Home

  Images

  Equipment

  Resources

  Podcast

  Blog

  Contact

 


 
 
 FreeFind

Translation

Front Mounted Filter Design

Front mounted Ha filter

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 peakFree Spectral Range 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.

Ait Spaced Solar Etalon
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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


 © Greg Piepol