Download first results of the qa_2plates procedure

THE QUALITY ASSURANCE OF THE GSCII DATABASE:
FIRST RESULTS OF THE QA_2PLATES PROCEDURE
by
R.Pannunzio, R. Morbidelli, M. G. Lattanzi and A. Spagna
Osservatorio Astronomico di Torino
Internal Report no. 57/01
Pino Torinese, February 2001
ABSTRACT
This Report is the continuation of the study described in the previous OATo Internal Report no. 53/00, in
which were showed the procedures used for the evaluation of the reliability of the data produced both by the
pipeline and by the matching procedure on the GSCII plates processed at Torino.
In particular, preliminary results, obtained by selected samples of GSCII plate pairs of the CRA list, loaded in
our local COMPASS Database and processed with the QA_2PLATES procedure (see Internal Report no.
53/00), are given in this Report.
The first statistical results obtained on the whole sample of data produced by the QA_2PLATES till now
seem to show a good agreement with the expected astrometric and photometric accuracy for most pair of
plates.
The goodness of these results is a preliminary indication that both pipeline and object matching procedures are
reliable even if future astrometric and photometric calibrations could improve the astrometry and photometry
in order to give to scientific community the best that can be obtained from the analysis of these GSCII plates.
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INTRODUCTION
Some months ago at the Astronomical Observatory of Torino (OATo), an IDL procedure called
QA_2PLATES has been realized (see OATo Internal Report no. 53/00).
The purpose of this procedure was the extraction of the parameters of two overlapping plates loaded in our
local COMPASS Database, by means of a C++ interface (CIQA), in order to perform a selected series of
statistics with the parameters obtained by the previous steps of the GSCII project (pipeline and object matching
procedures).
The QA_2PLATES procedure has been refined and checked several times in OATo and now is in exportable
version. Recently a copy of this procedure has been delivered and implemented in the COMPASS database of
the Space Telescope Science Institute (STScI) of Baltimore (U.S.A.).
PRELIMINARY QA RESULTS ON GSCII PLATES REDUCED AT TORINO
The output of the QA_2PLATES procedure are essentially reports files and statistical graphics on the results
of the pipeline and object matching of the plates processed and contained in the local COMPASS Database.
In order to have a global check about the quality of the results furnished by the QA_2PLATES procedure we
have realised a mock-up IDL software (GLOBAL_QA) that collects all the information contained in the report
files of the previous software. The GLOBAL_QA software works on the basis of the structure of the output
produced in batch mode by the QA_2PLATES procedure.
On the other hands, the QA_2PLATES runs on all pair of plates starting from a list of source plates.
The batch procedure finds all the overlap plates of each source plate and for each pair performs all the
statistics foreseen by the routines of the QA_2PLATES.
For each examined pair of plates this code creates a directory of first level labelled with the name of the source
plate and each ones of these directories gives origin to a series of subdirectories of second level labelled with
the names of the overlap plates, where each ones of these contains all the report files and figures produced by
the routines of the code.
In this context the QA_2PLATES has been ran in batch mode on a sample of 67 GSCII source plates of the
CRA list.
For each considered source plate, several overlapped plates have been found and for all 67 source plates, 840
combination of plate pairs are resulted. In this context we want to remark that in our COMPASS Database, at
present, not all the overlap plates are connected with the GSCII project, because at the beginning the Database
has been populated with GSCI plates only , while the GSCII plates are loaded sequentially only after the
pipeline processing.
The fist result obtained by this software has put in evidence that not all the plate pairs have given reliable
results both in pipeline and in the objects matching, for which in some cases the parameters extracted from the
Database are resulted completely wrong (null values or inconsistent values). In a such way the plate pairs
having reliable results are 708 only.
If we take into account the GSCII pair of plates only , the number furthermore decreases to 335 pairs.
All the above numerical considerations are reported in the following Table 1. In addition the same Table 1
also gives the number of pair of plates having same area but different colour (usually plates quasi-completely
overlapped) or plates having same colour but different area (usually contiguous plates in the same pass-band
with a small percentage of overlap ) .
With the same criteria the combination of pair of plates having both different colour and area or having same
colour and area, are taken into account. For each one of these combinations in Table 1 two columns ,are
reported .The first column gives the global number of coupling (GSCI and GSCII plates), while in the second
column the GSCII coupling only, are shown .
