Tuesday, 8 July 2014

A Guide to Filters

There are a wide range of filters available on our cameras. Different cameras have different filters. These filters have strange names, which may be confusing if you've never seen them before. I dedicate this post to explaining what these filters are and when to use them.

Before I get to the filters, there are a few things you should be clear on if you aren't already:
The different cameras (Galaxy, Cluster, Constellation) have completely separate optical paths.
Each 'camera' has its own light-gathering apparatus (a telescope and two camera lenses respectively), its own filter wheel full of filters and its own CCD sensor. This means that filters available on one camera might not be available on another.
The CCD chips are black and white sensors. Not like in the cameras you are used to.
Once again: the sensors are black and white. The sensor returns values of bright vs dark for each pixel. It doesn't distinguish between red, green, blue or any colour of light. Just lots of photons vs not many photons.

Now we can talk about filters.

Why do we need filters?

You can get an image without them. But with them you can do more things. With no filter, you get a simple black and white image. This would be a good image. Every photon that enters the telescope makes it to the CCD, so you get the maximum light gathering power for your exposure time (all filters block some light). But maybe you don't want a black and white image.

No filter
(Levelled the same
as later images)

Making a colour image

So how do you get a colour image? You need to take three images and you need to use filters. You probably know that any colour of light can be made by mixing red, green and blue light in the right ratio, and that this is how colour screens produce colour using red, green and blue pixels. This is not physics but biology: our eyes use three different sorts of cells on our retina sensitive to red, green and blue light to guess the wavelength of incoming light. 

We can do the same with our camera by making it alternately sensitive to red, green and blue light, using the appropriately tinted filter.This gives us three black and white images, representing the redness, greenness and blueness of the target. These can then be tinted and combined into a single colour image for display on your screen. This is done automatically when you order a BVR Colour or RGB colour image.

Colour filters can be used on their own too. Some objects look clearer or more interesting in a particular colour band. The cost of course is that you are getting less light for the same exposure time, as some photons are rejected by the filter, making fainter objects look noisier.



I have seen many people confused by the filter names. On constellation and cluster cameras, the filters are R, G and B. On Galaxy they are called (Johnson) R, V and B. Why V? V stands for 'visual'. It is actually a green filter. It gets to be called visual because green is the wavelength our eyes are most sensitive to.

But why not call green green? Reddish, greenish and blueish are vague ideas. A vaguely correct filter will produce a nice colour image for your eye, but astronomers doing photometric research (numerically measuring the brightness and/or colour of stars) need slightly different and more precisely defined filters. To distinguish the photometric filters from standard ones, we name them differently.

Beyond the Visible

Our CCD is sensitive to a slightly wider spectrum than the human eye. It can also see ultra-violet and near infra-red light. On Galaxy Cam only, there is an IR filter. You can use this to image in infra red, but this isn't as interesting as you might think as space is quite cold. Nebulae mostly vanish in infra red. It can be used for photometry. The photometric filters actually came as a BVRI set.

Infra Red

Narrowband Filters

Galaxy Cam has two interesting narrowband filters: O-III (Oxygen 3) and H-Alpha (Hydrogen Alpha). These are for emission nebulae. I touched on this subject in my post on the Triffid Nebula.

Most of the nebulae you know about don't glow because they are hot. The gasses in nebulae glow because atoms are 'excited' by having their electrons either energized or knocked off them completely by radiation from nearby stars. As the electrons return to their original state, they give out light of a very specific wavelength. This wavelength is specific to the atom involved (Hydrogen, Oxygen, Nitrogen etc) as well as the particular energy level the electron is knocked into.

The narrowband filters allow only a narrow range of wavelengths to pass, corresponding to a particular element. The black and white image returned through a H-alpha shows lots of glowing hydrogen vs no glowing hydrogen. Areas of nebulae that look red in a colour image will likely show up strongly through a H-alpha filter. The less common green colour in nebulae results from oxygen.

As well as being kind of cool, this can really improve images. These filters allow a large portion of the light from the nebula to get to the CCD while blocking almost all the stray skyglow/moonlight/starlight that might otherwise make the image looked washed out or hazy. Compare the red image above with the H-alpha below, and the green with the O-III. (All the images in this post have the same exposure and post-processing settings).

Most, if not all, of the Hubble Space Telescope's nebula images you have seen have been made by taking three or more narrowband images for different elements, and tinting and combining these in the same way you usually would with R G and B.

Since stars become fainter through H-Alpha, galaxies (which are made of stars) will become very faint or invisible. Galaxies with a lot of active star formation (and so many bright nebulae) may show something through this filter.


Neutral Density Filters

There is only one thing you should use a Neutral Density filter for. That is the Moon. In astrophotography, it's usually all about getting as much light as possible: using the biggest telescopes; looking through the least atmosphere; with the most sensitive CCDs; with very long exposure times.

Not so with the Moon. The Moon is so bright that even with the shortest exposure possible, an unfiltered image would overload our sensitive CCDs. The neutral density filter uniformly cuts out light across the spectrum. The most powerful reduction is needed for Galaxy Cam. The ND3 filter on galaxy cam lets 0.1% of the light through. And you still need the minimum exposure time to get a good image. The other two cameras have ND2 filters, which let 1% of the light through.

To reiterate, don't use this for deep-space objects. You just waste 99.9% of that valuable light (and telescope time).



This is a summary of the filter options and what they mean.

R All Cameras Red light
G (or V) All Cameras Green light
B All Cameras Blue light
Infra Red Galaxy Cam
RGB or BVR All Cameras Automatic colour image
Clear or None All Cameras Lets all light through

(for emission nebulae)
H-alpha Galaxy Cam Shows only light from excited Hydrogen atoms
O-III Galaxy Cam Shows only light from doubly ionised Oxygen

Neutral Density
(For the moon)
ND3 Galaxy Cam Blocks 99.9% of light
ND2 Cluster/Const. Blocks 99% of light