There’s More to the Eye than Specifications

Lunt Solar products are a lot more than the specification that you see when comparing manufacturers.

Lunt was started in 2007. Lunt had the advantage of being able to completely re-invent the Solar Telescope from the ground up. We were able to improve on issues that we had seen in previous products and introduce significant technologically advanced improvements to etalon tuning and overall filter design.

For example:

Etalons are the heart of the Solar Telescope. Our Doppler True tuning technology using Barometric Pressure Tuning has no equal.

Lunt continue to keep our eyes on safety. Our products are designed with redundant safety filters in place. Safety is our first priority and has no compromise.

In addition to comparing advertised etalon specification, it is important to look at how the product itself is designed and tested.

Lets take a Lunt 50mm Solar Telescope as an example:

The first ERF:

Immediately after the objective we have a full aperture ERF. This ERF is designed to accept the full aperture of the objective. Because our etalon is sized to match the objective diameter we do not need to aperture down the ERF.

Our ERF is a red looking full UV blocking filter that we have added a IR blocking dielectric coating to one surface and a less than 0,1% anti reflection coating to the second surface. This filter blocks all harmful UV and IR radiation to safety specifications.

The red glass is precision polished prior to coating.

It should be noted that the IR coating is fairly reflective and is therefore placed in all systems at a slight angle to avoid back reflection to the eyepiece.

It should also be noted that the red glass is generally held loosely in the retainer so it has room for any thermal expansion.

The Etalon:

The etalon is not designed to be a safety filter but because of its design it does reflect IR and UV light back up the OTA.

The first ERF is placed in front of the etalon in order to remove any heat load from the etalon.

Lunt etalons are matched to the diameter of the objective. Our etalons are clear aperture (no center obstruction) for high magnification, high contrast viewing.

The Blue Glass Filter in the Blocking Filter Nose:

This filter acts to reduce the overall brightness of the solar image to a comfortable level and to absorb IR radiation if the front ERF should ever fail.

The Blue glass is coated with a narrow band AR coating centered at 656nm to eliminate back reflections. It is also placed in the retainer at a slight angle.

The 90 degree diagonal “mirror”:

Not a mirror at all.

The 90 degree “mirror” that is internal to the diagonal is actually a Long Wave Pass Filter that is designed to reflect the wavelength of 656nm to the eyepiece and absorb all wavelengths above that CWL.

The Blocking Filter:

The blocking filter is designed to allow the CWL of 656.28nm to pass and to block out all other “combs” of the etalon transmission. It has a fairly wide band of blocking to 1×10-5.

While not considered a safety filter, it would reflect a significant portion of UV and IR.

The final Red Filter:

The final red filter serves 2 purposes.

It is a piece of the same red glass that makes up the first ERF filter. It is placed in this position to stop eye reflections that would occur when the user looked into the eyepiece due to the very highly reflective surface of the blocking filter.

This red glass is coated with a very narrow-band AR coating centered at 656nm. And while additional IR blocking is not needed it would absorb IR by default.


All Lunt Solar products go through several levels of testing.

The final test of a Lunt Solar product is to set it up outside and look at the Sun (weather permitting). We do this to 100% of all our products.

The Solar Telescope has to pass a long list of acceptance specifications before it can be cleared for shipment to the customer.

If you have any questions or comments please feel free to contact Lunt Solar. We would love to hear from you..

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