Anecdotal Mechanical and Optical Review of
A full description of the appearance and functionality
of the ETX can be obtained at the Astronomics.com website. Here,
the ETX instruction manual can be downloaded. Familiarity with the
maksutov-cassegrain type scope in general and the etx/questar type
configurations in particular, will facilitate understanding the
Saturn: wow! The cassini division is visible each
time, every time (once thermal equilibrium has been reached) as
a thin dark line nearly through the whole ring system at this time.
The shadow of the disk of the planet on the obscured ring is clearly
visible. Polar darkening, crepe ring against the planetary disk
and the equatorial band are visible. The moons are visible but not
spectacularly so. The ETX gives a beautiful crisp view of the ring
system and cassini at 40X. The best views in terms of sharpness
are around 100X. A nice crisp view continues to be delivered up
to 210X (c. 70X per inch of aperture) with a concomitant three-dimensional
Jupiter presents a very nice sharp disk up to 210X.
It is in the 100X to 135X range that the ETX performs best on the
Jovian detail, revealing two distinct belts, with indications of
belt splits, bars and fragments of belts elsewhere on the planet
and some indications of festoons. The GRS and eclipse shadows should
be easily visible. The features, though visible, seem to be slightly
washed out. The use of a yellow filter helps. The moons appear as
four beautiful bulls-eyes.
Venus presents a nice gibbous disk at this time with
no color (chromatic aberration) present.
Mars even though it is six arc seconds in size, presents
a definitive disk and a hint of a dark surface feature.
The double star Castor is beautifully presented at
100X as two headlights encircled by a noticeable diffraction ring.
The doubled nature of eta Orionis is clearly seen.
With a 32mm plossl providing a 1.3 degree FOV, the Great Nebula
in Orion is attractively framed. At 100X the trapezium presents
four perfect tiny airy disks with first diffraction rings. At 180X,
in the clearest and most stable of skies, there is a hint of “e”.
With the 1.3 degree FOV of the 32mm plossl, the entirety
of the Pleiades can be taken in. Stars are pinpoint across the field,
as is to be expected at this long focal length.
At fifty feet, both through a window and without a
window, views of juvenal blackbirds are excellent, revealing the
detail of the coat as well as its iridescence. It is easy to see
a sparrow work a seed’s covering apart with its thin red tongue
to obtain the seed and leave the chaff. Though very pleasing, this
scope is of limited “birding” potential because of its
small FOV. Some nearer objects would not be properly framed.
When using attachments such as a diagonal and visual
back in the rear port or an extension tube (2 inches) in the upright
port, objects as near as three meters away can be focused on. Even
at this near distance focus is sharp. Observations of a bubble in
a window pane at ten feet illuminated by sunlight revealed numerous
details that could not be seen at all upon close inspection with
the unaided eye in full light conditions.
Sometimes point-light sources (sunlight reflecting
off of crevices in ceramic or glass, stars) appear as tiny perfect
airy disks surrounded by a bright first diffraction ring that is
of uniform width but of slightly differing brightness at one end.
I would call this 95% collimated. This level of collimation probably
provides a degree of detail and contrast that in my eyes would be
indistinguishable from 100% collimation. I obtain this degree of
collimation inside the house at sun reflected point light sources,
sometimes under the sky when the scope has reached full thermal
equilibrium and at other times by pressuring the internal diagonal
mirror in one specific direction (not advisable) or by loosening
and tightening the meniscus ring. At other times there is a perfectly
round airy disk with a discontinuous first diffraction ring. The
ring is fully formed on one side going to a less well formed, fainter
and at times disconnected appearance at the other side. Sometimes,
more diffraction rings are noticed on one side of the disk than
on the other. Slightly out of focus images of point-light sources
reveal the central circle/point as slightly off center. I would
consider this to be 90% collimated. Under most conditions this provides
perfectly acceptable views such as those described above under Saturn
and Jupiter, the descriptions of which were secured while observing
the same under these conditions. Nevertheless, improvement of the
collimation from what I am calling 90% to 95% collimated provides
a noticeable difference in detail and contrast.
