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Old 18-05-2018, 06:59 PM
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billdan (Bill)
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Join Date: Mar 2012
Location: Narangba, SE QLD
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Hi Martin,

If you were told to only use an APSc camera, maybe your plumbing (i.e. focuser barrel diameter) is too narrow.

The converging light beam as it approaches the chip is dependent on your focal ratio (f6). A full frame (36x24mm) sensor has a diagonal measurement of 44mm. So if you want to fully illuminate that sensor then the focuser barrel must be wide enough to achieve this.

Going from the sensor towards the objective, the light cone increases by distance divided by focal ratio. So at 30mm from the sensor the light cone has increased by 5mm or is now 49mm wide, at 60mm its 54mm wide and at 90mm its 59mm wide. As you can see a 2inch (50mm) focuser will severely cut off that light cone. With an APSc sensor of 30mm diagonal at a distance of 90mm away the light cone is 45mm wide, well within a 2inch focuser barrel.

To be safe you should have a 3 inch (75mm) focuser for a full frame camera, it all depends on how much vignetting you are willing to live with.

Cheers
Bill
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