TRU-BALANCE FILTERS
SIMPLIFY YOUR IMAGING
Great Filters Just Got Better - Generation 2 Now Shipping!

Astrodon Tru-Balance RGB filters have revolutionized CCD imaging. Their popularity is due to ease-of-use, high optical throughput and great resulting color for galaxies, star clusters and nebulae.  There are two varieties of Tru-Balance filters - E-Series and I-Series.

E-Series filters are designed to approximately equalize the flux of Kodak's full-frame (E-Series) CCD detectors, including compensation for the solar photon flux. This means that your RGB color combine weights will be approximately 1:1:1 within perhaps 10%.  This can never be perfect, but it does allow you to take equal time exposures for your RGB data and also just one corresponding dark exposure time.

 

 

 

Introducing Generation 2 Tru-Balance filters (LRGB2) in April, 2008 in all sizes. This is the first change in Astrodon filter design and fabrication since their inception.  We listened to your requests. 

 

• 1.25" mounted, 50 mm dia., 50 mm square
• ~1:1:1 color combine weights for G2V white-point for Kodak Full-Frame (E) detectors(patent pending)
• Best choice for backthinned and Sony detectors
• Equal RGB exposures and one dark time
• Better color separation (spectra)
• Better color rendition for galaxies based upon color theory
• Significant reflection and star halo reduction (see Alnitak image)
• Enhanced contrast for HII regions in galaxies
• Spectral "gap" to minimize effect of light pollution
• Highest efficiency blue filter with less UV
• Correct "teal" OIII color for planetary nebula
• More parfocal  - thickness 3+/-0.025 mm*
• Parfocal with Astrodon's high-performance narrowband and near-IR filters  
• Striae-free 1/4 wave fused silica substrates
• 30 arcsec parallelism
• Ultra-hard and durable sputtered coatings

Astrodon filters are known for being parfocal on most systems.  The new thickness tolerance of +/-0.025mm (25 microns) for Generation 2 LRGB fitlers is a factor of 2 better than Gerneration 1 filters, and should be parfocal for systems down to f/3.5, IF your optical system is well color corrected. We have reduced the UV contribution in the lumincance and blue filters for this reason to reduce star bloat  The Clear (no near-IR blocking) may produce bloated stars if your optical system has poor near-IR focus.   Use the near-IR-blocked Luminace filter in this case. The Clear filter is likely a better choice for reflectors, such as Ritchey-Cretiens, rather than refractors or camera lenses.

Narrowband (NB) filters  enhance contrast of emission objects by accepting only a narrow range of wavelengths around the emission lines of hydrogen (H-a, 656 nm), oxygen (OIII, 501 nm), sulfur (SII, 672nm) and others.

They can be used to image when the moon is up, thereby extending imaging time. They can be used in light-polluted locations. 

 

 

 

The narrow range of wavelengths is defined as  the FWHM (full-width at half- maximum intensity).  Narrower filters decrease the background noise.  However, narrower filters are more difficult to manufacture consistently, and are thus more expensive.  Furthermore, it is difficult to maintain high transmission through the bandpass of the filter as it becomes narrower. If the peak transmission decreases as the filter is made narrower, the emission signal decreases and the gain in S/N (signal-to-noise) is not realized.  Astrodon has achieved this goal of high transmission for narrower filters. 

 

Astrodon has evolved its narrowband product line from 6 nm to 5 nm FWHM, significantly lowered the prices on all filters, and added the ultra-narrow 3 nm FWHM filters for H-a, OIII and SII. We added a Red Continuum filter (645 nm, 5 nm FWHM) that produces a star map without the H-a or SII emission in order to subtract stars from emission images.  Astrodon NB filters are renowned for not producing halos around stars and not leaking NIR light.

The 3 nm FWHM filters are stocked and are not custom ordered.  The 3 nm OIII filter provides the best protection from the effects of moonlight, producing half the background signal of a 90% T, 6 nm FWHM filter, resulting in an increase in S/N of ~21%.  This will even be larger in comparison to wide 7 or 8 nm FWHM filters. 

• 28 mm dia., 50.4 mm dia, 49.7 x 49.7 mm square
• 3 mm (+/-0.05) thick
• Parfocal to f/4 and parfocal with Astrodon LRGB
• H-a, OIII, SII, Red Continuum
• 5 nm FWHM filters guaranteed >90% transmittance (typically 94-97%)
• 3 nm FWHM filters typically 90-92% (scan included)
• >4 O.D. (out-of-band blocking) 300 - 1150 nm
• 1/4 wave, striae-free fused silica substrates
• 30 arcsecond parallelism
• sides are blackened
• hardest available sputtered coating for durability
• 3 nm filters can be used on systems to f/3
• Designed to have lower blue spectral shift with faster optics

Astrodon-Schuler UVBRI photometric filters have become a popular, low-cost addition for astronomical photometric measurements.  The American Association of Variable Star Observers (AAVSO) orders the Astrodon-Schuler V filter for its members.  Many of the filters go to universities for research. Based upon Bessell's 1992 modification of the  Johnson-Cousins design, each filter is made made with several Schott and Hoya colored glasses.  Several of the Schott glasses have been discontinued over the years, such as KG4, that was used in the Is filter.  The current formulations used in Astrodon-Schuler photometric filters are listed below. 

UV	1.0 mm Schott UG-1 1.0 mm Schott BG-39 2.0 mm Schott WG-295
B	1.5 mm Schott BG-25 1.0 mm Schott BG-39 1.5 mm Hoya L-38
V	2.0 mm Schott BG-39 2.0 mm Schott GG-495
Rs	2.0 mm Schott OG-570 2.0 mm Schott KG-3
Is	2.0 mm Schott RG-9 2.0 mm Schott WG-295

 Features

• 28 and 48 mm dia.mounted, 50 mm square
• 4 (+/-0.1) mm thick
• No A/R coating to maintain low cost
• Colored glass
• No spectral shifting with faster optics
• Special metal sleeves for 48mm filters to fit in 50mm filter wheels

Astrodon-Schuler filters are thicker (4 mm) will NOT be parfocal with Astrodon LRGB, narrowband or NIR filters (3 mm).  It is noted that the phosphate glass, Schott BG-39, is prone to surface crystallization due to humidity over time that leads to frosting of the glass.  This primarily affects the V filter, since the BG-39 glass is exposed.  The surface of the BG-39 glass incorporated into the Bu filter is not exposed. It is sandwiched between the other two glasses.  Common anti-reflective (MgF2) coatings will not mitigate this problem. More expensive, hard (e.g. BBAR) coatings may help. An alternative solution is to add 1 mm of Schott colorless WG-295 glass over the BG-39 glass in the V filter, but that would mean it is no longer parfocal with the other filters. This is not a ubiquitous problem, but does occur from time to time. Please contact Astrodon

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 Meffei 2, a galaxy in the plane of the Milky Way that is obscurred by dust and hence, very faint.  This image was made entirely in the NIR with no visible light!.
 

Goals

• Produce tri-color images entirely in the NIR

• Color combine weights for NIR1, 2, 3 are ~ 1:1:1.2.

• 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.

• 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.

• Mix NIR with RGB of globular clusters to enhance the appearance of cooler stars.

• 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.

Features

• 1.25" mounted, 50 mm 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)
• Set of 4 (NIRL123)
• NIR Luminance sold separately
• ~1:1:1 for KAF3200ME and similar detectors