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globular clusters ( gcs ) are among the oldest known stellar systems . they typically have ages similar to those of their host galaxies , making them ideal probes into galaxy formation and evolution . the properties of gcs vary significantly . however , individual clusters contain populations of stars with similar ages and metallicities . this makes them unique locations for studying stellar evolution .    the milky way s gcs still represent the best studied gc system . while the study of these clusters has led to many advances , the milky way contains relatively few gcs ( @xmath0150 gcs : * ? ? ? * ) , many of which have high foreground extinction , making them hard to study . by determining the properties of extragalactic gcs , we are able to study a more diverse population and ensure our current conclusions are not biased by the milky way s clusters being atypical .    for extragalactic gcs , it is very difficult to resolve individual stars in the clusters . however , it is possible to estimate many properties of a gc from its integrated light . for example : the masses of gcs can be estimated by assuming a mass to light ratio ; combined optical and near infrared colours of gcs can be used to ( at least partially ) break the age and metallicity degeneracy and estimate these parameters ( e.g. * ? ? ? * ; * ? ? ? * ; * ? ? ? * ) ; and their structural parameters can be estimated by fitting their density profiles ( e.g. * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? the colours of gcs and gc candidates are also very useful in selecting genuine gcs from stellar asterisms and background galaxies . good multi - wavelength photometry of gcs is therefore highly desireable . the proximity of m31 , and its relatively large gc population compared with the milky way ( @xmath0400@xmath1 * ? ? ? * ) , makes it the ideal location to study extragalactic globular clusters . its clusters have been the focus of many studies dating back to the early work of @xcite and @xcite . however , attempts to study its clusters have faced several challenges . photometry of these clusters is complicated by many of them being projected against the bright and non - uniform structure of m31 itself . the galaxy s proximity also results in the gc system extending over a wide region of the sky , with clusters recently found beyond 4@xmath2 from the centre of the galaxy @xcite . this means that surveys with large fields of view are required in order to study the gc system . it is also difficult to confirm gcs in m31 based on spectroscopy alone . this is because the velocity distribution of its gc system overlaps that of milky way halo stars .    over the past decades there have been several large catalogues of m31 s gcs including those of : @xcite . in addition to these catalogues many new clusters and candidates have been proposed ( e.g. * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? these studies have made considerable progress in removing contamination from the cluster catalogues due to either background galaxies ( e.g. * ? ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ) or stars and asterisms both in the milky way and m31 itself ( e.g. * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? however , despite this work , it is likely that there remains significant contamination in the current catalogues of m31 clusters , especially at the faint end of the gc luminosity function . these studies have also resulted in a large number of unconfirmed candidate clusters ( currently over 1000 : * ? ? ? * ) whose true nature remains uncertain . it has been known for many years that some of the proposed gcs in m31 have very blue colours . recent work has identified that a large number of the clusters in the current catalogues are young clusters ( e.g. * ? ? ? * ; * ? ? ? * ; * ? ? ? * ) . a comprehensive catalogue of young clusters in m31 has recently been published from the spectroscopic survey of @xcite . compared with the young open clusters in the milky way , these clusters have relatively high masses ( @xmath3 ) , akin to the young clusters observed in the large magellanic cloud . a recent _ hubble space telescope ( hst ) _ study of 23 of these young clusters suggested that on average they are larger and more concentrated than typical old clusters @xcite . most of the clusters studied by @xcite are found to have dissolution timescales of less than a few gyr and are therefore not expected to evolve into typical old globular clusters . whether these clusters are massive open clusters , young globular clusters or a mix of both , it is clear that they represent a different population to the classical old gcs also observed in m31 ( which are the focus of this study ) . we therefore distinguish between the two populations in our classifications and conclusions . while these previous studies have provided a wealth of information on the m31 gc system they have also resulted in a rather heterogeneous sample . for example , the excellent and commonly used revised bologna catalogue ( hereafter rbc ) of m31 gcs by @xcite includes photometry from