January 1984, Poul E. Nissen
General description
The main components are:
The transmission coefficients as measured on the 84 cm telescope, December 1983, were
Filter Transmission
------ ------------
u 0.1098
v 0.0997
b 0.0947
y 0.0923
Hß Narrow 0.0940
Hß Wide 0.0940
The coefficients may change with time, because of e.g. varying amount
of dust on the filter. Therefore, if the neutral density filter is
used in connection with observations of primary standard stars,
accurate transmission coefficients should be determined on each night
by observing stars of suitable magnitudes with and without the filter.
As the reflectivity of the grating and the beamsplitter in the Hß section depends on the direction of polarization of the light, polarized stars may deviate systematically from unpolarized stars in the measured indices. For a star with a polarization degree of 0.10 mag the maximum deviation is of the order of 0.092 mag. In order to avoid this effect a depolarizer can be pushed into the beam before the entrance slit. It consists of a super achromatic lambda 2 retardation plate, which rotates with a frequency of 1.5 Hz. It causes a light loss of about 10%.
No. Size Arc sec at 1.5m (f/13.5) -- ------ ------------------------ 0 3.0 mm 31" 1 2.0 mm 20" 2 1.2 mm 12" 3 open - 4 0.6 mm 6" 5 0.45 mm 4".5
The grating-angle may be changed by turning a micrometer screw, the position of which is shown on a meter. Half a turn of the meter -i.e. 0.1 units - corresponds to a wavelength shift of the spectrum of 10Å. Increasing number on the meter corresponds to increasing wavelength for a given point on the exit-slot unit.
The long wavelength of the edge of the b-slot (4801Å) happens to be very close to the wavelength of a strong Cd line at 4799.9Å. This can be used to find the correct setting of the grating. Illuminate the dome wall with a Cd spectral lamp. Wait 5 min for the lamp to heat up. Point the telescope against the wall, and measure the signal in the b-channel, while turning the grating meter in steps of e.g. 0.02 units from 0.3 to 0.6. Use the small diaphragm No. 5. The result of two scans is shown in Figure 3. As seen the Cd line is centred on the edge of the b-slot for a meter position of 0.471. The correct setting is then obtained by adding 0.011 units corresponding to the difference between 4801Å and 4799.9Å.
Alternatively the grating setting may be found by inserting the narrow Hß slit in the v-band and scanning a star with a suitable strength of the Hß absorption line. The minimum in the signal will then determine the grating setting. This method, however, requires good seeing and a very accurate traching of the telescope.
It is recommended to determine the grating setting each time the photometer is mounted on the telescope. Normally the setting remains the same for a whole observing period. The effect of nightly temperature variations is negligible small.
5. Optical filters and exit slot
The spectral response of the photometer is defined by the filters and - in case of uvby - the exit slots. The wavelengths of the slot edges correspond to filter transmissions of 15%. Table 1 gives effective wavelengths, half-widths and peak transmissions of the filters as well as the wavelengths of the slot edges. Detailed transmission curves of the filters are available on request.
Table 1
lambda Delta Peak
eff. lambda trans. slot-edges
------ ------ ----- ----------
u 3505Å 330Å 76% 3324-3686Å
v 4110 170 83 4006-4222
b 4685 183 89 4572-4801
y 5488 235 90 5346-5636
HßN 4864 30 70
HßW 4865 137 82
Table 2 provides various information about the photomultipliers applied in the photometer.
Table 2
Channel PM.No. Resist. in Discrimi- Dark at Standard Source
Hv-distr. nator 5°C Diaphragm # 1
------- ----- -------------- -------- -------- ---------------
u 7953 390 kOmega 0.1 17 c/s 4100 c/s
v 32043 470 1.5 9 8650
b 9336 470 1.1 15 145000
y 32054 0 1.5 13 260000
HßN 7965 470 1.6 14 33500
HßW 7915 0 1.4 3 26100
Spares 9333 470
9331 470
9277 470
Note: High-voltage HV = - 1200 V
The discriminators have been set at a level, where the dark count is less than about 20 c/s at ambient temperature of 5°C, and where the signal is a slowly varying function discriminator setting. A typical example of signal and dark count as a ful, where the dark count is less than about 20 c/s at ambient temperature of 5°C, and where the signal is a slowly varying function discriminator setting. A typical example of signal and dark count as a function of discriminator setting is shown in Figure 3. It should be noted that the dark count rate increases quite significantly when the ambient temperature raises above 10°C. At 15°C the dark count rates will be a factor 2-5 higher than the values listed in Table 2.
The count rates of the photomultipliers can be checked by measuring the signal of the standard light source through diaphragm No. 1 and compare with the count rates listed in Table 2. The intensity of the standard source (a tritium Hß light source) decreases slowly in time with about 10% per year.
The expected count rates for the photometer mounted on the 1.5 meter telescope is given in Table 3. The count rates have been computed from measured count rates with the Swedish 60 cm telescope on La Palma.
Table 3
Expected count rates for the 6-channel photometer
mounted on a 1.5m telescope for a V=10m.0 star
B1 A0 F5
------ ------ -----
u 20 000 6 000 4 000 c/s
v 14 000 12 500 7 000
b 11 500 11 000 8 500
y 6 500 6 500 6 500
HßN 3 200 2 400 2 400
HßW 2 400 2 300 1 800
The count rate for the photomultipliers must not exceed 106 c/s. Higher count rates may damage the photocathodes. It therefore follows from Table 3 that the brightness limit for the uvby is:
V = 5.3 for A and F stars
and V = 5.7 for B-type stars.
For Hß the brightness limit is: V = 4.0 for all stars.
Furthermore, it is recommended that the count rate does not exceed about 300 000 c/s in order to avoid large dead-time corrections. It means that none of the primary standard stars can be reached in uvby. In Hß a few may be observed.
9. Dead-time correction The observed count rate (No c/s) and the true count rate (Nt c/s) are connected by the equation:
No
Nt = ------------
1 - No tD
where tD is the dead-time constant. It has been determined by
measuring the transmission coefficient of the neutral density filter
for stars near the brightness limit and for fainter stars of the same
spectral type. Individual values are given in Table 4.
Table 4
Dead-time constants in units of 10e-9s = 1 ns
(after April 15, 1994)
u 80 ± 15 ns
v 125 ± 10
b 115 ± 10
y 70 ± 15
N 70 ± 10
HßW 65 ± 10
The average value is 78×10e-9 sec, but it seems that the value for HßW is significantly higher.
10. The photon counting system
is described in a manual by D. Hiriart et al. December 1989, Reporte Técnico # 66, Instituto de Astronomía, UNAM, "Sistema de Fotometría Danés-PC".
Most of the primary uvby and Hß standard stars may be observed through the neutral density filter. Accurate determination of the transmission coefficient for that filter will then allow transformation to the standard system.
For late A and F-type stars it is, however, recommended to select well observed (nobs > 3) stars from the recent "Catalog of four-colour uvby and Hß photometry of A5 to G0 stars brighter than 8m. 3" by E.H. Olsen (Astron. Astrophys. Suppl. 54, p.55, 1983). Some Population II stars should be included in order to get a good transformation of the m1 index.
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