Observation: NOT OBSERVING Either Not observing, Waiting, On Source, On reference, Scanning etc.

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1 JODRELL BANK OBSERVATORY 7-M RADIO TELESCOPE: OBSERVING MANUAL The Jodrell Bank internet Observatory (JBiO) is a web interface to Jodrell Bank's 7-m radio telescope. The telescope itself is actually controlled by a VAX computer running a VMS operating system. The software which handles communications between the telescope and the web server will only be running at the times designated for internet observing. Access to the telescope is obtained from the web page 1. The Telescope Monitor When the internet observing software is running, the telescope sends information about its current status to the web server and this can be displayed using the Monitor link at left. If you click on this link the monitor information will be displayed in this page. If you like, you can open it in a new window by right-clicking the link on the menu bat at left, and selecting Open in New Window. This page will refresh automatically every 10 seconds with the latest data from the telescope. You can see the time at which the information refers to at the top of the screen. If it looks like this is not updating you can try manually refreshing (or reloading if using a Netscape browser). If this does not give recent (i.e. within last seconds or so) information then check whether the telescope is supposed to be performing internet observations at the moment. The current schedule is summarised on the Home page of this website - click on the Home link on the menu bar. The only other thing that might stop observing is mechanical failure of the telescope (thankfully rare) or weather (high wind or heavy snow) - you can check on the local weather conditions by going to the Jodrell Bank Live page at and looking at the details of one of the local telescopes (Lovell, Mark II or 42ft). An annotated example of the Monitor screen is shown below: Control: Local What is controlling the telescope - when using the web interface this should read Local Time (UT): 12:32:27 13/12/2004 This is the time (according to the Telescope Control Computer) at which the information was sent to the web server - hh:mm:ss dd/mm/yy - this is Universal Time, remember your time zone may be different from this; all observations must be set up in UT MJD: Modified Julian Date. The number of days since midnight on 17 November 1858 Observer: Name of current observer plus a 10 digit number uniquely identifying this observation made up from time at which the observation was made in yymmddhhmm format Coord system: GALACTIC Either galactic, equatorial or Alt-Az Coordinates: (Long, Lat) if galactic; (RA, Dec) if equatorial; (Alt, Az) if Alt-Az. Motors: ON If the telescope is working these should be ON! Observation: NOT OBSERVING Either Not observing, Waiting, On Source, On reference, Scanning etc. Telescope: TRACKING Either Tracking, Off Source, Calibrating, etc. Received power: 0.21 Total power currently being received in db (i.e. a change of 3 in this number is a factor 2 in received power) Observing Freq: (inc. LSR & vel) In MHz - should be around but usually has an offset due to the motion of the telescope with respect to the Local Standard of Rest (LSR), or because of observing a source with a given Doppler shift (from its velocity) or if a frequency offset has been explicitly requested for calibration (as in this case) Secs. Remaining: 0 Time left on an observation Azimuth Elevation Azimuth is degrees east of North, Elevation is degrees above the horizon Actual: Where telescope is pointing now Demanded: Where telescope wants to be pointing Errors: Difference between actual and demanded positions - significantly different from zero only when telescope is driving towards a new position Offsets: Only non-zero if the telescope has offsets set away from its nominal pointing position (e.g. while scanning) Right click to manually refresh Last updated: Mon Dec 13 12:32: This is the time according to the communications software at which the monitor information was recorded

2 2. Webcam Clicking on the Webcam link in the menu bar will launch a separate window in which you will get an image of the 7-m telescope which updates every 30 seconds. The view is towards the east (Figure 1). The date/time at which the image was taken is shown at the top of the image - if it does not appear to be updating then right click just below the image and select Refresh (in Internet Explorer, or in Netscape select Reload Frame ). It isn't essential to look at the Webcam image but it is sometimes nice to convince yourself that when the Monitor tells you the telescope is slewing to a new set of coordinates it really is moving! 3. Skymap Figure 1: Live webcam image of the JBO 7-m Clicking on the Skymap link gives a live map of the sky above Jodrell Bank Observatory, as shown in Figure 2. This is looking up at the sky so North is at the top, South at the bottom, East at the left and West at the right. The concentric circles are of elevation above the horizon labelled every 15 degrees. The plane of the Milky Way (galactic latitude zero degrees) is shown as a curved line running across the sky. It is labelled with galactic longitude in degrees. Various radio sources (and some visible constellations) are also indicated. The sign of galactic latitude can be determined by noting that Cas A and Tau A are both at negative galactic latitudes (i.e. towards the Galactic South Pole). Hence positive galactic latitudes will be on the other side of the galactic plane from these two sources. The position of the 7-m telescope is shown as a target-like crosshairs symbol. When the telescope is asked to move to a different position another set of crosshairs appears which shows the demanded position. The image updates automatically every 10 seconds. If the image fails to appear you can just wait 10 seconds until the next automatic refresh. Figure 2: The Skymap image shows a representation of the sky above Jodrell Bank Observatory, and the current pointing position of the 7-m radio telescope.

