An article in the author’s Astronomy Digest – http://www.ianmorison.com
This is a step by step work through of processing a fairly tricky data set of the Orion and Running Man Nebulae imaged by a colleague. The subframes were taken using a Sony A5000 camera and Televue 4 inch, f/5, telescope. Thirty, 15 second exposures at an ISO of 1,600 had been aligned and stacked in Deep Sky Stacker using the 2x dither mode and saved as a 32 bit Tiff file. (Over 700 Mbytes in size!) The short exposures ensured that the central, very bright, part of the Orion nebula surrounding the Trapezium stars was not over exposed.
[The updated version of Affinity Photo can also align and stack subframes and produces excellent results as covered in the ‘Aligning and Stacking …’ article in the digest and described in Imaging Tutorial (1)]
An introduction to the workspace
This section for newcomers to the program and describes those features of Affinity Photo that are used in the tutorial.
The tools palate is a vertical bar on the left hand side of the image space. The required tool symbol is simply clicked upon to activate. Those used in this tutorial are the ‘crop tool’, the ‘selection brush’ that lies below it and the ‘paint brush’ below the colour disk.
Across the top of the workspace lie some drop down menus:
In the ‘File’ submenu, one can open and export (rather than save) a file, in the ‘Edit’ drop down menu one can ‘undo’ or ‘redo’ adjustments that have been made to the image. One can also access the ‘Inpaint’ tool.
In the ‘Document’ drop down menu one can convert the format to, say, reduce size or change the bit-depth of an image.
In the ‘Layer’ drop down menu one can ‘Duplicate’ a layer or ‘Merge Down’ two or more layers.
In the ‘Filters’ drop down menu one can go into the ‘Blur’ and ‘Sharpen’ drop down menus to apply the desired effect.
At the top right one can select the colour of the paint brush. Below are a set of adjustment tools. Clicking on the levels symbol, for example, three options are brought up such as, in this case, ‘Default’, ‘Darken’ and ‘Lighten’. The default modes are used in this tutorial and, when clicked upon, bring up the adjustment window. After adjustment, to immediately apply its effect as will be done in this tutorial, one clicks upon the ‘merge’ tab.
The use of layers
When a tool, such as ‘Levels’ is used to alter an image, AP produces an ‘Adjustment Layer’ that sits above the preceding layer and these will build up as more tools are used unless they are ‘merged’ after each tool is applied. Advanced users will keep multiple layers active – which has the advantage that any single adjustment layer can be altered later if desired without affecting any of the other adjustments made. This is quite hard to describe in a tutorial and so I will use the approach where each adjustment layer is merged immediately after its application so only one layer remains active. If necessary, within the ‘Edit’ drop down menu one can backtrack as many steps as one wishes to get back to an intermediate step that one is happy with. At certain points within the editing process I will ‘export’ the current version of the image so that, should I need to leave the program or should it crash, I will be able to continue on from that point. I have found that, occasionally, AP has crashed when, as a novice, I have made some mistakes in using it. On a couple of these occasions, this required a hard reset of the computer. The program also crashed when it somehow ‘disapproved’ of two out of 45 tiff files that I had been attempting to stack. [I had to find them by trial an error.]
Importing and cropping the file.
To reduce the processing time, the image was cropped to only cover the part of the image covering the two nebulae. The crop tool was used to select the desired region and the blue ‘Apply’ tab at top left clicked upon to make the crop.
Reducing in size
In the ‘Document’ drop down menu, the ‘Resize Document’ command was used to halve the resolution – making it equivalent to the original resolution of the camera.
First stretching of the image
The use of 32 bit data allows one to extract the very faint detail in the image – but it does increase the processing time and two useful functions (‘Inpaint’ and ‘Median Blur’) cannot work on 32 bits. If one first stretches the data to lift up the fainter parts of the image one can then reduce the bit depth to 16 bits without losing any information contained within the image. [In some cases, it may be worth working in 32 bit mode throughout and the ‘work-around’ for the Inpaint and Median Blur functions that cannot be used in 32 bit mode is described below.]
