From jeg@astro.princeton.edu Thu Jun 1 08:36 EDT 2000 Received: from minos.Princeton.EDU (jeg@minos [128.112.24.49]) by astro.princeton.edu (8.9.3/8.9.3) with ESMTP id IAA08032; Thu, 1 Jun 2000 08:36:17 -0400 (EDT) Received: (from jeg@localhost) by minos.Princeton.EDU (8.9.3/8.9.3) id IAA14474; Thu, 1 Jun 2000 08:36:16 -0400 (EDT) Date: Thu, 1 Jun 2000 08:36:16 -0400 (EDT) Message-Id: <200006011236.IAA14474@minos.Princeton.EDU> From: jeg@astro.Princeton.EDU To: hogg@sns.ias.edu, schlegel@astro.Princeton.EDU Content-Type: text Content-Length: 19093 Status: O THE NEAR-IR SCATTERING PHENOMENON IN THINNED SITe CCDs J. E. Gunn 20 July 1997 Some time ago Don York was told about an alarming result discovered by Rich Reed at NOAO during tests of thinned SITe CCDs like ours for STIS. In these tests, Reed saw extended scattering wings in the near infrared, which he attributed to light going through the thinned device and scattering from the indium solder interface behind the translucent ceramic substrate which supports the thinned membrane in these chips. In what follows we shall call this phenomenon, whatever the cause, rear scattering (because it is certainly associated with the return of flux from the rear of the device.) Reed kindly sent me his data in graphical form, and I have finally gotten around to analyzing it in the context of the SDSS. It would appear that the data are well described by the phenomenon which Reed proposed; light scattering in a translucent medium is attenuated by a factor 1/r due to the 2-dimensional geometry and by an exponential optical depth factor due to scattering and losses (into the ccd or absorption by the indium or the ceramic). The data are well fit by the radial function Bs/F = (f/2*pi*r0) * exp(-r/r0)/r, where Bs is the surface brightness of the scattering wing, F is the total flux, and f is the fraction of the total which is scattered. Reed studied the phenomenon at several wavelengths using what was essentially a line source, and his curves are well represented by the projection of this function in one dimension. The parameters f and r0 are reasonably well represented by the interpolation formulae r0 = 50 lam^2, lam = wavelength in microns f = exp(11.51*(lam-1.05)), lam < 1.05 1 lam > 1.05 representative values are lam r0(pix) f 6000 18 .006 7000 25 .016 8000 34 .065 9000 41 .18 10000 48 .56 Here r0 is in pixels for the 24u pixel device; it presumably scales with the inverse of the pixel size and in some complex way which depends on the exact details of the transport upon the thickness and opacity of the ceramic substrate, the thickness of the silicon membrane, the reflectivity of the indium film, and the efficiency of the antireflection coating on the CCD. The scattered fraction f presumably goes to unity at the wavelength of the bandgap of silicon, about 10500 A (at which wavelength the quantum efficiency goes essentially to zero anyway.) and the behavior, in which the log of f is almost linear with wavelength, supports this conjecture. It is perhaps curious that the relation is more nearly exponential with wavelength than frequency, and the interpolation formula for it is to be regarded as just that. There in reality will be a small thermal tail extension to 1-f to longer wavelengths, which we ignore here. As one can see from the table, at 1 micron the effect is catastrophic, with nearly 60 percent of the flux in the scattering halo; by 9000, the fraction is still 18 percent, 8000, six percent, and 7000, only 1.6 percent. The effective wavelengths of the Sloan filters for this effect are considerably longer than the flux effective