a) Set the declination to trace the winter Sun's path and rising and setting points in

Transcription

1 Astronomy 3130 Spring 2017 Observation Lab 1 (Beta Version) Getting around the sky and observing with the 6 doghouse telescope As an observer, a primary skill is getting your telescope pointed to an appropriate target and centering it in your eyepiece/instrument. In the modern world the telescope and control system takes care of virtually all of this overhead and your target magically appears close to the center of your instrument field of view. To some extent this lab is about gaining an appreciation for what happens behind the curtain as you assume the role of the telescope control computer. The lab is also oriented toward giving you familiarity with the celestial coordinate system of Right Ascension and Declination and positioning an equatorial mount telescope on the sky using that coordinate system. All activities here make use of the 6 refracting telescope in the doghouse at McCormick Observatory. The TA(s) will be your guide to proper use of the facility. First some generals activities/tasks: 1) Measure (only as accurately as great care for the glass will allow) the objective (main lens) diameter of the doghouse telescope. No touching the glass. Yes, it is a 6 telescope, but is that really the clear aperture? In this class, if you make a measurement be sure to attach a wellreasoned (or measured) uncertainty where appropriate. 2) Measure, crudely, the focal length of the telescope by measuring the distance from the objective to the eyepiece. 3) Catalog the focal lengths of the eyepieces in the eyepiece box. For each eyepiece calculate the magnification given the focal length of the telescope. (Basic trick for later: Acquire your target in the widest field (typically lowest magnification) eyepiece that you have, center it in that eyepiece, and then put in higher magnification if you want it.) 4) Set R.A. so the telescope is on the meridian. Use the level to get the telescope pointed to the horizon as exactly as you can. Measure the declination reported on the declination wheel (or use the declination vernier/magnifier if you are so inclined). From this measurement you should be able to derive your latitude presuming the telescope is properly installed. Make the measurement to a precision of 0.1 degree as best you can. 5) Use the above process for measuring of your latitude to explore the statistical nature of the observation. Have each partner in your lab group make the measurement (re-leveling the telescope each time). Cycle through the group three times for a total of measurements. Look at the distribution of answers. What is the standard deviation of the ensemble of all 12? Are there individuals whose results stand out as biased? 6) Identify the polar axis of the telescope. Is it pointed at the North Star and thus parallel to the earth's rotation axis? 7) Set the declination of the telescope to 0. Release the RA clamp and use the telescope to trace the celestial equator across the sky. Where is due east? west? a) Set the declination to trace the winter Sun's path and rising and setting points in

2 the same way. b) Do the same for the summer Sun at ) Install the lowest power eyepiece. Identify the finder telescope. Scan around a bit comparing qualitatively the view in the finder and in the main eyepiece. Find a bright star a bit south of the celestial equator and near transit (maybe Rigel?). Try eyeballing the star by lining up the star with the edge of the telescope. Can you get things close enough with this technique to acquire the star in the finder? in the main eyepiece? 9) Set the hour angle/ra dial so that it reads hour angle (reads zero when the telescope is on the meridian). Fix the telescope declination to the declination of Betelgeuse. Check the sidereal time and calculate the hour angle for Betelgeuse. Point the telescope to this hour angle. Were you close? Use the set dec and scan in RA technique to acquire the star and center it in the finder. 10) Measure the field of view of the finder. Center Betelgeuse in the finder and read the declination (which should be close to the declination of Betelgeuse). Move the telescope in declination only until Betelgeuse is at the edge of the finder field of view measure the declination. Do the same for the other edge of the field of view and use the declination readings to determine the size of the finder field. 11) The table below contains the R.A. and Dec for the hands and feet of Orion. You should already be pointed toward Betelgeuse. Set the adjustable R.A. wheel to read the proper R.A. as opposed to hour angle. In turn point by coordinates to each of the other three stars returning to Betelguese letting each member of the group have a try (or two by going through the sequence more than once). After first setting by coordinates for each star, note how close you came to centering the star in the finder. From the envelope of these observations, how good was your basic ability to use coordinates to get the star centered? Were some members of your group better at it than others? If so, what were they doing to get better results? Consider the following steps as defining two experiments worthy of being written up as a scientific paper (the standard format for presenting your observational conclusions from these and future observational exercises). You can include the more rote measurements above as an appendix to your paper. Those two experiments are: 1) Demonstrate that eyepiece field of view is inversely proportional to magnification. 2) Demonstrate that the effective drift rate of stars on the celestial sphere decreases toward the pole with the cosine of the declination. 1) Select two of the higher power eyepieces and test whether the field of view is inversely proportional to the magnification by timing the drift of a star across the eyepiece with the clock drive turned off. Center Betelgeuse in a low power eyepiece and focus the telescope. Switch to one of the two high powered eyepieces of your choice. Offset the telescope in RA so that the star is at the eastern edge of the field of view. Turn off the clock drive and time how long it takes the star to move to the western edge of the eyepiece (it will take two people, an observer and a timer,

3 and some coordination to make this measurement). Later on convert this time to arcseconds knowing that the stars should drift at a rate of 15 arseconds/second * cos(declination). The drift time should be short in a high power eyepiece so it should be possible for each group member to make 3-4 observations of the drift time and get some statistical estimate of the uncertainty. Repeat everything for the second eyepiece and show that the field of view scales as expected. 2) Perform the same drift experiment, this time for three stars at different declination Betelgeuse and two others (conveniently you selected those two other stars as part of your prelab). For these observations you will use a video-rate electronic camera and display in place of the eyepiece. You may need to acquire each star visually in a low power eyepiece first to get things centered before inserting the electronic eyepiece if the finder is not well enough aligned. Since this experiment is looking at how things scale (i.e. a proportionality) with cos(declination) you can get away with not knowing the absolute field of view of the electronic imager. You will be timing the passage of a star between two fiducial marks on the screen with the telescope drive turned off. It will be the ratio of these timings that will enable you to verify the cos(declination) effect. If you do want to calibrate the scale (and who wouldn t) you can point the electronic camera at the Trapezium Cluster (Theta Orionis) in Orion and use the finder chart below and a ruler to establish the screen scale in arcseconds per millimeter. If you crank up the integration time on the electronic camera you might get an enhanced view of the Orion Nebula and pick up stars too faint to see in the eyepiece. The table below identifies the fainter stars in the Trapezium field (and comes from Make two marks on the acetate (not the screen itself!!!!) with the whiteboard marker and time how long it takes each of your three targets to drift from one mark to the other (you might want to take a look at the drift rate before making the marks so that you can select a spacing that is not too short (leading to poor timing) or too long (leading to a very long night)). As before, multiple measurements should permit you to place empirical uncertainties on your values. Star Name R.A. (J2000.0) Declination Betelgeuse (α Orionis) 05:55: :24:25 Bellatrix (γ Orionis) 05:25: :20:59 Rigel (β Orionis) 05:14: :12:06 Saiph (κ Orionis) 05:47: :40:11

4

5 The video-rate electronic camera and its control paddle. The knob at the top of the paddle controls the exposure time. 6 on the high side is the shortest exposure with exposure time increasing as the dial rotates counterclockwise. If the frame is completely white, the integration time is too long rotate the dial counterclockwise. Looking into the imager you can see the light sensitive detector. Whatever scene the telescope objective casts onto this surface will appear on the TV display (if you are having trouble seeing the camera output on the monitor, press the video button on the display until the right mode comes up).

6