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Section 2.2 Contents of the Tycho Catalogue

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2.2. Contents of the Tycho Catalogue
Over view of the Tycho Catalogue: The Tycho Catalogue provides astrometr y (positions, parallaxes and proper motions) and two-colour photometr y (in B T and VT ) for more than one million stars brighter than VT = 11.5 mag. The median precision (standard error) is 25 mas in position and 0.10 mag in the B T - VT colour index. These values apply at the median magnitude VT = 10.5 mag for stars of median colour index B T - VT ' 0.7 mag. The Tycho Catalogue contains 1 058 332 entries: comprising 1 052 031 entries (stars) obser ved by Tycho, supplemented by the 6301 entries from the Hipparcos Catalogue and Par t C of the Double and Multiple Systems Annex that were not obser ved by Tycho. The Tycho Catalogue contains roughly 40 000 stars brighter than VT = 9 mag which are not contained in the Hipparcos Catalogue. For these stars the median precision is 7 mas in position, parallax and annual proper motion and 0.019 mag in B T - VT . Double stars with separations larger than 2 arcsec and with moderate magnitude difference are usually resolved. The Tycho Catalogue, and its associated Tycho Epoch Photometr y Annex, is strictly an obser vational catalogue. It contains data derived exclusively from the Hipparcos satellite's star mapper obser vations, with the exception of cer tain cross-identifications. The reduced data comprise two par ts. The main catalogue (the Tycho Catalogue, or TYC, described in this section) contains the astrometric and summar y photometric data for each star. The Tycho Epoch Photometr y Annex (described in Section 2.6) contains `epoch photometr y' (photometr y at each epoch of obser vation) for a subset of stars obser ved with sufficiently high signal-to-noise ratio. In structure, the Tycho Catalogue and its Epoch Photometr y Annex resemble the corresponding machine-readable par ts of the Hipparcos Catalogue and associated Epoch Photometr y Annex. Solar system objects obser ved as par t of the Tycho experiment are contained within the general annex of solar system obser vations (see Section 2.7).
Tycho Catalogue Completeness: Some Hipparcos Catalogue stars were not obser vable by the star mapper. The dynamic range of the star mapper detector resulted in non-linearity was not obser vable). The faintest Hipparcos Catalogue stars fell below mapper detectors. Stars in ver y dense clusters and other dense fields thus leaving the resulting Tycho Catalogue incomplete in such regions `R' in Field T42). Section 1.1.1 gives fur ther details of the relationship Catalogues and entries. at the brightest magnitudes (Sirius the detection threshold of the star could not be obser ved by Tycho, (see, for example, entries flagged between the Tycho and Hipparcos

All 6301 single star entries and double and multiple star components contained within the Hipparcos Catalogue but not obser ved by Tycho have never theless been included in the Tycho Catalogue for completeness, and assigned a corresponding TYC number. In these cases (flag `H' in Field T42), the truncated astrometric and photometric descriptor taken from the Hipparcos Catalogue (Fields H35) has been included in the Tycho Catalogue at the appropriate location (Fields T35). The position of the entr y taken from the Hipparcos Catalogue (Fields H89) is also included in Fields T89 to assist cross-identification between the catalogues. Among the 120 000 stars in the PPM catalogue brighter than 9.0 mag, about 120 are not contained in the Tycho Catalogue (see Chapter 17, Volume 4 for details).

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The following table gives the number of stars in TYC and the number of TYC stars not included in HIP, along with the corresponding median standard errors for stars within the given inter vals of VT magnitude (the column `All' also including entries for which VT is not available). Systematic errors in astrometr y are less than 1 mas and 1 mas/yr, although the external standard errors (the true accuracies) may be up to 50 per cent larger than the quoted standard errors for faint stars. In photometr y, systematic errors may reach the level of the quoted standard errors for faint stars. The photometr y for about 20 000 stars is considered to be uncer tain, for example when the standard errors are larger than 0.3 mag. Inter val of VT Median VT , mag N (TYC) N (not in HIP) <6.0 5.38 4553 4 67.0 6.63 55 78.0 7.62 3485 89.0 8.62 910.0 9.61 1011.0 10.58 515029 506720 27.2 34.3 33.5 0.084 0.064 0.117 0.098 >11.0 11.19 205275 39.2 49.6 48.6 0.128 0.122 0.200 0.171 All 10.47 934901 24.6 31.2 30.2 0.074 0.057 0.104 0.087 <9.0 8.33 119882 40055 5.6 7.2 7.0 0.014 0.012 0.019 0.017

