wpo - testing deepsky / OIII filters via a simple spectroscope

homelinkgrism spectroscope    other spectroscopes
This simple spectroscope comprises a lightweight aluminium tube containing a transmission grating and 1mm wide slit in black card to admit a sample of light with the camera lens focusing the slit as seen in the viewfinder/LCD.  The combo is pointed at a white card or sky illuminated by sunlight with the filter under test held immediately before the slit to modify the light.  When pointed direct at fluorescent lamps, TV etc emission lines are recorded.

Using a narrow 0.1mm slit the spectroscope has sufficient resolution to record absorption lines in the spectrum of daylight for calibration.  The spectroscope is probably too insensitive for nighttime use.   More sample spectra to follow.

simple spectroscopecard slitspectrum of daylight

Spectral transmission of deepsky filter and emission lamps
The following selection shows nine deepsky filters and four emission lamps with daylight spectrum [reflected sunlight] for comparison.   The filters by Lumicon, Thousand Oaks, Astronomik and TeleVue transmit primarily in the blue/green region to accentuate planetary nebulae [PNe] OIII emission lines at 4959A and 5007A and emission nebulae Hb line at 4861A [Lumicon Hb 1985].  Generally filters are spot-on in their transmission as stated.  The TeleVue OIII #2 is a marked improvement but is still not as tight as the other OIII filters.  A number of filters have slight red and blue 'leaks' which would go unnoticed in visual use.  Tilting a filter seem to shift the transmission band bluewards.

The Thousand Oak OIII filter seem to transmit only OIII at 5007A but as this line is typically 3x stronger than 4959A OIII line, there is probably little loss of light.  Lumicon market a deepsky broadband [1984], Swan band [comet - 1998] and UHC filters [1984/2001] covering Hb+OIII.

Filters immediately below sampled at BAA Deep-sky Section Meeting on 2005 Mar 5 at Northampton - UK.  All filter block LP sodium [Na] and transmit dual OIII at 4959A and 5007A [latter brightest in PNe] and Hb at 4861A. Lumicon OIII and Lumicon UHC block Hb.   Lumicon UHC has similar transmission to Lumicon Swan-band filter and TeleVue similar but more even transmission to TeleVue OIII #2.
deepsky filter test @ BAA DSS meeting 2005 Mar 5
deepsky filter test


Domestic fluorescent lamps glow at discrete colours or wavelength to simulate the effect of white light.
Neon lamp from domestic appliance and computer monitor included.
domestic fluorescent lampsneon lamp + TV monitor

The Didymium filter is regarded at the sodium light buster blocking yellow - here weakly at other wavelengths too.
The spectrum of both low-pressure [LP] and high-pressure [HP] sodium lamps seems essentially identical except that HPS redistributes the energy from the main sodium line at 589nm [which is suppressed]  into the other emission lines in blue and green to give a whiter light.  The Didymium filter is ineffective against HPS.
didymium filter blocks sodium light

text & images copyright Maurice Gavin - July 2003/Mar 20054006 6mar05