The first time I took a picture of the sun with the Lunt, the result was a bit disappointing. The sun had become a somewhat fuzzy disk with limited surface detail and a prominence was barely visible at nine o'clock (see figure 1). The picture was in stark contrast to what I had observed visually with the telescope, which was fitted with the standard Lunt zoom eyepiece. The recording was made with FireCapture, stacked with AutoStakkert! and coloured with Paint Shop Pro. For the surface I had used a shorter exposure time than for the prominence. After processing in AutoStakkert! I superimposed the two images, cropping out the sun from the prominence image, so that the shorter exposure showed some surface detail.

Before I took the picture I had admired many images from other astrophotographers and I found them without exception better than the one I made. I am now almost two years later and I am now also able to take reasonable pictures of the Sun, although there is still room for improvement. Initially I slowly managed to improve my methods myself, but the greatest progress is due to Edith van Werkhoven, for which I am grateful!

As described above I use both a SkyWatcher Esprit 150ED apochromat and a Lunt LS80THA solar telescope. The Esprit is equipped with a Baader ND5.0 solar foil filter, just one of those that is mounted in front of the telescope. The Lunt has a single pressure-tuned etalon. I use both telescopes focally, so photos are not taken through the eyepiece, but directly with the camera at the focal point, just like with deep-sky photography.

With the Esprit I have so far only taken pictures of the entire solar disk, using a ZWO ASI1600MM Cool Pro camera. With the Lunt I make both full solar disk and detail images. To make the latter, I use a TeleVue 2″ 4x PowerMate.

To be able to focus, the PowerMate must come after the blocking filter (this is a B1200). There are blocking filters that have straight-through housings, but it is expected that most users, like me, will use a diagonal with blocking filter. In order to connect a Barlow to the diagonal, Lunt supplies a special adapter that can be screwed onto the blocking filter. To this end, the 1.25″ eyepiece holder is first unscrewed, after which the 2″ Barlow adapter can be mounted. Once it is in place, the Barlow can be inserted and finally, with a 2″ to 1.25″ adapter, the camera is inserted into the Barlow (see figure 2).

What I especially learned is that everything stands and falls with the quality of the raw data, so good focus and ditto seeing are crucial for a sharp photo like in figure 3! For good seeing it is best to image in the morning and preferably on a not too hot day. Of course there is a chance of better seeing as the sun's height increases, but at the same time this causes more heat and therefore the chance that the atmosphere becomes more unstable. In terms of seeing, the best periods for solar photography are spring and autumn. In that respect there is no difference with planetary photography: it is mainly a lot of trying and hoping that you will find a good moment. Image 7 was taken in the morning of September 20, 2019, when I made a second attempt in the afternoon, the same sharpness was no longer achievable due to the seeing.

Focusing is easiest when some details are visible, preferably in the form of sunspots or a large prominence, but the edge of the solar disk can also be used to focus. To check the focus, I make a crop that is as small as possible and put it on the monitor to fill the screen. Then I pay a lot of attention to focusing. It is a matter of not focusing too quickly and in small steps and waiting patiently in between to see whether the blur is caused by the focusing (the image remains blurry) or by the seeing (the image occasionally becomes sharper) is caused.

For the recordings I use FireCapture, using the same exposure time as with all other astrophotos: just keep an eye on the histogram and fill it to about 70-80%, leaving room to stretch in the post-processing without the risk of clipping. Then I make SER-videos of 1 to 2 minutes, depending on the seeing and the details I want to see. I adjust the focus between the recordings, as even though it looks good, resetting the focus can make the difference between a reasonable and good sharp image. Sessions longer than 2 minutes increase the chance that prominences smear too much due to their movement. Of course it is possible to cut or process the videos in parts afterwards.

I use three pieces of software for data processing. The stacking is done with AutoStakkert!, the prominences are made visible with ImPPG and finally the colour is added and the image sharpened with PaintShop Pro.

Let me start by saying that stacking data is not always necessary. Figure 3 is an example of a photo constructed from a single frame of a 2-minute movie. The plane appeared on more than 10 frames, the one shown here had the best composition. To make prominences clearer, stacking is recommended. In AutoStakkert! is it possible to temporarily give the frames a linear stretch, so that the prominences can be seen. This brightening has no effect on the output of AutoStakkert!, but allows APs to be placed in the area of ​​the prominence. Then stacking can be completed in the usual way. Figure 4 shows the 30% result of 2584 frames stacked with 41 APs. The result does not look promising yet, but that will change in the next step.

Once the stack is made, I use ImPPG to get the most out of the data by stretching it non-linearly. To do this I start ImPPG, set the work area to the full photo (fifth icon from the left below the menu bar) and then turn on the histogram (fourth icon from the left, in ImPPG this is called Tone Curve). The window of this histogram can be stretched as desired. The bigger it is made, the easier it is to play with the Tone Curve to bring out details nicely.

Initially, the Tone Curve appears as a straight diagonal line in the histogram. Click this line to add points and then drag it to stretch the image. As shown in figure 5, I use multiple points to get as much detail as possible. At the same time, I make sure that the stretch curve remains as smooth as possible to ensure that there are no strange transitions in the photo. The non-linear stretching may result in a slight edge around the Sun, the trick is to keep it as narrow as possible and to make it blend into the surface of the Sun as smoothly as possible. If necessary, the image can be sharpened with an unsharp mask, but this can also be done later. It is not recommended to do it here and to do it again later in other software, because otherwise there is a chance of an 'orange peel'-pattern in the areas with the prominences. Once satisfied, I save the image as a 16bit TIFF for the final steps in PaintShop Pro.

In PaintShop Pro I do the final steps. I first colourise the Lunt images with the Colorize-tool (Adjust -> Hue and Saturation -> Colorize). I chose Hue 23 and Saturation 201.

Figure 6 shows what this stage looks like. Until colouring, there is no difference between the processing of photos taken with the Lunt or the Esprit. However, I colour the Esprit photos in a different way, because the Colorize-tool does not give the desired result. In PaintShop Pro I put a layer over the photo, give it an orange color (RGB 255, 151, 34) and then set that layer to the colour mixing mode and an opacity of 100%. The result can be seen in figure 8. The colours used in both the Lunt and the Esprit shots are my personal preference, everyone is of course free to choose different values.

The image can then be sharpened. I do this either with Adjust → Sharpness → Sharpen Even (1 or 2 times) or with the unsharp-mask (Adjust → Sharpness → Unsharp Mask…). Finally, I crop the image and paste in a scaled Earth, as shown in figure 7 and figure 8.

Further reading: