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Figure 9: The left image is a simulation of what a focused star looks like through a Ronchi grating. The right
image is a simulation of what an out of focus view looks like through a Ronchi grating. The right image shows
2 full lines. The more lines you see, the farther the Ronchi screen is from the focal plane.
Step 4: Setting Up the Test
Starting with a telescope that is fairly well collimated, center a bright star and replace the low
power eyepiece with the Ronchi Ocular that was provided with the telescope. Make sure the
Ocular is seated all the way into the adapter. If the Ronchi is nulled (you can see no lines, or
one Ronchi line covers the entire aperture; see Figure 9) This means that your spacing is
already set; you may proceed to Step 4. This is not uncommon because the primary-to-
secondary spacing is set at PlaneWave and you most likely will not need to adjust it.
If you see two or more lines, you should move the secondary toward or away from the
primary.
Determining the Direction to Move the Secondary
To determine which way to move the secondary, gently slide the Ronchi Ocular out of the
spacing tube. If the Ronchi reaches null as you do this, the focal plane is too far back and
you need to move the secondary mirror
away
from the primary. If the number of lines grows
as you slide the Ronchi out of the spacing tube, the focal plane is too far forward and you
need to move the secondary
toward
the primary.
Remember that the secondary mirror acts as a magnifier. So a small change in secondary
spacing can translate into a much larger change at the focal plane. For example, for every
1mm the secondary spacing is changed on the CDK17, the focal plane moves 4.4mm. You
want to keep the error at the focal plane to 5mm or better, which equals just under two lines
on the Ronchi. This corresponds to an error in primary-to-secondary spacing of about +/-
1.1mm. See table below for spacing tolerance for all CDK models.
