Front Mounted Filter Design
Front mounted Ha filters use many
of the same components as rear mounted filters.
The ERF serves the same purpose as
mentioned before, to block unwanted UV and IR (if coated). Immediately
following is the air spaced etalon. This unique device is made up of two
precision polished plates of fused silica. The plates are coated for
reflectivity and precisely spaced with either a central obstruction or
along the edge. Front mounted etalons do not use mica as a spacer.
The bandpass of the filter is directly
affected by the flatness of the glass, reflectivity of the
coating and how parallel the two surfaces are.
The distance between the two surfaces
determines the Free Spectral Range (FSR) of the etalon, or
how far apart each transmission peak
is from one another.
The blocking diagonal (BF) contains an
Induced Transmission Filter (ITF) and blocking filter. An
ITF will further block unwanted IR and works with a blocker
to trim the unwanted off band peaks. For example, if
the FSR is 10A (from one peak to the next), the BF would
pass only a 6A transmission and block the other peaks.
Light exiting BF contains only the Ha
centerline transmission.
Lunt Solar Systems air spaced etalon
An knurled knob on the filter housing allows you to tilt the filter.
Tilting changes the wavelength of the primary bandpass peak,
allowing the filter to be
"tuned" to see deeper into the chromosphere or fast moving
solar phenomena. When tilting, light travels a longer distance through
the etalon. It actually broadens the filter passband
a bit, which can
result in a reduction of contrast if carried too far. Turn the knob a
little too much and you get a white light view of the sun (colored red).
A feature moving
towards the observer will have its light shifted towards the
blue end of the spectrum, while a feature moving away will
have its light shifted deeper into the red part of the
spectrum.
Other factors that affect etalon
performance:
Temperature (more so in rear designs)
Air Pressure (some Lunt Telescopes use this to tilt)
Glass
thickness
Angle of incoming light (tilt)
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
