Observational Astronomy

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1 Observational Astronomy Instruments The telescope- instruments combination forms a tightly coupled system: Telescope = collecting photons and forming an image Instruments = registering and analyzing the image The instruments must be optimized for the telescope (and vice- versa) Main types of instruments: Imaging (camera) Recording image of celestial objects to measure shape (spatial) and relative brightness (energy) Spectroscopy (spectrometer) Dispersing incoming light to measure intensity as a function of wavelength Photometry photometer Measuring light intensity in different bandpasses Mixed Narrow band imaging or spectral reconstruction of images 1

2 Camera Simplest form = a detector (a CCD) is placed directly at the focal plane of the telescope Advantage: no additional optics = high throughput Disadvantages: filters in the converging beam + optical thickness modified the focus Filters must be very large and need to be extremely good optically (any defect will affect the image quality at the focus) Alternative: collimating the input beam so that the filters are in a parallel beam Forms a real exit pupil: positioning filters at the exit pupil, theirs sizes can be minimized and transmission not affected by field angle o Focus unaffected by differences in filters and needs for high quality less stringent A stop can also be placed at the exit pupil to prevent stray light or prevent infrared emission to reach detector (cold stop) 2

3 Exit pupils and Stops Two stops limit the ray bundle passing through a telescope: The aperture stop, limits the ray that enter the telescope The field stop, limits the extent of image (FOV) The two stops must be designed such that all the intermediate optic components between them accept all oblique rays entering the aperture When this condition is not realized we have vignetting: a fading of the image near the edges of the field Optical system design: Object space: located in front of first optical element of the telescope o Entrance pupil: is the image of the aperture in the object space Image space: located after the last optical element of the telescope o Exit pupil: is the image of aperture in the image space Pupil: is any intermediate image The exit pupils contain all the rays that reach the image, whatever the angle of ray bundles Different field angles do not shift over the pupil as a function of the field angle Ray fans (comprised of all field angles) passes through a minimum diameter at the pupil Optimal location for deformable mirrors, fine steering mirrors (minimizes size of these elements) or filters (guarantees that their characteristics remain unchanged as a function of the field angle) 3

4 Photometer Instrument that measures the brightness of a single source within a given spectral bandpass A single- cell detector (photomultiplier tube) is required IMPORTANT: Photometry is now done mostly with CCDs Photometer used only for very high precision or high- speed photometry of bright objects and for inexpensive systems In photometer the detector is not placed at the focus To spread uniformly the ratio of detected to incoming photons over the field, a Fabry lens is used to re- image the primary mirror onto the detector o Equivalent as placing the detector at the exit pupil A diaphragm is place at the focal plane of the telescope to block unwanted radiation from the surrounding sky and reduce the background 4

5 Polarimeter Polarization is generally small (a few %) and consequently difficult to measure Simplest way to measure it Placing a birefringent material in the incoming beam of light and rotates it to determine the maximum intensity in each direction = direction of polarization Disadvantage: polarization could be cause by optics Alternative: Introduce a calibrated phase shift (retarder) in the beam as far upstream as possible Then use fixed polarizer downstream to measure polarization The phase shift variation can be obtained by rotating a retardation plate or by using a fixed retarded plates of various values mounted on a wheel Measuring polarization = extremely difficult Polarization created by the telescope must be calibrated by observing standard sources Care must be taken also to avoid polarization effect due to non- normal incidence Polarimeters cannot be placed at Nasmyth or Coudé focus, because of folding mirrors 5

6 Dispersing spectrometer Dispersion of white light into constituents can be done either through a prism (low dispersion) or a diffraction grating (reflector types high dispersion) Grating: glass plate rules with fine parallel, equally spaced grooves so that light can only be reflected between the grooves o Equivalent to Young s slits : diffract the light producing destructive interferences except for specific directions which are a function of wavelength Dispersing elements must be fed by a parallel beam (collimator) to avoid mixing wavelengths Hybrid device: objective grism (grating + prism) Transmission grating is ruled or glued onto the surface of a prism The prism deviation compensates for the grating dispersion angle such that the output beam remains aligned with the input beam Typically placed into the filter wheel of an imaging system to add spectroscopic capabilities Can be used in converging beam 6