many different authors using different telescopes and in some cases different ( homogenised ) filters . this has been previously noted by @xcite ( hereafter c09 ) who recently published new v - band photometry for many rbc sources which were located in the local group galaxy survey images of m31 ( lggs : * ? ? ? while this work provides excellent deep v - band photometry , the survey does not cover the outer clusters and candidates and does not provide colour information . the most complete set of optical colours , derived in a consistent manner , are still that of the barmby catalogue @xcite . this work presented self - consistent ubvri colours for many of their clusters . however , it is incomplete in some of these bands and only provides new photometry 285 clusters . for these reasons we chose to produce new , self - consistent , optical photometry for the proposed gcs and gc candidates in the rbc using images from the sloan digital sky survey ( sdss ) . the excellent calibration and large field of view of this survey is ideal for studying such an extended system . details of this photometry are presented in section 2.2 . the study of m31 s gcs in the near infrared ( nir ) is very useful both for confirming genuine gcs and for estimating their ages and metallicities ( e.g. * ? ? ? * ; * ? ? ? * ; * ? ? ? the first major survey of m31 s gcs in the nir was by @xcite who used pointed observations of individual clusters to obtain k - band photometry of 228 of their clusters . more recently @xcite obtained nir photometry in the j , h and k - bands of 279 of their confirmed gcs from the 2 micron all sky survey ( 2mass ) . the spatial coverage of 2mass makes it ideal for such a project . however , the survey is relatively shallow and has relatively poor spatial resolution . we have obtained new deep k - band photometry using the wide field camera on the uk infrared telescope to determine the k - band magnitude of m31 s gcs across the entire gc luminosity function . some results of this survey are already published in @xcite . in addition to providing the first k - band photometry for 126 gcs marked as confirmed in the rbc , the excellent spatial resolution of these images is very useful for removing stellar sources from genuine clusters , and for investigating the density profiles of the clusters . details of this new k - band photometry are presented in section 2.3 , while the classifications of the proposed clusters and candidates are considered in section 3 . the proximity of m31 makes it the ideal location for studying the structure of extragalactic gcs . determination of the size and density of gcs is very useful in investigating : stellar evolution ; galaxy formation and evolution ; constraining n - body simulations ; and investigating exotic objects in gcs like x - ray binaries , blue stragglers and horizontal branch / extreme horizontal branch stars . the structural parameters for some of m31 s gcs have been measured using _ hubble space telescope ( hst ) _ : faint object camera ( foc ) images of 13 clusters @xcite ; wide field planetary camera ( wfpc2 ) images of 50 clusters @xcite ; and advanced camera for surveys ( acs ) and space telescope imaging spectrograph ( stis ) images of 15 and 19 clusters respectively @xcite . in @xcite we presented the results of fitting psf convolved king models @xcite to the ground based wfcam images used here in order to estimate the structural parameters for 239 clusters . in section 4 we discuss the structure of m31 s gcs . in the following analysis we consider all the gcs and gc candidates listed the rbc ( their class 1@xmath48 and 2 objects respectively ) . based on the original catalogue of @xcite , this catalogue has been regularly updated to include the results from most new studies . this version of the catalogue ( v3.5 ) includes the newly discovered gcs in the outer regions on m31 @xcite and the new gcs and candidates from @xcite ( hereafter k07 : their class a and b@xmath4c objects respectively ) . we also consider the catalogue of c09 which includes some additional clusters and gives updated locations and classifications for many of the objects in the rbc based on images from the lggs or digital sky survey and/or hectospec spectroscopy . this combined catalogue is used to identify the known gcs and candidates in the following analysis .        to obtain self - consistent optical photometry of m31 s clusters and candidates we extracted images of m31 from the sdss archive . since m31 is at a relatively low galactic latitude of -21@xmath2 , it is not included in the standard survey field . however drift scan images of m31 were obtained by the sdss 2.5 m telescope @xcite in 2002 as part of a special run during a period when the survey s primary field was not available @xcite . the runs used ( 3366 , 3367 , 6426 and 7210 ) provide images in the five sdss bandpasses ( _ ugriz _ : * ? ? ? each of the observations takes images in these bands simultaneously meaning that they are taken under the same atmospheric conditions . the seeing for different observations varied significantly between 1.1 - 2.1@xmath5 in _ g _ ( meaning that faint