Im­age Pro­cess­ing

Colouris­ing the Sun

Sky at Night Magazine - - IN THE MAGAZINE - With Gary Palmer Gary Palmer is an ex­pert so­lar imager. See more of his shots at­larsys­temimag­

The qual­ity of so­lar images pro­duced by am­a­teurs has come a long way in the past few years, so much so that they are com­pa­ra­ble to some satel­lite images. While so­lar tele­scopes have stayed pretty much the same, cam­eras have evolved mas­sively.

Many as­tronomers start out in so­lar imag­ing us­ing hy­dro­gen-al­pha tele­scopes and colour cam­eras, and they have a hard time gain­ing de­tail in their images. This is be­cause of the nar­row­band na­ture of the scope, which means a colour cam­era only uses a frac­tion of its chip. Mono­chrome cam­eras pro­duce much more de­tailed images, as they can make full use of their sen­sors. Of course, to achieve a colour im­age, we need to use pro­cess­ing soft­ware.

Colourised mono images are of­ten de­scribed as hav­ing ‘false colour’, and most of the time this is added in a pro­gram such as Pho­to­shop or GIMP. The fi­nal colour you pick is a mat­ter of taste, but you have to be care­ful not to over­sat­u­rate the colour as this will lead to a loss of de­tail. As a guide, lighter colours such as pale yel­low and a pale or­ange work best for keep­ing de­tail in the fi­nal im­age; darker reds can hide it, par­tic­u­larly around sunspots.

There are two ways to add colour in Pho­to­shop. The first is to use Vari­a­tions (Click Im­age > Ad­just­ments > Vari­a­tions) – note this op­tion is only avail­able on Mac OS when run­ning Pho­to­shop in 32-bit mode.

A new panel will open and the colours can be added by click­ing on the re­quired red, green or blue tabs. A live view of the colour can be seen at the top of the page, in the screen named ‘Cur­rent Pick’. Fine amounts of colour can be added by click­ing mul­ti­ple times on a colour. Fur­ther changes can be made af­ter­wards us­ing Colour Bal­ance (Im­age > Ad­just­ments > Colour Bal­ance).

The sec­ond way is to use a process called Duo­tone; if you are pro­cess­ing tiff images, you’ll need to con­vert them to 8-bits for this to work. Do this by click­ing Im­age > Mode > 8Bits/Chan­nel.

Tiff or not, the im­age also needs to be greyscale to start with (Im­age > Mode > Greyscale). Now you can set it to Duo­tone (Im­age > Mode > Duo­tone), which will open the Duo­tone op­tions dialog box. In th­ese op­tions, change the type tab from Mono­tone to Tri­tone.

This al­lows you to pick three tones for your im­age. Black is au­to­mat­i­cally in the first slot, and you need to keep it to main­tain con­trast. Se­lect one of the blank boxes and choose a suit­able colour; re­peat for the sec­ond. When done, you will need to turn the im­age back to RGB colour (Im­age > Mode > RGB Color). If you are work­ing on a promi­nence and a sep­a­rate sur­face im­age, do not com­bine the lay­ers when asked. As with the pre­vi­ous tech­nique, colour bal­ance can now be used to ad­just the colours for each layer to your pref­er­ence.

Gary’s fi­nal pro­cessed im­age shows rich de­tail on the dy­namic sur­face of our star

Your start­ing shot should be mono­chrome; the fi­nal colour you choose is up to you

In Vari­a­tions, click on a colour more than once to in­cre­men­tally add more to your im­age

In Duo­tone, al­ways keep black as one of your inks to main­tain con­trast

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