henry link
Well-known member
Hi dipped,
I got your PM. Sorry for the delay in responding to your post. Thanks for doing the test.
I have a few questions:
What kind of artificial star did you use and how far was it from the scope? I ask because an 85mm scope needs a very small artificial star to form a genuine point source at a short distance. Also spherical aberration is usually worse at close distances.
When you say "inside focus" do you mean moving the focus in the direction of infinity?
How far out of focus were the patterns you evaluated? In other words how many rings could you see? I ask that one because the non-circular appearance of things like astigmatism, coma and pinching is most evident at only a few rings out of focus and tends to even out and become more circular at higher ring counts.
I'm not sure I know what you mean by "vignetting" at close focus. If there is a loss of clear aperture you can check it be shining a collimated flashlight beam through the eyepiece end from about a foot or more behind the eyepiece. Then measure the diameter of the bright circle projected on a flat surface just in front of the objective lens. I wouldn't expect any loss in the Gavia since it uses a focusing lens rather than a moving prism.
Henry
I got your PM. Sorry for the delay in responding to your post. Thanks for doing the test.
I have a few questions:
What kind of artificial star did you use and how far was it from the scope? I ask because an 85mm scope needs a very small artificial star to form a genuine point source at a short distance. Also spherical aberration is usually worse at close distances.
When you say "inside focus" do you mean moving the focus in the direction of infinity?
How far out of focus were the patterns you evaluated? In other words how many rings could you see? I ask that one because the non-circular appearance of things like astigmatism, coma and pinching is most evident at only a few rings out of focus and tends to even out and become more circular at higher ring counts.
I'm not sure I know what you mean by "vignetting" at close focus. If there is a loss of clear aperture you can check it be shining a collimated flashlight beam through the eyepiece end from about a foot or more behind the eyepiece. Then measure the diameter of the bright circle projected on a flat surface just in front of the objective lens. I wouldn't expect any loss in the Gavia since it uses a focusing lens rather than a moving prism.
Henry