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Samir Kharusi | all galleries >> C14 Hyperstar - Initial Tests > Vignetting
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Vignetting
Samir Kharusi

Vignetting

Azaiba, Muscat Oman

The above are line profiles along the diagonals of flats, absolute corner to absolute corner, to check out the vignetting. The Baader 2" OIII filter requires a UV/IR Blocker to cut out IR. Consequently I have to use it in conjunction with a Hutech pop-in UV/IR Blocker Front Filter inside the DSLR, i.e. two filters simultaneously, a pop-in FF inside the camera and a 2" inside the Hyperstar. The 2" Baader 7nm H-alpha filter does not leak IR. Hence I do not have to use a pop-in Front Filter with it. Conclusion from the above profiles? Hutech pop-in Front Filters increase the severity of the vignetting somewhat. One should also be aware that some "vignetting" is inevitable with most digital sensors and a high cone angle lens or OTA. Recall all that talk about acute angles of incidence at the corner pixels giving difficulties to Leica, etc when one tries to use extreme wide angle lenses? Well, the current DSLRs now have micro lenses on each pixel that overcome much of the effect, but probably not all. And an f/1.9 light beam does have a wide angular make-up. Nevertheless flats should be able to take care of it. However, to make sure that there is ample Signal to Noise Ratio in the corners one has to increase the lengths of subexposures a bit (to make sure that we stay in the desirable skyfog-statistics-limited regime over the whole frame, not just the middle), and of course also the integration time.

Hutech modded Canon 20D,C14 Hyperstar 675mm f/1.9
Flats, illuminated through milky white acrylic sheet. full exif

other sizes: small medium original
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