300 to 1000 nm transmission of some Barlow lenses, coma correctors, ADC and camera window

Spectrometer measurements (Ocean Optics HR4000)

Clavé 2
× Barlow
APM 2.7
× coma corrector, Vixen 2× short Barlow, APM 1.8× Barlow
Baader FFC, Astrophysics 2
× Barlow
Siebert 5.3× UV, 
Siebert 4× UV, Siebert 2×UV, Siebert 2×
Takahashi extender 1.5, Takahashi extender 1.6, Takahashi TOA 645 coma corrector
Televue Powermate 2.5× and 4×


ZWO 462 window

Vixen Fluorite 55 mm f/8

Celestron Starbright XLT coating (Schmidt plate only)

NB : peak transmission is normalized to 100% for divergent or convergent lenses (Barlow). Absolute transmission is provided for zero power optics.



ASH data

Transmission measurements made by Christian Buil

These curves are probably more accurate than the measurements made with the HR4000 spectrometer.


Transmission measured between 320 and 520 nm with UVEX spectro : Clavé 2× (27 mm and 50 mm versions), Televue 2× and Televue PowerMate 2.5×, APM 2.7× coma corrector.


Total transmission measured in the Astrodon UV bandpass

Barlow lens UV Transmission
APM 1.8× 45%
Vixen 2× 58%
Takahashi extender 1.5× ED 68%
Astrophysics 2 (old model) 64%
2.7× APM coma corrector 75%

Protocole :

- Measurements made with a Mewlon 250, ASI 290 camera and Astrodon UV filter.
- Source of light : Venus high in the sky (> 55°), coronal sky.
- Reference measurement : made at the Mewlon 250 focus (i.e. with no Barlow, only reflective optics)
- Barlow transmission evaluated by comparison with the measurement made with no Barlow lens (difference in effective focal length is taken into account in the calculation).

Accordingly, UV transmission is only valid for :
- the ASI290 response profile  in UV,
- the Astrodon UV filter transmission profile (bandpass = 320 to 385 nm).

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