BBC Sky at Night Magazine

Processing

How to transform a familiar view of a gas giant by using a methane filter

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At the 2019 IIAPY awards I was fortunate enough to take third place in the Planets, Comets and Asteroids section for my image ‘Black Saturn’. This image was taken through a methane filter, a device that makes this favourite object look quite eerie with the familiar view being

replaced by one showing a planet with sooty belts on a dark disc surrounded by a set of much brighter rings.

Methane (CH4) is a major component of the gaseous atmosphere­s of both Jupiter and Saturn. It makes its

presence known by absorbing light in several narrow

spectral bands, including one in the near-infrared

(near-IR) centred on a wavelength of 890 nanometres. Imaging through a filter with a narrow band pass (allowing it to capture a small defined bandwidth and exclude others) centred on this wavelength allows

you to see where the methane is by which areas look

dark. Such images are scientific­ally useful as they can show features and activity not visible in normal

optical images.

Methane filters used to be costly, but in recent years lower-cost Chinese filters have come onto the market, allowing for

the imaging of Jupiter and Saturn in this

wavelength. As a result, images of Jupiter

taken in the methane band are now much

more commonplac­e. Even so, detailed

images of these planets in this band are

difficult to take, mainly due to the lack of

light. There are a couple of reasons for this. Firstly, the methane band in the near-IR is narrow and to get good contrast the

methane filter also needs to be in the same

narrow band pass. Secondly, the spectral response of planetary cameras falls off in the infrared band; with CMOS cameras

having peak responses of only 25-40% at 890nm, the methane filter band pass.

To compensate for the lack of light, methane images tend to require much longer exposures, making them more

prone to smearing by atmospheri­c movement. The

low light also means that images tend to suffer badly from noise or unwanted artefacts. On top of this the

longer wavelength reduces the resolution of the scope significan­tly, as the size of the diffractio­n disc is directly proportion­al to the wavelength. It’s an

interestin­g challenge getting decent surface details on Jupiter using methane-imaging techniques, while

detail on Saturn is even more so as it has a much lower

surface brightness due to its greater distance from the

Sun. It requires even longer exposures and there’s a

likelihood of more noise. Good Saturn images, taken in the methane band and showing disc details, are rare.

Back in black

My image of Saturn was taken on a night of good seeing in early August 2018. Even though the planet was visible at only 15° altitude from my suburban location, the image was relatively steady in my 444mm Dobsonian telescope, so I thought I’d have a go at imaging in methane and see if I could capture

belt detail and surface features on the disc. Using a

low-noise ZWO camera with a mono chip sensitive

in the infrared (a Sony IMX290 sensor) and operating

at f/12, I had to use a gain of just over 500x and

a 32-millisecon­d exposure to get a bright image

on the preview screen which – however – meant it was very noisy (Stage 1).

Swapping to a 610nm filter with a much greater light transmissi­on than the methane filter, but

with the same thickness, allowed me to get the

focus on a much less noisy image – with less

unwanted artefacts (as shown in Stage 2). Swapping

back, I then captured one methane video of 5,700 frames and another of 8,000 frames, giving a total exposure time of 7.3 minutes – a long-enough

accumulate­d exposure time to help reduce noise

to tolerable levels.

Processing involved picking the best 50 per cent

of frames and stacking in AutoStakke­rt!3, where

I selected a very high ‘Noise Robust’ value of 8 to

stabilise the frames. I manually placed alignment

boxes over the disc and rings of size 104 (Stage 3).

Stacking was then followed by wavelet processing in RegiStax using gaussian wavelets

(Stage 4 screenshot) to yield two images to then combine together, by using WinJUPOS to derotate them.

The combined image was then imported into

PaintShopP­ro where the noise level was reduced by using a gaussian blur of 0.5 pixels and a Topaz DeNoise plug-in. I often use

another great plug-in, Astra Image, to add a bit of

deconvolut­ion, which helps

bring out detail. But it also boosts the noise, so I had to apply the

finishing touches with some

more gaussian blurring

to smooth it. My finished

‘Black Saturn’ image was now ready to enter into the

IIAPY competitio­n.

 ??  ?? The final image of Black Saturn, ready to submit to the IIAPY
The final image of Black Saturn, ready to submit to the IIAPY
 ??  ?? ▲
Stage 1: one of the best raw frames from the video showing the high levels of noise – unwanted artefacts – that are present
▲ Stage 1: one of the best raw frames from the video showing the high levels of noise – unwanted artefacts – that are present
 ??  ?? Martin Lewis is a planetary imager. He was shortliste­d at the IIAPY in 2019 for ‘Black Saturn’
Martin Lewis is a planetary imager. He was shortliste­d at the IIAPY in 2019 for ‘Black Saturn’

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