How it is possible to see from Table 1 the last four particular combinations are much reduced in number with
respect to the total amount of data but however are sufficient to put in evidence possible systematic and
accidental errors in astrometry or photometry depending on the colour or on the percentage of overlap.
Even if the number of reliable plates is only 708 against 840 (see column 1 of Table 1 ) the effective number
of wrong plates is lower, because usually a wrong plate is combined with several overlapped plates.
The adopted criteria for the selection of reliable plates can be summarized in the following points:


only pairs of plates having at least 50 matched objects have been selected
the pairs of plates with () , () and (mag) having null values or values greater than 2 arcsec or
2 mag for both stars-stars and non stars-non stars combination of classifications, have been rejected
3
We remember that all the astrometric statistics performed by the QA_2PLATES and consequently by the
GLOBAL_QA are in standard coordinates. In addition when in the text is indicate  ,  and mag we want
to remark that these variables are the average values of all the differences in  ,  and mag between each pair
of matched objects of a single pair of plates (intended as source – overlap plate), while the () , () and
(mag) are the standard deviations with respect to the average values of  ,  and mag for a single pair
of plates.
On the basis of the constraints above quoted only two GSCII plates are really wrong, in particular the plate
A343 (survey : XO323) and the plate G0R9 (survey : S020), while the other wrong plates are combination of
GSCI plates (usually with larger errors and few objects) with GSCII plates (more accurate and populated) .
Table 1
The sample of processed plates
•NUMBER OF SOURCE PLATES
067
•NUMBER OF PLATE PAIRS
840
•PAIRS OF GSC I-II/GSCII PLTS WITH RELIABLE RESULTS
•PAIRS OF GSC I-II /GSCII PLTS WITH SAME COL.DIFF.AREA
•PAIRS OF GSC I-II /GSCII PLTS WITH SAME AREA DIFF.COL.
•PAIRS OF GSC I-II /GSCII PLTS WITH DIFF.COL.- DIFF.AREA
•PAIRS OF GSC I-II /GSCII WITH SAME COL. AND SAME AREA
708
335
127
059
091
064
479
212
011
000
The first figure gives the spatial distribution of the source plates with their overlap plates on a planisphere in
galactic coordinates. How it is possible to see in Fig. 1 the source plates (+) are preferentially distributed
around the galactic plane and to the galactic poles together to the corresponding overlap plates (). In this way
we have the possibility to compare the accuracy of the QA results in two peculiar part of the sky.
GALACTIC REPRESENTATION
OF THE GSCII PLATES AT OATO
Source (+)
Overlap ()
FIG. 1
In the following two histograms of Fig. 2 are presented the distributions of the outliers percentage with respect
to the whole matched objects found in each pair of plates both on the whole sample of reliable plates
(GSCI+GSCII) and on the GSCII pairs only, respectively.
4
We have considered as outliers all the pair objects whose ,  and mag in absolute value are greater than
4 arcsec and 4 mag.
The analysis of the first histogram of Fig. 2 shows that most outliers are confined in less of one percent of the
matched objects, while a residual part of plates has a percentage included between 1% and 5%. The situation
is decidedly better for the GSCII plates only, because there are very few outliers with a percentage greater than
1% . The explanation of the low outliers percentage in the GSCII pairs with respect to the whole sample of
plates is in the different procedure of the astrometric and photometric calibrations adopted for GSCI and
GSCII plates.
Percentage of outliers
FIG. 2
Comparison GSCI + GSCII and GSCII plates
( case ) stars – stars
FIG. 3
5
In order to have reliable statistics from all the parameters collected by the report files of the QA_2PLATES
procedure we have decided to take into account only the parameters connected to the stars-stars and non
stars-non stars combination of classifications.
The histograms of Fig. 3, show the frequency of the plate pairs both for all the whole sample of plates (GSCI
and GSCII plates) and for GSCII plates only as a function of the () found for the stars-stars combinations
on each pair of plates .
In both cases the average () is about 0.5 arcsec while their standard deviation () are about 0.2 arcsec
even if the global distribution (left histogram) seems slightly better than the GSCII distribution (right
histogram).
We want to remark that the average () are approximately larger than the average () of a single plate by a
factor 2, because the average () is the combination of the average () of the source and overlap plates.
Of course the same conclusions seen above can be extended to the average () and (mag) and to the
() of the same parameters too.
