Lunt Solar do not use central obstruction sun filters for telescopes.. Here are a few reasons why.
The challenge to early etalon sun filter making was to figure out how to re-produce a ultra high precision flat on a mass production scale. Typically etalon sun filters required a clear aperture for laser and imaging work. They were hard to produce and very expensive. However, due to the placement of the etalon on the front of a telescope system it was recognized that at this position the solar filter was simply collecting light and an obstruction would not significantly effect the performance. Much like the obstruction on a SCT.
The early work-around to mass produce these optics was to place a foot at the center of the “less than perfect” etalon plate to “pull” the etalon plates into parallel. This allowed the manufacture of lower precision optical flats which could be more easily produced.
In larger optics this has no noticeable effect. The % of obstruction is small when compared to the overall collection area of the optic. However, on smaller diameter etalon sun filters the % is much increased and has a noticeable effect. The obstruction can be seen at low magnifications and makes low magnification imaging difficult. The obstruction can be seen at low magnification as a dark spot through the eyepiece.
The original idea worked because the etalon sun filter was typically placed in front of the objective. Today, etalon sun filters are typically placed internal to the optical system. In this case obstructed etalons are now part of the imaging path and have an effect on the overall performance of the system.
Obstructions are at the center of the etalon sun filter and are typically 12mm+ in diameter. The etalons are smaller at this location and the obstruction takes up a much higher % of the area. The obstruction is placed at what is considered the “sweet spot” of the optical system. Removing the higher quality central portion of the optical path. (due to roll off at edges and spacer uniformity). An etalon sun filter performs best when the light rays are at 90 degrees. ie: at it’s center.
Over the years Lunt Solar have developed polishing techniques that produce ultra high quality flat optics on a very large scale. We produce optics that are typically lambda/50 all the way to the edge of the optic. (no roll off) We have developed these techniques as a direct result of the need to place the etalon internal to the optical system. This reduces the overall cost of the manufacturing (smaller etalon sun filters) and provides significant savings to the customer without the loss of performance.
We do not compromise the performance of the etalon sun filters or the optical design.
Lunt Solar do not use central obstruction solar filters in any of our products. Our internal etalons are 100% unobstructed and allow for optimum performance.