wavelengths because of the very strong wavelength dependence; the effective wavelengths, scattering lengths, and scattered flux fractions are approximately fil lameff(u) r0 f r' 0.64 20 .009 i' 0.78 32 .049 z' 0.95 44 .31 The photometric camera is, of course, not using thin chips in the z band, but the monitor telescope is. The effect is likely to be worse, in fact, for the MT chip because its UV-optimized coating is less efficient in the red and infrared than the normal vis-AR coatings and more reflective for the scattered flux returning to the device from below. Preliminary estimates for the MT throughput confirm this, with the u', g', r', and i' efficiencies pretty close to the estimates, but the z' efficiency about a factor of 2 low. This will not be a problem provided the effect is sufficiently uniform over the chip. This is, in fact, likely to be the case, because the primary QE is not dependent on the effect, but if the rear scattering is not uniform one must be very careful in the interpretation of flat fields for calibration, which measure in effect the total PSF including the rear scattering wings. It may be necessary to use stars to calibrate the "flat field" in z' with the MT, and perhaps in i' as well, though probably not. The use of aperture magnitudes for the MT, necessary because of the undersampling, makes the operational consequences of the phenomenon negligible; the measurement of fluxes can proceed exactly as it is being done now. The consequences for the survey imaging data are a little more severe and may have some software impact. The z' chips are not affected at all, since they are thick. In u' and g' the effect of rear scattering is negligible; in r', the small-angle scattering wings of the PSF generated by the atmosphere and optics dominates the wings of the PSF everywhere, but if the amplitude of the rear scattering is as Reed measured it, the wings are significantly modified at radii of 20 pixels or so. In i' the rear scattering dominates the PSF from 10 to over 100 pixels radius, and the simple power-law model for the PSF wings which the pipeline uses currently will not work. This has little or no impact for photometry, but severly affects the subtraction of the wings of bright stars; in brightness, the range is from about 5.e-4 to 3.e-7 of the central intensity; the flux, again, is about 5 percent of the total. In the following table is presented some expected radial profiles of stars including Kolmogorov seeing and observed small-angle scattering, and the rear scattering wings as calculated here. The first column is the radius in units of the half width at half maximum, here assumed to be 1.25 pixels (0.5 arcsec); the next column, kolpsf, is the Kolmogorov seeing psf of unit central intensity, including observed small-angle scattering wings (from Racine). The next three columns, rscatt, iscatt, and zscatt, are the expected rear-scattering wings in r', i', and z', respectively, for a thinned device in this seeing. These wings are to be compared with the Kolmogorov psf decreased by the unscattered fraction, 0.99, 0.95, and 0.68, respectively, for the parameters discussed here. The pixel scale assumed here is that for the imaging camera with 1 arcsecond seeing, and so the z' column is of academic interest only, but the severity of the effect we would have to live with had we chosen thin chips for the z' band is properly frightening. r kpsf r'scatt i'scatt z'scatt 0.100 9.909e-001 1.542e-003 5.260e-003 2.501e-002 0.126 9.870e-001 1.533e-003 5.231e-003 2.488e-002 0.158 9.807e-001 1.519e-003 5.188e-003 2.468e-002 0.200 9.705e-001 