WSO Observer's Guide

Introduction to Observatory Operation

This guide gives basic instructions on how to run the Wilcox Solar Observatory. It will be helpful to read this document in its entirety before beginning. There are two observing modes: meanfields and magscans. Both measure the magnetic field of the Sun. Meanfields measure the average magnetic field of the whole Sun and take about half an hour to complete. Magscans scan over the visible disk of the Sun and make a map of the magnetic field at its surface. These take about two and a half hours. Ideally, one would take at least one magscan and as many meanfields as possible every day. The weather, however, and occasional technical difficulties, may make this impossible. In that case, it is up to the observer to optimize the use of the available observing time. Strategies for doing so will be discussed below. The observer must also keep a written log of all observations run. Finally, at the end of each day of observation, the observer must run the shutdown procedure.

To run each type of observation, follow the step by step instructions given below (see the sections entitled How to ...). Both types of observations are initiated in the comproc window on daystar (the computer sitting on the table). After that, a new window will pop up in which the computer will guide you through the setup of the observation. The computer will either ask you to input something in the observation window, or it will direct you to take some action, in which case it will light the blue OPWAIT light. Once you have completed the action, press the button by the blue light until the light goes out. This tells the computer that you have done what it has asked and to go on to the next step. There are three of these lights/buttons in the observatory: one at the top of rack 2, one on the observing table beside the phone, and one in the dome. They are all equivalent. You will also sometimes interact with the computer using the four toggle switches at the top of rack 2.

Observatory Overview

This section gives a brief description, for the non-scientist, of how the different components of the observatory work together to observe the magnetic field of the Sun. It also explains the setup procedures common to both types of observations. For this section, one should note the cardinal directions. The wall of the observatory behind the electronics racks runs exactly east-west. The angle opposite this wall points south; the direction of the Stanford main campus is (approximately) north.

