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Mission/category:    

Frequently Asked Questions
(last update: January 31, 2013)

Mission

Proposing

Data Retrieval

Catalogs and Search Pages

Data Handling, Analysis, and Quirks

Multiwavelength Surveys

Documentation

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Mission

  • Should I send technical questions to the MAST helpdesk?
    Yes. For best service send questions on technical issues (i.e. not relating to the GO proposal program) to the MAST helpdesk at archive@stsci.edu. These will be screened within a work day and forwarded to the appropriate Kepler Project office if MAST should not or cannot answer them.

  • Can Kepler be pointed to observe another region of the sky?
    No. The Kepler field of view (FOV) is centered on the sky position RA = 19h22m40s and Dec +44o30'00". This pointing will not be altered during the course of the mission.

  • Are there other sources of public information that describe photometric or spectroscopic observations of objects in the Kepler field?
    Readers should consult the Related Sites link in the left gutter of the MAST/Kepler home page. We expect this list of links to grow with the mission as ground-based exoplanet hunters post their results.

  • What are the spatial properties of a point source on the Kepler focal plane?
    The focal plane scale of the instrument is about 3.98 arcsec/pixel. Although Kepler is not meant to be an imaging instrument, each detector module has been focused independently. Some changes in focus and image quality are possible during the mission lifetime. According to the Kepler Instrument Manual, the goal is for 95% of the light ("Encircled Energy") to fall on an area of 7 pixels or less, and the latest information from the project indicates that the actual 95%-circle is 4.2 arcsec (a little more than 1 pixel) . Note that actual image characteristics may change subtlely from quarter to quarter, or even within quarters, because of differential aberration of starlight. The aberration effect causes shifts up to +/-0.25 pixel and can shuffle the distribution of flux among neighboring pixels. In its extraction of flux from the stellar image, the Kepler pipeline processing software should correct for this effect.

  • Where can I go to find recent status reports on the Kepler satellite?
    NASA provides an official site for mission updates.The homepage of the Kepler Asteroseismic Science Consortium (KASC) also lists mission status activities of the satellite and data collection process. An Elapsed Mission Clock is furnished on this site to provide browsers of just where "today" fits into the mission lifetime.


Proposing

  • How do I determine what the schedules and writing instructions are for the next Guest Observer (GO) cycle?
    NASA releases a ROSES (Research Opportunities for Space and Earth Sciences) document annually that announces the research programs its grants offices are supporting and the schedule and instructions for the proposal cycle. MAST recommends that users consult the official announcement at the NASA 2012 ROSES site. For details on the Kepler GO proposal Cycle 5 see the Table 3, Appendix D7. At this writing NASA has not announced the proposal due date for this proposal cycle.

    Information from the GO office takes precedence over this response.

  • Where do the targets on the Kepler Target Search page come from?
    Objects visible on the Target Search page were provided from a list of objects, the great majority of which are stars, in the Kepler Input Catalog. See the question under "Catalogs" on the KIC.

    Information from the GO office takes precedence over this response.

  • What is the procedure for proposing for extragalactic or other objects not returned in the Kepler Target Search results page?
    GO proposers are referred to the Kepler GO program web site for information. In general, there is no prohibition on targets within the appropriate magnitude range. However, for extended objects the number of pixels required to cover an object is a limited resource for the Project. Proposers interested in proposing these types of targets should consult the Proposal instructions and FAQs posted by the Kepler GO Office for guidance in proposing such targets.

    A FAQ from the GO target informs users of important consequences of requesting customized apertures, e.g. for extended sources.

  • What are the Kepler "seasons" all about?
    Kepler was launched into an Earth-trailing heliocentric orbit of a little longer than a year (372.5 days), and this fact has a number of consequences. First, the spacecraft slowly becomes more distant from Earth. This fact ultimately places a hard limit on the mission lifetime. Second, in order to maintain orientation of the solar array toward the Sun, the satellite must roll on its pointing axis four times a year. Each of these intervals is called a season or quarter. The Project designates four seasons, numbered 0, 1, 2, 3, which repeat each Kepler year. New "quarters" start with the beginning of data collection for each new season and run continuously (1, 2, 3, 4, 5, etc.). The exception to this statement is that the Project has designated the commissioning period, ending on May 12, 2009 as "Quarter 0." This quarter has no analog in the seasons assignments. To find out the predicted and actual season start dates (which differ from one another by as much as a few days) users may consult MAST's seasons table, See also the Quarter calender table under the Data Retrieval section. Date for seasons further into the the future will be posted as new predicted and actual dates are communicated by the project.

    During a given season the CCDs are fixed in position such that light from any object in the FOV falls on the same group of pixels. Kepler's 42 CCDs (contained in 21 modules) have a four-fold symmetry such that when the satellite rolls 90o a new CCD sees almost exactly the same area of the sky as an old one did during the previous season.

