MTF expert neededPosted By Paul Hyndman |
Uhhh...
A mask consisting of multiple openings may create lots of angst when used for normal viewing. Multi-holed masks have been around for quite a while (a notable three holed version is the Hartmann mask) and are extremely useful for imagers wanting to determine the precise location of critical focus, but likely to drive a visual observer nuts!
Perhaps it would help to unravel the mystery of how multi-holed masks work (mind you, I make no pretensions about being an optical guru, but have used a few of these):
In a nutshell, 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, an off-axis image will move across the field of view. This perhaps is most apparent when the image is a point source of light (a star).
Focusing a single image can be somewhat ambiguous, as there is often some latitude or "sweet" spot where small changes seem to have little or no effect. When multiple off-axis 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... 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. More holes or larger size is not always better though.
The farther apart the openings, the more dramatic their effect (and hence making it easier to determine critical focus). Openings nearer the center will have little movement even with large changes in focus, while those nearer the edge will show greater movement even with small changes in focus. Generally, imagers want the bulk of the opening area as close to the edges of the aperture as possible.
Bigger openings = brighter targets, but to be bigger means that the bulk of their area is moved closer to the scope's central axis, diminishing the off-axis effects.
Soooo.... for imagers it is a balance of enough total (and individual) area to be bright enough to work with, but not so large as to minimize their off-axis effect. Too small = not bright enough; too big = hard to detect any changes.
And for visual usage... even the smallest of focus deviations (those that would otherwise be imperceptible as a single image) will break out into individual images, creating blur.
Then we have the additional diffractive elements this contributes to the image... but that's a whole 'nuther kettle of fish! Cardboard is cheap (or free) though, so play with it!
)
Paul
http://www.astro-nut.com/
A mask consisting of multiple openings may create lots of angst when used for normal viewing. Multi-holed masks have been around for quite a while (a notable three holed version is the Hartmann mask) and are extremely useful for imagers wanting to determine the precise location of critical focus, but likely to drive a visual observer nuts!
Perhaps it would help to unravel the mystery of how multi-holed masks work (mind you, I make no pretensions about being an optical guru, but have used a few of these):
In a nutshell, 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, an off-axis image will move across the field of view. This perhaps is most apparent when the image is a point source of light (a star).
Focusing a single image can be somewhat ambiguous, as there is often some latitude or "sweet" spot where small changes seem to have little or no effect. When multiple off-axis 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... 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. More holes or larger size is not always better though.
The farther apart the openings, the more dramatic their effect (and hence making it easier to determine critical focus). Openings nearer the center will have little movement even with large changes in focus, while those nearer the edge will show greater movement even with small changes in focus. Generally, imagers want the bulk of the opening area as close to the edges of the aperture as possible.
Bigger openings = brighter targets, but to be bigger means that the bulk of their area is moved closer to the scope's central axis, diminishing the off-axis effects.
Soooo.... for imagers it is a balance of enough total (and individual) area to be bright enough to work with, but not so large as to minimize their off-axis effect. Too small = not bright enough; too big = hard to detect any changes.
And for visual usage... even the smallest of focus deviations (those that would otherwise be imperceptible as a single image) will break out into individual images, creating blur.
Then we have the additional diffractive elements this contributes to the image... but that's a whole 'nuther kettle of fish! Cardboard is cheap (or free) though, so play with it!
)Paul
http://www.astro-nut.com/
