Astrophotography filters are necessary for capturing the astral objects in the sky. If you try to capture the night sky without using filters, you will see a very muddy and grainy image. This is down to a number of reasons.
Light pollution from the earth enters your frame. Also, different wavelengths of light come from different objects in the sky, some of which we want to keep. This allows us to focus on those specific objects.
These filters include broadband, narrowband and line filters, all with different purposes. Depending on your subject, you will need to consider the type of light you are receiving.
Here, we present the many filters that are available for DSLRs, CMOS and CCD sensors.
Styles of Filter
There are styles of filters that clip-in to the body of your camera. There are benefits to having these, such as being able to stack filters without stacking them together. One goes into the camera body, the other twists onto the lens.
They are a little more difficult to get to, as you need to remove the lens every time you want to change the filter. This makes your images more prone to accidental focus failure.
Lens mounted filters screw onto the front of the lens or flat plane for CCD/CMOS sensor systems. These are easier to twist on and off, meaning you don’t need to separate any equipment to change them.
These are more often used with DSLR camera lenses.
Light pollution suppression (LPS) filters suppress the common emission lines generated by artificial lighting. They also allow the important nebula emission lines to pass, thus enhancing the contrast of astronomical objects, particularly emission nebulae.
The most recently introduced version (D2) has a bandpass designed to cope with the increasing trend of society’s switch to LED lighting.
IDAS – Unlike other light pollution suppression filters, IDAS (Ion-Gun Assist Deposition) filters achieve balanced colour transmission. The unique Multi-Bandpass Technology (MBT) process makes this possible.
The balanced transmission allows colour photographs with a minimal colour cast to broadband emission objects. These include stars, galaxies and globular clusters.
City Light Suppression (CLS) filters block the light of the spectral lines of mercury and sodium-vapour lamps. They also let the largest part of the visible light and H-alpha emissions pass.
All the important emission lines, as well as the spectral region, can pass through the filter.
Narrowband filters are basically the same as the Line filters, yet they allow H-alpha. H-Beta and OIII to pass through together.
They also darken the background skyglow significantly without touching the nebula. They are often of considerable help when observing in mild to moderate light pollution.
The Ultra-High Contrast filter allows the transmission of nearly 100% of the radiation from both O-III and the H beta lines. Though the H-alpha-line is not intended for visual observation, it is important when using the filter.
All annoying, scattered light from other wavelength sources, including local artificial light pollution, is reliably filtered out. With this strong blocking of the sky, an unexpected wealth of detail becomes visible for gas nebulae and planetary nebulae.
The UHC-E Filter increases the contrast of emission nebulae and comets and blocks the light of typical streetlights as well as airglow. you would use this for deep-sky observation of emission nebulae and comets under light polluted skies. It’s particularly suitable for small scopes.
A hydrogen-alpha filter is an optical filter designed to transmit a narrow bandwidth of light, generally centred on the H-alpha wavelength. H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm in air.
H-alpha light is important to astronomers as it is emitted by many emission nebulae. You can use it to observe features in the Sun’s atmosphere, including solar prominences and the chromosphere.
Hydrogen-Beta (486nm) lets the light of the H-beta emission line pass nearly unhindered while blocking the remaining spectral range to the extent that the eye is dark-adapted.
By the use of the H-beta filter, the contrast increases so strongly, that even observing the Horse Head Nebulae with telescopes of 10″ – 12″ becomes possible.
Oxygen (OIII – 496nm and 501nm) has been very specifically designed for the visual observation of gaseous and planetary nebulae. The extremely narrow pass band of the two OIII lines brings a substantial contrast gain, even under best observation conditions.
On faint supernova remnants and faint planetary nebulae, the OIII filter will often make the difference as to whether the object is visible or not.
All semiconductor sensors in digital cameras produce inherently monochrome images. But some astronomy cameras and all DSLR cameras produce colour images. This happens by using a matrix of red, green, and blue filters, called a Bayer filter, in front of the sensor.
Colour digital cameras can work well in astronomy imaging of planets and deep-sky objects, especially for beginner photographers. What cameras offer in convenience and ease, they take away in image sharpness and colour fidelity.
To get the sharpest possible colour images, many experienced imagers prefer using monochrome cameras. Instead, they take several images of a celestial object, each with a separate colour filter placed in front of the camera. Each image combines the colours using software to produce a single, true, colour image.
These are possible in two variations. UBVRI (Ultra-violet, blue, visible, red and infrared) and LRGB (Luminance, Red, Green and Blue)
The IDAS HEUIB-II (H-α Enhanced UV IR Blocking) filter is for enhanced spectrum DSLR’s and one-shot colour CCD astronomical cameras.
Besides removing the IR and UV ends of the spectrum, it isolates the common H-alpha line relative to the surrounding extreme red end of the spectrum. The IR and UV often create out of focus refractive optics.
The removal makes it easy to bring out the faint H-alpha emission nebulosity of astronomical photos. It does this while retaining the background colour balance of broadband photos.
IDAS UHB-RS (UV/heat blocking, reflection suppression) solar filters are the first innovation in many years in the area of broadband filters. These fix internal reflections and light scattering.
These filters suppress reflections within the filter (1/10,000 reduction). Also, it cuts out internally reflected light coming back towards the filter from the imaging sensor and lens elements.
A Dark Frame removes the dark signal from the light frames. You use this to capture one or more images taken at the same exposure length, ISO and ambient temperature as the light frames.
A body or lens can is sufficient, but those astrophotographers using systems without a mechanical shutter will need a dark frame filter.