From erik Mon Mar 5 15:14:29 2001 Subject: CUO-82.txt sky background To: MPERRYMA@estsa2.estec.esa.nl (Michael Perryman) Date: Mon, 5 Mar 2001 15:14:29 +0100 (MET) X-Mailer: ELM [version 2.5 PL2] Content-Length: 24196 Status: OR Multi-colour photometry of the diffuse sky background with GAIA =============================================================== GAIA-CUO-082 Erik Hoeg 18 Jan. 2001 Issue #2 27 Feb. 2001 with comments by Kalevi Mattila Full text on http://www.astro.ku.dk/~erik/gaia/82.background ABSTRACT: The report discusses the possible multi-colour photometry of the sky surface with the Astro and Spectro telescopes of GAIA. This photometry makes use of the data in the star patches for stars, thus no extra telemetry is required, only a proper data reduction on the ground. The ordinary photometry of faint stars will give good results for at least 500 million spots on the sky. The precision per spot for the mission average values are given. At a surface brightness of e.g. uV=21.0 mag/arcsec^2 a precision of 0.001 mag in G and about 0.01 mag is obtained in all medium width photometric bands for spots of 7 arcsec diameter. Based on comments from K. Mattila we conclude that GAIA photometry is suited to tackle the whole range of astrophysicallu interesting diffuse sky components. The required accuracy and low level of straylight can be realised by proper design of the photometric data reduction and by introduction of a conical baffle in the Spectro telescope. MACP: Please note the required conical baffle in Spectro (see Section 5) and the corrections in Table 3. 1. Introduction ----------------- The reader is assumed to be familiar with the GAIA Study Report (GSR), but for the sake of other readers I give here some explanations: Astro telescopes : two telescopes of aperture 1.70x0.70 m^2 Spectro telescope : aperture 0.75x0.70 m^2 ASM : Astrometric sky mapper in the Astro telescopes A1BBP : Broad-band photometer in the Astro-1 telescope A2BBP : Broad-band photometer in the Astro-2 telescope AiBBPn : The n-th CCD in the BBP of the Astro-i telescope SSM : Spectrometric sky mapper in the Spectro telescope MBP : Medium-band photometer in the Spectro telescope sample : The value read from the CCD, sometimes binned from several pixels. The word "sample" also designates the area on the sky covered by these pixels. Basic and transmitted samples are defined in Table 1 patch : the area on the sky covered by the samples for one star The present report discusses the possible multi-colour photometry of the sky surface with the Astro and Spectro telescopes of GAIA. This photometry makes use of the data in the star patches for stars, thus no extra telemetry is required, only a proper data reduction on the ground. The photometric data analysis is described in GSR-Sect.9.4.2. The ordinary photometry of faint stars will give good results for at least 500 million spots on the sky. The GAIA photometric system in BBP+MBP is not finally decided, but it will contain about 5+10 bands. The bands shown in Table 2 are sufficiently representative, and were used since the count rates were readily available. The G band is very wide as obtained without any filter. The detection and measurement of sky surface brightness in the Astro sky mappers were discussed in ref.1 as summarized in the following section. Use of the Spectro telescope was not discussed in ref.1. The ref.1 could still be useful to consult in matters of detection of excess surface brightness, ie. relative to a global background. This is not required in the present discussion which is only concerned with utilization of the data routinely collected for the stars. 2. The report ref.1 = SAG_CUO_39 ---------------------------------- The detection and measurement of sky surface brightness in the two main GAIA telescope sky mappers was discussed in ref.1, almost three years ago. A new issue no.2 is now available where the notation has been updated for better readability. The assumptions on noise and light fluxes are still valid. The sensitivity of detection of excess surface brightness is discussed. The sensitivity to the parameters readnoise, global background and the uncertainty of the global background is illustrated in tables. It appears, for instance, that all normal galaxies brighter than about V=18 mag may be detected and that four-colour Sloan photometry at a resolution of 0.2 arcsec may be obtained for their central parts, provided the data can be accomodated in the available telemetry. Astrophysicists were invited to discuss the scientific case of the proposed nebula detection and 12-arcsec mapping with GAIA. The nebula or galaxy detection has been pursued, but the 12-arcsec mapping was not recommended further. 