Svbony has released an interesting light filter against light pollution — SV240 Multi-Narrowband Filter Galaxy & Nebula NIR 2″. It passes the ranges H-beta + OIII, H-alpha, and the near infrared range. According to the manufacturer, the filter is universal, since it allows you to shoot not only emission nebulae, but also galaxies due to an additional signal in the infrared range, but this requires a color camera with good sensitivity to infrared rays (imx462, imx464, imx585 sensors), while the protective glass in the camera should be just AR (without IR-cut coating), and the telescope used should have good chromatic aberration correction at near IR range.
Declared characteristics from the manufacturer’s website:
| Brand | SVBONY |
| Model | SV240 |
| Name | Multi-Narrowband Filter |
| Size | 2-inch |
| Substrate Thickness | 2mm |
| Clear Aperture | 44mm |
| Wavelength Range | 300-1100nm |
| FWHM | 24nm & 20nm & 115nm |
| Blocking | >OD5@300-450nm；>OD4@560-620nm；>OD2>690-750nm； >OD3@940-1100nm |
| Peak Transmittance | T＞95%@486.1nm；T＞92%@495.9nm；T>85%@500.7nm；T>95%@656.3nm； Tavg>95%@830-870nm |
| Surface Quality | 60/40 |
| Surface Parallelism | 1/4λ |
| Thread | M48x0.75 |
| Single Thread | Yes |
| Net Weight | 12.5g / 0.44oz /0.027lb |
APPEARANCE
The filter is supplied in a cardboard package and a plastic box for storage and transportation. The filter frame is metal. Thread only on one side (at the outlet). The filter surfaces are mirrored. When held up to the light, the image is painted greenish-blue. There are no defects in the coating.
MEASUREMENTS
filter weight — 13.6 grams
filter weight in a plastic box — 34.1 grams
frame diameter — 50.73 mm
input diameter — 43.90 mm
output diameter — 44 mm
filter frame thickness (without thread) — 5 mm
thread height — 2 mm
thread diameter at the outlet (male) — 47.77 mm.
TESTING
I checked the filter’s transmission using a spectroscope. In terms of transmission in the blue-green range, the Svbony SV240 2″ filter turned out to be very similar to the Optolong L-Enhance 1.25″. In the red range of the spectrum, the SV240 is wider. There is transmission of radiation in the near infrared range, as stated by the manufacturer.
A FEW PRELIMINARY REASONS
1. When shooting multispectral (IR + visible range), you may need to use a mirror or mirror-lens telescope, since the infrared and visible radiation of a lens telescope may not be focused at the same point.
2. Certain questions arise regarding the white balance setting, since the infrared component will be mixed into the visible range. For example, when shooting Jupiter in the visible range using a color astronomical camera Svbony SV505C (imx464 sensor) with an IR-cut filter (blocks IR, passes the visible range), I set the white balance values to 134 for the red channel, 108 for green and 194 for blue. But more on that later.
For shooting with an IR-pass 850 nm infrared filter (passes IR, blocks visible range), I set the values to 128, 128, 128 to get a roughly monochrome image.
PHOTO TEST OF THE ORION NEBULA
Object: Orion Nebula M42.
The following equipment was used for testing:
-Sky-Watcher BKP150750 telescope
-SharpStar 0.95x 2″ coma corrector
-Celestron CG-4 mount
-Svbony SV226 filter drawer
-астрономическая камера QHY5III462C.
IR-cut and 850 nm filters were installed between the camera and the coma corrector. Svbony filters were installed on the coma corrector.
Shutter speed 300 ms, gain 468. White balance R, G, B: 66, 56, 106.
Examples of single frames with dark frame subtraction:
It is clear how well the city light is blocked. This is a narrow-band filter (7 mm for H-alpha and OIII is stated).
Here the image is a little brighter, because the signal from H-beta has been added to OIII.
The image is even brighter, and on all channels – the IR component has been added. It is not much, but it is there.
White balance R, G, B: 64, 64, 64
What about IR pass? Unfortunately, I don’t have an IR-pass filter for 790 nm, but I do have an 850 nm one. The shutter speed is the same, 300 ms, white balance R, G, B: 64, 64, 64
It turned out very dark. Considering the bandwidth, there should also be an IR component from 790 nm – it is possible that even with the same white balance coefficients the image will go into red, so to get a “monochrome” for the galaxies, you will need to adjust the white balance a little.
Let’s increase the shutter speed to 2000 ms with the same gain of 468:
Faint stars, invisible because of the bright nebula, immediately appeared.
PHOTO TEST OF THE NEBULA “THOR’S HELMET”
Shooting conditions: city center, LED lights.
Equipment:
-Sky-Watcher Evoguide 50ED telescope
-Sky-Watcher AZ-GTi mount
-Svbony SV226 filter unit
-Svbony SV240 multi-bandpass 2″ filter
-QHY IR-cut 1.25″ filter
-QHY5III462C astronomical camera.
Stacking 452 frames of 5 seconds each.
PHOTO TEST ON GALAXIES
For me, this was the most intriguing part of the testing. Note: I used an IR-sensitive camera with a Sony imx462 color sensor for testing. Shooting conditions: city center (LED lights around), haze, new moon.
Equipment:
– Meade 70mm f/5 ED quadruplet apo telescope
– Celestron CG-4 mount
– QHY5III462C astronomical camera.
First I took a picture with IR-cut QHY filter, shutter speed 10 seconds, white balance (R, G, B) 66, 56, 106. You can see the light background from the overexposure.
