ASTRONOMY 221 SARA IMAGING EXERCISE Spring 2011 Observing Exercise 4 Introduction: The use of the SARA telescopes at Kitt Peak, Arizona and Cerro Tololo, Chile, permit us to observe fainter objects that we could from VU. This is because the SARA telescopes are larger (36 and 24 inches, respectively) and are on better sites. Exercise: This exercise is designed to give you an opportunity to use one of the SARA telescopes to get images of interesting and fainter objects that can be done from VU. There are three parts to this exercise 1. You are to prepare a proposal requesting the objects that you would like to observe and why. You can choose to observe (a) a class of objects: (i) nebulae planetary nebular, star formation regions, supernova remnants, reflection nebulae, (ii) star clusters - open or globular, or (iii) galaxies; or (b) you can choose objects from a variety of classes. This will then require that you make a target list, with coordinate, and finding charts. Make a good case so that you are sure to get time! Due to the fact that we are hurrying to get on the telescope this Tuesday, we will postpone the writing of a formal proposal. 2. You will be assigned observing time on Tuesday, April 5 (waxing crescent moon); back-up Wed., April 6. This will hopefully be completed this week Wednesday. If you want to make color images, then observe your object(s) with 3 filters. 3. You will reduce the data to make your scientific images. For the final report, include the usual Introduction, Observing section, Reduction section, and a short Summary & Conclusion. I plan for us to start the reduction on Monday, April 11. The final report will be due on Fri, Apr 15. Although you will observe with a partner, you should each submit a separate proposal and report.
Preparation of the Observing Target List You want to observe with either the 0.9-m SARA-N or the 0.6-m SARA-S telescope. However, first you need to go through the steps necessary to prepare for and execute a successful series of observations. These steps are outlined below. Professional astronomers must be well prepared to use a telescope for research. Observing time is precious and highly sought after. This exercise will introduce you to some of the procedures involved in this preparation. Goals: 1. To adequately prepare your group to use observing time on a research telescope. 2. To perform your desired observations and complete them in the time allotted. 3. To learn how professional astronomers use telescopes to perform astronomical research. 4. Finally to reduce your images to get nice, (colored,) and scientifically useful images. Equipment/Software Needed: Computer, web browser, printer, calculator. Procedure: The procedure for this lab will be completed in four parts: (i) preparing an appropriate list of targets, (ii) printing out finding charts for each object, (iii) preparing an observing log, and (iv) observing your object. CHOOSING YOUR TARGETS: Before you use the telescope, you need to know which object(s) you are planning to observe. Your lab instructor will provide you with a list of potential targets. From this list you need to decide on two primary target and at least two back-up targets. A. The Right Location: In this portion of the lab you will determine a range in coordinates for appropriate targets for your group s observing time. Not all targets on your list will be easily observable at all times of night. This means that different observing groups will likely be choosing their targets from different subsets of the list provided. The first limit you must obey is declination. Recall that this astronomical coordinate is analogous to latitude on the Earth. The north celestial pole is at +90 degrees declination, the equator is defined as 0 degrees declination, and the south celestial pole is at -90 degrees declination. The range in declination that you can observe depends on your location on the Earth. The SARA-N telescope is located just west of Tucson, AZ at a latitude of +31.95 o degrees and the SARA-S telescope is in the foothills of the Chilean Andes at a latitude of -30.15 o. Q1. Based on this location, at what angle above the horizon is the north (south) celestial pole? Q2. What is the declination at your zenith? The best targets for observing are generally more than about 40 degrees above the horizon, or within 50 degrees of the zenith. Q3. Based on this and your answer to Q2 above, what is your preferred range in declination for observing targets? to
The second, and more involved limit is right ascension (RA). Recall that RA is analagous to longitude on the Earth, but goes from 0 to 24 hours. You must determine what RA is near your meridian at the time you will be observing. The value of R.A. that is on the meridian at any given time is called the sidereal time. The following procedure will walk you through the steps of calculating the sidereal time for your scheduled observing time. Step 1: Convert local time to Universal Time (UT). For central standard time (CST), add 6 hours to the local time to find UT. For central daylight savings time (CDT), add 5 hours to the local time to find UT. Local time (24 hr clock) + 6 (5) hours = UT = + = Now convert this to hours and decimal hours: UT = Step 2: Convert solar interval to sidereal interval. Recall that a sidereal day is approximately 4 minutes shorter than a solar day. Therefore, a sidereal hour/minute/second is shorter than a solar hour/minute/second. This means that in a given amount of time, more sidereal seconds will have passed than solar seconds. The conversion factor to change from solar time (UT from step 1) to sidereal time is 1.00273791. UT x 1.00273791 = Sidereal time = x 1.00273791 = Step 3: Determine the Greenwich Sidereal Time (GST) at