Antlia D-ERF

To expand my solar imaging capabilities with my relatively large refractor (130 mm aperture), I ordered a full-aperture ERF filter with 150 mm diameter glass from China. Currently, such filters, which are available to us in Russia through AliExpress, are made by a company called Antlia. Previously, I purchased two other solar filters from this manufacturer: a narrowband 5-angstrom Calcium (CaK) filter and a full-spectrum one which, for some reason, they call “Dualband.” I wrote a review on those, which you can read here.

For telescopes with an aperture larger than 100 mm, Antlia recommends using these filters only in combination with an external protective energy rejection filter (ERF). Accordingly, they produce several sizes of these filters with mounting rings designed to fit different tube diameters. In my case, I planned to install the external ERF on my TS-Optics Photoline APO 130 mm telescope.

The filter that arrived from China was mounted in an absolutely massive housing designed for telescopes with tube diameters up to 190 mm. Perhaps one day I’ll try it on a telescope with a 150–180 mm aperture, but since I don’t currently own one, the housing seemed too bulky and heavy. Including the glass, the filter weighed over 2 kg, which I found unacceptable.

After removing the glass from the housing, I took measurements and sketched out a lighter and more compact housing made from ABS plastic, which I then had 3D printed. I decided to secure the retaining ring for the glass with decorative bolts for hunting knives (found on Ozon). The result was a neat and snug-fitting filter, only slightly larger than the telescope’s outer dimensions and weighing just 630 g. If anyone’s interested, I can share the 3D file that shows the glass retention design.

The ERF filter reflects solar energy in the ultraviolet and, more importantly, in the infrared range, allowing the telescope to remain pointed at the solar disk for extended periods. Heated air inside the tube interferes with achieving sharp images. In addition, focusing sunlight at the end of the optical tube poses a risk of damaging equipment like cameras, filters, Barlow lenses, or specialized filters such as hydrogen-alpha etalons. The larger the telescope’s aperture, the greater the risk of overheating.

There are modified large Schmidt-Cassegrain telescopes—8 inches and up — for which Baader makes an ERF coating on the corrector lens. This series is called Triband SCT. The price of such a telescope, essentially a slightly modified Celestron SCT 11”, is €6500 in Germany. There are also a couple of other companies that offer similar setups. Using a powerful SCT reflector allows for significantly closer views of the Sun and reveals very impressive details, especially in the hydrogen-alpha band. On the other hand, the larger the aperture, the more noticeable the effects of “seeing,” i.e., atmospheric turbulence. If you’re a true purist, it’s probably best to haul such a telescope up into the mountains. In this hobby, there’s no limit for enthusiasts!

Buying Chinese ERF glass specifically for my existing telescope and installing it in a 3D-printed housing was a relatively low-cost solution, although large-diameter ERF filters are expensive regardless.

On AliExpress, there are currently two types of external ERFs available: the Antlia Solar Discover Dualband ERF (glass diameter 130/150/200 mm, max. outer tube diameter 250 mm), and the second is the Startrip Ha-ERF filter (glass diameter up to 240 mm). The Antlia D-ERF is universal and suitable for imaging in both calcium and hydrogen-alpha bands. The Startrip is only suitable for hydrogen-alpha; the calcium bandwidth is blocked. Unfortunately, the Antlia is only available with its heavy housing, while the Startrip can be purchased either with or without a housing.

One more note. On online forums, I’ve come across information that in calcium imaging, people sometimes use an aperture “mask” — a cover with a circular cutout to reduce the aperture. For example, the 130 mm opening can be reduced to 100 mm. I haven’t tried this myself, but I’ve read that it slightly increases contrast.

The first test results using special filters and the external Antlia D-ERF were positive. There was no negative effect of the external thick glass on final sharpness of the images and I was able to use a higher-quality Barlow (telecentric) lens without worrying about overheating the equipment at the end of the large 130 mm tube. Before this, I had only imaged the Sun with an 80 mm telescope, and the resolution was noticeably worse. I’ll continue experimenting. Hopefully, this short post will be interesting and useful to amateur astronomy enthusiasts.

Here are the first few images of the Sun using Antlia Solar Discover CaK 5Å, Dualband, and Antlia D-ERF filters:

Questions: @telemonk

Astroblog: @astrographic