User's Guide to the Kast Double Spectrograph

Table of Contents

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

Data Archive
Mt. Hamilton Homepage

Setup Procedures

This section is not exactly a checklist, but rather an orderly discussion of what needs to be done in the usual case, with some hints. We do assume that you have read the descriptive material which precedes. The aim here is to be more complete than the one page checklist, so if you have never observed with the Kast you can get a good idea of what to expect, or if you haven't observed with it for six months or so, you can quickly remind yourself of how things work.

Obviously what we present here are only suggestions. There may be many other ways to accomplish the same thing, and some of those may be better for you. We don't pretend to be exhaustive nor to stifle your creativity, but merely to tell you enough so that you can do everything you need to do in a least one reasonable way.

First Principle | Checkout | At the TUB | Start Software | Trial Integration | Spectroscopic Windows | Order Separators | Spectral Coverage | Direct Windows | Slit Center | Focusing | Record your Setups | Start a Log | Wavelength Calibration Spectra | Direct Flats | Dispersed Flats | Opening the Dome | Focus Star | Object Coordinates | TUB Rotation | Prepare to Expose | Exposing | End of Night

First Principle

Some of the most experienced observers in the world observe with the Shane, but in recent years the telescope has become available to a much wider community, some of whom are not so experienced. It is to this less experienced segment of the community that we address the following advice.

The Shane is not a difficult telescope to use, because the instrumentation (and particularly the Kast) is user friendly, and knowledgeable help is usually readily available. However, if you have not observed with a world-class telescope before, please appreciate what a valuable commodity observing time is on such an instrument. It is only fair to yourself, the TAC, and the UC astronomical community if you try hard to maximize the use of the time which has been assigned to you.

With this in mind, we state here the First Principle of Big-Time Observing: try to keep the telescope busy at all times. Come to the telescope as fully prepared as possible. It should be a very unusual circumstance that required you to make a finding chart during the night. The time to plan your strategy is not between observations, but the week before, or before the night begins, or at the very latest, during a prior exposure. When one observation has ended, you should already know what you are going to do next, so you can quickly start a new data taking cycle. If you are debating what to do next, it's usually better to start something, so at least some data in rolling in while you're fretting over the next move; but if you're planning ahead well, this will rarely happen. If the next move depends on the outcome of the present observation, don't wait for the observation to conclude to consider the alternatives; have your contingency plans already made, so you are prepared to make a quick decision.


If it's your first time with the Kast, or if it's been a long time and you feel you need a refresher, contact the person who will check you out in advance to arrange a time to meet (you may contact all support astronomers using the email and the proper person will get in touch with you). This service is free, required on your first visit, and strongly recommended if you have any doubts later. Our point of view is that we want to do everything we can to make the science successful, and to protect the equipment for the entire community.

At the TUB

Go out to the telescope, where there are five things to do.

There are four things to look at, and one to set up for yourself. The look at items are

  1. Make sure the CCD temperatures for both chips are reasonable.
  2. Check that the x-y stage settings are properly set on the blue side (probably to the nominal 10.0).
  3. Check the dichroics to be sure the ones you want are installed.
  4. Check the grating tray contents to be sure the ones you want are installed.
Usually the telescope technicians have set up the instrument as stated on your telescope proposal, so they may be able to confirm the setup without your having to inspect the instrument yourself. The one thing to set up on the TUB is to insert any user filters you'll need. Contact a telescope technician or support astronomer if you need assistance or instruction on inserting filters.

Start Software

From the user account on gouda, karnak, or shanevnc start the following software:

Although it has never been clearly necessary, it just seems to us to be good practice to re-calibrate the spectrograph controller at the beginning of each run, or following a power failure. This is done by clicking Actions - Re-calibrate all in the Kast motor control software.

When you get a data taking window, update the header with the observer's name on the 2nd Level tab for each data taker. This information will be added to every image header.

Check the data directories /data/kastred and /data/kastblue. If there are data from the previous observer's run still in the directory, contact a Support Astronomer or Telescope Technician who will make sure the data are properly archived before clearing the directory for you. Please note that data frames can be overwritten, so it is important to clear the directory before recording any images or make sure you use observation numbers different than any in the directory.

Trial Integration

As soon as possible after you have things started, do a short integration to be sure everything is ok. The most complete check is to read the whole window for both chips (click Whole CCD in each data taker). Be sure the shutters open and close, and that the chips read and display correctly. The point is, of course, to find any serious problems early enough to get them fixed before observing time.

Spectroscopic Windows

Set up the spectroscopic windows. This is best done by illuminating a full length slit with the blue flat field lamp, mirror cover open, diagonal mirror position 3 or 4, and reading out the whole chips (or you can just check the windows left by the last observer if they look reasonable). In particular for the gratings, the dispersed illuminated areas may not be the same, although they should be close. You will probably choose to use one window which covers only the area of the overlap, or one all-inclusive window.