  1. The Dome
    • At the top of the observatory is a rotating dome. Usually the position of the dome and the dome door are controlled by the computer, but there are manual controls upstairs as well. Note that the dome cannot rotate a full 360 degrees (the door cannot face north). At the beginning of an observation, the computer will rotate the dome to one of its limits, and from there it will position the dome so that sunlight comes through the dome door and falls on the primary mirror (primary because light hits it first; this mirror may also be called the first flat). During an observation, it is important to keep the hatch leading up to the dome closed; otherwise birds can get into the observing room. At the apex of the dome is a precipitation sensor; its purpose is to close the dome door automatically in case of rain.
  2. Coelostat
    • The primary mirror is mounted in what is a called a coelostat. The coelostat is mounted on a sled that can be moved independently along the cardinal directions. When setting up an observation, you will first put the coelostat in the correct position east to west. The correct position is one of three fixed locations along the east-west axis. These are marked and will be referred to as east, middle, and west positions. During setup you will tell the computer which position you are putting the coelostat in so that it knows where to position the dome. Next you will loosen the clutch on top of the coelostat and rotate the mirror until the sunlight reflecting off the first flat is centered on the second flat. Holding the primary mirror in this position, reengage the clutch, taking care not to overtighten it. NEVER TOUCH THE REFLECTIVE SURFACE OF THE MIRRORS. The mirrors may appear dirty to you, but dust is easier to wash off than finger prints, so please be careful. Now, if the sunlight isn't perfectly centered north to south on the second flat, you can move the coelostat north or south so that it is. Usually this is only a small adjustment that is made gradually throughout the year. Close to the winter solstice, around December 21, you will not be able to move the coelostat as far south as necessary. This is a known problem; don't worry about it. Also, positioning the first flat is made easier if you rotate the second flat to face it. Lastly, make sure that you turn on the coelostat drive motor using the toggle switch below the primary mirror. You should be able to hear it running.
    • In the summer months (from about May 1 to August 15), the Sun is high enough in the sky that the shadow of the second flat will not fall upon the primary mirror throughout the entire course of the day. During that time, one can leave the coelostat in the middle position all day. During the rest of the year, however, the shadow of the second flat will fall on the primary mirror around local noon. Local noon is the time at which the Sun is highest in the sky. This time varies throughout the year, but one can take it to be at about 12:15 Pacific Standard Time at Stanford. (Keep in mind that this would be 1:15 Pacific Daylight Time. Under current U.S. law, daylight time begins the second Sunday in March and ends the first Sunday in November.) If the shadow of the second flat will fall on the primary, the coelostat must be in the east before half an hour before local noon, and in the west after half an hour after local noon (east, the direction of sunrise, in the morning; west, the direction of sunset, in the afternoon). Sometime within half an hour of local noon, then, the coelostat will usually have to be moved. This is very important to bear in mind when running a magscan during this time of year. A magscan begun in the morning must be started early enough so that it finishes by 12:45 at the latest. A magscan run in the afternoon can be started no earlier than 11:45. Keep in mind that these times are approximate and that the key issue is whether or not the secondary casts a shadow on the primary. If it doesn't, then the observation is fine. On the other hand, an observation started with the coelostat in the east position will be no good if it doesn't finish before the primary is in shadow. A sliver of shadow can be tolerated, but if the shadow encroaches on the primary by more than an inch or so, follow these guidelines: allow a magscan to finish, but notify the monthly data analyzer by email. A meanfield that is still running at that time should be terminated. Use your best judgement, and if you are in doubt, just email the monthly data analyzer (contact information is posted near daystar).
  3. Second Flat and Guiding Table
    • The coelostat drive motor rotates the primary mirror at a constant rate that approximately follows the Sun. To precisely center the sunlight on the instrument, motors on the second flat move it so that equal sunlight falls into each of the four outer photodiodes on the guiding table below. But before it can do that, you have to adjust the second flat so that light falls on at least one of these diodes. The position of the second flat is roughly indicated by the four lights surrounding the CENTER button in the 2ND FLAT SERVO panel in the middle of rack 1. If the lights along one axis are green, then the mirror is centered in that direction. (This is also true if both lights on an axis are not lit. This is a bug, but for the time being regard both lights unlit as being the same as green.) Otherwise, a blue light indicates that the mirror is between the center position and the limit indicated. A red light means that the mirror is up against one of its limits. Before adjusting the second flat, it is usually a good idea to center it in its range of motion by pressing the CENTER button in the middle of rack 1 or the same button on the indicator in the dome. This step is necessary if any of the lights show red. See the section on troubleshooting to see what else may be necessary.
    • The second flat moves on two axes. It is usually easiest to first line it up east to west and then, if necessary, align it north to south. Again, never touch the reflective surface of the mirror. Ideally, you should be able to do the alignment using the diode indicator at the base of the second flat post. When light falls on one of the diodes, the corresponding indicator upstairs shows red. However, if the sunlight isn't very bright, or if you are far out of alignment, the indicator may not be much help. In that case you can look through the observing lens and try to center the image directly on the instrument, or you may find it easier to look through the hatch and judge how far you need to move the second flat to hit one of the diodes on the guiding table. Note that although it is necessary to hit only one of the diodes, it is best if you can do the alignment as precisely as possible by hand before turning it over to the guiding system.
  4. Lens and Light Path
    • From the second flat the light passes through a set of three apertures in the base of the dome platform. The westernmost of these is unused, and the one on the northeast directs light to the guiding table. The remaining tube holds the lens for the instrument, and it is this tube that is changed for the different observing modes. For a meanfield, one uses the meanfield lens. This lens produces an image high above the level of the observing table, so that the light falling on the instrument is actually out of focus. The reason is because this mixes the light from all over the Sun, allowing us to measure the average magnetic field of the entire Sun. Another way to think of it is that the meanfield lens lets us see the Sun in the same way we see other stars, which are too far away for us to resolve.
    • For a magscan, one uses the image lens. This lens produces an image at the level of the entrance aperture on the observing table. This is the same level where one places the 'three minute square aperture'. Hence, the instrument will measure the magnetic field of the small patch of the Sun that falls through the square aperture. By moving the guiding table, we cause the second flat to move the image of the Sun across the square aperture. Thus we scan over the Sun's surface, measuring the magnetic field of each small patch.
    • Above the entrance aperture to the instrument is a green filter, and above this is the KDP (these initials stand for potassium diphospate). It is the block with two cables coming out of it. The KDP circularly polarizes the light passing through it depending on the voltage across it. By measuring the frequency difference between the two polarization states, we infer the strength of the magnetic field. At the beginning of every observation, the system uses the movable filter above the KDP to make sure that it is functioning properly.
  5. Diffraction Grating and Phototubes
    • Below the floor of the observing room is a 75 ft pit, at the bottom of which is a diffraction grating. A diffraction grating is a little like a prism; it splits the light falling on it into its frequency components. We take only a tiny slice of this spectrum and measure the spectral lines within it. The light gets reflected back up to the level of the observing table and passes through two exit slits. The exit slits direct light from the two wings of a spectral line into the photomultiplier (PM) tubes, which lie right below the phone on the left side of the observing table. PM tubes generate a voltage proportional to the intensity of the light falling on them. By measuring the difference of the intensities of the two wings, we infer the exact frequency of the spectral line, which will be slightly different for the two polarization states (as discussed above).

