6.2) Check the distance between mirrors
Two important definitions needed to understand this section.
•
The
BFL
is the
back focus length
, i.e. the distance between the flat back plate of the scope
and the focal plane, when the telescope is focused at infinite.
•
M1M2
is the distance between mirrors (surface to surface). M1M2 is a short form for
“distance between Mirror 1 – the primary - and Mirror 2 – the secondary”.
Any telescope has his design M1M2 and BFL. When shipped from our plant, M1M2 is correct
within a fraction of a millimeter, and the BFL is correct within 1 mm. When you change M1M2, you
also change the BFL (if the secondary mirror is a bit more
closer
to the primary than it should be,
the BFL will be significantly
longer
). Just as an idea, if you change M1M2 by 1 mm, the BFL will
change by 4-9 mm, depending on optical scheme.
When you ordered your telescope, Officina Stellare has (usually) provided the whole
optical train
,
i.e. all the mechanical parts needed to keep, at the same time:
•
The telescope focused at infinite.
•
The camera sensor (or eyepiece focal plane) at the design BFL.
•
M1M2 at the design value.
The focuser, in theory, is used only to compensate for dimension changes due to temperature,
different filters thickness, eyepiece model when working visually, and some other minor factors.
Optical performances are guaranteed
only for minimal changes of M1M2
. If you have an electric
motor fit on your secondary mirror, the movement permitted by the software is always very small –
a few millimeters at most. This does guarantee that your scope can always perform within
specifications.
If you change M1M2 outside the design range, severe degradation of optical
performance may occur.
Let's see how to check and change, if needed, M1M2.
For this step
you need to focus on a real star
, using a
camera (or an eyepiece) where is
clearly marked
the
position of the focal plane. Reflex cameras (both
traditional or digital) are perfect for this job. A standard
symbol on the camera body always tells you where the
focal plane of the camera is. Look the photo at right: the
symbol looks like the planet Saturn with edge-on rings.
The straight line marks the exact position of the sensor
(or film, i.e. the focal plane).
When the star is focused,
this is also the position of the focal plane of the
telescope
. Bring the star to a sharp focus (your eye
judgment is good enough). Use the focuser, not the secondary mirror movement, to change
focus. If you don't have a standard focuser (you only have the BFL control on the secondary
mirror) see the note at the end of this procedure.
Measure the distance between the backplate of the telescope and the focal plane (symbol on
your camera) using a tape meter. If it is within 4 mm of the declared BFL (
Back focus length
)
of your scope, interrupt this procedure and jump to step 6.3, secondary mirror tip-tilt.
If you need to correct the BFL, there are different procedures for different telescope
configurations: follow the appropriate procedure for your configuration:
CASE A: you have both the BFL electric motor on the secondary mirror AND a regular
focuser:
use the focuser to move the mark on the camera body to the correct distance from
the back plate of the telescope. The star will look a bit out-of-focus. Use the BFL motor to
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