User's Guide to the Kast Double Spectrograph


Table of Contents


Introduction
Quick Reference
Hardware Overview
Common Path
Blue Side
Red Side
Detector Characteristics
Software
Kast Controller
Data Taking System
Position Angle
Arc and Flat-field Lamps
Diagonal Mirror
Kast Focus
Eventsounds
Telescope Offset
Setup and Observing Hints
Setup Procedures
Observing Hints
Calibration Lamp Spectra
Exposure Time Calculator

Mt. Hamilton Homepage

Observing Hints

Direct Imaging | Flat Fields | Arc Lamp Calibrations | Overscan Subtraction | Acquisition and Guiding | Centering on very faint objects | Scripts | Polarimetry


Direct Imaging

Direct images may be taken on both sides.

Use "Open" for a wide open decker, and "Open" for a wide open slit. The unvignetted field of view is about 145 arcsec square (185 pixels at ~0.78 arcsec/pixel, 337 pixels at ~0.43 arcsec/pixel).

A suggested blue-side window using the direct mirror is number of rows = 325, number of columns = 325; start row = 850, start column = 850. For the red side, with the tilted flat mirror in the grating tray at grating tilt 8800, use window 200, 200, 130, 500. 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 in the User Filter Wheel Assembly.


Arc Lamp Calibrations

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

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

See Calibration Lamp Spectra for sample Kast spectra.


Overscan Subraction

The number of overscan columns per amplifier is listed in the COVER keyword in the FITS header of Kast data. COVER should be 32 for Kast data (though it is possible for it to be some other value, it is very unlikely).

The red CCD uses a single amplifier for readout. The overscan region is the 32 rightmost columns of the image.

The blue CCD uses two amplifiers for readout. Hence there are two overscan regions, each 32 columns wide. The total of 64 columns of overscan are at the righthand side of the image. The first 32 columns of overscan are for the left hand side of the image, the second 32 columns are for the right hand side of the image.

The IDL script kastbias.pro will properly identify the data and overscan regions based on the FITS header information, subtract the overscan region(s) from the data and write out a new file containing the overscan subtracted data with an updated header.

Syntax:
kastbias,InputFitsFile,OutputFitsFile[,/legendre]

InputFitsFile is the raw data file with overscan regions.
OutputFitsFile is the new data file containing the overscan subracted data.
/legendre is an optional keyword to do a third order legendre fit to the overscan region. If one does not use this keyword the overscan value subtracted from each row is simply the mean of that row's overscan values.

Examples:
kastbias,'r100.fits','r100new.fits'
kastbias,'b346.fits','newimage.fits',/legendre


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 gouda or karnak by clicking on its icon or selecting it from the Root Menu (depending on window manager). 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 telescope offset routine to move the telescope so as to center the object in the slit for a spectroscopic observation.

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.


Scripts

There are currently two scripts available to take loops of exposures on the red and blue sides of kast. These scripts are run from the command line on gouda, karnak, or shard. As scripts are added they will be documented here. If you need a specialized script, please contact a support astronomer (sa@ucolick.org) in advance of your run to see if your needs can be accomodated.


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 selecting a rotation angle for the waveplate from the kast controller software. This will give you a split spectrum (one polarization on the top, the other on the bottom). For certain calibrations you will also have to select the 'filter' - which is actually a polarizing filter (not be confused with the 'polaroid' filter, which is only good for wavelengths < 7300 Angstroms) - in the Upper Filter Wheel.

The waveplate is rotated to any one of four positions (or 16 if you choose "More Options") from the Kast motor control GUI.

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 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: 0.0, 45, 22.5, 67.5 degrees.

It is helpful to note that when using the polarizing filter and a waveplate rotation 0 degrees puts all the light of a calibration lamp in the upper spectrum, 45 degrees in the lower spectrum, and in 22.5 and 67.5 degrees 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)

Note that direct imaging polarimetry is also possible with Kast, though the field of view is reduced to about 40 arcseconds by the polarimeter. The necessary standards and calibration procedures are very similar to that for spectropolarimetry.

Further reading on polarimetry:

Miller, J. S., Robinson, L. B., & Goodrich, R. W. 1988, in Instrumentation for Ground-Based Astronomy, ed. L. B. Robinson (New York: Springer-Verlag), 157 "A CCD Spectropolarimeter for the Lick Observatory 3-Meter Telescope" The basic reference for the instrument design and data reduction strategy. A couple of the equations (particularly for the errors) have typos in them.

Goodrich, R. W. 1991, PASP, 103, 1314 "High-efficiency 'superachromatic' polarimetry optics for use in optical astronomical spectrographs" A good description of the design of similar polarimeters.

http://www2.keck.hawaii.edu/inst/lris/polarimeter/manual/pol_v3.ps The LRIS polarimeter manual by Marshall Cohen (and updated by Aaron Barth). A description of a similar instrument that also describes the data reduction process.

Schmidt, G. D., Elston, D, & Lupie, O. L. 1992, AJ, 104, 1563 The best polarization standards, if you throw out the ones they mark as variable (!!). Used to calibrate HST.


Support Astronomers (sa@ucolick.org)
Last modified: Thu Mar 22 18:20:59 PDT 2012