That's how it works.

Observing Strategies and Useful Commands

On an ideal day, one would normally run one magscan as close to noon as possible, and as many meanfields as time allowed. This ideal is often achieved during the summertime; days are long, skies are usually clear, and the coelostat can be left in the middle position all day. In fact, one can sometimes run two or three magscans in a day and still get plenty of meanfields. However, generally speaking, one should follow these guidelines:

  1. Meanfields
    • We want as many meanfields as possible (see exception below). Start them as soon as possible in the morning, which would ideally be about 25-45 minutes after sunrise. Run them as late as possible, which if weather permits is until the observation stalls close to sunset. Only take time out to run the magscan(s). But if the weather is questionable and you have to choose between meanfields and a magscan, run meanfields (see exception below).
  2. Magscans
    • We want to run magscans as close to noon as possible. This is because this is the time of day when the Sun is brightest and its position is changing most slowly. Hence, close to noon, the position will change the least during the course of the scan. However, you can run a magscan at any time, if needed (for example, if you expect the weather to be clear only in the morning). Also feel free to run a magscan at any time if your schedule demands it. Furthermore, if the weather is questionable or you are otherwise pressed for time, you can run a time critical magscan, which takes less time. But again, if you have to choose between a couple of meanfields and a time critical magscan, always run meanfields. The exception is when we haven't had a magscan in 3 days. Then get a magscan even if you get no meanfields. Generally speaking, the longer it has been since you have run a magscan, the higher the priority of running a magscan becomes. The minimum we would like to get in any given day is one meanfield and one (time critical) magscan. Hence, if you have run at least one meanfield, try to run a magscan, especially if you didn't get one the day before or don't expect to get one the day following. On the other hand, if you are confident that you will only miss one day of magscans, then don't worry about it. Also keep in mind the fact that for most of the year the coelostat must be moved near local noon (see section on coelostat position above).
    • If sky conditions are extremely bad, you can consider running time critical meanfields. In opposite circumstances, when there is plenty of sunlight, it's not necessary for you to get more than about 10 meanfields. Running more than 10 is fine, but if you are able to get 10 and still run a second (or third) magscan, then do that instead. This will only happen in the summer months.
  3. Update

    • The easiest way to monitor the progress of an observation, either a meanfield or a magscan, is to run the command 'update' in a ksh window on daystar. You can also do this while away from the observatory. Just telnet into daystar (login: ### , password: ########) and enter 'update'. The first section of output from 'update' is the day's meanfields and magscans. This is simply the output of the unix command 'ls' for the files containing the observation data. Don't worry if you are unfamiliar with unix. For each file, it lists a line of information. All the way to the right is the file name, which has a MF for meanfields and a MG for magscans. To the left of this is a date and time that the file was last written to, and to the left of that is a file size. For meanfields, the file size grows in increments of 1600 up to a complete file size of 12800 (the first 1600 and the last 1600 are automatic, they do not take any time). This is useful to know when deciding whether to let a meanfield finish, or to kill it. That is, if the filesize is 1600, the meanfield only just started, and you might as well kill it if you need to. If the filesize is 9600, the meanfield is just about finished and you should just toggle the 'stop at end of meanfld' switch in the top of rack 2. For a time critical meanfield, the total filesize will only be 9600. For magscans, the total file size is 61680, or 46016 for a time critical magscan. This should tell you how far along the scan is. Keep in mind that if clouds pass in front of the Sun and the intensity gets too low, the observation will stall. It will start up again once the clouds pass by, but this may cause observations to take longer than expected.

