http://www.astro.ku.dk/~erik/gaia/78.3G.phot.txt http://www.astro.ku.dk/~erik/gaia/78.3G.fig.ps.gz The 3G photometric system ========================= SAG-CUO-78 E. Hoeg, V. Straizys, V. Vansevicius 29 Aug. 2000 ABSTRACT: The 2G photometric system was proposed in CUO_72. Discussions started during the workshop in Vilnius (see PWG-VIL-001) have led to a new version of the system described below. The BBP bands have been revised and some of the MBP. There are 5 bands in the BBP and 9 bands in the MBP. All of them shall be defined by interference filters giving as far as possible steep sides and low wings, and the steep sides should not coincide with strong spectral features. A graphical overview of all photometric systems relevant in discussions of GAIA is given as CUO_78.3G.fig.ps.gz which is an update of CUO_72...ps. ===== Introduction It is the intention to use the BBP and MBP photometry simultaneously in the classification of stars. This leads us to omission of the 20 nm y-band from MBP. The saved integration time is used for another MBP band, and the continuum measurement by the y-band shall be replaced by use of the broad Vg band. The v and b bands are broadened from 20 to 30 nm. The bandwidth effects resulting from the use of broader bands will be studied. If the effects due to the use of Vg instead of y are too large, the y band may be introduced again. It is therefore still listed in Table 2. ===== The 3G-BBP system of 5 bands The 3G-BBP is a 5 band system as that of 2G. It differs somewhat from that of 2G with respect to the positions and widths of the bands. The Bg, Vg, Rg and Ig bands now cross each other at the 50 per cent transmission point. A gap of 30 nm between Ug and Bg excludes the Balmer jump in order to make the bands astrophysically more significant. The blue Bg limit at 400 nm is more safe not only due to the Balmer jump, but also for G-K-M stars, which possess steep drop down at 400 nm. The red limit of Ig at 900 nm excludes the atmospheric H_2O band, and we loose nothing by shortening Ig from the red side because of the low sensitivity of the CCD, and the effects of possible variation of the CCD sensitivity in time are diminished. Table 1. The 3G-BBP system. The system is very similar to UBVRcIc. We list the two half-maximum wavelength (HMW) of each filter in a separate column. The QE of the CCD is given for the wavelength in the Centre column, and also for the red tail of the CCD which partly defines the red side of the Ig filter. The system is designed for the CCD#1B in BBP. Names Centre HMW FWHM QE t Notes nm nm nm % s BBP, CCD#1B 290 3G33B Ug 330 80 58 0.86 370 400 3G45B Bg 450 100 84 0.86 500 500 3G55B Vg 550 100 80 0.86 600 600 3G68B Rg 675 150 74 0.86 750 750 64 3G83B Ig 825 150 43 0.86 900 Red tail of CCD#1B: 850 35 900 22 950 13 ---- BBP total 4.30 s ===== The 3G-MBP system of 9 bands The 3G system is similar to the 2G. It differs from the 2G in the following respects: y is omitted; v and b are broadened from 20 to 30 nm; P is shifted from 380 to 375 nm; The integration time of p3 is changed from 2*3 to 3 s; the integration times of v and Z are changed from 3 to 2*3 s. Table 2. The 3G-MBP system. The 3G-MBP system is similar to the 1G-MBP system described first in CUO_58 of 11 Feb 1999, and in CUO_71, and it is similar to the 2G system. The system is designed for the CCD#1B in the part with <550 nm. The more red-sensitive CCD#2 is assumed for >550 nm. Names Centre FWHM QE t Notes nm nm % s MBP, CCD#1B SSM0 Gw 550 500 - 0.004 G band, bright stars SSM1 Gw 550 500 - 3 G band = no filter 3G35 u 345 40 69 3*3 3G38 P 375 30 77 2*3 3G41 v 405 30 84 2*3 3G46 b 460 30 84 3 3G52 Z 515 20 82 2*3 2G55 y 545 20 80 0 Omitted in the 3G system MBP, CCD#2 3G66 S 655 20 84 3 3G81 p1 810 40 90 3 3G88 p2 875 30 82 3 3G94 p3 938 20 65 3 ---- MBP integr. of colours 14*3 s = 42 s SSM 3 s Spaces 11*1 = 11 s N+1=11 filters * 1 s Spare time 4 ---- MBP total transit time 60 s = 2 deg ------------------------------------------------------------------- Fig. 1. A graphical overview of all photometric systems relevant in discussions of GAIA. It is an update of CUO_72...ps. The figure is placed on http://www.astro.ku.dk/~erik/gaia/78.fig01.ps -------------------------==========================-------------------