Hartmann Mask and Off-Axis Mask
UPDATE 2008: I now know that a Bahtinov Mask
is a much, much better tool than the traditional two-hole Hartmann Mask. So click here to read all about the exciting new Bahtinov Mask.
Good focus is important for viewing objects in your telescope, but when
you photograph them good focus is CRITICAL. So click
here to read all about the exciting new Bahtinov Mask.
Many otherwise excellent astronomical photos captured with great effort
are actually out of focus. If they were properly in focus they would turn from being
good photos to sensational ones. So click here to
read all about the exciting new Bahtinov Mask.
Judging focus by eye is not good enough for astrophotography because your
eye can deal with a 3% error in focus. Your eye and your brain work together to
compensate, and your mind sees things in focus when in reality (as captured by the camera)
they are not. So if your eyes can't be relied on when you want precise focus then
how can you tell? Answer: Use a Bahtinov Mask.
Oh, alright. If you want to learn all about a Hartmann Mask then
read all about it below:
Mr McGoo the home-made Hartmann Mask focuser can also
be used as an Off-Axis Mask
The Hartman Mask
The Hartman Mask is a simple but clever device for checking for
approximate focus. Parallel light beams from a distant star travel down the optical
tube having passed through the holes in the mask. The beams are converged by the
telescope optics (lens or mirror). The point at which they cross is the focal plane,
which (for prime-focus photography) is the exact spot where you want your camera's film
(or CCD chip) to be. So by adjusting the focus in and out you can tell when you are
in focus - when the two beams converge. The diagram below shows the simple case of a
refractor (glass lens) telescope with a Harman mask with two holes in it. The
principle is the same for reflectors where mirrors are used instead of a lens. When
the two beams from the distant star overlap you're in focus.
While the Hartman mask is very, very useful it has a couple of problems:
a) The light beams from the two holes will actually wobble around due to
atmospheric turbulence (seeing) such that judging the point where the two dots overlap can
be a big problem. On nights of good seeing however, the Hartmann mask can give
you quite precise focus when focusing a CCD camera live on-screen (computer or TV).
This is because when the focus point is reached you will notice that suddenly several dim
stars previously not seen or noticed will appear out of the dark background as if by
magic. The dim stars become noticeably brighter when light from the two beams
combine to their maximum intensity (smallest area) and light up the dim stars' pixels.
Generally on most nights I use a very bright star to get approximate focus and then
spend a lot longer looking for the focus point where unseen dim stars suddenly appear.
b) Usually focus with the Hartman mask is not exact. It is often a
subjective decision that you make, deciding when the dots are smallest and
brightest. It is possible when you are near focus to move the focus wheels quite a
bit and not see any noticeable change in the position or size of the dots. So where
is the focus point? Somewhere in the middle you guess. You'll have to
practice. The problem is more of an issue with fast telescopes (f/4 or f/5).
To help with this problem you can also use three or four holes in the mask instead of just
two. You can also make one of them a different shape, but judging the exact
focus point will still remain a problem. Some folks use triangles instead of
circular holes to combine the Hartmann mask effect with diffraction focusing. When
near focus the diffraction pattern from the edges of the triangles will be clearest.
c) Because it's a mask, it reduces the light from the star being
focused. You don't get the benefit of your telescope's full aperture. You
therefore must use bright stars when focusing with a Hartman mask.
For a better alternative to the Hartmann Mask try the Bahtinov Mask which most folks think is much better.
If you want a chuckle then check out my home-made Chicken-Wire (Wire Mesh) Diffraction Focuser
Off-Axis Mask
By plugging one of the holes (with a well-fitting plastic coffee tin lid)
my mask becomes an off-axis mask. The telescope then has the same focal length but
will have a much smaller aperture. It collects a lot less light. Less aperture
is useful in some situations. The view is a little less susceptible to bad
"seeing" conditions and so is useful for high magnification planetary or lunar
observing.
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