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the motivation and concept of measuring @xmath1 and @xmath2 at lhc with the method based on the optical theorem have been discussed at length elsewhere  @xcite .
the central equations expressing @xmath1 and @xmath2 in terms of the measured inelastic and elastic rates , @xmath4 and @xmath5 , and the extrapolation of the differential elastic rate to the optical point , @xmath6 , are @xmath7 where @xmath8  @xcite will as a first step be taken from theory . at a later stage , a measurement at @xmath3 km can be attempted ( see last section of this article ) .
this article aims at giving an update on the expected performance , taking into account recent studies for the reduced centre - of - mass energy of 7tev and the running experience in 2010 .
the roman pot ( rp ) stations at @xmath9220 m from ip5 and the forward gem telescope t2 had already been fully operational in 2010 , whereas the second half of the rp spectrometer 
i.e. the @xmath9147 m stations  as well as the csc telescope t1 were installed during the technical stop in winter 2010/2011 , thus completing totem s detector apparatus ( figure  [ fig : apparatus ] ) in time for the running season 2011 .
the inelastic trigger acceptance predicted by simulation for the totem detector configuration is given in table  [ tab : triggerlosses ] .
.__trigger losses at @xmath10tev , requiring 3 tracks pointing to the ip .
the cross - sections given are the central values of rather wide ranges of predictions  @xcite . _ _ [ cols="<,^,^ " , ]
the @xmath11 m optics , studied in detail for @xmath12tev  @xcite , extends the measurable @xmath13-range to values of the order @xmath14 and thus enables an elastic extrapolation with an uncertainty at the 0.2% level .
consequently , the precision in @xmath1 improves to @xmath15% .    scaling the optics properties to @xmath10tev shows an even further enhancement of the acceptance reach to low @xmath13 ( figure  [ fig : coulomb ] , bottom block of curves ) .
inserting the rps to 8 or 6 @xmath16 would give access to the elastic scattering zone dominated by the coulomb interaction and thus permit a measurement of @xmath17 , avoiding any need for theoretical input to the @xmath1 determination via the optical theorem . at @xmath12tev
this opportunity will not be offered .
however , the current version of the @xmath18 m optics for totem is only compatible with operation at @xmath19tev to 14tev  @xcite . at lower energy
the two main limiting parameters are the minimum strength allowed in the insertion quadrupoles and the aperture
. one way to avoid these limitations would be to loosen the constraints on the phase advance by abandoning the condition to have `` parallel - to - point focussing '' in both transverse projections .
alternative optics at very high @xmath20 , compatible with operations at @xmath21tev are under study .
9 totem collaboration : technical design report , cern - lhcc-2004 - 002 ; addendum cern - lhcc-2004 - 020 . g. anelli et al .
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physics and beam monitoring with forward shower counters ( fsc ) in cms , cms - note-2010 - 015 .
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. + m. m. block , e. m. gregores , f. halzen and g. pancheri : photon proton and photon photon scattering from nucleon nucleon forward amplitudes , phys .
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totem collaboration : early totem running with the 90 m optics , cern - lhcc-2007 - 013 / g-130 , 9 march 2007 .
h. burkhardt : high - beta optics , these proceedings .
+ h. burkhardt and s. white : high - beta optics for the lhc , lhc project note 431 , may 2010 .
with the installation of the t1 telescope and the roman pot stations at 147 m from ip5 , the detector apparatus of the totem experiment has been completed during the technical stop in winter 2010/2011 . 
 after the commissioning of the dedicated beam optics with @xmath0 m , a first measurement of the total pp cross - section @xmath1 and  simultaneously  the luminosity @xmath2 will be possible in the upcoming running season 2011 . 
 the precision envisaged is 3% and 4% for @xmath1 and @xmath2 , respectively . an ultimate beam optics configuration with @xmath3 km 
 will later reduce the uncertainty to the 1% level .