Intra and extra focal images of point light sources
reveal a high degree of similarity. Though high optical quality
can also be obtained in some optical configurations while also producing
dissimilar intra/extra focal images of point light sources (of which
the maksutov might be one), the presence of this similarity in a
90mm F13.8 maksutov cassegrain is a good sign. The famed double
dark ring that maksutov owners are familiar with in star testing
is of nearly the same dark hue on both sides of focus; the extra
focal being only slightly more washed out. Both defocused images
nicely collapse down to the Berrevoet’s circle, and then to
the Barbour Dot and finally to the airy disk with diffraction rings.
There is no indication of astigmatism in the extra and intra focal
images. There is a slight indication of astigmatism in focused point
light sources in slightly unstable air or in thermal disequilibrium.
However, this is well below diffraction limitation if it is present
A similar sized refractor may take 15 minutes to
a half hour to equalize a thermal difference of fifty degrees
Fahrenheit. Maksutov cassegrains of this size may take between
thirty minutes and an hour for the same. Good views can be obtained
between 45-60 minutes, with everything that can possibly be seen
in a 90mm maksutov being within the visual threshold as one comes
nearer to thermal equilibrium. I have adopted the technique of
uncapping the meniscus lens, pointing it down, opening the rear
port, inserting a visual back and a mirror diagonal and pointing
it facing down to faciliate cooling and to minimize dust entering
I have not used the provided 8X21 finder. However,
I suspect that it is nearly worthless for optical and mechanical
reasons. The 8X25 right angle finder is adequate. It provides a
more comfortable position for viewing. It also provides a nice apparent
field of view. However, the limited FOV and the lack of light gathering
surface somewhat limits its ability as a finder. If one is comfortable
with using charts and star-hop-ing from bright objects, this should
be adequate. Another challenge with this finder is that the use
of the three double screws can prove challenging. This challenge
is augmented by the fact that the #774 original carrying case was
not designed with this finder in mind. The wall of the case jams
against the finder. This in turns tends to change the direction
pointed. One final negative about the finder is that under cold
conditions, the glue holding the objective cell to the tube and
the lens to the diagonal holder tends to deteriorate, resulting
in these being pulled off. One other deleterious feature is that
two dust caps were not provided with the finder.
There are knobs that allow one to manually adjust
both in right ascension and declination. There is a rod-attached-knob
that can be used to tighten the declination setting and a knob provided
to tighten the right ascension setting. In order to use the electric
clock drive this knob must be tightened. The drive tends to drift
a bit until it catches. Usually this drift is less than the FOV
of a 32mm plossl and just about the FOV of a 12.5mm ortho. If one
attempts to center the object by loosening the right ascension lock
knob and using the other knob, the drift will be experienced again.
I have learned to slightly adjust one of the table-top tripod legs
and then re-adjust the declination. I have had little difficulty.
It is not difficult to open the back and insert the
batteries. A suggestion given elsewhere is very useful; use the
central leg fully screwed in and secured to pull the bottom plate
off once the three screws are removed.
Aesthetics and Portability:
The metal tube of the OTA has a beautiful royal blue/purple
sheen. Though invisible in lamplight, it is particularly attractive
in daylight on the picnic table, on the office desk or a window
table in the living room.
For those who appreciate small complete packages,
this scope is very ideal. The scope, its mount, its drive and needed
accessories can be carried in one hand. Within its overhead compartment
allowable hard case, everything one needs can be packed. This includes
the scope (ota, mount, drive), eyepieces, diagonals, legs, filters,
tools, etc. Everything one needs for a night of observing can be
stored in this one convenient case.
All the controls and easy to use and convenient. The
addition of an EZ-Focus cord and a Visual Back increase the convenience.
The plastic components, though they seem substantial enough, do
cause one to treat things quite gently for fear of stripping some
screw or cracking a housing. On the other hand, there are a number
of ETX users who can claim up to eight years of use since the
first ETX arrived on the market with no durability problems.