9550 27750

78029 211107 36511 182773 6.7 8.6 8.3 12.9 16.4 16.0 0.036 0.027 0.049 0.041

205934 1052031

Median standard errors in astrometr y (mas): Position (J1991.25) 1.8 2.6 4.0 Parallax 2.5 3.6 5.3 Proper motion/yr 2.3 3.3 5.0 Median standard errors in BT 0.003 VT 0.003 B T - VT 0.005 B-V 0.004

photometr y (mag): 0.006 0.010 0.018 0.005 0.008 0.014 0.008 0.014 0.024 0.007 0.012 0.020

Tycho Photometr y: Tycho photometr y was obtained in two colour bands, B T and VT , closely corresponding to B and V in the Johnson UBV system. The Tycho colour index, written B T - VT or (B - V )T , is not explicitly given, but may be simply derived from the difference of the published magnitudes. Approximate values of the Johnson V magnitude (Field T5) and colour index B - V (Field T37) are also provided, derived by the transfor mations in Section 1.3, Appendix 4. Because it is a strict obser vable, and unaffected by the uncer tainties inherent in such a transfor mation, the Tycho colour index rather than the derived Johnson colour index is recommended for use whenever appropriate. A simple linear transfor mation from the Tycho B T , VT magnitudes to B , V magnitudes in the Johnson photometric system is: V B-V

' '

VT - 0.090 (B T - VT ) 0.850 (B T - VT )

[2.2.1] [2.2.2]

In the inter val - 0.2 < B T - VT < 1.8 mag the systematic errors of this simple transfor mation do not exceed 0.015 mag for V and 0.05 mag for B - V for unreddened main-sequence stars. Proximity Flag: In analogy with the Hipparcos Catalogue, Field T2 provides a `proximity flag' giving a coarse indication of the presence of nearby objects (within 10 arcsec). These may be separate Tycho Catalogue entries, or separate Hipparcos Catalogue entries (or components). Further infor mation on nearby objects can be derived from an examination of the relevant Hipparcos or Tycho Catalogue entries, or from Field T49 indicating unresolved duplicity.

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Reference Stars: A distinct flag, Field T10, indicates whether the Tycho Catalogue entr y is considered as a `recommended' astrometric reference star. This flag is assigned to classification of the entr y as an astrometric reference star since there are several considerations, aside from (possible) duplicity, which may disqualify the object from being classified as a Tycho Catalogue astrometric reference star. Fur ther details are given under Field T10. Hipparcos Catalogue objects not obser ved by Tycho (i.e. where Field T42 contains `H') are flagged as dubious Tycho astrometric reference stars even though the object could be a well-behaved, single object within the Hipparcos Catalogue. This distinction has been preser ved in order to maintain the consistency of the Tycho Catalogue as an independent obser vational catalogue. Most of the Tycho Catalogue magnitudes have magnitudes derived from photographic sur vey pl Selection according to the criteria adopted under about 520 000 Tycho photometric reference stars sufficient accuracy for calibration of ates in colour bands near to B or V . Field T34 results in the availability of suitable for such purposes.

Transits, Detections, Measurements: In the ter minology of the Hipparcos Catalogue, a star `transit' is defined as a crossing of the star across the main modulating grid (2688 slits covering a field of view of approximately 0 9 в 0. 9). In the ter minology of . the Tycho Catalogue, a `transit' refers to the crossing of the star across a star mapper slit system (either of a set of four ver tical or inclined slits of 40 arcmin length, located at the edge of the main field of view, and used primarily for the satellite real-time attitude determination). Such a transit is defined irrespective of whether or not such a crossing yields `useful' astrometric and/or photometric infor mation. The transit yields useful astrometric and/or photometric infor mation when the star is not too faint, when the background was below a cer tain limit, when an accurate attitude deter mination was available, and when the obser vations were not per turbed by nearby bright stars. Details of the measurements and reductions are given in Volumes 2 and 4. All relevant transits related to a given star have been combined to provide the astrometric data and the summar y photometric data contained in the main Tycho Catalogue, and individual transit records (providing `epoch photometr y') are contained in the Tycho Epoch Photometr y Annex. The summar y photometric data provide median magnitudes for bright stars and `de-censored mean magnitudes' for fainter stars, and a set of parameters and flags giving an over view of the variability. The Tycho Epoch Photometr y Annex includes details of each transit including background, obser vation epoch, and related quantities and flags. In practice, the detection process giving a signal amplitude and a transit time was carried out on a signal where the photon counts in the B T and VT channels had been added, forming the so-called T channel. The ter m `detected transit' is used to refer to a transit containing a significant signal belonging to the relevant star, and this signal itself is called a detection. When a signal was detected above a signal-to-noise ratio of 1.5 in the T channel an estimation (or measurement) of the signal amplitude was carried out in the B T and VT channels separately whenever possible. If no such measurement was available, a flag in the first two bits of Field TT13 (see Table 2.6.2) indicates that the magnitude could not be measured in one or other of the separate channels.