Comparison GSCI + GSCII and GSCII plates
(  case ) stars – stars
FIG. 4
A similar distribution is shown in Fig. 4 in which are represented in abscissa the ().
Comparing Fig. 3 with Fig. 4 we can remark that the () are slightly better than the corresponding ()
both for the left and for the right histogram. The greater accuracy showed in the  coordinates with respect to 
is explained by the fact that in  there are probably residual errors in the telescope driving.
If we consider the same distributions of Fig. 3 but referred to the non stars-non stars pairs we have the
situation shown in Fig. 5.
In this case the average () are practically coincident in both histogram but if we compare these values
with those of Fig. 3 we note that the average values of the non stars-non stars are about one and half times
greater than the corresponding stars-stars pairs.
The reason of this is probably due to the irregular shape of this images that in many cases are not perfectly
circular.
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are very different from the corresponding values of Fig. 3 (0.7 arcsec against 0.5arcsec
Comparison GSCI + GSCII and GSCII plates
( case ) nostars – nostars
FIG. 5
In order to have a direct comparison of the previous figures, in Fig. 6 the distribution of the () for GSCII
plates only for stars-stars and non stars-non stars combination, are repeated. In this case it is clear that the
average () is better for the stars-stars combinations for the reasons above seen, but the comparison with
the first plots of Fig. 3 and 5, where the whole sample of plates (GSCI + GSCII) has been considered, shows
no evident variations .
Comparison GSCII plates
( case ) stars/stars and non stars/non stars
FIG. 6
7
Comparison GSCII (  )
star-star case, same color
FIG. 7
Fig. 7 shows the distribution of the () and () for GSCII plates only, for stars-stars combination but
referred to plates having same colour and different area . The average values of the two histograms shows a
better accuracy in the  coordinates as already seen in the above statistics. On the other hands when
overlapping plates of the same colour are considered, the percentage of overlap between this kind of plates is in
general small, as the most plates considered in our statistics. In fact a large number of plates pair has different
area and colour, and consequently a small percentage of overlap (see Table 1), for which the statistics of Fig. 7
are very similar to the previous one on the global data.
Comparison GSCII ( )
star/star and non-star/non-star case
same color
FIG. 8
Fig. 8 represents the distribution of the () for the GSCII plates only having same colour but different area
for the stars-stars and non stars-non stars combination.
8
The results of these statistics are very similar to those obtained with the previous global statistics for the same
reasons given for the Fig. 7. In particular is confirmed that the stars-stars combination is slightly better than
the non stars-non stars combination.
A very interesting statistics is shown in Fig. 9 in which are compared the distributions of () for the GSCII
plate pairs of the stars-stars combinations both for plates having same area but different colour (left
histogram) and plates having same color but different area (right histogram)
Comparison GSCII ( ) star/star case
same area/diff. color and same color/diff. area
FIG. 9
It is clearly evident that the first histogram of this figure (different colour and same area) has an average ()
lower than the corresponding average () of the second histogram (same colour and different area). The
motivation of this appreciable difference is probably due to the fact that the astrometric accuracy is function of
Comparison GSCII ( ) non-star/non-star case
same area/diff. color and same color/diff. area
FIG. 10
9
the position on the plates of the matched objects. In other words, matched objects near to the plate centres
show a lower dispersion of the () or () with respect to the same dispersions of the matched objects at the
edges of the plates, probably due to residual distortions far away by the plate centres, not completely corrected
by the astrometric calibrations.
This situation is really verified when the plates are partially overlapped, namely when the plates have same
color but different area.
In Fig. 10 the same distributions of Fig. 9 are shown, but referred to the non stars-non stars classification.
The average () in both histograms are very similar among them and consequently seems that the
astrometric accuracy is independent by the colour of the plates or by the percentage of overlapped area.
A possible explanation of this should be in the low astrometric accuracy found in the position of the non
stellar objects that hides the greatest accuracy usually given by the plates with a total overlap with respect to
the other ones with a partial overlap (even if of the same colour).
As far as the standard deviations by the average () or () are concerned, it is possible to see that in all
the previous figures the () are all included in the range 0.15 – 0.20 arcsec. This indicate that the astrometric
quality of all the considered plates is comparable.
