Antlia CaK 3nm 393.3nm filter review

The Sun is the only star where amateur telescopes can be used to view or photograph details in the stellar atmosphere. In 1814, the German physicist, optician, and inventor Joseph Fraunhofer first observed lines in the spectrum of the Sun and assigned a nomenclature to the most noticeable lines. Some of his notations are still used today. There are special telescopes and light filters with which you can photograph the Sun in various spectral ranges. I decided to purchase one of these filters for photographing the Sun in the calcium line (CaK, or K-line) – Antlia CaK 3nm 393.3nm 1.25″.

IMPORTANT!
1. Antlia CaK 3nm 393.3nm filter is NOT INTENDED FOR VISUAL OBSERVATIONS! IRREVERSIBLE LOSS OF VISION IS POSSIBLE! FILTER FOR ASTROPHOTOGRAPHY ONLY!

2. Antlia CaK 3nm 393.3nm light filter cannot be used without first filtering sunlight, otherwise you can damage the filter and camera! It is necessary to use an aperture filter (I recommend Baader AstroSolar Photo ND 3.8), or a Herschel wedge + an additional ND filter.

The filter is available only in a 1.25″ form factor, which is quite enough for shooting the Sun with most lunar and planetary astronomical cameras. The filter is supplied in a very impractical magnetic box that tends to open up.

Отчёт о тестировании, страница 1
Filter scan report, page 1
Отчёт о тестировании, страница 2
Filter scan report, page 2

WHAT DOES THIS LIGHT FILTER ALLOW YOU TO CAPTURE?

Using Antlia CaK 3nm 393.3nm you can photograph the photosphere, granulation and partly the chromosphere of the Sun. When compared with photospheric filters such as Baader Solar Continuum or Player One Photosphere, the images obtained with Antlia CaK show better visibility of flare fields—hot areas of the photosphere. There is also information about photographing prominences using Antlia CaK. Narrower-band filters (Lunt CaK, Quark CaK) provide better image contrast and actually work across the chromosphere, but the price of these filters is much higher.

Baader Solar Continuum vs Antlia CaK
Baader Solar Continuum vs Antlia CaK

Baader Solar Continuum:

Antlia CaK:

Baader Solar Continuum:

Antlia CaK:

If you have a chromospheric telescope and a 540 nm photospheric filter, then purchasing a calcium filter will allow you to obtain unique multispectral images of the Sun.

The Sun, December 2, 2023
The Sun, December 2, 2023

I also used the Antlia CaK filter for something other than its intended purpose — to photograph the cloud cover of Venus. The filter works and highlights the Venusian clouds, but due to the narrow bandwidth the image is very dim and requires increasing the shutter speed and camera gain.

Венера, 23 февраля 2023 года
Venus, February 23, 2023

RECOMMENDATIONS FOR USE

1. To shoot through the Antlia CaK 3nm 393.3nm filter, use a monochrome camera, since color cameras will only have a quarter of the pixels working, which will significantly reduce detail and complicate the processing of the material. I use a QHY5III178m monochrome camera.
2. When using the full-aperture protective filter Baader Astrosolar Photo (ND 3.8), no additional filter is needed; just screw the Antlia CaK into the camera and install it in the telescope.

3. When using a Herschel wedge, it is necessary to use an additional eyepiece 1.25-inch ND filter (ND96-09 or denser), which is screwed in after the CaK filter. It is advisable to have a set of ND filters of varying densities.

Lacerta Herschel wedge 1.25″ review
Lacerta Herschel wedge 1.25″ review

TEST WITH SPECTROSCOPE

HOW TO COLOR A MONOCHROME IMAGE?

Through a monochrome camera, the image is produced in shades of gray. It can be left the same – the monochrome is completely self-sufficient. However, if you want to color the image, this can be easily done using a graphics editor (for example, Adobe Photoshop). You need to convert the image from monochrome to RGB, then go to the levels adjustment, select the green channel and move the central slider to the right.

Example of an image in false color:

Солнце, 15 апреля 2023 года, 10:37
The Sun, April 15, 2023


SUMMARY

The Antlia CaK 3nm 393.3nm filter fully copes with its task, allowing you to obtain images of the Sun in the calcium line. Yes, the filter’s passband is very wide, but the images of the Sun are still noticeably different from the Continuum range (540 nm). If you don’t have the money to buy a Lunt CaK, but really want to photograph the Sun in calcium, the Antlia CaK 3nm 393.3nm will be an excellent choice. I bought an Antlia CaK 3nm 393.3nm filter on Aliexpress.
My solar images taken through this filter can be viewed at the link.

There is also a solar wedge with a built-in calcium filter at a very attractive price.

Let me remind you that it cannot be used through a calcium filter for visual observations. ONLY FOR ASTROPHOTOGRAPHY!

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