gcs could appear as point sources in some of these images ) . the 3@xmath6 detection limits of these images were verified to be similar to the standard survey ( @xmath7 ) . these data were found to cover and detect 92@xmath8 ( _ gri _ ) , 90@xmath8 ( _ z _ ) and 73@xmath8 ( _ u _ ) of the 1558 clusters and candidates in the current rbc . two of these gcs were saturated in the _ r _ and _ i _ bands and one was saturated in the _ g_-band . we do not provide new photometry for these clusters but good photometry is already available for these very bright clusters from previous studies . we extracted all images covering the locations of confirmed and candidate clusters from the sdss supplemental archive . figure [ fig : images ] shows the coverage of these data and demonstrates that most known clusters and candidates ( red circles ) are covered . these images have been processed through the standard sdss pipeline @xcite which both reduces the raw images and produces a catalogue of sources in each image . since the sdss extraction and photometry routines are not designed to work in crowded fields ( like m31 ) , the default catalogues produced by the pipeline can not be used for photometry of the clusters . instead we performed photometry on the images as described in the next section . the photometric zero points for these images were calculated using the calibration coefficients produced by the pipeline . these calibrations place the magnitudes on the ab photometric system [ @xcite ; the _ u_-band zeropoint has previously been found to be slightly offset from the ab system by @xmath9  mag @xcite , this correction is _ not _ applied to our photometry ] . this calibration is known to give magnitudes accurate to @xmath00.01  mag . , height=317 ]    catalogues of all sources in each of the _ ugriz _ images were produced using the program sextractor @xcite . this detected and located every source in each filter , performed initial aperture photometry , and gave an estimate of the stellarity of each source based on the psf of its host image . sources were identified using a minimum detection area ( detect@xmath10minarea ) of 3  pixels and a detection ( detect@xmath10thresh ) and analysis threshold ( analysis@xmath10thresh ) of 1.5@xmath6 . this threshold was chosen to ensure that the majority of the clusters profiles were included in the analysis and to help to separate genuinely extended objects from merged sources . this source catalogue was matched to our combined catalogue of known m31 gcs and candidates ( described in section 2.1 ) based on astrometry . we identified all objects within 3@xmath5 of the locations quoted in the rbc and ( separately ) to their locations in c09 . some genuine clusters in the sdss images may not appear extended due to the poor angular resolution of some of the images . also , we wish to provide photometry for potentially misclassified stars in addition to the extended clusters . the m31 gc catalogue was therefore matched to sources with stellar profiles in addition to those with extended profiles . in the few cases where multiple sources were located within 3@xmath5 of the quoted location , priority was given first to sources flagged as extended and then to the closest source to the quoted location . figure [ fig : pos_error ] shows the difference between our positions and the positions quoted in rbc ( solid ) and c09 ( open ) . we find excellent agreement between our locations and those of c09 but find that the difference in the positions of many sources in the rbc are greater than 1@xmath5 . the errors in the positions of some sources in the rbc were noted and discussed by c09 . we note the strong agreement between our locations and those of c09 and use their locations to identify gcs and candidates . photometry of all clusters and candidates was obtained using sextractor s simple aperture photometry . we also considered using the iraf : apphot routines to perform the aperture photometry but sextractor was found to deal better with contamination from neighbouring sources . this is a significant problem when using aperture photometry to obtain magnitudes of extended sources in a crowded region like m31 . to minimise the effects of neighbouring sources within the gc aperture , sextractor masks all other sources detected in the aperture and replaces them with pixels from symmetrically opposite the source .    for background estimation we considered the use of both local and global solutions . to produce a global estimate of the background sextractor produces a smoothed background map for each image . we chose to create this with a back@xmath10filtersize of 3 and a back@xmath10size of 64  pixels . by examination of the background maps produced by sextractor , this method was found to give a good estimation of genuine background variation ( due mainly to structure in m31 itself ) without subtracting flux from the sources of interest . this was compared with the photometry produced using local backgrounds ( calculated around the isophotal limits of the sources ) . in most cases good agreement was found between the two methods . however the local background estimates were found to deal better with the most strongly varying background regions ( near the center of the galaxy and its spiral arms ) . for this reason local background estimation was used for the final photometry .    