3. Measurements and precision ------------------------------- Some characteristics of the various detection chains are given in Table 1. The last line gives the typical sky areas. The sky areas per patch obtained from the Spectro telescope are much larger than from Astro, and the total noise per sample is much smaller. This, together with longer integration times, leads to a much better precision for surface photometry with the Spectro telescope, as will be seen in the Tables 2a and 2b. For the MBP in the Spectro telescope the patch covers an area of 7.0x2.0 arcsec^2, and the mission average is therefore obtained over a circle with diameter 7 arcsec, centred on a star. The precision per spot on the sky given in the Tables 2a and 2b is calculated from formulae derived in ref.2. Let b be the counts per sample from the sky during the integration time, and r be the total noise from the reading of a basic sample. The standard error of the sample is then sg_b1 = sqrt(b+r^2) with b calculated as b = S A t / n_b where S is the count rate per sample, A is the sky area of the patch, t is the integration time per sample, and n_b is the number of samples per patch containing background. The standard error per sample of the mean value for the mission is sg_b = sg_b1 / sqrt(n_b * n_p) where n_p is the number of patches collected during the mission. Usually one patch per scan of the star is collected, but sometimes (for P and u) 2 or 3 patches are collected (see the last line of Table 2). The signal-to-noise value is then R = b / sg_b and the standard error in magnitudes is sg_bm = 1.086 / R. These errors, multiplied with a margin factor 1.2, are given in Tables 2a and 2b. It appears from the last line that there are about 100 scans per star per band during the 5 year mission time. The standard error per scan is therefore about 10 times the values in Table 2. This must be taken into account when a variable sky background is encountered, especially in the zodiacal light. 4. Conclusion --------------- The precision per spot for the mission average values are given. At a surface brightness of e.g. uV=21.0 mag/arcsec^2 a precision of 0.001 mag in G and about 0.01 mag is obtained in all medium width photometric bands for spots of 7 arcsec diameter. Inside each spot a resolution about 0.5 arcsec will be obtained, with correspondingly lower photometric precision. Astrophysicists are invited to look at this performance and comment on the scientific value. 5. The comments by K. Mattila ------------------------------- A first version of this report was sent to K. Mattila and his comments to the astrophysical aspects are included below, after the Tables. Mattila's two concluding question should be answered with yes. 1. A 100 spots from the instruments can be averaged to obtain an accuracy of 0.5 S10 with the MBP. This requires a careful calibration of GAIA photometry and this accuracy goal should be kept in mind when the photometric processing is designed and developed. 2. The instrument straylight from stars plus diffuse sky components should be less than 0.5 to 1 S10. This is a requirement for the baffling of the telescopes. This is achieved with a conical baffle extending from the focal plane of the Astro telescopes, discussed in Section 3.2.5 in the GSR and shown in Figure 4.2/6 in the GAIA CTS Study Final Report. The Spectro telescope has no such baffle at present (see Figure 5.2/1) but it seems possible to introduce it. This baffle seems to be required even for the sake of the MBP and RVS. This should be studied soon. REFERENCES: GSR: GAIA Study Report 2000, ESA-SCI(2000)4 - references are given simply as eg. G-Table 3.16, or G-Fig.3.7 ref.1: E. Hoeg, C. Fabricius, J. Knude, V.V. Makarov 1998, SAG_CUO_39 of 25 June 1998, GAIA surveys of surface brightness. Issue 2 of 12 Jan 2001: http://www.astro.ku.dk/~erik/gaia/39.2.surface.txt ref.2: Hoeg et al. 1999, Baltic Astronomy 8, 25 %----------------------------------------------------------------------- Table 1. Some characteristics of the various detection chains. Partly from G-Table 3.16, but corrected for A2BBP, SSM1, MBP (see Table 3). Note that A1BBP and A2BBP means the BBPs in respectively Astro 1 and Astro 2. Basic samples are samples read with or without binning from the CCD. The size of transmitted samples and patches are taken from G-Fig.3.7. Transmitted samples are basic samples or are coadded from these. The typical sky area is given for faint stars. Stars disturbing the sky area are detected down to V=23 mag by analysis on the ground of the data from AF17 and SSM1, and this number of stars is quite low. There are eg. only 0.15 stars with V<25.5 mag in a typical 3 sq.arcsec area at galactic latitude b=5 deg, according to ref.2, Table 1. Parameter ASM1 AF17 A1BBP A2BBP SSM1 MBP The basic samples and patches: Pixel size (um) 9x27 9x27 9x27 9x27 10x10 10x10 Pixel size (arcsec) ...... 