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.
Specifications declared by the manufacturer:
|8g / 0.3oz /0.02lb
|15g / 0.53oz /0.03lb
|Aviation Aluminum 6061T6
Filter frame diameter: 31 mm
Light diameter: 26 mm
Filter thickness: 7.5 mm
Frame thickness (without thread): 5 mm
Internal thread: none
External thread: 28.3 mm.
The spectrum of the Sun obtained using a simple spectroscope and a QHY5III462C camera:
The white area on the right is the wavelength range from 800 nm. As a reference filter, I used an H-alpha filter with a declared bandwidth of 12 nm. By the spectrum, Svbony SV183 IR Pass 685 nm 1.25″ is indeed an IR-pass filter, but the transmission of radiation starts already at about the wavelength H-alpha plus 6 nm, that is, about 663 nm. However, there may be some measurement error caused by a slight inclination of the filter relative to the spectroscope or light source.
I tested the Svbony SV183 IR Pass 685 nm 1.25″ filter with a QHY5III178m monochrome camera on several telescopes – Meade 70mm Quadruplet APO, Levenhuk Ra R66 ED Doublet Black, Celestron NexStar 8 SE. In all cases, the filter showed a reduction in turbulence compared to shooting through a color camera with an IR-cut filter.
Source video sample:
The result of stacking frames and sharpening:
-Meade 70mm Astrograph Quadruplet APO Refractor
-Meade LX85 mount
-Barlow lens NPZ 2x
-Svbony SV183 IR Pass 685 nm 1.25″ filter
Processing: stacking of 100 frames from 3006 Autostakkert, wavelets and deconvolution with AstroSurface.
Location: Russia, Anapa, backyard.
You can see my other images taken with the 685 nm filter from Svbony at the links:
Svbony SV183 IR Pass 685 nm 1.25″ – inexpensive and high quality filter for infrared photography. I recommend it to all owners of monochrome cameras, as well as mirror or mirror-lens telescopes, who want to get detailed images of the lunar surface and planets in the near infrared range.
You can buy a Svbony SV183 IR Pass 685 nm 1.25″ light filter in the official Svbony store on Aliexpress
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