San Pedro M

San Pedro Mártir Observatory

SINGLE-CHANNEL PHOTOMETER CUENTAPULSOS

ON THE 0.84-m TELESCOPE AT SPM

OPERATING MANUAL

Written by: Petr Harmanec (hec@sunstel.asu.cas.cz)
Version 1.1 November 4, 2000

(minor editing by: Michael Richer, richer@astrosen.unam.mx, 20 Feb. 2001)

The photon-counting photometer Cuentapulsos is currently equipped with with an RCA 31034 photomultiplier and sets of UBV and uvby filters installed in a rotating wheel. It can be used on either the 0.84-m or the 1.5-m telescope. It is controlled by the program FOTOM, run on a PC computer under DOS. The program warns the user if the object is too bright and would give more than 610,000 counts per second. It does not permit integration in such situations. It is possible, however, to use an attenuating filter (factor 14.29) which then allows observations of stars up to about magnitude 3 (depending also on the high voltage used). The photometer is cooled by dry ice. The data are read into the computer memory and stored into a file defined by the user. The configuration at the 0.84-m telescope dome is the following: The control computer and drives of the telescope are in a closed cabinet on which there is also a clock showing accurate local time. On a table to the left of the cabinet, there are: the control keyboard of the computer which controls the telescope and (currently) three other computers which are - in the order of a growing distance from the cabinet:

the control computer of the Cuentapulsos photometer,
the control computer of the photoelectric guider, and
the control computer of a CCD photometer.

Arriving prepared:

If you are going to observe a larger number of stars, it is practical to have prepared in advance the control file of the photometer computer with the compulsory name

OBJ.DAT

which has to be copied into the directory CPULSOS in the computer memory.

The data in this file must be specified in a fixed format, shown in the following example:

_5 (total number of objects, with four records each)

_12 (star identification, number or a text like COMP)

15_55_33 (right ascencion [hours, min., sec.])

_42__33__44 (declination [deg. with sign, arcmin, arcsec])

15_0 integration time and number of multiple integrations

_13

16_2_50

_46___2___3

10_0

_15

15_58_28

_43__12__37

20_0

_16

15_52_42

_42__26__56

15_0

-1

_0__0__0

__0__0__0

10_0

Note that the character "_" denotes here a space between the numbers; there must be one space between the r.a. values but two spaces between decl. data (do not ask me, why!). It is desirable to have all coordinates defined for the same reference epoch otherwise you would have to remember to change the epoch every time you would be entering the coordinates for a given star. The telescope control computer calculates the instantaneous coordinates from the coordinates specified for a given epoch.

Getting started

Before you start preparations for observing, check the humidity conditions in the dome. You can consult the meter inside the dome but it is also advisable to have a look at the locally measured meteorogical data, refreshed every 5 minutes, at

http://132.248.4.31 .

You are not allowed to open the dome if the humidity exceeds 85 per cent.

The first thing you should do is to refill the cooling box of the photomultiplier with a cube of dry ice. There is a pressure bomb with CO₂ on the ground floor, in the room to the right from the entry to the dome. The bomb is connected to a special wooden box. You have to unscrew the valve on the bomb to the point when the attached manometer shows about 3 atmospheres and wait about two minutes until the wooden box is filled with dry ice and the gas begins to flow out violently. Then close the valve and remove the box. The cube of dry ice fits perfectly into the cooling box of the photometer. It takes 10-15 minutes before the photomultiplier is stabilized at a working temperature of about -65° C. You can still safely work if the temperature has risen to about -50° C during the night. Note, however, that this particular photomultiplier cannot be used without cooling because of its high dark current.
Next you should turn on the switch ENCENDIDO located directly on the box of the photometer. This will activate the power supply of the photometer for the motor that rotates the filters and for illumination of the field and diaphragm.
Another critically important thing is to switch on the control computer of the photometer and set it to the correct time. The internal clock obviously stops every time the computer is switched off. You must set the time at the beginning of each observing night. At the time of writing, the clock on the cabinet with the telescope control computer shows summer or winter local time, depending on the season. It is therefore advisable to set the time of the control computer of the photometer to UT but do not forget to set the date to UT also (i.e., the European date, usually one day ahead when you are starting at the evening). To do so, you must first leave the control program FOTOM which is activated every time the computer is switched on. Use the "F10" key and reply "S" when asked. Then you can set the date and time with the usual DOS commands

DATE and TIME .

Note that

UT = local time + 7 hours for the summer time, and

UT = local time + 8 hours for the winter time.

The choice of UT is also assumed in the conversion program PEDRO22 which allows you to convert the observations into input data of the photometric reduction program HEC22. For a complete photometric reduction software and its detailed description see Harmanec and Horn (1998) and also anonymous ftp sunstel.asu.cas.cz to directory phot. This program is based upon a detailed discussion of the strategy of photometric observations by Harmanec, Horn and Juza (1994). The principal programs PEDRO22.EXE and HEC22.EXE, together with the complete software manual and a sample data and results, are available in the control computer of the photometer in the REDUCT directory. (You can activate Norton Commander for an easier work.)

Before your first night, it may also be a good idea to check that there is enough free memory in the computer.

After you set time and date, return to the program FOTOM, best by running autoexec.bat again.

