The Story Behind
Recently my family of telescopes was enlarged by an ETX-90. It came with an Identi-view solar-filter and two eyepieces, the usual 26mm SP and an additional 12.4mm SP. For solar observation the 26mm eyepiece appears to be the ideal choice... really?First tests on terrestrial targets already revealed a rather disturbing reflex of the primary mirror just in the middle of the field of view when using the 26mm Super-Plössl eyepiece. When observing homogeneous bright objects this reflex even becomes stronger. On smaller objects, "nice" psychodelic interference effects occur at the same position. Just to ensure I did a collimation test (on stars) beyond the region of trouble, infra- and extrafocal diffraction pattern showed collimation.
The 12.4mm SP did not cause a behaviour like describe above (first hint to me the eyepiece being the source of troubles).
Super-Plössl (SP) & "Modified Achromat" (MA, Kellner)
Some theory for starters:- The Kellner eyepiece design (origin of the "Modified Achromat",
see Meade's catalogue) makes use of a plane anterior (first) surface
for the first lens. The Kellner eyepiece design is carrying a bad name
for being a source of ghost-images.
- A (Super-) Plössl designed eyepiece incorporates two
achromatic lens arrangements. The anterior (first) surface of it's
first lens is concave.
26mm SP vs 25mm MA
Obviously the first surface of a Plössl-type eyepiece, if all
parameters (surface-radii) are "matched", will create an in-focus
reflection on the two mirrors of a Maksutov-type telescope. In case of
the ETX-90, the 26mm Super-Plössl appears to meet all requirements
for the above mentioned effect.It appears that a reflection off the first surface of a Kellner design (which certainly occurs too) does not lead to an image in the focal plane (of the eyepiece).
Have a look at the images of our closest available star, taken on eyepiece projection...
 |
 |
| ETX-90 with
26mm Super-Plössl |
ETX-90 with
25mm MA (Kellner) |
Both pics are sections of the original size. A few days between the different photographs. In both cases I tried to center the sun as good as possible. Have a click on the pics to see some more details....
The images of the sun do not appear in
their best possible shape as the
digital camera used (Olympus C-100, my favourite for snap-shots of all kinds)
insists calculating a focus based on some pattern
computations, causing this lack of focus and thereby hiding several
smaller sun spots visible in the scope.