If we take into account the statistics on the average (mag) for the stars-stars case both for all the sample of
plates and for the GSCII plates only , we have the distribution represented in the following two histograms of
the Fig. 11, respectively
Comparison GSCI + GSCII and GSCII plates
( magnitude) star–star
FIG. 11
The average values of (mag) are practically coincident in both distributions around to about 0.5 mag
while the () of about 0.2 mag is slightly better on the whole sample of data.
A little bit different is the distribution of the average (mag) of Fig. 12 . The histograms show the
comparison between the whole sample of plates and the GSCII plates only for non stars-non stars pairs.
The average value of (mag) is greater than the values of the corresponding histograms of Fig. 11 .
10
The motivation of this lower accuracy is still due to the shape of the images that for non stellar objects is
much irregular with the consequent imprecision in the magnitude determination.
Comparison GSCI + GSCII and GSCII plates
( magnitude) non-star/non-star
FIG. 12
The first histograms presented in Fig. 13 is referred to statistics conduced on the (mag) of the GSCII plates
only for the stars-stars combination of the plates having same colour but different area, while the second
histogram gives the same statistics but for plates having same area and different colour.
It is evident that the plates of the same colour show an accuracy on the average (mag) double with respect
to the plates having the same area.
The reason of this difference between the two histograms depends on the fact that the accuracy in magnitudes
is correlated with the colour of the plate pairs .
On the other hands if the plate pairs have the same colour, the photometry of the objects of the source and the
corresponding objects of the overlap plates should be theoretically equal , apart the accidental errors , while for
plate pairs having different colour each matched object shows a colour index depending on the intrinsic colour
of the objects and on the colour difference between source and overlap plate.
If we take into account that these systematic effects in (mag) are alternatively positive and negative (in the
case where the source becomes the overlap plate and the overlap becomes the source) the average value of
(mag) will be greater than the corresponding value of the plate pairs with the same colour.
11
Comparison GSCII ( mag.) star/star case
same color/diff. area and same area/diff. color
FIG. 13
The same statistics of Fig. 13 are conduced in Fig. 14 where the non stars-non stars case is represented.
Also in this situation the average (mag) of the first histogram has an accuracy twice as much with respect to
the same value of the second histogram.
Comparison GSCII ( mag.) non-stars/non-stars case
same color/diff. area and same area/diff. color
FIG. 14
12
The grater accuracy in the plate pairs of the same colour with respect to those with the same area are
practically independent by the object combinations. Furthermore, in this case, comparing the (mag) of the
non stars-non stars combination of Fig. 14 with the corresponding values of Fig. 13 it is possible to see how
the accuracy of the stars-stars coupling is always better of the accuracy of the non stars-non stars pairs for
the same motivations seen before.
Comparison GSCII plates()
star/star and non-star/non-star
FIG. 15
Another interesting set of statistics is the distribution of the average values of the ,  and mag of all
GSCII plate pairs. From the theoretical point of view if for each pair of plates A-B (in the sense A= source
plate and B = overlap plate) there were the corresponding coupling B-A the distributions of the ,  and
mag should have a mean value equal to zero whatever are the astrometric and photometric errors of the
plates.
On the contrary the astrometric and photometric accuracy can be put in evidence looking at the dispersion of
the ,  and mag around the null value.
However the  deduced by these distributions in some cases is not realistic because if the astrometric
calibrations are made using different catalogues there is the possibility that the systematic effects introduced
by these inhomogeneous catalogues can enlarge the distribution and consequently the .
A similar situation is also verified for the photometry because not all the pair of plates have the same colour
and in addition not always the same photometric catalogue has been used for calibration. The consequence of
these two conditions produces an enlargement of the  with respect to normal conditions (plates of the same
color calibrated with the same photometric catalogue) .
In order to have homogeneous material the following statistics have been performed with the GSCII plates
only. However not always the astrometric and photometric calibrations have been made with the best
astrometric and photometric catalogues such as the ACT and the TYCHO2/GSPCII catalogues respectively.
The statistics of Fig. 15 show the distribution of  found on each GSCII plate pairs, for the stars-stars (left
histogram) and for the non stars-non stars (right histogram) combinations respectively.
It is possible to see from this figure that the average  in both histograms are very close to zero, while the  is
about 0.4arcsec in both distributions . In effect the histograms of the figure are very far from a gaussian
distribution, because the peaks are very sharp and consequently the standard deviations  result much larger of
the real mean dispersions of the histograms
.Probably in the tails of the histograms are included the values obtained by different catalogs or by pair of
plates with a low percentage of overlap and consequently with appreciable systematic and accidental errors.
