To determine the internal precision of the combination VP-Observer, a long term test was carried out. Measurements of a constant, uniformly illuminated surface corresponding to a sky brightness of about 21.0 mag/arcsec² taken over a six-month period, show a rms scatter of 0.05 magnitudes. This value seems to indicate a good eye threshold constancy over long periods of time. Nevertheless, experience suggests that, under typical field conditions, night sky brightness can be measured with a precision of 0.1 mag.

**Table 1:** Journal of V, CCD observations for personal zero point calibration
Telescope dia. [cm]	Field	Total	Sky	VP	Zero
Site	Center	Extinction	brightness	Attenuation	Point
		[mag]	[mag/arcsec²]	[mag]	[mag/arcsec²]
15.5	R.A. $18^{\rm h}12^{\rm m}$	0.24	21.2	6.0	27.2
Balma Lodge	Dec. $+41^\circ$ 50'
25.5	R.A. $18^{\rm h}12^{\rm m}$	0.36	20.1	7.1	27.2
Torre Luciana Obs.	Dec. $+41^\circ$ 50'
100	R.A. $15^{\rm h}39^{\rm m}$	0.16	21.1	6.0	27.1
VAO	Dec. $+0^\circ$ 15'

On the other hand, calibration of VP-Observer was performed by simultaneous measurements of night sky brightness using both the VP and CCDs (broadband V photometry). This is necessary because the personal eye threshold may differ from the typical value. This procedure should also account for invisible faint stars that nonetheless contribute to the sky background. The necessary corrections are included in the personal zero point (ZP). A journal of CCD observations for zero point calibration is given in Table 1. Results from aperture photometry show that the author's mean zero point is 27.2 mag/arcsec². Thus, sky brightness is found subtracting A, given by Eq. (1), from this value, this is:

where $m_{{\rm sky}}$ is the sky brightness in mag/arcsec², ZPis the personal zero point and A is the magnitude attenuation measured by the VP. No colour terms were included in Eq. (2). A yellow filter, such as Wratten No. 15, rejecting the blue wing, could probably improve the spectral band consistency, but this must be confirmed experimentally to justify its use.

**Table 2:** Zenith night sky brightness at selected sites in Venezuela and Italy. Typical values refer to the mode of the distribution
Site		$m_{{\rm sky}}$		Sky brightness
(Latitude, Longitude, Altitude, Population)	Date	[mag/arcsec²]		increase since 1973
		typ.	min	[mag]
Caracas
$10^\circ 30\hbox{$.\mkern-4mu^\prime$ }$ 4N, $66^\circ 54\hbox{$.\mkern-4mu^\prime$ }$ 1W, 900 m, 1822000	28, 30 Nov. 1997	18.0	-	-
Mérida	22 Jan. 1999
$8^\circ35\hbox{$.\mkern-4mu^\prime$ }$ 5N, $71^\circ 8\hbox{$.\mkern-4mu^\prime$ }$ 6W, 1500 m, 171000	20, 21 Feb. 1999	19.6	-	-
	19,23,24,25,29 Oct. 1997
El Valle (Mérida)	3,22 Nov. 1997
$8^\circ 39\hbox{$.\mkern-4mu^\prime$ }$ 7N, $71^\circ 6\hbox{$.\mkern-4mu^\prime$ }$ 5W, 2000 m,-	20, 21, 23, 25 Dec. 1997	20.7	20.9	-
	4 Jan. 1998
	17, 21 Oct. 1998
	6, 17, 18, 24 Nov. 1997
VAO	27 Jan. 1998
$8^\circ 47\hbox{$.\mkern-4mu^\prime$ }$ 3N, $70^\circ 52\hbox{$.\mkern-4mu^\prime$ }$ 0W, 3600 m,-	16 Oct. 1998	21.1	21.3	-
	18 Dec. 1998
Florence	26, 27 Aug. 1998
$43^\circ 46\hbox{$.\mkern-4mu^\prime$ }$ 5N, $11^\circ 15\hbox{$.\mkern-4mu^\prime$ }$ 0E, 49 m, 402000	19 Sep. 1998	17.5	-	1.4
	19, 26 May 1998
Luciana (Florence)	16 Jun. 1998
$43^\circ 36\hbox{$.\mkern-4mu^\prime$ }$ 6N, $11^\circ 16\hbox{$.\mkern-4mu^\prime$ }$ 1E, 401 m,-	16 Jul. 1998	20.1	20.3	0.7
	26, 27 Aug. 1998
Siena	15 Jul. 1998
$43^\circ 19\hbox{$.\mkern-4mu^\prime$ }$ 0N, $11^\circ 19\hbox{$.\mkern-4mu^\prime$ }$ 9E, 345 m, 57000	14 Aug. 1998	19.3	-	1.6
	1, 17, 19, 20, 21, 24, 25 Apr. 1998
Le Tolfe (Siena)	17, 22, 30 May 1998
$43^\circ 20\hbox{$.\mkern-4mu^\prime$ }$ 8N, $11^\circ 20\hbox{$.\mkern-4mu^\prime$ }$ 3E, 330 m,-	22 Jun. 1998	20.0	20.3	-
	15 Jul. 1998
	24 Aug. 1998
S. Omero (Teramo)	29,30 Mar. 1998
$42^\circ 47\hbox{$.\mkern-4mu^\prime$ }$ 2N, $13^\circ 48\hbox{$.\mkern-4mu^\prime$ }$ 1E, 217 m, 5300	21, 22, 23 Aug. 1998	19.2	19.7	2.1
Balma Lodge (Cuneo)
$44^\circ 13\hbox{$.\mkern-4mu^\prime$ }$ 9N, $7^\circ 45\hbox{$.\mkern-4mu^\prime$ }$ 0E, 1883 m,-	24, 25, 26, 27 Jul. 1998	20.9	21.2	0.6

3 Tests and calibration