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Old 15-02-2021, 08:18 PM
Mickoid (Michael)
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Join Date: Feb 2016
Location: Melbourne, Australia
Posts: 1,439
Taking the inside out

On the advice of Iceinspace member "Bojan" I decided to put his recommendation into practice. Having struggled with removing CA inherent on most cheaper lenses, I tried stopping down a few f-stops from wide open. While it did significantly remove CA, the blades of the internal diaphragm produce unsightly ( to some ) spikes of light forming on bright stars. The amount of spikes depended on the amount of blades contained in the diaphragm - the more blades, the more spikes. Bojan's advice was to make an external aperture. The example below was shot last night using an old Pentax Takumar f4 300mm lens using an aperture I created from the plastic lid of a tennis ball container. Ignoring the below average example of NGC 2244 (less than 60min exposure from light polluted front yard), the test was to prove the advantage in adding an external aperture to the optical train before light reaches the lens. Not only is there virtually no CA, there are no star spikes because the homemade aperture has a round hole without blades. Thanks Bojan, this will be the way I shoot widefield in the future!
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Last edited by Mickoid; 16-02-2021 at 06:57 PM.
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