Quote: I have seen in the past other references that also mentioned that this was the correct way to measure back focus: from the first possible point where the focal plane can be accessed.from the flat at the rear of the SCT port. In other words, if you put a flat card across the rear opening, this would be 0mm.
Quote:Good evening edz!
No, I meant 5%, as 198^2/203^2 makes 0.95
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Quote:In the on-axis condition, which is what Ed's tests have been restricted to, the location of the entrance pupil is irrelevant in any event.
Quote:So where is the exit pupil? There is none for a single point source.
Quote: A major difference however was that In the flashlight aperture test that aperture showed very strong vignette at the edges, a quite obvious darkening of the field edge in the projected circle of light, all the way down to an aperture of 107mm. However, in my daylight measures, that outer darkening could not be seen at all.
Quote: Quote: A major difference however was that In the flashlight aperture test that aperture showed very strong vignette at the edges, a quite obvious darkening of the field edge in the projected circle of light, all the way down to an aperture of 107mm. However, in my daylight measures, that outer darkening could not be seen at all.
This gets to the crux of what I have been talking about. This flashlight test is based on measuring the diameter of something based on its shadow, and if the entrance pupil is far away and the light source isn't a point, and has a tapered beam, then the shadow will be blurred and have an unknown size.
What you describe as vignetting isn't vignetting at all - it's just an artifact of the way you are measuring aperture, as depicted in my diagram above. There is no "vignetting" of the entrance pupil. It is what it is - and the goal here is to measure it accurately.
Similarly, on the viewing side, if the aperture is greatly reduced by a stop somewhere, you would never notice it as vignetting per se - but as dimming of the central part of the field due to the reduced size of the entrance pupil. The vignetting might even be reduced by the presence of the stop, in terms of percentage illumination loss at the edge.
So this gets to my point about the flashlight test. It should be pretty accurate if the stop is near the front of the telescope, but if the stop is near the focus, then the shadow becomes very ill-defined and not a good way to measure the size of the entrance pupil. That is shown in the apparent blurring and "vignetting" of the shadow you are trying to measure.
Quote:This time I focused on the moon.edz
Quote:the problem with attaching a 2" SCT diagonal to the C6 is that the focus knob interferes with the ability to rotate the diagonal to preferred viewing positions. The base of the diagonal hits the knob and allows rotation only thru a small arc.
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