3 4. Setting up an Observation The process of setting up an observation will seem complicated and lengthy at first, but once you have tried it a few times it is actually quite straightforward and only takes a few minutes. Clicking on the Setup Obs link will access a page which allows three types of observations to be set up. You will be asked for your username and password when you first click on Setup Obs. There are a number of stages which are dealt with in detail below: 1. Decide on your target i.e. its coordinates 2. Check when your target is available for observation i.e. when it is above the horizon 3. Check the schedule to see which timeslots are available. Remember that these timeslots are in Universal Time (UT) 4. Set up all the details of your observation (e.g. coordinates, reference, integration time, timeslot etc.) and submit to the web schedule 5. When you're happy your observation is correct, flag it for submission to the telescope scheduling computer 6. If all goes OK sit and wait for the observation to occur in the timeslot you requested 7. Once the observation is finished view the data, carry out any analysis required and think about whether another observation is necessary 5. Target Choice The position of a target will be expressed in either galactic or equatorial coordinates. Galactic coordinates are longitude and latitude with respect to the plane of our own Galaxy, the Milky Way. Basically anything along the plane of the galaxy is at latitude 0 degrees, 90 degrees is perpendicular in a direction above the plane and -90 directly below the galactic plane. Longitude 0 degrees is towards the galactic centre increasing in the direction the Sun is moving, to the left as viewed from the Earth. The position of an object in the equatorial coordinate system is expressed as Right Ascension (RA) and Declination (Dec). These can be understood as a projection of the Earth's longitude and latitude (respectively) projected up onto the sky. Hence objects directly above the North Pole have a declination of +90 degrees, above the South Pole of -90 and above the Equator of 0 degrees. The Right Ascension is measured in units of time with 24 hours marking 360 degrees since the Earth rotates once in 24 hours. Zero hours RA is defined by the position of the Sun at the Vernal (Spring) Equinox as it passes over the equator on its way from the Southern to the Northern sky. This means that objects with 0h RA are overhead at around midday on 21 March. Here, technically we should say on the meridian rather than overhead - the meridian is the line on the sky running from north to south and passing directly overhead the observer. This changes by about 2h every month (making 24h in a year) due to the Earth's motion around the Sun. Hence towards the end of June (3 months after 21 March) objects of 6h RA will be on the meridian around midday and around midnight (12h later) objects of 18h RA will be on the meridian. Once you have the coordinates it is useful to know at what time of day or year the object is visible above the horizon. This is the next step in the procedure. 6. Target Track By clicking on the Target Track link in the menu bar at left you can access a utility which will convert between galactic and equatorial coordinates and also display the elevation above the horizon of a given object during a particular day of the year as viewed from Jodrell Bank Observatory. Check the elevation of a position with galactic coordinates (120, 0) (i.e. longitude 120 degrees, latitude 0 degrees) for 18 December You should find it is at a reasonable elevation all day. Now try coordinates (120, -40). This will be at too low an elevation during the morning hours. Note times will be expressed in Universal Time (UT) - effectively Greenwich Mean Time (GMT). It is important that you assess whether the observation you intend to make will be completed in the time slots you allocate to it. Each 10 minute slot must include time for the telescope to slew to its new target. If you submit an observation on the other side of the sky from one done in the previous time slot, it is probable that there will not be enough time to complete both the slew and the observation. Such observations will stay in the queue indefinitely and will never complete. This may be alleviated by allocating more than one time slot to a given observation. In all cases, try to be sensible about what you are asking the telescope to do. 7. Show Queue & Show Archive We have broken up the periods in which the telescope is available for internet observing into 10 minute slots. These are all expressed in Universal Time. You will normally be allocated 10 slots of observing time. When you submit an observation, the software will detect whether you have exceeded your maximum number of observing slots. Any single observation can be allocated to from 1 to 4 consecutive slots i.e. no single observation can take longer than 40 minutes. Also, even if an observation only takes 5 minutes the next

