Astro Filter: Photographing stars at night with a modified camera

Die Infrarot Fotografie zeigt bekanntlich vertraute Landschaften in einem abstrakten, künstlerischen und neuen Licht. Bei der Astro- oder Sternfotografie geht es aber eher darum, die Objekte möglichst “natürlich” abzubilden, so wie wir sie auch sehen. Das ist vor allem für Nachtlandschaften wichtig, bei denen auch noch ein spannender Vordergrund zu fotografieren ist.

Advantages of an astromodified camera

Modern digital cameras have excellent sensitivity combined with good noise performance and high dynamic range. Yes, you can even capture the starry sky in acceptable quality with your smartphone. So what on earth is there to improve in this kind of photography? To answer this question, we need to talk about the areas of the spectrum that we can no longer see with a normal camera.

What is H-Alpha radiation?

A shining object emits light. This can be a candle, an LED lamp or a star. But the light is not the same: depending on the light source, it differs in its spectrum - simply put, in the distribution of colors. What we perceive as white light is actually a mixture of all spectral colors. What a spectrum looks like, i.e. which colors it contains, depends, among other things, on the composition of the light source at the atomic level. By far the most common atom in the universe is hydrogen (symbol H), and now it gets interesting again.

When hydrogen is excited (e.g. by heat or radiation) it emits light. Not white light, but very discrete wavelengths (i.e. colors). The strongest color is the so-called H-alpha radiation - it has a wavelength of exactly 656.28 nanometers (nm). This wavelength corresponds to a deep red in the spectrum visible to us. While we can just barely see this wavelength with our eyes, most digital cameras unfortunately have difficulty doing so.

Astrophotography with a normal camera

Depending on the manufacturer and model, the H-alpha sensitivity of a digital camera is only between 10 and 30 %, typically around 15 %. This makes these spectral ranges practically invisible in the already very dark night. The reason for this is the UV/IR blocking filter (also called Hotmirror), which is installed by the manufacturers directly in front of the sensor. This is necessary to ensure good color reproduction during the day, but is just as much a thorn in the side of Astro photographers as it is for Infrared photographers. Therefore, the blocking filter is removed during astromodification and replaced by a filter that allows the H-alpha radiation to pass.

Astromodified camera vs Normal camera

Auch wenn der Anteil von Wasserstoff im Universum sehr groß ist, so ist die Verteilung nicht gleichmäßig. Es gibt Objekte im Nachthimmel, die einen sehr geringen Anteil an H-Alpha Strahlung abgeben und es gibt Objekte, die fast ausschließlicht in diesem Bereich abstrahlen (z.B. einige Nebel). Eine erhöhte H-Alpha Empfindlichkeit macht diese Bereiche auf den Bildern überhaupt erst sichtbar, was zu einem viel interessanteren und “farbenfroheren” Bild führt. Was durch einen Umbau allerdings nicht wesentlich verbessert wird ist die Grundempfindlichkeit der Kamera. Obwohl theoretisch mehr Licht auf den Sensor gelangt, fällt der tatsächliche Helligkeitsgewinn in den Bereich unter 1/3 EV.

Which filters are suitable for an H-alpha conversion?

For astrophotography a pure IR camera with e.g. 630 nm infrared filter is not the best choice. Of course, after the conversion, the cameras are sufficiently sensitive to be able to capture the stars at night, and they also see the H-alpha regions of the spectrum. However, as described at the beginning, astrophotography is typically interested in imaging the natural conditions. But if you want to hunt for stars with an infrared camera, I don't want to stop you at all.

Full spectrum conversion for astrophotography

A Full Spectrum conversion with 280 nm long pass filter opens the camera for the complete spectrum from UV to IR, including the H-alpha range. Such a camera can be operated without additional filters with an actual gain in sensitivity. On the other hand, one is very flexible and can use pre-screw filters on the lens or telescope to narrow the spectrum. Multispectral imaging with narrow bandpass filters and rather scientific analyses become possible, you really have the maximum freedom with such a camera.

