The purpose of SAGA is to obtain accurate Strömgren photometry for stars in the magnitude range 10≤y≤14, where Kepler is able to detect oscillations in thousands of red giant and a few hundreds of selected main sequence stars.


 

The uvby Strömgren photometric system is designed such that different colour indices indicate different stellar atmospheric parameters. The y magnitude is essentially the same as Johnson V for non M-type stars. The color (b-y) is well suited to measuring effective temperatures. The m1 index, defined as (v-b)-(b-y), reflects the depression from metal lines around 4100Å and thus allows us to infer stellar metallicity. Finally, the c1 index, (u-v)-(v-b), indicates the scale of the Balmer discontinuity. For B- and A-type stars, c1 indicates effective temperature. For late-type stars, it reflects stellar surface gravity. Note that it also has metallicity information for stars cooler than the Sun.

Using a combination of short and long exposures, SAGA achieves photometric errors of less than 0.03 mag in all bands for stars with y≤14, the accuracy required to determine robust parameters from Strömgren indices. However, useful stellar properties can still be derived down to 16th magnitude, a regime important for planet-candidate host stars.


 

The first SAGA data release covers a stripe centered at Galactic longitude 74° with latitudes from about 8° to 20°. About 60% of the pointings were observed more than once on at least two different photometric nights. Each pointing has an overlap between 3 and 10 arcsec with its neighbor, allowing us to verify the photometric zero-points remain constant over the observed region.

Our Galactic longitude of 74° includes the open cluster NGC6819, which provides a useful calibration benchmark. This latitude is also only moderately affected by dust and obscuration. Similarly, regions with b below 8° are very close to the Galactic plane, where crowding and reddening make it more difficult to use photometric data.

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Left: The Kepler field as seen via the 42 CCDs on board of the spacecraft in RA and Dec. Overplotted in black is the stripe of stars with uvby magnitudes from the first run of SAGA. The red circle marks the position of the open cluster NGC 6819.
Right: Kepler field (defined with continuous black lines) plotted in Galactic latitude (b) and longitude (l). Vertical dashed lines mark the observed stripe, and the open circle indicates the position of NGC 6819. Linear color code corresponds to different values of reddening 0<E(B-V)<1 using the Schlegel et al. (1998) dust map. The tick mark in the upper bar identifies E(B-V)=0.3, the highest value in our observed stripe.

 

The Strömgren photometric system was designed to provide reliable stellar parameters, in particular metallicities. Thus, even compared to multi-fiber spectroscopy, wide-field Strömgren imaging is a very efficient way to study Galactic stellar populations.

SAGA aims to uniformly observe all stars across the Kepler field down to a V of 15-16th magnitude, whether or not they have seismic information. This has the advantage that no pre-selection is made on targets.

All stars with Strömgren photometry are the cross-matched with stars in the Kepler field of view, i.e. ≈15,000 giants from the Kepler Asteroseismic Science Consortium and over 500 dwarfs. For the initial SAGA stripe, we detect about 95% of the available seismic targets. Further cross-matching of the sample with optical and infrared broad-band photometry reduces the completeness to 93%.

Note that although SAGA includes essentially all stars with seismic information, the seismic target selection criteria of the Kepler satellite are quite complex.


 

Strömgren photometry was obtained with the Wide Field Camera (WFC) on the 2.5m Isaac Newton Telescope on La Palma.

The camera comprises four 2k x 4k thinned EEV CCDS; the coverage of each of the chips is 22.8 x 11.4 arcmin for a total field of view of 34 x 34 arcmin. Pixel size is 13.5 microns, corresponding to 0.33 arcsec/pixel, making the instrument ideal for wide field optical imaging surveys.


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The Nordic Optical Telescope and the Isaac Newton Group of Telescopes at the Roque de los Muchachos Observatory (Credit: L. Casagrande).