STC Duo Narrowband Astro clip in filter

STC Duo Narrowband Astro clip in filter

I was an astrophotographer first, before I was a photographer.  And I don’t mean photographing tiresome Milky Way galaxy backgrounds with different foreground landscapes.   I was imaging planetary, solar and deep space objects.   It is a highly complex niche world of photography that will leave you sleep deprived and financially impoverished.  Many books have been written on the subject.  I even wrote one in 2015, Astro Imaging Projects for the Amateur Astronomer – A Maker’s Guide.

I live in the metropolis of Toronto, which some of you may be surprised to learn is now the fourth largest city in North America – after Mexico City, NYC and LA.  The light pollution in the city is intense, but it is possible to conduct planetary and solar photography from my front lawn and driveway.  It’s just not the optimal place on the planet to do so, for geographic reasons such as the proximity of the Great Lakes.   But to image deep space objects like galaxies and nebulas, one has to drive about two hours into the countryside so that the skies are dark enough that these faint objects become visible to the camera sensor.  At the same time you have to carry a lot of gear with you and portable power.  Set it all up and stay awake all night.  And then tear it all down at dawn and make the drive home in time to go to work.  Cause it’s never clear and cloud free on weekends.  And in Toronto we have this added thing … called winter.

It is possible to image certain emission nebulas from the within the city and one frigid winter I was out every clear night imaging from my front door steps with long USB cables snaking back into the warmth of the house where my laptop would be controlling the camera and telescope mount as well as writing image data to its hard drive.   I would let it run all night and go to bed and awaken a little earlier to put away the gear and have a look at all the data I had collected.  Processing that data into an image would take several more happy hours of computer interaction.

Nebulas are large gaseous structures of active star formation.  Nebulas can also represents the expelled corona of a dying star or one that has gone super nova (exploded).  Emission nebulas are a further subclass that actively emit light at certain specific wavelengths associated with the fall of electrons from higher quantum levels to lower ones of certain elements.  The two most common are called hydrogen alpha (Ha) and occurs in the red spectrum near the IR border and oxygen III (OIII) in the green/blue region.   A Ha narrowband filter is a special imaging filter that passes narrowly at that specific wavelength.  By imaging with a dedicated monochrome astronomical camera, I would spend one to several nights (because the filter is so restrictive and the resultant image will be so faint) obtaining data for a Ha image and similarly for an OIII image.  By mapping the Ha data to the red channel and the OIII to the green and blue channels, one can construct a false color image that would be similar to what the nebula would look like to human eyes, if you were to observe the phenomenon close up from the observation deck of a starship.  Even though some emission nebulas, like the Great Orion Nebula , are visible through binoculars and even the naked eye, they are so dim that the human eye cannot perceive color, only gray levels.

The restrictiveness of these narrowband filters allows them to punch through light pollution but images need long exposures.  A composite of hours of exposure.  This is why I greeted the announcement that STC was introducing a narrowband clip in filter that passed at both the Ha and OIII wavelengths with excitement.  I had been waiting for a filter like this for many years, possibly being amongst the first amateur astronomers to propose this filter design nearly a decade ago.   With recent improvements in sensor technology found in DSLR and mirrorless cameras, this would allow simultaneous collection of all colour data reducing the time and effort to perform narrowband imaging.  That the clip in filter would fit all EM cameras (except the first generation E-M5), the Pen F, the EPL7 and newer, and all Panasonic bodies meant a wide range of m43 lenses and adapted lenses could be used instead of a telescope.

OLYMPUS DIGITAL CAMERA
STC Duo Narrowband Astro clip in filter in place (E-M1.1)

The only question remaining is how good are the optical properties of these filters given that STC is a relatively unknown company.  Sense-tech Innovation Company was founded as a Taiwan optical and chemical company in 2010 whose core products are a large variety of photographic filters featuring optical coatings that they developed.  Their website shows technical specifications of a bandpass of about 15 nm at full width at half maximum (FWHM).  This is very promising in terms of fighting light pollution yet being wide enough to allow imaging with a reasonably short long exposure.  I ran the filter through my spectrophotometer and despite differences in transmission magnitudes I also obtained a FWHM of about 20 nm.

STCNarrowProfile3

Any discrepancies with the STC transmission profile I attribute to using my 1970s era Hach portable water testing spectrophotometer unit which may no longer be fully calibrated or functioning to specifications but it has the singular virtue of having a large sample chamber that will easily accommodate the STC clip in filters.

Screen Shot 2019-11-14 at 9.53.33 AM
Hach DR-EL/2 portable spectrophotometer

 

And how well does it work?  Very well indeed.

NarrowbandM42
The Great Orion Nebula, 2.5 hours of 2 minute subexposures. STC filter, E-M1X, ISO400, 0.71x focal reducer and AP Traveler refractor operating at 430mm f/4 … from my front lawn in Toronto.  The same photographic gear and settings are used in the following narrowband images.
composite
The Three Faces of Orion – (from left to right)  Orion Nebula from Toronto, Orion Nebula from a dark site in the countryside, Orion Nebula from Toronto with STC filter.
60x2minute STC duopass astro filter  1/2 Moon 45 degrees east of Nebula
Horse Head (dark) Nebula & Flame Nebula:  in proximity to Altinak, one of the stars comprising Orion’s Belt.  60x2minute STC duopass astro filter 1/2 Moon 45 degrees east of Nebula likely responsible for lens flare in image.
27x3minutes subs
Rosette Nebula  –  90 minutes of 3 minute subexposures.
OLYMPUS DIGITAL CAMERA
EQ6 equatorial mount, guidescope, laptop tracking celestial target as it moves across the sky from East to West.

The performance of the STC duo narrowband astro filter exceeded my expectations, there is serious detail and depth from the emission nebula images taken from within the city limits of Toronto.  It has reawakened my passion for astro imaging from which I have taken a break over the past few years to pursue other interests.  Without a permanent observatory setup, it literally takes an hour to just set up all the supporting equipment needed to image deep space objects.  Using my Olympus mirrorless body means that the setup is less complicated and post processing also simplified.

Stay tuned for Part 2 – STC Astro Multispectral filter evaluated and thermal dark signal production by Olympus bodies.

The image at the very top of the article is Messier 76 – The Little Dumbbell nebula is the gaseous shroud of a dying star photographed from my front door steps in 2010 using the false color narrowband approach described in the text.

There have been some comments about the quality of the images so let me explain.  Two hours is the bare minimum exposure for some of these targets, longer exposures would result in improved IQ.  As a result, I had to apply significant noise reduction on the images to clean up a lot of noise and the images suffered.

 

If you press [HOME] on the upper right you can see the entire list of topics in this blog.

 

 

 

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