Context :

Aims : Our aim is to measure the time delay between the two gravitationally lensed images of the z _ { qso } = 1.547 quasar SDSS J1650+4251 , in order to estimate the Hubble constant H _ { 0 } .

Methods : Our measurement is based on R -band light curves with 57 epochs obtained at Maidanak Observatory , in Uzbekistan , from May 2004 to September 2005 .
The photometry is performed using simultaneous deconvolution of the data , which provides the individual light curves of the otherwise blended quasar images .
The time delay is determined from the light curves using two very different numerical techniques , i.e. , polynomial fitting and direct cross-correlation .
The time delay is converted into H _ { 0 } following analytical modeling of the potential well .

Results : Our best estimate of the time delay is \Delta t = 49.5 \pm 1.9 days , i.e. , we reach a 3.8 % accuracy .
The R -band flux ratio between the quasar images , corrected for the time delay and for slow microlensing , is F _ { A } / F _ { B } = 6.2 \pm 5 % .

Conclusions : The accuracy reached on the time delay allows us to discriminate well between families of lens models .
As for most other multiply imaged quasars , only models of the lensing galaxy that have a de Vaucouleurs mass profile plus external shear give a Hubble constant compatible with the current most popular value ( H _ { 0 } = 72 \pm 8 km s ^ { -1 } Mpc ^ { -1 } ) .
A more realistic singular isothermal sphere model plus external shear gives H _ { 0 } = 51.7 ^ { +4.0 } _ { -3.0 } km s ^ { -1 } Mpc ^ { -1 } .