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Valid and Invalid Transits for Photometr y: Transits were used for Tycho (mean value) photometr y irrespective of whether or not the object was actually detected in the predicted `transit inter val' of a few arcsec length for the corresponding slit group-- the condition for using the transit being simply that the relevant data inter val was considered to be `valid'. Such a transit inter val could contain several detections (either real detections due to the predicted star or to another star, or false detections due simply to photon noise) or it could contain no detection at all. Cer tain transit inter vals were considered as `invalid', and subsequently excluded from use in Tycho photometr y, for a variety of reasons: (a) if the satellite attitude was poorly known, or if (attitude-control) jet firings were affecting the satellite attitude estimation at the moment of the obser vations; (b) if the detector background was high, for example as a result of a passage of the satellite through the van Allen radiation belts--a higher background was acceptable for astrometr y than for photometr y; (c) because the star crossed the star mapper slit system too close to the end of the slit, or to the 90 angle of the inclined slits--in such cases, attitude uncer tainties may have made it infeasible to distinguish between `uncaptured' transits, and transits where the signal was below the detectability threshold.

Non-Detections and De-Censored Magnitudes: A valid transit inter val was classified as `non-detected' or `censored' if it contained no detection, in the T channel, close enough to the predicted transit time for the relevant star. The criterion for rejection was that all residuals of detections in the astrometric adjustment of the transit inter val were larger than given limits.
Limits used in astrometr y for the rejection of detections were ju j > 1.0 arcsec or ju j > 3 u , where u is the difference between the obser ved and computed transit times (conver ted to an angular distance using the instantaneous satellite scan speed across the slit group), and u is the standard error of u . A single limit was used in the de-censoring analysis, ju j > 0.6 arcsec. Since transit detection was based on preliminar y predicted transit times, which were sometimes in error by a large amount, the real transit occasionally occurred outside the predicted transit inter val. Such detected transits were not assigned to the correct star and were thus lost, even when the improved transit times were introduced at a later stage. As a consequence, non-detections were occasionally associated even with bright stars. This problem was accommodated within the mathematical model for the de-censoring analysis by assuming that there was a probability of 6 per cent that a predicted Tycho star transit resulted in a non-detection even for a of 6 per cent `spurious non-detections', and users of the deficiency. Photometric standard star obser vations were analysis and for correcting final small biases, as described bright star. This is referred to as the assumption Tycho epoch photometr y should be aware of this used for checking the validity of the de-censoring in fur ther detail in Volume 4, Chapter 9.

The use of non-detected transits has two reasons. First, because detectability depends on the signal-to-noise ratio of a given transit, mean or median magnitudes have not simply been constructed from the detected transits--rather, a `de-censored mean magnitude' in B T and VT was constructed, using model-based inferred magnitudes in place of transits which were either not detected in the T -channel, or detected but not measured in the B T or VT channels. All valid transits were thus taken into account, whether detected or not (see Volume 4 for details). Second, non-detected transits may be relevant in variability studies, where it may be impor tant to identify whether a photometric data point is absent because the object's magnitude fell below the threshold at that epoch, or simply because no data were acquired at that epoch. But a non-detection is not always

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an indication that the star was too faint to be detected due to the 6 per cent spurious non-detections described above. For bright stars with B T 8.5 mag and VT 8.0 mag a median magnitude was derived from the measured signal in the B T and VT channels respectively. This median magnitude is equivalent, within 0.005 mag, to a de-censored mean magnitude because bright stars resulted in ver y few non-detections. The median magnitude was adopted for bright stars since the median could also be constructed for variable stars, while the de-censoring analysis was based on the assumption that the star is constant.