to determine the total luminosity of each cluster , we produced curves of growth from _ g_-band photometry obtained through apertures with a radii in the range @xmath11 with 0.6@xmath5 increments . these were used to determine the aperture size required to enclose the total cluster light . the best aperture was determined independently for each object . this method ensures that we measure the total cluster luminosity correctly for the largest clusters . while the use of smaller apertures for smaller clusters maximises the signal to noise and minimises the contamination from nearby sources . the aperture size used to determine the total magnitude of each cluster is quoted in table 1 . the average aperture radius used was @xmath05.8@xmath5 , with 87@xmath8 of the apertures @xmath128.2@xmath5 . the _ ugriz _ colours of the clusters were measured through 4@xmath5 apertures . we also measured the colours using the aperture determined for the total _ g_-band magnitude . this confirmed that there were no significant aperture effects due to the use of smaller apertures . for the final colours we chose to use the smaller aperture size in order to maximise the signal to noise and minimise the contamination from nearby sources .    the statistical errors across the gc luminosity function are in most cases less than 0.05  mag in _ gri_. in general the _ u _ and _ z _ bands have slightly larger errors as they have slightly lower signal to noise . however , there are additional systematic errors which need to be considered . firstly , the errors on the zero point calculation are estimated at 0.01  mag . this error dominates over the statistical errors for many of the bright clusters . the other significant source of error for some of the clusters is due to contamination from nearby sources and the error on the background estimation . in the bluer wavelengths there is significant small scale structure in m31 . for clusters projected against the densest regions of the galaxy , this makes background subtraction difficult as it can vary on scales smaller than the cluster of interest . the issue of background estimation is found to be particularly significant for the _ g_-band photometry . in this filter there is significant small scale structure across m31 ( this structure is less significant in the shallower _ u_-band images where the statistical errors are larger ) . in order to estimate the error on our background estimation we repeated our photometry with apertures 1.8@xmath5 larger than the aperture used for the total magnitudes . with perfect background estimation , the determined flux should be the same through both apertures ( within the photometric errors ) . the difference in luminosity of the cluster through each aperture can therefore be used to give an estimation of the error on the background estimation . in most cases this estimated error is quite small , with a median value @xmath00.015  mag , but for a few clusters it can reach @xmath00.1  mag . this additional error is combined with the calibration and statistical error and included in table 1 as @xmath13 . the error on the _ ugriz _ colours should be less affected by these effects . this is because they are often taken through smaller apertures , also we expect that possible errors on the background level in each filter should , at least partially , cancel . for this reason the errors on the colours quoted in table 1 are only the statistical errors .          [ cols="<,<,^,>,>,>,>,<",options="header " , ]      + @xmath14 many of these rbc _ class 1 _ clusters are flagged separately in the rbc as potentially young clusters .      these colours clearly identify the population of very blue clusters that have been noted by previous studies . it can be seen that our colours are in excellent agreement with the spectroscopic classifications of c09 . we also find good agreement with the confirmed clusters in the rbc which are flagged as potential young clusters ( flag yy@xmath151,2 or 3 in the rbc ) . this flag is based on the work of @xcite . most of the previously identified young clusters are much bluer in _ g - r _ than any gc in the milky way . using a similar method to @xcite , we define all objects with _ _ g - r__@xmath160.3 to be young clusters .    