Then, without changing the shooting parameters, I changed the IR-cut filter to a Svbony SV240 2″. A little refocusing was required.
You can see the bluish tint of the galaxies due to the white balance. I increased the shutter speed to 30 seconds and set the balance to 64, 64, 64:
The color of the galaxies and background has become more neutral. Impressive. Let’s look at the histogram with white balance 64, 64, 64:
The histogram shows that the red and blue channels are almost the same intensity, the green is slightly shifted to the right. You can slightly reduce the value of the green channel, but I decided to return to the white balance of 66, 56, 106 and look at the histogram:
Conclusion – when shooting galaxies with the Svbony SV240 2″ filter, it is necessary to set the same (or almost the same) white balance values.
I then pointed the telescope at the famous Leo triplet and set the white balance again to 64, 64, 64.
Galaxies are visible and almost without color cast. Very good.
Next I started shooting a series of frames.
IMPORTANT. When shooting flat field shots, I shot two series of shots – one with a white balance of 64, 64, 64, and the other with 64, 41, 64, using the histogram to ensure that all three humps were at approximately the same level. I used a laptop screen as a flat panel. Flats with a white balance of 64, 64, 64 gave a grid structure after calibration in DeepSkyStacker:
When calibrating with flats 64, 41, 64 the image is without a grid artifact. In general, when shooting flats I always try to set the white balance so that the image is without color cast, and the histogram humps of different colors are approximately at the same level.
You may need to set slightly different white balance values, but stick to the same values when shooting, and use the histogram as a guide when shooting flat shots.|
And here is the Leo triplet obtained after calibration, addition and transformation of the histogram:
There is potential, and if you accumulate enough signal (at least a few hours), you can get a decent result even in urban light conditions. I did not notice any problems with focusing, parasitic glare or halos with my ED telescope, but in other lens telescopes with different chromatic aberration correction the image may be completely different.
PHOTO TEST OF THE STAR CLUSTER M13
In this test I used a small 50 mm ED-telescope with two lenses. I installed the Svbony SV240 2″ filter BEFORE the telescope lens via an adapter.
Equipment:
-Sky-Watcher Evoguide 50ED telescope
-Celestron CG-4 mount
-QHY5III462C astronomical camera.
See what happened.
We remove the IR-cut filter, but leave the white balance the same:
We install the Svbony SV240″ light filter in front of the lens and…
…a surprise in the form of elongated stars. At first I thought there were some problems with tracking, then I turned the filter and realized that the elongated stars had also turned.
Let’s set the white balance to 64, 64, 64 and increase the shutter speed to 2 seconds:
I tested this filter, as well as the Svbony SV220 2″ (prototype) – the stars are also elongated with it. I found my old Baader UHC-S 2″ filter and did not see any deformation of the stars. I tried to loosen the filter frame, but it did not help. A separate test with an artificial star confirmed the effect of the filter position on the image. At the same time, in the subaperture mode, when installing the filter close to the matrix, the shape of the stars is not distorted even when using a camera with a small pixel of 2.4 µm. So the Svbony SV240 filter, and perhaps the SV220, should not be installed in front of the lens. At the same time, I did not notice any defects on the filter or swirls in the glass. Perhaps this is some kind of wedge-shaped glass. Just keep this in mind.
Here is another full-aperture test of the Orion Nebula through a 50 mm ED telescope:
VISUAL TEST FOR EMISSION NEBULAES
Having already seen the manufacturer’s stated graph, I immediately realized that this is a potentially interesting filter for visual observations of emission nebulae due to the transmission of OIII and H-beta. In fact, this is a good UHC filter with a narrower bandwidth than budget UHC and CLS filters. H-alpha and IR components will not be visible, and they are not needed for visual observations in this case. For testing, I used a Sky-Watcher Dob 14″ SynScan GoTo telescope with a 30 mm 70 degree eyepiece under a dark country sky.
Objects:
-Orion Nebula
-planetary nebula M27
-planetary nebula “Helix”
-nebula “Veil”
-nebula M17 “Omega”
-nebulae M8 and M20
The Svbony SV240 2″ filter demonstrated excellent improvement in the visibility of emission nebulae during visual observations, darkening the background, muting the stars and highlighting the nebula itself. The stars are colored blue-green – this is normal. In urban light pollution conditions, the filter will also work during visual observations, but much worse than in a dark sky. Also, there is no point in using the filter for visual observation of galaxies and star clusters due to significant signal attenuation.
SUMMARY
The Svbony SV240 2″ filter effectively suppresses urban light pollution. Without the filter, the shooting result was much worse. When shooting emission nebulae, I still recommend using an additional IR-cut filter, if it is not built into the protective glass of your camera. You can try shooting with and without the IR-cut filter – if there is no difference, then you can leave only the Svbony SV240 2″ filter when shooting. When shooting galaxies with a camera sensitive to IR radiation, the blocking IR filter must be removed, and the same white balance values (R, G, B) must be set in the shooting settings.
Svbony SV240 2″ is not suitable for working in full-aperture mode (in front of the lens), since the filter introduces distortions into the shape of stars. That is, it should not be installed on a photo lens in front or on binoculars. In subaperture mode (when installed near the matrix), I did not notice any distortions in the shape of stars. During visual observations with the filter installed in front of the eyepiece, at low magnifications, I did not notice any distortions in the shape of stars.
I can confidently recommend the Svbony SV240 2″ filter to astrophotographers and fans of visual observations of emission nebulae, but strictly for subaperture use.
You can buy a filter in the official Svbony store on Aliexpress.
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