Parasites: Some transits have been flagged as disturbed by a `parasite', i.e. a fairly bright star which was close in transit time to that of the star considered, according to calculations based on the stars in the Tycho Input Catalogue Revision, described in further detail in Volume 4. Such transits were rejected in the astrometric adjustment, and (par tly) in the de-censoring analysis since these analyses were sensitive to outlying obser vations. They are however included in the Tycho Epoch Photometr y Annex (flagged in bit 2 of Field TT13, see Table 2.6.2) if none of the conditions (ac) discussed under `Valid and Invalid Transits for Photometr y' also caused a rejection in the astrometric adjustment. The flag was not used in the construction of median magnitudes since the median is only weakly affected by outliers, and since such transits in fact often do not suffer from any significant photometric disturbance.

Number of Transits: The number of valid transits for a given TYC entr y, including the non-detections, is denoted by Ntransits . The Tycho Epoch Photometr y Annex contains Ntransits transits for the selected stars, and the number is given in Field TH4 of the corresponding header record (see Section 2.6). The final astrometric and photometric results for each star have typically been constructed from different numbers of star transits in each case--individual transits having been used, or rejected, for the final catalogue for a variety of reasons. The number of transits used in the astrometr ic adjustment, Nastrom , is given in Field T29 of the main Tycho Catalogue. It excludes non-detections and detections affected by parasites. The number of transits used in Tycho mean value photometr y, Nphotom , is given in Field T43 of the main Tycho Catalogue. Nastrom and Nphotom are about 25 per cent less than Ntransits . The number of valid transits was slightly lower for photometr y than for astrometr y because a higher background was acceptable in astrometr y. The process of photometric de-censoring used both detections which were unaffected by parasites, and the non-detections. Therefore, for stars brighter than VT ' 10 mag with few non-detections the ratio Nphotom /Nastrom ' 0.80, while for fainter stars with many nondetections the ratio may be as large as 1.5. For median magnitudes only detections were used, including those affected by parasites, since these were too few to have any significant effect on the median.

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Format of the Tycho Catalogue
Unlike the main Hipparcos Catalogue, the Tycho Catalogue and the Tycho Epoch Photometr y Annex are not made available in printed for m, but only as a machinereadable version. The Tycho Catalogue for mat is largely identical to that of the Hipparcos Catalogue up to and including Field H39/T39. This approach is intended to facilitate use of both catalogues. Thereafter, additional Tycho Catalogue fields are included with astrometric and photometric infor mation and cross-identifications to other catalogues. Flags indicate variable stars, detected duplicity or multiplicity, or notes. Entries of the Tycho Catalogue are given in the sequence of the hierarchical TYC13 identifier (Field T1), i.e. corresponding to organisation according to the GSC (Guide Star Catalog) region number (and not in sequence of right ascension). The same conventions and units are adopted for the Tycho Catalogue as for the main Hipparcos Catalogue. Thus, definitive values of right ascension and declination are expressed in degrees and decimals of a degree, while all other astrometric parameters, and errors, are expressed in milliarcsec, even though some digits may not be significant. The catalogue epoch is J1991.25, and positions and errors are given for that epoch. Astrometric correlations are also provided. All parameters are expressed within the reference system ICRS (see Section 1.2.2). Positions are also provided in conventional sexagesimal units, with truncated precision. Similar approaches have also been adopted for the categorisation of variability data. Double and multiple star detection differs significantly between the Hipparcos main mission and Tycho, and fields dedicated to summar y double and multiple star data in the main Hipparcos Catalogue are accordingly absent in the Tycho Catalogue.

Fields T02: Tycho Identifier/Proximity Flag
Field T0: The machine-readable (main) Hipparcos and Tycho Catalogues include a character indicating whether the associated record is derived from Hipparcos (H) or Tycho (T) data. Field H1/T1 then provides the Hipparcos Catalogue number (HIP) or the Tycho Catalogue identifier (TYC) accordingly, with the interpretation of subsequent fields being, in par t, catalogue dependent (see Tables 2.1.1 and 2.2.1). Field T1: TYC identifier The designation of the object within the Tycho Catalogue uses the Guide Star Catalog (GSC) numbering system (a region number, designated here TYC1; and a number within the region, designated here TYC2) followed by a Tycho Catalogue specific component number (TYC3). As well as giving a cross-identification to an impor tant star catalogue, this designation system has the advantage of giving a rough position indication. For objects contained in the GSC, TYC1 and TYC2 are identical to the identifiers defined by the GSC numbering system.