1.498e-003 5.122e-003 2.438e-002 0.251 9.543e-001 1.468e-003 5.025e-003 2.393e-002 0.316 9.295e-001 1.424e-003 4.882e-003 2.327e-002 0.398 8.920e-001 1.362e-003 4.677e-003 2.231e-002 0.501 8.358e-001 1.276e-003 4.394e-003 2.098e-002 0.631 7.541e-001 1.165e-003 4.024e-003 1.924e-002 0.794 6.423e-001 1.030e-003 3.572e-003 1.711e-002 1.000 5.014e-001 8.793e-004 3.063e-003 1.470e-002 1.259 3.435e-001 7.248e-004 2.540e-003 1.223e-002 1.585 1.973e-001 5.797e-004 2.047e-003 9.888e-003 1.995 9.173e-002 4.529e-004 1.615e-003 7.836e-003 2.512 3.505e-002 3.481e-004 1.256e-003 6.129e-003 3.162 1.237e-002 2.642e-004 9.682e-004 4.756e-003 3.981 4.668e-003 1.983e-004 7.407e-004 3.671e-003 5.012 1.943e-003 1.470e-004 5.625e-004 2.818e-003 6.310 8.743e-004 1.073e-004 4.232e-004 2.150e-003 7.943 4.082e-004 7.675e-005 3.146e-004 1.626e-003 10.000 1.964e-004 5.352e-005 2.302e-004 1.216e-003 12.589 9.713e-005 3.612e-005 1.651e-004 8.966e-004 15.849 4.929e-005 2.339e-005 1.154e-004 6.488e-004 19.953 2.557e-005 1.437e-005 7.803e-005 4.584e-004 25.119 1.352e-005 8.260e-006 5.064e-005 3.143e-004 31.623 7.257e-006 4.369e-006 3.120e-005 2.075e-004 39.811 3.942e-006 2.080e-006 1.799e-005 1.306e-004 50.119 2.161e-006 8.675e-007 9.555e-006 7.741e-005 63.096 1.193e-006 3.062e-007 4.572e-006 4.253e-005 79.433 6.621e-007 8.761e-008 1.918e-006 2.124e-005 100.000 3.687e-007 1.924e-008 6.823e-007 9.404e-006 For the MT pixel scale, the star image is more concentrated than in the case of the camera, so the effect on the psf is somewhat smaller; the fraction of light lost to scattering is the same, but it is spread out over a much larger area compared to the star image (though the profile of the scattered light is the same in PIXELS, modulo a trivial change in convolution due to seeing). The table above for the MT becomes (the unit of radius is still 0.5 arcsec) : r kpsf r'scatt i'scatt z'scatt 0.100 9.909e-001 3.794e-004 1.294e-003 5.960e-003 0.126 9.870e-001 3.771e-004 1.287e-003 5.929e-003 0.158 9.807e-001 3.737e-004 1.276e-003 5.882e-003 0.200 9.705e-001 3.686e-004 1.260e-003 5.810e-003 0.251 9.543e-001 3.612e-004 1.236e-003 5.703e-003 0.316 9.295e-001 3.504e-004 1.201e-003 5.545e-003 0.398 8.920e-001 3.350e-004 1.151e-003 5.317e-003 0.501 8.358e-001 3.140e-004 1.081e-003 5.001e-003 0.631 7.541e-001 2.867e-004 9.899e-004 4.586e-003 0.794 6.423e-001 2.535e-004 8.787e-004 4.077e-003 1.000 5.014e-001 2.163e-004 7.536e-004 3.504e-003 1.259 3.435e-001 1.783e-004 6.249e-004 2.914e-003 1.585 1.973e-001 1.426e-004 5.036e-004 2.357e-003 1.995 9.173e-002 1.114e-004 3.973e-004 1.867e-003 2.512 3.505e-002 8.564e-005 3.091e-004 1.461e-003 3.162 1.237e-002 6.500e-005 2.382e-004 1.133e-003 3.981 4.668e-003 4.878e-005 1.822e-004 8.748e-004 5.012 1.943e-003 3.616e-005 1.384e-004 6.716e-004 6.310 8.743e-004 2.639e-005 1.041e-004 5.124e-004 7.943 4.082e-004 1.888e-005 7.740e-005 3.876e-004 10.000 1.964e-004 1.317e-005 5.664e-005 2.899e-004 12.589 9.713e-005 8.886e-006 4.061e-005 2.137e-004 15.849 4.929e-005 5.753e-006 2.838e-005 1.546e-004 19.953 2.557e-005 3.534e-006 1.920e-005 1.093e-004 25.119 1.352e-005 2.032e-006 1.246e-005 7.491e-005 31.623 7.257e-006 1.075e-006 7.675e-006 4.946e-005 39.811 3.942e-006 5.117e-007 4.427e-006 3.113e-005 50.119 2.161e-006 2.134e-007 2.351e-006 1.845e-005 63.096 1.193e-006 7.533e-008 1.125e-006 1.013e-005 79.433 6.621e-007 2.155e-008 4.719e-007 5.061e-006 100.000 3.687e-007 4.734e-009 1.679e-007 2.241e-006 It is seen here that scattering has little effect on the psfs except in z', dominating only between about 30 and 60 pixels in i' and even so not dominating by very much. In z', the