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Below is a comparison of the Point Spread Function
of each design and each of the three focal ratios: f8, f10 and
f12. Again, this is at the edge of a 2.0" diameter field.
RMS spot size (in microns) at the 2.0" diameter edge is
given for each PSF just below the image. |
Please note that the below PSF's are not an "actual
size" representation of the star's size at the edge of the
2.0" field. It is merely a tool in analyzing the star's
shape and overall size versus other PSF sizes for other designs
or focal ratio systems. |
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f8 Classical Cassegrain |
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f10 Classical Cassegrain |
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f12 Classical Cassegrain |
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30.7 |
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25.2 |
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22.8 |
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f8 Ritchey-Chretien
Cassegrain |
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f10 Ritchey-Chretien
Cassegrain |
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f12 Ritchey-Chretien
Cassegrain |
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26.9 |
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23.6 |
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22.3 |
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In reality the above differences in spot sizes from
say the f8 compared to the f12 are mostly buried in seeing. The
exception would be for those who own land on a Peruvian mountaintop...
The difference between two systems that sit next to each other
above are definitely buried in the seeing. |
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You can see from above that as you go faster with
the focal ratio of the system the RMS spot sizes suffer a greater
spread of the star image as you jump from each progressively
faster focal ratio. The difference in RMS spot size from the
f10 to f12 is very slight, while the jump from f10 to f8 is relatively
large in sizes. If planetary and/or very narrow fields are desired,
then the f12 with it's 36.7% obstruction (5.50" secondary
with 7.3" baffle diameter) will yield slightly higher contrast
images (at least visually) than the f10 variation that uses a
41.0% obstruction (6.3" secondary with 8.2" baffle
diameter). The f8 variation has a 46.9% obstruction (7.5"
secondary with 9.375" baffle diameter) and would not be
an ideal instrument for planetary work, at least visually. |
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Even a fast parabolic mirror, say f3.0 like the above
Classical Cassegrains use, will work for planetary use. As long
as the object is placed on-axis and the optics are very well
collimated. Most optics will yield the best optical quality directly
down the center of the optical path (on-axis), no matter what
their design. Cassegrains can be designed to offer wider fields
at the cost of increasing the size of the on-axis star sizes
but this is typically not how commercial optics are produced.
Professional sky survey type work that is using 10" CDD
arrays needs this type of design change to the optical perscription
but otherwise almost all instruments will have their best optical
quality on-axis. |
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You can also see from above that as the focal ratio
slows from f10 to f12, the general shape of the star for the
Classical Cassegrain begins to become more round in nature. At
f12 the difference between CC and RC is very, very slight. |
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All of the above off-axis PSF star images are shown
with the optical system focused on a star at the middle of the
optical axis (on-axis). If the optics are focused in (toward
the primary) slightly, about 0.3mm in the case of the f10, the
PSF at the edge of the field is reduced. The PSF on-axis increases
but as long as those are kept at or below the airy diameter,
they will not experience any measureable difference in their
size. Again, this is a function of seeing limiting what can and
cannot be detected, even with a CCD camera. Continue one page
forward to see graphical illustrations of this defocus method. |
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With any of the above designs, whether it be CC or
RC, f8, 10 or 12, a field flattener
can greatly reduce the PSF of the stars in the outer 2/3rds of
the field. Even without such flatteners though the above spot
sizes are very small. A 1" square CCD chip will give stunning
images with even the f8 version of the RC. |
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