some of the proposed young clusters have colours which are consistent with being old clusters . however , these objects are also consistent with being young clusters with reddened colours . these clusters are also found in high density regions of m31 and look similar to the other young clusters we have observed . we therefore choose to keep the previous ( spectroscopic ) classification for these clusters and suspect that their colours may be reddened . it can also be seen from figure [ fig : colours ] that two clusters classified as old by c09 ( b386 and phf7 - 1 ) have very blue colours . we reclassify these two objects as young clusters . figure [ fig : colours_final ] shows the colours of all confirmed and candidate clusters following the removal of all stellar objects based on their stellarity flag or visual examination of the cluster images . we have also removed those objects identified in the previous section as being as young clusters . it can be seen that , having removed these objects , the colours of the confirmed clusters are now consistent with the colours of the old gc system of the milky way . the clusters extend to much redder colours , but this is consistent with the expected reddening due to extinction from m31 . the grey points in the bottom panels of figure [ fig : colours_final ] show the colours of the remaining candidate clusters after the removal of non - extended objects . comparison with the confirmed old clusters shows that many of the candidates have colours consistent with being old clusters . these clusters are flagged as old candidates in table 1 and should be considered the strongest candidate clusters . we also identify candidates with very blue colours , consistent with the other young clusters identified . these are flagged as young candidate clusters in table 1 . it can be seen that , despite removing objects identified as stars , the colours of many of the candidate clusters are inconsistent with being either old or young clusters . as we are uncertain of the classification of these objects , we retain their classification as candidates . however , it is likely that many of these candidates are either background galaxies or unresolved asterisms . recent studies of the halo of m31 have identified a population of very extended clusters @xcite . these clusters have half light radii much greater than the majority of clusters in m31 . for a description of these clusters we refer the reader to @xcite . seven of these clusters are located in our sdss images and can be identified in table 1 from their names which have the prefix hec ( ` halo extended cluster ' ) . our colours of these clusters were found to be less reliable than the other clusters studied . this is because they are resolved , due to the diffuse nature of the clusters , into multiple sources . the colours for these clusters were therefore re - measured through 12@xmath5 apertures using the iraf : apphot task phot . a smaller aperture of 8@xmath5 was used for hec11 due to a bright neighbouring star . this method gives reliable results for clusters in the halo of m31 where there is little contamination from neighbouring sources and the background is relatively smooth . none of these extended clusters are identified in the inner regions of m31 , although detecting such extended and faint objects in front of the m31 would be very difficult . these clusters are identified in figure [ fig : colours_final ] as open green points . it can be seen that the colours of these clusters are now consistent with the other old gcs in m31 . the errors on the colours of the hec clusters are larger than those of the other gcs . this is due to their diffuse nature and the use of large apertures , which increases the total sky background . our final classification is based on : the stellarity of the object ; its colours ; visual examination of the object in our 6 bands ; velocity information and classifications from previous studies . table 1 lists these classifications for all gcs and candidates . for comparison we also include the previous classifications from the rbc and c09 . for consistency we have tried to keep our classifications similar to those used in the rbc . if we have no reason to reclassify the sources , we keep the original classifications ( where available from c09 , which were found to agree best with our classifications , otherwise from the rbc ) . the classifications used are : +   + _ 1 : old globular cluster : _ extended and has colours consistent with the milky way s gcs . its velocity is confirmed from previous work ( k07 , c09 or the rbc ) to be consistent with being in the m31 gc system , or the object is confirmed from high resolution _ hst _ images . + _ 2 : candidate cluster : _ not confirmed , but previously proposed as being a cluster or candidate and is found here to be extended ( or have uncertain stellarity ) . candidate is sub - divided , depending on whether its colours are consistent with being an old cluster ( _ 21 _ ) , consistent with being a young cluster ( _ 23 _ ) or inconsistent with being a cluster ( _ 2 _ ) . + _ 3 : young cluster : _ has colours consistent with being young . if previously classified , may appear as a resolved asterism in k , but looks like a cluster in the sdss images . + _ 4 : background galaxy : _ previously classified from spectroscopy by c09 or identified from our visual examination . + _ 5 : hii region : _ from previous classification of c09 . + _ 6 : stellar source : _ object appears to be a single stellar source or a previously unresolved asterism of stellar sources . + the total number of sources of each class are shown in table [ tab : compare_class ] . for reference we include whether these objects were previously classified as : clusters or candidates in the rbc ( rbc 1 and rbc 2 