Order Separators

Select any order separating filters you may require. On the red side, you will need to suppress second order if you go beyond twice the effective cut-on point of the dichroic you use. If you are not using a dichroic, the glass in the lenses cuts off at about 3800 A, and therefore will suppress any second order below 7600 A, but beyond that you will need to suppress with an order separator.

Spectral Coverage

Check the wavelength coverage with the line lamps. Sample spectra are available in the Control Room to help you identify the wavelength region.

Direct Windows

Determine where the direct windows are. Use decker setting "Open" for a wide open decker, and slit setting "Open" for a wide open slit, no disperser. A suggested blue side window is 325, 325, 850, 850. For the red side, with the tilted flat mirror in the grating tray at grating tilt 8800, use window 350, 390, 2020, 424. On the blue side the window may change if the dewar x-y stage is changed from its nominal positions. Check direct windows with the top lights, not the TUB lights.

Slit Center

If you will be using the direct mode to find and center faint objects, you will need to know the row and column number of the center of the slit. Close the slit to spectroscopic size, and with illuminations from the dome continuum lamps (not the tub lamps!), take a direct (undispersed) frame. Use the cursor to find the center and record it for later use and/or enter it into the Telescope Offset GUI.


On each side, focusing is accomplished by moving the collimators. If you are using the spectrograph, focus with a narrow slit; if direct, make an artificial star with the small slit and decker. There are a number of ways to analyze the quality of the image, however, we recommend the use of kastfocusp for achieving an accurate focus for a dispersed image for most cases.

Don't forget to look at the whole spectrum at some point, in case the focus is not flat; you may need to make some compromise. The Fairchild and Hamamatsu CCDs should be very flat, though the focal plane is slightly curved. If a spectral region of particular concern to you is less well focused than most of the chip, you may choose to adjust the focus to favor that area, or make a compromise, or (on the red side), move the grating tilt to move that part of the spectrum to a better place. Also consider that for observing you'll probably use a wider slit than that you focused with, so small focus differences will wash out anyway.

Record Your Setups

It is useful both as a convenience and as a precaution to record your setups. You might want to do this both on paper and for future reference, and on the disk for quick recall during the night. The Kast controller allows you to store as many setup as you desire (please store your setups in your directory in /u/user/observers/).

Start a Log

Instrument specific logsheets are available on-line and can be printed out as needed at the telescope.

Wavelength Calibration Spectra

Use mirror position 2, dispersed setup, and record image to disk. For the best solution, use a two-pixel slit or greater to avoid aliasing. With the currently installed CCDs, the 0.5" width slit is recommended.

Most people who are doing "ordinary" spectroscopy (that is, not trying for accurate radial velocities), do a set of line lamps at the beginning and (just as a check) another set at the end of the night. Remember that between San Jose and night sky emission, there are comparison spectra available for zero point corrections on every frame, except possibly the shortest exposures.

Direct Flats

Use the Red dome lamp (unless very narrow band), make sure the mirror cover is open, the diagonal is in mirror positon 3, and use your direct imaging setup.

Dispersed Flats

Wait until after sunset to do dispersed flats so as to avoid the possibility of getting solar features in your flat if that might make a difference for your science program. Use the blue or superblue dome lamp, mirror cover open, mirror position 4 (or 3), dispersed setups.

Opening the Dome

It is ordinary practice to open the dome in mid- to late afternoon, weather permitting, in order to let the inside air reach equilibrium with the outside. If you're working in the control room (local or remote) setting up, the night assistant will probably ask you when it will be convenient for you to have the dome opened. If you are not around, the night assistant will normally open the dome anyway as soon as the outside temperature begins to be less than that inside the dome.

Focus Star

At the beginning of the night, the night assistant will choose a bright star with good coordinates from Apparent Places, and set to it first. You may focus the telescope if you wish, or the NA will do it for you. The NA will put the focus star in the center of the slit and reset the telescope coordinates, then will mark the location on the TV in mirror position 2 which corresponds to the center of the slit.

Object Coordinates

You may give the object coordinates to the NA one by one, or as a complete list, whichever is convenient for you. An electronic starlist is a convenient and efficient way to give coordinates to the NA. The NA will run the coordinates (any epoch) through a computer program which will correct for precession, nutation, aberrations, refraction, the cost of living index, and (most important of all) flexure. Usually the telescope points well, but it would be wise to come with well-prepared charts to avoid errors.

TUB Rotation

You may wish to choose some preferred positon angle for the TUB, perhaps to get more than one object at once, or to position the slit along some nebulosity, or (most likely) to position the slit along the angle of atmospheric dispersion. The pa program can help you determine the positon angle of the spectrum due to the atmospheric refraction. Ask the TO to rotate the TUB for you.

Prepare to Expose

Before each exposure, be sure the spectrograph setup and selection is appropriate. Set the integration time and object name. Try to anticipate the need for an offset guide star so the NA can be setting up that while you are setting up the spectrograph.


The autoguider is nearly always used and will be set up by the night assistant, so there is not usually much that's required of you during an exposure, except to check the guiding now and then.

End of the Night

Support Astronomers (
Last modified: Sat Mar 6 16:19:21 PST 2021