dichroic specifications (Vik Dhillon, 12/12/2000)





I requested quotes from 3 different companies: The best specification and cheapest quote has come from CVI Technical Optics - £6288.6 +delivery in 8-10 working weeks. The specifications of the dichroics are: Here are the final curves for dichroic 1 and dichroic 2 in WORD format and an accompanying email from Helmut. Note that these are theoretical curves (see below for actual measured curves).

Here is a plot showing the SDSS filter band-passes and the dichroic cut-points, with an explanatory email (to Tom).

Just prior to coating, I noticed that CVI had made the substrates the wrong dimensions - the optical engineers had not received the final set of dimensions from Helmut. This caused a slight delay whilst they recut the substrates to the correct size. The coated dichroics were finally delivered in December 2001. Here are the measured curves for dichroic 1 and dichroic 2 in EXCEL format. Here are the measured curves for dichroic 1 and dichroic 2 in ASCII format. Here is an accompanying email from Helmut. Here is a plot showing the dichroic curves together with the filter bandpasses (plotted using the program "dichroic_curves").



The cut-point of dichroic 1 falls at 380nm and the cut-point of dichroic 2 falls at 558nm. Whereas dichroic 2 is within the 1.5% manufacturing tolerance quoted by CVI (it is actually 1.2% off), dichroic 1 is not (it is 2% off specification). CVI offered either to recoat dichroic 1 or give me a discount on it if I keep the original. I looked at the resulting throughput, as shown in this plot (computed using the program "throughput").



The u-band throughput is approximately 7% below the value it would have been had dichroic 1 been on specification. Given the importance of the u-band to the ULTRACAM project, the great time and expense we have put in trying to keep it at a maximum, and the much greater band-pass and efficiency of the g-band, we were unable to accept dichroic 1 and sent it back to CVI for recoating. The recoated dichroic arrived in the middle of January 2002 and the cut-point falls perfectly at 387nm. Here is the final curve for dichroic 1 in ASCII format.