[vsnet-chat 5965] Re: Crude USNO B1.0 B1-R1 to B-V transformation

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

Re: Crude USNO B1.0 B1-R1 to B-V transformation

   John Greaves has provided the information (proven to be forwardable to
the list).

===

USNO A blue and red are often used as estimators of colour for stars. 
This despite no guarantees that the observations were 
contemporaneous.  Though this is not a problem for constant stars, it 
can of course be a problem for variable stars.

USNO B1.0 is newly photometrically calibrated with respect to Tycho2 
BT and VT photometry.

A May 2002 copy of loneos.phot, a critically compiled list of 
photometry by Brian Skiff, was used as a source of Johnson V and B-V 
photometry.

A 'list' of coordinates was generated using it and the USNO B1.0 was 
investigated via the CdS VizieR service, selecting objects within a 
search radius of 1 arcsec where data for both B1 and R1 was available 
(technically, where both B1 and R1 were non-null).

ASSUMPTIONS:

i) B1 and R1 are contemporaneous POSS I observations (same night 
'back to back' exposures).  This has been the case in every instance 
that I have double checked over the past few months via also 
requesting USNOFS images for various tasks.  I have no concrete proof 
of this always absolutely being true, unfortunately.

ii) Despite differences in source astrometry, within 1 arcsec matches 
are assumed to be real matches and that any false cross 
identifications would only result in general scatter and not affect 
any trend within the data.  Indeed, a large number of false matches 
would simply lead to a mess in the intended plot.


DATA RETURNED

The loneos file consisted of ~ 33,000 objects, 500+ entries were 
removed due to having no B-V value recorded.

Only single matches were kept, which resulted in the manual deletion 
of about a dozen "double hit" objects.

This resulted in 22,500+ matches with USNO B1.0 B1-R1 data.

An arbitrary decision was made to investigate only B1-R1 values, once 
calculated, between 0 and 4.  This led to the removal of very nearly 
600 negative B1-R1 values (including a dozen ridiculously large ones 
of up to -8) and about a dozen B1-R1 values between +4 to +10.

The final sample consisted of 21,911 objects.

Plotting loneos B-V against B1-R1 led to attached graph BVBR.gif 
(interference effects due to the number of points have led to 
artificial patterning in the plot).

A linear fit gave B-V = 0.235 + 0.411(B1-R1) with correlation 
coefficient, R^2, of 0.59.

Adopting a zero intercept gave B-V = 0.556(B1-R1) with a correlation 
coefficient, R^2, of 0.50.  (The white line in the graph).

Personally, I feel this difference in fit is sufficiently small to 
feel happy about adopting the latter, considering the large scatter 
in any case.

B-V values were calculated from B1-R1 using this factor of 0.556 and 
an 'observed minus calculated' plot derived, which was plotted 
against loneos V, the attached graph 'OminusC'.

Three points deserving comment can be seen from the graph.

i) the few objects with V between 8 and 10 have very little scatter.  
This is simple enough to explain.  These objects are probably seeded 
from Tycho2 and B1-R1 are in fact direct from BT and VT.

ii) the relationship between B1-R1 and B-V seems to be _remarkably_ 
independent of magnitude from bright to faintest, except...

iii) ...there is a strange kink in the O-C plot from around V = 11.5 
to about 12.5, centred on V =12, in the sense that calculated B-V is 
just that bit redder than true B-V relative to other magnitudes.  The 
kink indeed seems to show a definite structure/trend, arcing out and 
returning to the general trend in a symmetrical manner.  Whether this 
represents the point where images just start to be overexposed, with 
a swap from one type of calibration method to another, or the 
practical lower limit of Tycho2 (there are known problems with Tycho2 
at the faint end, which is about here), or something else altogether, 
I do not know.

The mean of these approximately 22,000 O-C values is 0.05 and the 
standard deviation is 0.24.

0.05 isn't too bad an offset, so in general the relationship

B-V = 0.556(B1-R1) +/- 0.24

allows USNO B1.0 B1-R1 'colour' to be used to derive an approximate 
value for B-V.