4 observation will not begin until the start of the next 10 minute slot. Of course it may be that another observer has already appropriated a slot you were interested in using. Click on Show Queue to display a list of all observations which have been queued for the current session. An example is shown below: Status User Submitted Scheduled Slots Type Source Details Submit Delete 2 TOBR Dec-14 14: :20 2 Spec 105, 10 Details Submit Delete We see here that user TOBR has queued one observation. It was submitted at 14:45 on 14 December and is scheduled to take place at 10:20 on 18 December and lasting for two 10 minute slots i.e. you would be able to submit another observation to start at 10:40. You can access more details of each observation by clicking on the Details link. The status field can contain one of 7 numbers: 0: Observation just entered, waiting to be submitted to telescope queue 1: Observation submitted for scheduling, waiting for confirmation from telescope scheduling software 2: Observation now queued and waiting to be done 3: Observation in progress 4: Observation completed 8: Observation rejected by telescope scheduling software - already scheduled 9: Observation rejected by telescope scheduling software - invalid format Please note that it is possible that an observation which is classed as having begun (status 3) may not complete perhaps due to another observer scheduling an observation before yours has completed or a telescope/software break down. You can always resubmit an observation perhaps allowing an extra slot to ensure your observation completes. We will periodically remove observations at status 3 which have never completed from the queue. Any observation which has been completed is automatically removed from the queue and then appears at status 4 (completed) at the bottom of the list obtained by clicking on the Show Archive link on the menu bar. In this list, data should be available for any observation by clicking on the Data link. Try this on the archive list for any observations with a status 4 green code. An example of the result is shown in Figure 3 where plotting of the data is achieved using a java applet. You must have java installed and enabled for this to work. What you see is a plot of brightness temperature in each of 1024 channels in a bandwidth of 5 MHz. Emission from HI should show up at MHz (here labelled in units of 10 3 MHz i.e. GHz). In general the emission can be Doppler shifted to frequencies above and below the rest frequency hence the spread in the peak shown in the example plot. In this case the hydrogen emission is the complex multiply-peaked structure. The curved background is just due to the continuum radio emission from the sky plus receiver noise. If you like, you can zoom in on a region of interest by clicking and dragging out a rectangular box, click on Fill to zoom back out to the whole graph again. You can also view the data by clicking on View datafile. You can also change the horizontal axis to velocity by changing the selection button below the graph. This is calculated using the (nonrelativistic) Doppler shift equation with a rest frequency of MHz. Another useful feature is that you can perform a temperature integral under the emission line by clicking on points on the continuum (the sloping background) to left and right of the line. You may occasionally see narrow spikes of interference or a pattern of spikes near to 1420 MHz as in the example in Figure 3. It is possible to smooth the data by entering a number into the field labelled Median Smooth. If the number 5 is entered (and carriage return hit) then the plotting program replaces every value in the spectrum by the median of 5 numbers centred on that value this has the effect of removing noise spikes. Note that when you first submit an observation it is at status 0 with a red exclamation mark i.e. only just entered onto the queue (we shall see how to do this in a moment). If the observation is left at status 0 it will not occur since it has not been passed to the telescope scheduling system. In order to give you time to check your observation or discuss with colleagues the submission to the telescope is a separate process initiated by clicking the Submit link at the right of the table. Although you will be able to see details and view data for any observation you will only be able to submit your own observation by entering a password which you set when the observation was initially placed on the queue. Submission changes the Status to 1 and then the Figure 3: Example of a spectrum taken with the 7-m

