Visual observations ASTB01 Lab 1

Transcription

1 Visual observations ASTB01 Lab 1 Lab instructors: Giorgi Kokaia Matthäus Schulik giorgi@astro.lu.se schulik@astro.lu.se Lund Observatory Lund University

2 1 Introduction The laboratory exercise visual observations is aimed to give you an overview of the starry sky, to show you different types of objects with a telescope, and to discuss some fundamental concepts of astronomical observations. The exercise is divided into two parts: first, there are some theoretical questions and computer exercises to help you plan your observation nights, and to consider some basic properties of the objects you will see. Afterwards, you will do observations with one of our smaller teaching telescope. Your report should discuss both parts of the laboratory, to show the knowledge gained from both theoretical considerations and the practical experience. 1. Planning the observations First, answer the questions in section Then, the tasks in section can be solved using a planetarium program (XEphem), which is installed on the computers in the computer rooms of the Observatory. It allows simulations of the night sky for a proposed observational date and time and can be used to make a list of objects to consider for your actual observations. 2. Observations of the night sky (from the rooftop of the department) The observational exercise with telescopes require a clear sky, which in Lund is hard to predict until the day of observations, as the weather changes quickly. For planning purposes you must in advance sign up for the observations and be prepared to show up if the weather is good. It is very important to use the few clear evenings, so you must have very good reasons for not participating, if you have signed up for that evening! 1.1 Place and material The computer exercise is made in one of the computer rooms, Lyra or Hercules, in the Western corridor. The actual observations are made with a teaching telescope placed on the roof terrace between the two smaller telescope domes on the Eastern side of the Astronomy building. The meeting point for observations is the main entrance door, Sölvegatan 27. You meet there at the given time and day with your lab instructor. To guarantee a sufficiently dark sky, the starting time of the observations is changed continuously Table 1: Telescope properties Telescope model primary mirror, Effective diameter(mm) focal length(mm) MEADE LX Table 2: Eyepiece properties Focal length(mm) apparent field of view(deg) during the term. The most important thing concerning the observations is to dress sufficiently warm, in particular on hands and feet; the observations are made at night under a clear sky, and it is often much colder than you would imagine! For the observational exercise you should bring this manual, tables, and printouts from the computer exercise, and paper and pen. Anyone, who has a star chart of better quality than the computer printouts should absolutely bring that as well as binoculars for those that have such. Torchlights with red light, not ruining the dark adapted eyesight, are available for loan. 1.2 Telescope The reflective telescope used in the exercise is a Schmidt-Cassegrain design, see Figure 1b. As for a classical Cassegrain telescope a secondary mirror isusedtoreflectthelightthroughaholeintheprimary mirror, but since the primary is spherical, a thin correcting lens of Schmidt type is added. This design results in a very compact, easy to use telescope. The optical focal length is about 5 times longer than the physical length of the telescope tube. More details about the existing equipment are given in Table 1 and 2. The telescope has equatorial mounting, with computer controlled motion in both axes. The control system of the telescope is set-up in the beginning of the evening by pointing the telescope towards one or two known (bright) stars. Subsequently, the telescope can be automatically slewed to any object by giving its designation (e.g. NGC or Messier number) or coordinates. 2

