SEE_ALSO

## NAME

filterphot -- Compute instrumental or standard magnitudes and colors.

## USAGE

`filterphot infile1 infile2 inmask1 inmask2 outfile outmask option coeff1 `
`coeff2 coeff3 datafill`

## DESCRIPTION

This task computes the instrumental magnitudes and the colors of objects from their intensities and can also used to transform these magnitudes and colors to standard photometric systems. Four methods are available, and the user selects the operation to be performed; these operations are described below.

- Instrumental magnitudes can be computed from intensity measurements.
- The instrumental magnitude (IM) is calculated from the instrumental
intensity (I) according to the following formula:
IM = -2.5 * LOG (I) + IM0

Where IM0 is the magnitude zero point constant (the

`coeff1`parameter) and LOG represents the common logarithm function.

- Instrumental colors can be computed from two intensity measurements.
- The instrumental color (IC) is calculated from the instrumental
intensity measurements (I1) and (I2) obtained through different filters
according to the following formula:
IC = -2.5 * LOG (I1 / I2) + IC0

Where IC0 is the color zero point constant (the

`coeff1`parameter) and LOG represents the common logarithm function.

- Instrumental magnitudes can be transformed to a standard photometric
system. The standardized magnitude (SM) is calculated from the
instrumental magnitude (IM) and an appropriate (i.e., standard) color
(C') according to the formula:
SM = a + b * IM + c * C'

where a, b and c are magnitude transformation constants (i.e., a =

`coeff1`, b =`coeff2`, and c =`coeff3`; see parameters below).

- Instrumental colors can be transformed to a standard photometric
- system. The standardized color (SC) is calculated from the instrumental
colors (IC) and (optionally) a certain apropriate color (C') based on
the formula:
SC = r + s * IC + t * C'

where r, s and t color transformation constants (i.e., r =

`coeff1`, s =`coeff2`, and t =`coeff3`).This procedure assumes that the input measurements are calibrated count rates. Sometimes, the transformation of magnitudes or colors to a standard photometric system requires a formula more complicated than those which have been implemented. Multiple invocations of this procedure can be used to effect the transformation. For example, in the Stromgren uvby system, the metallicity index m1 is defined as m1 = (v-b) - (b-y). In order to get m1, we need to run this task three times: first to get the (instrumental) colors (v-b) and (b-y) separately from the observed intensities (

`option = instcol`), then to recalculate m1 (instrumental) from (v-b) and (b-y) using`option = instcol`.The following table summarizes how the input and output files are related to the operation option:

(Option) Output file Input file 1 Input file 2 coeff -------- ----------- ------------ ------------ ----- (instmag) Inst. mag Inst.intens (or mag) none 1 (instcol) Inst. colors Inst.intens (or mag) Inst.intens (or mag) 1 (stdmag) Std. mag Inst.insten (or mag) Inst.colors 3 (stdcol) Std. colors Inst.colors Inst.colors 3

Note that the second input file is not required if the third magnitude/color transformation constant is zero (0.0). Thus the relevant equations for each case would be:

Option Equation ------- ----------------------------------------------------------- instmag Output = -2.5 * log(Input1) + coeff1 if input's BUNIT is not 'MAG' OR = input1 + coeff1 if input's BUNIT is 'MAG' instcol Output = -2.5 * [log(Input1) - log(Input2)] + coeff1 if inputs' BUNITs are not 'MAG' OR = input1 - input2 + coeff1 if inputs' BUNITs are 'MAG' stdmag Output = coeff1 + coeff2*Input1 + coeff3*Input2 if input1's BUNIT is 'MAG' OR = coeff1 + coeff2*[-2.5*log(Input1)] + coeff3*Input2 if input1's BUNIT is not 'MAG' stdcol Output = coeff1 + coeff2*Input1 + coeff3*Input2

Note: The BUNIT in

`infile2`must be contain the value "MAG" if the`option`parameter is set to either "stdmag" or "stdcol" and the BUNIT in`infile1`must be set to "MAG" if`option = stdcol`. The character string in BUNIT is always treated as "MAG" even if, for example, it contains "MAG**2"--the file is treated as magnitude measurements rather than intensities.

## PARAMETERS

- infile= "" [string]
- Name of the first input data file (templates are allowed).

- infile= "" [string]
- Name of the second input data file (templates are allowed).

- inmask= "" [string]
- Name of the first input mask file (optional).

- inmask= "" [string]
- Name of the second input mask file (optional).

- outfile = "" [string]
- Name of the output data file (this can be a template if multiple files are being processed).

- outmask = "" [string]
- Name of the output mask file. There will always be an output mask file for each corresponding output data file.

- option = "" [string, allowed values = instmag | instcol |
- stdmag | stdcol]
Operation to be performed (as described in the following chart, and in more detail above).

Value Operation --------------------------------------------------------------- instmag Compute Instrumental Magnitudes instcol Compute Instrumental Colors stdmag Transform Instrumental Magnitudes to standard system stdcol Transform Instrumental Colors to standard system

- coeff= 0. [real]
- First transformation coefficient.
Notice that only transformation to standard magnitudes or colors ("stdmag", "stdcol") requires 3 constants, while instrumental options ("instmag" and "instcol") require only one constant (i.e.,

`coeff1`). Note that`coeff1`always corresponds to the offset (zero point) term, while for the "stdmag" and "stdcol" options,`coeff2`is the factor multiplying the first file and`coeff3`is the factor multiplying the second file.

- coeff= 0. [real]
- Second transformation coefficient (see description of
`coeff1`).

- coeff= 0. [real]
- Third transformation coefficient (see description of
`coeff1`).

- datafill = 1. [real]
- Fill value for bad data in the output data file.

## EXAMPLES

1. Calculate instrument magnitudes from the input file infile1.c0h and put the results in im.hhh and its data quality file immask.hhh. Use -10. as the magnitude zero point:

ti> filterphot infile1.c0h "" "" "" im.hhh immask.hhh "instmag" -10.

2. Calculate instrument colors from the input files b.hhh and v.hhh. The output file is bminusv.hhh and its data quality file is bminusvmask.hhh. Use 0.2 as the color zero point:

ti> filterphot b.hhh v.hhh "" "" b-v.hhh b-vmask.hhh "instcol" 0.2

## BUGS

## REFERENCES

## SEE ALSO