scattering dominates beyond about 8 units, 4 arcsec, 5 pixels, radius. For a just saturated star, the annulus between 12.5 and 25 pixels (10 and 20 arcsec) has a signal which is enhanced by 14 and 5 DN, respectively, at its inner and outer boundaries. Using such an annulus for the sky signal would depress the measured flux by about 2.5 percent; it happens that about 2.5 percent of the flux is also scattered within 5 pixels, so the net result of using a disk of 5 pixels radius for the stellar aperture and determining the sky from an annulus with inner and outer radius 12 and 25, which is a reasonable set, is that the scattering roughly cancels, and leaves one measuring the unscattered fraction, about 70 percent of the total. Recall that it would appear from the preliminary MT data that the UV-coated chip suffers rather more than these parameters would indicate in z', but until better data are available, detailed calculations of the form of the psf and the effect on the shape of the z' band cannot be done. We CAN calculate the effects if the current paramters were correct, and in that case the 'vital statistics' of the MT passbands are, with no scattering, (these are slightly different from those last distributed because the MT mirror coatings have changed since then to bare aluminum on the secondary and a quartz-overcoated aluminum film on the primary.) MT FILTER QUANTITIES, NO ATMOSPHERE 1 bare 1 230nm SiO2, no rear scatt. fil lbar wid qt qtdll sig efwhm u' 3505 630 1.919e-001 3.220e-002 0.0569 469 g' 4744 1415 4.348e-001 1.207e-001 0.0899 1003 r' 6217 1385 4.945e-001 1.065e-001 0.0653 955 i' 7626 1533 4.406e-001 7.921e-002 0.0588 1055 z' 9061 1392 2.396e-001 3.689e-002 0.0573 1221 MT FILTER QUANTITIES, 1.2 AIRMASSES 1 bare 1 230nm SiO2 no rear scatt. fil lbar wid qt qtdll sig efwhm u' 3547 584 9.758e-002 1.513e-002 0.0536 447 g' 4772 1379 3.619e-001 9.458e-002 0.0885 994 r' 6227 1374 4.495e-001 9.412e-002 0.0649 951 i' 7629 1530 4.079e-001 7.161e-002 0.0591 1061 z' 9038 984 2.254e-001 3.175e-002 0.0584 1242 MT FILTER QUANTITIES, NO ATMOSPHERE 1 bare 1 230nm SiO2, 'std' rear scatt. fil lbar wid qt qtdll sig efwhm u' 3505 630 1.919e-001 3.220e-002 0.0569 469 g' 4744 1415 4.338e-001 1.205e-001 0.0899 1003 r' 6216 1384 4.911e-001 1.057e-001 0.0652 954 i' 7616 1522 4.304e-001 7.584e-002 0.0585 1048 z' 8955 1194 2.100e-001 2.818e-002 0.0506 1067 MT FILTER QUANTITIES, 1.2 AIRMASSES 1 bare 1 230nm SiO2 'std' rear scatt. fil lbar wid qt qtdll sig efwhm u' 3547 584 9.758e-002 1.513e-002 0.0536 447 g' 4772 1380 3.610e-001 9.443e-002 0.0885 993 r' 6225 1374 4.449e-001 9.334e-002 0.0649 951 i' 7618 1519 3.993e-001 6.855e-002 0.0588 1054 z' 8927 968 1.980e-001 2.435e-002 0.0506 1063 For contrast, the quantities for the photometric camera are DSS FILTER QUANTITIES, NO ATMOSPHERE 2 shiles Al bare, 'std' rear scatt. fil lbar wid qt qtdll sig efwhm u' 3501 619 2.394e-001 3.956e-002 0.0564 464 g' 4741 1419 5.425e-001 1.508e-001 0.0902 1006 r' 6220 1385 6.110e-001 1.318e-001 0.0653 955 i' 7613 1519 5.267e-001 9.258e-002 0.0585 1047 z' 9150 1348 1.374e-001 2.166e-002 0.0640 1377 DSS FILTER QUANTITIES, 1.2 AIRMASSES 2 shiles Al bare, 'std' rear scatt. fil lbar wid qt qtdll sig efwhm u' 3543 567 1.237e-001 1.853e-002 0.0530 442 g' 4770 1387 4.501e-001 1.181e-001 0.0887 996 r' 6229 1373 5.569e-001 1.164e-001 0.0649 952 i' 7615 1513 4.924e-001 8.370e-002 0.0588 1053 z' 9134 950 1.291e-001 1.867e-002 0.0659 1417 Note that though the bands for the MT and the camera match very well for u', g', r', and i', the z bands do not match well; this was always the case, but the mismatch now is much