11.1.5 HIPO Sensitivity Spreadsheet

A spreadsheet exposure time and SNR estimator for HIPO is available here. This spreadsheet accounts for transmission of all optical components from the atmosphere through the telescope, and inside HIPO, ending with the quantum efficiency of the detectors. It accounts for shot noise on the target, a possible background object (important for occultation work), and sky brightness as well as read noise. The wavelength dependence of the stellar flux is simply modeled as blackbody radiation, with the effect that there are systematic errors in the predictions that vary with wavelength and spectral type. The spreadsheet has been checked against a number of spectrophotometric standard stars with calibrations traceable through Vega to blackbody sources. Observed signals of these standards with the deep depletion CCDs is typically ~10% higher than predicted but in the Johnson I and Sloan i' bands the observed signals are ~30% higher than predicted.

THE SPREADSHEET SNR ESTIMATION DOES NOT ACCOUNT FOR SYSTEMATICS THAT DOMINATE AT HIGH SNR VALUES (~500 OR HIGHER) ON TIMESCALES OF ~10 MINUTES OR LONGER. THIS REMAINS A RESEARCH AREA! PLEASE CONTACT THE PI IF YOU HAVE QUESTIONS ALONG THESE LINES!

The spreadsheet is operated from its first tab, the Control Sheet. The next four tabs deal with optics that are common to both sides of HIPO (e.g. the atmosphere and telescope optics), optics peculiar to the blue or red sides, and the special case of the bare CCD. The last two tabs deal with blackbody and sky fluxes and the final combination of all the previous calculations that eventually appear back on the Control Sheet.

The spreadsheet is operated by filling in the yellow (most commonly used) and orange (less commonly used) cells to account for the star brightness and effective temperature, observing circumstances, CCD operations (integration time, binning, photometric aperture diameter), optics selections, and CCD choices. Results are returned in the light green cells. The main results are the signal levels from target, sky, and background object and the various noise contributions modeled by the spreadsheet. Scintillation noise is very difficult to measure and its value is not well determined yet. Finally the overall S/N ratio and fractional error are given. An estimate of the peak pixel signal in a star image is given in cells G34-H36. This is highly dependent on focus state and image jitter, and somewhat dependent on altitude so should be only used as a guide. The peak signal for the target star is also given in units of full well (assumed to be 100,000 electrons) in cells G35-H35. Note that full well is not affected by binning in a CCD47-20 since this CCD has no summing well. It is sometimes convenient to know the flux-weighted effective wavelength of a given filter/star combination. These are returned near the top center of the Control Sheet for the blue and red sides. For convenience the names of the beamsplitters, filters, and windows, or an indication if they have been removed, is given in the green cell above the selection cell for each optical element. The same applies to the CCD selection cells.

The optical elements modeled, in order, follow:

  • Atmosphere: Atmospheric extinction is a combination of ozone (assumed to be 300 Dobson units) and Rayleigh scattering appropriate for 41,000 foot altitude. This matches observations quite closely. The usual sec(Z) airmass correction is applied.
  • Telescope: The condition of the aluminum coatings on the telescope optics can be changed using cells J43-L43. At present the only tertiary mirror available is the dichroic tertiary and cell L43 has no effect. Cell B26 allows selection of either the dichroic tertiary or the not-yet-available aluminized one. If the aluminized tertiary is selected the aluminum coating condition in L43 does have an effect. The SOFIA coatings are maintained in excellent condition so cases 0 and 1 are the only reasonable ones to use.
  • Fore optics: If the tub is to be evacuated the gate valve window must be installed. Whether this window is installed or not is modeled by selecting 1 or 0 in cell C26. The dichroic reflector used in the FLIPO configuration is controlled by cell D26. It can be left out (0), be the new Reynard dichroic (1), or the old Lick dichroic (2). The Reynard dichroic has about half the emissivity of the Lick one and similar optical performance so is likely to be preferred in most cases.
  • The dichroic beamsplitter internal to HIPO is controlled by cell E26 and can be left out (0) or be one of the two presently available ones (1 or 2). If it is left out no light reaches the blue side CCD.
  • Filters for the blue, red, and bare CCD cases are controlled by cells F26-H26. There is a short explanation of what the various codes mean to the right of the cells used to select the various optics. New filters are certain to be added from time to time.
  • CCD selection is made using cells F30-H30 with 0 being the original standard silicon CCDs and 1 being the newer deep depletion CCDs.The deep depletion CCDs have substantially higher QE and will be preferred in most cases in spite of their greater cosmic ray sensitivity. For UV work the thin blue CCD has better sensitivity than the deep depletion CCD.

    A helpful figure showing the total signal per micron of bandwidth as a function of wavelength is shown in the upper right of the Control Sheet. It gives a graphical representation of the total signal accounting for everything from the star and sky through the CCD. The vertical axis on this graph autoscales.

    An informational box related to the systematic error introduced by the blackbody approximation for stellar fluxes is given in the lower right of the control sheet. Errors at the ends of the wavelength range for very red stars can be ~50% but are 30% or less for most cases.

    Another informational box at bottom center provides a crude estimate of effective temperature given either spectral class or B-V for a target object.

    Finally the revision list is given at bottom left. This spreadsheet will be updated from time to time as optics or detector selections change, or as additional information becomes available. The sky brightness estimate is currently a weak area and may be changed relatively soon.