Astrophotography by Anthony Ayiomamitis

Processing Images of the Sun

Our closest star provides a unique opportunity for the amateur astronomer and astrophotographer to engage himself with this wonderful hobby during the daytime. In fact, for the amateur whose nighttime activity is restricted due to family, work or any other complication, the sun provides a viable alternative for the active pursuit of astronomy during the day.

Although the acquisition of images is very exciting, it really represents only one half the battle required to produce a stunning result and which will reward the photographer for his efforts under the sun and, perhaps, during adverse conditions. It should be emphasized that good technique is most vital during the acquisition of a GREAT raw image so that the ensuing processing will be as trivial and least-evasive as possible. In order for this to become reality, we need razor-sharp focusing, proper exposure and a suitable recording format.

Razor-sharp focusing can be influenced by a variety of factors including image scale, seeing and transparency (as usual), the type of focuser used (ex. rack-and-pinion vs a feather-touch focusing device), the mount's stability etc. Rather than delve into these technical issues, I would prefer to concentrate on the physical process itself where I seek out the smallest possible sunspot visible on the solar disk and which I use to gauge my focusing. If we are poorly focused, small details such as very fine sunspots will be lost from our field of view; in contrast, as we converge towards the critical focus zone, smaller and finer detail will become available and which we can use to derive the optimal focus point.

Once we are focused, we must establish the proper exposure. The availability of digital cameras has proven to be a phenomenal tool for the astrophotographer, for testing and results are immediately available for perusal and study. My preferred modus operandi is to use the lowest possible ISO setting (ex. ISO 100) so as to ensure that I have the lowest possible electronic noise (this ISO setting may have to be increased upward if, for example, we have a windy day and we require fast exposures to "freeze" the field of view as much as possible). I will then proceed to take 2-3 images of the sun at various exposures which I consider to be possible candidate exposure times (for example, anything from 1/2000 to 1/250 sec). By looking at the histogram produced for each image on the camera's LCD, I will seek a histogram which has as much dynamic range as possible and simultaneously covers approximately three-quarters of the maximum ADU (or the horizonal distance of the histogram). Once I have narrowed down my available choices, I will use the zoom feature of my EOS 300D to examine surface detail, for a properly exposed image will have indications of solar disk detail in the form of grain and simultaneously record both large and small suspots and sunspot groups.

The third and final requirement for a great raw image is the recording format. Today's digital cameras have the consumer in mind and, as such, their primary purpose is to perform software adjustments which should produce pleasant consumer photos. One such software adjustment is the white balance which can prove to be detrimental to our solar images. In order to ensure that we have the original image at our disposal with no software adjustment or intervention, it is critical that we use the RAW image format when writing our images to the camera's flash card. Once downloaded onto our computer, these images can be converted to TIFF format (16-bit !) using the camera's external software (usually available on an accompanying CD) and proceed with our proposed processing.

Once a (good) raw image has been acquired, we are presented with the second challenge, namely the proper processing which will maximize the amount of information that is hidden within our raw image file. Below I describe ten "easy" steps which I use incessantly in processing ALL of my solar images. In fact, one could write a Photoshop script to pursue the steps described below without any worry that human intervention would be required.

Processing Sequence
Step 1: Examine image at 100% size to assess quality

A quick view of our image converted into TIFF format and examined at 100% is most encouraging since we see indications of solar surface detail (grain and plages), large and small sunspots and penumbral detail. It is a good idea at this point to crop our original image so that we remove any unnecessary periphery and thereby restrict the processing to only the solar disk itself (it also helps with computer memory since we are now processing a smaller image involving the solar disk and nothing else). At this point, my cropped image files involving the solar disk exclusively are approximately 1600x1600 pixels.

It should be pointed out that conversion from RAW to TIFF format be performed at 16-bit and that ALL processing steps below also be performed at 16-bit. Only when producing a final (JPG compressed) image for web presentation do we convert to 8-bit!

Please click on the image to the right for a larger format.
Step 2: Convert to Grayscale

We now convert our image into grayscale and where it will take the first form of life. We will now have our first indications of the dark limb effect, solar granulation and many other (crude) details.