worse than before; there was roughly a 100A mismatch in the effective wavelengths before, and with scattering taken into account the mismatch is 200 A. The calculated effect on the total throughput, qtdll, is a bit smaller than we estimated, about 25 percent vs 31, doubtless because of the extreme assymetry of the z' passband. The z' response (system DQE, in these tables with no atmosphere) with wavelength for the camera, MT with no scattering and MT with scattering, with the scattering model employed here, is DSS z' response functions lam camera MT_noscatt MT 7800 0.00001 0.00001 0.00001 7900 0.00009 0.00019 0.00018 8000 0.00073 0.00158 0.00149 8100 0.00447 0.00973 0.00912 8200 0.01856 0.04024 0.03739 8300 0.04607 0.09775 0.08998 8400 0.07862 0.16013 0.14585 8500 0.10580 0.20480 0.18430 8600 0.12359 0.22716 0.20166 8700 0.13326 0.23310 0.20374 8800 0.13715 0.23044 0.19787 8900 0.13639 0.22408 0.18855 9000 0.13266 0.21609 0.17764 9100 0.12671 0.20614 0.16499 9200 0.11904 0.19429 0.15078 9300 0.11021 0.18105 0.13555 9400 0.10047 0.16632 0.11943 9500 0.09021 0.15027 0.10274 9600 0.08001 0.13345 0.08609 9700 0.07039 0.11668 0.07022 9800 0.06171 0.10028 0.05548 9900 0.05428 0.08437 0.04208 10000 0.04773 0.06876 0.03009 10100 0.04174 0.05384 0.01987 10200 0.03596 0.04002 0.01168 10300 0.03008 0.02779 0.00572 10400 0.02411 0.01820 0.00198 10500 0.01846 0.01159 0.00000 10600 0.01344 0.00722 0.00000 10700 0.00943 0.00433 0.00000 10800 0.00675 0.00249 0.00000 10900 0.00527 0.00141 0.00000 11000 0.00463 0.00075 0.00000 The systematic shift to the blue is seen clearly in this set of tables. Thus it would appear that the net result of the effect is that we will be driven to complicate the form of the model psf used for fitting for purposes of star subtraction in the photometric pipeline in i' and perhaps in r' as well, and may have to complicate the flat-fielding calibration for the MT in z' and perhaps i'. Parenthetically, it would appear that the choice of thick chips for z', which was an entirely economically driven one, was fortuitously the best one as well; the real quantum efficiency (for stars) of the thick chips is roughly as high as that for the thinned ones and the lost light does not wander to other places in the device. It is also unfortunately the case that the dependence of quantum efficiency on wavelength through the z' band has been grossly incorrectly measured, because the measurements include both the primary (which is all that is relevant for stars) and rear-scattered components and the rear-scattered fraction is large and very wavelength-dependent. To calculate the correct effective stellar passband for the MT in z' will require observations of bright stars with the telescope to determine the scattering amplitude. The already large and now larger difference between the MT and the camera passbands in z' raises the issue of how the z' magnitude should be defined; for the other bands there should be no systematic differences of any important magnitude, but in z' it would appear that it might be best to define a synthetic z' magnitude which fits the instrumental response of the camera better, so that z' and i' images from the camera will have measured flux ratios which are on the `system'. The lever arm in i'-z' for the camera is about 1500A, and for the MT is about 1300A, so a color (i' - z')_syn = 1.15(i'-z')_MT would approximately match the camera instrumental colors for not-too-complex spectra. I would like to thank Rich Reed for access to the rear scattering data and some very helpful information on its quantitative properties and method of acquisition.