IDAS Type 4 BGR+L filter sets are designed for tri-color (BGR) or quad-color (BGR+L)
imaging, with special attention to features which address accurate rendition of colors as
viewed by the human eye:
The bandpasses are designed with astronomical imaging in mind. The passband rolloffs are
sufficiently gentle and overlapping to allow intermediate colors (especially the prominent
emission nebula lines) to be reproduced so that the elusive goal of "true color" can be
approached.

The red (R) filter includes some blue (B) band response to match the response of the human
eye. This allows violet frequencies to be reproduced correctly as violet (purple). Note that
traditional filters which do not include B response in the R filter incorrectly represent violet
frequencies as pure blue.

The B/G filter cross-over point is set slightly below 500nm to correctly balance the colors in
the two OIII emission lines (500.7nm and 495.8nm).

All filters utilize the unique IDAS Ion Gun Assisted Deposition (IGAD*) coating technology for
superior coating durability (quartz hardness) and safer cleaning. IGAD coatings also improve
temperature and humidity stability of the filter performance, reducing spectrum shifts down to
+/-1nm from the +/-3 or 4nm shift of standard coatings.

The unique bandpass arrangements of the IDAS Type 4 filters allow several types of imaging
to be done with combinations of filters:

Conventional BGR+L imaging is accomplished by using the IR blocking L filter in conjunction
with each of the component color filters (BGR). The Clear filter is used for the L-component so
that the focus position is the same as for each of the other components.

Near IR (NIR) "tri-color" (false color) imaging can be done by using an optional IR-pass / visual
cutoff filter (R700) in conjunction with the BGR and clear filters. Note that the imaging sensor
and optics must also be capable of imaging IR wavelengths.

One application of NIR false color imaging is to planetary imaging. The methane absorption in
NIR allows gas giant features to be seen with more contrast, as seen in a Galileo probe view
of the Great Red Spot on Jupiter.

NIR imaging also allows for better "penetration" of areas dominated by interstellar dust due to
the reduced scattering of longer wavelengths. The Trapezium region of M42 is a good example
of an area benefitting from NIR imaging.

Full-band False Color imaging can be done by using the BGR and clear filters without
additional filtering. This allows IR-capable imaging chips to utilize their full spectral capability.
Note that the imaging optics must also be capable of properly imaging IR wavelengths.

Prices:

BGR4-28.6 Type 4 filter set, 28.6mm (1-1/4") - £299.00
BGR4-37 Type 4 filter set, 37mm - £299.00
BGR-52 Type 4 filter set, 52mm - £359.00

The Astronomy Shop

1 Tone Hill. Wellington

Somerset TA21 0AU

United Kingdom

Telephone: +44 (0)1823 665510