URSULA is a multiband astronomical photoelectric photometer which minimizes errors introduced by the presence of the atmosphere. It operates with two identical channels, one for the star to be measured and the other for a reference star. After a technical description of the present version of the apparatus, some measurements of stellar sources of different brightness, and in different atmospheric conditions are presented. These measurements, based on observations made with the 91 cm Cassegrain telescope of the Catania Astrophysical Observatory, are used to check the photometer accuracy and compare its performance with that of standard photometers

Debiase, G. A.

Fedel, B.

Paterno, L.

Santagati, G.

Ventura, R.

English

NASA Technical Reports Server

N89- 1331 2
PRECISION LIMITS OF THE TWIN-BEAM k2/LTIBAND 'URSULA'
ADAPTIVE PHOTOMETER
G.A. De Biase
Istituto Astronomico, Universita' di Rome la Sapienza, Italy
L. Paterno'
Istituto di Astronomia, Universita' di Catania, Italy
B. Fedel, G. Santagati, R. Venture
Osservatorio Astrofisico di Catania, Italy
Abstract
URSULA is a multiband astronomical photoelectric photometer which
minimizes the errors introduced by the presence of the atmosphere; it
operates with two identical optical channels, one for the star to be
measured and the other for a reference star. This renders the signal
ratio independent of atmospheric transparency variations, thus
eliminating the major source of error, essentially for bright stars.
Scintillation error cannot be eliminated, unless the light beams are
within the correlation radius of turbulence, so that light variations
are in phase. Following this concept, four machines have been
constructed for four Italian Astronomical Observatories in late '70s,
among which that operating at the Catania Astrophysical Observatory
stellar station (1720m asl) has been improved during the past years.
The photometer has been used for measuring a wide field of
astronomical variable phenomena. After a technical description of the
PRI_?_DING PAG_ BLANK NOT FILM]_D
17
https://ntrs.nasa.gov/search.jsp?R=19890003941 2020-03-20T04:46:38+00:00Z
present version of the apparatus, we present some measurements of
stellar sources of different brightness, and in different atmospheric
conditions, by using the 91 cm Cassegrain telescope of the Catania
Astrophysical Observatory, to check the photometer accuracy and
compare the latter with that of standard photometric techniques.
Introduction
In 1974, an italian technological group (URSA) discussed the
possibility of designing a photometer for high accuracy multiband
stellar observations.
The idea was of constructing a flexible instrument suitable for
observations of a large variety of stellar phenomena without modifying
the system hardware. Moreover the instrument was thought for
minimising errors caused by variations of atmospheric transmittance
during the measurement times of the star and the reference star, thus
eliminating the most important error source of ground based
astronomical photoelectric photometry.
With these premises, the URSULA photometer has been designed with
two optical channels, one for the star to be measured and the other
for the reference star, for eliminating the atmospheric transmittance
errors, and integrated to a computer for the control of all the
operations. The only parts of the photometer not integrated to the
computer were in fact the optical band multiplexers and the detectors
with their discriminators and amplifiers.
Four instruments for the Catania, Kilano, Napoli and Trieste
Observatories were constructed and tested in laboratory through
1975-77, while the first astronomical observations were carried out in
1978. A detailed description of these instruments can be found in
De Biase et al. [1978, 1980a,b].
The photometer installed at the Mount Etna stellar station of the
Cstania Astrophysical Observatory was subsequently improved.
In the following sections we shall describe the concept of this
instrument, suggest possible implementations to improve its efficiency
and give some example of use.
Astronomical photoelectric photometry error sources
The atmospheric effects are the most serious sources of accuracy lack
for ground based astronomical photoelectric photometry [Young 1974].
These effects can be divided into two main classes:
- those deriving from the turbulent nature of the atmosphere
which show stationary spectrum, ranging from a few tenths of Hz
to a few hundreds Hz, and give rise to the well known phenomenon
of stellar scintillation;
- those deriving from large scale inhomogeneities of the
atmosphere not averaged by the telescope aperture which show non
stationary spectrum, ranging from nearly zero to a few tenths o5