5 web software should pass it onto the telescope within less than a minute. If the telescope control system is happy with the observation details requested the status flag will change to 2 and you need then only sit and wait for the observation to take place. When you submit an observation to the queue, it is important that you supply your username correctly. The software will not allow you to perform observations using an unrecognised username. When submitting observations, the password field is used only to identify your observations later on, so that you can delete or submit them at will. It is not, and need not be, the same as the password you used to first access the Setup Obs page. In fact, this password field can be left blank, but that would mean any user could submit or delete your observations. The Show Queue web page does not automatically update you will need to manually refresh it by clicking on the link at top or bottom of the queue display - you should see the status to change to 2 when your observation is accepted by the telescope and then to 3 when the observation is running and finally 4 when completed and the data are available for viewing. Within about 30 seconds the observation is then moved to the Archive List. You do not have to wait online for observations to complete; you can submit observations days in advance. You do not even have to be online when the observation takes place, although it might be fun to see the monitor as your observation occurs. You can simply connect back sometime later and view the resulting data. If at some point you decide you don't want a particular observation you can click on Delete and by supplying a password remove it from the queue. At the moment it isn't possible to remove an observation after scheduling on the telescope. 8. Setup Obs In order to set up an observation and place it on the queue you will need to click on the Setup Obs link. There are detailed instructions on this page itself. Other than the obvious parameters to which we have already referred such as target coordinates and scheduling time you will also need to define a reference observation in order to correct for the response of the receiver or to subtract background emission. There are two types of reference observation: Frequency-switched observations (used generally for observations of our own Milky Way) in which the reference has the same coordinates as the source (or target) but the observing frequency is shifted by several MHz. This moves the HI emission outside the band and the reference observation therefore just defines the instrumental response which is then automatically subtracted from the source observation. Position-switched observations in which no frequency offset is made but the reference coordinates are shifted by several degrees from the target direction. This provides a spectrum of HI in a neighbouring line of sight to that to the target and when it is subtracted can be used to minimise the contribution of emission from the Milky Way when making observations of other galaxies. 9. Sequence of Events During an Observation A single observation consists of the following sequence of events which can be followed on the monitor: 1. Just before your observation is scheduled to take place the telescope will be in the middle of repeating the observation last scheduled. 2. When the time arrives for your observation the status will switch to: Aborted Run not set as the scheduling system swaps to your observation. 3. Once the observing parameters are set the status will show Waiting Off source (slewing) Source as the telescope slews to your target coordinates (note difference between demanded and actual az/el - assuming previous observation was not already pointing in the direction you require). 4. When the demanded position is reached status changes to: Integrating Normal The observing frequency should be close to with a small correction due to the telescope's velocity with respect to the Local Standard of Rest (typically a few 10's of km/s) or a larger correction due to a source velocity (if set) - note a 100 km/s velocity produces a shift of about 0.5 MHz. 5. When the countdown reaches zero the calibration process begins and status will show: Done

6 Calibrating You should note that first the received power will increase as a calibration level from a noise diode is added in. Then it decreases as a zero level is checked. 6. The telescope will then move on to the reference position showing: Waiting Off source (slewing) Reference Note the error in demanded/actual position. If this is a frequency-switched there is no actual change in position but coordinates are checked so this status will show briefly. 7. Once the telescope is happy it is pointing at the reference position then status will switch to: Integrating Normal If this is a frequency-switched observation then the frequency will change to the previous value plus the offset (+/- 3 MHz). 8. Once the integration is complete there is a calibration process on the reference: Done Calibrating 9. The reference spectrum is then subtracted from the source spectrum and the result is returned to the web server for you to analyse. 10. This whole sequence of (source:calibration):(reference:calibration) will now repeat until a new observation is scheduled.

