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transiting planets , which transit in front of their host stars , are especially important research objects among exoplanets , as when combined with radial velocity measurements , they can provide us various information about the nature of exoplanets such as the mass , radius , density , orbital obliquity , and atmosphere .
most of transiting planets have been discovered by transit surveys which monitor the brightness of hundreds of thousands of stars .
several groups have worked or been working on ground - based transit surveys @xcite , and corot@xcite and kepler@xcite have performed space - based transit surveys . moreover , the second epoch mission of kepler , namely k2@xcite , is now ongoing , and next generation space missions tess@xcite and plato@xcite have been approved to be launched around 2017 and 2024 , respectively .
the biggest problem for transit surveys , especially for ground - based ones , is commingling of false positives with candidates of transiting planets .
the cause of false positives of transiting planets are eclipsing binaries , as they mimic planetary transits by grazing or by being mixed in another bright star s brightness .
thus it is important to conduct follow - up observations after transit surveys in order to validate candidates as true planets by eliminating false positives .
high precision multi - color transit photometry is known to be useful for such follow - up observations to discriminate whether transit - like dimming is caused by a true planet or by an eclipsing binary @xcite .
this is because a true planet is almost dark in all wavelength , while an intervening body of an eclipsing binary is bright itself and its brightness changes significantly with wavelength .
thus false positives caused by eclipsing binaries can be spotted by observing significant wavelength dependence in transit depths .
on the other hand , transit depths of a true planet also have wavelength dependence .
most of the wavelength dependence comes from the stellar limb - darkening , but the apparent planetary radius also has weak wavelength dependence which reflects the nature of its planetary atmosphere .
high precision multi - color transit photometry is known to be useful to measure the weak wavelength dependence in transit depths to study atmospheres of transiting planets .
this kind of study is known as transmission spectroscopy , and numbers of multi - color transit observations for this purpose have been reported so far .
multi - color simultaneous cameras are very fruitful for the studies described above for two reasons .
first , multi - color simultaneous cameras provide not only higher efficiency but also more feasibility to accomplish aimed studies than single - color cameras , since observable transits from a specific ground site are very limited .
second , simultaneity of multi - color transit photometry is important to avoid systematic differences of transit depths due to luminosity change in host stars possibly caused by existence of starspots , plages , stellar activity , and so on .
for the reasons , multi - color simultaneous cameras such as grond , busca@xcite , ultracam@xcite , sirius@xcite , and mitsume@xcite have been actively used for transit observations .
considering the fact that more interesting transiting planets will be discovered in the near future by advanced ground - based surveys , and also space - based surveys , like k2 , tess , and plato , developments of new multi - color simultaneous cameras are highly desired .
we here report a development of such an astronomical instrument named muscat ( multi - color simultaneous camera for studying atmospheres of transiting planets ) , which is now installed on the 188 cm telescope at okayama astrophysical observatory ( oao ) in japan .
the rest of this paper is organized as follows .
we first describe designs of the optical system of muscat and its components ( sec .
[ sect : design ] ) , and introduce the control system of muscat ( sec .
[ sect : control ] ) .
we then report characteristics and performances of muscat shown in engineering observations ( sec .
[ sect : test ] ) .
we discuss on some capabilities for future upgrade of muscat ( sec .
[ sect : discussions ] ) , and finally summarize this paper ( sec .
[ sect : summary ] ) .
we have designed muscat considering the following conditions . as we plan to use muscat for validations of transiting planets discovered by transit surveys , at least 2 colors are necessary to discriminate eclipsing binaries from transiting planets .
considering the cost and available research grants , we adopt a design for a 3-color simultaneous camera with the 3 colors in optical wavelength where ccds are sensitive . for transit observations , it is important to obtain good comparison stars in the field of view ( fov ) to achieve high precision transit photometry . for the reason , we have designed the muscat fov as wide as possible for the 188 cm telescope .
we have also took care of the throughput ( tp ) of the instrument to achieve high photometric precision . to achieve higher sensitivity to the utmost extent ,
we have carefully selected and designed the muscat optical system including astronomical bandpass filters , dichroic mirrors , and ccds .
we report a development of a multi - color simultaneous camera for the 188 cm telescope at okayama astrophysical observatory in japan . 
 the instrument , named muscat , has a capability of 3-color simultaneous imaging in optical wavelength where ccds are sensitive . 
 muscat is equipped with three 1024@xmath01024 pixel ccds , which can be controlled independently . 
 the three ccds detect lights in @xmath1 ( 400550 nm ) , @xmath2 ( 550700 nm ) , and @xmath3 ( 820920 nm ) bands using astrodon photometrics generation 2 sloan filters . 
 the field of view of muscat is 6.1@xmath06.1 arcmin@xmath4 with the pixel scale of 0.358 arcsec per pixel . 
 the principal purpose of muscat is to perform high precision multi - color transit photometry . for the purpose 
 , muscat has a capability of self autoguiding which enables to fix positions of stellar images within @xmath51 pix . 
 we demonstrate relative photometric precisions of 0.101% , 0.074% , and 0.076% in @xmath1 , @xmath2 , and @xmath3 bands , respectively , for gj436 ( magnitudes in @xmath6=11.81 , @xmath7=10.08 , and @xmath8=8.66 ) with 30 s exposures . 
 the achieved precisions meet our objective , and the instrument is ready for operation .    
 : norio narita , astrobiology center / national astronomical observatory of japan , + 2 - 21 - 1 osawa , mitaka , tokyo 181 - 8588 , japan ; tel : + 81 422 - 34 - 3543 ; fax : + 81 422 - 34 - 3527 ; + e - mail :    2