Jargon Busting - Filters

During my recent photographic journeys to the Moon and Venus, I’ve been reading up articles about the use of filters. To be honest, I found it all very confusing and I had to make extensive notes to understand what each filter should do and when it would prove to be useful. In essence, this blog post wrote itself. 

Is Infrared good or bad?

Particularly confusing are IR filters, variously named IR pass, IR block, IR cut, IR 850 and whether or not this was suitable (or even damaging) for the camera I was using. 

The collection of deep sky, planetary & guide cameras I own (all ZWO), all have an anti reflective window over the sensor which allows all UV, the visual spectrum and IR through. These cameras are highly sensitive to IR (I now understand), so if the IR is not ‘cut’, then problems can occur; lack of sharpness in the images and bloating/ halos around stars. Alternatively, you can harness this sensitivity to your advantage on certain targets.

But what filter do I need for various applications? Do I want to cut IR or isolate it remove the effects of poor seeing (or atmospheric distortions)? I already own a handful of these filter types but this process has been a lesson in when to use them. Let’s bust the jargon with a full review:

Physical shape

Filters tend to be round, threaded rings which screw into a nosepiece or other accessory. These follow the standard 1.25” and 2” sizes common is astronomical equipment. The prices of filters jump considerably between the two sizes. A single high-end 2” filter can cost hundreds of pounds. Also available are clip filters which are flattened discs that sit into the body of a DSLR. The filters described below are the traditional type, with most also available in the clip format where they are suited to astrophotography. Finally, unmounted filters are also available which comprises the glass without the threaded ring. These can be mounted into a filter wheel and may help avoid vignetting - to me they just look unnecessarily fiddly.

Visual Contrast filters

Coloured filters are used for visual astronomy to bring out greater detail and contrast in objects. Different colours respond better to different objects and each colour typically has a Wratten number signified with a hashtag, e.g. #23a is light red, good for picking out the Mare on Mars. Filter sets of 3-6 are sold to give a range although many colour filters only become useful with larger apertures. 

Use for: visual contrast on planets and the moon.

RGBL

Stands for Red Green Blue and luminance. Mono sensors are more sensitive to light than colour versions, so if you have the time, imaging in mono per colour band, then combining is best. In this way, you build up the colours that a colour camera (often called a ‘single shot’ colour camera) would give, but takes 4x as long! 1 hour in colour would need 1 hour in each of RGB plus an hour of  luminance (which covers the middle of the visual spectrum). As a guide, blue is a shorter wavelength (just beyond Ultra Violet or UV) followed by green then red up towards Infared (IR).

Use for: mono camera imaging. 

A bit more about Luminance

Many filter manufacturers like Astronomik have different types of L filters to suit different optical systems, say L1/2/3 covering slightly different wavelengths.

Filter wheels

With this many filters already to swap out, the filter wheel is a handy tool. Placed in front of the camera, the wheel is rotated (manually or electronically) placing a different colour between telescope and camera whilst remaining parfocal. Filters wheels typically have 3, 5, 7 or 8 slots available. 

Use for: visual or imaging runs. 

Filter Trays

An alternative to a filter wheel is a filter tray. The body sits in the image train before the camera and different filters can be quickly interchanged by loading different trays. With the same body you can swap between 2” and 1.25”. As the trays are slender, note that not all filters fit in all trays. Baader filters, for example tend to be quite chunky , possibly because they can be threaded together and therefore need a greater thickness for the threads.

Clear filters /Focusing Filter

A clear filter does nothing at all other than prevent dust reaching the filter and keeping your image train parfocal. By introducing a piece of glass (like a ‘proper’) filter would you can swap between filter and no filter without refocusing.

Use: as a protective cover or as the ‘no filter’ position on your filter wheel.

Moon glow / neutral density

These visual filters help to provide balance between the very dark and very bright areas on the lunar surface so more detail is visible. As soon as the moon cycle is 3-4 days old, the bright areas are too bright to observe without a filter to tame the contrast to a more comfortable range. 

Use for: visual lunar observing. 

Polarising Filter

Described as providing greater contrast for planetary or solar observations, especially in conjunction with a Herschel Wedge (specialist solar viewing diagonal). A variable polarising filter has a rotating element to change the apparent brightness.

Use for: lunar and solar observing.