0.037 x 0.111 .... 0.5 x 0.5 Sample bins (pixels) 2x2 2x12 1x8 6x8 1x1 1x4 Patch size (basic samples) 1x1 30x1 16x1 10x1 1x1 14x1 Total noise assumed (e-) 10.9 6.5 6.2 5.0 5.3 3.0 The transmitted samples and patches: Sample (basic samples) - 1 1 1 1x3 1 Patch size (transm. samples) - 30x1 10x1 10x1 14x3 8x1 Patch size (arcsec) - 2.2x1.3 2.2x0.9 7.0x2.0 0.6x0.9 7.0x4.5 Patch area (arcsec^2) 2.9 0.54 2.0 31.5 14.0 Sky area, typical (basic samples) 26 10 8 30 10 Sky area, typical (arcsec^2) 2.6 0.3 1.6 30 10 %----------------------------------------------------------------------- %----------------------------------------------------------------------- Table 2a. Diffuse surface brightness measured with 6 broad bands and Stromvil u. Standard errors for the mission mean value per spot on the sky. Baseline assumptions are given at the end of the table. Detector AF17 A1BBP A2BBP A2BBP A2BBP SSM1 MBP Band G U U V Ic G u Centre [nm] 700 366 366 550 800 700 345 FWHM [nm] 800 50 50 100 140 800 40 Ssurf uV uI m/a^2 m/a^2 mag mag mag mag mag mag mag 17.0 16.0 0.001 0.024 0.006 0.002 0.003 0.000 0.001 18.0 17.0 0.001 0.055 0.011 0.004 0.004 0.000 0.002 19.0 18.0 0.002 0.133 0.022 0.006 0.008 0.000 0.003 20.0 19.0 0.003 0.329 0.049 0.012 0.014 0.001 0.006 21.0 20.0 0.006 0.819 0.116 0.024 0.030 0.001 0.010 22.0 21.0 0.012 2.052 0.283 0.052 0.067 0.002 0.022 23.0 22.0 0.028 5.148 0.701 0.123 0.160 0.003 0.049 24.0 23.0 0.067 12.926 1.753 0.301 0.395 0.006 0.117 %----------------------------------------------------------------------- Sky n_b= 26 10 8 8 8 30 10 [samples/patch] Sky A= 2.6 0.3 1.6 1.6 1.6 30 10 [arcsec^2] uI=15.0:S= 38970 1418 1418 8391 6369 17193 332 [e-/s/arcsec^2] t= 0.9 0.9 0.9 0.9 0.9 3.0 3.0 [s/sample] uI=21.5:b= 2.21 0.02 0.16 0.95 0.72 32.54 0.63 [e-/sample] RON: r= 6.5 6.2 5.0 5.0 5.0 5.3 3.0 [e-/sample] n_p= 133 67 67 67 67 100 100*3 [patches] The count rates S are derived from a file from LL of 6 Feb 1999 which uses the QE of CCD#1B, see G-Fig.3.24. The background is assumed to have the spectrum G2V. Standard errors include a margin factor of 1.2. %----------------------------------------------------------------------- %----------------------------------------------------------------------- Table 2b. Diffuse surface brightness measured with seven medium bands. Standard errors for the mission mean value per spot on the sky. Detector MBP MBP MBP MBP MBP MBP MBP Band P v b Z y S 800 Centre [nm] 380 405 460 515 545 655 800 FWHM [nm] 26 19 18 21 23 20 40 Ssurf uV uI m/a^2 m/a^2 mag mag mag mag mag mag mag 17.0 16.0 0.002 0.002 0.002 0.002 0.001 0.002 0.002 18.0 17.0 0.002 0.003 0.003 0.003 0.002 0.003 0.002 19.0 18.0 0.004 0.005 0.004 0.004 0.004 0.004 0.004 20.0 19.0 0.007 0.009 0.007 0.007 0.006 0.007 0.007 21.0 20.0 0.013 0.016 0.013 0.012 0.011 0.013 0.012 22.0 21.0 0.027 0.033 0.027 0.024 0.022 0.025 0.023 23.0 22.0 0.062 0.073 0.058 0.051 0.046 0.055 0.050 24.0 23.0 0.149 0.174 0.137 0.119 0.107 0.129 0.116 %----------------------------------------------------------------------- Sky n_b= 10 10 10 10 10 10 10 [samples/patch] Sky A= 10.0 10.0 10.0 10.0 10.0 10.0 10.0 [arcsec^2] uI=15.0:S= 321 392 502 581 649 535 596 [e-/s/arcsec^2] t= 3.0 3.0 3.0 3.0 3.0 3.0 3.0 [s/sample] uI=21.5:b= 0.61 0.74 0.95 1.10 1.23 1.01 1.13 [e-/sample] RON: r= 3.0 3.0 3.0 3.0 3.0 3.0 3.0 [e-/sample] n_p= 100*2 100 100 100 100 100 100 [patches] %----------------------------------------------------------------------- %----------------------------------------------------------------------- Table 3. Section 3.7.4, Table 3.16 - Modifications >From MP on 9 Jan. 2001: 3.7.4. Table 3.16. Modifications: * total chips AF01-16: from 136 to 160 * total chips AF17: from 34 to 10 * outputs/chip BBP2: from 8 to 1 * Video chains: AF01-16: from 180 to 160 * Video chains: AF7: from 24 to 10 Corrected by EH below Further modifications by EH on 10 Jan.: * Video chains: AF17: from 24 to 10 * Pixel size: SSM0/1 from * to 10x10 * TDI period: