Single filter transform

Community General Discussion
1

I did read a lot about single filter transformation and still learning… Can someone confirm if my understanding of the topic is correct or not with the following?

I did resume the formula as:

Vs = (vs - vc) + Tv_bv * [(B-V)s - (B-V)c] + vc

Vs = standard magnitude of the variable star (V filter)
vs = instrument magnitude of the variable star (V filter)
vc = instrument magnitude of the comparison star (V filter)
Tv_bv = Epsilon-V (calculated with TG on M67)
(B-V)s = catalog B-V for the variable star
(B-V)c = catalog B-V for the comparison star

If i’m looking at a red star with the following information:
vs = 10.240, vc = 10.740, Tv_bv = 0.2, (B-V)s = 2.0, (B-V)c = 0.8
The result is Vs = 10.480 (from the instrument magnitude of 10.24)

If i’m looking at a blue star with the following information:
vs = 10.240, vc = 10.740, Tv_bv = 0.2, (B-V)s = 0.2, (B-V)c = 0.8
The result is Vs = 10.120 (from the instrument magnitude of 10.24)

If both B-V are the same, both instrument and standard magnitude will be the same.

If my process is correct, please can you confirm also the following:

Bs = (bs - bc) + Tb_bv * [(B-V)s - (B-V)c] + bc
Rs = (rs - rc) + Tr_vr * [(V-R)s - (V-R)c] + rc
Us = (us - uc) + Tu_ub * [(U-B)s - (U-B)c] + uc
Is = (is - ic) + Ti_ri * [(R-I)s - (R-I)c] + ic

Thanks.

2

The last term in the above formula (vc, instrumental mag of the comp) is not correct. It should be Vc, catalogue magnitude of the comp.

3

Thanks for the reply. Now, I see the mistake I did.

4

Most of the variable stars in the VSX does have APASS-DR9 B, V, g’, r’, i’ and B-V.
How I can calculate coefficient if U, R and I are not available? Can we pick any catalog from SIMBAD?

5

“Pick any catalog…” One can do very well for g,r,i by adopting the ATLAS ‘refcat2’ catalogue for stars fainter than about mag 11 or so. It is query-able through VizieR item j/apj/867/105. These are transformable to BVRI using the relations in Kostov & Bonev:

The Johnson U or Sloan u are problematic simply because the data do not exist. If you think you can get good transformations in U/u with various standard fields, why not go ahead and start making new comp star values in fields of interest (multiple frames on multiple nights, etc)?

\Brian