Astrodon Photometrics Johnson-Cousins Filters



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We have very limited stock on Johnson-Cousins filters and if your selected filter is in stock you should purchase immediately. The rest of the Astrodon Johnson-Cousins filters are on long term backorder and demand far exceeds supply. We are no longer taking additional backorders. We do not expect more photometric filters until March-June 2021. We are working to get them faster.

You may click the “Get an alert” button to be notified when an out of stock filter is back in stock. 

Standard sizes are 1.25″, 31mm, 50mm round, and 50mm square. The 1.25″ is 27.5 mm of filter glass mounted in a 1.25″ filter cell. The 31mm is 27.5mm of filter glass mounted in a 31mm ring. More information here:

Unlike other Astrodon filters, the Photometric filters (Johnson-Cousins and Sloan) do not come as 36mm of unmounted glass. For those who want 36mm round for use in a 36mm filter wheel, 27.5 mm of filter glass can be mounted in our 36mm adapter. More information on 36mm mount here:

We also have a limited quantity of 12mm and 25mm (call for pricing and availability).

100%-Coated for long-term durability and consistency of research with the highest throughput.


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Astrodon Photometrics UVBRI Filters_3U, B, V, Rc, Ic filters have been the standard for photometric measurements for decades. They have evolved over time as technology changed. H. Johnson in the 1950s and A.W.J. Cousins in the 1970s designed these filters for photomultiplier tubes (PMT). M. Bessell in the early 1990s selected colored glasses to match the Johnson-Cousins designs for CCD cameras of the time. However, Bessell’s designs were based upon colored glasses available at the time. Some of those glasses (e.g. Schott KG-4 used in conjunction with Schott RG-9 to make the “I” photometric filter have been discontinued. Lastly, PMTs of the time limited light past 900 nm, whereas modern CCDs are sensitive to 1100 nm. Thus, to truly match the Johnson/Cousins Ic filter, a dielectric coating must be used to block light past 900 nm for CCD systems. This cannot be achieved with colored glasses. We use Ic and Rc (c = Cousins) to designate that we match the Johnson/Cousins designs with our coated filters, as closely as possible.

Astrodon Photometrics U, B, V, Rc, Ic:

  • Achieve the highest transmission possible for fainter objects
  • Use hard-oxide durable coatings (no V filter fogging) for durability
  • Include A/R coating and construction that minimizes halos around stars
  • Do not depend upon colored glasses being discontinued
  • Includes Ic filter (which must use a 900 nm cut-off coating)
  • Proud supplier to Las Cumbres Observatory Global Telescopes, AAVSO, Caltech/Palomar, NASA, MIT, Harvard, Apache Point, U. Valencia, and research organizations worldwide
  • Proudly made in the U.S.A.

Internal absorption of light in colored glasses used to make traditional filters limits the final throughput. For example, the traditional UV filter made with colored glasses typically does not transmit more than 60-70% at the peak. The Rs filter, 70 -80%. Astrodon Photometrics 100%-coated filters exceed 95% (see actual scans below).

Schott BG-39 glass (with Schott GG-495 glass) used in the traditional photometric V filter may degrade and become cloudy due to surface crystallization from exposure to humidity. It needs to be covered by a clear glass to protect any exposed surface, making the filter thicker.

With the advent of high performance, hard and durable dielectric coatings, both high transmission in excess of 95% and durability can be achieved. Filters last longer and transmit more light. Durability is critical for long-term, consistent research.

Astrodon Photometrics U, B, V, Rc, Ic filters adhere to the Johnson/Cousins/Bessel designs as closely as possible. The following is a spectral compilation of actual scans of the Astrodon Photometrics UBRcIc filter set. ​

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What’s in the box?

Photometrics U, B, V, Rc, Ic Filter

Photometrics U, B, V, Rc, Ic Filter Wavelength Graphs

Astrodon Johnson-Cousin Filter Scans

Johnson-Cousin Filter Comparison

Astrodon Photometrics UVBRI Filters_johnsonscousinsdata

Johnson-Cousins Details

  • Spectral profiles designed after U, B, V, Rc, Ic Johnson/Cousins
  • Improved B filter (B***) extending blocking >1100 nm and eliminating small light leaks that affected measurements of reddest stars in our initial filters
  • 100% coated – no colored glass
  • Peak transmission guaranteed > 90%
  • <0.01%T(ave) typical out-of-band 300 – 1100 nm; additionally =<0.1% Tabs for B***
  • Striae-free, single fused silica substrates
  • 1/4-wave propagated wavefront prior to coating
  • <0.5 arcminute substrate parallelism
  • 3.000 +/-0.025 mm substrate thickness
  • Parfocal with all Astrodon filters
  • Edge-blackened
  • 1.25″ mounted, 49.7 mm dia. and square unmounted
  • Other sizes available on a custom basis for filters as large as 150 mm
  • Proudly made in the U.S.A.


 Substrate:                     3.0mm +/-0.025mm fused silica

Filter Size:                    25.4mm +/-0.1mm diameter

Clear Aperture:             Central 23.4mm diameter minimum

TWF:                           </= 1/4 lambda per any 25mm square area before coating

Parallelism:                   </=0.5 arcminute before coating

Coating:                        To edge of part

AOI:                            0 degree

Scratches:                     </= 0.06mm width; no limit on length

Particles/Bubbles:         </= 0.4mm dia.

Pinholes/Voids:             </= 0.2mm dia.

Edge Finish:                  As fabricated

Chips:                           </=0.5mm

Edge Treatment:           Edges blackened with Enthone M-0-N black epoxy ink.

Thickness </= 0.1mm. It may extend 0.5mm onto top and bottom surfaces.



T = 50%: 391.3 nm +/- 3.1 nm

350 nm – 410 nm: match target curve by design, will not evaluate

360 nm – 375 nm: Tmin >= 90%; 430 nm – 1050 nm: Tave <=0.01%

Coating has been redesigned to minimize back reflections with theoretical values < 0.3% average.



T = 50%: 377.0 nm +/- 5.7nm; T = 50%: 473.0 nm +/- 3.8 nm

365 nm – 525 nm: match target curve by design – will not evaluate

410 nm – 425 nm: Tmin >= 90%

300 nm – 350 nm: Tave <=0.01%; 550 nm – 1100 nm: Tave <=0.01%; 560 nm – 1000 nm: Tabs <=0.1%

Back reflections with theoretical values < 0.4% average by design.



T = 50%: 490.0 nm +/- 7.3nm; T = 50%: 589.2 nm +/- 4.7 nm

478 nm – 640 nm: match target curve by design, will not evaluate

518 nm – 532 nm: Tmin >= 95%

300 nm – 460 nm: Tave <=0.01%; 670 nm – 1050 nm: Tave <=0.01%

Coating has been redesigned to minimize back reflections with theoretical values < 0.3% average.



T = 50%: 567.3 nm +/- 8.5nm; T = 50%: 717.1 nm +/- 5.7 nm

550 nm – 850 nm: match target curve by design – will not evaluate

594 nm – 615 nm: Tmin >= 95%; 300 nm – 525 nm: Tave <=0.01%

900 nm – 1050 nm: Tave <=0.01%; 860 nm – 900 nm: Tave <= 1.0%



T = 50%: 721.8 nm +/- 10.8nm; T = 50%: 889.8 nm +/- 7.1 nm

705 nm – 900 nm: match target curve by design – will not evaluate

758 nm – 850 nm: Tmin >= 95%

300 nm – 680 nm: Tave <=0.01%; 930 nm – 1050 nm: Tave <=0.01%

Astrodon Filter 1.25 Cell Customer Drawing