Since approximately July 2017, I have been the owner of a special filter for observing and photographing the Sun – Baader Solar Continuum 1.25″. This is an interference filter that transmits part of the spectrum in the green region at a wavelength of 540 nanometers and a half-bandwidth of 10 nanometers. It is now on sale no, and it was replaced in 2022 by an updated version with a narrower band (7.5 nm) and IR blocking. The filter housing has an M28.5 thread, both external and internal. The filter is small and the thread height is small, so be careful not to drop it when unscrewing it from the eyepiece, T-adapter, or other filter.
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.
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Equipment:
-Coronado PST H-alpha 40 mm telescope
-Celestron Nexstar SE mount
-Deepsky IR-cut filter
-QHY5III178m camera.
Stacking of 250 frames.
The Sun, July 12, 2023, 10:27
Equipment:
-Celestron 102 SLT telescope (optical tube)
-Celestron Nexstar SE mount
-Lacerta Herschel prism
-ND3 filter
-Baader Solar Continuum filter
-ZWO ASI 183MC camera
Stacking of 100 frames.
The Sun, July 12, 2023, 10:31
Equipment:
-Celestron 102 SLT telescope (optical tube)
-Celestron Nexstar SE mount
-Lacerta Herschel prism
-Antlia CaK 3nm 393.3nm filter
-ND96-09 filter
-QHY5III178m camera.
Stacking of 100 frames. Panorama from 2 images.
Sunspot AR 3363, July 12, 2023, 10:40
Equipment:
-Celestron 102 SLT telescope (optical tube)
-Celestron Nexstar SE mount
-Lacerta Herschel prism
-Svbony SV216 Barlow lens
-Baader Solar Continuum filter
-ND3 filter
-QHY5III178m camera.
Stacking of 250 frames.
Sunspot AR 3363, July 12, 2023, 10:37
Equipment:
-Celestron 102 SLT telescope (optical tube)
-Celestron Nexstar SE mount
-Lacerta Herschel prism
-Svbony SV216 Barlow lens
-Antlia CaK 3nm 393.3nm filter
-ND96-09 filter
-QHY5III178m camera.
Stacking of 250 frames.
I decided to write my impressions of the 685 nm IR-pass filter from Svbony. I bought it in January 2021. The main task is to shoot the Moon with a monochrome camera, since moving to the near infrared range allows you to slightly reduce turbulence when shooting. With a very stable atmosphere, you can set a green filter, while the resolution will be higher than in the red and IR ranges, however, such an atmosphere in combination with the visibility of the Moon is rare, and I get 99% of the shooting of the Moon in monochrome just with red or IR-pass filter. Yes, I also have an 850 nm IR-pass filter from ZWO, but with it the resolution drops noticeably, and the Sony imx178m sensor in my monochrome astronomical camera (QHY5III178m) is not very sensitive in the IR range.
The 685nm filter can be used quite successfully with high infrared sensitivity color sensors, especially Sony imx462 or imx464 sensors. Another possible application of IR-pass filters is flare suppression when shooting galaxies, since LEDs in IR shine much much weaker than in the visible range.
The 685 nm filter may be of interest for shooting Uranus and Neptune at large apertures (from 250-300 mm) due to the higher contrast of atmospheric details.
Of course, it makes no sense to use IR-pass filters for visual observations, since the eye sees very poorly in this range. Moreover, such filters are forbidden to be used when observing the Sun in order to avoid deterioration or loss of vision.
Note that mirror or mirror-lens telescopes are best suited for NIR imaging as they have little or no chromatism. The Barlow lens in the IR range can also add chromatism, which will negatively affect the sharpness of the image. In any case, my tests on mirror-lens telescopes, ED refractors and apochromats, as well as with a 2x Barlow lens, did not reveal a noticeable drop in sharpness due to uncorrected chromatism. For shooting with an achromatic refractor and a monochrome camera, you can try a red or green CCD filter – they do not pass the infrared range, as well as the blue region with chromatism.
The Svbony SV183 IR Pass 685 nm filter is made in 1.25 inch format, but there is also a 2 inch version on sale. From the outside, the surface of the filter seems to be a mirror-like feature of interference filters, but a bright light bulb looks dark red in the light. At the same time, absorbing IR-pass filters look black from the outside. However, interference filters have higher light transmission than absorption filters, and can also have a sharper cutoff of the spectral range.
Svbony SV183 IR Pass 685 nm 1.25″
The filter Svbony SV183 IR Pass 685 nm 1.25″ is supplied in a cardboard box with a plastic box inside. I have no complaints about the quality of making – the surfaces are clean, without cracks, scratches and scuffs.
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The filter housing has an M28.5 thread, both external and internal. The filter is small and the thread height is small, so be careful not to drop it when unscrewing it from the eyepiece, T-adapter, or other filter.
