Date: Sat, 3 May 2003 14:02:58 -0400 (EDT) From: "Wesley A. Traub" Reply-To: "Wesley A. Traub" Subject: transfer function of IO To: Jean-Philippe.Berger@obs.ujf-grenoble.fr Cc: kern@obs.ujf-grenoble.fr, malbet@obs.ujf-grenoble.fr, wtraub@cfa.harvard.edu, sragland@cfa.harvard.edu JP: We need some help from you and Pierre and Fabien, regarding the complex transfer function of the integrated optic component currently at IOTA. Sam has been using some narrow-band filters, each about 1/3 the width of the standard H-band filter, centered in the short-wavelength 1/3 of H, the center 1/3, and the long-wave 1/3. In other words, we are resolving the H band by a factor of about 3. On a given star, Sam notices that the closure phases are repeatable to about 1 degree or better, which is good. In the short wavelength band, the closure phases from the "left" and "right" outputs of the beam combiners (by analogy with a classical Michelson combiner) are the same to about 1-2 degrees. The same is true of the middle wavelength band, but the closure phase value here is different by roughly 10 degrees. For the long wavelength band the "left" and "right" outputs are not the same; they differ by about 10 degrees, and are also different than the closure phases of the other beams by about the same amount. Graphically, and schematically, it looks like this: 200 deg L c R l o s R u 190 deg r LR e L p h 180 deg a s short middle long e lambda lambda lambda <-----------H-band---------------------> Also, the magnitudes of the raw visibilities vary systematically, roughly as V(short) ~ 40%, V(middle) ~ 60%, V(long) ~ 40%. None of this surprises me, because single-mode fiber couplers are well-known to have strong chromatic properties, due to the fiber geometry itself, ane also due to the dispersion of the materials. However we are having a hard time calibrating the data, because the color of the star seems to have a large effect. Again, this is to be expected because of the wavelength-variation of the coupling efficiency and phase shift. The problem is that we cannot easily find calibrators of the same color as the target stars. *** Therefore we would like to have measured (and theoretical) values of the complex transfer function as a function of wavelength across the entire H band.*** Can you give us some guidance on this last point? In principle I could calculate this myself, but it is a big effort, and I probably would not have the right parameters to use anyhow. I assume that you have done all this, or that it is in a thesis or paper somewhere. We would very much appreciate guidance on this issue. We have a lot of data now, and need the guidance rather soon. The best would be if we could hear from you before Sam starts his observing run on 7 May, so he can use the result to guide his choice of targets and filters. Electronic or faxed information would be most welcome. We could measure these properties ourselves, but it will take some time and effort to set up the optics. It will be a big project. By the way, I spent a very pleasant three days in Paris this week, visiting Meudon, playing in the coronagraph lab, giving a talk, and dining (Le Procope is not a bad place!). Jean Schneider and his wife took me to the roof of Paris Observatory, and the old refractor. Very pleasant. Someday I hope to get as far as Grenoble to visit your labs (and restaurants)! Thank you for any advice you can offer on our IO problem Best wishes, Wes -------------------------------------------------------------- Wesley A. Traub wtraub@cfa.harvard.edu Harvard-Smithsonian Center for Astrophysics 60 Garden St., Cambridge MA 02138 http://cfa-www.harvard.edu/cfa/oir/IOTA/ tel 617-495-7406; cell 978-852-6390; fax 617-496-0121 Travel: FIRST meeting, Logan Utah, 13-14 May (Eclipse mtg, JPL, 21-23 May) Keck Interf. Sci. Working Group mtg, USNO, 2 June NASA Origins Subcommittee mtg, Wash. DC, 1-2 July Michelson Summer School, lecturer, Pasadena, 7-11 July IAU General Assembly meeting, Sydney, Australia, 13-26 July Balloon flight, Ft. Sumner NM, September OSA mtg, inv. talk, Tucson, 5-9 Oct. --------------------------------------------------------------