Hz, and give rise to variations of atmospheric transmittance, so
affecting the mean value of the signal during the measurement
time.
Besides these two temporal effects, the atmosphere introduces an extra
spatial effect called seeing. The effect o5 seeing on extra
atmospheric point sources is to spread the luminous energy over an
I_
area much larger than the diffraction pattern. Therefore a larger
diaphagram in the center of which the stellar image is formed must be
used, so allowing a sizeable fraction of sky background to be
collected together with the signal, with the consequence that the
signal to noise ratio is lowered. These atmospheric effects are
essentially important for bright star measurements, since they alone
determine the accuracy that can be achieved in a photometric
measurement. For faint stars, photon noise and photomultiplier dark
current noise become also important.
The expression for the total relative error of a ground based
astronomical photoelectric measurement is thus given by:
+ +
where the suffixes s,r and b refer to the source, reference source and
background respectively, p represents statistical photodetection
(signal and background) and scintillatlon errors, and S the errors
caused by atmospheric transmittance variations.
The importance of the above errors depends on the photometric
operation mode.
The standard single detector single beam technique is based on
separate subsequent measurements of source plus background (S+Bs),
source background (Bs), reference source plus background (R+Br), and
reference background (Br). In this case all the terms of the
expression (1) concurr to RE. This measurement procedure can be
classified as 'fully serial mode'
The optimum operation mode would consist in simultaneous
2o
measurements by four detectors of S+Bs, Bs, R+Br, Br. In this case
only statistical intrinsic errors given by the 9 terms in expression
(1) are retained (scintillation error can be minimized only if source
and reference source are observed within the turbulence correlation
radius so as the light fluctuations are in phase); these decrease with
the square root of the measurement time. Therefore the optimum
photometric mode coincides with the 'fully parallel mode'
The two detector twin-beam URSULA astronomical photometer uses an
operation mode based on the simultaneous measurement of S+Bs and R+Br
and a subsequent simultaneous measurement of Bs and Br. In such a way
the most important _ error (_s,r) vanishes, because the variations
of atmospheric transmittance equally affect both the source and
reference source, and, unless measurements are made in dramatic sky
conditions, the influence of trasmittance variation on background is a
second order effect. Scintillation error can be minimized under the
same constraint as before. We define this mode as 'parallel-serial
mode'.
Band
multiplexer
sw2 oe,ec,or
N
Fig.l - Operation modes of a multiband photoelectric photometer.
In Fig.1 the general procedure schemes described above are summarized.
The switch SW1 defines the operation mode, while the switch SW2
selects the optical bands to be measured. In Fig.2, the relative
21
error is plotted as a function of the visual magnitude mv (extra
atmosphere) of the source for the fully parallel operation mode (curve
I), parallel-serial mode (curves 2,3) and fully serial mode (curves
4,5). All curves refer to a Im diameter telescope with 2_
scintillation, atmospheric transmittance q_ = 0.8, lOs measurements
time and 0.I efficiency; a reference source m v = i0 has been used.
101l
RE
10 0 -
-1
10
162
cKrve i
6 8 10 12 14 16 20
Fig.2. Relative error RE as a function of stellar magnitude for
various operation modes and different atmospheric
transmittance variations_/
For a wide discussion on these curves see De Biase et al. [1978] and
Belvedere and Paterno' [1976].
:TF _"_C,._, QUALITY
OF' POOR QUALITY.
Fig.3. The URSULA photometer of the Catania Astrophysical
Observatory working with the Cassegrain 91cm telescope
The Catania Observatory URSULA photometer
At the Mount Etna stellar station (1720m asl) of the Catania
Astrophysical Observatory a URSULA two-channel photometer is operative
since early '80s. The photometer is assembled on a 91 cm Cassegrain
telescope and its overview picture is shown in Fig.3, while the
optical and mechanical scheme is outlined in Fig.4. As it has been
said, it consists of two identical measure channels (A and B) working
by photon counting. On both channels two EMI 9658 R photomultipliers
have been set up, cooled at about -20°C through Peltier cells; at this
temperature the dark current is o_ about 8 count/sec.
A
colour filters
neutral filters
eveniece
dlaohacram --_
" '->
diaoha_ram _-'_ __
eyepiece / " I
" " fi!ters_ [
neutral
colour filters
I
B