7 JODRELL BANK OBSERVATORY 7-M RADIO TELESCOPE EXERCISE 1 JODRELL BANK OBSERVATORY 7-M RADIO TELESCOPE EXERCISE 1: BEAM PATTERN OF A RADIO TELESCOPE Exercise 1: Aim You will scan the 7-m telescope across a bright radio source (Cassiopeia A) in order to measure the telescope s beam-width. This defines the region of the sky from which the telescope is sensitive to radio waves. It depends on the telescope diameter and the frequency of observation. You will compare the measured beam-width to the expected value for the 7-m telescope. Scanning with the 7-m Radio Telescope Click on Setup Obs (entering your username and password if required) and then on Total Power Scan. In this experiment we will use the 7-m telescope to measure the total power received in a 5MHz band centred on the hydrogen line at 1420MHz. Follow the instructions by entering your username again on the web form (and a password if you wish) for identifying your observations. Then click the button labelled Equatorial. This will enable us to pick radio sources from the list under Target Details in the Equatorial Coordinates box. Select the radio source Cassiopeia A. This is the remnant of a supernova explosion in our Galaxy and is always visible above the horizon. You can check visibility by clicking on Target Track in the navigation bar, selecting the Equatorial coordinate button, clicking on the Cas A button, and entering today's date. Then click on Submit. You should then see a graph showing the elevation of Cas A above the horizon at Jodrell Bank Observatory during the day. Leave the next button checked as an Azimuth scan. This means the telescope will scan through the target in one axis only, one that is parallel to the horizon. Now select the number of degrees to offset by before the telescope starts scanning. We will use -5 degrees as a reasonable starting value. We also need to say how long we want the scan to take. In effect this sets the scan rate. So for example if we say the Integration Time is 300 seconds and we'd asked for a scan starting at -5 degrees then the whole scan will cover -5 to +5 degrees i.e. 10 degrees in total in 300 seconds, so the scan rate is 2 degrees per minute. Select 300 for the Integration Time. Now select the time slot in which you wish the observation to take place. These are every 10 minutes, so either pick the nearest next slot or one sometime in the future (try to avoid picking one in the past!). The time is Universal Time at Jodrell Bank Observatory so you can find out the current time from clicking the Monitor link in the navigation bar. You also need to say how many 10 minute slots you'll need. You are able to pick more than 1 to ensure a longer observation is completed, but setting this to 1 should suffice, as long as the telescope isn t scheduled to be on the other side of the sky when your observation begins! Then click Add to Queue. If all the details were correct, a screen will appear saying your observation has been added to the queue. However you must now actively submit it to the telescope for observation. Click on Show Queue. Your observation should appear as status 0 with a red exclamation mark. If you are happy the details are correct you can click on Submit (entering the password you supplied on the Setup Obs screen; if indeed you did supply one, otherwise leave password box blank). If something was wrong you can delete it before submission. The observation will now appear in the queue at status 1. If you manually refresh the queue you should see the status change to 2 once the telescope control computer has received the information. It is now just a matter of waiting until the observation takes place. The Observation Watch the Monitor, Skymap and Webcam when the time approaches for your observation. At the allotted time these should show the telescope slewing towards the demanded coordinates. Note that the telescope will first be sent to the offset position of -5 degrees from the Cas A coordinates. When the telescope pointing error reaches zero, the observation begins. As you watch, the telescope will scan through Cas A from -5 degrees to +5 degrees of its position in the azimuth axis. After about 5 minutes (assuming you set Integration Time to be 300 seconds) the Acquisition status will change to Done and after some calibration the scan is complete. Refresh the telescope queue to see that your observation has now changed to status 4 and you can click on the Data link to see the results. Alternatively, look under the Archive list after the observation completes and click on the Data link for your observation.

