Hydrogen Alpha Filter
Components
The first thing a light ray hits
in a Ha system is some sorts of Energy Rejection Filter
or ERF. The ERF's, used with rear mounted filters, are usually made of #25 Wratten glass that
is optically flat. Front mounted hydrogen alpha filters use proprietary
glass ERF's.
The ERF function is to block any
unwanted Infrared and ultraviolet wavelengths. This helps to
protect filter from deteriorating and stops excessive heat from knocking
the filter
off band.
After the light passes through the
telescope in a rear mounted design, it must be straightened
prior to striking the filter. Ha filters must contend with the incident angle of light passing through
them.
The sun’s apparent diameter half a degree. Straight light enters the
scope as well as a multitude of other angles of light. Rear mounted filters require near
straight light to come on band. A combination of aligned lens in the Telecentric position are
used to straighten
out the rays.
A Telecentric system typically
consist of a divergent lens, usually a 2x barlow, a spacer and some sort of convergent doublet sold by AP or TeleVue. A Telecentric
adds about 3x magnification to the system which enlarges the image size. They also
reduce vignetting.
Light exiting the doublet should be at around f/30 (depending on the
aperture of the ERF) to ensure the entire disk of the sun is on band.
Coronado filters use a rear
mounted blocking diagonal. These filters are specially matched to the
etalon to provide the best (and safest) possible image. The BF5, 10 and
15 diagonals are angles
and the BF30 is a 2" strait through filter.
Why Ha? |
The View |
What you see |
The Chromos |
Ha Emission |
Bandwidth |
Main Designs |
Ha Components |
Rear Filters |
The Etalon |
Front Filters |
Coronado filters |
DayStar Filters |
Solar Spectrum Filters
