8 May 2022 (© N. de Hilster & Starry-Night.nl)
When solar imaging with an H-alpha telescope Newton-rings can occur due to interference between flat surfaces of the etalon and camera-window. In this white paper I describe a method to easily overcome this issue using an ADC.
Recently I realized that with the QHYCCD QHY163 (which has the same image chip as the ZWO ASI1600 MM Pro Cool) and 2x Barlow I can probably make full-disc images of the Sun with the 80mm Lunt. So the TeleVue 2x PowerMate was mounted to the diagonal, the camera inserted, and aimed at the Sun. It did indeed fit well, but unfortunately it also produced Newton rings. Now, I knew that they can be solved with a tilt adapter, but since I don't have one at home, I first looked on the Internet to see if this really could be solved with this camera.
While searching the internet, I found a page on the SolarChat forum discussing an alternative: the dispersion wedge. Now, I don't have that at home either, but I do have the alternative mentioned below: the ADC and there are no fewer than two dispersion wedges in it. And no, we don't have to demolish it, just include the ADC in the optical train.
So I removed the camera, and placed the ADC in between the PowerMate and camera (see figure 2). Figure 1 shows the image on the far left without ADC and in the middle with ADC, but with the levers still at 0° (the position where the ADC does nothing). As can be seen, the ADC in that position gives little to no improvement. On the far right of the first image we see what happens as soon as the levers are set at a mutual angle of 90° (the wedges are now perpendicular to each other, see figure 2): the Newton rings have completely disappeared!
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).
An additional advantage is that the Sun is displayed slightly larger (approximately 2.5%). It follows from the theory that with this camera (pixel size 3.8 micrometres) we may use a telescope with a focal ratio of approximately 3.8 x 3 = 11.4. The Lunt is f/7, with the 2x PowerMate this becomes about f/14, so that's very close. The ADC makes this about f/14.3. Without ADC, the pixel scale is approximately 0.692″/px, with ADC it becomes approximately 0.675″/px.
The disadvantage of this optical train is its length, as can be seen in figure 2. Another disadvantage is that the exposure time has to be slightly increased. Without ADC I came to approximately 0.73ms today, with ADC this became approximately 0.97ms, so a loss of approximately 33%. This increase is hardly noticeable in the data quality as can be seen in below image:
If you have any questions and/or remarks please let me know.
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Balancing system Camera centring errors Collimator construction Dome Azimuth Calculation Filter focus-offsets RC Collimation Removing lens artefacts Removing Newton-rings Solar Seeing Monitor (DIY) Stability measurements