respectively ) ; clusters or candidates by k07 ( k07 a and k07 b@xmath4c respectively ) ; old or young clusters by c09 ( c09 old and c09 young respectively ) . it can be seen that we have reclassified 10 previously confirmed clusters in the rbc as likely stellar sources . we also reclassify 6 of these objects as candidate clusters , as we are uncertain of they nature , or they lack spectroscopic confirmation . some of the candidate clusters in the rbc are confirmed to be old or young clusters . this is based on the new spectroscopic confirmations by c09 . we are also able to classify many of the candidate clusters in the rbc as stars . in most cases we find good agreement with the new classifications of c09 . their catalogue includes fewer objects because they do not provide classifications for the whole gc system . all objects classed as young clusters by c09 are retained in our classification . we have reclassified many of the confirmed clusters from k07 as likely stellar sources . we have also been able to classify nearly half of the cluster candidates from this catalogue as being stellar . we believe this is due to our improved spatial resolution compared with the images used for this previous catalogue . we identify and remove 8 objects from the catalogue of k07 which are within 2@xmath5 of another previously identified object in the rbc and we believe are now duplicated in the rbc . a further 5 objects from the catalogue of k07 appear to be associated with objects in the catalogue of c09 . the names for these objects in table 1 are the combination of their identifications in each catalogue . figure [ fig : gclf ] shows the gc luminosity function ( gclf ) for all confirmed gcs ( solid bars ) and young clusters ( open bars ) with k - band photometry . these clusters are not corrected for extinction . however , extinction is not very significant in the k - band where the maximum correction for the most extreme case of b037 is only 0.5  mag ( the width of the bins used ) . the peak of the gclf is found to be at k@xmath014.2  mag . the k - band luminosity of a cluster is a useful estimate of its mass . this is because , in addition to being less effected by extinction , the k - band mass to light ratio ( @xmath17 ) is less effected by metallicity than optical bands . the mass to light ratio of a 12  gyr cluster in the k - band has previously been estimated to be 0.9@xmath16__m / l__@xmath161.3 for metallicities in the range 0@xmath18[fe / h]@xmath18 - 2 @xcite . to estimate the peak mass of the old gcs in m31 , we assume a k - band @xmath17 ratio of 1.1 for all clusters ( as the metallicities are not known for all of the clusters ) . at the distance of m31 ( 780  kpc : * ? ? ? * ) and assuming the k - band magnitude of the sun to be m@xmath19=3.29  mag [ m@xmath20=3.33 : @xcite ; m@xmath19=m@xmath20 - 0.04 : @xcite ] , this implies a peak mass of m@xmath21 . this is slightly higher than that found for milky way gcs ( e.g. * ? ? ? * ) . however , this difference is relatively small compared with the expected uncertainty in the peak mass . this is due to errors in accurately estimating the peak in the gclf combined with errors on the distance to m31 and the value used for the mass to light ratio .    for the fainter gcs , it is likely that masses estimated from their integrated k - band luminosities are less accurate due to stochastic effects . stars at the tip of the red giant branch at the distance of m31 are expected to reach magnitudes of k=17.5 @xcite . while stars this bright are relatively rare , the integrated light of some fraction of these faint clusters can therefore be dominated by a relatively low number of these stars . it is also likely that some of the faintest clusters in m31 are missing from our catalogue . these clusters should be detected in our data . however , identifying these faint clusters in front of m31 would be very difficult .    as expected the proposed young clusters peak at fainter magnitudes than the old gcs . some of these clusters are found to be relatively bright , reaching luminosities similar to the peak of the gclf . this suggests they are more massive than typical young open clusters in the milky way . while this is in agreement with the conclusions of other work ( e.g. * * ) , it should be noted that our conclusions based on this k - band luminosity are limited . the @xmath17 ratio of these clusters is likely to be significantly lower than the @xmath17 ratio for the older clusters in the galaxy . also stochastic effects in these young and faint clusters are likely to be significant in the k - band . the excellent spatial resolution of these wfcam images allows us to investigate the structure of m31 s old gcs . this can be done by fitting psf convolved king models to their profiles . the results of this fitting to 239 _ class 1 _ clusters from the rbc are already presented in @xcite ( hereafter p09 ) . for convenience , we have included these structural parameters , where available , in table 1 . here we review these data and use it to consider the structure of m31 s gc system .    