DOI: 10.17617/3.xiaakm
title: Ground-based observations of comets and other small solar system bodies, obtained with a focal reducer in the years 1989 - 2003

description: &lt;p&gt;
The archive consists of ground-based astronomical images, conducted with astronomical telescopes in the years 1989 - 2003. In 1989 CCDs were just becoming available for use in astronomy. As compared with photographic plates, the CCDs of this time had a size of only a few centimeters. But they were about an order of magnitude more sensitive than photographic plates that were traditionally used for astronomical imaging (As an example see the Palomar Sky Survey https://skyserver.sdss.org). This allowed for the first time to take astronomical images in narrow spectral bands transmitting only light emitted by atoms or molecules of a single species and in this way strongly facilitating the interpretation of the observed images. But early CCDs were surrounded by bulky structures for readout and for cooling them to the temperature of liquid nitrogen. Only Cassegrain telescopes had an easily accessible focal plane able to accept the early CCDs. But their focal length is too large for the small-size CCDs. A new device was needed to adapt the CCD to the Cassegrain focal plane of a telescope: the focal reducer. Focal reducers are now commonly used to adapt CCDs to Cassegrain telescopes. They consist of a collimator, a parallel beam and a short-focus camera lens generating an image reduced in size by the ratio of the focal lengths of camera and collimator. The parallel beam provides space for dispersing devices like narrow-band filters or grating prisms. A focal reducer adapts the angular size of the sky recorded by a single pixel of the digital receiver to the size of the average atmospheric seeing element. All images in the present data set are taken with a focal reducer of all lens design. The collimating optics was built by the company Carl Zeiss Jena (German Democratic Republic) and the short focus camera lenses by Carl Zeiss Oberkochen, at that time the western counterpart of Zeiss Jena. In 1995 a color divider was introduced in the parallel beam after the collimating optics. This allowed to generate simultaneously a red and a blue image of the observed object in selected filters (Two-Channel Focal Reducer). A detailed description of the equipment, in particular of the devices that can be put into the parallel beam like filters, Fabry-Perot-Interferometers, and Wollaston prisms used for polarimetry, is contained in Publication 59 (Jockers K. et al.: Exploration of the solar system with the two-channel focal reducer at the 2m-RCC telescope of Pik Terskol Observatory, Kinematics and Physics of Celestial Bodies, Supplement 3, 13-18, 2000).
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The aims of the observations depend on the objects observed. The main interest was devoted to comets. The cometary ions CO&lt;sup&gt;+&lt;/sup&gt;, H&lt;sub&gt;2&lt;/sub&gt;O&lt;sup&gt;+&lt;/sup&gt; and CO&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; have important transitions in the visual and near-UV spectral regions and are therefore well observable from the ground in the cometary ion tails. The corresponding neutral molecules are cosmologically important parent molecules of the nucleus. The molecular ions were observed in order to derive their abundances and in this way get information about the abundance of their neutral parents. This necessitated the derivation of the atmospheric extinction as function of wavelength using observations of standard stars. Most of the observed standard stars were taken from the book by Glushneva, I.N., and Voloshina, I.B., Spectrofotometriya yarkikh zvezd', Nauka, Moskva, 1982. From the emission bands of the neutral cometary coma, only CN was observed.
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Asteroids and planetary satellites have a solid surface and emit in the visual and near-UV spectral region a continuous spectrum. We observed the reddening of the continuum of these bodies with respect to the solar spectrum as function of phase angle Sun-Object-Observer. In addition, we attempted to determine the phase curve of their polarization. From these observations conclusions were drawn on the properties of the regolith on their surfaces.
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At the start of creation of this archive (2022) the data were located on 25 CD-ROMs (Compact Disk - Read Only Memory) in chronological order. The images on a single CD-ROM have been taken in a comparatively small time-interval. Often, but not always, they correspond to a single observing campaign. The organization of the present archive still reflects the original CD-ROMs including their directory substructure. Data taking at the telescope was computer controlled. This was required as CCDs are digital devices. The images were written in the so-called FITS (Flexible Image Transfer System) format. Metadata and image data are conserved in a single file. First come the metadata in ASCII format. They have standard names. Most important are NAXIS1 and NAXIS2 denoting the dimensions of the image. After the name of the metadatum follows an equality sign "=" and the value. Then follows a "/" and after this as a commentary an explanation of the quantity. The header ends with a single line containing the word "END". After this follow the digital data of the image. This format has been maintained in the archive. 
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The directory structure of the archive is as follows: presented in the file "directory-overview.pdf". 
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At the top level of the directory structure there is the main archive called "Jockersdata" and the directory "Introduction" containing the necessary files introducing the data and making them accessible. 
