astrophysical lab course solar telescope and spectrograph · astrophysical lab course solar...

Astrophysical Lab Course

Solar Telescope and Spectrograph

The first day at the Schauinsland Observatory is about getting familiar with the telescope andthe spectrograph. This is required for the experiments "‘Magnetic fields in Sunspots"’ and "‘SolarRotation"’. All but the last steps in this experiment can be done without sun light using spectrallamps.

Tasks

1. Determine the spacing of the grating using known spectral lines.

2. Focus the spectrograph for different wavelengths (Na-D & Hg lamps, FeI 630 nm using theHartmann-method.)

3. Calculate the linear dispersion and the focal length of the lens using the Na-D lines.

4. Measure the resolution of the spectrograph with the line profile of the Na-D lines use thespectral lamp. Compare your measurement to the theoretical resolution.

5. Find and measure some important solar spectral lines.

Questions for preparation

• What characterises a good location for a telescope?

• What are the pros and cons of coelostat telescopes?

• What are the differences, advantages and disadvantages between refracting and reflectingtelescopes?

2 Solar Telescope and Spectrograph Manual

• What is the diameter of the solar images given a certain focal length?

• Why are the apertures of solar telescopes small (compared to night time telescopes?).

• Sketch the optical path of a grid spectrograph.

• How does the width of the slit relate to the spectral resolution?

1 Physical Background

Depending on the reflection angle there is a phase difference between the wavefront reflected lightfrom the individual grid lines. If this difference is a multiple of the wavelength λ, light interferesconstructive. This leads to:

g (sinφi + sinφr) = mλ. (1)

Where φi and φr are the angles of incident and reflected wavefront, respectively and g is thegrating constant. In case of a Littrow -spectrograph φi = φr = φ holds and Eq. 1 simplifies to

2 g sinφ = mλ. (2)

DispersionDifferentiation of eq. (1) with respect to the wavelength (for a fixed angle) results in the angulardispersion in form of:

∂φ

∂λ=

m

g cosφi. (3)

Together with the focal length f of the collimator lens the linear dispersion

∂x

∂λ= f

∂φ

∂λ=

fm

g cosφA(4)

in the focal plane follows.

ResolutionTwo spectral lines can be separated from each other if the maximum of the first line coincideswith the first minimum of the second line (Rayleigh-criterion). The theoretical resolution R of agrating can be calculated from this minimal wavelength distance ∆λ.

R =λ0∆λ

= Nm. (5)

The Raylight uncertanity ∆f gives a limit on how accurate the focus has to be measured. V isthe focal ratio.

∆f = 2 · V2 · λ (6)

Astrophysical Lab Course Kiepenheuer-Institut für Sonnenphysik, Freiburg

Manual Solar Telescope and Spectrograph 3

2 Instruments

2.1 The Telescope

The solar telescope at Schauinsland uses a coelostat system to feed the sunlight into the fixedtelescope (Figure 1).

8.5

m

5 m

2. CoelostatenspiegelC1

Linse

45°−Umlenkspiegel

Primärfokus

Spektrographen−obektiv

Gitter

SpektrographLabor

Innerer Turm, entkoppelt

Äusserer Turm

Spektrographenspalt

Spektrographenausgang

KuppelSonne

Zwischenabbildungssystem

8 m

Figure 1: Schematic figure of the tower telescope at Schauinsland.

A Coelostat system consists of two plain mirrors (Fig. 2). The first (C1) can be rotated aroundan axis parallel to rotational axis of earth. The name comes from the latin coleum = sky. Itsimage is static in the focal plane through the day. The second mirror (C2) has to be adjusted atthe beginning of the observation so it feeds the light vertically into the telescope. It is thereforeadjustable in height and can be rotated in two axes. This mirror is also used to fine tune theposition of the solar image in the optical lab and find targets on the sun.The position of the second mirror depends on the on the declination δ, the angular distance

Kiepenheuer-Institut für Sonnenphysik, Freiburg Astrophysical Lab Course


Höhe

Neigung um 2 Achsen

O

W

N S48°

Himmelsnordpol

Rotations−achse

Äquatorebene

C1

C2Sonne

Spiegel−normale

δ

Sonnenbahn

Horizont

Figure 2: Scematic figure of a Coelostat system.

between sun and equatorial plane.The solar telescope on Schauinsland is a refractor with an aperture of 45 cm and a focal length

of 13.5m. It measures 8.5m from the lens to the ground floor, a plain mirror directs the lightinto the oprical lab where the primary focus is located. If you block the beam, (e.g. with a backcardboard) you can see an image of the sun.

Comissioning of the telescope

Do never touch optical surfaces like lenses or mirrors.

• Open the dome.

• Remove the telescope cover, then remove the window covers and the lens covers. Start fromthe top so dirt or dust does not fall on optical surfaces.

• Start the tracking motor. There is a main power switch at the desk in the ground floor.Switch on both racks. The first mirror (M1) moves now.

• Remove the cover of the plain mirror in the cellar. Open the wooden door to the opticallab.



• Move the dome until the sun shines on M1. You might want to put on sunglasses.

• There are multiple levers that make M1 and M2 movable. Open the M1 lever and rotatethe mirror until the sunlight is reflected on M2.

• Adjust the height of M2 until it is illuminated symmetrically. Open the M2 leaver and tiltit until th light falls into the optical lab.

• Move M2 in small steps using the hand control in the ground floor until the Sun is centredon the optical axis.

3 The optical bench

There are two lenses mounted in the optical lab. The first collimates the light, the second focusesit onto the spectrograph slit. You can put different optical elements there. If you are workingwith spectral lamps then put them onto the optical bench just before the slit.

• Open the cover of the slit.

• Put a field stop in the primary focus (F1).

• Remove the cover of the collimator lens.

