Some things cannot be improved. You can only make them better.
Filters for astrophotography are constantly being improved to adapt them to modern imaging systems and changing environmental conditions. Our CMOS-optimized narrowband filters are the best example for this development and research process. There are other demands for those filters which are used for science instead of pretty pictures: they must deliver data that are comparable even with decades-old data sets. New developments must therefore only improve production and handling without changing the filter characteristics themselves.
The filters used today for scientific photometry with digital sensors go back to the filter sets introduced by Johnson and Cousins, which were developed in the 1950s-1970s and which were adapted in the 1990s by M. Bessel for CCD cameras - in each case using the techniques available at the time.
The original UBV filters consisted of coloured glass, partly even without protective coating. Therefore, the V filter, for example, worked wonderfully in dry observing sites. However, because its BGT39 material was exposed to the environment without protection and had hygroscopic properties, it aged if there was high huminity: so, under less favourable conditions, it quickly got hazy.
Another feature of the original filters was that the Cousins Ic bandpass filter, for example, was open at the red end, so the sensor or film - instead of the filter itself - limited the sensitivity at the red edge of the spectrum. At the blue end of the spectrum, on the other hand, it was the atmosphere (humidity, altitude of the observing site) rather than the filter that set a limit. Despite these disadvantages, thousands of observations were made with UBV filters and the filter systems based on them.
Whoever develops filters today has many more possibilities to influence the spectral transmission characteristics. While the original filters consisted of coloured glasses and protective glasses of different thickness and transmission, which on top of that were not resistant to ageing, we can now manufacture dielectric filters whose characteristics allow very well-defined transmission windows - but they are steeper than the original transmission windows, and depend on the focal ratio. On fast telescopes with f/2-f/4 they work somewhat differently than on slow telescopes with f/8, and since the angle of incidence of the light distributed over the field of view also changes, there are now some new problems on modern telescopes with large CCD sensors that were unknown on old, slow telescopes with film cameras. A modern photometric filter therefore does not have to provide perfectly delineated transmission windows with steep edges, but data that are comparable to the old data sets (even if only by using reliable conversion algorithms).
The UBVRI system was developed in the 1950s for the 0.9m McDonald Observatory telescope and initially consisted of a Johnson UBV filter set, later extended by Cousins to include the R and I filters for the red region of the spectrum. The filters of the extended UBVRI system according to Johnson and Cousins include:
- U – Ultraviolet, with a transmission window between about 320 and 400 nm
- B – Blue, with a transmission window between about 400 and 500 nm
- V – Visible, with a transmission window between about 500 and 700 nm
- R – Red, with a transmission window between about 550 and 800 nm
- I – Infrared, with a transmission window between about 700 nm and 900 nm.
According to the state of the art at that time, the filters had different thicknesses and transmittance values as well as no sharply defined edges (if any). Their spectrum therefore corresponds to a curve instead of a plateau.
Bessel filters for the digital age
In 1990, M. S. Bessel looked into the subject in order to find a filter combination for the new CCD cameras that were becoming increasingly widespread at the time. These Bessel-UBVRI filters are still standard today and are also the most widely used in amateur circles such as the AAVSO. The V-filter in particular has proved to be an inexpensive introduction to photometry: It corresponds approximately to the visual brightness; since only one measured value is used, this is optimal for getting to know the techniques.
Since 2010, Baader Planetarium has offered photometric UBVRI filters according to Bessel, which protected the coloured glasses from ageing with a dielectric coating and at the same time provided the desired transmission properties. This is not at all a matter of course: unfortunately, comparisons by the AAVSO have shown that not every modern "Bessel filter" actually shows the irregular flanks of genuine Bessel filters, but instead there are often plateaus with steep flanks. These steep slopes are easily achievable with dielectric filters, but the data obtained with them are simply not comparable, or only with difficulty, with the old data sets. Compare for yourself: You can find the transmission spectra in the product description.
New generation of Baader Planetarium UBVRI filters
Die We could not further improve the characteristics of our previous, Bessel-compatible UBVRI filters - but we could make the filters better. The new generation of Baader Planetarium UBVRI filters, introduced at the end of 2021, is a modern filter set that is both fully compatible with the characteristics of the original Bessel filters and at the same time meets modern demands for contemporary filters:
- The photometric filters now have the same thicknesses as all other Baader filters:
- 2 mm filter thickness for all filters up to size 2" mounted (1.25", 31 mm, 36 mm, 2")
- 3 mm filter thickness for all sizes above (50.4 mm, 50 x 50 mm, 65 x 65 mm)
- This means that they fit into any commercially available filter wheel, for which the original filters were often simply too thick and could not be fixed securely.
- They are parfocal with all other Baader filters of the same size and now also available in sizes up to 100x100mm - large enough even for modern, professional sensors, or optimal for easy handling in the common filter mounts for amateur telescopes.
- The coating of all photometric filters is manufactured in the same technology as the CMOS-optimized narrowband and LRGB filters. The lifetime Life-Coat™ guarantee also applies here.
The next generation: SLOAN filters
In 2000, a new era in photometry began: with the Sloan Digital Sky Survey (SDSS), a 2.5 m telescope at Apache Point Observatory (New Mexico, USA) surveyed one third of the sky in five wavelengths. A camera with a total of thirty large CCD sensors with 2048×2048 pixels each was used, each in five rows of six sensors. In front of the sensors was a u'g'r'i'z' filter set for five non-overlapping wavelength windows around 355.1, 468.6, 616.6, 748.0 and 893.2 nm; light pollution such as the airglow at 558nm and the mercury line at 546 nm were masked out. This system was also used for the Pan-STARRS project; the broad data base of these two surveys makes them very well suited for calibrating images. Another advantage are the wider transmission windows, which allow fainter objects to be measured in a shorter time.
Even though the six Sloan filters have a different characteristic than the UBVRI filters, there are now the appropriate algorithms to compare the data with each other or to convert them into each other. Therefore, the Sloan photometry system is gaining more and more importance. At present, UBVRI filters still dominate in amateur circles, but we not only offer unmounted Sloan filters for professional telescopes, but also in the sizes and mounts commonly used by amateurs. In the coming decades, it is expected that the Sloan system will also become more widespread in amateur circles, which is why Baader Planetarium's Sloan filters are also available in the common filter sizes for amateur telescopes as well as in the large format 100x100 mm. The SLOAN z' filter is currently not included in the scope of delivery, as its function can usually also be taken over by the UBVRI -I- filter.
Ready for the future
At a time when common filter types often disappear from the market without warning, with these filters we want to offer all users a way to build on previous data obtained with UBVRI or Sloan filters. Long-term availability makes it possible for amateurs to enter with a limited budget and a small set of filters to expand in the long term. Therefore, we have resisted the temptation to improve the filter lines and have done our utmost to improve the filters in durability and applicability.