3 a 2 Theory of telescope observations 1a 1b a c b b c d e Astronomical objects are normally small and faint, that is why one is primarily interested in sharp images and the ability to collect many photons. It turns out that both the lightcollecting ability and the resolving power (image sharpness ) are improved for larger telescope apertures and telescope development is aiming at ever increasing primary mirrors. For earth-bound telescopes the atmosphere has a very large disturbing influence on the resolution and the theoretical resolving power can usually not be fully achieved. 2.1 Light-collecting ability Figure 1: Telescope principles. 1a- Refractor(lens telescope) (a) Objective, (b) Focus, (c) Eyepiece. 1b - Reflector (Schmidt-Cassegrain, the type we are using). (a) Schmidt corrector lens, (b) Primary mirror, (c) Secondary mirror, (d) Focus, (e) Eyepiece. 1.3 Literature In Fundamental Astronomy (Karttunen et al.), chapter (excluding transformation formulas), , 3.1 and 3.2 should be studied before the exercise. Extra literature for those interested is, e.g., Seeing Stars (Kitchin, Forrest) or Burnham s Celestial Handbook (Burnham). 1.4 The report Cooperation and discussion during the different parts of the exercise is encouraged, but the report needs to be written individually. In the report all questions from the computer exercise should be answered, and all night sky observations should be described. Tables and plots from the computer exercise should be clearly labelled, and they must be explained and commented on in the text. The description of the observing evening should describe the actual observations, not only tables of the observed objects. The final report should be handed in not more than two weeks after the observations. The amount of light that can be collected (in a certain time interval) from an object is proportional to the light-collecting area of the telescope and it is thus primarily the size of the objective (primary mirror) that decides how faint objects one can observe. The telescope used in the exercise with an aperture of 305 mm collects about (305/7) 2 = 1900 times as much light than a dark adapted eye (with an entrance pupil diameter of around 7 mm). One would then be able to see 2.5 lg1900 = 8.2 magnitudes fainter point sources than with the unaided eye. With the canonical magnitude limit of 6th magnitude for the unaided eye, it should be possible to observe stars down to 14th magnitude with this telescope. However, transmission and reflection losses in the optics, as well as light pollution at the observation site, make the limiting magnitude somewhat brighter. 2.2 Resolving power and image quality Ideally (no atmosphere), the resolving power (resolution) of a telescope is determined by diffraction effects. The aperture defined from the objective/primary mirror gives rise to a diffraction pattern in the focal plane when observing a distant point source (star). The diffraction pattern consists of a central bright area, the diffraction disk, surrounded by a number of concentric, successively fainter diffraction rings, see Figure 2. The radius b of the first dark ring (between the diffraction disk 3

4 Figure 2: Negative image of the diffraction patterns of two well resolved stars in a binary. The distance between the centra of the diffraction disks is here larger than the value b of the Rayleigh criterion. and the first bright ring) is given by b = 1.22 λf d, where λ is the wavelength of the light, f is the focal length of the telescope and d is the diameter of the primary mirror. If we, for example, observe a binary, we will see two diffraction disks in the focal plane. These could partly overlap each other but still be distinguished. The limit (Rayleigh criterion) for them to be distinguished as two stars is called the resolving power(or resolution) of the telescope and is reached when the distance between the centra of the diffraction disks is equal to b. In angular units (radians or arc seconds) we have θ R = b f = 1.22λ d,rad = 138 d[mm],arcsec where we have assumed that λ = 550 nm, corresponding to the green light where the Sun has its maximum intensity and the human eye has its highest sensitivity. In the rightmost expression d must be expressed in mm. This ideal Rayleigh limit is valid if the stars are almost equally bright, while a fainter secondary star obviously has to be further away from the primary component to be discernible. If we disregard problems related to optical quality and adjustment, it is in practice almost always the atmosphere which limits the resolving power. In the Earth s atmosphere there are turbulence and temperature variations, that constantly make the images tremble or being smeared out. This negative effect on image quality is usually called seeing, and it varies from time to time and from one night to the other depending on meteorological conditions. There is also a systematic variation, making the seeing worse closer to the horizon, where the path of the light through the atmosphere is longer. For the naked eye the influence of the atmosphere can be noticed through the twinkling of the stars (scintillation). The size of the seeing varies within a large interval, but usually the smearing of a stellar imageisoftheorderofonearcsecondorlarger, i.e. much larger than the Rayleigh limit for all but the very smallest amateur telescopes. Generally, the seeing is better at higher altitudes oversealevel, butitisalsoaquestionoffinding places with even temperatures, minimal turbulence and low humidity. Such specially chosen sites, where many large telescopes are placed, are e.g. high mountains on Hawaii, Canary IslandsoralongthecoastinChile. Ifonewantsa better resolution/image quality than the seeing permits, one must use an advanced technique (adaptive optics) which by using guide stars, fast computers and flexible mirrors is aimed to compensate for the influence of the atmosphere in real time. For special purposes, e.g. observations of planetary surfaces and binaries, it has been possible already from the 19th century to make visual observations with resolutions down to 0.1 arc second. The human eye has a great ability to distinguish details which just momentarily can be seen in an otherwise boiling image. However, such observations can be quite subjective (as e.g. is shown by the example of the non-existing channels on Mars). 2.3 Magnification and field of view A common misunderstanding is that the quality of a telescope is given by its magnification. The magnification of a given telescope is determined by the choice of eyepiece and depends on the requirements of the specific observation and the atmospheric conditions. Note that the question of magnification becomes relevant only when the telescope is equipped with an 4