  • Are all objects in the Kepler field available for proposing in the current GO cycle?
    According to a the revised Project policies, dated Dec. 18, 2009, all objects in the Kepler Field of View are now available for GO observations, as long as the purpose of them relates to non-exoplanetary science. Proposers should check with the GO Office for the suitability of proposing bright objects.

    Because of this revised policy the purpose of the Availability Flag has been changed. Its value now equals 0 for KIC targets not observed or planned to be observed, =1 if observations are planned and/or taken but not yet archived, and = 2 if data for the target have already been archived at MAST.


Data Retrieval

  • How do I retrieve Kepler data?
    First, locate the data of interest via the Kepler Data Search & Retrieval page. On this form specify one or more search criteria. Typical search selections are based on time of observation (MJD, at mid-cadence), target name (e.g., 2MASS id), proposal id (aka "investigation id"), stellar parameters, such as Teff, or target position in the sky. Note that kepler_id and target name are different identifiers. For standard target names the resolver (i.e., SIMBAD or NED) will return the target coordinates, which will then be used in the search. Uploads of target lists follow the same formats allowed for most other MAST missions. Lists are uploaded by clicking the File Upload form on the link at the upper right of the search page and selecting the list tab. See the Help for this form and the Kepler Archive Manual for further details.

    Initiate the search by clicking on the Search button. The results of the search will be displayed.

    To initiate data retrieval, click in the open box next to the desired data in the Search Results table. Submit the list of data by clicking the "Submit marked data for retrieval" tab. Data retrieval follows in the same way as for the HST and FUSE missions. The retrieval Options page is displayed. The user may request the data be staged to an ftp area or sent directly to the user's computer. Either way, notification is sent by email when the process is completed. Help on the retrieval options is available by clicking on any of the field option links. As of this writing the options for data products include light curve and pixel target files, in each case for whole Kepler quarters.

    Problems and questions should be directed to the archive help desk at archive@stsi.edu.

  • How do I determine when the proprietary date for a target expires?
    As of October 28, 2012, with a limited exception, there is no longer a proprietary period on Kepler data, including objects monitored for exoplanet searches by the Project. The exception is the GO cycle 3 data, which retain their original proprietary period. GO data for subsequent cycles will have no proprietary period.

  • Why/when should I register to retrieve data?
    You must register if you want to retrieve proprietary data, or if your existing archive account name is different from that on your proposal. The system that handles the proprietary rights for Kepler data requires an archive account and password be entered before retrieving proprietary data. This will ensure that proprietary data are sent only to the GO or Kepler Team member authorized to retrieve the data.

    Note that registered users must also be authorized to receive proprietary data. For Kepler,an automated system has been set up so that authorization for the PI and co-Is of a given investigation will be handled by software for all PIs and co-Is that have registered before their data are taken using the same name as on their proposals.

    Use the on line form to register (i.e., get an archive account and password).

    GOs and Kepler Team members may register as soon as they receive an investigation_id/proposal_id from the GO Office or the Kepler Project. You must register and be authorized before retrieval of your proprietary data will be successful.

    PIs and co-Is who do not register in time will need to follow the standard archive registration policy as follows. The PIs should request authorization for themselves when they register for their account. Only the PI for an investigation may authorize anyone to access the data. If a Co-I wishes access to their data, they must have the PI on the investigation send e-mail to archive@stsci.edu stating the investigation ID and the identities of anyone who should be able to retrieve the data.

  • What is the policy for public release of images over the whole Kepler field of view ("Full Frame Images") and calibration files?
    The Kepler project typically takes one Full Frame Image (FFI) per month, and these are calibrated and sent to the MAST archive for public use. Although originally carried out about once per month, the delivery schedule has been changed to once per quarter, meaning that generally three FFIs are delivered at a time. Note as stated elsewhere that a set of FFIs (the "Golden Eight") were taken over a span of a day or so early in the telescope's commissioning period. The FFIs may be retrieved at the FFI retrieval site.

    All calibration files, which are known as the Focal Plane Characterization Model, are public. Users should note that the calibration is applied across the field of view, not on a target by target basis. Thus the Focal Plane Characterization Model is applied to cadence data, not light curve or target pixel data. The times of the FPC Model and the FFI files are expressed as MJD (at mid-cadence).

  • May I request data over arbitrary lengths of time?
    Yes, but note well how the time is specified. Recall that light curves are produced on a quarterly basis. If the input start time in the query falls within a quarter, all light curves whose start times occur before the input start time and whose end times occur after the input start time, will be missed. For example, a query where start time > 26 May 2009 will miss all data with cadence start times of 12 May 2009.

  • What are the start dates for the Kepler quarters?
    The planned start dates (UTC) for the Kepler quarters are given in the Table below. The planned end times are typically one day before the start date of the next quarter.