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In the few cases where no corresponding GSC object exists close to the Tycho Catalogue object, values of TYC1 and TYC2 consistent with the GSC numbering system were created with the assistance of the Guide Star Catalog team. The component number (TYC3) gave necessar y flexibility for addition of objects during the Tycho Catalogue production discovered close to the GSC object designated by the values of TYC1 and TYC2. The TYC3 numbers have been assigned starting with 1 and increasing with the VT magnitude. Components with TYC3 > 1 are always located within a distance of 15 arcsec from the corresponding star with TYC3 = 1. The recommended TYC designation contains a hyphen between the TYC numbers, e.g. for the first star in the Tycho Catalogue the designation is `TYC 1131', corresponding to `GSC 113'. The TYC identifier of the first entr y in the catalogue is ` 1 13 1'.
The maximum value for each of the TYC numbers is as follows: TYC1 = 9537; TYC2 = 12119; TYC3 = 4. The number of entries with each of the occurring TYC3 numbers is as follows: 1 = 1 051 966; 2 = 6334; 3 = 31; 4 = 1. The entr y with 4 TYC3 components is TYC 13276061 to 4 = HIP 30075 ABCD of which the components C and D were only measured by Hipparcos.

All stars contained within the Hipparcos Catalogue but not obser ved by Tycho have also been included in the Tycho Catalogue for completeness (see Field T42) and assigned a corresponding TYC number. Field H31, which appears as the first column of the right-hand page of the printed Hipparcos Catalogue, contains the Hipparcos Catalogue (HIP/HIC) identifier. This is retained for the Tycho Catalogue, so that Fields T31 and T51 (which gives the Hipparcos component identifier, if any) provide the links between the Hipparcos and Tycho Catalogues. Field T2: Proximity flag This field provides a coarse indication of the presence of nearby objects. If non-blank, it indicates that there is one or more distinct Hipparcos Catalogue entries (or distinct components of the system if double or multiple), or one or more distinct Tycho Catalogue entries, in either case irrespective of magnitude, within 10 arcsec of the position given in Fields T89. The ter m `distinct' means simply that a proximity flag is not assigned to a Tycho Catalogue entr y cross-identified with the same (single) star contained within the Hipparcos Catalogue. The flag is assigned according to the following hierarchy (i.e. if `H' and `T' both apply, `H' is adopted): H : there is one or more distinct Hipparcos Catalogue entries, or one or more distinct components of the relevant Hipparcos Catalogue entr y, within 10 arcsec of the position given in Fields T89. This includes the cases where two Hipparcos components are merged into one Tycho entr y (see also Field T51); T : there is one or more additional Tycho Catalogue entries within 10 arcsec of the position given in Fields T89.
The number of entries in each of these categories is as follows: H = 12 812; T = 5851.

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Fields T37: Descriptor
Fields T34 provide an approximation to the position given in Fields T89, and are included as a convenient way of object identification. Fields T57 provide an approximate but rather homogeneous indication of the V magnitude in the Johnson UBV system, along with a source flag. Fields T34: Positional identifier: truncated coordinates (epoch J1991.25, ICRS) The approximate right ascension and declination are given in conventional sexagesimal units with truncated precision, for epoch J1991.25, and within the reference system ICRS. Fields T34 are rounded values of the positions given in Fields T89, and are included as a convenient way of object identification. Since the identifier is strictly constructed from the Tycho position, identifiers for entries contained in both the Hipparcos and Tycho Catalogues will not necessarily be identical. Field T5: V magnitude The V magnitude is given mainly for identification purposes (see Field T7). Nor mally this V magnitude is in the Johnson UBV photometric system, although this is not the case for some entries as specified in Field T7.
The field is blank for 3217 entries, all faint (Hp > 10.9 mag) Hipparcos double star components. Although Tycho obser vations give magnitudes in the photometric bands referred to as B T and VT , the magnitude identifier has been transformed from the available B T and VT data to a magnitude in the V band, for consistency with the information given in the Hipparcos Catalogue, using the transformations defined in Section 1.3, Appendix 4. Magnitudes are given to two decimal places, with typical systematic errors of about 0.01 mag, although much larger systematic errors are expected especially for red stars, i.e. with B - V > 1.5 mag. The standard error is usually ' 1.1 VT , but may be calculated more accurately using Equation 1.3.21. The magnitude is ver y uncer tain if Field T57 is not blank, in par ticular if it contains `M'.

