Removing lens-cell artefacts

When I created the first full LRGB image with my SkyWatcher Esprit 80ED I noticed some points that required attention. Those points were the minor issue of determining the correct flattener-to-camera distance, but above all issues with the shape and quality of the stars. The first test image can be seen in figure 1 and shows Sadr surrounded by an asymmetrical halo in which as well irregular diffraction spikes are visible.

A while ago, a thread on a Dutch forum mentioned that the Esprit 80ED sometimes suffers from spikes around stars and from asymmetrical halos around it. The replacement copy that the owner received at the time also appeared to suffer from this and a member on another Dutch forum also once owned such an Esprit (the replacement copy was fine).

A while ago I had one of those telescopes that was mentioned in above thread in front of the collimator, with which it could indeed be shown that this copy was not in order. Closer inspection showed that a rubber ring (see figure 2) in the lens cell was not mounted correctly and may have caused the asymmetric halos, while the spikes seen in that photo were likely caused by pinched optics. Pinched optics are lenses that are mounted too tight in the lens cell and a regular diffraction pattern is created that corresponds to the number of adjustment screws around it (the ones that hold the lenses in place (see figure 3)). These screws are thus not the collimation screws, but the screws with which the lenses are centred in the lens cell. Similar to a triple spider, six diffraction spikes can be seen with a telescope that has three adjustment screws.

The Esprit 80ED I used to image M31 had an improved lens cell, which is recognizable by the absence of the rubber ring. So I didn't expect any problems like in the telescope mentioned above. Yet it also turned out that that specimen had an asymmetrical halo, in which diffraction spikes were also recognizable (see figure 1). However, these diffraction spikes did not form a regular pattern and are due to irregularities along the inner edge of the lens cell.

Since the star-field was regular everywhere and almost no coma was visible even in the corners (something another Esprit 80ED did show), I found it interesting to find out where those artefacts originated from. If I could solve them, this telescope potentially would produce very nice images.

The diffraction pattern is known to be caused by irregularities on the inner edge of the lens opening and I suspected that the asymmetrical halo also came from there. The front part of the lens cell is like a tube with parallel walls. The idea was that, if that part is not completely perpendicular to the telescope axis, this inner wall can sometimes work as a reflector. To test this, I cut a 79mm diameter circle from a piece of thick paper and mounted it to the lens cell with some tape (see figure 4).

Then I took an image of Altair with this aperture-disc (see figure 5). What is striking is that the star had become nicely symmetrical, but that the number of diffraction spikes had increased drastically. Since the latter must come from the periphery, it cannot but be traced back to my poor tinkering. The most important thing was that progress had been made: apart from the diffraction spikes the halo was nicely symmetrical, and so were the stars in the rest of the image.

So the next step was to make an aperture-disc with a very smooth inner edge. Fortunately I own a well-equipped workshop and had a piece of left-over aluminium sheet of 3mm thickness. First I cut an approximately round disc from it and then clamped it in the lathe to turn it into an aperture-disc with an opening of 79mm. The front of the opening was chamfered at an angle of 45 degrees to prevent unwanted reflections. A rim of one and a half millimetres high and one millimetre thick was left on the outside, so that the disc fits neatly centred around the lens cell (see photo at the top right in figure 6). Finally, the inside was sanded with waterproof sandpaper (180 grit) and then polished with 600 grit.

The test with this aperture-disc had to be done in front of the collimator due to a lack of clear skies (see middle row of photos in figure 6). Clearly can be seen how the image improved. At the far left the fanning out star can be seen with a diffraction pattern, in the centre the star has become symmetrical but the diffraction pattern has become more intense due to the mediocre cutting, and at the far right we finally have a nice round star. The next day I was able to repeat the test using Altair and the difference became even more apparent (see photos at the bottom right of figure 6 and figure 7).

All in all it was quite a search, but I am very satisfied with this final image (see figure 7). By using the aperture-disc, the telescope has become a fraction slower. Instead of 400mm f/5, it is now 400mm f/5.06, a difference that no one will notice, but which makes a world of difference in the resulting images. Since the rest of the image is also very good, I thought it would be better to use the telescope in this way than to exchange it for a new one, especially since an earlier one suffered from coma along the diagonal and one may wonder if a next Esprit 80ED would not suffer from this issue again.

Incidentally, a SkyWatcher USA employee indicated in an email that the asymmetrical image could be a problem with the coating. He gave no further explanation, but most likely he meant that the coating does not extend all the way to the lens cell. If the lens (or coating) is not completely centred, light would indeed enter more from that side and cause an asymmetrical image. Whether this is also the case with this telescope cannot be determined. However, it was mentioned that it would have been solved by SkyWatcher in the meantime. As this particular telescope was brand new, I wonder if this is indeed the case.

It only remains for me to close this paper by thanking the supplier, Robtics from Leidschendam, for the enormous service they provided. Even now I could exchange the telescope, the fact that I did not do so was my own choice, as explained above.