The Type-4 Tri-Color filter set from IDAS with four 31 mm filters.
The IDAS Type 4 BGR+C and BGR+L filter sets are designed for three-color (BGR+C) or four-color (BGR+L) imaging, with special attention to features which address accurate rendition of colors as viewed by the human eye:
- The band-passes are designed with astronomical imaging in mind. The passband roll-offs 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 represent violet frequencies as pure blue.
- The B/G filter cross-over point is set slightly below 500 nm to correctly balance the colors in the two OIII emission lines (500.7 nm and 495.8 nm).
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 +/-1 nm from the +/-3 or 4 nm 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. (see the Gallery Tab for 6/26/1996 image processed by Cornell Univ.)
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 benefiting 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.