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″.
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.
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.
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.
My first shot of Mars in 2020 – a year and a half passed my previous shooting. The visibility conditions of Mars are gradually improving; now it can be observed quite high above the horizon closer to the morning. The angular size of Mars is currently small – about 11 seconds of the arc, but by mid-October 2020 it will be 2 times larger than now.
-Celestron NexStar 8 SE telescope
–long Barlow lens 2x
–extender tube NPZ
–ZWO ir-cut filter
-ASI ZWO 183MC camera (100 fps).
Stacking 1000 frames from 11908 with Autostakkert.
Location: Russia, Anapa, backyard.
Also, at the end of the capturing, I managed to visually test the Celestron Mars filter with Meade HD-60 6.5 mm eyepiece. When using this filter, the planet’s color changed to pinkish, however, the visibility of the darkening on the disk of Mars significantly improved.
The O-III light filter (read as “o-three”) is an astronomical accessory designed to improve the visibility of gas nebulae, namely planetary nebulae, supernova remnants and regions of active star formation. The O-III filter passes the green region of the spectrum around the doublet of the spectral lines of doubly ionized oxygen O2+ (or O III), highlighting these lines (495.9 and 500.7 nm) against a darker background of the rest of outer space. Perhaps this is one of the most useful filters for visual observation of nebulae.
The filter comes in a colored cardboard box with a magnetic lock. Inside the box there is a filter in a plastic box and a bag, sealing material, as well as a filter bandwidth graph.
Venus visibility conditions are getting better every day. The brightness of the planet is so high that under favorable conditions it can be found with the naked eye during the day. The angular size at the time of the capturing was 16.72 “, the angular distance from the Sun was 42 degrees. It is interesting that the distance from the Earth to Venus was exactly 1 astronomical unit (150,000,000 km).
Details of Venus’s cloud cover are best obtained when shooting with a light filter that transmits only ultraviolet light. There are special filters for shooting Venus (for example, Baader U-Venus), but they are very expensive. As a budget solution, I decided to use a bunch of ZWB2 filters with a diameter of 20.5 mm and a NPZ SZS-22 1.25″.
ZWB2 transmits ultraviolet radiation with a maximum at 365 nm. I bought it without a rim – an inexpensive 1.25″ filter perfectly fit as a housing. The disadvantage of the ZWB2 filter is that it also transmits infrared radiation, to which the camera is also sensitive. To reducing it, a second filter is used – the NPZ SZS-22.