You are an expert at summarizing long articles. Proceed to summarize the following text: 
we live in a dusty universe ( hoover 1998 , private communication ) , and correcting for the dust extinction and reddening affects almost all aspects of optical astronomy . for us , observing from within the milky way , it is of crucial importance to know how much galactic dust there is towards various objects . burstein & hailes ( 1982 ; hereafter : bh ) constructed an all - sky reddening map , used extensively by the astronomical community . recently , schlegel , finkbeiner & davis ( 1998 ; hereafter : sfd ) published a new all - sky reddening map , based on the cobe / dirbe and iras / issa maps . this map is intended to supersede the bh map in both the accuracy and the spatial resolution ( @xmath3 ) . also , unlike the bh map , which presented the @xmath1 values only for the @xmath4 regions , the sfd map extends all the way to the galactic plane . there are many instances when one would like to be able to identify low - reddening regions near the galactic plane . as an example , the expected microlensing optical depth increases strongly towards the galactic center ( e.g. stanek et al . 1997 , their figure  12 ) . in this paper i show the usefulness of the sfd map for selecting low - reddening regions close to the galactic plane . in section  2 i compare the predicted and the observed reddenings and arrive at the approximate scaling relation . in section  3 i show the re - scaled reddening map for central region of our galaxy and discuss its applications for microlensing experiments . with its high spatial accuracy , the sfd map is potentially very useful for selecting low - reddening windows close to the galactic plane . however , as discussed by sfd , at low galactic latitudes ( @xmath0 ) most of the contaminating sources have not been removed from their map . also , no comparisons between the predicted reddenings and observed reddenings have been made in these regions . it is therefore important to attempt such a comparison . harris ( 1996 ) surveyed the vast literature on the galactic globular clusters ( gcs ) , producing an electronic catalog of gcs with reasonably well - known properties , among them the reddening @xmath1 . many galactic globular clusters are extensively studied objects , for which it is possible to accurately determine their extinction and reddening using a variety of methods ( burstein & heiles 1978 ; webbink 1985 ; zinn 1985 ; reed , hesser & shawl 1988 ) . however , many gcs in harris ( 1996 ) compilation have been poorly studied , in which cases the reddening estimates are rather inaccurate .    there are 147 gcs in the catalog of harris ( 1996 ) , out of which i selected 32 clusters with @xmath0 . i then excluded eight clusters with the distance from the galactic plane @xmath5 , to ensure that each cluster remaining in the sample probes most of the reddening along the line of sight ( see discussion in stanek  1998 ) . for the remaining 24 clusters i used the program `` dust_getval '' included with the sfd reddening map to obtain the value of reddening predicted by the sfd , which i hereafter call @xmath6 this was done by obtaining 49 values of @xmath6 on a uniform @xmath7 grid with @xmath8 spacing centered on the cluster ( @xmath9 part of the sky ) , which allowed me to obtain an average value of @xmath6 as well as its standard deviation @xmath10 . at this point i excluded additional five gcs with @xmath11mag , indicating large reddening gradient across the cluster ( stanek 1998 ) , which might strongly affect the determined reddening . this leaves me with 19 gcs with which i can test the sfd map for the @xmath0 region . i denote the gcs reddenings taken from the catalog of harris by @xmath12 . additional comparison comes from using the reddening map of baade s window ( bw ) region ( stanek 1996 ) . this map was constructed using the method of woniak & stanek ( 1996 ) and the ogle project data ( szymaski & udalski 1993 ; udalski et al . the zero point of baade s window reddening map was re - determined recently by gould , popowski & terndrup  ( 1998 ) and alcock et al . i obtained 240 values of @xmath6 on a uniform grid with @xmath8 spacing , falling within the bw map . for the same grid i also obtained the values of @xmath13 using my map of the reddening . using the relations between the selective extinction coefficients given by sfd ( their table  6 ) and also the coefficient of @xmath14 found by stanek ( 1996 ) , i then converted the @xmath13 reddening to @xmath15 using relation @xmath16 .    