  4. Display
    • Another useful command that can be entered in a ksh window on daystar is 'display'. This is related to the 'display' command that one enters in the comproc window (see section on troubleshooting below). The 'display' command that one enters in the comproc window opens a display window that (usually) shows the values of variables called 'count', 'intens', and 'zeeman' once per second. The 'display' command that one enters in the ksh window shows whatever is being displayed in the display window until it is killed by pressing ctrl+c. This is useful if you want to remotely monitor what intensity the instrument is seeing. If it is below a certain threshold, the observation stalls.
  5. Dome
    • Finally, if necessary (such as if rain is imminent), one can close the dome from a ksh window on daystar or remotely. The command is 'Dome close'. If it is severely overcast, but you think it may clear up later, it is often a good idea to close the dome. You can open it again with 'Dome open'. Use 'Tell dome' for the computer to tell you the current state of the dome door.

How to Run a Meanfield, Step by Step

Follow these instructions to run a meanfield. If something happens that is not described in this section, or if there is not already a comproc window open, refer to the section on troubleshooting.

  1. Make sure no observations are running. If one is running, either allow it to finish or stop it. You may want to ensure that the 'stop at end of meanfld' toggle switch is in the position you want it in (see end of this section), as well as the toggle switch for time critical observations.

  2. Type meanfld in the comproc window. A meanfld window will pop up.

  3. The computer prints some setup information (some of which is needed for the log) and then prompts with 'observer:'. Enter your name. The computer then prompts with 'remove optical bench covers, open PM tubes' and lights the blue light. Remove all tape cases, foil, etc. from the light path and guiding table diodes. Turn the PM tube shutter (behind the phone) to the open position. Press the button by the blue light until the light goes out.
  4. The computer prompts with 'coelostat position:'. Enter either 'w' for west, 'e' for east, or 'm' for middle (see the overview section for an explanation of the coelostat position). The computer will now ask you to change to the mf lens and adjust the second flat. It also moves the dome to the appropriate position and opens the dome doors. Once it has done so it will light the blue light. This is a good time to center the second flat, if necessary. Go up to the dome. Make sure the meanfield lens is in and that is it flush against the other lenses. Put the coelostat in the right position (east, west, or middle), align the primary mirror, and make sure the power to the primary mirror is on (you should be able to hear it running). Align the secondary mirror so that the light shines on at least one of the guiding table diodes downstairs. Press the button by the blue light until the light goes out. You will hear the motors in the secondary mirror as it centers the image of the Sun on the guiding table below. Don't forget to close the hatch when you are finished in the dome.
  5. The computer will do a few seconds (to a few minutes, depending on light levels) of setup and then prompt with 'balance and zero' and light the blue light. Adjust the PM Coarse Balance (on rack 2) so that the DC Difference is zero. (Do not accidentally adjust the dial labeled 'dark current cancel'; this is a common mistake.) Note the coarse balance value for the log, and press the button by the blue light until the light goes out.
  6. The meanfields are now running, and will run continuously until you stop them. To stop the meanfields, you can type 'stop meanfld' in the comproc window, but it is better to toggle the switch labeled 'stop at end of meanfld' in the top panel of rack 2 so that the red light beside the switch is on. In that case, the meanfields will stop after the current one completes. The computer will then light the blue light. You needn't do anything, just press the button by the blue light until the light goes out (and flip the 'stop at end of meanfld' switch to off). The meanfld window will close. (Note that if you stopped the meanfield by typing 'stop meanfld' in the comproc window, the computer will close the meanfld window without lighting the blue light.) Also, unlike magscans, meanfields can be switched to time critical in the middle of an observation. This can be useful if an observation is near finishing, but you are in a hurry.

How to Run a Magscan, Step by Step

Follow these instructions to run a magscan. If something happens that is not described in this section, or if there is not already a comproc window open, refer to the section on troubleshooting.