• For demonstration you can use the prism to redirect the light towards the wall. You willsee the spectral colors.

• Remove the prism and mount a prefilter on the optical bench.The reflecting side of theprefilter should point towards the sun. Be careful, do not touch the glass of the prefilters,they are expensive.

• Switch the slit-jaw camera and the monitor on. If you use a prefilter you might have toremove a neutral filter before the camera.

• You can move M1 until you see a sunspot on the slit jaw.

• Focus the the telescope (by moving the lens with the hand control) until you see a sharpimage.

4 The Spectrograph

The spectrograph on the Schauinsland is a Littrow- or Autocollimationspectrograph. Instead of acollimator- and a camera-lens (as in a Czerny-Turner spectrograph) there is only one lens whichdoes both (Fig. 3).The refraction grid can be rotated around the vertical axis to select the spectral range and

the autocollimator lens can be moved parallel to the optical axis to focus the spectrograph. Thecreated spectrum is deflected into either an ocular or a CCD camera.



Primärspalt

visuelle BeobachtungAnschluss CCD

bewegliches Auto−kollimationsobjektiv

Beugungs−gitter

Dreh−winkel

φ

Umlenk−spiegel 1

Umlenk−spiegel 2

Sekundärspaltund Photomultiplier

PM

Figure 3: schematic model of the spectrograph at Schauinsland. Top view.

Comissioning

• If you work with sunlight you need a perfilter to select the spectral range of interest.

• Swith on the clock and rotate the grid to zero. Check if the grating is set to zero. Lookthough the ocular and see if you see the zero order m = 0 (reflection).

• If not, set the spectrograph to zero. To do so, rotate the grating until the clock is set tozero, then switch the clock off and rotate until you see the zero order and switch the clockon again.

• Move the grid until you see the spectral line of interest. There is a folder whith the theoreticalpositions of the grating for various lines.

• Move the autocollimator lens to focus the spectrograph. Fixing a hair across the slit helpsto focus and align the images.

• Once you see the spectral line, you can switch to the PCO camera.

• The transmitted wavelength of the prefilter depends on the angel the line shines through it.Tilt the prefilter slightly until the maximum transmission is centred on the spectral lines(the image gets brightest).



Figure 4: This figure shows a raw image of the iron lines at 630.15 and 630.25 nm, recordedwith the PCO camera at Schauinsland. The two broad lines are the solar iron lines,the narrow ones next to them are telluric oxygen lines. The black horizontal line is ahair taped across the slit. Most of the dirt and artificial brightness variations can becorrected for in the data analysis (see the experiments solar rotation and magnetic fieldsin sunspots).

Fokus

intrafokal extrafokal

d

DPunktobjekt

paralleles Licht

von einem

Figure 5: Sketch of the focus measurement with the Hartmann-method.

Hartmann MethodThe Hartmann method is used to find the focus of the spectrograph. Reduce the height of thespectrograph slit with the designated metal bar to about 1mm. We have a point source now.Cover the spectrograph lens asymmetrically with a piece of cardboard so the upper and loweropen part have different sizes.Move the lens of the spectrograph. You will see two defocused images (of different brightness

due to the asymmetry) if you are out of focus. Those images will move and flip once you passthe focus. Note the positions of the two bright points for about 5 positions before and after thefocus (10 in total), also note the position of the lens for every image. Fit a straight line to thepositions of the images. The zero crossing of the fit marks the position of the focus. You can dothis by hand using a sketch and a ruler. Note the focus position of the spectrograph.



to 3.:Measure the distance of the two Na D lines (0.6 nm) with the ocular. Calculate the linear disper-sion with Eq. 4. You need the order of diffraction.

to 4.:Record the Na D lines in different orders with the PCO camera. Calculate the ∆λ using thedispersion and the width of the line (Full Width Half Maximum). The size of the grating is 175× 250mm the pixel sixe of the PCO2000 camera is 7.4µm.

to 5.:Observe the solar (iron) and the terrestrial (oxygen) spectral lines in the 630 nm range in differentorders. Record the Ca+ lines (393.4 nm and 296.8 nm) in a single order. Integrate over slit to getan average quiet sun spectrum. If possible scan the lines in the umbra of a sunspot as well andcreate an average umbral spectrum. Why do the lines have so different shapes? What effects doinfluence the shape and the position of a line?

Some other important Fraunhofer lines worth looking at are:

1) single ionized calcium (Ca+)H: 396.849 nmK: 393.368 nm

2) neutral sodium (Na)D1: 589.594 nmD2: 588.997 nm

3) neutral hydrogen (H), Balmer-seriesHα: 656.282 nmHβ : 486.134 nmHγ : 434.048 nmHδ: 410.175 nm

4) neutral iron (Fe)Fe 557.609 nm (g=0)Fe 630.15 nm Fe 630.25 nm

5) neutral magnesium (Mg)Mg 518.36 nmMg 517.27 nmMg 516.73 nm



Figure 6: Splitting scheme of the mercury lines (in mÅ)



5 Literature

• A. Bhatnagar and W. Livingston, Fundamentals of Solar Astronomy, World Scientific Pub-lishing (2005) [Kapitel 2: Modern Solar Observatories, Kapitel 7: Solar Optical Instrumen-tation]

• P. Massey and M. Hanson, Astronomical Spectroscopy, Online (2011), http://lanl.arxiv.org/abs/1010.5270v2

• Online Spektralatlas der Sonne, Ephemeridenberechnung: Observatoire de Paris, http://bass2000.obspm.fr

• Handbuch der Physik, Band XXIX, Artikel von Stroke (S. 454 ff.), von Stroke (S. 609 ff.)und von Bahner (S. 300 ff.)


astrophysical lab course solar telescope and spectrograph · astrophysical lab course solar...

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