Stars extending out in the corners of your photo often indicate image field curvature in your refractor. The Flat4 from William Optics blurs that field, and with this William Optics Flat4 adapter you connect the flat surface to the extension tube of your focus lens.
Which adapter you need depends on the telescope and the focusers. Check beforehand whether the thread and diameter of your extension tube match and whether the adapter is compatible with your telescope. You can find other adapters at veldvlakkers for refractors.
Pay attention to the distance between the flat surface and the sensor. It must be correct; otherwise the stars at the edge will still be stretched out. Take a test image and especially look at the corners. If you have a 4-inch refractor from William Optics, this will help. a Bahtinov mask for 102 and 103 mm when focusing.
If you are using a 3 inch focus lens, then there is a conversion adapter to William Optics.





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