    Quarter Planned Start Date Actual Start Date
    1 May 13, 2009May 13, 2009
    2 June 20, 2009Jun 18
    3 Sept 17, 2009Sept 18
    4 Dec 17, 2009Dec 19
    5 Mar 19, 2010 Mar 19, 2010
    6 June 23, 2010 June 23
    7 Sept 23, 2010 Sep 23
    8 Dec 22, 2010 Dec 22, 2010
    9 Mar 24, 2011Mar 24, 2011
    10 June 27, 2011 Jun 27
    11 Sept 29, 2011Sep 29, 2011
    12 Dec 29, 2011 Jan 5, 2012
    13 Mar 28, 2012 Mar 29
    14Jun 28, 2012Jun 28
    15Sep 28, 2012Oct 4
    16Dec 29, 2012 TBD
    17Mar 29, 2013 TBD
    18Jun 28 TBD


  • How do I find public data in the archive?
    To find public data in the Kepler archive, go to the Data Search and Retrieval page. In the "Release Date" field on this page, enter the less than sign followed by the current date in the form MMM dd YYYY. For example < Oct 30 2009. The results page will show all Kepler data with a release date prior to the current date. By definition, these data are public. Users are also directed to a public download page, either from a direct link at the top of the MAST/Kepler home page or by navigating to the public light curve page, which details a few methods of downloading public data.

  • What is a Golden FFI?
    The "Golden Eight" FFIs were obtained during commissioning after the telescope had been focused, thermal stability obtained and pointing established. The Kepler Project considers these 8 FFIs to be the best FFIs Kepler will ever obtain. Here are the golden FFI filenames.

    • KPLR2009114174833
    • KPLR2009114204835
    • KPLR2009115002613
    • KPLR2009115053616
    • KPLR2009115080620
    • KPLR2009115131122
    • KPLR2009115173611
    • KPLR2009116035924


    These files may be retrieved from the archive via the FFI search form. The size of a FFI file is approximately 390 MB.

  • What is a Custom Aperture File Observation and how do I find them?
    The vast majority of Kepler observations are taken with one of the standard apertures. However, for bright stars, where there is saturation and bleeding along the CCD columns, multiple objects that can not be reasonably separated into individual apertures (e.g., a star cluster) and objects that do not appear in the KIC, a custom aperture is defined and used for the observation. These observations, called custom aperture observations or CAF observations, are handled differently from usual Kepler observations and are assigned a kepler_id greater than 100,000,000 (for example, 100001645).

    The following apply to different subsets of CAF observations.

    • For many CAF observations, the kepler_id changes from quarter to quarter, even if the target and investigation are the same.
    • For CAF observations of clusters, the kepler_id may remain the same for a number of quarters.
    • For CAF observations of multiple targets and non-KIC targets, the coordinates change from quarter to quarter, as they are the geometric center of the aperture.
    • For CAF observations of objects with KIC ids (usually bright stars), the coordinates in the archive are taken from the KIC and remain the same from quarter to quarter, although the kepler_ids may not.
    • For CAF observations of bright targets the kepler_id is supposed to change from quarter to quarter. However, kepler_id = 100001645 violates this rule.

    For the archive user, a cone search is an obvious tool for finding CAF observations. In the case of a target with different coordinates in each quarter, the difference in the coordinates needs to be considered when specifying the cone search radius. In the case of bright objects, say 16 Cyg, the coordinates provided by the resolver (SIMBAD at CFA, for example) may differ from those in the KIC and so require a larger search radius to find the observations. We recommend the user try a range of radii. The current default is 0.02 arcminutes.

  • I have the kepler_id (aka KIC id) for CH Cyg. Why don't I see the custom aperture observations when I search on the kepler_id?
    At this time, there is no way to search for the CH Cyg kepler_id (11913210) and have the CAF observations returned. A cone search is required. For this example, enter CH CYG as the "target name" and use "Simbad at cfa" as the resolver. As of August 12, 2011, 27 rows are returned, ranging from a single Q0 observation through Q8 with 4 observations. The default search radius (0.02) was used.

    While it appears that not all of these observations are of CH Cyg (why are there 3 long cadence observations in Q6?), all observations are of CH Cyg. This is an artifact of the scheduling system, which requires a long cadence for any short cadence. Since each short cadence in Q6 has a different kepler_id, 3 long cadences, each with a different kepler_id, were produced. These 3 long cadence files are basically the same. (Cosmic rays can be calculated differently for the three files however.)

