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Quality RGB Filters for Less: Astronomik Deep-Sky RGB Filter Review

Astronomik Deep-Sky RGB filters with mounting cells. Credit: Astronomik

With an increasingly broad range of simple yet capable equipment, astrophotographers can pursue a wider range of targets than ever. With dramatic improvements in camera technology, along with significant cost reductions, monochrome cameras are now available at a comfortable price point.

Astronomik’s Deep-Sky RGB series of filters, which I’ve been testing for a few months, is one of the new reasonably priced accessories available for monochrome cameras.

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L-RGB image of M81 and M82 captured with the Astronomik Deep-Sky RGB filters. Credit: Rouzbeh Bidshahri

Pricing

Deep-Sky RGB series filters start at EUR 217-242 ($215 to 242 at the time of writing) for the smaller 27mm and 1.25-inch sizes. The popular mid-sized 36mm filters suitable for APS-C sensors cost $333, and the large 50mm “unmounted” size for full frame sensors cost $500.

That means an Astronomik 50mm filter set will set you back only about one-third what you’d pay for high-end brand offerings. Astronomik sells the RGB set separately from the Luminance filter, allowing users to choose the best option based on the color correction of their telescopes and light pollution at the imaging site.

Packaging and Shipping

The filters arrived in separate plastic cases with padding, something even higher priced brands don’t always offer. The filter cases had a quality control sticker, and each individual filter case had a its own serial number printed on it, which I really like.

Filter cases with serial numbers and seals. Credit: Rouzbeh Bidshahri

Mounting

I use a 43m full frame sensor, which requires 50mm round unmounted filters. While a 2-inch filter has a diameter of 50.8mm, the M48 mounting holes take up space toward the outside edge, leaving only 44mm clear aperture. Because the light cone is converging towards the focal plane, it’s larger than 43mm at the position of the filter.

I purchased the 50mm round unmounted filters, which turned out to actually be mounted in a cell. This could be confusing since they look similar to the 2-inch filters, but they have no mounting threads like the 2-inch filters do. Also, the mounting is just a thin protective ring that doesn’t restrict the clear aperture. After mounting on the inner lip of a filter wheel carousel, the clear aperture will be about 47-48mm.

50mm round unmounted filter with cell.  Credit: Rouzbeh Bidshahri

The Astronomik 50mm round filters measure as follows:

  • Outer Diameter: 49.83mm
  • Inner (clear) Diameter: 47.15mm
  • Protective ring Thickness: 3.6mm
  • Glass thickness: 1mm

I really like the Astronomik protective ring design as it makes handling filters much easier. Also, the 3.6mm thickness makes it easier to secure them a in filter wheel compared to a plain 1mm glass filter. Having the metal outer ring means the mounting screws put pressure on the ring and not the actual glass, which can crack if overtightened.

Furthermore, the ring acts as an edge mask that eliminates the possibility of any light leaks or reflections from the outer edge of the filter.

Astronomik mounting cell used to secure the filter in a typical filter wheel carousel. Credit: Rouzbeh Bidshahri

Sizes Available

Astronomik offers the RGB filters in 7 sizes covering almost everyone’s needs.

  • 1.25 inch and 2 inch: Threaded with M28.5 and M48 threads, respectively.
  • 27mm: Plain glass without threads or protective cell.
  • 31mm, 36mm, and 50mm: Round filters housed in a thin metal protective cell.
  • 50x50mm: Glass filters only without cell.

Color Balance

Astronomik state that they designed the new Deep-Sky RGB with a maximum transmission of 95% and a spectral response to achieve 1:1:1 color balance. Having a better color balanced filter set makes image processing a lot easier. Minimal post processing and adjusting is required as the RGB color image has a more neutral color profile to begin with.

At first glance, the color profile of the filters certainly didn’t seem balanced. Looking at the published transmission graphs, the total green bandwidth is narrower than that of red and blue. From that data, we can measure the green bandwidth, centered at 530nm, to be 18% narrower than the red bandwidth centered at 645nm.

Astronomik Deep-Sky RGB filter transmission graph. Credit: Astronomik

However, we have to understand how our imaging sensors respond to light, described as the quantum efficiency of the pixels, i.e., how well they convert incoming photons (light) into electrons (signal).