  1. Make sure no observations are running. If one is running, either allow it to finish or stop it. If you want to run a time critical magscan, toggle the 'time critical obs.' switch in the upper panel of rack 2 so that the red light beside the switch is on. If you are not running a time critical magscan, make sure this switch is off.
  2. Type 'magscan' in the comproc window. A magscan window will pop up. If you want to run more than one magscan, type 'magscan n' where n is the number you want to run.
  3. The computer prompts with 'Please insert three minute square aperture' in the magscan window and lights the blue light. Insert the square aperture so that the screws in it are aligned east to west (parallel to the observing table). It will be flush when aligned properly. Press the button by the blue light until the light goes out.
  4. The computer prints some setup information (some of which is needed for the log) and then prompts with 'observer:'. Enter your name. The computer then prompts with 'remove optical bench covers, open PM tubes' and lights the blue light. Remove all tape cases, foil, etc. from the light path and guiding table diodes. Turn the PM tube shutter (behind the phone) to the open position. Press the button by the blue light until the light goes out.
  5. The computer prompts with 'coelostat position:'. Enter either 'w' for west, 'e' for east, or 'm' for middle (see the overview section for an explanation of the coelostat position). The computer will now ask you to change to the image lens and adjust the second flat. It also moves the dome to the appropriate position and opens the dome doors. Once it has done so it will light the blue light. This is a good time to center the second flat, if necessary. Go up to the dome. Make sure the image lens is in and that is it flush against the other lenses. Put the coelostat in the right position (east, west, or middle), align the primary mirror, and make sure the power to the primary mirror is on (you should be able to hear it running). Align the secondary mirror so that the light shines on at least one of the guiding table diodes downstairs. Press the button by the blue light until the light goes out. You will hear the motors in the secondary mirror as it centers the image of the Sun on the guiding table below. Don't forget to close the hatch when you are finished in the dome.
  6. The computer will then go through a centering process, which will take about 10 minutes. (This is actually your last chance to toggle the 'time critical obs.' switch if you think it is necessary.) When it is done, note the percent scattered light and serial number for the log. The computer will prompt with 'enter sky condition (zero to five):'. Enter a number (0 is perfect, 5 is bad). (If you are running a time critical magscan, the computer will prompt you at this point to enter the integration time. Just enter the default value of 15.) The computer will then prompt with 'please focus spectrograph at mm110' and light the blue light. This is an action that is no longer taken. Do nothing, but press the button by the blue light until the light goes out anyway.
  7. The computer will do a few more seconds of setup and then prompt with 'balance and zero' and light the blue light. (This is where one could adjust the dark current cancels and zeeman zero. See appendix A for instructions. If you are not adjusting them, which you usually won't, be careful not to accidentally adjust those dials; this is a common mistake.) Adjust the PM Coarse Balance (on rack 2) so that the DC Difference is zero. Note the coarse balance value for the log, and press the button by the blue light until the light goes out.
  8. The magscan is now running, and stops when the magscan (or magscans) is complete. At that time the computer will prompt with 'please remove the three minute aperture' and light the blue light. Remove the aperture and press the button by the blue light until the light goes out. The computer will then prompt with 'please focus spectrograph at mm75' and light the blue light. As before, take no action, but still press the button by the blue light until the light goes out. The magscan window will then close. If you need to terminate a magscan early, you can type 'stop magscan' in the comproc window.

How to Shutdown, Step by Step

  1. Make sure no observations are running. If one is running, either allow it to finish or stop it.

  2. Type shutdown in the comproc window. A shutdown window will pop up. It will be a short while before you are prompted to do anything, but if you like you may go ahead to the next steps in anticipation of the computer's requests.

  3. The computer prompts with 'cover optics, close PM tubes, don't forget the IRIS optics' and lights the blue light. Replace the tape cases, foil, etc. in the light path and on the guiding table. Turn the PM tube shutter (behind the phone) to the closed position. The IRIS optics no longer exist; don't worry about them. Press the button by the blue light until the light goes out.
  4. The computer prompts with 'cover dome optics, make sure mf lens is in' and lights the blue light. Go up to the dome. Make sure the the meanfield lens is in and that it is covered. Turn off the power to the primary mirror and turn it upside down. Make sure that the dome door closes. Press the button by the blue light until the light goes out.
  5. The computer prints the average daily meanfield; note this value for the log. Make sure to jot it down right away because the window should close as soon as the files are transferred to campus.

