Continued: (Field curvature with Flatteners and compressors)

Roland Christen

Field correction of the 140EDL refractor using the QuadTCC telecompressor:

In order to cover a wide field and compress the image to get faster focal ratios, we have the QTCC. This compressor was originally designed for our 130 GTX refractor for use with the 3.5" focuser. However, it has been requested for other scopes such as the TEC 140 to 180 triplets, and our own triplets of various sizes. The amount of back focus required depends on the scope's field curvature and some other factors. The faster the lens, the more back focus is required to get a flat field. So the question is what is the best back focus for my camera and my scope. The correct answer is - it depends....

For an example, below I have an analysis of the AP140 F7.5 refractor (which we made in limited quantities a long time ago). As the images below show, the optimum back focus changes as the size of the field changes. The reason for that is that the shape of the field curvature doubles back on itself, at first ever so slightly undercorrected and then as the limit is reached it goes quickly into overcorrection. Even a small change in the field size will produce significant deterioration of the corner stars. So the spacing becomes more and more critical as the chip size gets larger. Any tilt in the camera will cause one corner's stars to be perfect and the opposite corner to be quite out of focus with wonky star shapes.

Frame 1 shows the optimum spacing for a 42mm field. Frame 2 shows that with the same spacing the stars at the edge of a 51mm field go quickly out of focus. Frame 3 shows that decreasing the spacing by 1.25mm results in much better edge sharpness. For reference, the Airy Disc diameter at 140mmF5.39 is 7.2 microns.

Roland Christen

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