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Re: [vsx-dis] Fw: SkyGX + larger VSX FOV + GCPD

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  • Arne Henden
    ... It is a combination of factors: lower QE of the CCD, higher extinction, lower transmission of the filter, less flux from the star. Typically, at nofs a
    Message 1 of 6 , Sep 24, 2006
      On 9/24/06, Wolfgang Renz <w_renz@...> wrote:
      >
      > >> A major "difficulty inherent" (no matter the site they are done
      > >> from) is that the U band measurements alone takes about
      > >> as much time as the BVRcIc measurements take together.
      > >
      > > Is that really a problem when he's doing a sequence?
      > > ...
      > > I was thinking he didn't do U because the nights rarely let him,
      > > and as he was already at altitude, I assumed high altitude
      > > cirrus was the only problem left there.
      >
      > If one can calibrate twice as many fields in the same time,
      > then the decision is easy if the the U band is of no special
      > interest for the traget.
      >
      It is a combination of factors: lower QE of the CCD, higher extinction,
      lower transmission of the filter, less flux from the star. Typically, at
      nofs a U-band exposure is 6x the length of a V-band exposure. This
      means for both the target field and all of the Landolt/extinction fields,
      you have to spend 2x longer on each field if you want U-band calibration.
      So, in general, you get something less than half of the number of fields
      calibrated per night if you include U-band. While I can do it at nofs, and
      the results are good, I don't do it unless I know the requester can make
      effective use of U-band.
      >
      > >> If you complain about U band, you must complain about Ic
      > >> band too. Due to the Ozone (~770nm) and increased H2O
      > >> (~930nm) absorbtions at low altitude sites and the wide
      > >> spread usage of pure Bessell prescription Ic band filters
      > >> that don't have the required 880-900 nm short cut to match
      > >> the actual standard Cousins PMT PEP Ic band.
      > >
      > > Err, I was talking about Uncle Arne using Ic in Flagstaff, not
      > > general Ic work: I always say Rc is preferable, despite (actually,
      > > because of) the problems inherent with that intrinsic to stellar
      > > absorption and emission lines.
      >
      > I haven't seen a transmission plot of the filters Arne uses at Flag.
      > Maybe he can say if the Ic one is with or without short-cut.
      >
      The nofs Ic is an interference filter that closely matches Cousins Ic when
      used with a CCD. The Sonoita (SRO) Ic is a Bessell filter that has the
      red tail and gives poorer results on red stars. We're going to be replacing
      that filter this year.
      Arne
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