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I've been wrestling with this star elongation problem for some time and I think I may now understand it. So here is a Star Elongation Summary 101, feel free to tell me that I am wrong:
1. Let us assume you are using a refracting telescope
2. The faster the F number of the telescope (the smaller the F number), then the more curved will be the focal position across the field of view of the camera sensor (the camera sensor is, in principle, perfectly flat)
3. If we don't do any corrections, then if the stars are in focus at the centre of the camera's field of view, the corresponding point of focus for stars at the edge of the field of view will be in front of the camera's sensor, and hence the stars at the edge of the camera's field of view will be out of focus
4. To correct this curvature problem, we put a field flattener in front of the camera sensor. A field flattener is essentially a "reverse lens" - it is designed to reverse the forward curvature of the focal position across the field of view to ensure that everything is in focus from centre to edge when the photons hit the sensor.
5. For a field flattener to work correctly, it has to have two properties:
a: its degree of reverse curvature has to match the degree of forward curvature of your telescope (not necessarily exactly, but at worst within an error smaller than the resolution of your camera sensor). In other words, the faster your telescope is, the more reverse curvature your flattener needsto have.
b. the distance from the back of the field flattener to the camera sensor has to be an exact match, which should be specified by the manufacturer of the flattener. If the manufacturer of your flattener doesn't specify a spacing distance, then dump your flattener and buy a good quality one. If you can't match the flattener-to-sensor distance correctly, then you will either have radially elongated stars (too short distance) or tangentially elongated stars (too long distance at the edges of your camera's field of view)
If you have invested in a good quality fast refracting telescope, and a good quality camera, then you deserve it to yourself to buy a good quality flattener and a spacer adapter to get yourself the right flattener-to-sensor distance.
Joshua Bunn makes such adapters to order and he can be contacted on this website. I thoroughly recommend his workpersonship.
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