I selected 5 ISP images (same field, different bands) to calculate the zeropoint.

These were the 5 images, all 5 second exposures (typical for ISP):

||isp/20100225/o5252i0361o03/o5252i0361o03.chip01.fits||
||isp/20100225/o5252i0361o04/o5252i0361o04.chip01.fits||
||isp/20100225/o5252i0361o05/o5252i0361o05.chip01.fits||
||isp/20100225/o5252i0361o06/o5252i0361o06.chip01.fits||
||isp/20100225/o5252i0361o07/o5252i0361o07.chip01.fits||

I then used dvo, and the bright star catalog (catdir.synth.bright), to find some bright stars that I could match (by eye) to the images. I found 10 stars (the same 10) in g, i, and r band, only 9 (in z), and only 3 in y (I found 5 more to get a better estimate).

psPhot was used to get instrument mags for these stars, and the zero point was found by taking the median of  (catmag - instmag - 2.5log10(exp_time))

These are the ISP zeropoints I calculated:

||band || ZP  || stddev ||
||g  ||19.3 ||     0.3||
||  r  || 19.1  ||   0.3 ||
|| i||   18.4  ||   0.2||
||z||      17.2 ||   0.1 ||
|| y ||   14.0  ||   0.8||

Here are plots of ZP vs instrument magnitude, for the various bands:
[[Image(isp.zpoints-0.jpg)]]
[[Image(isp.zpoints-1.jpg)]]
[[Image(isp.zpoints-2.jpg)]]
[[Image(isp.zpoints-3.jpg)]]
[[Image(isp.zpoints-4.jpg)]]