the time of your observations (UT is also known as Greenwich Mean Time, or GMT). To find this value, you will need to add the sidereal time found in step 2 above to the sidereal time at 0 hours UT at Greenwich on the day of interest. You will be provided with the Greenwich sidereal time at 0 UT for the observing dates. Sidereal Time + (GST @ 0 UT) = (GST @ time of observation) = + = Step 4: Correct for the longitude of your observations. The SARA-N telescope is located at a longitude of 7h 26m 28s (or 7.441 hours) west and the SARA-S telescope is located at a longitude of 4h 48m 54s (or 4.721 hours) west. To find the Local Sidereal Time (LST) at the SARA telescope during your observations, add your result from step 3 to the above longitude. (GST @ time of observation) - 7.441(4.721) hr = LST = - 7.441 (4.721) = Now convert this back to units of hours, minutes, and seconds: LST = You finally have your Local Sidereal Time! This is the RA of a celestial object that is on your meridian at the time of your observations. Q4. It is best to make observations within about 2-3 hours of the meridian, so your preferred range in RA is: to B. The Right Size and Brightness: In addition to making sure your targets are in the right location in the sky, you also want to make sure that the telescope and instrument you are using are capable of observing them. For example, the field of view of the camera on the SARA telescope is roughly 10 x 10 arcminutes (or 1/6 of a degree on a side). You will be observing extended objects. Stars appear just as points of lights through a telescope. Other objects are large enough that they do not look like points of light; they are extended. Some objects are much too large or much too small to give good results in our camera. So you will need to make sure that whatever target you choose is not so big, or so small, that it will not work well with the telescope and instrument you will be using. Also, some objects are too faint to easily observe in a short period of time. You and your partner will have 90 minutes to use the telescope, so you must be sure that your object is bright enough to observe in this timeframe. One way to estimate this is described below. In general, determining if an object is bright
enough depends specifically on the telescope, instrument, amount of water vapor or clouds, and other factors, so your lab instructor will be available to help you with any cases where you are unsure. Table 1 is provided for you to record your preferred targets. You will need one or two primary targets and at least two secondary targets (though you may have more). You will also need to record the coordinates of each target in both RA and Declination. To find these coordinates you will use information available via the internet. Open a web browser and go to the following website: simbad.u-strasbg.fr/simbad/ and click on the by identifier link under Queries. Here you should type into the Identifier box the name of one of the targets from the list given by your instructor and click on submit id. The resulting page provides a large amount of information on the object, including several values for the object coordinates. Use the ICRS coord line to record the RA and Declination (the information will be in the format hh mm ss.s dd mm ss.s where h stands for hour, m for minutes, and s for seconds in the RA coordinate and d stands for degrees, m for minutes, and s for seconds in the Declination coordinate). Take a few minutes to explore the rest of the information provided and see if you can determine what each item provides. FINDING CHARTS: The Simbad information page described above will also lead you to the next requirement for observing. While modern telescopes typically point very accurately, you need to confirm that the telescope is in fact pointing to the object you wish to observe. The way to do this is to produce a finding chart. A finding chart is often an image of a region of the sky which contains the object you wish to observe. In the Simbad information page, scroll down to the Plots and Images section and click on Aladin applet. This will launch a Java application and provide you with an image that can be used as your finder chart. The image will also be helpful in determining if the given object meets the requirements in section B above. When you find an object for which you wish to observe, then print a finding chart. Typically the images produced with the Aladin applet are between 13 x 13 arcminutes and 15 x 15 arcminutes, which are appropriate sizes for our needs. When you have identified at least four targets (two primary and two secondary), make sure that you have the necessary information filled out in Table 1 and have a printed finding chart for each. Under Notes place other info, such as type of target, something interesting about it, etc. TABLE 1: TARGET LIST (epoch 2000.0) Object RA Declination Notes
OBSERVING LOG: While you are at the telescope and performing your observations, it is important to be thorough and complete in recording what you are doing. This helps in many ways and you will use this information next week when you analyze the images that you obtain at the telescope. Attached is an observing log sheet that you will need to bring with you to your observing session. There is some preliminary information you can fill out beforehand (date, observers group members, etc.); in fact doing so will save time when you are observing. You will fill out the majority of the observing log during your observing session. Honor code: Spring Target List Southern Hemisphere M1 NGC 1825 NGC 2070 NGC2516 NGC 2906 NGC 2927 M42 NGC 2808 NGC 3119 NGC 3201 NGC 3293 NGC 3372 Greenwich Sidereal time at 0h UT on April 05/06 12h 51m 48s April 06/07 12h 55m 42s