Focusing and Focusing Masks
For this discussion we must presume that the seeing
conditions are sufficiently good to make a sharp focus possible. If
the seeing conditions are very good then the limiting factor will be the
quality of the primary mirror and the other optics. Also we must have a
well made focuser mechanism otherwise maintaining the optics in position won't
Depth of Focus
The depth of focus depends on the F number. On short
focus instruments the light converges at a steep angle making a very small range
in which the image is in perfect focus.
Schmidt cameras are noted
for being particularly difficult to focus. Being of very low F number and
having a large image field its almost impossible to keep from having the star
images form tiny rings over some portion of the photographic plate.
To the other extreme the
36 inch refractor, being F19 is relatively easy to
and Telescope, March 1995,
has an article showing the wave patterns at focus. If all else is perfect,
which is rarely the case, the wave nature of light is the ultimate determinant
of the depth of focus for a particular telescope.
Well, I look through the telescope and focus in and out
until the image looks sharp.
That's simple and simple is good but what if we have a glass photographic plate
onto which to focus our star.
Years ago I watched an astronomer take a set of exposures through the
Telescope to find focus.
Each exposure was followed by a small adjustment of the focus micrometer and
recording of the number from the plate holder. The plates had to be
developed and examined under a magnifier to find the setting but the focusing
mechanism was stable enough to return to the same position for the best focus
( Best to approach focus from the same side every time in case there is play in
the mechanism. )
Photographic plates are rarely seen now days. Most CCD
software can be set to a very short repetitive mode so the focusing can be done
in 'real time' watching the monitor. Some software can work with an
electronic focuser to set best focus and even determine seeing.
Amazingly sometimes great planetary images can be taken
while holding a digital camera to the eyepiece. If the focus is set so the
camera senses the object is less than infinity ( less than or at infinite
distance ) then the camera auto focus can do its job. These types of
images cannot be 'stacked' because each image is stretched or distorted slightly
Shown below are images showing use of an 'aperture mask'
and the resultant effect when focusing in and out through focus with the mask in
place. ( Don't forget to remove the mask before recording the image with
the telescope. )
Focusing Masks Designed to Generate Diffraction
In the picture
at right notice that each image has
in a cross shape. These spikes are produced by a
vane spider. If one were trying to determine the location of a comet
in relation to a known star on a photographic plate using a 'measuring
these spikes could be useful since the center of the over exposed star image is
hard to determine.
Diffraction can also be used for focusing.
A possible design for such a mask is shown in image
Commercially made masks use a different design to produce a much brighter
diffraction pattern by putting more light into the desired
and less light into what would normally be desired, a pin point image.
is a diffraction mask made by Farpoint for use on a 10 inch Meade Telescope.
The mask would probably work on other telescopes if the center section
were taped over.
Use of a Zone Mask to generate a Diffraction Pattern
Since much of this website is devoted to mirror making its worth
mentioning another use of a diffraction mask,
Refer to daft.htm for the background article.
If you read
realize why there is a problem with
knife edge testing with a zone mask.
The problem results from the fact that the focal point of the individual zones
of the zone mask is not exactly at the crossing point of the light coming
from the two zones. I don't want to 'throw you a curve' as many fine
telescope mirrors have been made without any attention being paid to this
But if you must account for every detail involved then this is something of
Presumably the advantage of the Daft test is that it does not depend on
the knife edge but on the interference pattern generated by the combining of light coming
from the two holes for the zone
of interest. ( The other zone holes
being masked. )
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