CASPROF Photopolarimeter

The CASPROF (CASLEO - PROFOEG) photopolarimeter has been at the "Jorge Sahade" telescope since mid-1996 and it has been available to users since September 1997.

CASPROF was designed and built at CASLEO with funds provided by the Galactic Photometry and Spectrography Program (Programa de Fotometría y Espectroscopía Gal´ctica - PROFOEG). The optical elements were donated by Dr. G. Coyne of the Vatican Observatory.


Technical Description

CASPROF can function as a photometer or as a polarimeter. In this instrument, the light collected by the telescope enters the photopolarimeter and goes through the set of neutral filters, the diaphragm wheel, the filter wheel, the Wollaston prism, and then passes on to the two photomultipliers. A Glan prism can also be introduced into the light beam.

Remote visualization of the object is achieved by means of a CCD or TV camera installed in a way that makes it possible to inspect the field around the diaphragms, which are placed on a reflecting surface.

The instrument can accommodate three neutral filters which are controlled separately (1.1, 2.6 and 6.0 magnitude attenuations). The UBVRI filters are mounted on the filter wheel, which has three other free spaces available. The diaphragm wheel is designed to hold a set of eight diaphragms of different diameters.

The instrument has a manual and an automatic shutter which opens up at the start of an integration. The cold box accommodates the two photomultipliers, both of which have an operating temperature of -30oC, which can be achieved by using dry ice.

The system is controlled from a PC which processes the data and organizes the operations required by the user. For the instrument to work as a polarimeter, a rotating and a fixed half-wave plate are added to the optics of the photometer. The rotating half-wave plate rotates at a constant speed of 10 turns per second changing to another step or position every one millisecond, taking 100 steps in each turn. In each position of the half-wave plate, the outgoing pulses of each photomultiplier are counted. These pulses accumulate in 32 bits buffers on the PC. In the polarimeter mode, the light beam of the star reaches the Wollaston prism which divides the beam into two parts which are directed towards the photomultipliers located in a cold box after going through a field lens and a Fabry lens located before each phototube.

A typical observation night

Before the start of the observation it is necessary to:

1) Make sure the operator has created a personalized subdirectory, within which a new subdirectory should be created for each observation night. The control program and acquisition of CASPROF must start every night from the latter subdirectory to properly save the files with the corresponding observations.

2) Make sure that when mounting the polarimeter on the telescope, the coaxial cables for both channels are properly connected (they are marked as "Channel A" and "Channel B"). It is enough to conduct this connection verification at the start of the shift.

3) No less than two hours before the start of the observation, the corresponding high voltage sources must be connected and it should be verified that the photomultipliers are refrigerated.

4) If you prefer, you can familiarize yourself with the operation of the control program. This program has a menu which makes it possible to set up the polarimeter and conduct the observations. The following possibilities are available:

4.1) "Archivo" ("file", Alt-A), which makes it possible to "Abrir" ("open", Alt-A o F3) and "Cerrar" ("close", Alt-C o Alt-F3) a file, or "Salir" ("exit", Alt-S) the program.

4.2) "Mecanismos" ("Mechanisms", Alt-M), which allows the user to operate with the "Rueda de Filtros" ("filter wheel", Alt-R o F5), the "Rueda de Diafragmas" ("Diaphragm wheel", Alt-D o F6), the set of "Filtros Neutros" ("neutral filters", Alt-F o F7) or the "Glan" ("glan prism", Alt-G o F8).

4.3) "Adquisición" ("acquisition", Alt-D), which make it possible to "Configurar" ("set", Alt-C) the observation, or "Integrar" ("integrate", Alt-I).

5) If you are working with faint objects, you may replace the TV camera normally used for guiding by the CCD ST7 camera, which will make it possible to gain 2-3 magnitudes by integrating some seconds. In this latter case, it is important to control, before the first observation night, both the pointing of the camera and its focus. This can be quickly achieved by observing the interior of the illuminated dome and controlling the sharpness of the diaphragm image.

During the observation:

a)Make sure the control and acquisition program is started at the correct subdirectory for that night. This will make it possible to use the observation files in an organized manner.

b) Make sure that the operator has in fact removed the manual shutter in the head of the polarimeter and that the focus obtained for the guiding camera throughout the afternoon is the most suitable one (if your objects are faint, it may be convenient to slightly defocus the diaphragm so that any mark or dirt is out of focus and does not affect the observation).

c) Before making a series of observations it is necessary to decide which filter and diaphragm will be used, or whether it is necessary to use a neutral filter. These tasks are achieved by using the "Mecanismos" ("Mechanisms") menu mentioned above. The options available are the following:

- Filtros: ("filters") UBVRI broadband filters or "sin filtro" ("no filter").

