In a serie of papers (first: Maitzen & Hensberge 1981; for the time being last: Maitzen 1993) 38 open clusters have been surveyed for the presence of magnetic peculiar stars using $\Delta a$ -photometry in the conventional photomultiplier technique. The advantage of this approach is both a relatively high accuracy of the detection index $\Delta a$ (external scatter for normal stars 0.003 to 0.005mag) and its immediate availability, hence detection of CP2 stars at the telescope. Using photoelectric photometry, on the other hand, the magnitude limit is about V=12 mag for work with a 1m telescope. This limit corresponds to a vicinity of only about 1000pc around the Sun. Even in this area we can reach only the hotter sections of CP2 stars depending on the amount of interstellar absorption.

If one aims to study the behaviour and appearance of peculiar stars over a significant range of galactocentric distances - speculating about the possible influence of different degrees of metallicity and/or galactic magnetic field strengths - then it is imperative to increase the actual magnitude limit by at least two or three magnitudes. Discarding the availability of 4m telescopes as highly unrealistic we are left with employing the CCD-technique. Maitzen et al. (1997) have prepared the ground by showing that for field stars in the range 8<V<10 mag $\Delta a$ -values obtained with CCD-photometry at a 60cm telescope exhibit essentially the same error level as conventional photoelectric photometry.

Aside from the basic advantage of CCD-photometry - i.e. simultaneous recording of many individual objects not only saving time but also increasing precision - the second benefit becomes more and more obvious when observing clusters at larger distances with decreasing angular separations of their members. In this case stellar magnitudes can be derived only by fitting of individual point-spread-functions.

This paper presents the first three open clusters, NGC 2169, Melotte 105 and NGC 6250 measured in the $\Delta a$ system.

Table 1: Observation log
Cluster Site Nights # $_{g_{\rm 1}}$ # $_{g_{\rm 2}}$ #_y

NGC 2169 L. Figl Obs. 1 5 3 3

Melotte 105 ESO 4 9 12 10

NGC 6250 ESO 6 19 20 19

UTSO 1 2 2 2

**Table 1:** Observation log
Cluster	Site	Nights	# $_{g_{\rm 1}}$	# $_{g_{\rm 2}}$	#_y
NGC 2169	L. Figl Obs.	1	5	3	3
Melotte 105	ESO	4	9	12	10
NGC 6250	ESO	6	19	20	19
	UTSO	1	2	2	2

**Table 2:** Characteristics of the used filters
Filter	$\lambda_{\rm C}$	Bandwidth	Transmission
	[Å]	[Å]	[%]
$g_{\rm 1}$	5027	222	66
$g_{\rm 2}$	5205	107	50
y	5509	120	54

1 Introduction