T90histogram.eps Acta Polytechnica Vol. 52 No. 1/2012 Ultra Low Dispersion Spectroscopy with Gaia and Astronomical Slitless Surveys R. Hudec, L. Hudec, M. Kĺıma Abstract The ultra-low dispersion spectroscopy to be applied in the ESA Gaia space observatory and the ground-based objective- prism plate surveys represent a similar type of astrophysical data. Although the dispersion in plate surveys is usually larger than in the Gaia blue and red photometers (BP/RP), the spectral resolutions differ by a factor of 2–3 only, since the resolution in ground-based spectra is seeing-limited. We argue that some of the algorithms developed for digitized objective-prismplates can also be applied for theGaia spectra. At the same time, the plate results confirm the feasibility of observing strong emission lines with Gaia RP/BP. Keywords: astronomical spectroscopy, low dispersion spectroscopy, slitless spectroscopy, Gaia. 1 Introduction The ESA Gaia is a primary astrometry ESA mis- sion to be launched in 2013. The satellite pay- load consists of a single integrated instrument, the design of which is characterised by a dual tele- scope concept with a common structure and a com- mon focal plane1. Both telescopes are based on a three-mirror anastigmat design. Beam combi- nation is achieved in image space with a small beam combiner. Silicon-carbide ultra-stable ma- terial is used for the mirrors and the telescope structure. There will be a large common fo- cal plane with an array of 106 CCDs. The large focal plane also includes areas dedicated to the spacecraft’s metrology and alignment measure- ments. Three instrument functions/modes are de- signed: (i) Astrometric mode for accurate mea- surements, even in densely populated sky regions of up to 3 million stars/deg(), (ii) Photometric mode based on low-resolution, dispersive spectro- photometry using Blue and Red Photometers (BP and RP) for continuous spectra in the 330–1 000 nm band for astrophysics and chromaticity calibration of the astrometry (Jordi and Carrasco, 2007), and (iii) Spectrometroscopic (RVS) mode for high res- olution, with grating, covering a narrow band: 847–874 nm. The expected limiting magnitude is 20 in pho- tometric mode2. In this contribution, we discuss the data expected to be provided by the BP/RP photometers, and show that they will represent ultra-low dispersion spectra which can be used in various astrophysical projects. We compare this data with analogous data provided by plate sur- veys. 2 Gaia photometers and simulations In this paper we focus on the “photometric mode” RP/BP. Use of the dispersive element (prism) gen- erates ultra-low dispersion spectra. One disperser, called BP for Blue Photometer, operates in the 330–660 nm wavelength range; the other, called RP for Red Photometer, covers the 650–1 000 nm wave- length range. The dispersion is higher at short wave- lengths, and ranges from 4 to 32 nm/pixel for BP and from 7 to 15 nm/pixel for RP3. It should be noted however that the photometric CCDs are located at the edge of the focal plane, where the quality of the images is more sensitive to aberrations than astro- metric images (Straizys et al., 2010). The BP and RP spectra will be binned on-chip in the across-scan direction; no along-scan binning is foreseen. RP and BP will be able to reach ob- ject densities on the sky of at least 750 000 objects deg−2. The obtained complex images can be simu- lated by the GIBIS simulator (Figure 1). GIBIS is a pixel-level simulator of the Gaia mission intended to simulate how the Gaia instruments will observe the sky, using realistic simulations of the astronom- ical sources and of the instrumental properties. It is a branch of the global Gaia Simulator (GaiaSimu) under development within Gaia Coordination Unit 2: Data Simulations. 1http://sci.esa.int/gaia/, 2011 2http://sci.esa.int/gaia/, 2011 3http://sci.esa.int/gaia/, 2011 27 Acta Polytechnica Vol. 52 No. 1/2012 Fig. 1: Left: The prism astronomical survey plate with 1.8 degree prism (Case/PARI), Right: The RP image simulated by theGaiaGIBIS simulator (visualizationDS9). This simulated image illustrates the imagewingsmentionedbyStraizys et al., 2006 Fig. 2: Image analyses of objects on low dispersion spectroscopic plates of the La Pas German Bolivia southern sky survey. Digitised (using a USB microscope) star image (left), the spectrum (2D) plot (centre), and the 3D plot (right). The 3D plot allows some details to be studied that are not included in the 2D plot, such as the image distorsion (image wings) caused