Plus: Flexible mounting options; superb sharpness and contrast
Minus: Box cannot hold filter once feet are in place; user must buy a case
Summary: The Baader ASTF offers a temperature-compensated cell to prevent the AstroSolar film from becoming stressed, so it remains sharp. It worked very well.
Who Is It For? Avid solar observers and photographers looking for the sharpest white-light solar images.
My standard setup for white light solar viewing had been a Takahashi TSA-102 refractor with a full aperture glass solar filter. While I heard Mylar filters can provide better performance, I had always been skeptical.
When I wanted to equip my Lunt Solar Systems 152-ED refractor with a solar filter, I chose a Baader ASTF (MSRP: $91), short for Astro Solar Telescope Filter. This series uses Baader AstroSolar film said by Baader to have better quality than standard Mylar, because it is “ion implanted” and coated with a tough, color-neutral metallic layer on both sides of the film. The claim is greater durability and sharpness.
However, for best sharpness the plastic film should remain loose, even wrinkled. It should not be taught. Uniquely, the ASTF filters feature a temperature-compensated cell (the less expensive ASSF and ASBF filters, for spotting scopes and binoculars respectively, lack this feature). In the ASTF filters, the cell matches the thermal properties as the AstroSolar material, allowing the film to float within the aluminum frame within a range of temperatures from -22° F to +122° F (-30° C to +50° C), keeping it stress-free and optically sharp.
In my tests, the temperature-compensated cell worked as advertised, as the safety film retained its unstressed shape and never became stretched or tight.
I was pleased with quality of the ASTF filter. The lens cell was rigid and the three rubberized feet were robustly engineered. I was concerned the feet wouldn’t hold the filter securely enough against strong winds. While the filter kit does include securing straps for a secondary level of protection, these require attaching Velcro patches to the dew shield, which I was not fond of doing.
The spacing of the feet is adjustable for a range of tube or dew shield diameters. The two trios of slots also allow the feet to fit either on the inside or the outside of a dew shield, making for a flexible system.
Despite my attempts to knock it off, I found that the ASTF filter stayed securely in place with just the rubberized feet. Given the length of the feet, pulling at any one side of the cell would not make it come off.
THE SOLAR VIEWS
Solar observing through the 152mm refractor using the ASTF exceeded all my expectations. The level of additional detail was astoundingly better compared to the 102mm refractor, using the standard glass solar filter. Details were more abundant and more etched. Surface granules appeared as sharply defined dark specks peppering the solar surface. Penumbral filaments around sunspots were individually visible. Faculae were bright and distinct near the solar edge, but also visible on the interior of the solar disk, an unexpected surprise.
The image was bright and neutral in color, with the solar disk white, not orange or yellow. I was able to push magnifications to 200x or more.
For a more equal comparison, I was able to fix the ASTF to the 102mm refractor. Switching back and forth from the glass filter to the ASTF I was surprised at the difference. The image was significantly brighter through the ASTF by, in my estimate, two f-stops. Individual details of sunspot umbras and penumbras, and well as of filaments and faculae were all more etched. In the glass filter, granulation appeared as an indistinct roughness. In the ASTF granules looked like well-defined specks.
It was like the 102mm refractor had a veil lifted from it and was operating like a larger aperture telescope. The Baader ASTF completely outclassed the glass filter.
In conclusion, the Baader ASTF filter proved to be excellent in performance, engineering and construction. It is now a permanent part of my equipment for white light solar observing.
Original Review: Cloudy Nights
With files from Alan Dyer