When It Comes to Filters, Shiny Side Up Is Only the Way to GoColin Haig
Which way do my filters go?
Shiny side up! We want to reflect or block the light that we don’t want from the telescope and pass only what we do want down to the detector: your camera. Cameras such as the SBIG STC-7 come with a complete set of filters—you just need to install them. Other cameras and filter wheels are often purchased separately from the filters, allowing you to choose specific ones for your imaging task.
Most optical glass filters have an anti-reflective coating on both sides to improve the air-to-glass-to-air transmission of light. Astronomical filters take that a step further, allowing only the colors (wavelengths) of light that you want to pass through.
Usually, when installing a filter, the most reflective side faces the telescope and the least reflective side faces the camera sensor.
This is usually obvious if it is a threaded filter, as the filter threads on the bottom normally face the camera, and the top of the filter faces the telescope.
Some threaded filters have a small arrow or dot pointing out which side has the coating (opposite to the light path) or lettering on the side. Generally, these are oriented so that the lettering is readable and the threads are at the bottom, which go toward the camera side of the carousel.
If you have unmounted filters (bare glass, no metal holder) and can’t tell, simply compare how shiny the two sides are. Again, shiny side up, toward the telescope.
What happens if I get this wrong?
A telltale sign that a filter has been installed backward are “ghost stars”: faint echoes of bright stars in the image from your detector. Light will pass through the filter, hit the reflective coating on the back side, and may then bounce inside the filter glass and pass through again toward the camera. Really bright stars may reflect off the camera’s optical window, hit the reflective coating, and reflect back down through the window. If you have perfect mechanical/optical axis alignment and top-quality coatings on the camera chamber window (as you do with SBIG cameras), some light may still be reflected by the glass cover on the detector, although you may not notice this.
I’m not talking about solar filters that go on the front of telescopes. Solar filters must reject a lot of heat and infrared light, so their coatings are usually on the sun-facing side, before the telescope optics and far from the camera.
Optical filters tend to come in two compositions: absorptive and dichroic (also known as interference) filters. Absorptive filters are often made of a tinted glass, where the color of the glass has been modified by adding chemical compounds during its fabrication. Absorptive filters have lost favor to dichroic filters for many applications due to cost and better light transmission. Dichroic filters have thin layers of coating deposited on clear glass. The process for applying multiple layers of coating really came to the forefront in the 1970s, with companies such as Asahi Optical (originators of the Pentax brand) buying patents for the technology and commercializing it for film camera lenses. This commercially successful multilayer coating rapidly became affordable for amateur astronomy with the advent of dielectric coating techniques in the 1990s and earlier.
Heat-absorbing glass and some photometric filters (such as the Johnson-Cousins Bessel V) usually have a special glass composition. The composition of the V filter glass tended to have problems with high humidity, as the glass would absorb moisture and cause the coating to flake. Most photometric filters also have an anti-reflective coating on them. Some current photometric filters are created with dichroic technology that allows more light to pass through at the wavelengths of interest, so these are more efficient, allowing shorter exposures and thus improving the signal-to-noise ratio for your image data.
If you have somewhat dark skies because you aren’t in an urban area, the typical set of filters are LRGB: luminance (for all white light), red, green, and blue, respectively. Most LRGB filters today are coated to block infrared (IR) and ultraviolet (UV) light, as many telescopes are not corrected to bring those wavelengths into focus and ensure a good color image. The luminance filter looks like clear glass and has coatings that block IR/UV light. The best way to tell what filter you have is to look through the filter at a white light source (not the sun!). Otherwise, you will get a reflection of the wrong colours.
Narrowband filters are especially useful for fighting light pollution and imaging faint nebulae. They tend to have very reflective coatings on one side, and they work both by reflecting the light you don’t want and passing an extremely narrow range of wavelengths—the light you do want.
The “bandwidth” of the filter is a measure of how wide a colour range the filter will pass, and you will sometimes see this specified as a measurement like 8 nm or 7 nm. Some vendors offer narrower bandwidth like 5 nm or 3 nm. This is a very narrow range that is usually not worth the high price for modest amateur telescopes (e.g., 16 inches and under) and can have a negative impact on fast optical systems (e.g., less than f/5.6) due to the angle of incidence of the incoming light. Narrowband filters are excellent for nebulae. Planetary nebulae are often brilliant emitters in Oxygen III (a green wavelength). Extended nebulae like the North America Nebula are best served with a Hydrogen α filter (in the red). The Sulphur II (SII) filter is a darker red, and that is how you can tell it apart from the Hydrogen α filter.
Most narrowband filters have reflective coatings on one side that are very obvious, almost mirror-like.
For planetary observation—which is often done with a complementary metal-oxide semiconductor active-pixel sensor (CMOS APS) camera—specialty filters, such as a methane filter for Neptune, are available. For the moon, some people use a neutral-density filter to dim down the image. These filters come in several evenly dark shades but are no longer commonly used.
The same shiny side up reasoning applies whether you have a charge-coupled device or CMOS APS in the camera. Eyepiece filters work the same way: the shiny side faces the stars.
So, keep looking up, and remember, filter shiny side up—toward the stars!