Please click on the image to the right for a larger format.
Step 3: Unsharp Masking ... (100%, 1.0, 0)

At this point I apply the first round of unsharp masking. Since the image scale for this image is approximately 1.27"/pixel, I have the flexibility to pursue a small radius in the unsharp mask so as to enhance the finer details as much as possible. If I note no improvement and/or processing artefacts at this point, I will begin to doubt the quality of my original image. In contrast, if such a fine unsharp mask produces an improvement, I will have further evidence that my original (raw) image is very good and will be able to withstand further processing.

Please click on the image to the right for a larger format.
Step 4: Brightness/Contrast ... -30 / +50

This particular step is quite significant and will have a serious impact on the final image. Our interest is maximizing contrast throughout the solar disk while maintaining a pleasant amount of brightness for the disk itself. As we increase contrast, we automatically increase the brightness of the overall image and which we can reverse by purposely lowering the brightness via the slider. I usually proceed by lowering the brightness first (ex. -30) and then follow by increasing the contrast (+50). These values are arbitrary and it is best to experiment back and forth with suitable values and combinations for both brightness and contrast. Once this step is completed, our image should start to look even more appealing.

Please click on the image to the right for a larger format.
Step 5: Resize to 65% ... and Unsharp Masking ... (100%, 1.0, 0)

My first step in preparing an image suitable for web presentation involves doing an initial resize of the original file so that it is now 65% of the original. I then perform a second round of unsharp masking and it is at this point that our image begins to take a "Wow" look.

Please click on the image to the right for a larger format.
Step 6: RGB mode and false colouration

In order to add some spice to our image, we can insert some false colouration using some experimentation for the midtones involving each of the RGB components. We first must convert our image from Grayscale to RGB via the Image->Mode->RGB menu sequence (note that there will be no visible difference in the image on our canvas). We then proceed with Image->Adjustments->Levels and where a new window appears on our screen showing the histogram for our image to this point. We will use the drop-down menu to individually select each of the Red, Green and Blue channels and where we will proceed to adjust the midtone slider for each of these channels. Following lots of trial and error, I have established that the slider for Red, Green and Blue with values of 1.70, 0.72 and 0.14 respectively yields a very pleasant RGB image for my image train involving an Astro-Physics AP160/f7.5 EDF refractor and my Canon EOS 300D used in prime focus. These values are best for my particular optical train and you will need similar experimentation for your setup so as to derive similar optimal values.

Please click on the image to the right for a larger format.
Step 7: Finetuning ... size and sharpness

We are VERY close to our final masterpiece. At this point I will resize the image a further 65% (equivalent to approximately 45% of the original since this is the second reduction by 65%) and apply a very FINE round of unsharp masking. For this particular example, I used (100%, 0.3, 0) for my final sharpening.

Please click on the image to the right for a larger format.
Step 8: Finetuning ... brightness and contrast

We are now even closer to our final masterpiece since I really like the detail and sharpness at this point but we could use a slight adjustment to the brightness and contrast so that we have a more pleasing final image. This step may be iterative and require slight tweaking a handful of times. With this example, we will simply proceed with the following adjustments: brightness @ -08 and contrast @ +08.

Please click on the image to the right for a larger format.
Step 9: Finetuning ... aaaaaalmost there

We are very close to our final result and final version of our masterpiece. We simply need another round of very FINE unsharp masking (100%, 0.3, 0) and we are done.

Please click on the image to the right for a larger format.
Step 10: Mission Accomplished

What have we accomplished in the previous nine trivial steps? The answer is available by clicking on the image to the right.

One final comment: when we have a VERY good initial raw image, our job in processing this image becomes both a trivial and entertaining exercise. To this end, it is vital that we INVEST ten to fifteen minutes at the start of each session to get our focus as perfect as possible; to establish our proper exposure with bracketing one stop as a further precautionary measure; and ensure that we record a RAW image on our flash card for processing and without any on-board camera software doing what it thinks is right for our image!

Happy shooting!!!