Planetary filters have been around for a long time. For many years simple, dyed in glass colour filters were used productively by keen observers of the bright planets Mars, Jupiter and Saturn. In general, the would-be observer would choose a filter with a colour opposite to that of the feature he/she wished to observe. And while these filters are still used productively by seasoned observers today, in recent years a new type of filter has come to the fore; the interference filter; designed to selectively block unwanted wavelengths and transmit selected wavelengths with high efficiency.
The interference filter is made out of successive layers of dielectric materials, with thicknesses ranging between one-quarter and one-half of the target wavelength. The coatings are deposited onto an optically flat glass surface in a vacuum using ion beam sputtering technology. The unwanted wavelengths are nulled by the reflections generated by the dielectric materials, which undergo destructive interference and thus are removed from the optical path. Because the designer can cut off and enhance any visible wavelength or waveband at will, they offer the potential to experiment in new ways with the human visual system.
A good planetary filter ought to accomplish a number of things.
1. Glare reduction, which almost invariably leads to an increase in perceived image quality.
2. Exaggerating differences in brightness between the various coloured features of a planetary image.
3. Overcoming to a greater or lesser degree, the image distorting effects of the atmosphere
3. Enabling observers to study different levels of a planetary atmosphere.
4. Enhancing the resolving power of a telescope.
In the last few years, Televue has brought to market a series of interference based filters designed to enhance details on the bright, extended objects like planets. Their latest product, the Bandmate Planetary (BPL) Filter, has different dielectric layers applied to each side of the optically flat BK7 substrate. At the present time, Televue only produce the filter in a 1.25 inch format, which is threaded to mate to most any eyepiece. Some have questioned why not a 2 inch version? I believe the company reasoned (correctly) that most planetary eyepieces are going to be of the smaller, 1.25 inch variety. When viewed in bright daylight, the filter gives a pinkish tinge to objects and seems to enhance brown, yellow and red tones.
Overall, the light transmission of the filter is good. Comparing it with a light yellow colour filter, a Baader Fringe Killer and the Baader Contrast Booster, the Televue BPL showed less light transmission than the all of them bar the Contrast Booster. Light transmission is important in determining the size of a telescope that would benefit from its use. If the telescope is too small and gathers only a limited amount of light, the filter will dim the image too much. For this reason, the benefits of filters are best revealed in larger aperture instruments.
I tested the filter on a number of telescopes, with apertures ranging from 80mm to 200mm My target was invariably the bright planet Jupiter, now well placed in the sky for observation. In my 80mm f/5 achromat, the filter imparted a reddish tone to the planet. Visible chromatic aberration was reduced but the image was quite dim, making any gains in contrast marginal at best.
When tested with a 5 inch refractor, the effects of the Televue BPL were more readily observed. The greater light gathering power of the instrument allowed the filter to transmit a decent amount of light to the eye. The modest amount of secondary spectrum seen in the unfiltered image was effectively removed with the filter and the elaborate banding of the giant plant was more easily seen. Close examination of the image under optimal magnifications revealed many shades of orange, yellow, fawn and dark brown. The Great Red Spot (GRS) was much easier to see with this filter than without it. The bright zones surrounding the belts came through in a plain white.
When I tested the filter on a 17cm f/16 Maksutov Cassegrain and a 8-inch f/6 Newtonian, its effects were most effectively manifested. Like all good filters, it reduced glare and greatly enhanced the dark Jovian belts. Under the best conditions, the filter brought out very subtle features, such as ovals and festoons in the planet’s southern hemisphere. The GRS was very obvious and with appropriate magnification, some substructure could be seen within it. Shadings in the polar hoods were also more easily seen with the filter.
When compared to other filters with a proven track record to enhance certain Jovian features, the Televue BPL edged ahead in terms of the amount of Jovian atmospheric details revealed. All in all, the images struck a nice balance between image brightness and the telescope’s contrast transfer.
The fact that the filter passes longer wavelengths better than shorter wavelengths may help stabilise the image. This is because of Rayleigh scattering which predicts that for a given sized particle, light is scattered in inverse proportion to the fourth power of wavelength. This means that the air is less turbulent when viewed at longer wavelengths. This effect was not vigorously tested however.
What is clear from my tests is that the Televue BPL filter works very well indeed for larger aperture ‘scopes. Personally, I would not recommend it for ‘scopes less than about 5 inches in aperture but will work brilliantly with larger Dobsonians and catadioptric telescopes. It will also work extremely well in suppressing the chromatic aberration of inherent to larger achromatic telescopes and would thus be a useful tool used with old fashioned classical refractors housed in observatories up and down the country.
Planetary imagers will also likely benefit from using this filter, especially large aperture instruments. Because of its reduced light transmission, slightly longer exposures may be necessary in order to achieve optimal results.
In summary, the Televue BPL filter is an excellent filter to divine as much detail from Jupiter’s massive and ever changing atmosphere as is conceivably possible. If you’ve never seen the Great Red Spot, you will with this filter! Saturn and Mars should also benefit from its use. It is more expensive than other filters on the market but I believe the extra cost incurred is well worth it when you see what the filter can achieve. Albert Nagler has definitely done his homework with this exciting new product. Highly recommended!
Typical UK Retail Price: £113.
Typical US Retail Price: $140.
Neil English is author of Choosing and Using a Dobsonian Telescope.