These are often referred to as light pollution rejection filters but their effect is more encompassing than that alone.
The Baader UHC-S is among our popular series of Nebula Filters. The UHC-S filter is a multi bandpass filter made so that only some portions of light in the visual the spectrum are transmitted. This UHC-S will pass a total of about 100 nanometers within the 400 to 700 nma range, with high percentages of transmission of that light in the portions of the spectrum commonly emitted by emission or diffuse nebula (the Veil, North American, or Dumbell Nebula for example), and planetary nebulae (Ring Nebula for example). By rejecting most other light this will cause these nebula to appear more clearly against a blacker background of space.
Right: Baader 2 and 1.25 inch UHC-S filters. When viewed parallel to the eye then the filter element appears like a silvered mirror. (136,193 bytes).
The UHC-S performs a function somewhere between that of the very narrow bandpass Baader Oxygen III, and others that are broad bandpass filters. This Nebula Filter is recommended by Company Seven to our customers who have a suitable telescopes: including premium quality Apo refractors of 80mm and larger aperture, Achromatic refractors of 100mm and larger, or reflectors of 8 inch (20cm) or larger).
Filters take away but they can not add - diminishing some parts of the spectrum
so that we might see what is otherwise obscured...
How Do Nebula filters Work?
These filters are a helpful accessory used with telescopes and binoculars to better view or photograph some types of the faint objects outside the Solar System. Filters of the UHC-S variety specialize in showing diffuse emission and planetary nebula; they are not designed to improve the views of objects outside these categories.
Street lights, especially the poorly baffled ones, cast a greenish or golden glow upon the otherwise black night sky. Particulates and humidity in the atmosphere refract and reflect that light rendering the sky background brighter, hence the fainter objects become harder to see against the brighter background. Nebular filters filter out a high percentage of the street lighting effects thereby making the background of the night sky appear blacker; faint objects stand out better against the darker background. While these are obviously beneficial for those who live near sky light polluted suburban areas, Nebular filters are helpful even when observing at darker rural settings since they also filter some of the naturally occurring background glow. Note, these filters can not help to attenuate light from neon sources since these emit across a wide bandwidth.
Narrower, and narrower bandpass filters become more and more specialized and may actually cut down the light coming from some of the brighter emission nebula; for example M42 the Great Nebula in Orion looks great with no filter, better with a broad band nebula filter, but much of the extended portions of the nebula disappear when observed with a very narrow band filter.
In the way these filters work, they will still absorb some light from desired object. And there are objects that they can not help one to see better including stars, star clusters and galaxies since these emit light from all wavelengths including those emitted by street lights. And so a pronounced darkening of these objects will be observed with the filter in place.
Observers with smaller telescopes will do best to operate at the lower magnifications, on the order of 10x or less per inch of aperture. While those with larger apertures or more efficient mid to large refractors will do well to span from low to higher magnifications.
Nebular filter coatings present a metallic, mirror like quality. As such one should use care to use the filters away from bright sources of stray light an with a well baffled telescopes. A darkening hood or blanket over the observer's head may help to keep reflections coming back to the observer, and darken the sky background seen even further.
How Are These Constructed?
The filter is made with a disc of transparent optical glass that has been ground to be plane parallel and then polished until very smooth. The glass undergoes a vacuum deposition coating process where numerous micron-thin coatings of various elements are applied one upon another in a precise order and thicknesses.
The filter is then housed in a machined aluminum cell by a thread in retaining ring. The filter name, size and part number are in silver, and this mirror like print when combined with the fine print make this information difficult to read under dim red lighting. Company Seven recommends our customers keep these filters in well organized accessory cases, so that you know which filter is which. It helps to become familiar with how the elements appear too (read descriptions below).
Nebula Filters are made up of micron thin layers of materials deposited onto an otherwise transparent optical glass window. The coatings are calculated to transmit desired wavelengths (portions of the visible spectrum), while simultaneously rejecting a high percentage of other wavelengths. Depending on the type of filter (broad band pass or narrow band filters including UHC-S) these filters pass light in the nebula lines which may include UHC-S, Hydrogen Alpha, and Hydrogen Beta lines of the spectrum. In a more perfected nebula filter a higher percentage of light is transmitted in those desired wavelengths, as much as 80 to 90 percent being typical.
When tilted while exposed to bright light then both surfaces of the UHC-S filter element will appear like a silvered mirror. When viewed with no backlighting and parallel to the eye from either side, then the filter element appears to impart a light green cast. When tilted with respect to the observer's position this light green changes to magenta gradually becoming deeper in color, as the tilt increases even farther then the intense magenta cast diminishes to a light pink.
The Baader UHC-S filter is made available in diameters of 1.25 (31.7 mm) or 2 inch (50.8 mm) so that it can be installed in line onto many accessories, or directly onto most popular eyepieces.
What Does This Do For Me?
This filter is among the better mid transmission nebula filters on the market as such it effectively blocks the entire visual spectrum with the exception of the O-III transmission lines. By means of its sophisticated optical fabrication and coating technologies it eliminates the annoying colored halos that often surround stars when seen though some other O-III filters. It is a surprisingly long lived, durable filter featuring the same ultra-hard, extremely durable vacuum-deposited dielectric coatings employed on the Baader UHC-S-S filter.