  6. Now you are done with the shutdown. Turn off the computer monitors and observing room lights for the night.

How to Keep the Log

Every time you start meanfields or magscans, you need to enter a line in the log. If you start a new page of the log, be sure to write the year in the upper right corner.

Annotate anything interesting or unusual in the log. It may turn out to be of interest later for discovering anomalies.

The following are the columns you need to enter:

  1. Day this is the day number of the current year. You can read it from the GMT clock, which is in the very top of rack 3. You only need to make one entry in this column per day.

  2. Date this is the local date, day and month. You only need to make one entry in this column per day.

  3. Start time this is the time you started the observation in universal time (UT). UT is 8 hours ahead of Pacific Standard Time (7 hours ahead of Pacific Daylight Time). This is the time that the GMT clock indicates. But it is most easily read from the observation window at the beginning of any observation. It is indicated on the line that begins with 'Setup time:'.

  4. Observer your name.

  5. Obs. Type either 'MF' for meanfields or 'MAG' for magscans.

  6. Coelostat Position - either 'E' for east, 'W' for west, or 'M' for middle.

  7. Lens either 'MF' for meanfields or 'IM' (image) for magscans.

  8. Coarse Balance read this number from the Coarse Balance dial after you have performed the 'balance and zero' part of the observation.

  9. Dark Current Delta Cancel, Dark Current Sigma Cancel, Zeeman Zero if you are only running WSO for a few days, don't worry about these. To learn how to set them, see the appendix.

  10. Serial Number entered only for magscans. Read it from the magscan window.

  11. Sky Conditions enter a number from 0 (perfect) to 5 (nearly too bad to do the observation). It helps if you also give a couple words describing why you chose the number you did. For magscans, also write the percent scattered light here. Also write 'T.C.' if you are running a time critical observation. Also enter any other information that you feel is relevant to the observation, such as any problems you may have had. This is also a good place to record when the mirror and/or lenses are cleaned or the tire in the dome is changed.

  12. Daily MF this the the average meanfield value for the day. Read it from the shutdown window, and record it on the last line for that day.

Troubleshooting

This section explains how to handle the most common errors you will encounter while running the observatory. It is imperative that you read this entire section before attempting to fix any problem. If something happens that you are not familiar with and the instructions in this section don't fix it, don't hesitate to call someone for help. A contact list is posted near daystar.

Appendix A How to set Dark Current Cancels and Zeeman Zero

Look at the panel in rack 2 above the magnetograph power supply. You will see dials labeled \Sigma DARK CURRENT CANCEL, \Delta DARK CURRENT CANCEL, and ZEEMAN ZERO. These dials should be adjusted about every two weeks, although they won't always change in that time. A good guideline is to check them every time you start a new page of the log. You do so during the 'balance and zero' phase of a magscan. When the computer lights the blue light, turn the PM tube shutter to the closed position. To unlock a dial, lift the small lever on its right side. First, unlock and adjust the \Sigma dark current cancel until the intensity shown in the display window on daystar becomes zero. Next, unlock and adjust the \Delta dark current cancel until the meter labeled DC DIFFERENCE (the same one you read when adjusting the coarse balance) reads zero. Finally, unlock and adjust the zeeman zero until the value of zeeman shown in the display window becomes zero. It may be difficult to get this last one to exactly zero; fluctuations of ± 1 or 2 are alright, but try to make the average as close to zero as you can. Make sure the dials are all locked and record their values in the log. Also, be sure not to touch the dial labeled ZEEMAN GAIN; its value should always be set to 2.0. You can now open the PM tube shutter and proceed with the magscan by adjusting the coarse balance dial in the usual way.

Appendix B How to Clean the Primary Mirror

Appendix C How to look at the spectrum and the bottom of the pit

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Based on the ''Paper'' Edition of the WSO User's Guide by Tim Larson, 2009. Uploaded to wiki November 3, 2009

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