8 JODRELL BANK OBSERVATORY 7-M RADIO TELESCOPE EXERCISE 1 Measurements & Analysis In the resulting data plot, you should see a scan showing the total radio power rising and falling as the source moves through the beam. If the source is unresolved i.e. a point source, then this effectively maps out the beam shape. The plotting applet allows you to click on a point either side of the peak and subtract a sloping background - the colour of the curve changes from red to blue. You can then estimate the full width of the beam at half the peak power - the half-power beam-width (HPBW) in degrees. Measure it directly from this graph by eye. You can redisplay the original data by selecting the appropriate box. You can also click on the graph and drag out a box to zoom into any region. Click on the Fill button to zoom back out. Make a note of your estimated beam-width for the 7-m telescope. The expected HPBW (in radians) of a radio telescope is proportional to the ratio of the wavelength λ and the diameter D of the telescope (when both are measured in the same units, e.g. metres). The proportionality constant, η, typically lies between 1 and 1.4 and its exact value depends on the aperture illumination for the particular telescope in use. Find the value of this proportionality constant from your observations, given that the frequency of observation is MHz and the telescope diameter is 6.4 metres. You may need to recall that frequency ν, wavelength λ and the speed of light c are related by the expression c = νλ, that c = 299,792,458 m/s and 1 degree = radians. The plotting applet can also plot the theoretical beam profile for a uniformly illuminated circular aperture of diameter 6.4 metres if the appropriate checkbox is selected. Select the original data again, click this check box and refit the background. A real aperture tends not to be uniformly illuminated resulting in a slightly larger beam-width and lower sidelobes. Of course, if the actual source were extended then the profile would appear even broader. Compare your scans with the theoretical profile to assess whether this expectation is correct.

9 JODRELL BANK OBSERVATORY 7-M RADIO TELESCOPE EXERCISE 2 JODRELL BANK OBSERVATORY 7-M RADIO TELESCOPE EXERCISE 2: HI EMISSION FROM THE DISC OF THE MILKY WAY GALAXY Exercise 2: Aim In this experiment you will take a spectrum of HI from the Milky Way Galaxy. In particular, you will make observations that will support the existence of spiral arms and determine the velocity of HI peaks corresponding to emission from those spiral arms. Introduction Due to the rotation of our Milky Way galaxy, along any given line of sight different regions of hydrogen gas we observe will be moving with different velocities with respect to us. Therefore, the emission from hydrogen which is generated at MHz is actually observed at a range of frequencies due to Doppler shift. This produces a profile in the spectrum which varies in appearance for different regions of the galaxy. To measure these HI (neutral atomic hydrogen pronounced H-one ) profiles, we pass the signals into a high-speed digital sampler from which we can use a Fourier transform technique to obtain a spectrum of the radio emission. Don t worry if you don t understand what a Fourier transform is it s basically a way of getting information on the different frequencies present in a signal by sampling the signal strength at a very high rate. When plotting spectra, it is convenient to work in temperature on the y-axis and frequency or velocity (computed using the non-relativistic Doppler formula) on the x-axis. A useful rule-of-thumb is that, at this observing frequency (approx 1420 MHz), the 5 MHz bandwidth of the 6.4m telescope receiver corresponds to a velocity width of approximately 1000 km/s. All the spectra you will measure are in fact rather weak and are superimposed on a large system temperature (the background noise level due to the receiver) which has a curved shape. We require some way of subtracting this, leaving just the spectra. One way of doing this would be to point the telescope at the target of interest, collect the signal for a while (called integration) then move a few degrees off to one side to obtain a spectrum of a reference position, and then subtract the two spectra. Whilst this position switching method is fine for observations of objects outside our own galaxy, when looking at HI in our galaxy there is virtually no clear part of the sky. In this case, we obtain a reference spectrum by changing the observing frequency whilst pointing at the same position. Such a frequency switching mode can be achieved by changing the frequency of the local oscillator (signal generator). In this case the background spectrum which is subtracted is then just a section of the spectrum to one side of the MHz emission line region. A Spectrum from Galactic Position (120, 0) For your first observation of HI in the Milky Way Galaxy we will select a position in the galactic plane that is visible all the time from Jodrell Bank Observatory (technically objects like this that never set are called circumpolar - they rotate around the celestial pole without ever going below the horizon). In galactic coordinates a good choice would be a longitude of 120 degrees and latitude of 0 degrees. The latitude ensures we are exactly in the plane of the galaxy, whilst the longitude is chosen to ensure the position is never below the horizon. You will now need to check using Show Queue what observing slots are available - an on-source integration time of 120 seconds should be sufficient