  • I did a cone search, and these multi-quarter, multiple object CAF observations of kepler_id 100000935 were returned. Why is the angular separation the same when the coordinates for each observation are different?
    MAST uses a Hierarchical Triangular Mesh (HTM) to improve search times. There is one set of HTM indices per target. The indices are updated quarterly when the new Kepler Target Catalog (KTC) is received. All angular separations shown in cone search results are based on deviations from the HTM position. If the entered/resolved coordinates are for the latest KTC received, all entries for a given target are returned with a 0.0 angular separation. Unfortunately the latest KTC coordinates are NOT necessarily the coordinates for the latest archived data, and there is no way for the user to determine which quarter's data corresponds to the HTM indices.


Catalogs and Search Pages

  • What is the "KIC" and how does it relate to what I see on the MAST/Kepler search pages?
    The KIC, or Kepler Input Catalog, is the primary source of information about objects observed as part of the ground-based Kepler Spectral Classification Program (SCP) in preparation for the selection of Kepler PI and GO targets. The KIC lists objects down to 21st magnitude, but it is not complete to this limit. Light from only about 1/3 of these objects, some 4.4 million, falls on the Kepler CCD detector. A small number of the KIC objects are calibration objects distributed across the sky. For this reason the full KIC should never be used for Kepler target selection. (By "target" we mean any fixed celestial object observable by Kepler). Information from the KIC is combined with data in two other catalogs to allow searches on the Kepler Target and Data Search pages; see next FAQ.

    MAST provides a portal to those who want to see the contents of the full KIC. Also, Release Notes for this compilation have been posted by the SCP team.

  • Are there other Kepler catalogs delivered to MAST?
    The Project delivers to MAST the Kepler Target Catalog (KTC) and the Characteristics Table (CT). The KTC is basically an observing log, containing both the observed and planned targets. The KTC catalog is delivered to MAST quarterly. The CT contains object-specific information pertaining to the Kepler objects, such as the column and row pixel location. The CT was delivered early in the project mission, and a new version was delivered and made public in June, 2011 (note that some new fields were added and others deleted!). Access to information in these catalogs is via MAST's Kepler Search and Retrieval page and Kepler Target Search page. (See for example the "Field Descriptions" link on these pages.)

    In addition to these catalogs, plans are being made to enhance the effective magnitude limits of targets to stars fainter than the KIC routinely covers (i.e. fainter than 17th magnitude). General users as well as Kepler GO and ADAP program proposers should stay tuned to the MAST's High Level Science Products page for these enhancements.

    Please pay attention to news updates on the MAST and Kepler news corners for the announcements of formally delivered catalog material.

  • Are there objects visible to the Kepler detectors that are not in the KIC?
    Yes. The KIC includes objects as faint as 21st magnitude, although its coverage is not complete to this brightness level. In particular, variable sources and extended sources with low surface brightness may not be in the KIC because they were not in the ground-based catalogs that were used to make it.

  • What are the objects with kepler_ids greater than 10,000,000?
    There are two types of objects with kepler_ids greater than 10,000,000. The vast majority are CCD monitors, which are engineering targets defined by the Kepler Project. The remainder are astronomical objects that have been or will be observed by Kepler, but which do not appear in the KIC, or groupings of KIC objects that can not be adequately resolved by Kepler. Examples include just a few GO targets, faint flare stars, planetary nebulae, and subclusters of stars. The Project is expecting to provide only positional information for such objects.

  • What is the "Kepler magnitude"?
    The "Kepler magnitude" is not to be confused with satellite flux measurements from the satellite. It is a magnitude computed according to a hierarchical scheme and depends on what pre-existing catalog source is available: SCP, Tycho 2, or photographic photometry, in order of preferred selection. For SCP objects (see next FAQ) the Kepler magnitude is further defined according to which of the Sloan gri magnitudes are available for a given star. These dependencies can be found in the Brown et al. (2011AJ....142..112B) paper. When all gri magnitudes are available the relations depend on the gri magnitudes and (g-r) color according to the following prescription: if (g-r) ≤0.3 then kepmag = 0.25g+0.75r, while if (g-r) >0.3 then kepmag = 0.3g+0.7i. We note that for the small fraction of objects for which Sloan photometry is not available the Kepler magnitude is taken from photographic catalogs. The errors of photographic Kepler magnitudes are typically ±-0.2-0.3 mags.

  • What is the Spectral Classification Program (SCP)?
    Early on, the Kepler Project realized the need for homogeneous ground-based observations that would provide information about all the stars in its detector's field of view. The SCP, a project commissioned to the Harvard-Smithsonian Center for Astrophysics (led by Dr. D. Latham), realized the solution to this need by providing colors through copies of the filters used for the Sloan survey (plus an additional "metallicity" filter). "SCP objects" are defined for which the SCP_ID field is listed; its value is the 2MASS identifier (TMID). For an object to be an SCP object it must have 2MASS JHK colors. However, the SCP program succeeded in observing nearly all "SCP objects" in at least one of the 4 ground-based griz filters of the Sloan photometric system. Thus, in practical terms SCP stars are those that have been observed by the 2MASS project and through at least one of the Sloan filters. Some 98% of KIC objects on the Kepler detectors have been observed through one or more of the Sloan filters or had synthetic g and r magnitudes determined from the B and V band photographic photometry of earlier catalogs.