by the optics that are used 3 Ultra low dispersion spectral plate databases Low Dispersion Spectroscopy (LDS) astrophysics was evolved and performed at numerous observatories be- tween ca 1910 and 1980. In our project we have anal- ysed the oldest LDS plates at the Carnegie Observa- tories, in Pasadena, CA, USA. These LDS plates were taken in 1909 with excellent quality, albeit limited FOV. Mostly LDS with Schmidt telescopes (plates with objective prism) were used for various projects e.g. QSO, emission line and Halpha surveys, star classifications, etc.,though some LDS surveys were performed with refractors. This technique was how- ever little used after 1980. The plate databases were previously mostly evaluated only by manual meth- ods, hence the application of advanced computer methods to this data can yield many new (and prob- ably unexpected) results. Some of these surveys are listed below (the dis- persion data is given in the next section, and we also note that many other similar surveys exist): (1) Schmidt Sonneberg Camera. Sky survey (se- lected fields) with a 50/70 cm Schmidt telescope. No online access yet, but the scans can be provided upon request (http://www.stw.tu-ilmenau.de/). (2) Bolivia Expedition Spectral Plates. These plates offer homogeneous but not full coverage (90 southern Kapteyn’s Selected Areas Nos. 116–206 were covered with plates representing 10 × 10 grad each, hence 9 000 square degrees in total) of the southern sky with spectral and direct plates, directed by the Potsdam Observatory. The plates are stored at the Sonneberg Observatory (http://www.stw.tu- ilmenau.de/) and were taken between 1926–1928, in total about 70 000 prism spectra were estimated and published in Potsdam Publications, see Becker (1929) and following papers. See Figure 2 for an example of this type of LDS data. (3) Hamburg Quasar Survey. A wide-angle objective prism survey searching for quasars with B < 17.5 in the northern sky. The sur- vey plates were taken with the former Hamburg Schmidt telescope, located at Calar Alto/Spain since 1980. Online access (http://http://www.hs.uni- hamburg.de/DE/For/Exg/Sur/index.html). (4) Byurakan Survey. The Digitized First Byurakan Survey (DFBS) is the digitized version of the First Byurakan Survey (FBS). It is the 28 Acta Polytechnica Vol. 52 No. 1/2012 Fig. 3: LDS spectra of two starburst galaxies on PARI Institute plates Fig. 4: Examples of objects with prominent spectral features on LDS spectra, PARI Institute plates. Left: QSO, Right: WN8h star V378 Vel. The image on the right is an example of LDS with higher spectral resolution. For these plates numerous emission and absorption lines are visible and various algorithmes are to be developed and applied largest spectroscopic database in the world, pro- viding low dispersion spectra for 20 000 000 objects on 1139 FBS fields = 17,056 deg−2. Online ac- cess (http://byurakan.phys.uniroma1.it/). Sky cover- age: DEC > −15 deg, all RA (except the Milky Way). The prism spectral plates were taken by the 1 m Schmidt telescope. Limiting magnitude: 17.5 in V. Spectral range: 340–690 nm, spectral resolution 5 nm. (5) Spectral survey plates in the Astrono- mical Photographic Data Archive (APDA) lo- cated at the Pisgah Astronomical Research In- stitute (PARI), USA, e.g. Case QSO-Survey (http://www.pari.edu/library). Telescope: 61/91 cm Burrell Schmidt at Kitt Peak, 1.8 deg prism, plate FOV: 5-degree by 5-degree, limiting B magnitude: 18, emulsion: IIIaJ Baked, spectral range: 330 nm to 530 nm (Figures 3, 4). (6) Karl Henize H-alpha plate collection (located since 2010 at PARI) — Michigan-Mount Wilson Southern H-alpha survey (Henize, 1954). A newly (in 2010) re-discovered highly valuable plate collec- tion. 290 high quality plates 15 × 15 inches taken in 1950–1952 in South Africa by dedicated telescope by Karl Henize. Telescope aperture D25 cm, dispersion 45 nm/mm at Halpha, various filters used (Henize, 1954). The spectral dispersion of Gaia BP/RP was pre- determined with no chance to interfere with the re- quirements of the scientific community. An impor- tant question is whether the dispersion of these de- vices is sufficient to detect and to study bright spec- tral features/emission lines. This question was an- swered e.g. by the extended work of US astrophysi- cist and NASA astronaut Karl Henize, who spent a large part of his scientific career on low disper- sion spectroscopy with an objective prism. We have found the original low dispersion spectral plates that he took about 60 years ago in South Africa and we have analysed them extensively. We found and inves- tigated these plates (probably the complete Henize collection) in the PARI (Pisgah Astronomical Re- search Institute) Institute, NC, USA. The plates show numerous examples of objects with very promi- nent and very wide emission lines, which he found in a very extended time-consuming and laborious project. 