One of the advantages of CCD photometry when compared to photoelectric detectors arises from the possibility of measuring more than one star simultaneously. This point is specially interesting in highly populated regions, as star clusters, where usually we are interested in several (or all) stars present in the CCD frame.

In order to transform instrumental CCD measurements to the standard system, it is necessary to observe a suitable set of standard stars. Quite often several stars are detected in the neighbourhood of a primary standard star, but they cannot be used in the reduction procedure due to the lack of standard photometry. So, the advantage arising from the two-dimensional character of CCD detectors is lost.

UBVRI standard stars by Landolt (1983, 1992) are widely used. They constitute an internally consistent and homogeneous realization of the Johnson-Cousins photometric system. Their location close to the celestial equator makes them accessible to telescopes in both hemispheres.

Table 1: Observational run and chip specifications
Period Nov. 1991, Oct. 1993 Dec. 1993 Dec. 1994

Observatory CAHA OAN CAHA

Telescope 1.23 m 1.52 m 1.23 m

Type: GEC#10 THX 31156 TEK#6

Size (pixels): $385\times576$ $1024\times1024$ $1024\times1024$

Pixel size: 22 $\mu$ m = 0.46'' 19 $\mu$ m = 0.33'' 24 $\mu$ m = 0.50''

Field of view: $3.0'\times4.4'$ $5.6'\times5.6'$ $8.6'\times8.6'$

Gain: 5.7 ${\rm e}^{-}$ /ADU 3.5 ${\rm e}^{-}$ /ADU 4.3 ${\rm e}^{-}$ /ADU

RON: 2.3 ADU 2 ADU 1.5 ADU

Dyn. range: 65535 ADU 65535 ADU 65535 ADU

Linear up to: 40000 ADU 50000 ADU 45000 ADU

Bias level: 260 ADU 240 ADU 261 ADU

Overscan: right-left right-left right-left

**Table 1:** Observational run and chip specifications
Period	Nov. 1991, Oct. 1993	Dec. 1993	Dec. 1994
Observatory	CAHA	OAN	CAHA
Telescope	1.23 m	1.52 m	1.23 m
Type:	GEC#10	THX 31156	TEK#6
Size (pixels):	$385\times576$	$1024\times1024$	$1024\times1024$
Pixel size:	22 $\mu$ m = 0.46''	19 $\mu$ m = 0.33''	24 $\mu$ m = 0.50''
Field of view:	$3.0'\times4.4'$	$5.6'\times5.6'$	$8.6'\times8.6'$
Gain:	5.7 ${\rm e}^{-}$ /ADU	3.5 ${\rm e}^{-}$ /ADU	4.3 ${\rm e}^{-}$ /ADU
RON:	2.3 ADU	2 ADU	1.5 ADU
Dyn. range:	65535 ADU	65535 ADU	65535 ADU
Linear up to:	40000 ADU	50000 ADU	45000 ADU
Bias level:	260 ADU	240 ADU	261 ADU
Overscan:	right-left	right-left	right-left

Table 2: Quantum efficiency of the detectors at the central wavelengths of UBVRI filters
wavelength (nm)

Detector 360 440 550 650 800

GEC#10 17% 19% 31% 49% 43%

TEK#6 50% 60% 65% 70% 60%

THX 31156 17% 15% 28% 38% 32%

**Table 2:** Quantum efficiency of the detectors at the central wavelengths of *UBVRI* filters
	wavelength (nm)
Detector	360	440	550	650	800
GEC#10	17%	19%	31%	49%	43%
TEK#6	50%	60%	65%	70%	60%
THX 31156	17%	15%	28%	38%	32%

The purpose of this paper is to provide standard UBVRI-CCD photometry for stars in the neighbourhood of several Landolt standard stars. This will allow the use of several reduction-useful stars from a single CCD frame. The stars presented in this paper are grouped in 11 different fields, each one containing at least one Landolt star.

1 Introduction