People often ask about filter size and vignetting - and the main thing referenced is f/ratio. To some extent f/ratio plays an important role - but in certain common situations, the location of the exit pupil will completely change things. This is particularly true with DSLR lenses and SCT's with a reducer.
Here is an app that will show how rays actually arrive on a sensor when you know the basic geometry of the setup - including the distance of the exit pupil from the sensor:
Here is a simple example showing how people might think a 15mm f/2 DSLR lens might work with a 30mm sensor and a 31mm filter that is 17mm from the sensor:
Normally people think of the "aperture" as being one focal length away from the sensor - so for 15mm f/2 it will be a 7.5mm disk 15mm away from the sensor, as shown in blue.
But you know this can't be right because the last element of a DSLR lens - including one as short as 15mm fl - is much farther away from the sensor.
In fact, the rays arriving on the sensor are coming from the exit pupil of the lens - and that pupil has a size and location that could be very different from the assumed 7.5 and 15mm. For DSLR lenses the location of the pupil may be around 90mm - which means at f/2 the size of the exit pupil would be 45mm - and the light arriving from the lens actually looks like this:
This is a completely different situation - and results in vignetting.
For an 8" f/10 sct you might think the pupil is 200mm in diameter and 2000mm from the sensor - like this:
In fact, the exit pupil of a cassegrain is right near the secondary - so in this case it would be about 500mm from the sensor - and its diameter would be 50mm. So the actual situation would be slightly different:
But where it is particularly important is when you add a reducer to speed it up to f/7. You might expect the wider cone to be a bigger problem - but the reducer will bring the pupil much closer to the sensor. I don't know exactly where it is for 8" f/7, but it could be perhaps 200mm from the sensor - and the result would be:
The app includes some links and more descriptions of what is going on - but you should be able to model vignetting more accurately for your system - as long as you know where the exit pupil is. And unless it is a refractor or Newtonian with no reducing elements - the pupil will likely not be at the distance of one focal length - and that needs to be factored in when considering vignetting.
Woops - the app isn't drawing the layout correctly. Wait for version 1.1.
OK it should be fixed now. The diagrams and vignetting were correct but the aspect ratio wasn't drawn 1:1 so the drawing was stretched. It should be 1:1 now.
Edited by freestar8n, 11 August 2018 - 07:13 AM.