Field T6: This field (coarse variability flag for HIP) is blank for Tycho Field T7: Source of magnitude given in Field T5 The V magnitude, considered primarily as a magnitude `identifier' (although with astrophysical relevance) has usually been derived from the available B T and VT data, as described under Field T5. Otherwise, if non-blank, the field has the following meaning: B : no VT magnitude is available, therefore the B T value was adopted; D : derived from approximate B T and VT magnitudes, corresponding to flag `D' in Field T36; T : only an approximate VT magnitude has been derived, as indicated by flag `T' in Field T36; in these cases, the V magnitude given in Field T5 is simply the approximate VT magnitude given in Field T34; V : no B T magnitude is available, therefore the VT value was adopted;
The number of entries in each of these categories is as follows: B = 79; D = 3314; T = 1333; V = 173.

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Fields T830: Astrometric Data
The data in Fields T830 are the same in both for mat and meaning in the Tycho Catalogue as in the Hipparcos Catalogue, i.e. position, proper motion, parallax, astrometric standard errors and correlations, and solution flags. Positions are expressed in degrees. Annual proper motion components, trigonometric parallaxes, and all astrometric errors (including those of positions) are expressed in milliarcsec. Angular measures are along a great circle, including the factor of cos for positional error in right ascension, for proper motion in right ascension, and for proper motion error in right ascension. If parameters such as the proper motion or parallax were not calculated (e.g. for a close companion having too few obser vations) the corresponding field will be blank.
The right ascension and declination are referred to the catalogue epoch, J1991.25, as described in Section 1.2.6. Corresponding standard errors, also at the catalogue epoch, are given in Fields T1418. Positions and proper motions are referred to ICRS, as described in Section 1.2.2. The astrometric positions and their errors can be propagated to the standard epoch J2000.0, or to any other epoch, within the ICRS frame, by the methods described in Section 1.2.8. [Since for many Tycho Catalogue entries the proper motion will have a standard error larger than the expected proper motion itself (µ ' 20 mas/yr for a star of spectral type F5 and V = 11 mag), a more accurate position for faint stars at a different epoch may often be predicted by assuming a zero proper motion.]

Fields T89: Equatorial coordinates (epoch J1991.25, ICRS) The right ascension, , and declination, , are expressed in degrees for the catalogue epoch J1991.25, and with respect to the reference system ICRS.
ICRS is consistent with the conventional coordinate system at J2000.0, previously realised by the FK5 Catalogue. The source of astrometric data is detailed in Field T42. If only the position is given (i.e. if Field T42 is non-blank), Fields T1113 and T1628 will be blank, and sometimes also other fields. Formal positional standard errors are still given in Fields T1415 if Field T42 = `P' or `R', although they will be less reliable than the standard errors corresponding to a full astrometric solution. If Field T42 = `H', the position given in Fields T89 is the position taken directly from the Hipparcos Catalogue.

Field T10: Reference flag for the astrometric parameters This flag indicates that the astrometric parameters in Fields T830 refer to: : a recommended astrometric reference star, having good Tycho astrometric quality, and not recognised as double; X : a dubious astrometric reference star in the context of the Tycho Catalogue.
A star is flagged as a dubious astrometric reference star either because the quality flag in Field T40 is Q 6, or because Fields T2 or T42 are non-blank, or because Field T49 = `D' or `S' indicating possible duplicity, or because Field T57 = `J', `K' or `L' indicating dubious astrometr y. The case where Field T42 contains `H' (Hipparcos entr y not obser ved by Tycho) is systematically classified as `dubious' within the context of the Tycho Catalogue, even though it could be a well-behaved, single object within the Hipparcos Catalogue. The number of entries in each of these categories is as follows: = 886 621; X = 171 711. The catalogue thus contains nearly 900 000 recommended astrometric reference stars.

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Field T11: Trigonometric parallax The trigonometric parallax, , is expressed in units of milliarcsec. The estimated parallax is given for ever y star, even if it appears to be insignificant or negative (which may arise when the true