so you should only need one slot (the observation will take about 5 minutes: 2 minutes on source plus 2 minutes on reference plus about 1 minute for calibration overheads etc.). You will need to pick a slot a little in advance of the current time (you can find out the current time according to the telescope by looking at the Monitor page). Select a slot a few minutes ahead at least, to give time for you to confirm it (see below) and for the software to insert the observation into the telescope queue. In the Setup Obs section (after supplying your username and password if required) select the first option a simple 21cm spectral line observation. On the web form, input your username again and, if you wish, a password to identify your observations. Set the galactic longitude and latitude to be (120, 0) and the integration time to be 120 seconds. The system automatically sets up a frequency-switched observation with a default -3 MHz offset. Set the slot in which you wish the observation to take place. Remember the time is in Universal Time. Select 1 for the number of slots required. Click on Submit to add the observation to the queue. Once you have inserted your observation in the queue check the details from the Show Queue page. Your observation will appear at status 0 with an exclamation mark next to it. If you're happy, click on the Submit link on the Show Queue page. Status should change from 0 to 1 and then, if the observation is

10 JODRELL BANK OBSERVATORY 7-M RADIO TELESCOPE EXERCISE 2 successfully queued, to 2 a few minutes later (you'll need to refresh the Show Queue screen to check if the status has changed). You should be able to use the Monitor, Skymap and Webcam to watch the telescope move and your observation take place (status will be 3 in the list while the observation is running) and when it is completed the status should change to 4 and the data should then be available for viewing. Note that for the spectrum observation, the telescope first integrates on the target position, then integrates at the same position with the frequency offset added. When the observation completes, it is moved from the Queue list to the Archive list under Recent Observations click on Show Archive to show the data and click on the Data link to see your spectrum. Measurements & Analysis The HI line profile should show a curved background level with a large peak of emission due to hydrogen around a frequency of MHz. This peak should show some structure due to Doppler shifts of hydrogen in several spiral arms along that line of sight through the galaxy. We can measure the velocities of each of the spiral arms visible in your spectra. There should be three clear peaks in the hydrogen line corresponding to three spiral arms. For the purposes of this project it will be sufficient to simply measure the positions of the peaks by zooming in and noting the velocity value at the highest point of the peak. On the data plot you can choose whether to plot Frequency or Velocity along the horizontal axis in the pulldown menu. You can also carry out a temperature integral by clicking to the left of the emission line and then to the right - a line is drawn and the emission above this is integrated. This can be done on either the frequency or velocity plot; the units of the integral whose value is displayed in the third box are then K MHz or K km/s respectively. The values of the start and end point of the integral on the horizontal axis are also shown in the first two boxes. You can also click on the graph and drag out a box to zoom into any region. Click on the Fill button to zoom back out. To measure the velocities of three peaks, display the spectrum with velocity as the horizontal axis then zoom in to the region near the top of one of the peaks by clicking at the top left of a small rectangle centred on the peak and then dragging to the bottom right of a rectangle. Estimate the velocities of all three peaks by eye, noting that the labelling on the axis may need to be multiplied by 100 to get the actual velocity in km/s. You should find the velocities are about -100, -55 and 0 km/s (with a little shoulder at around -10 km/s). Look at the schematic picture of the spiral arm structure of the Milky Way in Figure 1 (left) and work out which spiral arm corresponds to which peak in your spectrum. Note that because the galactic disc does not rotate as a solid body (in fact many parts of the disc rotate at almost the same velocity) this means that the Sun is overtaking the stars and gas which are farther away from the centre of the Galaxy than itself i.e. we are overtaking on the inside. Hence when viewing at a longitude of 120 degrees we are looking at hydrogen gas which we are approaching and hence the Doppler-shifted velocities are negative! Furthermore, if we assume that all the material in the disc is actually moving at the same speed in a circle then the component of velocity along the line of sight is smaller for the gas which is farther away. Remember that the velocity plotted on our spectra is the relative velocity between ourselves and the cloud i.e. V 2 -V s for cloud 2 in Figure 1. This relative velocity is therefore larger for the clouds of gas which are farther away in this direction. You should therefore be able to conclude that the peak at -100 km/s is due to the Outer Arm, the one at -55 km/s the Perseus Arm and the one at 0 km/s to hydrogen in our Local Arm which is moving with the Sun. Figure 1: A schematic representation of the spiral structure of the Milky Way Galaxy and the components of velocity