7 Spectral resolving power (R): the capacity to distinguish between two wavelengths (4.1.1) R = λ Δλ Where λ is the mean wavelength Different range in resolution: Low resolution R < 100 Mid- resolution 100 R 1000 High resolution R > 1000 Resolving power of grating spectrometer (4.1.2) R = mλw θ Dd m = integer = grating s interference order W total width of the grating d is the spacing between adjacent lines on the grating θ is the angular size of the entrance slit projected on the sky, λ is the mean wavelength, D is the diameter of the telescope Δ λ apart: For diffraction- limited system, the slit width is optimal when its angular size on the sky is equal to the angular size of the image! θ = λ D (4.1.3) R = mw d A larger slit would reduce resolution, as well as letting in sky background A smaller slit would reduced the flux of the source, hence reducing sensitivity For ground- based telescope, limited by seeing, the image size is larger than λ D For a slit width equal to the seeing disk σ : (4.1.4) R = mλw σ Dd As D increases, the width of the grating has to increase in the same proportion in order to maintain the same resolution o Very large telescope (30 M or more) will have huge grating 7

8 When λ is not negligible with respect to slit width (ex. IR) diffraction effects become important o Resulting blur introduces extra background o Fore- optics system producing a cold pupil image of the slit is needed to reduce the extra background It is generally advantageous to artificially widen the spectra in order to make the spectral features more visible o Accomplished by moving the image along the slit controlling the telescope pointing or wobbling a glass plate in front of the slit Comparison spectrum is also generally added to the detector to calibrate the spectrum of the astronomical source in wavelength o Generated by a lamp filled with a gas that produces a large number of lines of known wavelengths (thorium or helium- argon) o It is also possible (although limited) to use atmospheric emission lines (like OH in IR) Échelle gratings: grating with step instead of rulings Allows small portions of the spectrum to be stacked one on top of the other Solves the problem of long narrow spectra, as produced by mid to high resolution spectrograph, compared to small size squared CCD Disadvantages: overlapping orders + limited spectral range in each order Need a low- resolution disperser to separate the orders 8

9 Very high resolution spectrograph Fabry- Perot spectrometer Formed by two highly reflective, very close plates (étalon) Placed in a collimated beam, create multiple reflections in the gap between the plates This produces destructive interferences, except for a specific wavelength o Function of the gap width + incidence angle of the incoming light The light emerges from the etalon in a circular pattern and is imaged onto a detector The spectrum is explored by changing the gap width of the etalon Advantage: Works on extended sources Produces extremely high spectral resolutions R ~ 10 4 or larger Disadvantage: very narrow spectral coverage - used to study emission line profiles 9

10 Fourier transform spectrometer (FTS) Physical principle similar as the Fabry- Perot Light from a source is fed to a Michelson interferometer and the output signal is recorded as one of the mirrors is scanned For monochromatic light the intensity recorded vary as a cosine law of the scanning distance due to successive constructive and destructive interferences: cos 4π x λ ( ), where x is the distance the scanning mirror is moved For polychromatic beam the recorded intensity is the sum of all the cosine terms, and its spectrum is extracted by an inverse Fourier transform Advantage: Extremely high resolution R > wide spectral coverage (multiplex advantage) Disadvantage: S/N suffers from photon noise of the full spectral range covered, rather than band analyzed The fact that measurement requires a continuous motion of the mirrors is an obstacle for space telescope 10

11 Detectors To detect as efficiently as possible each photon collected by the telescope and instrument Until late 1970: Photographic plates: Low quantum efficiency (QE = Number of detected photons/ Number emitted photons) Noisy, affected by fog = natural formation of silver grains in the absence of light Linear for limited range of exposure number of photons saturates with time in a complex way, due to different chemical reactions Quantitative analysis requires digitalization of the plates (complicated expensive in cost and time) Photocathode devices (photomultipliers and electron- beam detectors video type): QE 10 times higher than photographic plates, but complicated electronics and utilization, and very limited in size Single pixel detectors in the IR Breakthrough in the mid- 1970s = Solid state imaging detector Charged Couples Devices (CCD) + IR arrays = lcose to ideal detector o Very high QE ~ 99% o Linear the number of photons is proportional to the exposure time o Intrinsically digitized image can be analyzed using computers 11