2 Observations and data-reductions

The observations presented here were made by using the 1.03 metre Dall-Kirkham-type telescope (f/8.45) of Tuorla Observatory with a SBIG ST-8 CCD-camera and a standard V-band filter. A small number of observations were also taken on Roque de los Muchachos (La Palma, Canary Islands) at the 2.56 metre Nordic Optical Telescope (NOT) with standard B-, V-, and R-filters. Corrections for dark-current effects, additive effects (bias) and multiplicative effects (flatfield) were applied. Due to the relatively small (5$^\prime$34 $^{\prime\prime}$$\times$3$^\prime$42 $^{\prime\prime}$) field of view of the 1.03 metre telescope separate frames of comparison stars were exposed for Mrk 421 and 4C 29.45. For the other objects normal differential photometry methods were applied. Exposure times with the Tuorla 1.03 meter telescope were between 60 and 240 seconds for objects brighter than 16 mag. For fainter objects, we have combined several exposures to achieve a sufficiently high signal to noise ratio.

Photometric measurements were made with the normal IRAF-routines and applying differential photometry. The error, ${\sigma}_{\rm total}$, associated with each measurement was assumed to consist of two components: ${\sigma}_{\rm total}$ = ${\sqrt{{{\sigma}_{\rm photon}}^{2} + {{\sigma}_{\rm cal}}^{2}}}$. The first component, ${\sigma}_{\rm photon}$, consists of the photon noise of the object, the error in sky measurement, and the background photon noise in the measurement aperture, summed in square. The second component, ${\sigma}_{\rm cal}$, was obtained from the standard error of the mean of the magnitude difference between the object and its comparison stars. Typical total errors were between 0.03 and 0.06 mag.

The objects in our sample have been chosen by the following selection criteria: 1) radio-loud, $\gamma$-bright blazar (Fichtel et al. [1994]; von Montigny et al. [1995].; Thompson et al. [1995]),

2) optically bright enough (V $\leq$ 18 mag) to be observed with a telescope of one metre aperture. Several objects in our sample were targets of EGRET-pointings during the years 1995, 1996, and 1997. Some blazars which have not been detected in $\gamma$-rays were also selected for our sample so as to compare their variability properties to those of $\gamma$-bright blazars. Some objects, like OJ 287, were monitored as a part of the OJ-94 campaign (Sillanpää et al. [1996]a-c). A list of the objects including their names, coordinates (epoch 2000.0), redshift, classification, number of datapoints observed at Tuorla, variation scale in the V-band ($\Delta V$), detection in the $\gamma$-region (Fichtel et al. [1994]; von Montigny et al. [1995]; Thompson et al. [1995]; Catanese et al. [1997]), and comparison stars with their references is given in Table 1.

 

Table 1: Objects in Tuorla quasar monitoring campaign

Name	R.A.(2000)	Decl.(2000)	z	class.	N obs.	$\Delta V$	$\gamma$-detec.	comp. stars ref.
S2 0109+224	01 12 05.8	+22 44 39	unknown	BL Lac	38	1.97	No	8
3C 66A	02 22 39.6	+43 02 08	0.444	BL Lac	74	1.16	Yes	4, 5
AO 0235+164	02 38 38.9	+16 37 00	0.94	BL Lac	7	*	Yes	10
OF 038	04 24 46.7	+00 36 07	unknown	BL Lac	16	1.10	No	8
S5 0716+714	07 21 53.3	+71 20 36	unknown	BL Lac	108	1.58	Yes	5, 11
PKS 0735+178	07 38 07.4	+17 42 19	0.424	BL Lac	41	1.68	Yes	10
PKS 0736+107	07 39 18.0	+01 37 04	0.191	HPQ	19	0.59	No	10
OI 090.4	07 57 06.7	+09 56 35	0.660	BL Lac	38	1.03	No	8
OJ 287	08 54 48.8	+20 06 30	0.306	BL Lac	120	1.68	No	5
S4 0954+658	09 58 47.2	+65 33 54	0.367	BL Lac	35	*	Yes	9
Mrk 421	11 04 27.2	+38 12 32	0.031	BL Lac	47	*	Yes	14
OM 280	11 50 19.2	+24 17 54	0.200	BL Lac	26	0.63	No	5
4C 29.45	11 59 31.9	+29 14 45	0.729	HPQ	11	1.60	Yes	10
ON 325	12 17 52.0	+30 07 01	0.237	BL Lac	37	0.83	No	5
ON 231	12 21 31.7	+28 13 58	0.102	BL Lac	73	*	Yes	6
3C 273	12 29 06.7	+02 29 08	0.158	LPQ	54	0.30	Yes	10
3C 279	12 56 11.1	-05 47 21	0.538	HPQ	45	1.5	Yes	12
PKS 1424+240	14 27 00.5	+23 48 00	unknown	BL Lac	25	0.21	No	5
4C 38.41	16 35 15.5	+38 08 05	1.814	LPQ	5	1.61	Yes	13
3C 345	16 42 58.8	+39 48 37	0.595	HPQ	40	1.03	No	10
OT 546	17 28 18.6	+50 13 11	0.055	BL Lac	25	0.59	No	4
3C 371	18 06 50.7	+69 49 28	0.050	BL Lac	26	0.79	No	7
BL Lacertae	22 02 43.3	+42 16 39	0.069	BL Lac	52	1.34	Yes	3
4C 31.36	22 03 14.5	+31 45 42	0.297	LPQ	20	0.63	No	12
CTA 102	22 32 36.4	+11 43 51	1.037	HPQ	9	2.01	Yes	2
3C 454.3	22 53 57.7	+16 08 53	0.859	HPQ	25	0.78	Yes	1
OY 091	22 57 17.3	+07 43 12	0.190	BL Lac	15	0.52	No	4

(1) Angione ([1971]) (2) Balonek, http://quasar.colgate.edu/$^{\sim}$tbalonek/optical/CTA102compstars.gif (3) Bertaud et al. ([1969]) (4) Crane ([1977]) (5) Fiorucci et al. ([1996]) (6) Fiorucci et al. ([1998]) (7) McGimsey et al. ([1975]) (8) Miller et al. ([1983]) (9) Raiteri et al. ([1999]) (10) Smith et al. ([1985]) (11) Takalo et al. ([1994]) (12) Tritton et al. ([1973]) (13) Villata et al. ([1997]).