Which filters are suitable for an H-alpha astro 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.

For astrophotography, you have the choice between a full spectrum filter and a so-called astro filter. Both filters have advantages and disadvantages, but before we go into detail, here is a brief overview:

Full spectrum conversion for astrophotography

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

However, the colors produced by a full spectrum camera are clearly "off" due to the mixture of various wavelengths and require extensive post-processing if no additional filters are used. With long focal lengths and telescopes, this can usually be corrected relatively easily using a white balance. In addition to the shifted colors, other weaknesses of the lens will also present themselves. UV and IR light, which hit the sensor in certain proportions, are diffracted to different degrees. In general, an increased level of chromatic aberrations and color fringing is to be expected, which is why "naked" photography without additional filters is problematic.

However, as soon as photos are taken with wide-angle lenses, there are further problems - mainly due to the omnipresent light pollution. With a full-spectrum camera, this takes on a greenish tint instead of the usual yellow-orange hue. In addition, depending on the light pollution in the immediate vicinity, a clear purple coloration of the vegetation can occur.

For night landscapes with wide-angle lenses, we cannot recommend a full-spectrum conversion because the infrared components of the light pollution negatively affect the image. However, using it on a telescope can be useful, especially with additional filters.

Camera construction with a "real" astro filter

A "real" astro filter, specifically referred to as a luminance filter (often also called an H-alpha filter), is a UV/IR blocking filter that only allows the visible spectral range to pass, just as the original blocking filter does. However, the astro filter is open a little longer in the near infrared range, allowing the H-alpha spectral range to reach the sensor unhindered. Quality is of course also the top priority here, we use the Astrofilters of the company Optolong for our Astro Modifications.

A "real" astro filter has almost no problems with chromatic aberrations, because no UV or infrared areas enter the optical path. Furthermore, there is no untypical discoloration of the sky or vegetation due to light pollution, the landscape in the image appears natural and balanced. Of course, there is a color shift into red, but this can be corrected quickly and easily with a simple white balance - even with wide-angle lenses.

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.

Full spectrum conversion simply combined with an astrofilter

The great attraction of a full spectrum camera is that you can, in principle, simply screw any filter in front of the lens and start photographing. So why don't we offer screw-on astro filters?

Unfortunately, astro filters work on a different principle than infrared filters (interference instead of absorption), which has a decisive disadvantage: the filter effect is angle-dependent. As long as the light hits the filter surface vertically or almost vertically, astro filters are great and deliver very good and precisely defined results - with normal lenses, telephoto lenses and telescopes from 50 mm, there are no restrictions on the use of screw-in filters.

Unfortunately, astro filters are unsuitable as screw-in filters on wide-angle lenses. With wide-angle lenses, the light hits the filter at the edge of the image at a relatively acute angle, resulting in a different filter effect. What can be observed is a color shift from the center of the image to the edges - a very annoying effect that unfortunately cannot be easily corrected afterwards.

A full spectrum camera cannot therefore be used for night photography with an astro filter without further ado, at least not with wide-angle lenses. We strongly recommend using a camera with a permanently installed astro filter for wide-angle images to ensure the best possible image quality.