5 p in a b pou f objective f eyepiece Figure 3: Rays from two infinitely distant point sources in a refracting telescope. eyepiece. For scientific observations telescopes are often used directly with cameras or other instrumentation. From geometric optics we find for the magnification of an astronomical telescope G = β α = f objective f eypiece = p in p out, where β is the angular size of the image seen through the telescope, α is the angular size of the object seen without telescope, f objective the effective focal length of the objective, f eyepiece the focal length of the eyepiece, p in the diameter of the entrance pupil (=objective) and p out thediameter oftheexit pupil. The useful magnifications depend on the size of the primary mirror and the type of object one wants to study. The field of view decreases with increasing magnification and one often has to start with a relatively low magnification in order to find the object in the first place. As a rule of thumb the field of view is around (45/G), based on a typical apparent field of view (45 ) in the eyepiece. An important concept is the normal magnification, which only depends on the diameter of the primary mirror. In practise, all observed objects have a non-negligible angular extent (even point-like stars, due to diffraction and seeing). It can then be shown that the largest possible illumination (W/mm 2 ) on the retina of the eye is reached at a magnification where the exit pupil isthe same as the pupil diameter of the eye ( 7 mm at night). This magnification is then called the normal magnification. For a lower magnification the exit pupil will be larger, and some of the light will fall outside the eye, i.e. we are only using part of the light available. If, instead, the magnification is increased, the image on the retina will be larger and the flux per unit area will be lower, and the object seems fainter. From the formula above the normal magnification is given by G normal = p in d[mm], p eye 7 and it will increase linearly with telescope size (diameter). If the normal magnification is so large that the field of view is smaller than the angular size of the object, it is better to use a smaller telescope, or in extreme cases (the Milky Way, large comet tails etc.) no telescope at all. When studying small and bright objects it is advantageous to use magnifications much larger than the normal magnification, partly in order to see details better, partly to make the background sky darker. At night (low light levels) the eye can resolve objects more distant than 2-3 arc minutes ( arc seconds). If one wants to study objects close together, e.g binaries, one has to make sure that the magnification is sufficient to match the resolving power of the eye. 3 Execution 3.1 Preparatory exercises 1. Describe the two most common coordinate systems used in astronomical observations, the equatorial (α/δ) and the horizontal (alt/az) system. How are these coordinates defined and when are the different systems mainly used? 2. Calculate the theoretical resolving power and the normal magnification for the telescope to be used. Also calculate the magnification and field-of-view (compare to 5