Fig.4. Optical and mechanical diagram of the URSULA photometer
Setting up the photometer occurs manually, utilizing three degrees of
x_.,_._ ._¢_
O.p POOR QUALITy.
freedom (in addition to those of the telescope):
1 - rotation of the axis of the photometer (joining the stars to
be measured),
2 - shifting of detector A along the axis,
3 - shifting of detector B along the axis.
The useful field of the instrument is 10' with a minimum distance
among stars (due to mechanical limitations) of 4' As can be seen
from Fig.4, the instrument allows the introduction of neutral filters
(to be used with intense sources) and the choice of a certain number
of diaphagrams.
traneducer
t,me gate T
! 1
Fig.5. The concept of hardwareless URSULA photometer.
All the electronic parts, except the detectors,
are integrated to the computer
The URSULA photometer is widely integrated with its control computer:
as a matter of fact, the only external electronic parts are the
photomultipliers with the respective discriminators, as can be seen
25
from Fig.5. The photon pulse counting is carried out by two real time
clocks used as counters.
In its first version, the instrument was based on a DEC PDPII/IO
computer with 16 KW memory, which conditioned the development of the
running software, consisting of a stand-alone program written in
assembler language.
A c ou nt s ------_ RTC
--4RIO]--
comman0s 0,0,n er 
rl
PDPII-24
memory ']
disks
tape
Fig.8. Configuration of the control computer at
Mount Etna stellar station
The present structure of the photometer control program is summarized
here. Upon starting, one can initialize the time scale, which can be
either internal or synchronized with an external one. During the
initial dialogue the operator selects the optical bands, the computer
26
asks whether or not a preliminary measurement of the star, reference
star, and background is needed, and successively the operator will
assign the total measurement time for each band and the ratio between
the source and background measurement
requested to insert:
- cycle time (elemental measurement
times. The operator is
time within the complete
measurement in all the selected optical bands),
- measurement time (the duration of a complete measurement),
- background repetition rate (the number of complete measurements
to be executed on the sources before shifting the telescope to
measure the sky background),
- time distribution within the optical bands.
At this point the measurements start and the values of the outputs of
the two channels are recorded on magnetic tape (time, star and
reference star or respective backgrounds). The system is,
furthermore, able to point out some run time errors (for instance, a
wrong colour filter displacement).
Differences between the present apparatus and the first
installation mainly occur in the addition of the photomultiplier
cooling system and in the control computer change into a DEC PDPll/24
system equipped with disks and magnetic tape, as shown by the block
diagram of Fig.6.
Examples of use
In the following figures some examples of the photometer performances
are shown. One of the first measurements carried out by the URSULA
27
photometer is shown in Fig.7 [Rodono' et al. 1979]. This measurement
has revealed a small amplitude long lasting pre-flare activity in
YZ CMi. Also in previous observations, using traditional photometers,
such a kind of small amplitude activity was suspected, but no firm
conclusions could be derived, since such small variations could also
be caused by transmittance variations occurring between the star and
reference star measurement delay.
Fig.?. One of the first observations with URSULA photometer
carried out in January 1978 shows a pre-flare activity
in YZ ¢_i undetectable with standard techniques.
Observations shown in Figs.8a,b have carried out recently at the
stellar station of the Catania Astrophysical Observatory with the new
version of the URSULA photometer.
The figures show observing runs of 1500s for the same two stars
(mv = 10, 10.2) in two very different atmospheric transmittance
variation conditions. They show the behavior of the signals
(counts/s) of the two stars measured in the photometer channels A and
B together with their ratio A/B as a function of observing time. In
28
15 --_ l I
r."
10
t-_
<-
El]
E--
Z
O
E_)
0
5OO
m (B) = 10.0
v
m (A)= 10.2
v
B/A
___._ _.L • Z [
1000 1500 2000
TIME (s)
&
15
co
r._
co10
c:3
v
l,')
E--
:C)
O
O
A/B
I
m (A)= 10.0
v
m (B)= 10.2
v
/t
/%
0 I ¢ i J I z i i I J L z ..... _ __
500 1000 1500 2000
TIME (s)
b
Fig.8. Signals in the channels A and B and their ratio A/B are
plotted vs observing time for the same two stars in different
transparency conditions: a) _/_ = 10_, 0 _ (A/B) = 1.2%;
b)_/_ > 50%, _ (A/B) = 3.g_
29
