Astrodon Near IR-Luminance Filter - 31mm
Amateur imagers are producing incredibly deep and detailed tri-color images of deep sky objects that rival or even exceed the quality of film images taken with professional equipment only 20 years ago. CCD cameras have quantum efficiencies (QE) approaching 90% that can bring out detail in faint objects. CCD detectors as large as 35 mm film with 12 million pixels are readily available to amateur imagers. High resolution images are taken with plate scales of 0.4 – 1.0 arcseconds/pixel on modest backyard telescopes with apertures ranging from 25 – 50 cm. Mounts can be actively guided to <0.2 arcseconds all night. Detectors in these imaging cameras are made from silicon (Si). They cover the ultraviolet (UV <400 nm), visible ( vis 400 – 700 and short-wavelength near-infrared (nir) spectral regions up to about 1100 nanometers (nm; nm =1.1 microns).
Professional astronomers use expensive CCD detectors made of different materials for NIR measurements that extend beyond the Si range. For example, J- and K-band images are taken at 1250 and 2200 nm, respectively. The 2MASS survey carried out 30 years ago was done in the NIR. The “2” in 2MASS means 2 microns (2000 nm). The readily available photometric I filter covers the NIR region, also shown in the figure below. NIR commonly refers to the region from 700 to 2800 nm and infrared (IR) for longer wavelengths. Tri-color images are typically made by placing red (R), green (G) and blue (B) filters between the telescope and the monochrome CCD camera. They are combined in software to form the RGB color image. The color images are generally taken at lower resolution by grouping (binning) pixels in the CCD camera, thereby permitting shorter exposures. The lower resolution RGB image is then used to “colorize” the deeper, higher resolution (unbinned) luminance image. This forms a high resolution LRGB image.
There are two types of luminance filters; NIR-blocked (L) and unblocked (C = Clear). L is designed to match the spectral coverage of the RGB filters. One school of thought is that this is the best combination for color images because of the spectral matching. The other school of thought is that one wants all the photons possible. C adds about 40% more signal from the NIR (using the Kodak KAF3200ME detector, for example) to produce a deeper luminance image. The question is how the additional NIR signal relates to the RGB image that will colorize it. This is still debated. Lastly, narrowband filters bring out detail in nebula and supernova remnants from the emissions of hydrogen (H-alpha or H-a) at 656 nm, oxygen (OIII) at 497 and 501 nm, sulfur (SII) at 672 nm, and others. It is common to add H-a into the RGB of an emission nebula to enhance structural detail. All of these colors and emission lines lie within the VIS spectrum.
More on Astrodon Near-Infrared Tricolor Filters
The NIR region remains largely unexplored by amateur imagers. Perhaps this is due to the impression that the Si CCD camera is not sufficiently sensitive in the NIR. Perhaps it is not clear what can be imaged in the NIR. Narrowband imaging has only recently become popular, allowing imagers to take images from backyard equipment like the Hubble Space Telescope’s “Pillars of Creation” (Messier 16). NIR imaging may follow this recent narrowband evolution and become another tool in the imager’s toolkit. This image is of Maffei 2, a galaxy in the plane of the Milky Way that is obscured by dust and hence, very faint. This image was made entirely in the NIR with no visible light!.
Some of the goals of Astrodon Near-Infrared Filters are to produce tri-color images entirely in the NIR. Color combine weights for NIR1, 2, 3 are ~ 1:1:1.2. Now you can examine objects highly obscured by dust in the plane of the Milky Way, such as the IC342/ Maffei group, that are difficult to image at VIS wavelengths. The Astrodon NIR filters permit you to explore faint, extended red emission (ERE) nebula. Uncover stars and other details in nebula that are obscured by bright VIS emissions from H-a, SII, OIII or other elements! What's more, you can mix NIR with RGB of globular clusters to enhance the appearance of cooler stars and minimize terrestrial light pollution from sodium and mercury street lamps and oxygen skyglow for imagers living in suburban or city locations. Use the NIR luminance (no visible light) to explore galaxy structure in comparison to VIS luminance data.
Astrodon Near-Infrared Tricolor Filters Features
- 1.25" unmounted, 31mm diameter
- 3+/-0.05 mm thickness
- Parfocal with other Astrodon LRGB and Narrowband filters*
- NIR Luminance (>700 nm)
- NIR123 (700-800, 800-900, >850 nm)
- NIR Luminance sold separately
- ~1:1:1 for KAF3200ME and similar detectors
Astrodon Product Number: NIRL-31R