoin 2 or vice versa; you only have to remember which way you want to go. It works, and it's really nice. Here are two important hints: 1) be sure to turn off the autoguider during moves, and 2) all of the experienced observers take another direct image after the move to verify that the telescope moved as desired.

Another option is to use the telescope offset program to move a premeasured distance from a bright offset star. It is possible to make very long moves this way (something like 32,000 arcsecs!), but it's not very fast, so keep it reasonable; a few arcmins is no problem. The program (under Z-5) accepts RA in either units of time or arc. Accuracy is roughly 0.1 arcsec over an arcminute; less for longer moves.

Vista Scripts

Vista, a Lick developed data reduction package, can also control the data taker, allowing basic scripting of certain observing tasks, such as taking multiple exposures on one or both sides, or rotating the waveplate between exposures. For most purposes, the existing scripts are sufficient.

The existing Vista scripts for Kast are:

kastloopred - Take a series of exposures with the red side.
kastloopblue - Take a series of exposures with the blue side.
kastloopboth - Take a series of exposures with both sides. Take note that this script does not take exposures on both sides simultaneously. Exposures are taken serially (e.g. once the red side exposure finishes, the blue side exposure will begin).
exppl - Take exposures on one or both sides and rotate the waveplate in between exposures.

Note: All these scripts assume that you are using selection 1 in the data taking software.

To use these scripts you must first start Vista on shanepc by typing vista at an xterm prompt. Vista should give you a GO: prompt.

You will then have to load the procedure by typing rp procedurename, e.g., rp kastloopred. You then start the procedure by typing go. Most scripts will prompt you for some information, e.g. "How many exposures?".

You may write your own scripts. All vista scripts are in /procedure directory on shanepc. Look at existing scripts to understand the syntax of the scripting language. The command that interacts with the data taking software is dtake. Dtake can take a number of arguements. Examples of the most important:

dtake set plate=3 ccd=1
will set the waveplate to position 3 (out of 4 possible rotations) and the CCD to 1 (blue).
dtake start sel=1 ccd=2
will start an exposure with CCD 2 (red) and selection 1 from the data taker.
dtake wait
will force the script to wait until the current exposures are completed before continuing.

Polarimetry

To do polarimetry the polarimeter module must be installed in Kast (currently this is the default). Put the waveplate into the light path by setting the user filter to the 'in' position from the kast controller software. This will give you a split spectrum (one polarization on the top, the other on the bottom). You will also have to select position 1, 'filter' - which is actually a polarizing filter (not be confused with the 'polaroid' in position 2, which is only good for wavelengths < 7300 Angstroms) in the Upper Filter Wheel.

The waveplate is rotated to any one of four positions from the data taking software option L.

Using the dichroic with the polarimeter is not recommended. It introduces uncalibrateable wiggles over the several hundred angstroms in the vicinity of the dichroic crossover.

The polarimeter shifts the spectrum on the CCD, so you will have to define a new Window (DTS option D) to get all the data.

Focusing in polarimetry mode is nearly the same as for regular spectroscopy. However, you should use the centerline option in kastfocus to choose the center row of the top or bottom spectrum for focusing otherwise the kastfocus program will assume the center row, which lies between the two polarization spectra.

Data-taking proceeds as in regular observing, (including TUB rotation as necessary), except that you will want to take exposures with the waveplate in each of its four rotations (0, 22.5, 45, and 67.5 degrees). Most observers take data with the waveplate rotation in the following order for efficiency: 1, 3, 2, 4.

It is helpful to note that waveplate rotation 1 puts all the light of a calibration lamp in the upper spectrum, 3 in the lower spectrum, and in 2 and 4 the spectra are evenly split between the two.

Additional calibrations are required for polarimetry: Polarizance test, Polarization standard star, and Null standards. (Descriptions courtesy of Ryan Chornock, UC-Berkeley)

Polarizance test: Observe a low polarization standard star with a polarizing filter in to produce 100% polarized light at a fixed position angle. This does two things. The first is to make sure that you measure 100% polarization (more or less) when you should. The second thing is to get the angle correction curve as a function of wavelength. The half-wave retarder has a fast and slow axis that are perpendicular to each other, with light polarized along the slow axis being retarded by 180 degrees of phase relative to the fast axis (by definition). However, the position angle picked out by the fast axis is a function of wavelentgh that varies by +/- 5 degrees over the optical range (see Goodrich 1991). You want to remove this variation so that an object whose intrinsic polarization angle is constant with wavelength is measured to be so. The polarizance test gives you the shape of this angle correction curve, but to set the zero point, you have to observe a polarization standard star. Some people observe dome flats through the polarizing filter to measure the angle curve (effectiveness of this method is not known by the Mt. Hamilton techinical staff).
Polarization standard star: These are generally bright, relatively high-polarization stars with a cataloged polarization angle that has been shown to be constant. Observe one of these to set the zero point of the angle correction curve. A second one is nice to double-check the answer. Also, you can double-check that you measure the right degree of polarization.
Null standards: Objects of intrinsically low polarization (generally < 0.1%) used to check the instrument. If you measure <0.1%, great! You don't need to any more calibrations. Generally we don't find instrumental polarization to be a problem. Sometimes, for reasons we don't understand, we do measure noticeable instrumental polarization (at several tenths of a percent level). You can use the null standards to remove the instrumental effects from your object observations, but that can be tricky.

User's Guide to the Kast Double Spectrograph

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