from our new classifications it is found that 186 of the 239 clusters studied by p09 are confirmed as old gcs . we have also determined the structural parameters for an additional 27 , newly confirmed , gcs with wfcam images . these parameters were determined using exactly the same method as p09 . this results in a sample of the 213 old gcs with structural parameters ( representing over half the confirmed gcs identified in m31 ) . the reliability of these structural parameters , was investigated by p09 and they were found to be consistent with those obtained for 33 clusters whose parameters had already been measured using higher spatial resolution _ hst _ images they also found that the structural parameters obtained from fitting different wfcam images of the same gc were consistent . however , the reliability of the parameters was found to decrease significantly for clusters with k@xmath1815  mag . we note that this decrease in reliability for the fainter clusters is in agreement with the signal to noise limit proposed by @xcite . the half light radii of the clusters was found to be the most reliable parameter ( as found by other studies e.g. * ? ? ? errors are not available for these parameters , but we believe them to be relatively reliable for most clusters with k@xmath1615  mag .    figure [ fig : param ] shows the concentration [ @xmath22log@xmath23 , core radii ( @xmath24 ) , half light radii ( @xmath25 ) , tidal radii ( @xmath26 ) and core density ( @xmath27 ) of m31 s old gcs . for comparison the same parameters for the milky way s gcs are shown on the bottom row @xcite . before comparing these populations there are two important differences which need to be considered . firstly , the m31 parameters are based on the k , rather than the v - band luminosity of the clusters . as a result we expect offsets in parameters such as the core luminosity density due to the different mass to light ratios . however , this is unlikely to have a significant effect on the size of the clusters @xcite . secondly , due to a lack of wfcam data , the sample of m31 gcs does not include the most central or distant gcs in the galaxy . it can be seen from figure [ fig : param ] that there are a lack of core collapsed gcs in our sample compared with the milky way . this can be partially explained by the exclusion of the innermost gcs ( where most core collapsed gcs are located in the milky way ) . taking this into account , we still find fewer of these clusters than expected . potentially some faint core collapsed clusters may be missed by gc surveys as they would be the most difficult clusters to resolve . however , it is likely that these clusters are present , but have their concentrations underestimated . this is because their core radii will be much smaller than the psf of our images , making it very hard to deconvolve and measure them . comparison of the core radii of the clusters does show that we are missing , or overestimating , the core radii of some of the very smallest core radii clusters . a lack of core collapsed clusters can also be seen in similar profile fits to _ hst _ images of cen  a clusters @xcite . allowing for these effects , we see no strong evidence for differences between the structure of the old gcs in m31 and milky way .    ) . bold points show the median values for clusters binned on @xmath28 . the line demonstrates the relationship found by @xcite between r@xmath29 and @xmath28 . , height=321 ]    in the milky way , it is known that some gc properties are related to position in the galaxy . in figure [ fig : rgcp_param ] we plot the structural parameters of m31 s gcs as a function of their projected galactocentric radius ( @xmath30 taken from the rbc ) . to help identify potential trends in these data , we have binned the clusters into groups of 25 and determined the median value of their parameters for each group ( bold points ) . it can be seen that the luminosity of the gcs appears to decrease slightly with @xmath28 . this is mainly driven by a deficit of faint clusters in this region combined with the most massive clusters being centrally located . the relatively low number of faint clusters in the central regions is likely due to selection effects , as it is very difficult to identify these clusters projected against the dense central regions of m31 . it is therefore likely that some of these clusters are missing from our catalogue . selection effects can not explain the lack of very massive clusters in the outer regions . however , there are very few of these very bright clusters . the distribution also suggests that more central clusters have smaller , more concentrated cores . this relationship may be expected from the evolution of the gc system as central gcs are expected to evolve more quickly due to greater interactions with their host galaxy . this effect is observed in the milky way s gcs @xcite . however , we again caution that this is a weak trend , and that the same selection effects could potentially prejudice us against extended , low density clusters in the inner regions . the half light radius of the clusters can be seen to increase with @xmath28 . this has previously been observed for a smaller number