13
Comparison GSCII plates (and)
star/star
FIG. 16
If we compare the distribution of the  and  for the stars-stars combinations we have the situation
presented in the two histograms of Fig. 16 .
It is clearly evident that the average values are centred around the zero in both histograms, while the .is two
times more accurate in  distribution . Probably the lower accuracy in  is due to the driving errors of the
telescope as already seen in previous statistics.
Comparison GSCII plates()
star/star diff. color/same area and same color/diff. area
FIG. 17
14
In Fig. 17 are presented the statistics of the  for the stars-stars combinations of GSCII pair plates having
same colour and different area (left histogram) and same area and different colour (right histogram).
Both distributions are centred around the zero value but the dispersion of the data in the left histogram is about
four times greater than the dispersion of the right histogram. From this figure we have another confirmation
that the astrometric accuracy is strongly dependent on the position of the overlap zone with respect to the
centre of the source plate. In fact the astrometry is better for plates having same area (with a percentage of
overlap close to 100%) even if of different colour.
Comparison GSCII plates()
non-star/non-star diff. color/same area and same color/diff. area
FIG. 18
Comparison GSCII plates( mag )
star/star same color/diff. area – diff. Color/same area
FIG. 19
15
Similar situation is repeated in Fig. 18, where the same statistics of Fig. 17 are reported, but referred to the non
stars-non stars combinations.
If we compare these histograms with those of Fig. 17 we can remark that the plates having same area in both
figures have an astrometric accuracy about four times greater than the corresponding accuracy of the plates
having same colour but different area. In addition the standard deviations of the first histograms of Fig. 17 and
18 as the second histograms of the same figures show a slight and strange preference for the non stars-non
stars combinations.
Another interesting statistics is shown in Fig.19 where the distribution of the mag of the stars_- stars
combination of the GSCII pairs are plotted both for plates having same colour (left histogram) and same area
(right histogram).
The analysis of this figure shows how the accuracy in mag is strongly dependent by the colour of the plate
pairs. Indeed in the case of plates with same color different area the distribution of the data is centred around
the zero with a  of about 0.3 mag, while in the second histogram where the data are referred to plates with
same area but different colour it is possible to see that the average value is always centred around the zero
but the distribution shows two symmetric peaks at about 1.2 mag before and after the null value.
The explanation of the first histogram it is easy , because if the plates have the same colour the difference in
magnitude between source and overlap plate on the same objects should theoretically be zero .Of course this
situation is never verified and consequently the  of the distribution put in evidence the accidental errors and
the .residual systematic errors due to the adoption of different photometric catalogues for the calibrations of the
plate pairs.
Completely different is the situation of the second histogram of Fig. 19 . The symmetry of the two peaks with
respect to the zero indicates that in the considered sample of GSCII plates each source plate becomes an
overlap plate and vice-versa, with the consequence that the shift of the peaks of about 1.2 mag symmetric to
the zero is due to the different pass-band (usually Blue Band (J) and Red Band (F)) existing between the
source and overlap plates covering the same area of the sky.
Because the peaks are placed to +/- 1.2 mag it means that in general most stars of these plates should have a
color index (B-R) of 1.2 mag that corresponds to stars of the late main sequence ( spectral type G and K)
Same considerations can be deduced by the histograms of Fig. 20 where are repeated the statistics of Fig. 19
but for the non stars-non stars combinations . In fact the distributions of these two last figures are very
similar among them even if the dispersions in both distributions of Fig. 20 are slightly larger than the
corresponding ones of Fig. 19. Also in this case the more frequent spectral types of these non stellar objects
seem to be very similar to those of the stellar objects. Probably in some cases stellar objects have been
classified as non stars .or vice versa However in these last series of figures in particular for the cases of plate
pairs having different colour the (mag) of a distribution very far from a gaussian curve has not a realistic
meaning on the goodness of the photometry and consequently can be used for comparison with similar
distributions, only.
