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Condition: This is a 48mm Japanese-made IDAS LPS-D1 filter in excellent condition, with clean optics.
Light pollution suppression (LPS) filters are designed to suppress the common emission lines generated by artificial lighting, yet allow the important nebula emission lines to pass, thus enhancing the contrast of astronomical objects, particularly emission nebulae.
Unlike other light pollution suppression filters, IDAS filters are specifically designed for balanced color transmission using the IDAS unique Multi-Bandpass Technology (MBT) process. The balanced transmission allows color photographs to be taken with minimal color cast to broadband emission objects such as stars, galaxies and globular clusters. The comparison photos at right demonstrate (qualitatively) that the IDAS filter maintains better color balance than other filters which have been designed for visual use.
LPS 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.
CCD imaging can also benefit, because although CCD imagers can already shoot through light pollution to some extent, including an LPS filter to the setup gives an added (signal-to-noise) edge.
Note, however, that light pollution suppression filters are not a perfect substitute for dark skies. Refer to Hutech's discussion of the limitations and common misconceptions about light pollution suppression filters.
LPS filters are available for 1-1/4" and 2" (48mm) eyepieces, as well as in numerous sizes to accomodate most popular camera lenses. Each filter lot is tested and a plot of the spectral response of the lot is provided with each filter.
[Above] Spectral response (% transmission vs. wavelength in nanometers) of the IDAS Light Pollution Suppression filter. Major emission lines generated by artificial lighting are also indicated. Note that this plot represents the spec target for LPS filters. The spectral response of each individual filter will vary. This is measured, plotted, and shipped with each filter produced:
Above is a sample of the filter lot data plot which is shipped with each filter.
The IDAS production criteria for filters are as follows, arranged from left to right (short to long wavelength) as plotted above:| Wavelength (nm) | Pass/Block | Transmission |
| 365 | Block | 6.5% max |
| 405 | Block | 6.5% max |
| Pass | 60% min (avg) | |
| 436 | Block | 6.5% max |
| Pass | 92% min (avg) | |
| 546 | Block | 6.5% max |
| Pass | 75% min (avg) | |
| 590 | Block | 6.5% max |
| Pass | 75% min (avg) | |
| 623 | Block | 25% max |
| Pass | 80% min (avg) |
[Above] Off-axis spectral response (% transmission vs. wavelength in nanometers) of the IDAS filter.
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