order to plot all the curves in the same figure, the counts in the
channels A and B have been divided by proper factors, while the ratios
A/B are in scale.
6
CD
crJ
• single beam
double beam
• •
6 8 10 12 14
MAGNITUDE
Fig.9. Relative error RE vs. stellar magnitudes for
single beam standard photometry (filled squares)
and double beam photometry (open squares). The
data refer to only three observation nights
with different atmospheric conditions.
The different atmospheric transparency conditions are clearly seen
from the behavior of signals in the separate channels. In Fig. 8a,
transmittance variations not larger than i0_ - 15_ can easily be
estimated, while in Fig. 8b, variations larger than 50_ are present
during the observation time. In the first case, the accuracy of A/B
ratio is within 0.012, while in the second case, in spite of the
dramatic variations of transparency, the accuracy is within 0.039 that
3o
is not very different from the first case.
Fig.9 summarizes the results of different brightness star
observations carried out during three nights in which atmospheric
conditions were different. All measurements were taken with the two
channels, but in some cases a single channel serial mode measurement
has been simulated for comparing accuracies. In the figure the
relative error RE, normalized to lOs measurement time, is given as a
function of the magnitude of the stars for both single (filled
squares) and double (open squares) beam modes. It is possible to see
a well definite tendency of double beam points to align along an
accuracy level better than single beam points. These latter tend to
reach the double beam accuracy level in good photometric nights and
greatly diverge from this level in conditions of high transparency
variations, as in the case of 12.5 m star.
Outstanding completions
The occurrence that the URSULA photometer practically clashes with its
control computer allows a very great flexibility of the apparatus so
far not much used, due to the limitation of the system based on
PDP11/10. The present control computer (PDP11/24 working under RT11
operating system) allows a greater quickness in the software writing,
since, excluding a few command routines, it is being made in FORTRAN
language.
Presently, the following developments of the apparatus are being
accomplished:
- on-line continuous interpolation or extrapolation of the sky
31
background values through polynomial fitting,
- on-line control of the accuracy of both single measurement and
final result,
- on-line removal of wrong measurements (telescope tracking
errors, spurious electrical signals, etc.),
- stopping of the measure sequence on achievement of a prefixed
precision,
- optimization of distribution of the measurement times on the
various bands versus a prefixed precision,
- improvement of the instrument mechanics in order to allow
pointing of the stars by optical fibers.
The mechanical improvement is suitable, as the experience in using the
instrument has shown that the present mechanical structure makes the
pointing of objects difficult and reduces the useful field.
Conclusions
The current use of the URSULA photometer, as well as the already shown
examples, have demonstrated the capability of the instrument of
measuring with a sufficiently good accuracy in situations of
considerable variations of transparency. In particular, by the
accomplishment of the above described completions, the instrument will
be brought to the maximum efficiency, since its adaptive capabilities
will intervene completely [De Blase and Sedmak 1974]. It is to be
noted that this photometer is able to make a continuous quality
control of the measurements it is carrying out, in the framework of
Ba_de's [1987] considerations.
3P
A noticeable improvement of the whole system could he achieved if
one more optical channel is introduced for the continuous measurement
of the sky background, which presently is an important cause of lack
of precision for faint stars and even for bright ones in the case when
variations of sky brightness are high.
References
Baade,D.: 1987, ESO, scient, preprint 520
Belvedere,G., Paterno',L.: 1976, Astron.Astrophys. 51, 199
De Biase,G.A., Sedmak,G.: 1974, Astron.Astrophys. 33, 1
De Biase,G.A., Paterno',L., Pucillo,M., Sedmak,G.: 1978,
Appl.0pt. 17, 435
De Biase,G.A., Paterno',L., Pucillo,M., Sedmak,G.: 1980a
Publ. Oss. Astr. Trieste, 656
De Biase,G.A., Paterno',L., Pucillo,M., Sedmak,G.: 1980b
Publ. Oss. Astr. Trieste, 665
Rodono',M., Pucillo,M., Sedmak,G., De Biase,G.A.: 1979,
Astron. Astrophys. 76, 242
Young,A.T.: 1974, Methods of Experimental Physics, L.Marton Ed.,
Academic Press, New York, 12, 95
33



Precision limits of the twin-beam multiband URSULA

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