We describe here the most ambitious survey currently planned in the optical, the Large Synoptic Survey Telescope (LSST). The LSST design is driven by four main science themes: probing dark energy and dark matter, taking an inventory of the solar system, exploring the transient optical sky, and mapping the Milky Way. LSST will be a large, wide-field ground-based system designed to obtain repeated images covering the sky visible from Cerro Pachón in northern Chile. The telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg 2 field of view, a 3.2-gigapixel camera, and six filters (ugrizy) covering the wavelength range 320-1050 nm. The project is in the construction phase and will begin regular survey operations by 2022. About 90% of the observing time will be devoted to a deep-wide-fast survey mode that will uniformly observe a 18,000 deg 2 region about 800 times (summed over all six bands) during the anticipated 10 yr of operations and will yield a co-added map to r∼27.5. These data will result in databases including about 32 trillion observations of 20 billion galaxies and a similar number of stars, and they will serve the majority of the primary science programs. The remaining 10% of the observing time will be allocated to special projects such as Very Deep and Very Fast time domain surveys, whose details are currently under discussion. We illustrate how the LSST science drivers led to these choices of system parameters, and we describe the expected data products and their characteristics.

We report on our search for microlensing towards the Large Magellanic Cloud (LMC). Analysis of 5.7 years of photometry on 11.9 million stars in the LMC reveals 13 -17 microlensing events. A detailed treatment of our detection efficiency shows that this is significantly more than the ∼ 2 to 4 events expected from lensing by known stellar populations. The timescales ( t ) of the events range from 34 to 230 days. We estimate the microlensing optical depth towards the LMC from events with 2 < t < 400 days to be τ 400 2 = 1.2 +0.4 −0.3 × 10 −7 , with an additional 20% to 30% of systematic error. The spatial distribution of events is mildly inconsistent with LMC/LMC disk self-lensing, but is consistent with an extended lens distribution such as a Milky Way or LMC halo. Interpreted in the context of a Galactic dark matter halo, consisting partially of compact objects, a maximum likelihood analysis gives a MACHO halo fraction of 20% for a typical halo model with a 95% confidence interval of 8% to 50%. A 100% MACHO halo is ruled out at the 95% C.L. for all except our most extreme halo model. Interpreted as a Galactic halo population, the most likely MACHO mass is between 0.15 M ⊙ and 0.9 M ⊙ , depending on the halo model, and the total mass in MACHOs out to 50 kpc is found to be 9 +4 −3 × 10 10 M ⊙ , independent of the halo model. These results are marginally consistent with our previous results, but are lower by about a factor of two. This is mostly due to Poisson noise because with 3.4 times more exposure and increased sensitivity to long timescale events, we did not find the expected factor of ∼ 4 more events. Besides a larger data set, this work also includes an improved efficiency determination, improved likelihood analysis, and more thorough testing of systematic errors, especially with respect to the treatment of potential backgrounds to microlensing. We note that an important source of background are supernovae in galaxies behind the LMC.

The MACHO Project is a search for dark matter in the form of massive compact halo objects (MACHOs). Photometric monitoring of millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge is used to search for gravitational microlensing events caused by these otherwise invisible objects. Analysis of the Ðrst 2.1 yr of photometry of 8.5 million stars in the LMC reveals eight candidate microlensing events. This is substantially more than the number expected (D1.1) from lensing by known stellar populations. The timescales (t) of the events range from 34 to 145 days. We estimate the total microlensing optical depth toward the LMC from events with days to be based upon our eight event sample. This exceeds the 2 \ tü \ 200 q 2 200 \ 2.9~0 .9 1.4 ] 10~7 optical depth, expected from known stars, and the di †erence is to be compared q backgnd \ 0.5 ] 10~7, with the optical depth predicted for a "" standard ÏÏ halo composed entirely of MACHOs : q halo \ 4.7 To compare with Galactic halo models, we perform likelihood analyses on the full eight-event ] 10~7. sample and a six-event subsample (which allows for two events to be caused by a nonhalo "" background ÏÏ). This gives a fairly model-independent estimate of the halo mass in MACHOs within 50 kpc of which is about half of the "" standard halo ÏÏ value. We also Ðnd a most prob-2.0~0 .71.2 ] 1011 M _ , able MACHO mass of although this value is strongly model dependent. In addition, the 0.5~0 .2 0.3 M _ , absence of short duration events places stringent upper limits on the contribution of low-mass MACHOs : objects from 10~4 to 0.03 contribute of the "" standard ÏÏ dark halo.

