Intruments: Meade SC 20cm f/10

It's a good optic, very versatile and suits about for all the fields of the observation and the astronomic photography. In the image above it's mounted inside two strong aluminum rings, and with a Meade 9x60 finder with illuminated eyepiece and 90° prism, a true luxury to center the objects.
The principal problem of the optics is the mirror shift, or the bending of the principal mirror that vary in different positions in the sky, it causes moved stars in long focus and long exposure imaging. A solution is to block the mirror, as described in the wonderful book of Robert Reeves "Wide-Field Astrophotography", page 429. The modifications need the insertion of three plastic thin plates around the mirror at 120°, that are folded up by three screws so that the plates block the mirror. After the modifications the tests on the field have shown that  the problem is reduced a lot, but non eliminated. I can take CCD images of 30 minutes exposure guided with a refractor in parallel. But be aware that the blocking force is very critical: too much will ruin the optical performance aiming to large stars, while too small will not correctly fix the mirror.
That's why some time after the modification, I've made another one using a different principle: blocking the central tube with six M5 grains (4 on the part in front of the main mirror, and 2 behind). The result is much better considering that the mirror is not deformed, but I've found that in my OTA the primary mirror shift is not the only on the optical system. So for high quality deepsky works I cannot exceed 5 min exposures, more will show trailed stars due to differential flexure between main scope and guiding optics. See the images below for details.

The first modification with lateral plastic foils (2001)

Left: one of the plastic foils attached inside the tube: in the rest position it does not touch the mirror.

Right: external view: at left the screws that block the foil, at right the screw that push the foil toward the mirror.

The second modification with grains on the central tube (2003)

The main mirror (203 mm diameter) with the four M5 threaded holes on the central tube.

The back of the main mirror, with other two threaded M5 holes. The white glue was used from the manufactured to prevent the blocking ring to unscrew.

An approximate drawing of the modifications (left) and the internal of the tube after the modifications (right).

Detail of the M5 grains (left) that can be screwed with a screwdriver from outside the tube thanks to six holes (right).

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