MBP from * to 4.1 * Sample bins: SSM0/1 from * to 1x1 * Sample bins: MBP from 1x1 to 1x4 * Patch size: BBP2 from 16x1 to 10x1 * Patch size: SSM0/1 from * to 1x1 * Patch size: MBP from 1x1 to 14x1 - Proposal: AiBBP means the BBP in Astro-i, thus : * Parameter : A1BBP replaces BBP1 * Parameter : A2BBP replaces BBP2 - Question: Is the total noise of 5.3 e- for SSM0/1 correct ? All the asterisks in the column indicate uncertain assumptions. - After the MBP a CCD SSM2 should be added for NEO detection according to CUO-77. This could be included in the table simply by the modification of Parameter: from SSM0/1 to SSM0/1/2. %----------------------------------------------------------------------- ********************************************************************** Date: Fri, 23 Feb 2001 15:28:53 +0200 (EET) From: Kalevi Mattila To: E Hoeg Copenhagen Subject: Re: Surface brightness Dear Erik, Here are some comments on the GAIA diffuse sky background photometry. I am very sorry that I am so late with my comments. I had a lengthy problem with a cold. Please consider the two questions of my "Conclusions". If the answer to both of them is "yes" it might be meaningful to continue with this kind of discussion, and it might then be meaningful for me to attend your GAIA workshop in Copenhagen in March. Best regards, Kalevi =================================================================== Kalevi Mattila Observatory Phone +358-9-19122947 P.O. Box 14 Fax +358-9-19122952 Taehtitorninmaki University of Helsinki FIN-00014 Helsinki, Finland e-mail mattila@cc.helsinki.fi =================================================================== ---------------------------------------------------------------------------- COMMENTS ON "MULTICOLOUR PHOTOMETRY OF THE DIFFUSE SKY BACKGROUND WITH GAIA" --------------------------------------------------------------------------- (SAG-CUO-082 by Erik Hoeg 18 Jan. 2001) 1. Scientific interest in diffuse sky background photometry from space ---------------------------------------------------------------------- There are four main components of interest. I give for three of them at 380 nm also an estimated range of values in units of stars of 10th magnitude per square degree (=S10). 1.0 S10 = 27.8 mag/sq.arcsec. (for the unit, see e.g. Leinert et al. A&AS 127, 1-99, Jan. 1998). Zodiacal light (ZL) 30 - 100 S10 Diffuse Galactic Light (DGL) (= scattered light from interstellar dust) 1 - 50 S10 Line emission (H_alpha) from ionised galactic gas Extragalactic Background Light (EBL) (not yet detected!) 0.5 - 2 S10 Zodiacal Light: I think there are no major open scientific issues about the ZL itself which would make an extensive multifilter optical photometry justified. (This is my personal view and may be a bit pessimistic in the sense that the multifilter aspect provided by GAIA would still be unique and would provide an accurate measure of the colour variation of the ZL over the sky). However, ZL is the main foreground component for the diffuse sky background as seen from above the Earth's atmosphere. Therefore an accurate knowledge of ZL distribution over the sky and its variation with time are needed when studying the other components. Diffuse Galactic Light I, low galactic latitudes: About 1/3 of the (total) integrated galactic surface brightness at low latitudes is due to scattered light from interstellar dust. It is commonly called the Diffuse Galactic LIGHT (DGL). The DGL has previously been measured over limited regions of the sky, but already in several filters 350 - 700 nm. GAIA with its complete sky coverage and many filter bands could contribute substantially to this field. The essential scientific result expected is the albedo and and a rough shape of the dust scattering function. Diffuse Galactic Light II, high latitude cirrus: The Infrared Cirrus as mapped by IRAS over the whole sky has a counterpart in the optical wave band. In the optical the radiation is due to starlight scattered by interstellar dust. This Optical Cirrus has so far been measured only in limited areas. Scientifically one would expect very useful results on dust properties and their variations from cloud to cloud when the optical and IR cirrus measurements are compared. Line emission (H_alpha) from ionised gas: This can be accurately measured also from ground. There are several extensive sky surveys both on the northern and the southern hemisphere going on. Therefore I am not convinced that GAIA could produce unique observational material here through its H_alpha filter