Most manufacturers publish only the peak efficiency, which can be misleading. Better is an overall measure of the sensor’s efficiency at every wavelength as shown in a spectral response curve.

Quantum efficiency of the Sony IMX CMOS sensor used in many cameras such as the QHYCCD 268M. Credit: QHYCCD

The latest sensors are more efficient at recording green light than red. The popular Sony IMX sensor, for example, is 16% more responsive to green light than red. This is why Astronomik designed the green filter to allow 18% less light over that bandwidth. This compensation factor allows the final image to be very close to balanced with a 1:1:1 overall color reproduction.

Dealing with Light Pollution

Light pollution, unfortunately, is something most astrophotographers have to deal with. Narrowband filters can be in some cases to work around the wavelengths emitted by the lights but broadband targets still require the full RGB color spectrum to produce an accurate representation of deep sky objects.

Here, Astronomik Deep-Sky filters have a feature I really like – a “notch”, or narrow bandwidth, where light from around 580nm to 600nm is blocked.

Astronomik Deep-Sky RGB transmission graph showing notch blocking a high proportion of common light pollution sources. Credit: Astronomik

This is a range where we see orange, and it’s where a lot of industrial and city lights produce the most light pollution. The orange light of sodium vapor lamps is quite common, for example, as is the orange glow above cities seen from afar.

High pressure sodium vapor streetlight and its spectrum. Credit: Xiaopihar (above), CoolKoon (below) (Creative Commons)

Modern LED lights are more difficult to filter out as they spread their light over the entire visible spectrum, so broadband filters can’t filter out all the offending light pollution wavelengths. These filters are better equipped to mitigate some of the effects of light pollution, however, compared to filters without these notches.

Results

Results with the Deep-Sky RGB filters were very good. My image of the broadband target galaxy IC342 has the even color balance claimed by Astronomik. That’s after capturing equal sub-exposures of 300 seconds with an almost equal integration of 2.5 hours per filter.

The telescope used was a Planewave CDK and the camera was a the monochrome QHYCCD 600M with the Sony IMX455 CMOS sensor. The image not only looked visually neutral, the histogram showed a well-balanced RGB color profile.

RGB channels combined with basic screen stretch in Pixinsight showing a well-balanced RGB histogram profile.  Credit: Rouzbeh Bidshahri

The image above was a basic RGB stack after with luminance and hydrogen-alpha data added and post processing. The final image shows appealing colors.

IC 342, the “Hidden Galaxy” imaged through the Astronomik Deep-Sky RGB filters. Credit: Rouzbeh Bidshahri

Another target captured with the Deep-Sky filters, the Fireworks Galaxy NGC 6946 (below) also revealed pleasant colors.

NGC 6946, the “Firework Galaxy” imaged through the Astronomik Deep-Sky RGB filters. Credit: Rouzbeh Bidshahri

Final Thoughts

After several months of imaging with the Astronomik Deep-Sky RGB filters, I found them to perform very well without any issues like reflections or haloes. The images rendered also had good contrast.

Astronomik’s claim that these filters achieve a balanced color reproduction were evident with my monochrome camera. Processing was straightforward, rendering images with pleasant colors. I also found the light pollution blocking notch useful since my site is surrounded by streetlights.

Astronomik clearly put a lot of thought and effort into producing a good RGB filter set at a very attractive price point. They’re worthy of becoming a popular choice for many astrophotographers.

 

MSRP: $215 to $588

Website: www.astronomik.com

 

High-resolution versions of the images are available at:   rouzastro.com/astrophotography/

 

 

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About Rouzbeh Bidshahri

Rouzbeh Bidshahri is a mechanical engineer with a lifelong passion for astrophotography. He has tested dozens of telescopes ranging from 3 to 20 inches in aperture and has spent several years optimizing systems for very high-resolution planetary imaging in the sub 0.1 arcsecond/pixel range. He has contributed to several institutions such as ALPO (The Association of Lunar and Planetary Observers). His main area of interest has been designing and operating larger setups, and he is currently focusing on high resolution, long exposure photography for both broadband and narrowband deep sky imaging.

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