Justification for Ground-Based Measurements of Vesta Using the DAWN Framing Camera Filters
Webmaster: Bruce Gary, 2014.03.18

This web page describes a justification for ground-based measurements of Vesta using the Dawn Framing Camera (FC) filters. When Vesta observations were made the calibration of the FC filters was suspect, so an empirical calibration was made based on the Johnson-Cousins standard star system. Because the FC bands (#2 through #8) had different effective wavelengths and widths this method of calibration was subject to uncertainties that could not be straight-forwardly determined. It is possible to re-calibrate the Vesta results using ground-based measurements of Vesta with a duplicate set of the FC filters. This will entail creation of new magnitude system for each of the 7 narrow band FC filters, with carefully determined conversion coefficients for magnitude to flux equations. Vesta flux [watts/m2/nm] will be determined as a function of Vesta phase, which will provide the needed comparison with Dawn FC observations of Vesta versus phase angle. As a bonus, these ground-based measurements will produce Vesta albedo vs. wavelength (without the need of normalizing). These characterizations of the FC filters should also provide confirmation of the recently-published in-flight re-calibration of the Framing Cameras prior to orbit of Ceres.

Shortcomings of BVRcIc Scheme for Calibrating FC

The FC passbands are given below.

Figure 1. Framing Camera filter pass-band response as influenced by CCD QE and other optical transmissions.

A listing of the filter pass-bands (prior to CCD QE response) is given below.

Figure 2. Listing of filter pass-band center wavelength and width (columns 2 and 3). The effective wavelength column is for a sun-like star.

Are any of the FC pass-band locations similar enough to the Johnson-Cousins pass-band locations to enable stars with known JC magnitudes to be used for calibrating FC measurements?

Figure 3. FC filter pass-band locations in relation to the Johnson-Cousins pass-band locations. (All response functions are for use of a ground-based telescope and a specific CCD, to be described below. You may ignore the Ch# identifications since they are filter position assignments for the proposed ground-based system.)

The best match is FC8 (at 425 nm) and B-band, but FC8 is narrower, and this means that a target with a spectral slope (flux vs. wavelength) will have a different effective wavelength for the two bands. The same applies to FC2 and V-band. The other possible matches are even worse because effective wavelengths will have a greater dependence upon spectral slope. If a target's spectral slope is known then in theory corrections can be made, but each target with a different spectral slope will require a separate correction.

What about using the SDSS magnitudes for calibrating the FC?

Figure 4. FC filter pass-band locations in relation to the SDSS pass-band locations. (Ignore the Ch# identifications.)

The same comments, above, apply to any attempt to use star SDSS magnitudes for calibrating the FC.

The two graphs below show relative probability of photons incident at the top of the atmosphere producing photo-electrons (i.e., includes losses due to atmospheric extinction, telescope optical transmission and CCD QE). I'll refer to this as a filter band's "responsivity."

FC band responsivity (photo-electron production) and Vega spectrum.

FC band responsivity (photo-electron production) and solar spectrum.
Ground-Based Support for Using Asteroid Flux Measurements as a Check on Dawn FC Albedo Measurements

Both Vesta and Ceres have disk-averaged measurements (total Data Number, DN) for each FC filter. This information is for specific phase angle viewing geometries. If actual albedos for Vesta and Ceres were known as a function of phase angle for each FC filter then the Dawn FC measurements could be adjusted to agree with "ground-truth." By "ground-truth" is meant determination of albedo using ground-based observations with an identical set of FC filters, as a function of phase.

The goal of the proposed ground-based observations is disk-average fluxes [watts/m2/um] for a suite of observations that sample a range of viewing geometries. Fluxes can be compared with solar incident flux for each observing situation to determine albedo (for that FC band and at that phase angle). These data can be modeled, and predictions can be made for any Dawn disk-average DN, for the specific FC band and viewing geometry that applies. Any required adjustment to a FC channel will appear as a persistent coefficient that should be multiplied to the default Dawn FC flux in order to achieve agreement with the ground-based model flux.

The ground-based observations will require the creation of new magnitude systems for each FC band. Vega will be used as a primary standard for establishing zero magnitude (above the atmosphere). Secondary standard stars will be calibrated using Vega: two A0V stars (same as Vega) and several sun-like stars. These secondary stars will be located close to the position of Vesta and Ceres for their 2014 oppositions (they're close together in the sky, so their oppositions are in mid-April). This will permit easy calibration of Vesta and Ceres using the nearby (same air mass) secondary standards. A description of how this will be accomplished is in a white paper, "Deriving a Magnitude System," available from Bruce Gary (this web site's webmaster). All measurements will be made with a Celestron 11-inch (CPC1100), with a 10-position CFW and SBIG ST-10XME CCD, all inside a dome in my backyard that is controlled from inside my house.

Progress reports can be found at: index.html


This site opened:  2014.03.15 by Bruce L. Gary (B L G A R Y at u m i c h dot e d u).  Last Update:  2014.03.18