29 Acta Polytechnica Vol. 52 No. 1/2012 4 Ultra low dispersion images by Gaia RP/BP: algorithms and a comparison with plate surveys The algorithms for automated analyses of digitised spectral plates were developed by computer science students (e.g. Hudec, 2007). The main goals are as follows: automated classification of spectral classes, searches for spectral variability (both continuum and lines), searches for objects with specific spectra, cor- relation of spectral and light changes, searches for transients, and application for Gaia. The archival spectral plates taken with the objective prism offer the possibility to simulate the Gaia low dispersion spectra and related procedures such as searches for spectral variability and variability analyses based on spectro-photometry. We focus on the sets of spec- tral plates of the same sky region covering long time intervals with good sampling; this enables simula- tion of the Gaia BP/RP outputs. The main task is automatic classification of stellar objective prism spectra on digitised plates, a simulation and a feasi- bility study for the low dispersion Gaia spectra. The algorithms that have been developed and tested in- clude the application of novel approaches and tech- niques with emphasis on neural networks for auto- mated recognition of spectral types of stars, compar- ing them with atlas spectra. This technique differs from techniques discussed before (e.g. Christlieb et al., 2002, or Hagen et al., 1995). For the future, we plan to continue developing innovative dedicated im- age processing methods to continue our participation in data extraction and evaluation by providing exper- tise in the high level image processing with a focus on solving problems of data processing and data ex- traction emerging from the peculiar way that Gaia is functioning. The expertise available at the Depart- ment of Radioelectronics of the CTU Faculty of Elec- trical Engineering will be further used and developed in this direction. As illustrated in Figure 1, the Gaia BP/RP and LDS astronomical plates represent similar databases. The motivation for studies comparing these two databases is as follows: (1) A comparison of the sim- ulated Gaia BP/RP images with those obtained from digitized Schmidt spectral plates (both using disper- sive elements) for 8 selected test fields, and (2) A fea- sibility study for application for the algorithms devel- oped for the plates for Gaia. Dispersion is an impor- tant parameter, and is discussed later: (1) Gaia BP: 4–32 nm/pixel i.e. 400–3 200 nm/mm, 9 nm/pixel i.e. 900 nm/mm at Hγ, RP: 7–15 nm/pixel i.e. 700–1 500 nm/mm. PSF FWHM ∼ 2 px i.e. spec- tral resolution is ∼ 18 nm, (2) Schmidt Sonneberg Plates (typical mean value): the dispersion for the 7 deg prism 10 nm/mm at Hγ, and 23 nm/mm at Hγ for the 3 deg prism. (3) Bolivia Expedition plates: 9 nm/mm, with calibration spectrum, (4) Hamburg QSO Survey: 1.7 deg prism, 139 nm/mm at Hγ, spec- tral resolution of 4.5 nm at Hγ, (5) Byurakan Survey: 1.5 deg prism, 180 nm/mm at Hγ, resolution 5 nm at Hγ and (6) PARI prism dispersion: 150–340 nm at 450 nm. We see that the Gaia BP/RP dispersion is ∼ 5 to 10 times less than the dispersion of a typ- ical digitised spectral prism plate, and the spectral resolution of Gaia is ∼ 3 to 4 times less than for the plates. Note that for plates the spectral resolution is seeing-limited, hence the values represent the best values, and on the plates affected by not superior see- ing the spectral resolution is only ∼ 2 times better when compared to Gaia BP/RP. 5 Astrophysics with Gaia RP/BP spectro-photometry and LDS In our oppinion, the major strength of Gaia for many scientific fields will be in spectro-photometry, as the low dispersion spectra may be transferred to nu- merous well-defined color filters. As an example, the Optical Afterglows (OAs) of Gamma-Ray Bursts (GRBs) are known to exhibit quite specific color indices, distiguishing them from other types of as- trophysical objects (Simon et al., 2001 and 2004a, 2004b), hence a reliable classification of OAs of GRBs will be possible, in principle, using this method. The colors of microquasars