    Additional caveats are the following: (1) not all KIC stars observed by 2MASS are SCP objects; these do not have Sloan filter magnitudes, (2) the g and r magnitudes of many faint KIC objects were computed from photographic catalog magnitudes. To find out whether stars in your object list have been observed photoelectrically or photographically, highlight the "Keplermag_Source" item in the output menu of either of the Search forms and then click "Add."

    The colors and color-derived quantities shown in MAST's Kepler Search pages are derived from the SCP program. A copy of the SCP plan is available. And: as noted in another FAQ, users should be aware that the INT_KIS Sloan i magnitudes for Kepler objects are in serious disagreement with the KIC i magnitude. However, GO proposers should continue to use Kepler magnitude values for KIC objects listed in the KIC.

  • How do I search for an object by RA in decimal hours?
    The Kepler search forms have fields for entering Right Ascension and Declination, and they accept (only) decimal degrees or sexagesimal (hhmmss) formats. To search for RA in decimal hours, go to one of the "user-specified field" pulldown menus, select the entry "RA_hours (J2000)", and then enter values in the corresponding "Field Descriptions" box. Note that currently the File Upload search option accepts only RA in decimal degrees or sexagesimal format. For more help, click any of the form element labels on the search form.

    Note that the coordinates in the search results will always be displayed in sexagesimal format unless one clicks the "Hours" button in the "Output Coords" field element.

  • Is information about the physical characteristics of stars derived from the KIC reliable?
    Often not. Physical characteristics like the effective temperature are most trustworthy for cool stars. It is well known that this quantity is poorly calibrated for stars earlier in type than about F0. An recent published presentation by Batalha et al. (2010ApJ...713L.109B) shows that cool giants and dwarfs are statistically well separated. Unpublished information from the Project suggests that metallicity is not well determined. information on these topics has become available in the Brown et al. (2011AJ....142..112B) paper.

    Information from the GO office takes precedence over this response.


Data Handling, Analysis, and Quirks

  • What are the exposure times of short, long cadence time series data?
    Following the terminology given in the Kepler Instrument Handbook, the interval between reads of a given pixel on a CCD is called a "frame" (equivalently an integration time). The integration time consists of the "exposure time" (accumulated time of flux from a celestial source on the pixel), 6.02 seconds, plus a fixed read out time of 0.52 seconds. The default exposure time for a short cadence is thus 6.02 x 9 or 54.2 seconds. The cadence rate or integration time, is (6.02 + 0.52) x 9, or 58.9 seconds. The default exposure time for long cadences is thirty times 54.2 , or 1626 seconds. The cadence rate between starts of consecutive integrations is thirty times 58.9, or 1766 seconds.

  • What kind of time series (light curve) data product is the Kepler Project planning to release?
    The light curve is produced using simple aperture photometry (SAP). The light curve files contain two light curves. The SAP light curve is the flux in units of electrons per second contained in the optimal aperture pixels collected by the spacecraft. This light curve is the output of the PA module in the SOC pipeline. The PDCSAP light curve is the flux contained in the optimal aperture in electrons per second after the PDC module has applied its detrending algorithm to the PA light curve. Readers should also consult the target pixel file FAQ.

    Information from the GO office takes precedence over this response.

  • In what units are the time series light curves given?
    The time units are seconds (SI). The project has revised the initial units of "flux" from electrons per cadence" to electrons per second. These quantitites may be thought of as an instrumental flux unit closely approximating a linearized unit (for unsaturated pixels). There are no plans for the Project to produce a product in a calibrated absolute magnitude system.

    Information from the GO office takes precedence over this response.

  • UTC, barycentric time, MJDs, etc. - What does Kepler use?
    Kepler provides time in UTC and TDB (barycentric dynamical time, without the relativistic correction), formatted as MJD or (depending on where you are looking) reduced JD. There are several FITS keywords in the Kepler data headers that relate to time. For light curves and target pixel files, the relevant time keywords are in the extension header (BINTABLE). For FFIs, the time keywords are in the primary header. In the discussion below, the time is taken at the mid-point of the first and last cadences unless otherwise noted. Times expressed in UTC or MJD are geocentric, that is they are not corrected to the Kepler - Solar System barycenter. In the data, time is specified as a reduced form of TDB, expressed in Julian Data (so Barycentric JD or BJD). For light curves and target pixel files, this is BJD-2454833.0. See the Release Notes, given as the DATA_REL keyword in the header, specific to your data for more information.

    For light curves and target pixel files:
    The LC_START and LC_END keyword values are the UTC, given in MJD format, of the first and last cadence in the light curve or target pixel file. These keyword values are intended for the use of the DMC.