of gcs in m31 but over a greater range of @xmath28 by @xcite . the line included in this plot is the relationship found by @xcite not a fit to our data . this demonstrates the excellent agreement between the trend they identify and that found here for a larger number of clusters . a similar trend is also found for gcs in the milky way @xcite and in virgo cluster galaxies @xcite . unlike other cluster sizes , the half light radius of a cluster is thought to be largely unaffected by evolution . therefore this relationship may be related to the properties of the globular cluster system at the time of formation . our final catalogue includes 416 old gcs . where detected , we provide self consistent _ ugriz _ and k - band photometry for the proposed clusters and candidate clusters . we note the difficulty in providing accurate photometry for some of these clusters due to the complex background of m31 . we highlight the need for good spatial resolution in order to remove contamination from non cluster light when obtaining integrated magnitudes . where available , we find our photometry to be consistent with that previously published . from our multicolour photometry , we confirm the population of very blue clusters identified previously . we show that these colours are consistent with their spectroscopic classification by c09 as young clusters . we note that many of these clusters look like resolved asterisms in our k - band images . however , some of these are confirmed by hst images to be genuine clusters . higher spatial resolution optical images than available here are required in order to confirm their nature as genuine young clusters . we have identified that many of the confirmed clusters from @xcite are likely stellar sources ( we retain only 27 of their 111 confirmed clusters as old clusters ) . we also identify 10 confirmed clusters in the rbc as likely stellar sources . while we have considered the classifications from k07 and the rbc separately in this paper , we caution that _ all _ of the objects confirmed by k07 to be clusters are included as confirmed clusters in the current version of the rbc ( v3.5 ) . we also provide new classifications for many of the cluster candidates proposed by this previous work . we identify many of these candidates to be stars and reduce the number of unclassified candidate clusters to 357 .    taking extinction and selection effects into account , we find both the colours and structure of the old m31 gc system are consistent with the milky way s . we note a potential lack of both core collapsed and very extended gcs in our m31 sample . we caution that some ( or all ) of this effect may be due to selection effects in identifying these clusters , or difficulties in accurately measuring their parameters , rather than an intrinsic difference in the populations . we thank the referee of this paper , michele bellazzini , for his prompt , constructive , and helpful comments . we would like to thank mark rawlings , andy adamson and mike irwin from the ukirt / casu for obtaining these wfcam observations and assisting with questions related to the wfcam pipeline . we would also like to thank sebastian jester for his advise regarding the sdss and brian warner and alan shafter for useful communication regarding novae . adelman - 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we present an updated catalogue of m31 globular clusters ( gcs ) based on images from the wide field camera ( wfcam ) on the uk infrared telescope and from the sloan digital sky survey ( sdss ) . 
 our catalogue includes new , self - consistent _ ugriz _ and k - band photometry of these clusters . 
 we discuss the difficulty of obtaining accurate photometry of clusters projected against m31 due to small scale background structure in the galaxy . 
 we consider the effect of this on the accuracy of our photometry and provide realistic photometric error estimates . we investigate possible contamination in the current m31 gc catalogues using the excellent spatial resolution of these wfcam images combined with the sdss multicolour photometry . 
 we identify a large population of clusters with very blue colours . 
 most of these have recently been proposed by other work as young clusters . 
 we distinguish between these , and old clusters , in the final classifications . 
 our final catalogue includes 416 old clusters , 156 young clusters and 373 candidate clusters . 
 we also investigate the structure of m31 s old gcs using previously published king model fits to these wfcam images . 
 we demonstrate that the structure and colours of m31 s old gc system are similar to those of the milky way . 
 one gc ( b383 ) is found to be significantly brighter in previous observations than observed here . 
 we investigate all of the previous photometry of this gc and suggest that this variability appears to be genuine and short lived . 
 we propose that the large increase in its luminosity my have been due to a classical nova in the gc at the time of the previous observations in 1989 . 
 [ firstpage ]    galaxies : individual : m31 - galaxies : star clusters - globular clusters : general