It is an amazing experience to see a faint planetary nebula clearly revealed in the field of view - obvious even to a child,
when before then there was not even the slightest suggestion of its presence without the UHC-S filter in place
Diminished contrast results when definition is reduced or when light is spread out limiting the range of optical density and tone thereby making it harder to distinguish one object from another. The Baader Planetarium OIII is ideally suited for visual as well as CCD imaging applications due to it's unsurpassed contrast, it's ability to more clearly reveal variations in the intensity of an image. Compared to most other OIII filters the Baader filter improves upon:
- Since the transmission of the light in the OIII is so efficient, fainter nebula seem to appear brighter and the extent of nebulosity that can be seen is greater - you will see more of the fainter components.
- Since the transmission of the the other regions is so limited the background appears blacker, hence what is visible is more clearly contrasted against its background.
- Stars appear more natural, smaller, and more intense - stars are not bloated or halated by this filter. No image degradation is perceptible even at the highest magnifications.
Right: Baader UHC-S filter transmission curves. (145,382 bytes).
Features Of The Baader Filters:
The glass elements used to make the Baader Filters are free of striae, optically finely polished plane parallel to within 30 seconds of arc. This eliminates any double images or ghosting due to wedge errors in the glass. These filters tend to far supersede the quality of common color filter elements which are flame polished. The combination of their fine optical craftsmanship and a comparatively thin 2 mm profile allows these filters to be inserted anywhere into the optical path without introducing astigmatism or other noticeable defects into the image. For instance they can be installed in the front of a Binocular Viewer without degrading the image in any way.
The UHC-S Filter will thread into conventional slip fit eyepieces or accessories with the common 1.25 inch diameter x 42 tpi (28.5 x 0.6 mm) or 2 inch (50.8 mm) threading. These thread patterns have been tested and optimized to fit allmost all eyepieces on the market.
Stackable filters: each filter cell incorporates both a male and a female thread to allow stacking the filters to obtain even narrower passbands or special effects.
The filter cells features the largest clear aperture of any 1-¼ and 2 inch filters on the market - for complete freedom from vignetting at any focal length. The front crown of the cell incorporates milled notches which makes handling and threading the filter an easy operation in the dark.
For protection, each filter is provided with its own transparent plastic case. We are astonished however, that as outstanding as the filters are, plastic case latch tend to fail sometimes even when new. Company Seven therefore offer optional third party cases. And for the most discriminating clients, we even offer a custom made wood accessory case described below.
Examples of Imaging with the UHC-S Filter:
Above (Left to Right): M8 Lagoon Nebula, 2x 10 minute exposures. Helix Nebula
2x 10 minute exp.
Trifid Nebula 1x 10 min and 1x15 minute exp. combined.
Above: Bug Nebula, 2x 10 minute exposures. IC 4628 im Skorpion
2x 10 minute exp. M8 Lagoon Nebula taken during full moon.
All images by 10 inch f/5.6 reflecting telescope (note diffraction spikes), Canon 300d camera with ISO 1600 film.
Copyright Mr. Scott Alder, N.S.W. Australia.
Click on images to see enlarged view
Optional Wood Case
The optional wood case is foam lined with cutaway spaces for filters, eyepieces and other small accessories for your telescope. Please refer to our price list for current prices for the filter set and the optional case or contact Company Seven for more information.
Right: optional wood box for Color Filter Set (52,803 bytes)
Click on the image to see enlarged view (190,049 bytes).
FURTHER READING
Baader Planetarium Professional Astronomical Filters one page color illustrated overview of several Baader specialized astronomical filters including the UHC-S Narrow Band, Contrast Booster, Color Filter Set, UV-IR Rejection, Fringer Killer, Moon and SkyGlow, Solar Continuum, UHC-S-S High Transmission Nebular. Includes filter transmission curve graphs. Publication content by Baader Planetarium, all rights reserved. From Company Seven's Archives (127,011 bytes in Adobe .pdf).
CLEANING & MAINTENANCE INSTRUCTIONS:
- It is best to protect the filter from sharp edges or any item that could cause a scratch or abrade the glass.
- When not in use, store the filter in a its provided rigid container, and it may help you to keep them organized in a fitted carrying case.
- Dew may form on the filters, if this happens do not store a wet filter into its airtight container for long periods of time; whenever possible let the filter "dry out" in order to reduce the potential for problems developing associated with Fungus, etc.
- Draw any large, loose bits of foreign matter from the surface of the glass with an air bulb or small vacuum. Stubborn particles can be removed with a soft camel's hair brush. Throughout the year, but especially in cold climates when static become more of a problem we particularly recommend the use of a Staticmaster brush.
- Place a few drops of an approved lens cleaning solution (by Carl Zeiss or Kodak) or mild soap solution of clear dish washing liquid diluted with distilled water onto a cotton swab. Then gently wipe in a circular motion with little or no pressure. The filter can be damaged if there are tiny abrasive particles on the surface which are pressed in by rubbing hard.
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