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We report on a search for long duration microlensing events towards the Large Magellanic Cloud. We find none, and therefore put limits on the contribution of high mass objects to the Galactic dark matter. At 95% confidence level we exclude objects in the mass range 0.3 M ⊙ to 30.0 M ⊙ from contributing more than 4 × 10 11 M ⊙ to the Galactic halo. Combined with earlier results, this means that objects with masses under 30 M ⊙ cannot make up the entire dark matter halo if the halo is of typical size. For a typical dark halo, objects with masses under 10 M ⊙ contribute less than 40% of the dark matter.

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The nature of dark matter remains mysterious, with luminous material accounting for at most approximately 25 per cent of the baryons in the Universe. We accordingly undertook a survey looking for the microlensing of stars in the Large Magellanic Cloud (LMC) to determine the fraction of Galactic dark matter contained in massive compact halo objects (MACHOs). The presence of the dark matter would be revealed by gravitational lensing of the light from an LMC star as the foreground dark matter moves across the line of sight. The duration of the lensing event is the key observable parameter, but gives non-unique solutions when attempting to estimate the mass, distance and transverse velocity of the lens. The survey results to date indicate that between 8 and 50 per cent of the baryonic mass of the Galactic halo is in the form of MACHOs (ref. 3), but removing the degeneracy by identifying a lensing object would tighten the constraints on the mass in MACHOs. Here we report a direct image of a microlens, revealing it to be a nearby low-mass star in the disk of the Milky Way. This is consistent with the expected frequency of nearby stars acting as lenses, and demonstrates a direct determination of a lens mass from a microlensing event. Complete solutions such as this for halo microlensing events will probe directly the nature of the MACHOs.

Since July 1992, the MACHO project has been carrying out long-term photometric monitoring of over 20 million stars in the Magellanic Clouds and Galactic Bulge. Our aim is to search for the very rare gravitational microlensing events predicted if the dark halo of our Galaxy is comprised of massive compact halo objects (hereafter Machos). We have now analysed most of the rst year's LMC data, comprising 9.5 million light curves of stars with an average of 235 observations each. Automated selection procedures applied to this sample show 3 events consistent with microlensing, of which one is very striking (Alcock et al. 1993) and two are of modest amplitude. We have evaluated our experimental detection e ciency using a range of detailed Monte-Carlo simulations, including addition of arti cial stars to real data frames. Using a`standard' halo density pro le we nd that a halo comprised entirely of Machos in the mass range 3 10 4 to 0:06 M would predict > 15 detected events in this dataset, and objects around 3 10 3 M would predict 25 events; thus a standard spherical halo cannot be dominated by objects in this mass range. Assuming all three events are microlensing of halo objects and tting a naive spherical halo model to our data yields aMacho halo fraction f = 0:19 +0:16 0:10 , a total mass in Machos (inside 50 kpc) of 7:6 +6 4 10 10 M , and a microlensing optical depth 8:8 +7 5 10 8 (68% CL). We have explored a wide range of halo models and nd that, while our constraints on the Macho fraction are quite model-dependent, constraints on the total mass in Machos within 50 kpc are quite secure. Future observations from this and other similar projects and accurate measurements of the Galactic mass out to large radii should combine to give much improved constraints on the Macho fraction of the halo. * A similar calculation was carried out by Petrou (1981), but was not published.