Astrodon Johnson/Cousins U Filter
Description
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U, 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:
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.
Specifications
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. |
JOHNSON-COUSINS U+
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.
JOHNSON-COUSINS Bu+++
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.
JOHNSON-COUSINS V+
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.
JOHNSON-COUSINS Rc
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%
JOHNSON-COUSINS Ic
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%
In the Box
Johnson/Cousins Filter