1. Photometric Redshifts of objects in B0218+357 field Sanhita Joshi ANGLES Workshop, Crete 07/04/2005

2. What do we know about 0218+357 Double image, with Einstein Ring with radius 334 mas (Patnaik et al) Time delay of 10.5 ± 0.4 days (Biggs et al) Redshifts: Quasar – 0.944 (Lawrence et al) Galaxy – 0.685 (Browne et al) Galaxy position: (255±6, 119 ± 4) mas to A. –LENSCLEAN (Wucknitz et al) –With HST images (York et al) Hubble Constant: 73 ± 8 km/s-Mpc (York et al)

3. Need more work on 0218? The result can be (im)proved ! Any other object assisting lensing? – Redshifts of objects in field of view – 3D mapping of these objects Time delay

4. How to find photo-z For good enough accuracy, need data in 5 bands Wavelength range: –UBVRI from Keck – JK from UKIRT Fluxes compared with standard templates, using code HyperZ

7. Photometric Calibration Magnitudes should be corrected for –Zero point of the instrument – Atmospheric extinction (small) –Galactic extinction (small) Used a standard star for photometric calibration of J and K data

8. Why ‘Photometric’ Redshift Less time consuming Accuracy in z up to 0.1, which is sufficient for the purpose Objects with z > 1.0 can be neglected

9. Model spectrum and SED fits The spectra don’t fit to the model!

10. Sources of Error Sky was not photometric during Keck observations Some other systematic errors

11. What I will find ? Redshifts for objects in 0218 field (Non) – presence of any cluster assisting lensing