6 the size of the full Moon) for each of the eyepieces available. Explain all concepts. 3.2 Computer exercises 3. Read through section with practical recommendations. Start xephem. Choose the date and starting time when you plan to do the observations. Put the magnitude limit for stars to m=6, which is the faintest stars one are supposed to be able toseeonaverydarkandclearnight. Look at the sky (chart) and try to orient yourself. Can you find Polaris? Are the Moon or any planets visible? Do you recognize any constellations? If you have difficulties to recognize stars and constellations, then choose to show designations, names and the contour lines of the constellations from the Control Options menu. Finally, make a printout of the sky chart. 4. Pick out a smaller part of the sky (one or a couple of constellations), which you can study in more detail during the observation evening. Zoom in to a suitable size and print out (without help lines) and a magnitude limit of 6. In Lund the sky will rarely be dark enough for so faint stars to be seen. Change the magnitude limit to 4, make a new printout and note how many stars have disappeared. After observations you can decide which of the charts fit the appearance of the real night sky better. 5. Once again, choose magnitude limit 6, full skyview, butchangethetimetotwohours later. Howhastheskychanged? Arethere any bright objects which have set or risen? On the printout the equatorial coordinates at the centre of the chart (=zenith in Lund) are listed. Note how these have changed compared with the first printout. 6. Change date to one month later but to the same starting time as in task 3 (if there has been a change between daylight saving time and normal time or vice versa one has to adjust the time to the same UTC as in task 3). Make a new printout and compare with the printouts from tasks 3 and 5. Note the zenith coordinates. Explain differences and similarities. Note the position of Polaris on all printouts. Is it moving? Explain! 7. Now change date and time back to the original(task 3) and choose from Control Filter to show star clusters, nebulae and galaxies (deep sky objects). The classification of such objects and their true nature was unclear even into the last century, in particular the status of the galaxies. Choose one type at a time from the list below. Objects of different kind show very different large scale distribution on the sky. Can you qualitatively explain this from your present knowledge of their properties? Open clusters (Clusters O) Globular clusters (Clusters, C) Nebulae (Nebulae, N, F, K) Galaxies (Galaxies, G) 8. You should now try to find suitable objects of the types above for observations with the telescope. Thus, choose a much brighter limiting magnitude (< 10), and display the objects type by type once again. Write down details ( Name, R.A., Dec., constellation, magnitude, size) for one or two objects of each type (Rightclick with the mouse to get information about the object). Note that we will be observing from a roof platform with surrounding walls. Thus, only choose objects situated more than 20 degrees above the horizon. A classical catalogue with bright diffuse objects was compiled already in the 18th century by Charles Messier, and a sign that the object is reasonable is a M(essier) number (e.g. M57=Ring nebula). However, remember that some Messier objects are too large for our telescope, compare the size with our largest field-of-view. A modern version of the Messier catalogue is attached in the end of this manual. 9. Which planets are visible during the evening. Write down relevant data for these, e.g. R.A., Dec., constellation, magnitude and angular size. Also note rising andsetting times. Lookat animage of the sky later in the evening also. 6

7 10. Many of the stars are double or multiple, i.e. for the naked eye they look as a single star, but in the telescope they resolve into two or more components. A couple of interesting multiple stars are Castor (α Geminorum) and ε Lyrae. The relative positions for two brightest components of Castor are shown in Figure 4. There is also a third fainter component in a much wider orbit, and all of the three stars we see are spectroscopic binaries with short periods. Castor thus consists of in total six stars in a typical hierarchical configuration. In a similar manner ε Lyrae (Figure 5) is hierarchically made up of two close binaries, ε 1 and ε 2. Choose the constellation best suited for observation(gemini or Lyra) and estimate from Figure 4 or 5 the angular distance between the components (in arc seconds). Determine from your calculations under item 2 above if you will be abletoresolvethestarornot. Isthechoice of eyepiece of any importance? Practical recommendations for XEphem XEphem is a Linux program, which is started from a terminal window with the command xephem. (If the computer is in Windows mode, it has to be rebooted in Linux.) In the first window opened, the status window of XEphem, you can primarily change Location (usually Lund), Date and time of the observations (Local Date/Time). When you have changed the settings in the status window, you have to click on Update for the changes to take effect. Now, you can ask XEphem to show you how the sky looks like on the chosen location at the chosen time by choosing View Sky View. With the sliding controlattheleftsideoftheskychartthescale can be changed, allowing you to zoom in on one constellation. The controls under and to the right change the direction of view. When right-clicking on the mouse a window with information on the object pointed at will appear. This can also be used to zoom in on the object (Center + Zoom). In order to change the appearance of the sky 6 " " Figure 4: Castor A&B, apparent orbit of the secondary component (asterisks) relative to the primary component (circle in origo). The approximative orbital period is 420 years and the two components have magnitudes 2.0 and 2.8, respectively. Figure 5: The multiple system ε Lyrae. The components of ε 1 Lyrae have magnitudes 5.0 and 6.2, and for ε 2 Lyrae the magnitudes are 5.1 and 5.4, respectively. The orbits are quite uncertain, since only a minor part of the periods has been observed. 7

