Why is the Neodymium Filter only 1mm thick


Why is the Neodymium Filter only 1mm thick?
For me, this is a problem because a) focusing and (much important) b) backfokus, sinc I have the filters between coma corrector and camera sensor.
Will this change with the coming CMOS-filters? Or can you already offer an alternative (I have got an 8" F4)?
Thank you very much!

M13, captured with RASA 8", QHY 294C (with FCCT) and Baader Neodymium-filter 10x120s
© Christoph Kaltseis, 2021

Answer: First of all, the Baader Neodymium (Moon & Skyglow) Filter (various versions available) is primarily an absorption filter (just like the Semi-Apo filter), which reduces light pollution by neodymium ions in the glass melt which absorb the especially troublesome parts of the spectrum. Additionally (and in contrast to all copies which are offered nowadays) it has the dielectric coating of the L-filter (= UV/IR block filter), whereby outside the photographically important spectral range from 400 to 700 nm no light reaches the chip for which the telescope optics is not corrected in most cases. The result is that deep-sky images have stars that are almost half as large as those shot without the filter.

At the same time, this sophisticated filter design provides additional suppression/reduction of both the skyglow from common street lamps (mercury and sodium vapor lamps), but also the increasingly dramatic disturbing deep blue skyglow caused by the primary emission from modern LED cars and street lamps. This makes the Baader Neodymium (moon & skyglow filter) an excellent all-rounder filter for both visual and photographic applications.

The disadvantage is indeed the need to reduce the substrate thickness to 1 mm. If you want to use the neodymium filter photographically, you should not combine it with other filters, but only use the neodymium filter as an in itself combined anti-light pollution and L-filter with CMOS color cameras! Then you don't need additional deep sky filters for color cameras!

Many manufacturers do not consider this necessity of thickness reduction of the neodymium glass. As a result, exposure times increase unnecessarily and the finished image (as well as the visual impression) loses brilliance and is perceived as dull and uninteresting, precisely because too many neodymium ions are present at the double glass thickness of 2 mm. So, they "swallow" both desired and unwanted photons. In comparison tests of different copies - all offered as Moon&Skyglow - you can clearly see the difference.

The use of the selective absorption properties of the element neodymium (by the way, first used by Carl Zeiss as a contrast enhancer for photography) brings an additional photographic benefit that is usually not even mentioned. There are no halos even on the brightest stars and the natural impression of the sky background is completely preserved. With many other so-called "L-enhanced" filters, on the contrary, the stars are all discolored "dirty", the sky background appears in unnatural "pop colors" and halos "dominate the image".

Finally, your question about CMOS-optimized filters:
These are purely dielectric coated interference filters on fine optical, highly homogeneous glass, without any colored glass! They are manufactured in the standard thicknesses of our other interference filters and are the most suitable filters for all monochromatic CMOS chips to suppress light pollution highly effectively even from the center of cities, since they completely block ALL light - except the emission wavelength of a deep sky nebula. However, this comes at a dramatically higher cost than a relatively simple absorption glass.

For beginners with color CMOS cameras, the Baader neodymium filter therefore belongs to the inexpensive standard accessories to ensure a natural sky impression with sharp stars.

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