    The BJDREFI and BJDREFF keyword values are the integer and fractional part of the BJD reference date for that file.

    The TSTART and TSTOP keyword values are the exposure start and stop time, in BJD - BJDREF format. These times are TDB, expressed in JD format. This is referred to as the Barycentric Kepler Julian Date (BKJD) in the Data Characteristics Handbook.

    The DATE-OBS and DATE-END keywords give the TSTART and TEND values in UTC.

    For FFI files:
    The STARTIME and END_TIME keyword values are the UTC start and end times, in MJD format. They are found only in the FFI primary header. Since the FFI is a single readout (cadence), these times are the not mid-point values.

    The BSTRTIME and BSTPTIME keyword values are the TDB start and end times, in MJD format. They are found only in the FFI primary header. Since the FFI is a single readout (cadence), these times are the not mid-point values.

  • What is the reference time for Kepler data?
    For Kepler, the reference time for a cadence is the midpoint observation time. For a light curve or target pixel file, which is made from many cadences, the start time is the mid-point of the first cadence and the end time is the mid-point of the last cadence. The Data Characteristics Handbook, Section 6.2, contains a discussion of the Kepler times as does the Kepler Archive Manual, Section 2.1.2.

    The readout time for each cadence, which is recorded as the Vehicle Time Code (VTC), is produced within 4 ms of the readout of the last pixel of the last frame of the last time slice. The VTC is converted from spacecraft time to UTC (i.e., leap seconds accounted for) at the Mission Operations Center and the Data Management Center. The conversion to Barycentric Dynamical Time (TDB) is done at the Science Operations Center. In the headers, TDB is expressed as a reduced Barycentric Julian Date (BJD), with the offset from TDB given in the BJDREFI and BJDREFF header keywords.

    The Kepler Instrument Handbook, Section 7.3, warns users against assuming a precision of better than 9.0 seconds in the absolute time. Users who require temporal accuracy better than 1 minutes should carefully read the Kepler Data Characteristics Handbook (Section 6) and the associated Data Release Notes.

    For further details consult the Kepler Instrument Handbook.

  • What are target pixel files? Are they the same as cadence files? When will I need them?
    Target pixel files contain the pixels used to create the light curves. The data are packaged as a time series of images in a binary table, where each image is a single cadence. The intent of these files is to provide the data necessary to perform photometry on the raw or calibrated data when needed (or desired) to understand (or improve) the automated results of the Kepler pipeline.

    See the Kepler GO web site for tools that may be used to extract data from the target pixel files.

    Note that target pixel files are not the same as cadence data. Target pixel files are target-specific, while cadence data contain the pixels for every target.

  • Which FITS header keyword tells me the actual exposure time?
    There is no single keyword in the light curve headers that gives the actual exposure time contained in the light curve. The SOC processing may reject individual cadences, so users who require the actual exposure time must inspect the data to determine how many, if any, cadences were rejected. There are a number of keywords that describe the time spent acquiring the data. These are listed below.

    The Kepler integration time is composed of a fixed exposure time plus a fixed readout time. The exposure time for science data is 6.02 seconds, while the read out time is 0.520 seconds, for an integration time of 6.54 seconds per pixel. Individual reads of each pixel are summed on board before being written to the recorder. Long cadence data are summed for 30 minutes, (270 integrations) while the short cadence data are 1 minute sums (9 integrations), yielding total per pixel exposure times of 1625.4 seconds and 54.18 seconds,respectively.

    TSTART = observation start time in BJD-BJDREF

    TSTOP = observation stop time in BJD-BJDREF

    LC_START= mid point of first cadence in MJD

    LC_END = mid point of last cadence in MJD

    TELPASE = TSTOP - START

    LIVETIME= TELAPSE multiplied by DEADC

    EXPOSURE= time on source

    DEADC = deadtime correction



  • Which FITS header keyword tells me how many pixels were used to determine the aperture for observing an object? Which tells me how many we actually used?
    There are no keywords in the light curve or target pixel file headers that provide this information. However, the target pixel file has an aperture extension containing a single image that describes which pixels were collected by the spacecraft and which pixels are contained in the optimal aperture. The FITS standard requires a rectangular bounding box even though many target apertures are not rectangles. Therefore the image contains null pixels that were not collected (i.e., the image includes the extra pixels necessary to create a rectangular image). It is clear which pixels are which.

  • Where do I go to find the most recent report on data quirks, peculiarities, flaws, etc.?
    The Project expects to puts out at least one set of Data Release Notes each quarter. These reports are on the left gutter of the MAST/Kepler home page. The notes detail many of the vagaries of the data, including the latest information on noise sources, "argabrightening," loss of detectors, and so on. The notes are public even though almost all the data they refer to are proprietary. Users interested in attributes of data for a given quarter should start with the corresponding Release Notes version. However, it is useful to consult higher versions that may contain updates on tests on data properties. In February, 2011 the Project delivered the Kepler Data Characteristics Handbook, which is meant to be the definitive "static" document describing nonstandard properties of Kepler data.