8 chartsothatyougettheinformationyouwant, there are two important menus which you should familiarize yourself with. The first one changes the appearance and can be reached through Control Options. You can here choose the coordinate system to be used, turn on or off constellation lines and choose which and how many designations to show on the screen. The second menu, Control Filter is used to filter the different types of astronomical objects. You simply choose which type to be shown and to what limiting magnitude. Most of the functions from these menus can also be found as shortcuts in the toolbars. You can rapidly come back to the original sky chart using the menu History Sky above - annotated. A printout can be made at any time with Control Print. Don t forget to give a name to the printout, in order to differentiate it from the printouts from other students. 3.3 Visual observations The observational part of the exercise means you get to use a real telescope with an experienced astronomer as your guide. It is important that you take notes of what is shown and done throughout the evening! Ask the instructor about details (eyepieces etc.) or whenever something is unclear. It is important to let the eyes adapt to the darkness, since one then is able to see much fainter details. The dark adaptation is less disturbed by red light, and thus the torches available have red light only Observations without telescope Atthenightofobservationsstartwithgetting an orientation on the starry sky. Find (using the Big Dipper) the pole star, Polaris, so that you know the directions of the cardinal points. Use a star chart (e.g. the printouts from the computer exercise) to find some constellations along the celestial equator. This goes from east to west with its highest point in south. Often the Moon and/or a bright planet is available to help find the ecliptic plane. Also, try to find how the galactic plane is oriented using some suitable constellations. It is only at rare occasions that the sky is dark and clear enough, so that one can see bright partsof the Milky Way fromcentral Lund. Note how the stellar sky apparently turns during the evening. This can most easily be noticed by comparisons with some reference point, e.g. the water tower. Note the position relative to (disappearance behind?) the tower for some stars at different times. If a planet is visible one can compare its light with that of a star at about the same altitude. Besides brightness and possibly colour, are there any qualitative differences in the appearance? Observations with telescope Find and observe the diffuse objects you have picked out. If there are many students in the group it will probably not be possible to look at all the objects, since it would take too much time. Instead, one should try to agree on a few which are the most interesting and which are suitably located for observations. (A reason for excluding an object might e.g. be that the Moon is so close on the sky that its light will drown the light fromthe object) Make a short description of each object and comment on the appearance in the telescope. Which eyepiece was used? Why? Observe the bright planets which may be visible. Describe which details one can see on their surfaces. Close to Jupiter and Saturn several moons can be observed. Try to identify these. Note that we normally use a star diagonal (a mirror that reflects the image 90 degrees away from the optical axis) prior to the eyepiece, and the image will then be flipped. Observe a suitable binary, Castor (Figure 4) or ε Lyrae (Figure 5), with the telescope. Is it possible to resolve the individual components and with which eyepiece? If the moon is more than half it is best to save it to the end, since its strong light otherwise can destroy your dark adaptation. Observe it using different eyepieces and compare your estimated fields-of-view 8

9 with the diameter of the Moon. Notice that mountains and craters are best visible near the terminator (the border line between the sunlit and the dark part of the lunar surface). Why? 9

