4 Hole Aperture MaskPosted By Paul Hyndman |
"The 4 holes are superimposed one upon the other like stacked images taken with a camera when brought to focus. Because of this it actually acts like a one hole mask."
Ummmm, Nope... it DOES NOT act as a "one-holed" (AKA aperture) mask!
Multi-holed (AKA Hartmann mask/Scheiner disk) masks are *ONLY* used for focusing and are removed for viewing or imaging with good reason: the images they produce are extremely sensitive to changes in focus normally encountered.
About multi-holed masks:
Each opening acts as an individual off-axis telescope, producing its own unique image. As an eyepiece (or camera, etc.) is moved closer or further from focus, the OA image will move across the field of view. This is most apparent when the OA image source is a point source of light (a star).
Focusing a single image can be ambiguous (more forgiving!), as there is often some latitude or "sweet" spot where small changes seem to have little or no effect. When multiple OA images are used (ie; from a multi-holed focusing mask), the center of each focused "sweet-spot" (even if it is difficult to tell where each individual one might be) intersect at one unique point, (drum roll please...) "Critical Focus"!
So... here's the scenario: However many holes you have, you'll have separate images when out of focus. As you approach focus, those images converge. When you have a single image, Voilá, you have attained Critical Focus.
Thus the very attributes that make them ideal as focusing aids (diverging images), usually make them ill-suited for use as aperture masks. If left in place, any small changes in focus will cause the individual images to diverge (smear!)
But hey, if it works for ya', use it!
)
Ciao,
Paul
--- http://www.astro-nut.com ---
Ummmm, Nope... it DOES NOT act as a "one-holed" (AKA aperture) mask!
Multi-holed (AKA Hartmann mask/Scheiner disk) masks are *ONLY* used for focusing and are removed for viewing or imaging with good reason: the images they produce are extremely sensitive to changes in focus normally encountered.
About multi-holed masks:
Each opening acts as an individual off-axis telescope, producing its own unique image. As an eyepiece (or camera, etc.) is moved closer or further from focus, the OA image will move across the field of view. This is most apparent when the OA image source is a point source of light (a star).
Focusing a single image can be ambiguous (more forgiving!), as there is often some latitude or "sweet" spot where small changes seem to have little or no effect. When multiple OA images are used (ie; from a multi-holed focusing mask), the center of each focused "sweet-spot" (even if it is difficult to tell where each individual one might be) intersect at one unique point, (drum roll please...) "Critical Focus"!
So... here's the scenario: However many holes you have, you'll have separate images when out of focus. As you approach focus, those images converge. When you have a single image, Voilá, you have attained Critical Focus.
Thus the very attributes that make them ideal as focusing aids (diverging images), usually make them ill-suited for use as aperture masks. If left in place, any small changes in focus will cause the individual images to diverge (smear!)
But hey, if it works for ya', use it!
) Ciao,
Paul
--- http://www.astro-nut.com ---
