Getting the hubble constant from time delay in a gravitational lens

[cegec]

Homework Statement

I am working on a project for my cosmology course. I need to determine the Hubble constant from time delay in a gravitational lens. We are using QSO 0957+561 for our data. My question is how do I determine the relevant distances D_l, D_s? I have D_ls already. The equation for that is as follows:

Homework Equations

D_ls=\frac{2c}{H_{o}}\frac{\sqrt{1+z_{l}}(1+z_{s}^{2})-\sqrt{1+z_{s}}(1+z_{l}^{2})}{(1+z_{s})^{2}(1+z_{l})}

The Attempt at a Solution

Is it also possible to calculate the distances from the time delay? Or should I work strictly with the redshifts of the lens and the source?

thanks

 

[cegec]

i should have added that I also know the angular separation between the two images caused by the lens, the redshifts of both the lensing galaxy and the quasar, and the time delay between the two images. also, does anybody know which lensing model would be best for this scenario? I can use the point mass, isothermal sphere, and constant density sheet. I've heard isothermal is good.