Telecentric Lens
Because the sun is half a
degree in apparent diameter, light rays hit the ERF as
strait light and at a multitude of other angles (the
incident angle of light). The hydrogen-alpha filter requires
the light rays to be parallel to the filters surface as much
as possible. A combination of positive and negative lens,
properly spaced in the focal plane, straiten out the light
rays to allow the filter to perform at its best. The
placement of these lens are said to be in the "Telecentric"
position.
Baader Telecentric
The Baader Planetarium
has telecentric lens especially made for the Solar Spectrum
filters and was designed by Zeiss. Called the TZ-2 (2x) and TZ-4 (4x) , these lenses should bring
your system out to the desired f/30 focal length for a rear
mounted filter. The design delivers 99% strehl at the
Ha wavelength. Optics are coated for Ha only. Additional adaptors and extension tubes are
required. See Baader's
information
sheet for more info.
The AP155EDT requires
~75mm of back focus to be perfectly telecentric with the
Baader system.
These lens are sold by
Solar Spectrum, The
Baader Planetarium
and
Alpine Astro .
Astro-Physics
Telecentric System
The telecentric system
from AP consists of a convertible barlow (Barcon),
extension, and a telecentric lens. The three pieces together
act to form a nominal f/30 system. The magnification of the
telecentric system is around 3X.
I have found that adding
another 3" extension to the light train increases the
magnification without any noticeable dimming. This however,
probably takes the system out of the telecentric position.
Sold directly by AP.
This item is not on their website (Barcon and extensions
are).
Astro-Physics Telecentric Setup
This is what it
can feel like when you first try to get your scope to f/30!