  • Have any Kepler detectors been lost since the start of mission operations?
    Yes: All 4 channels of Module 3 (1 of 21) failed on Jan. 9, 2010 (MJD55205.745) during Quarter 4, and the Project has declared it permanently nonoperational. The impact on science observations is that about 20% of Kepler's field of view suffers a one-quarter loss of data per year. Further details of this announcement can be found in Section 6 of the Project's Data Release Notes #6 and in Section 4 of the Kepler Data Characteristics Handbook.

  • Are the flux contaminations given in the Target Search page reliable?
    There are two contamination values available in MAST. One is present in the Target Search database, and one is present in the Data Search database. The one in the Target Search database ("kct_contamination_season_{0,1,2,3}") were provided to MAST from the Kepler pipeline in June 2011. These values were designed for planning purposes only, and are based on 21 x 21 pixel square apertures centered on each target. The actual apertures used to extract light curves in the Kepler pipeline are usually different from this 21 x 21 pixel aperture, thus, use of this contamination estimate will not produce reliable results when used in conjunction with the Kepler pipeline fluxes.

    Instead, Users are strongly encouraged to make use of the contamination values present in the Data Search database ("sci_contamination"). These contamination values are based on the final photometric aperture and various seasonal differences for each dataset. The values of "sci_contamination" are defined as 1.0 - "sci_crowdsap", where "sci_crowdsap" is the value found in the FITS header keyword "CROWDSAP" in each dataset's light curve FITS file (*_llc.fits or *_slc.fits for long/short cadence, respectively).

  • Which of the available light curves should I use?
    Quarter 8 light curves and Quarters 0-7 light curves reprocessed in July, 2011 contain two types of light curves now called SAP_FLUX (formerly "raw") and PDCSAP_FLUX. The Project will soon provide tools via MAST that will enable users to apply corrections to the SAP_FLUX vectors to remove certain instrumental trends. These trends have been determined statistically by Principal Component Analysis. The PDCSAP_FLUX light curves will initially have some but not all of these trends removed. Future processing under Pipeline "Build 8" in early 2012 will include a best-attempt removal of the remaining trends. Thus, for the short term (late 2011-early 2012) researchers should consider using the provided tools to remove these trends themselves to the SAP_FLUX vector in case the PDFSAP_FLUX light curves do not do an adequate job. Users should stay tuned to the MAST News page to learn where to upload these tools. The Kepler project has posted documentation on the available tools on its website titled PyKE home page. Further details on the use of these tools will be found in the Archive Manual to be released in the summer of 2011.

    This subject is currently in a state of high flux. Questions should therefore be directed to the Project's GO or Science Office. Their replies should take precedence over this FAQ.

  • What information in the MAST Target Search form is different for targets having no Kepler magnitude entries?
    The delivery of a new Characteristics Table in mid-2011 excluded some 122,000 (usually faint) objects that have no Kepler or griz magnitudes. In order to permit users to propose for these targets. MAST has restored the data associated with these entries from the (frozen) KIC. However, this means that some columns found on the Target Search page now have different meanings for objects with and without Kepler magnitudes, or there may be no given values at all.
    One difference concerns "edge to CCD" values. For KIC targets with"null" Kepler magnitudes, this parameter refers to the distance of central pixel of its image to the nearest edge of the CCD it resides on, while for targets with Kepler magnitudes the edge value refers to the outer edge of the photometric extraction area computed for the image; the number Seasons_on_CCD is also based on this metric. Note well that these contamination values are predicted and are not measured.
    The contamination, crowding, fractional flux, and signal-to-noise ration (SNR) values are given only for targets with Kepler magnitudes. For targets without Kepler magnitudes the values provided by MAST on the Target Search form and results pages may be less accurate.

    Finally, objects included by catalogs other than the KIC (from the UKIRT, INT/KIS, and UBV surveys) have computed distances corresponding to their nearest KIC neighbors. However, targets are omitted if their positions do lie not at least 11 pixels from the detector edges. Surviving non-KIC entries are therefore guaranteed to lie on the detector (at least one season).

    Users should take care if targets not having Kepler magnitude are very close to the edge of a CCD. When in doubt users are encouraged to direct such questions to the GO Office.