Blue, 'early' type objects and red, 'late' objects should be easy 
enough to distinguish, though 'yellow' objects will still be 
problematic because the errors are large enough for them to be 
actually blue or red, and this over and above the fact that yellow 
objects can in any case be blue objects disguised by reddenning.

In any case, despite some small number of ridiculous results which 
are readily discounted (large negative or positive B1-R1 values), 
USNO B1.0 B1-R1 seems at least a usable set of data in first instance 
cases where an idea of colour would be useful.

It should not be considered rigorous though.  Yet it is preferable to 
not infrequent instances I have seen of blind usage of USNO Ax and/or 
GSC2.2 blue and red colours which should have been instantly obvious 
as peculiar due to their large value, and on investigation turned out 
to be due to being multi-epoch images of a variable star rather than 
being due to colour.


Comments and/or corrections to the logic route followed welcome.


John


John Greaves

===

I missed an assumption:

The assumption is that Brian Skiff's loneos.phot file, by the nature 
of the specific pecularities of selection for the stars in it, is 
essentially random in terms of magnitude (within it's range), colour 
(although it is probably relatively light in terms of quite red 
stars), RA and Declination.

John

John Greaves

===

The route for interrogating VizieR is more generally applicable (I've 
used it today for doing a CMC12 r' / USNO B1.0 R1 cross).  Besides 
which, somebody with better mathematical tools in their armoury may 
well get tighter results, especially wrt error estimation, on 
realising this is a feasible task data retrieval wise.



Incidentally.  From the same data: loneos Johnson B = USNO B1.0 B1, 
with standard deviation of 0.40 and a mean of -0.04 for the entirety 
of B-B1 from the dataset of twenty odd thousand used for the B-V 
relationship.


R1 is probably meant to be nearest Johnson R, at a guess.  I can't do 
anything about that, Johnson R being effectively dead as far as I can 
tell.

Comparison of about 5000 r' Carlsberg Meridian Catalogue Edition 12 
version 1.0 values in the range r' >13<17 against R1 gives a mean of  
-0.04 and a standard deviation of 0.5 for a derived nearly 5000     
r'-(R1+0.5) value dataset. As to how r' relates to gunn r relates to 
Cousins R relates to Johnson R in reality, I have no idea.  I merely 
did an arbitrary 0.5 shift because it kinda worked and fitted the 
pattern.

(Some other problems occured here also, as CMC12 r' can be too bright 
when stars are not resolved from crowding neighbours, giving 
erroneous magnitudes.  Also, I originally sourced the one request 
limit of 9999 CMC12 objects from VizieR, and it is interesting that a 
1" cross of that using VizieR only found ~5000 USNO B1.0 matches).


Not unsurprisingly V = 0.444 B1 + 0.556 R1 gave a standard deviation 
of around 0.5 for the twenty odd thousand derived (Vcalculated minus 
loneos Johnson V) values when I looked at it t'other day.

So, not much better than using GSC magnitudes then, but I was 
actually only really interested in getting some sort of colour 
relation out of it for estimatory purposes, anyway.


John Greaves



(thoughts on B and R magnitudes primarily from a thought by Bill Gray 
on the B-V = 0.556(B1-R1) factor when usual transformations relate R 
and B-V by a factor of around 0.66.  As can be seen, I follow the 
assumption that POSS I E plates _may_ have been intended to be 
nearest Johnson R, probably the only photometric system around at 
that time, rather than Cousins R which is used mostly nowadays 
(though Kron R is too I believe), with Johnson R itself having been 
intended to emulate traditional astronomical red plates when it was 
put together in the late forties... ...or maybe POSS I blue and red 
plates are just near Johnson B and R because of the definition of B 
and R by Johnson in the first place to be nearish to traditional red 
and blue astronomical plates???  The 'strong' and 'weak' theories of 
POSS I plate colours :-) .  At any rate, Johnson R (and I) are _not_ 
Cousins R (and I), Cousins R being where the 0.66 factor will have 
come from).

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