- Diafragmas: ("daphragms") 5.0, 11.3, 17.0, 22.6, 33.9, 45.2 arc seconds, "pasante" ("clear") or "ciego" ("blind").

- Filtros neutros: ("neutral filters") which provide 1.1, 2.6, y 6.0 magnitude attenuations, or "sin filtro" ("no filter").

- Prisma de Glan: ("Glan prism") which can be "In" o "Out".

Whenever you make a modification to the polarimeter mechanisms, you will have to wait for the program to show that such change has been made before moving on to another operation. Remember that objects brighter than V=9.0-9.5 must be observed through a neutral filter to limit the count number, and that if you are using a neutral filter to observe your object, this filter will also affect the image obtained by the guiding camera..

d) If you use a CCD ST7 camera for guiding, after positioning the telescope in the region of interest, take a relatively long image (30 to 60 seconds, depending on the magnitude of the object, with the option "grab" from the ST7 operation menu) to make sure there are no faint stars that contaminate the measurement, and choose the region where you will measure the sky contribution. From this point, you can take short successive images (3 to 10 seconds, with the option "focus" from the ST7 operation menu) for guiding.

e) For each observation series you may choose to observe only the object of your interest, or additionally observe the sky before each integration cycle, to discount its contribution from the measurements. Also, the program allows filtering or not the data to reduce the noise. These options are selected from "Adquisición", "Configurar" ("Acquisition", "Set"). In this menu you can also enter instrumental values for point zero magnitude, and the instrumental values for Stokes Q and U parameters, and for the position angle. On the whole, a 22.5 value for point zero magnitude will yield accurate magnitudes in most cases, while the instrumental values for Q, U and the position angle will have to be zero, as these values are not known in advance. In any case, these values will only affect the indicative instrumental values that the program shows after each integration and not the final results that will be saved in the corresponding files.

f) To make an integration, go to the corresponding sub-menu ("Adquisición", "Integrar" which means "Acquisition", "Integrate"), and enter the name you want to give the file where you will save the data, the integration time in seconds for each integration, and the number of integrations that you wish to make in the observation cycle. For example, "176425b" (this is the second of several files with observations of that star, that is the reason for the "b" at the end), 90 integration seconds and 4 integrations per cycle. If the program has been set to substract the sky measurement, when clicking "Accept", the program will ask you to direct the telescope to an area free of stars to obtain values for the sky. By clicking "Accept" one more time, the program will start the integration and will show you the counts on each channel and the integration time that has passed. When the integration has been completed, the screen will display the counts obtained for each channel in each of the 25 positions of the sheet, the total counts, and indicative values for Q, U, total polarization, position angle and sky magnitude. If you agree, when accepting the measurement, the program will require you to point the telescope towards the object. The process continues in the same manner as for the sky, but when the integration has been completed it will display not only the instrumental values for Q and U, position angle, etc. of the object, but also indicative values obtained for (Object - Sky). The integrations on the object will be repeated as many times as has been selected (4 in our case), and finally the program stops at the end of the cycle, ready for a new acquisition.

g) When the integration cycle has been completed, the subdirectory corresponding to that night will contain a number of files with the following information:

- A "176425b.c" file with the sky measurement. The file has a heading containing data about place, date, hour, coordinates, air mass, filter and diaphragm used, exposure time, a reference to indicate that it is a sky measurement, the list of counts obtained for each position of the sheet and the total counts for each channel (Fig. 1).

Fig. 1

- A series of "176425b.o?" files with the object measurements and with a structure similar to the structure of the sky file (Fig. 2). In the example, as four integrations were required in the cycle, four files will be created, one for each measurement (176425b.o0, 176425b.o1, etc.). It is important to point out that the files are numbered starting from zero, but in the files the integration is numbered starting from one (in the "176425b.o0" file, "Object 1" is indicated, and so on). It is also important to note that the sky contribution has not been substracted from the measurements saved.

Fig. 2

- A "176425b.dat" file, which contains a copy of the files generated during the integration cycle (in this case, 176425b.c, 176425b.o0, 176425b.o1, 176425b.o2, and 176425b.o3), but which also includes the preliminary values for Q, U, total Polarization, position angle and magnitude shown on screen at the end of each sky or object integration.

h) It is important to observe high polarization and zero polarization standard stars every night. In both cases, at least two differet objects must be observed, and in the case of zero polarization standard stars, about eight integrations must be made as a minimum (for example, two cycles of four integrations each) to reduce the error. The importance of observing at least two high polarization standard stars every night lies, among other things, in the fact that the only way of determining whether the polarimeter channels are inverted is by comparing the measurements of two objects with known Stokes parameters.

i) Even though many files are created every night, these files are too small and a diskette is enough to save a copy of the structure of subdirectories created to save the observations. To reduce the observations, you can use the REDPOL program which can be requested via e-mail from the person responsible for the instrument (the latest version available is 9.0).