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the earliest stages of star and planet formation are deeply shrouded in cocoons of gas and dust , usually impenetrable at shorter wavelengths .
mid - infrared radiation suffers relatively little extinction , and therefore provides a valuable probe for investigating these dusty beginnings .
the 5@xmath0m30@xmath0 m range is ideal for observations of the thermal continuum emitted by warm dust with @xmath1 100 - 300 k , often found in the circumstellar environment , and of the spectral features due to a variety of atomic ( e.g. , [ ne ii ] ) , molecular ( e.g. , @xmath2 , pahs ) and solid - state ( e.g. , silicates ) species .
the great promise of ground - based mid - infrared instruments is their high spatial resolution .
since imaging in this wavelength regime is ( almost ) diffraction limited , large ground - based telescopes have a significant advantage over satellite observatories such as iras and iso with their small primary mirrors .
however , ground - based mid - infrared observations are challenging , to say the least , for two primary reasons :    * the atmosphere is only partially clear in the 5@xmath0m30@xmath0 m wavelength range .
since the atmospheric opacity is mainly due to absorption by water vapor and carbon dioxide , transmission significantly improves with dryness of the site and altitude . *
the background emission is dominant over the astronomical sources and variable on short timescales , making it necessary to continuously measure and subtract the sky .
( a 300 k blackbody peaks at 10@xmath0 m . )
integration times are very short ( tens of milliseconds ) , and the use of chopping and nodding is essential .    as a result , observations are usually photon - noise limited and the sensitivity is relatively poor . even on an 8-meter telescope , it is difficult to detect a point source fainter than 1 mjy ( or mag@xmath311.5 ) in the broad n - band centered at 10@xmath0 m in a reasonable time . cooled space - borne observatories such as sirtf will have much better sensitivity .    while mid - infrared detectors have vastly improved over the past two decades , most arrays in astronomical use to date
have been limited to 128@xmath4128 pixels , providing a small field - of - view ( @xmath510@xmath410  ) on 8-meter class telescopes .
the new mid - infrared instrument on the eso 3.6-meter telescope , timmi2 , marks a step forward with its 320@xmath4240 si : as array developed by raytheon .
the other handicap of the current generation of mid - infrared instruments is their low spectral resolution , typically @xmath6100 at 10@xmath0 m .
however , comics on subaru achieves @xmath7 2000 , and visir on the vlt is designed to do long - slit spectroscopy up to @xmath7 30,000 .
despite these limitations , many important scientific results have been obtained in recent years through the use of mid - infrared instruments on the ground .
the advent of 8-meter class telescopes enhance the science prospects for the near future .
continuum mid - infrared imaging is useful for tracing the distribution of warm dust emission within compact hii regions while narrow - band ( e.g. , [ neii ] ) imaging can help identify ionizing fronts . for example , smith et al . (
2000 ) have studied the mid - infrared morphologies of the w49a complex , located on the far side of the galaxy and thus behind some @xmath3300 mag of extinction along the line of sight .
their images have sufficient resolution to investigate the nature of the dust emission from individual sources and make comparisons with the radio observations . as another recent example , chini et al .
( 2001 ) produced a 20@xmath0m/10@xmath0 m `` temperature map '' of the dust in the orion bn / kl complex , and identified the main sources of heating .
m , ( b ) 12.5@xmath0 m , and ( c ) 18.0@xmath0 m .
contours are plotted for 10 and 15 @xmath8 levels in panels ( a ) and ( b ) , and for 5 , 10 and 15 @xmath8 in panel ( c ) . from marengo et al .
( 2000).,scaledwidth=100.0% ]    one of the difficulties of probing the formation of protostars is a deficit of high spatial resolution observations .
large extinction ( @xmath9 10 mag ) usually prevents optical imaging while millimeter observations typically have larger beam sizes .
mid - infrared imaging and spectroscopy can help .
for example , marengo et al . ( 2000 ) have recently presented sub - arcsecond - resolution images of the high - luminosity young stellar object afgl 2591 and its circumstellar enviornment .
their images , at 11.7 , 12.5 and 18.0 @xmath0 m , reveal a knot of emission @xmath36  sw of the star , which may be evidence for a recent ejection event or an embedded companion star ( fig .
this knot is roughly coincident with a previously seen near - infrared reflection nebula and a radio source , and lies within the known large - scale co outflow .
marengo et al . also find a new faint nw source which may be another embedded lower - luminosity star .
the _ iras _ mid - infrared spectrum of afgl 2591 shows a large silicate absorption feature at 10@xmath0 m , implying that the primary source is surrounded by an optically thick dusty envelope .
first identified by herbig ( 1960 ) as higher - mass counterparts to young t tauri stars , these objects show many of the same signs of activity such as emission lines and large infrared excesses . over the years
, several authors have attempted to model their spectral energy distributions as dusty envelopes and/or disks ( e.g. , miroshnichenko et al . 1999 and references therein ) .
recent mid - infrared imaging by polomski ( 2001 ; fig .
2 ) and others reveal companions and complex circumstellar environments around many of these sources .          the recent discovery of a spatially - resolved disk around the nearby 10-myr - old star hr 4796a is a spectacular demonstration of the power of high - resolution mid - infared imaging  ( jayawardhana et al . 1998 ; koerner et al .
the surface brightness distribution of the disk is consistent with the presence of an inner disk hole of @xmath550 au radius , as was first suggested by jura et al .
( 1993 ) based on the infrared spectrum , and provides an important constraint on inner disk evolution timescales .
follow - up 10@xmath0 m and 18@xmath0 m observations on keck also revealed tentative evidence for a brightness asymmetry in the disk ( telesco et al .
2000 ; fig .
3 ) which may be the result of a forced eccentricity on dust particle orbits by a companion .
mid - infrared spectroscopy , even at a relatively low resolution of @xmath6100 , is a useful probe of disk mineralogy . in particular
, the 10@xmath0 m silicate feature of some disks appears to be rather similar to that of comets in the solar system ( e.g. , knacke et al .
there is also some evidence , albeit limited to small samples , for evolution of the silicate feature over timescales of several million years ( e.g. , sitko et al . 2000 ) .
one of the exciting prospects for the near future is nulling interferometry at 10@xmath0 m on the keck interferometer and vlti , with the possibility of detecting ( somewhat higher - surface - brightness ) counterparts of the zodiacal cloud around nearby stars .
hinz et al . (
1998 ) have already demonstrated nulling on an astronomical target with two segments of the mmt .
observations in the thermal infrared have the potential to identify the luminosity source in so - called `` ultra - luminous infrared galaxies '' determining the size of the emitting region . in many cases ,
the sources are unresolved , rather than extended , implying that they are either very compact starbursts or dust - enshrouded active galactic nuclei ( e.g. , soifer et al .
2000 ) . in some starburst galaxies , such as ngc 7469
, there is evidence for a starbursting ring surrounding an agn ( jayawardhana et al .
1997 ) and in the case of ngc 253 , a `` super star cluster '' ( keto et al .
imaging and spectroscopic observations in the mid - infrared wavelength range ( 5@xmath0m30@xmath0 m ) offer valuable insight into the origins of stars and planets . 
 sensitive new array detectors on 8-meter class telescopes make it possible to study a wide range of phenomena , from protoplanetary disks to starburst galaxies , in unprecedented detail . 
 i review the capablities of ground - based mid - infrared instruments ( e.g. , high spatial resolution ) and their limitations ( e.g. , poor sensitivity , small field of view ) using several examples in the field of star formation , and discuss prospects for the near future .