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 be possible.

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 focus.

Sky 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.




Focusing Methods

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 Crossley 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 setting.
( 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 differently. 

Focusing Masks

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 diffraction spikes in a cross shape.  These spikes are produced by a four 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 engine' 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 foc_mask.gif .

Commercially made masks use a different design to produce a much brighter diffraction pattern by putting more light into the desired diffraction pattern and less light into what would normally be desired, a pin point image.

Shown is a diffraction mask made by Farpoint for use on a 10 inch Meade Telescope.
Data Sheet

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,
the diffracting zone mask.   Refer to daft.htm for the background article.

If you read caus-w.htm you 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 factor.  But if you must account for every detail involved then this is something of interest.

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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