Allerdings sind die Farben, die eine Vollspektrum Kamera produziert durch die Mischung diverser Wellenlängen deutlich “daneben” und bedürfen aufwändiger Nachbearbeitung, wenn keine zusätzlichen Filter benutzt werden. Mit langen Brennweiten und Teleskopen ist dies in der Regel relativ einfach durch einen Weißabgleich korrigiert. Neben den verschobenen Farben werden sich zusätzlich weitere Schwächen des Objektives präsentieren. UV und IR Licht, die zu gewissen Anteilen auf den Sensor treffen, werden unterschiedlich stark gebeugt. Grundsätzlich ist mit einem erhöhten Maß an chromatischen Abberationen und Farbrändern zu rechen, weshalb das “Nackte” Fotografieren ohne zusätzliche Filter problematisch ist.

However, as soon as wide-angle lenses are used, there are further problems - mainly because of the omnipresent light pollution. This gets a greenish tint with a full spectrum camera, instead of the usual yellow-orange hue. In addition, depending on the light pollution in the immediate surroundings, a distinct purple coloration of the vegetation can occur.

For nightscapes with wide-angle lenses, I cannot recommend a Full Spectrum conversion unless a additional Clip-In filter is used (more on this in the next section). However, using it on a telescope can be useful, especially with additional filters.

Kameraumbau mit einem “richtigen” Astrofilter

Ein “richtiger” Astrofilter, oder auch Luminanzfilter genannt, ist ein UV/IR Sperrfilter, der nur den sichtbaren Spektralbereich passieren lässt, so wie der original verbaute Sperrfilter. Der Astrofilter ist jedoch im nahen Infrarotbereich noch etwas länger offen und ermöglicht so dem H-Alpha Spektralbereich ungehindert zum Sensor zu gelangen. Qualität ist natürlich auch hier die oberste Prämisse, wir benutzen die Astrofilters of the company Optolong for our Astro Modifications.

Ein “richtiger” Astrofilter hat so gut wie keine Probleme mit chromatischen Abberationen, weil keine UV oder Infrarot Bereiche in den Strahlengang gelangen. Außerdem zeigt sich keine untypische Verfärbung von Himmel oder Vegetation durch Lichtverschmutzung, die Landschaft im Bild wirkt natürlich und ausgeglichen. Natürlich kommt es zu einer Farbverschiebung ins Rot hinein, aber dies ist durch einen einfachen Weißabgleich gut und schnell korrigiert – auch mit Weitwinkel Objektiven.

All in all, the Astro filter has practically eliminated all the disadvantages of a full spectrum conversion (even if no increased overall sensitivity can be observed per se). However, one important disadvantage remains: These filters are relatively expensive and come with additional cost compared to a Full Spectrum camera. If you plan to buy more (bandpass) filters anyway, it would be cheaper with a full spectrum conversion, at least for the work on the telescope.

Last but not least, there is an excellent way to use a full spectrum camera at night as an Astro camera (and as an Infrared camera during the day): With Clip-In Filters. These are small filters that are attached directly in the bayonet between the lens and the sensor. Astro filters unfortunately do not work in front of the lens with wide-angle lenses (under 50 mm KB equivalent), so these filters must definitely be placed in front of the sensor. Excellent Clip-In filters can be ordered e.g. from the company Astronomik (Unfortunately not yet available for all camera manufacturers).

Which camera is best for a Astro conversion

Just like for an Infrared conversion, the following applies to an Astro conversion: The sensors of all manufacturers are practically identical in terms of their sensitivity in the H-Alpha spectral range. Much more interesting and important is the noise behavior and the dynamic range of the sensor - after all, we shoot in very dark light conditions with high ISO values. This is not much of a challenge for all cameras that have come onto the market since the mid-2010s. Unfortunately, older cameras will have a hard time in this discipline. So in short: Every reasonably modern camera is suitable for an Astro conversion.

However, there is a technical limitation, which is why not all cameras from the pricelist are available in the Shop for a Astro conversion . There are cameras that have an infrared LED installed internally, often this is part of the shutter mechanism. The stray light of these LEDs disturbs the image during a night shot and makes it unusable in the worst case. This problem affects e.g. all Sony Alpha 7 cameras of the 2nd generation, which is why they are not selectable for an Astro conversion.