The two tails of the comets
The most impressive tail of a comet is caused by sunlight which is reflected and scattered by the dust trail of the comet - also called dust trail.
In addition, there is often a gas tail, depending on the amount of gas emitted by the comet. This gas is ionized (excited to glow) by the energy of the solar wind. This ion tail shines preferably with the emission lines of OIII (501nm) and Cyan (511 and 514nm).
If you want a better contrast for the comet in the sky, you must consider the following with the choice of suitable filters:
There is no way to show only only the dominant light fractions of the dust tail (as with emission nebulae e.g. in H-alpha) and to simply block the other wavelengths, because it shines in all colors.
Here it it the same problem as with the observation of galaxies - you can only try to block or reduce the general light pollution and to let the rest of the light through as unblocked as possible. Of course, with weak comets the brightness of the dust tail will also be weakened somewhat, but the sky brightening/light pollution is drastically more strongly suppressed. This increases the contrast and the comet stands out better against the background of the sky.
These filters work better for brighter comets, since they tolerate a dimming of their light better than e.g. dim galaxies. Of course, this only helps if the main part of the interfering light is caused by artificial light pollution and not by the atmospheric twilight shortly after sunset. Thus, the classic broadband Deep Sky Filter (Baader UHC-S) is perfectly suited for the observation of all comets, especially in the late night. Narrow-band line filters, on the other hand, should be avoided with regard to the dust component.
Another part of the interfering light is the atmospheric airglow phenomenon and often the moonlight. Especially in dark locations far away from the city, a Moon&Skyglow filter (Baader neodymium filter with UV/IR blocker) can produce a better effect against these light sources than a pure Deep Sky filter. Such a contrast filter also provides a more natural image due to its more even spectral curve.
The Baader Semi-APO filter, which contains the same glass substrate as the Moon&Skyglow filter, offers a similar, but even more contrast enhancing effect. However, an elaborate coating system additionally dims the blue spectral component and makes the dust trail stand out even more against the background of the sky.
Especially in the bright twilight (also known as the Blue Hour), directly after the comet appears again from behind the sun (after the perihelion passage - when all comets are by far the brightest) it is especially important to dampen the high proportion of blue light in the atmosphere to increase the contrast. This is especially successful with the Baader FringeKiller filter. Most comets also have a blue-luminous ionized gas trail; therefore the broadband red filters often recommended for neutralizing the blue light will have no positive effect.
The Baader FringeKiller filter, which was actually developed for simple refractors, has the advantage that it suppresses the blue atmospheric glow but still maintains full transmission for OIII and Cyan, which are so important for the gas tail, especially because this filter does not attenuate the visual spectrum otherwise.
With regard to the ion or gas tail, it is possible to capture these discrete lines of ionized gas via narrow band filters with maximum contrast, but unfortunately these emission lines differ from comet to comet. Theoretically one would need a narrow-band line filter individually tailored to each comet's spectrum. Thus such a filter would only be commercially available much too late - after the passage of the comet. And it might not be equally effective for the next comet if the gas tail of this comet shines even in a slightly different range of the spectrum. The most probable, i.e. most promising, lines are the 500-514nm spectral bandwidth mentioned above. A wide OIII filter (Baader visual 10nm O III filter), which is slightly open towards the red, is suitable for emphasizing the gas tail in these wavelengths. Since this Baader filter was also designed for the use at high open telescopes (such telescopes produce a ZWL blue shift) it shows this also for comets favorable light redshift.
If we look at the above situation with the currently available filters, the following conclusion can be drawn:
The Baader Neodymium Moon&Skyglow filter is very well suited for twilight and for natural image reproduction - especially to improve the view of the dust tail. It would also be the first choice for gas-rich comets if it would not attenuate the cyan lines significantly at the same time. Nevertheless it dims the main part of the comet light only very slightly and should therefore be especially recommended as the cheapest entry into comet observation with contrast increasing filters.
Especially the Baader UHC-S filter which was developed against light pollution in cities should have the greatest effect as a universal filter, especially after dusk. Because in addition to its strong suppression of light pollution near the city, it also highlights the above-mentioned gas lines of comets through its wide OIII window.
Not yet verified in practice (due to lack of large comets), the Baader Semi APO filter should be highly efficient, especially at dusk. This filter combines all good properties of the Baader Moon&Skyglow filter, but in addition it also cuts the blue range of the spectrum, i.e. the twilight interfering light. Photographically, the perfect blocking of all UV and IR parts of the spectrum is an advantage for all those who want to do comet photography.
The Baader FringeKiller filter (a technically highly advanced minus violet filter) is also suitable for twilight, i.e. the 'blue hour', because it only weakens this blue light of the sky and simultaneously blocks the photographically harmful UV and IR spectral range without harming the important OIII and cyan components of the gas trail in any way. As it only affects the blue component and does not block it completely, the colour balance is maintained.
All of the above filters are particularly suitable for either highlighting the dust tail or emphasizing the gas tail in the same way. The last filter in this list is only intended to emphasize the gas trail more than the dust trail.
The visual Baader 10 nm OIII filter becomes interesting exactly for the observation of the gas trail not only visually but also photographically, especially if you take digital pictures with different filters and then combine them on the computer.
Theoretically one could also darken the sky brightness with polarization filters, but of course the position angle of the comets to the sun speaks against it. Therefor, polarizing filters will remain almost certainly ineffective.
The result of the above considerations can only be to react spontaneously to the respective situation and to have all filter options ready. Bright comets are rare - be prepared for all possibilities.
©Baader Planetarium GmbH 2018
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About the author: Martin Rietze
Martin has been an enthusiastic amateur astronomer for over 30 years. He has photographed 9 total solar eclipses and many other celestial objects, including many comets. From his countless trips abroad he brings back photos of the nightsky with fantastic scenery. His main passion, however, is volcano photography, for which he mainly travels around the world. His absolutely inspiring pictures can be found on his private website www.mrietze.com, one of his volcano pictures even made it to NASA APOD. Together with his colleague Michael Risch he has written chapters for "Color Foto" for the books Fotoschule (Photo School) and Extremfotografie (Extreme Photography).
As a trained electronic engineer, he is responsible at Baader Planetarium for the dome electronics and computer installation in observatories and research stations worldwide and has planned and built domes in the North Polar Circle and Antarctica. He is also very familiar with the CCD technology of DSLR cameras and all kinds of optics.