User`s guide

SECTION 5.0

JOHNSON UBVRI FILTERS

The UBVRI filter system established by Johnson is generally followed today and defines

color bands in the spectrum interval from 300 to 1200 nm. Table 5-2 lists the filter-detector

responses of this system as originally established by Johnson. Filter-detector response is

defined as the normalized product of filter transmission times detector response for each

wavelength interval. Table 5-3 lists the response functions of the UBVRI filters and Table 5-

4 lists the filter-detector responses for the filters used with the Texas Instruments TC-211

CCD device.

The OPTEC UBVRI filters are all made from combinations of Schott colored glass. The

glass types and thickness for each filter has been computer optimized for the best fit with the

Johnson standards.

As can be seen from Tables 5-2 and 5-3, the B, V, R and I filters match closely the standard

Johnson values and give correspondingly small correction coefficients. Blocking the red

leak of the glass used in the U filter results in loss of UV transmission from 300 to 350 nm.

The filter is still useful for comparison purposes even though it does not transform into the

standard Johnson system precisely. It is interesting to note that the U filter is the most

difficult one to transform accurately for all observers including those with photomultiplier

systems. UV transmission through the atmosphere varies considerably and any optical glass

in the telescope (corrector lens) or photometer will absorb light shorter than 350 nm making

accurate U magnitude determinations difficult at best.

For those persons interested in the exact filter transmission and detector responsivity, Table

5-4 has been prepared using the latest filter and detector batch data. Filter characteristics are

subject to change since we continually review our filter formulas and attempt to achieve

smaller transformation coefficients.

Flatness 6 fringes maximum

Surface Quality scratch and dig 80-50

Diameter 12.7 mm

Thickness 7 mm

Table 5-1. Physical Characteristics of Optec UBVRI Filters.