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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.
- 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.