Comparison GSCII plates( mag ) non-star/non-star same
color/diff. area – diff. Color/same area
FIG. 20
16
In the next figures accurate statistics, performed with steps of one magnitude on the same variables of the
previous figures, are reported
In the two distributions of Fig. 21 the average  and the corresponding standard deviations  for steps of one
magnitude as a function of the magnitude of the source plates both for stars-stars (blue filled line) and for non
stars-non stars (red dashed line) of the GSCII plates only, are shown. In the same figure with a parallel line to
the abscissa and with the same graphic symbols and colours ,the global value seen in the previous statistics ,are
drawn The distribution of  of the first graphic shows that the values are very closed to zero for each step in
magnitude both for the stars-stars and non stars-non stars combinations even if there is a small positive trend
for the stars-stars combinations with the increasing magnitudes, while for the non stars-non stars
combinations the trend is negative. More interesting is the distributions of the () of each step of magnitude
(right graphic) in which is evident the constant increasing for both distributions with the increasing of the
magnitude. In particular the minimum () equal to 0.2 arcsec is reached around the magnitude 11-12 that
roughly corresponds to the magnitude limit of the astrometric catalogues used for the calibrations, while for the
magnitudes close to the plates limits (20-21 mag) the () is about two times. Another particular feature of
this plot is the comparison of the trend of the stars-stars line with the trend of the non stars-non stars line.
These lines are practically parallel for quasi all the range in magnitudes and the line of the non stars- non
stars is more accurate of about 0.1 arcsec with respect to the stars- stars line.
FIG.21
21
Fig. 22 shows the same statistics of Fig. 21 but referred to the () and ().
The first plot of this figure shows that both distributions () are very close to the zero practically for all the
objects below the magnitude 19-20.
The second plot of this figure shows a rectilinear trend for the stars-stars combination for all the range in
magnitude, while for the non stars-non stars objects the trend is variable but however slightly better with
respect the stars-stars objects
Fig. 23 shows the distribution of the (mag) and (mag) respectively for steps of one magnitude. of the
GSCII plates.
The trend of the first plot shows that the average (mag) is practically close to zero until magnitude 19-20 but
for magnitudes fainter the trend for the stars-stars case and non stars-non stars case increases exponentially
up to magnitude 2.
As far as the (mag) is concerned it is possible to see that this value has a quasi constant increasing trend
toward fainter magnitudes starting from 0.6 mag for brighter objects until to 1.1 mag for magnitudes around
the values 20-21.
17
FIG. 22
FIG. 23
18
FIG .24
Another interesting set of statistics are those presented in the next figures 24 – 25 – 26.
These figures show the standard deviations of the average values of the variable () , () and (mag)
found in each step of magnitude both for the stars-stars case and for the non stars –non stars case and both
for the GSCII plates having same area but different colour (left plots) and for GSCII plates having same
colour but different area (right plots).
Fig. 24 shows the comparison of the () trends for step of one magnitude of the GSCII plate objects.
The first impression looking at both plots is that all the objects pairs show a decreasing trend of the  towards
fainter magnitudes when the plate pairs are quasi completely overlapped (left plot) but are of different colour,
while in the right plot, where the plates are only partially overlapped, but are of the same colour, the trend of
the  for both object combinations is increasing towards fainter objects.
FIG. 25
In other word the standard deviation in () seems in general equal in both plots for brighter objects (about
0.2 arcsec at magnitude 10-11) , while become more accurate for faintest objects (0.1 arcsec at magnitude 20
19
–21) in the case of plates quasi completely overlapped. In addition becomes less accurate for fainter objects
(0.5 arcsec at magnitude 20 – 21) when the plates of the same colour are only partially overlapped.
This probably means that usually in the centre of the plate, independently by the colour and by the magnitude,
the standard deviation in () is better with respect to the edges.
FIG. 26
On the contrary Fig. 24 shows that the () is more accurate on the faint objects for plates completely
overlapped, while is true the contrary for plates fairly overlapped with the same color where the best results on
this coordinate are given for the brighter objects.
Furthermore in this figure and for both plots the stars-stars and non stars- non stars pairs show similar trend
as a function of the magnitude even if the () of this last series of objects seems slightly better
Fig. 25 shows the same distributions of the Fig. 24 but referred to the standard deviation of the average ()
for steps of one magnitude. If we compare this two last figures it is clearly evident that in general the standard
deviation in () are slightly better than the corresponding (). However in both plots of this last figure the
trend is always increasing toward fainter magnitudes where was decreasing in the first plot of the previous
figure..
In Fig. 26 the distribution of the standard deviation of the average (mag) for steps of one magnitude on the
two kinds of object combinations of GSCII plates having same area (first plot) or same colour (second plot),
is presented.