10 4 Messier catalogue M Type Const. Mag. RA (h m) Dec. ( ) Distance Size M Type Const. Mag. RA (h m) Dec. ( ) Distance Size 1 BN Tau 8.2v kly 6 x4 56 GC Lyr 8.2v kly GC Aqr 6.3v kly PN Lyr 9.7p kly 86.0 x GC CVn 6.3v kly GX Vir 9.6v Mly 5.9 x4.7 4 GC Sco 6.4v kly GX Vir Mly 5.3 x3.2 5 GC Ser 6.2v kly GX Vir 9.8b Mly 7.4 x6.0 6 OC Sco 4.2v kly GX Vir Mly 6.5 x5.7 7 OC Sco 4.1v ly GC Oph 6.6v kly BN Sgr 6.0v ly 90 x40 63 GX CVn 9.3b Mly 10 x6 9 GC Oph 7.3v kly GX Com 8.8v Mly 10.1 x GC Oph 6.7v kly GX Leo 9.3v Mly 8 x OC Sct 6.3v kly GX Leo 8.2v Mly 9.1 x GC Oph 6.6v kly OC Cnc 6.9v kly GC Her 5.7v kly GC Hya 7.3v GC Oph 7.7v kly GC Sgr 7.7v kly GC Peg 6.0v kly GC Sgr 7.8v kly OC Ser 6.4v kly GC Sge 8.4v kly BN Sgr 7.5v ly GC Aqr 9.3v kly OC Sgr 7.5v (?) kly *C Aqr 9.0v GC Oph 6.6v kly GX Psc kly 10.5 x BN Sgr 9.0v kly 20 x20 75 GC Sgr 8.6v kly OC Sgr 6.5v ly PN Per kly 2.7 x GC Sgr 5.9v kly GX Cet 8.9v Mly 7.1 x OC Sgr 6.9v ly BN Ori kly 8 x6 24 *C Sgr 4.6v kly GC Lep 7.7v kly OC Sgr 6.5v kly GC Sco 7.7v kly OC Sct 9.3v kly GX UMa 6.8v Mly 27.1 x PN Vul 7.4v ly 8x6 82 GX UMa 8.4v Mly 11.3 x GC Sgr 7.3v kly GX Hya 7.6v Mly 12.8 x OC Cyg 7.1v kly GX Vir Mly 6.4 x GC Cap 8.4v kly GX Com 9.1v Mly 7.1 x GX And 4.8v Mly 192 x62 86 GX Vir 9.8b Mly 8.9 x GX And 8.7v Mly 8x6 87 GX Vir 9.6b Mly 7.4 x GX Tri 6.7v Mly 65 x38 88 GX Com Mly 7.0 x OC Per 5.5v kly GX Vir Mly 3.5 x OC Gem 5.3v kly GX Vir Mly 9.6 x OC Aur 6.3v kly GX Com Mly 5.4 x OC Aur 6.2v kly GC Her 6.5v kly OC Aur 7.4v (?) kly OC Pup 6.0v kly OC Cyg 5.2v ly GX CVn 7.9v Mly 14.3 x *2 UMa 9.1v ly GX Leo Mly 7.5 x OC CMa 4.6v kly GX Leo 9.1v Mly 7.6 x BN Ori 4.0v kly 85 x60 97 PN UMa 9.9v kly 3.4 x BN Ori 9.1v kly GX Com Mly 9.8 x OC Cnc 3.7v ly GX Com Mly 5.4 x OC Tau 1.6v ly GX Com Mly 7.5 x OC Pup 6.0v kly GX UMa 9.6v Mly 28.9 x OC Pup 4.5v kly GX Dra Mly 6.4 x OC Hya 5.3v kly OC Cas 7.4v kly GX Vir 8.5v Mly 9.3 x GX Vir 8.7v Mly 8.8 x OC Mon 6.3v kly GC Leo 9.2v Mly 5.4 x GX CVn 8.4v Mly 11 x7 106 GX CVn 8.6v Mly 18.8 x OC Cas 7.3v kly GC Oph 7.8v kly GC Com 7.6v kly GX UMa Mly 8.7 x GC Sgr 7.6v kly GX UMa Mly 7.6 x GC Sgr 6.3v kly GX And 8.9b Mly 21.9 x10.9 BN=nebula, GC=globular cluster, OC=open cluster, PN=planetary nebula, GX=galaxy, *C=stellar clustering, *2= binary or multiple star. The distances are given in light years (ly) or multiples thereof (kly, Mly) 10