photometry. (But I am less familiar with this field.) Extragalactic Background light: This is a very weak isotropic component of the diffuse sky background. So far there is no measured value for the EBL in the optical domain, only upper limits. One can estimate that the EBL is ca. 0.5 to 2 S10. This component is of great cosmological importance. Because of the (tentative?) detection of the far-IR EBL by COBE/DIRBE there is increased current interest also in the optical EBL. Being above the earth's atmosphere the main problem for EBL measurement is how to separate it from the two dominating foreground sky components, i.e. the ZL and the DGL (Cirrus). Thanks to its multi-wavelength capacity the GAIA Medium Band Photometer would be in a good position to perform the separation based on the different SEDs of ZL, DGL, and EBL. However, the GAIA photometric sensitivity may be a problem here. Another problem may be the stray radiation from the (open) telescope optics. I will discuss these problems briefly below. 2. Sensitivity of the GAIA medium band diffuse sky background photometry -------------------------------------------------------------------- >From the SAG-CUO-082 report I extract from Table 2b the following numbers for the diffuse surface brightness measurements in the 380 nm filter (BW = 26 nm): Ssurf Sigma Ssurf Sigma mag/sq.arcsec S10 S10 --------------------------------- 22.0 0.027 205 5.1 23.0 0.062 82 4.7 24.0 0.149 32 4.5 -------------------------------- The accuracies are (if I understood correctly) the mission-averaged values for some 100 scans per spot on the sky. The sky brightness (=ZL) ranges from 30 to 100 S10, i.e. 24 to 23 mag/sq.arcsec. Thus the two lowest lines in the table are to be used. As one can see the mission-averaged accuracy (one sigma?) is about 5 S10. When we compare this value with the ZL, DGL, and EBL surface brightnesses given at the beginning we must conclude that this accuracy is not sufficient for a meaningful measurement of any of the components. However, this accuracy is for a single spot of ca. 10 sq.arcsec size. There are some 500 million such spots measured over the sky. This means that the sky is covered with a grid of ca. 30 arcsec separation in each coordinate direction. This is much denser than needed for any of the diffuse sky components. IF IT IS CORRECT TO ASSUME that we can average 100 adjacent spots into one value and gain thereby a factor of 10 in the accuracy, then we would have an accuracy of 0.5 S10 and still a sufficiently dense grid over the sky with average spacing of 5 arcmin in each direction. This would make the GAIA medium band photometry sensitive enough to tackle the whole range of diffuse sky components, INCLUDING MOST IMPORTANTLY ALSO THE EBL. 3. The stray radiation problem ------------------------------ For the diffuse surface brightness (all sky) photometry it is very important that the stray radiation is suppressed to a minimum. Here I do not mean the stray radiation from the Sun which I believe will be low enough to be negligible for GAIA. However, there will be a stray radiation component coming from all the bright (and also faint) stars as well as from the off-axis sky surface brightness components. BECAUSE OF THE OPEN OPTICAL STRUCTURE OF THE GAIA TELESCOPES I SUSPECT THAT THIS STRAY RADIATION WILL BE A LARGE FACTOR STRONGLY INFLUENCING THE SKY SURFACE PHOTOMETRY. Unlike for the observations of small extended sources, such as (most) galaxies, the stray radiation cannot be subtracted by measuring an adjacent OFF position. On the contrary, one would be forced to measure the stray radiation "beam profile" and convolve with it a large fraction of the visible sky in the on-source direction. Such a predure would be very laborous, and one would not expect that it is accurate enough if the stray radiation level is substantial, i.e. more than about 0.5 to 1 S10. 4. Conclusion ------------- My preliminary conclusion is that GAIA diffuse sky medium band photometry can contribute interesting scientific results, if the following two statements are true: 1. One can average together some 100 spots to gain an accuracy of 0.5 S10 2. The instrumental stray radiation contribution from stars plus diffuse sky components is less than 0.5 to 1 S10. EH on 26 Feb.: These two questions are answered in the affirmative above, in the main CUO-82 report. The concerns rightly raised by Mattila, e.g. on straylight, can be solved by proper precautions. --------------------- end ------------------------------------------