Light pollution / Contrast enhancing / Ultra High Contrast/ Neodymium / CLS / L-Pro

These broadband filters work for both visual and astrophotography and basically make the sky darker and the astronomical objects brighter. Hence they ‘remove light pollution’, increase ‘contrast’ or perform ‘City Light Suppression’. However, when used for photography, allowing through short UV and long IR onto the sensor can cause star bloating and large halos. The ‘Pro’ versions of these filters are more expensive and also suppress UV and IR. All light pollution filters cut out common emission lines of street lighting and other nuisance wavelengths, shown yellow below.

Use for: visual or imaging. 

Fringe killers / Semi-Apo

Also called ‘minus violet’ filters these can remove the halos and bloat often seen around stars when using refractors. 

Use for: visual and imaging, especially with refractors. 

Narrowband filters

These exclude all light except narrow wavelengths apportioned to specific gas signatures. Common narrowband used for gas nebulae are Oxygen OIII and Hydrogen Alpha Ha. As such a small amount of light gets through, longer exposures are needed. Images can be created by additive means, e.g. layering some Ha data over an RGB image to pull out wisps of gas. 

Use for: astrophotography only. 

Dual narrowband (2, 3 or 4 combined)

These filters combine two or more passes into one filter to offer dual, triple of quad narrowband. These are simply for convenience rather than having to swap out different filters while imaging. Serious imagers would probably not take this short cut and only ever image in one band at a time. 

Use for: astrophotography only.

SHO Narrowband

This describes a set of three narrowband filters comprising the common usage SII, Ha & OIII.

Use for: astrophotography only.

Anti reflection (AR) window

If your camera sensor glass has an AR window then it allows all light through. If not, the glass likely is doing UV and IR cut. Most DSLRs have built in IR cut, so an Astro-modded camera has this element removed to increase their sensitivity. 

Original White Balance

If your DSLR has been modified, you may want to use a OWB filter to restore the original colours. These are typically clip in filters rather than threaded.

UV / IR cut

This filter cuts the short (UV) and long (IR) ends of the spectrum leaving the visual wavelengths to pass. For cameras without a protective IR window, this filter helps sharpen images and reduces star bloating. Without the IR cut in particular, the images will not appear sharp. This filter could be used in conjunction with a broadband light pollution filter to enhance the object without encouraging bloating of stars. 

Use for: visual or imaging with cameras without a built in IR cut window. 

IR and UV pass or cut filters work as opposites

IR block

This type of filter blocks all IR light allowing UV and visual wavelengths through. Particularly good for telescopes with a refracting element. 

Use for: cameras with sensitivity to UV. Imaging Venus cloud structures. 

IR pass

This filter blocks everything except the longer IR wavelengths. Could I suppose be called a UV/ visual cut filter! As this allows a smaller part of the spectrum, the effects of bad seeing are reduced are sharper images should result. 

Use for: imaging lunar features and planetary including Venus cloud structures. Also can sharpen guide stars. 

U filter or U-Venus

This is a pass filter for UV light, blocking everything else. A specific UV filter for Venus (various manufacturers have a Venus filter) allows a band pass around 200-300. 

Use for: Venus cloud structures. 

Planetary / Band Pass (BP) filters

Allow longer (higher number) IR wavelengths to pass in specific bands. For example 642 /742/ 807.

Use for: imaging of Mars, Jupiter & Saturn. 

Methane CH4

Specific wavelength for methane gas suitable for imaging Jupiter or Saturn. Wavelengths other than 890 are blocked. 

Use for: imaging Jupiter & Saturn. 

Atmospheric Dispersion Corrector (ADC)

An ADC sits between a telescope and colour camera (indeed immediately before in the image train) to compensate for blue and yellow colour fringing on planets in particular. Two levers adjust two internal prism that fractionally alter the light path. Fine tuning these levers removes the fringing and gives sharper images and edges.

Use for: colour cameras on lunar and planetary imaging.

Once you get up to 8”-10” apertures or more special visual filters can be used

H-beta Hydrogen Beta

H-beta is an example of a filter suited to larger apertures for visual use. An object like the Horsehead IC434 is visually very difficult in smaller scopes but in larger scopes, a filter like this brings out contrast and detail.

I hope this is a useful summary. As I learn more I’ll update it, hopefully with some examples. Happy hunting!