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Canon 200/2.8L can you get decent star shapes at f2.8


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The Canon 200/2.8L II USM has a reputation as a superb widefield lens... It is normally thought that it needs to be stopped down to around f4 using a front aperture mask to achieve decent star shapes across the full FOV with APS-C sized sensor... Or does it...

I have been playing about with a technique where instead of focusing on a star at the centre of the FOV you use on on a thirds point instead and then frame the shot....

I am using the FWHM based Auto focus routines in APT to perform the focusing...

Heres a quick pic form last night 20x120s + 8x180s unguided on my EQ3-pro using my BCF modified 1000D and Canon 200/2.8L at f2.8 ... The image is basically straight from DSS just resized and a true black border added in CS3... No tweaking of the star shapes or star reduction techniques have been applied...

Sadr%2020x120s%20%2B%208x180s%20200-2.8L

I have several other pics taken over a period of time using the same technique and they all show reasonable star shapes across the full frame...

It might be worth trying this technique on your kit.. you can also apply it to scope/field flattener combos... in particular SCT's with reducers fitted...

When I go back to this target I will alter the framing to properly include the Crescent neb - I have started photographically star hopping rather than using the goto now just to add to the excitment ...

Here's another dense starfield image which was the first test imaegs from a few weeks ago... 120s unguided subs

Oops%2060x120s%20Canon%20200%20at%20f2_8




Peter...

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