But before describing the faster method to produce an inverted solar image, let's first look at the quality of the data. In figure 1 of Part 2, I showed two full-disc recordings, one processed normally and the other processed inverted, but both starting from the same data. What stood out about the images was that the solar disk was not completely evenly lit, something that I did not understand at the time. Clearly visible is the light region (or dark in the inverted image) to the left of the centre of the Solar disc.

This exposure imbalance is caused by not positioning the camera correctly behind the etalon. The etalon consists of two glass plates that are at a very short distance from each other and therefore act as a filter. The light passing through an etalon is reflected between the two glass plates and by varying the distance from the light path, it is possible to amplify or cancel out certain frequencies. There are two ways to vary that distance: by forcing air between them (pressure tuner) or by tilting the etalon (tilt tuner).

Regardless of the type, the etalon has the characteristic that it has an area, approximately one degree in diameter, within which the attenuation/gain is more or less uniform and reaches a maximum, the so-called sweet-spot. The centre of this sweet spot marks the optical axis of the etalon. Now the Sun is about half a degree in diameter, so it fits perfectly within that area. But since the attenuation/amplification within the area is not quite even, the intensity of the image is determined by this. That is why it is important that the Sun surrounds the sweet spot center as well as possible. If you don't do that, you will see in the final result that the Sun is not equally bright everywhere. Centring is easily done by aiming the telescope at the Sun so that the maximum intensity coincides with the centre of the Sun. Temporarily overexposing the live-image helps with this (see below).

Now, however, it can happen that the optical axis of the etalon (and thus the sweet spot) is so far off to the optical axis of the telescope that it does not coincide with the centre of the imaging chip. I have tried to show this schematically in figure 2. The image chip is the red frame here. The Sun (orange disc) is completely against the left edge here and cannot go further to the left because it will then be outside the camera's view. However, the sweet-spot (indicated by a fading white-grey spot) is further to the left, so the Sun should actually be positioned within the yellow circle to to fall nicely within the sweet-spot. This can be achieved by simply moving the telescope to the right. However, the camera does not allow this and since the optical axis of the etalon is mechanically fixed to the telescope and camera and thus has a fixed orientation in relation to them, this means that the camera must be moved to the left, independently from the telescope and etalon, in order to centre the sweet-spot on the image chip.

However, no adapters are available for lateral movements, but we can try to achieve the opposite (i.e. moving the sweet spot towards the centre of the camera). With an Atmospheric Dispersion Corrector (ADC) it is indeed possible to control the optical axis. In an ADC there are two equal prisms that can be rotated relative to each other. If they are rotated 180 degrees relative to each other along their optical axes, then nothing happens and the light that falls on them goes straight through. However, as soon as the mutual orientation of the two prisms changes, the optical axis of the telescope is deflected and the image arrives at a different location on the image chip, so that the sweet spot can thus be 'pushed' towards the centre of the camera. This means that dispersion occurs because light of different wavelengths is deflected in different ways, but with an H-alpha telescope this does not cause any problems because the light we are trying to capture is almost monochromatic.

Aiming the telescope correctly and adjusting the ADC can be achieved by first slightly overexposing the image (see figure 3). The part of the Sun where the centre of the sweet-spot is located will therefore become uniformly white (see figure 3, A). Then we can steer the telescope in such a way that the sweet-spot coincides neatly with the centre of the Sun (the overexposed part of the Sun is then neatly centred within the Sun). The telescope is then correctly aimed, but the image of the Sun may not fall entirely within the image chip (see figure 3, B). To centre that too is a matter of adjusting the ADC. With the help of the two levers, the solar image can be shifted towards the centre of the image chip (see figure 3, C). It may be that this requires turning not only the two handles, but even the entire ADC. Once the Sun is again (nearly) centred in the frame, the sweet-spot will also be centred and the exposure can be adjusted normally again (see figure 3, D). In addition to centring the sweet-spot the use of an ADC also has the advantage that it eliminates the Newton rings that often occur in etalons (compare figure 3, A and B to C and D, for more info see this white-paper).

Once the Sun is neatly positioned in the sweet-spot using the above method and coincides with the centre of the imaging-chip, it can be captured. The first step in processing is simply stacking as discussed in Part 1. We open the result with ImPPG (see figure 4) in order to edit the photo in such a way that both surface detail and solar flares become visible (so this is done with a single image in which the histogram for the surface is approximately 90% filled). To start (figure 4, Step A), we set Iterations of the Lucy-Richardson Deconvolution to 0, which disables this sharpening method. The Unsharp Mask is disabled by setting the Amount to 1. These two operations are currently performed only within the black square (indicated by the red arrow). As this square is taken larger, the associated calculations take more time. With Lucy-Richardson Deconvolution and Unsharp Mask turned off, we can maximize the square by clicking on the corresponding icon (see figure 4, Step B). The last step to arrive at an inverted operation is to enable the histogram and click the inv-button at the bottom of the histogram (see figure 4, step C).

Once the image is inverted, it is possible to adjust it further. Figure 4 shows this in five steps. The histogram can be adjusted by clicking on it with the left mouse button, at wich location a point is added, which can then be dragged. Points can be deleted with the right mouse button. By first placing a dot in the middle and dragging it to about 10% from the top (see figure 5, A), followed by clicking the extreme left dot and dragging it down (see figure 5, B) we make the background dark again. We can make the prominences brighter by adding a dot at about 10% from the start of the histogram and setting it to about 70% intensity (see figure 5, C). Now the background may be a little too light, which we can adjust by placing a new point between the start and the previous point and pulling it down a bit (see figure 5, D). To remove the kink from the histogram, we put another point at about 20% of the histogram and pull it up slightly (see figure 5, E).

In this way, several points can be added to the histogram in order to further refine it. Once satisfied, it is possible to save this histogram for later use (File/Save Processing Settings). The best results are achieved by drawing the smoothest possible line in the histogram. Prominences on the surface can be further brought forward by very subtly adjusting the points. This last step is easier if the histogram is expanded to the full width of the screen and when zoomed in on the details you want to edit (see figure 6). To sharpen the image even more, the Lucy-Richardson Deconvolution and Unsharp Mask can be used, but of course this can also be done in another image processing package. Finally, we still have to mirror and rotate the image if, as in my setup, a diagonal has been used in the imaging and we can give the image a colour as described in Part 2. The result is then as below: