Tilt Correction with the Gerd Neumann CTU
For a good astrophoto, a similar level of sharpness across the field is crucial. Pinpoint sharp stars both in the center and in the corners. By the nature of optics, most telescopes have only a small image circle of full sharpness. Correctors such as field flatteners are used to expand the ‘flat’ image circle to cover the full surface of the camera sensor.
But a flat field alone is not enough. It is also important that the sensor surface is exactly perpendicular to the optical axis of the telescope. The slightest tilt of the sensor within its camera-housing can cause unsharpness around the edges of the image. Sensitivity to such deviations increases with sensor-size and faster telescopes. In today’s world where sensors getting bigger and telescopes becoming faster, sensor tilting issues pop up more often. To remedy this tilt, a so-called Camera-Tilting-Unit (CTU) can be placed in the optical path, which allows for tiny angular corrections of the camera relative to the telescope.
Camera Tilting Unit
With the acquisition of the full-frame ASI6200MM Pro camera, a slight sensor-tilt was observed when combined with the FSQ-106, which is an f/5.0 telescope. The ASI6200 comes with a tilting adjustment plate, but in combination with the EFW, this cannot be used. So the search was started for a CTU.
The choice was made for a Camera-Tilting-Unit from Gerd Neumann, which had very good reviews. The adjustments are made by three small screws, placed radially around the ring, allowing easy access with the whole rig assembled. These screws have a cone shape, essentially pushing the two metal plates further apart as they are pushed in. The plates are strongly held together by very tight springs. A full rotation of one screw separates the two plates 0.2mm apart. So sensor-plane adjustments can be made in the order of microns. The Gerd Neumann CTU’s come in various sizes with different connections. To reduce the chance of any vignetting, the largest version was selected. It has female M68 connectors on each side.
Optical train
Adding an M68-sized ring of 17.3mm thickness into the optical path required a re-design of the optical trains for each configuration where the ASI6200 is used. As an example, see the drawing for connecting to the FSQ-106. The Baader TAK-adapter brings the typical Takahashi M72 size down to M68. An extra 5mm spacer, necessary for some other configurations, was obtained from TS-Optics. The male-male M68 adapter comes with the CTU. The M68/M54 adapter had to be purchased separately from Gerd Neumann. The 0.5mm spacer is added to accomodate other configurations.
Both the 5mm M68 spacer and the M68/M54 adapter can get stuck in the CTU and because they are so thin, there is little grip on them. The best solution turned out to be to drill two little indents in the front end of these rings so that they can be loosened using a lens spanner.
Adjusting and monitoring the results
With the whole system setup, tilt adjustments were made. It is important that the 6 fixing screws are loosened. Without that, there is no play in the construction and no adjustments can be made. They can just rattle in their housing and once done, just loosely tightened. A hex key was used to adjust the little radial adjustment screws. It is important to remember how much each screw is rotated, so that fine-adjustments can be made. A hex key makes it easy to estimate the amount of rotations.
The actual adjustments are trial and error and require time and patience. An image is taken, analysed and based on the results, adjustments are made. Best is to start with reasonably large adjustments, like one or half a rotation, so that the effect is clearly visible. In the fine-tuning phase, rotations go down to an eighth of a turn. The starting-position was with all the screws dialed fully out, so no space between the two plates. The problem with that approach is that you can only dial them apart. So if one side needs to reduce in distance, the only way to do that is to increase the distance in the other two screws. With hindsight it might work better if you start by dialing all three screws in a full turn, so that you can push out and in dependent on needs.
There are different options to monitor the amount of tilt correction in the process. The tools used here are from PixInsight and ASTAP. The first one is the AberrationInspector in PI. It can be found under Script/Image Analysis. It essentially breaks an image up into small tiles along the edges of the image, and combines them into one panel mosaic image. In the interface you can adjust number and size of the tiles and colour and width of the separation lines between them. This is the most intuitive tool, as it just zooms in on the critical areas of an image. However, it is a very subjective method and not very suited to dial in the last bit of improvement.
The second tool that was used is the FWHMEccentricity script in PI, which also can be found under Script/Image Analysis. After selecting the proper file, the first thing to do is press the button ‘Measure’. After some time to calculate, the table populates with values for FWHM and Eccentricity. After the calculations are done, the button ‘Support’ becomes active and pressing that brings up two little graphical representations of FWHM and Eccentricity across the frame. These are very helpful in assessing the tilt. The goal is to have a relatively even distribution across the frame, with the lowest values around the center of the frame. With the tilting correction, the most you can influence is the distribution of the values. As absolute values go, these are also inherent to the optical performance of the system, seeing conditions, etc. FWHM values should ideally be below 2, and Eccentricity values below 0.45-0.5. The script was very helpful in assessing the effect of the adjustments.
The third and last tool used is from ASTAP, called CCD Inspector. It can be found under Tools/Image (CCD) Inspector. It calculates Half Flux Diameters across the image and plots the values at each corner. With an even distribution of FHD-values, the diagram should be a true square. Tilting shows up as a skewing of the square into certain corners. In addition, the software calculates a value for Tilt, based on HFD values. Also this tool turned out to be very helpful in fine-tuning the tilt-correction.
It is probably possible to dial in the adjustments with just one of these three tools, but they sort of all had their own function and the confirmation between tools helped finding in the end the optimal tilt correction.
AutoFocus
Between adjustments, regular autofocus routines were run. When pushing parts of the sensor towards or away from the telescope, the focus changes. In KStars/Ekos, the default is single star (subframe) autofocus. For tilting analysis, the location of the star being center or edge, is important. Therefore, Full Field autofocus area was used, with annulus settings between 15% and 65%. This gave both fast and consistent results, and has now become the default autofocus routine.
Conclusion
With larger sensors and faster scopes, it is worthwhile checking if there is any sensor-tilting noticeable. If so, adding a Camera Tilting Unit into the optical train is an efficient method to correct for it. Re-arranging the spacers/adapters can be a bit of a puzzle. And dialing in the proper amount of tilt is certainly a fiddly process. But with patience and trial and error, serious improvements in the final image can be achieved. It is well worth the effort. The CTU remains now fixed on the ASI6200 and moves along with the camera between OTA’s. With a reducer added to the FSQ-106, there are still some imperfections in the far corners, so perhaps an even further tweaking of the settings might help. But for now, the results show a worthwhile improvement.