Turn on the main switch of the high voltage source, located close to the control computer of the photometer. When the signal comes up, set the working voltage. (This will depend upon your observing program. For instance, a voltage of 1350V allows to observe blue stars as bright as 3.5 magnitude in V with the attenuating filter.)
Next, you have to prepare the 0.84-m telescope for operation. First you have to turn on the switch CONSOLA on the cabinet of the control computer. When the display inside the cabinet brightens and information on the telescope position, hour angle etc. appears on it, turn on also the remaining two switches: MOTORS and VENTILATOR. For a more complete description of the control of the 0.84-m telescope, see the manual dedicated to the telescope console program, Instrucciones del programa de control del telescopio de 0.84 m del OAN, written by Salvador Zazueta and which will eventually be available electronically. There is also relevant information in CCD Imaging at the OAN-SPM 84-cm Telescope by Alan Watson and Michael Richer.
Then use the switch CUPULA in the upper part of the cabinet and rotate the dome until its slit is above the ladder. Climb the latter and plug in the cord for the motors which open the upper and lower parts of the dome slit. Open the upper part of the slit first and then the lower one.
If it is your first night on the telescope, it is wise to check the zero point of the coordinate system which ensures that the control system points the telescope at your program star according to its equatorial coordinates. Use command

CENIT (for all commands, capitalization is mandatory)

on the keyboard which should direct the telescope towards the zenith. If this command will move the telescope to a position, which is obviously far from the zenith, you have to get the telescope to the zenith manually. There is a large wheel in the lower part of the telescope mount which fixes the motion of the telescope in hour angle. Release this wheel rotating it to the left. You can then move the telescope to the meridian position manually. Then you have to move the telescope in declination to direct it towards the zenith, using the command

FIJODEC [degrees] [arcmin] [arcsec]

tentatively in smaller increments after reading what value of declination is on the computer display. This value does not correspond to the real declination of the telescope at that moment, of course. But you know that pointing the telescope to a declination which is, say, for 10° higher than what the display shows, will increase also the real declination of the telescope for 10° . When the telescope is pointing approximately to the zenith, use a level (hanging usually near the thermometer on the wall) at two perpendicular directions to set the telescope exactly to the zenith using the handpaddle. You can do it manually in hour angle and using the hand-hold keyboard on the telescope for the motion in declination. Then fix the wheel of the hour angle motion again and use command

listo

to put the new origin of the coordinate system into computer memory. Then point the telescope at some very bright star using its equatorial coordinates for some specified epoch and commands

EPOCA [eqiunox]

AR [hours] [minutes] [seconds] (all integer)

DEC [degrees, with minus sign if appropriate] [arcmin] [arcsec]

ACT

Only the last command ACT will actually activate the clock drive of the telescope and will move it to the chosen position. Use the hand-held keyboard on the telescope to get the star into the centre of the pointer and than into the photometer diaphragh. (You must not move manually, releasing the wheel of the hour angle motion again at this stage, otherwise you would change the calibration of the zero point of the coordinates.) When the star is in the centre of diaphragm, make the final adjustment of the zero point of the coordinate system using command

CORR

The command CORR can also repeatedly be used at any time during the observations when one notes that the telescope is not pointing at the stars properly. Some drifts often happen when one moves from one position in the sky to another distant one.

Only when the dome slits are opened completely remove all covers from the telescope. You have to use the command

FIJODEC -30 0 0 (the character``-" has to be typed before the first non-zero number!)

which will move the telescope into a position where one can easily remove the covers of the main mirror after climbing the fixed ladder. Using the movable ladder, remove also the covers of the guiding telescopes.

In case of any unexpected and potentially dangerous motion of the telescope, you can stop it by switching off the MOTORS.
Now, you can point the telescope to your first program star using the commands ar and dec. Make sure that you first set the equinox of the coordinates which you are using with the command

EPOCA [equinox]

Note that the control program of the computer calculates instantaneous coordinates from your input data.

Observing

There are two handles on the photometer which allow you to view the star either in the field or through the diaphragm and to send the light to the photomultiplier once you have put the star into the centre of the diaphragm. Note that one of the handles can send the light to the photometer either directly or through an attenuating filter (in case you are observing bright stars). All these positions of the handles are identified by the words FOTOMETRO, OCULAR and FILTRO NEUTRO. Near the handles, there is also a switch for the illumination of the crosses in the field and the diaphragm. Always switch the light off before sending the stellar light into photometer.

You can control operations of program FOTOM via F keys in the upper part of the keyboard. With the first observation, started with F6 key, you are asked to specify the name of your output file to which all observations will be recorded (in DOS convention: up to eight characters, dot and up to three characters of extension). At the beginning and any time you make a mistake or need to type a remark, you can do so after pushing F9 key. Every comment (up to 30 characters per line) will be recorded into the output file.

Using F4 and F5 keys, one selects the star to be observed. Its identification, equatorial coordinates, and integration time are displayed on the computer screen. Unfortunately, there is no communication between the control computer of the photometer and that of the telescope. Therefore, one has to type the coordinates of the star to be observed on the keyboard of the telescope computer every time. After pointing at the star, you can start integration with F6 key, then change the filter using the F1 or F2 keys. The program displays the error of each measurement calculated from cumulative one-second integrations. One has, therefore, a control over the quality of the measured signal.

Finishing observations

Here, the procedure is opposite of that at the beginning. Decrease the high voltage gradually and switch off the source, move the telescope to the zenith using the CENIT command, cover the guider telescopes, move the handles of the photometer into the OCULAR positions. Rotate the slit of the dome to the ladder and close it. Lower the telescope with the FIJODEC -30 0 0 command, cover it with cover and plastics, and then return it to the zenith position. Write adios on the telescope computer keyboard and then switch off VENTILATOR, MOTORS and CONSOLA. Leave the program FOTOM with F10 key and S and do not forget to copy your data file onto a diskette. Switch off the lights, go to your dormitory, and have a good sleep...

References

Harmanec P., Horn J.,1998, Journal of Astronomical Data, CD-ROM No.4
Harmanec P., Horn J., Juza K., 1994, A&AS, 104, 121