The first plot shows that the (mag) is about one magnitude for brighter objects and attains about 1.4 mag
for fainter objects , while the situation is decidedly better in the second plot where the standard deviation is
only 0.2 mag for brighter objects and 0.7 mag for fainter objects
The greater accuracy of the (mag) of the second plot with respect the first is mainly due to the same color of
the considered plate even if the percentage of overlap between the plate is not high. .In other words the
accuracy of the photometry between two overlapping plates is strongly dependent by the the color of the plate,
in the sense that plates having same colour gives best (mag) even if the percentage of overlap is lower of
the plates having same area but different colour.
In order to give a global vision of the results obtained in this Report a summary of the statistical results in
Table 2 is reported
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Table 2
Results from GSCII plate pairs
STARS-STARS

MEAN VAL.GSCII ALL


 mag



 mag
+0.00 +0.55 -0.00 +0.49 +0.01 +0.47
+0.00 +0.72 -0.00 +0.69 +0.01 +0.59
+0.42 +0.20 +0.23 +0.17 +1.01 +0.20
+0.38 +0.18 +0.19 +0.17 +1.00 +0.23
MEAN VAL. GSCII SAME COL.
+0.02 +0.50 -0.00 +0.44 +0.04 +0.27
+0.02 +0.67 -0.01 +0.61 +0.03 +0.32
 OF GSCII
+0.51 +0.18 +0.22 +0.15 +0.27 +0.08
+0.48 +0.20 +0.26 +0.17 +0.34 +0.12
MEAN VAL.GSCII SAME AREA
+0.00 +0.38 +0.00 +0.36 +0.00 +0.54
+0.00 +0.64 +0.00 +0.63 -0.00 +0.61
 OF GSCII SAME AREA
+0.15 +0.16 +0.17 +0.15 +1.30 +0.20
+0.13 +0.20 +0.15 +0.20 +1.27 +0.18
MEAN VAL.GSCII DIFF.AREA - COL
+0.00 +0.61 -0.00 +0.54 -0.00 +0.51
+0.00 +0.76 -0.00 +0.72 +0.00 +0.65
 OF GSCII DIFF. AREA - COL.
+0.44 +0.17 +0.24 +0.16 +1.04 +0.19
+0.39 +0.15 +0.19 +0.16 +1.03 +0.20
 GSCII
DATA

NON STARS - NON STARS
ALL DATA
SAME COL.
CONCLUSIONS
To give a definitive answer concerning the astrometric and photometric accuracy of the processed plates
discusses in the present Report is not easy for a different series of reasons.
The main reasons that could introduce a degradation in the astrometric and photometric accuracy are
summarized as follows:
 The sample of plates is inhomogeneous in the sense that in some statistics have been considered plates of
the GSCII projects overlapped with GSCI plates
 The global statistics have been performed on plates taken both in galactic plane and in the galactic pole
and consequently with a different number of celestial objects depending on the region of the sky
considered
 The objects matching has worked on plate pairs having different band-pass or different percentage of
overlapping area
 The pipe line not always has run with the same astrometric or photometric catalogs for each pair of
matched plates
 The classifier probably not always has correctly identify the objects depending on the quality of the plates
or by the sky conditions among the overlapping plates or by the density of objects per plate (two narrow
stars sometimes are classified as non stars objects) or by the driving effects of the telescope and so on
 The object matching not always finds on two contiguous plates the same objects , especially when the
fields of the sky are crowded of objects
 For selected statistics the number of pair plates in some cases is hardly sufficient to have reliable results
However in order to perform reliable statistics we have subdivided the whole sample of plate pairs in sets of
homogeneous material both for verify some features of the plates processed and to put in evidence the true
astrometric and photometric accuracy.