  • How robust is the calibration of Teff values from KIC colors?
    The photometric calibration and temperature scale of stars in the KIC have been published by Brown et al. (2011) (2011AJ....142..112B) The catalog and calibrations established a photometric color-derived scale for effective temperatures for cool stars and enabled a differentiation between giants and dwarfs. It has been succesful for these purposes, particularly in the range 4500K < Teff < 6500K. In an analysis of color calibrations determined from M67 stars, 2MASS colors, the Yale Rotating Evolutionary Code, and high resolution spectroscopy, Pinsonneault et al. (2012ApJS..199...30P) found that for the range 4000K-6500K Teff's of KIC field dwarfs are about 215K too cool relative to temperatures found from calibration of infrared fluxes. This deviation is perhaps not surprising since Brown et al. estimate systematic errors in the KIC could be this large. Although the ultimate source of the systematics is still unknown, according to these authors, a significant component could arise from photometric errors and inclusion of other objects (including binary companions) in the photometric extraction apertures. Metallicity differences and extinction errors may play a secondary role.

    Pinsonneault et al. note similar systematic errors in warmer (mid A to F-type) stars. According to spectroscopic calibrations of a comparative handful of B stars (see (Mamejek 2010) and Lehmann et al. (2011) (2011A&A...526A.124L), the discrepancy in this calibration grows with increasing Teff.

    MAST offers no judgment on these claims. Users should go to these papers and follow-up literature to pursue this further. The new infrared, ultraviolet, and near-ultraviolet magnitudes that MAST is adding in its Enhanced Target Search page can enable users to test these calibrations with extended spectral energy distributions of KIC stars.

  • How do I determine the best masses for stars that are candidates for hosting planets?
    The Kepler Project will not compute stellar masses for inclusion in any of their Kepler Results Catalog tables. MAST and the Project recommend that users compute stellar masses directly from the radii (s_Rad) and log(g) in the Planetary Candidate table. (Do not use data provided in the KIC for this purpose.). Note that the Candidates table is part of the Kepler Results Catalog. An updated Planetary Candidates Table will be sent to NExScI in the fall of 2012, and from there to MAST. An updated Kepler Results Catalog, including the Candidates table, is expected in January of 2013. This FAQ was drafted in consultation with the Kepler GO office.


Multiwavelength Surveys

  • Did the Sloan (SDSS) project survey the Kepler field?
    No and yes. The original Sloan Survey (SDSS/DR7) covered 1/4 of the sky outside the Galactic plane and included very little of the Kepler field. However, to simulate the results of the Sloan survey the Kepler SCP team observed KIC objects with a set of Sloan-like filters. Note carefully from Pinsonneault et al. (2012ApJS..199...30P) that it is now known that there are both zeropoint and color effects affecting comparisons of results from the SDSS and KIC observations (even though the latter used a copy of the SDSS filters). The KIC catalog is comprised in large part from data taken from this program. Note that the Sloan photometric survey was undertaken in five filters, ugriz. The SCP team undertook a similar effort with its filter copies. However, because the u magnitude observations required relatively long integrations, these observations were abandoned after the first observing season. The project has delivered u-magnitudes of relatively few KIC objects (2997 of those appearing on the Kepler detectors). These are included as results of the Kepler Target search tool.

    In late 2012 MAST will add new u, g, r, i, and z magnitudes and potentially new targets from the extended part of the SDSS Project, Data Release 9. These data will be announced and will appear in MAST's Kepler Target Search form and CasJobs tool. Please stay tuned.

  • Has the GALEX project surveyed the Kepler field in the near-UV and far-UV?
    In the summers of 2009-2011 the GALEX Project surveyed much of the Kepler field. These survey products, including a cross-matched catalog of KIC objects observed by GALEX were added to the GALEX GR6 (General Release 6) in 2010, and a final supplement in 2012. MAST has made available two cross-match catalogs as a MAST-style interface form and for in-depth use in a "CasJobs" implementation. See the MAST news corner item, dated October 28, 2010.

  • Are there other UV-optical-IR missions that have surveyed the Kepler field?
    The 2MASS Survey, a survey of 70% of the full sky at the 3 infrared J, H, K bands (1.25, 1.65, and 2.17 microns), includes nearly all bright KIC targets, and is fairly complete down to KEPMAG 16 or 17. For all but faint KIC objects the 2MASS_ID often provides a good (though imperfect) substitute for Kepler_ID values in queries on the MAST search pages, particularly for cross-referencing to external catalogs.

    Users should be also aware of objects observed from the ground in the Kepler All Sky Automated Survey. This is a photometric survey dedicated to the ground-based study of optical variables in the whole observable sky (both hemispheres), including the Kepler field.

    Users should watch the MAST/Kepler News corner for developments on ingests of new ground-based photometric observations on the Kepler field.

  • Does the Digitized Sky Survey cover the Kepler field?
    Yes. Go to the Google Sky site and enter the following in the string: http://keplergo.arc.nasa.gov/tools/KeplerFOV.kml to see the location of the Kepler CCD "footprints" against a Digital Sky Survey map of the sky. (Courtesy of a tip on the Kepler GO website.)


Documentation


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