in figure [ fig : sfd ] i plot the relation between the observed values of the reddening @xmath17 , obtained using globular clusters with @xmath0 from catalog of harris ( 1996 ) and the reddening map of baade s window ( stanek 1996 ) , and the predicted reddening @xmath6 obtained using the sfd map . large symbols denote the clusters , with @xmath10 as errorbars , and small symbols denote points from baade s window . the observed reddenings @xmath12 extend from 0.25 to 1.75 . using linear least - squares fit to both the gcs and the bw samples i obtained the scaling relation @xmath18 , while using only the gcs i obtained a relation @xmath19 . since the typical error of the @xmath15 values is @xmath20mag ( stanek 1996 ) , i feel that using both samples gives a better estimate of the scaling relation , so hereafter i adopt @xmath21 as representing the `` true '' reddening for the @xmath0 region . it should be stressed here that there is no physical reason for _ one _ `` universal '' relation of the kind fitted above , so the adopted scaling factor of 1.35 should be applied carefully , only when there are no additional constraints on the reddening . also , as the comparison range of the @xmath12 extends only to 1.75 , the derived scaling might not apply in high reddening regions very close to the galactic plane ( @xmath22 ) . finally , for regions with @xmath23 this scaling factor _ should not _ be applied ( sfd ; stanek 1998 ) , as the sfd map is cleaned from the point sources in these regions . i can now apply the sfd map and the scaling factor obtained in the previous section to investigate the reddening close to the galactic plane . in figure [ fig : dust ] i plot the @xmath1 reddening map of @xmath24 part of the the galactic center . contours correspond to re - scaled @xmath25mag . as discussed above , the comparison range of the @xmath12 extends only to 1.75 , so the derived scaling of @xmath2 might not apply in high reddening regions very close to the galactic plane ( @xmath22 ) . also , the sfd map gives by its construction the total dust reddening along given line of sight , which for regions close to the galactic plane might have significant contribution from dust distributed beyond the observed source . the spatial structure of reddening seen in figure  [ fig : dust ] is very rich , both on large and small scales . some of the features , such as more dust extinction above the galactic plane than below the plane , are well known and can be observed even with a naked eye . there is a pronounced `` pinch '' in the reddening at the @xmath26 longitude , where some of the iso - reddening contours are @xmath27 closer to the galactic plane than elsewhere in the map . in a low - reddening window near @xmath28 the rescaled reddening is as low as @xmath29mag . also worth noticing is low - reddening region near @xmath30 with the rescaled reddening as low as @xmath31mag . it is interesting to compare the locations of the fields observed towards the galactic bulge by the ogle ii ( figure  [ fig : micro ] , upper panel ) and by the macho ( figure  [ fig : micro ] , lower panel ) microlensing experiments ( udalski , kubiak & szymaski 1997 ; alcock et al . both experiments cover similar areas on the sky ( @xmath32 ) and show significant overlap with each other , with ogle ii taking somewhat better advantage of low - extinction regions close to the galactic center ( such as the window near @xmath28 discussed above ) . however , ogle ii fields are much better distributed for applications of their data for the galactic structure studies , such as investigating the properties of the galactic bar with the red clump giants ( stanek et al . 1997 ) .    we might attempt to define an `` optimal '' selection of fields to be used for the microlensing studies . this selection naturally depends on the scientific goal of a given study . to maximize the observed microlensing optical depth during the lifetime of the experiment , i define `` reddening - adjusted optical depth '' as @xmath33},\ ] ] where @xmath34 is the standard optical depth for microlensing and the factor @xmath35}$ ] is used to reduce @xmath34 in regions of high reddening ( and hence extinction ) . the coefficient @xmath36 in the exponent can be adjusted to reflect , for example , a photometric band used for observations . to give an example , the microlensing optical depth is highest towards the galactic center , yet because of the high dust extinction there it is not a good place to monitor microlensing . in figure [ fig : tau ] i plot @xmath37 , using the rescaled sfd map as the source of reddening @xmath1 and the optical depth from the bar model e2 of stanek et al . ( 1997 , their figure  12 ) as the source of @xmath34 ( the contribution from the galactic disk to the microlensing was not included ) . i took @xmath38 for simplicity .    in practice , as current microlensing experiments operate in crowding - limited mode , the reddening - adjusted optical depth defined above is not optimal from their point of view . however , with the progressing development of image - subtraction techniques ( see recent papers by alard & lupton 1998 and tomaney 1998 ) one will be able to detect the microlensing events regardless of crowding .    to summarize , the sfd map overestimates the reddening in @xmath0 regions by a factor of @xmath2 , but it is a very valuable tool in investigating features in the reddening near the galactic plane . i limited myself to investigating the central part of our galaxy , but the sfd map should be also useful for similar studies in other parts of the galactic plane .
recently schlegel , finkbeiner & davis published an all - sky reddening map based on the cobe / dirbe and iras / issa infrared sky surveys . using the reddening map of baade s window and sample of 19 low - latitude ( @xmath0 ) galactic globular clusters 
 i find that the dirbe / iras reddening map overestimates @xmath1 at low galactic latitudes by a factor of @xmath2 . 
 i also demonstrate the usefulness of this high resolution map for selecting low - reddening windows near the galactic plane .