There are several ways of stretching the image one can either use the curves function or, as in this case, the levels/gamma method which works well with this data set.
The image is opened and the ‘Default’ levels adjustment tool is opened. Within its window, the gamma slider is moved to give a value of, say, 0.8. One will see that the fainter parts of the image increase in brightness more that the brighter parts – thus producing some image stretching. Two applications allowed the sky background level to become obvious.
Reducing the bit depth.
In the ‘Document’ drop down menu, the ‘Convert Format..’ function was used to reduce the bit depth from 32 to 16 bits.
Removing the sky background
The best way to do this is to make a new layer containing just the sky background and then subtract this from the original image. Affinity Photo has a magic way of doing this! The image is duplicated and the screen shows the upper layer. Using the selection brush tool, the areas containing the two nebulae are selected.
In the ‘Edit’ drop down menu, the ‘Inpaint’ function is clicked upon when, amazingly, they disappear.
The selection is removed by pressing ‘Esc’. Now in the ‘Filter’ drop down menu, ‘Blur’ is clicked upon and, in its dropdown menu, the ‘Median’ filter is selected.
This is applied with the maximum radius of 100 pixels and the stars disappear giving a smooth image of the sky background.
In the Layers blend mode drop down menu (at right beside the ‘Adjustment’ drop down menu), ‘Subtract’ is selected with an opacity of 100% and then clicking on ‘Merge Down’ in the Layers drop down menu produces an image with a totally black sky background. [Many astophotographers do not like a totally black sky background, so at the end of the imaging process, this can be addressed.]
Note: Removing the sky background in 32 bit mode
The Inpaint function and Median filter can only work in 16 bit mode so, before their use, the upper layer is changed to 16 bit depth, the inpainting and median filter applied before the layer is changed back to 32 bit depth and subtracted the from the image layer below.
Further Stretching of the Image
The same ‘levels’ procedure is applied again to bring out the fainter parts of the nebulae.
In this case there can be a problem in that the very bright central part of the Orion Nebulae surrounding the Trapezium stars can get blown out. If this is seen to happen one could use an alternative stretching method employing the ‘Curves’ function. If the greater part of the curve is locked to initial straight line curve and only some lifting done to the darkest end of the curve this can be avoided.
Alternatively, using the selection brush, one can make a selection of the central part of the Orion Nebula and, within the ‘Refine’ window, increase the feathering to, say, 90 pixels. If the pixel selection is now inverted (Select> Invert Pixel Selection), the application of the levels function will not apply to the originally selected region but, due to the feathering, there will not be an obvious boundary between the selected and de-selected regions.
This additional stretching may bring up the background level and use of the ‘black point’ slider in the levels command can reduce this if necessary.
Enhancing the Image
The ‘local contrast’ within the two nebulae can be increased by selecting them with the selection brush and using the ‘Clarity’ (Filters> Sharpen> Clarity) tool. The result is quite significant and greatly improves the image.
It could also be worth applying the ‘Vibrance’ filter (Filters> Sharpen> Vibrance) to the nebula regions.
One might also consider increasing the saturation within the two nebulae. This can be achieved by using the ‘HSL’ tab in the studio pane. The regions of the two nebulae are selected using the selection brush tool and the saturation can then be gently enhanced with adjustment made only to the selected areas.
Adding some background brightness
I desired, one can uniformly increase the sky background to that the sky is not totally black. The image is duplicated and the upper layer is painted a dark grey.
If the Layers blending mode is set to ‘Add’, the opacity of the upper layer can be adjusted to a low amount to brighten the sky background and give the final result.
As only 30 exposures were able to be stacked as wind shake had affected a further 30 subframes, the outer, very faint, parts of the Orion Nebula were barely visible, but otherwise this was an excellent image of the two nebulae – showing up the Running Man Nebula particularly well.