The preliminary conclusions about the accuracy obtained by the statistics can be summarized in the following
points:

The average standard deviation of the  ()and  () of each plate pairs is always better in all kinds of
statistics and in both coordinates for the stars-stars cases than for the non stars-non stars ones.(see the 
of Table 2 for a quick comparison). This expected results depends on the shape of the objects images that
in the non stars-non stars case are more irregular than in the stars-stars case. The same conclusions can
not be derived if we take into account the standard deviation of the average ()and () residuals of
each pair of plates, where the  values are comparable in both coordinates .The reason of this is due to the
residual systematic effects ,positive and negative of the double plate combinations (example: A-B and
B-A) .In this case the distribution results enlarged more or less in same way for all the kind of objects
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





The average standard deviation of the (), in all kind of statistics, is always better than the
corresponding standard deviations of the () independently by the object classifications.(see Table 2) A
possible explanation of this is in the driving errors of the telescope in right ascension
The astrometry attains in our statistics the maximum of the accuracy for any kind of classification when
the plates are fully overlapped even if of different colours This means that from the astrometric point of
view the position of the objects, as expected, are better at the centre of the plates with respect to the edges.
In fact contiguous plates of the same colour shows a worse astrometry probably because in the overlap
area at the edge of the plates there are some residual effects not corrected by the calibration model
On the contrary the photometry attains the maximum of the accuracy when the considered plates are of the
same color and this usually happens when the plates are contiguous with a partial overlap .Evidently the
distortion of the images to the edges of the plates don’t deteriorate the photometric quality of the objects
both stellar and non stellar
As expected the average values of  , and mag of all GSCII pairs are centred around the zero
independently by the colour, by the overlap percentage and by the object classifications for the reason that
each source GSCII plate becomes an overlap plate and vice versa
The astrometric accuracy as a function of the object magnitudes shows in general a better agreement for
the brighter magnitudes corresponding to the average magnitudes of the catalogues used for calibration.
This situation is particularly evident when are considered GSCII plates quasi completely overlapped and
with different colour. In fact the standard deviation in  and  for steps of one magnitude shows
average value of about 0.15 arcsec: This encouraging result indicate that probably the astrometry in the
central part of the plates should be slightly better than the value above quoted.
The photometric accuracy as a function of the object magnitudes shows in general a better agreement for
the brighter magnitudes corresponding to the average magnitudes of the catalogue used for calibration.
This situation is particularly evident when contiguous GSCII plates of the same colour with a small
overlap percentage, are considered. In this case the standard deviation in mag for steps of one magnitude
shows minimum values of 0.2 – 0.3 mag for objects brighter than 15 mag and for both objects
classifications . This results indicate that the standard deviation in mag are better ,as expected, for plates
having same colour even if the objects are in the edge of the plates
If we want to give a global evaluation of the results obtained by this limited sample of plates , it is possible to
say that in general both the pipeline and the object matching have furnished consistent data for most considered
plates. In fact the astrometric and photometric accuracy is that expected by the measurement of plates obtained
with Schmidt telescopes.
In particular the astrometric accuracy obtained from the objects placed near the plate centres and the
photometric accuracy of the objects taken on plates with the same colour gives a further confirmation that the
plates are in general of good quality and the used calibrations are reliable for all the plate size.
On the other hands if we consider that all the given standard deviations are relative to the differences between
the variables of the source and overlap plates, then the true standard deviation on the coordinate positions and
in the magnitude of a single plate will be better of a factor 2 with respect to the values reported in the present
paper
In addition the analysis of the results has shown that also the object classifiers has generally well worked
because the astrometric accuracy of the non stars-non stars objects , as expected, is worse of the
corresponding accuracy of the stars-stars objects.
FUTURE WORKS
When all the GSCII plates of the CRA list (about 400 plates) will be processed and analysed with the software
explained in this Report , it will be possible to give, in the next Report, for each considered plate, with a simple
algorithm based on the analysis of the obtained standard deviations ,an index of the accuracy both in the
position and in magnitude.
The software used for the present work is only a preliminary rough version , but should be improved and
integrated with other statistics in an exportable version in the next future when the sample of plates will be
more consistent.
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REFERENCES
PANNUNZIO R., MORBIDELLI R., SPAGNA A., 1999, Statistical studies on the results obtained with
the GSC-II pipeline at ESO/ECF
and STScI, Publ. OATo, Internal Rep. No. 47, 34 pgs
PANNUNZIO R., MORBIDELLI R., SPAGNA A., 1999, Q.A. Procedures in the GSC-II Project, Proc.
GSC-II Annual Meeting, Garching
(Germany), Oct 25-26, 1999
PANNUNZIO R., MORBIDELLI R., SPAGNA A., 2000, Astronomical Quality Assurance of the GSC-II
COMPASS Database (Phase 1),Publ. OATo, Internal Rep. No. 53, 31 pgs
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