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As already mentioned, the archive "Jockersdata" is split into 25 original directories. They are called "Terskol01" - "Terskol27" with "Terskol09" and "Terskol22" missing. The name "Terskol" refers to the observatory Pik Terskol in the Northern Caucasus. This is the observatory where many of the observing campaigns were conducted. In addition to the "Terskol"-directories there is a directory "Hand-written protocols", containing the protocols that were written on paper during the observations, as far as they are preserved. Depending on the observers present at the telescope, the protocols were written in German, English, or Russian.
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The directory "Introduction" provides information on the archive. Most important is the file "read_me_first.pdf". The directory "Photographs1517" shows ".jpg"- images of the pictures contained in the subdirectories "Terskol15" and "Terskol17". At that time the focal reducer was already in two-channel mode and one can look at the simultaneously obtained image pairs. The images should be considered as a preview, as a lot of the information contained in the original image files is lost. Some programs for image display, written in the programming language IDL, are included. They are explained in more detail in the file “read_me_first”. 
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Two catalogs have been constructed to help the user of the archive to find the data he/she is interested in. 
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A whole subdirectory is devoted to the Main Catalog. Using IDL (Interactive Data Language), the headers of the FITS-files of the images were extracted and LaTeX files were generated. The .pdf-files resulting from the execution of LATEX form the pages of the Main Catalog. Each subdirectory of "Jockersdata\Terskol01" - "Jockersdata\Terskol27" has a catalog file texoutnew with the same number. The individual pages of the main catalog refer to the original subdirectories found in the "Terskol01" - "Terskol27" directories. The original image file names have a prefix followed by consecutive numbering. The prefix is derived from the kind of observation contained in the file, like 'cr' = 'comet red channel', 'cb' = 'comet blue channel', 'sr' = ‘star red channel’, 'tfb' = ‘twilight flat blue channel’. Twilight flats are flat fields taken during twilight at low altitude. They are needed to correct for the varying sensitivity of the individual pixels of the CCD. In the Main Catalog each image has a line devoted to it. This line contains the file name and information extracted from the FITS header of the file, like frame dimensions, date, hour of the start of the exposure, exposure time, right ascension and declination (epoch of observation date), wavelength (if a tunable Fabry-Perot was used), and as comment the dispersing devices located in the parallel beam during exposure (see publication 59 mentioned above). In the early campaigns after introduction of the blue channel, the headers of the images in this channel are incomplete. They have been supplemented by auxiliary information, if available.
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The Overview Catalog is located in the directory "Introduction". Like the Main Catalog it is ordered according to the Terskol directories, but in the Overview Catalog a line is devoted to each file prefix. The range of file numbers belonging to a file prefix, start and end time of the exposures with this file prefix (date and hour), the observed object, and publication numbers of publications referring to this prefix are given. The objects are listed in both catalogs, but if one wants to get an overview about the observed objects the Overview Catalog is more useful.
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List of the numbered periodic comets of which observations can be found in this archive:
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2P/Encke&lt;br&gt;
4P/Faye&lt;br&gt;
19P/Borrelly&lt;br&gt;
21P/Giacobini-Zinner&lt;br&gt;
22P/Kopff&lt;br&gt;
23P/Brorsen-Metcalf&lt;br&gt;
26P/Grigg-Skjellerup&lt;br&gt;
29P/Schwassmann-Wachmann 1&lt;br&gt;
31P/Schwassmann-Wachmann 2&lt;br&gt;
45P/Honda-Mrkos-Paydushakova&lt;br&gt;
46P/Wirtanen&lt;br&gt;
47P/Ashbrook-Jackson&lt;br&gt;
76P/West-Kohoutek-Ikemura&lt;br&gt;
102P/Shoemaker (?)&lt;br&gt;
103P/Hartley 2&lt;br&gt;
109P/Swift-Tuttle&lt;br&gt;
141P/Machholz 2&lt;br&gt;
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List of the numbered asteroids of which observations can be found in this archive:
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11 Parthemope&lt;br&gt;
64 Angelina&lt;br&gt;
169 Zelia&lt;br&gt;
1439 Vogtia&lt;br&gt;
1620 Geographos&lt;br&gt;
1917 Cuyo&lt;br&gt;
2060 Chiron&lt;br&gt;
2100 Rha-Shalom&lt;br&gt;
2449 Kenos&lt;br&gt;
3797 Ching-Sung Yu&lt;br&gt;
4179 Toutatis&lt;br&gt;
33342 1998 WT24&lt;br&gt;
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In the "Related Publication" section the 17 most important publications related to the archive are listed. A list of all references is located in the directory "Introduction/Publications".
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description: IDL, LaTeX