From vsd@astro1.shef.ac.uk. Tue Sep 18 15:52:18 2001 Date: Tue, 18 Sep 2001 14:03:03 +0100 (BST) From: Vikram Dhillon To: Tom Marsh Cc: Mark John Stevenson Subject: Dichroics Hi Tom, I'm about to finalise the dichroic order. We should be getting them by the middle of next month, according to CVI, which is good news. I have to tell CVI this week: 1. what cut points we want, and 2. what kind of curve we want. 1. Cut points ------------- Attached is a plot summarising the situation. The filter profiles are for our actual filters (i.e. Asahi measured them for us before sending them). Taking these curves as final, one obtains cut points for the dichroics (calculated as the wavelength at which the transmissions of the filters cross over) of: dichroic 1: 3900.28 A dichroic 2: 5553.19 A The dichroic cut points themselves cannot be specified to better than 1-1.5%, according to CVI, due to manufacturing uncertainties (i.e. they cannot control the thicknesses of the layers perfectly). So I have plotted the cut point specified above as a vertical dashed line surrounded by 2 vertical dashed lines indicating the manufacturing error. We cannot take the above figures as the final cut-points, however, because Asahi measured the filters profiles in the attached plot using a collimated beam whereas the ULTRACAM cameras provide a f/2.3 beam. This shifts the bandpasses bluewards by approximately 0.7% for both the g' and r' cut-point wavelengths and by approximately 0.5% and 0.65% for the u' and g' cut-point wavelengths, respectively (as calculated by Mark). These shifted cut-points are indicated by the solid vertical lines. THESE are the dichroic cut points I think we should use, ie. dichroic 1: 3877.86 A (calculated assuming a mean shift of 0.575%) dichroic 2: 5514.31 A NOTE: There is also a shift due to changing temperature. The curves in the attached plot were measured at a temperature of 25 Celsius. However, I can't really account for this shift, as I don't actually know how much the curves shift due to changing temperature (it is a small effect compared to the f/2.3 shift, apparently) and we will be observing in a variety of different temperatures, of course. So, I have neglected to make this correction. You can see that it is an imprecise business and we are almost certainly going to have to take a throughput hit in one of the bands due to the cut-point not being perfect. 2. Shape of curve ----------------- There is a trade off between how sharp we make the cut and the peak reflectivity we get from the dichroic. If we want a really sharp cut, we pay a (small) penalty in reflectivity (manifest as a slightly more "bumpy" peak reflectivity profile). I'm keen to make the cuts as sharp as possible (especially given the likely errors in their position) - the resulting peak transmission loss is small (1% at most). Let me know if you agree. I hope to send the final specs off to CVI this week, so please let me know what you think of the above. Thanks, Vik. ------------------------------------------------------------------------------- Vik Dhillon phone: +44 114 222 4528 Dept of Physics & Astronomy fax: +44 114 272 8079 University of Sheffield email: vik.dhillon@shef.ac.uk Sheffield S3 7RH, UK web: http://www.shef.ac.uk/~phys/people/vdhillon ------------------------------------------------------------------------------- [ Part 2, "" Application/POSTSCRIPT 67KB. ] [ Unable to print this part. ]