may serve as another example: they display blue colors, with a trend of a diagonal formed by the individual objects. This method can be used even for optically faint, and hence distant objects. The Gaia BP/RP LDS will also provide direct valuable inputs for various fields of recent astro- physics. Figure 5 illustrates one of the examples, namely the value of LDS for analyses of OAs of GRBs. The emphasis is not only of the LDS spectral con- tinuum profile (reflecting the synchrotron radiation) but also on a study of wide redshiftet Lyman alpha breaks. The Gaia data will be supported by ground-based optical data with emphasis on robotic telescopes. This is part of the sub-workpackage supplementary optical observation in workpackage Specific Object Studies in the framework of CU7 Unit of Gaia. While this support will focus on supplementay photometry, we have also developed and tested methods involving the fast response LDS (Figure 6). This has scientific justification, as recently the LDS of OAs of GRBs are mostly delayed by 1–10 hours (Fynbo et al., 2009), hence they represent the afterglow optical emission, not the prompt optical emission. 30 Acta Polytechnica Vol. 52 No. 1/2012 Fig. 5: Examples of LDS of OAs of GRBs with Strong Intervening Absorbers (Fynbo et al., 2009). Evidently the wide redshifted Lyman alpha break will be observable by Gaia RP, and as a consequence there will be a possibility to study highly redshifted objects with Gaia RP up to z ∼ 7 Fig. 6: BOOTES WFS (with prism) constructed by themechanical shop, OndrejovAstronomical Institute (left) and the direct image (centre) and prism image (right). Direct Vision Prism is mounted on a 0.3m f/10 telescope, FOV 43′ ×28′, dispersion ∼ 4 Å/pixel at 4000 Å , ∼ 30 Å/pixel at 5500 Å, and ∼ 100 Å/pixel at 8000 Å, limiting magnitude 13.5 in 30 s (Spectrograph Mode) The correct color indices however cannot be calcu- lated without careful decontamination of the BP/RP spectra (Straizys et al., 2006, Straizys et al., 2010). The energy redistribution effect in the Gaia BP and RP spectra arising from contamination by wings of the image profiles was mentioned and investigated by Straizys et al. (2006), Montegriffo et al. (2007), and Montegriffo (2009). According to these researchers, the Gaia spectra may be used for classifying stars ei- ther after applying contamination corrections or by using standard calibration stars with known physical parameters and observed with the Gaia spectropho- tometers. In the latter case, there is no way to cal- culate the real spectral energy distributions, magni- tudes, color indices, color excesses or other photomet- ric quantities. The classification has to be made by matching the observed pseudo-energy distributions of the target and the standard stars, or by using pattern recognition algorithms (template matching) over the whole spectrum to estimate the astrophysical param- eters of stars. In addition, Gaia may be useful in the study of strong spectral time variations. It is known that certain types of variable stars (VS) such as Miras, Cepheids, and a few cases of other stars, mostly pe- culiar variables, exhibit large variations in their spec- tral types. This field is, however, little exploited, as these studies used to be very laborious (plates were mostly visually inspected) and limited, and hence no review on the spectral variability among VS exists. The ESA Gaia is expected to deliver data to fill this gap. 6 Recent Results Recently, we have digitised the full collection of Henize plates (southern MtWilson-Michigan H-alpha sky survey) and have found and analysed the North- ern Mt Wilson-Michigan H-alpha sky survey plates deposited at Carnegie Observatories in Pasadena, CA, USA. Selected plates from this collection have been digitized. The southern sky La Paz Bolivia Ex- pedition survey was recently fully scanned and de- posited at Sonneberg Observatory, Germany. In ad- dition, LDS plates located at various observatories (e.g. Sonneberg Schmidt, KPNO, Lick, Mt Hamil- ton, etc.) were investigated and some of them have been digitized. A technique for on site plate scan- ning (using a transportable digitization device) was developed and tested. This proved to be essential, as many of the large plate collections do not have any suitable plate scanner. For the LDS plates deposited at PARI, an ex- tended literature and in situ plate search was carried 31 Acta Polytechnica Vol. 52 No. 1/2012 out in order to correlate the literature records and the plates, and also to re-discover and re-investigate various objects with prominent spectral lines using modern methods, with emphasis on objects described many years ago in the literature. Some examples are illustrated in Figures 3 and 4. Advanced investiga- tion and visualization methods were also exploited, with examples shown in Figure 2. Algorithms for LDS data analyses, including neu- ral networks, were developed and tested, with em- phasis on automated star spectral type recognition. The parameters from various LDS plate projects were compared with those of Gaia BR/RP. In addition, various evaluation and visualization techniques have been developed and tested. The potential of Gaia BP/RP for astrophysical research was investigated with emphasis on objects with prominent colors and prominent spectral (and variable) spectral features. 7 Conclusion The ESA Gaia satellite will provide ultra-low dis- persion spectra by BP and RP, representing a new challenge for astrophysicists and for computer sci- ence. The nearest analogy is digitized prism spec- tral plates: the Sonneberg, PARI, Hamburg and Byu- rakan surveys. These digitised surveys can be used for simulation and for tests of the Gaia algorithms and Gaia data. Some algorithms have already been tested. Some types of variable stars are known to exhibit large spectral type changes — however this field is little exploited and more discoveries can be expected with the Gaia data, as Gaia will allow us to investigate the spectral behavior of huge numbers of objects over a period of 5 years with good sampling for spectroscopy. However, the data must first be de- contamined to be scientifically applied, as discussed above. Variability studies based on low dispersion spec- tra are expected to provide unique novel data, and can use the algorithms recently developed for auto- matic analyses of digitized spectral Schmidt plates. These variability studies may use either Gaia BP/RP data, or scanned plate data, or both. Then time cov- erage up to and exceeding 100 years can be achieved. The oldest LDS plates that we have identified in our project are stored at the Carnegie Observatories, Pasadena, CA, USA, and were taken in 1909. Acknowledgement Czech participation in the ESA Gaia project is sup- ported by PECS project 98058. The scientific part of the study is related to grants 205/08/1207 and 102/09/0997, provided by the Grant Agency of the Czech Republic. Some aspects of the project de- scribed here are a natural continuation of Czech par- ticipation in ESA INTEGRAL (ESA PECS 98023). The analyses of digitised low dispersion spectral plates are supported by MSMT KONTAKT Project ME09027. References [1] Becker, F.: Spektral-Durchmusterung der Kap- teyn-Eichfelder des Südhimmels. I. Pol undZone −75 deg. Potsdam Publ., 27, 1, 1929. [2] Henize, K. G.: The Michigan-Mount Wilson southern Ha survey. Astronomical Journal, 59, 325, 1954. [3] Hudec, L.: Algorithms for spectral classification of stars. BSc. Thesis, Prague : Charles Univer- sity, 2007. [4] Jordi, C., Carrasco, J. M.: The Future of Pho- tometric, Spectrophotometric and Polarimetric Standardization, ASP Conference Series, 364, 215, 2007. [5] Montegriffo, P., et al.: A model for the abso- lute photometric calibration of GAIA BP and RP spectra. I. Basic concepts. GAIA-C5-TN- OABO-PMN-001-1, 2007. [6] Montegriffo, P.: A model for the absolute pho- tometric calibration of Gaia BP and RP spec- tra. II. Removing the LSF smearing. GAIA-C5- TN-OABO-PMN-002, 2009. [7] Simon, V., Hudec, R., Pizzichini, G., Maset- ti, N.: A & A, 377, 450, 2001. [8] Simon, V., Hudec, R., Pizzichini, G., Mase- tti, N., Gamma-Ray Bursts: 30 Years of Discov- ery: Gamma-Ray Burst Symposium, AIP Con- ference Proceedings, 727, 487, 2004a. [9] Simon, V., Hudec, R., Pizzichini, G.: A & A, 427, 901, 2004b. [10] Straizys, V., et al.: Baltic Astronomy, 15, 449, 2006. [11] Christlieb, N., Wisotzki, L., Grasshoff, G.: A & A, 391, 397, 2002. [12] Hagen, H.-J., et al.: A & AS, 111, 195, 1995. [13] Fynbo, J., et al.: ApJSS, 285, 526, 2009. [14] Straižys, V., Lazauskaitė, R., Zdanavičius, K.: Baltic Astronomy, 19, 181, 2010. 32 Acta Polytechnica Vol. 52 No. 1/2012 René Hudec E-mail: rhudec@asu.cas.cz Astronomical Institute Academy of Sciences of the Czech Republic CZ-25165 Ondřejov, Czech Republic Faculty of Electrical Engineering Czech Technical University in Prague Technická 2, CZ-16627 Prague, Czech Republic Lukáš Hudec Miloš Kĺıma Faculty of Electrical Engineering Czech Technical University in Prague Technická 2, CZ-16627 Prague, Czech Republic 33