Optimizing the Orion 100mm, F/6 Achro

Optimizing the
Orion 100mm,
F/6 Achro
In Defense of the Small
Refractor - How to Hot
Rod Your Achromat
By Mike Gilmer
The Orion 100-mm achromat with 2-Speed GSO Crayford and WO
high-reflectance diagonal, on the Super Polaris mount with fixed
height, wood tripod, RACI finder in 6-point bracket mounted on third
100-mm ring and with "custom" illuminator installed.
I wrote this article to explore the benefits (and drawbacks) of using a small refractor as one’s primary viewing
instrument. Being limited by circumstance or budget to owning a single, inexpensive, small scope need not mean a
mediocre experience, especially if you
maximize the performance of a small refractor to provide the most viewing enjoyment possible. With some tinkering
and aftermarket parts, an average achromat can be “tuned” or “blue printed” to
provide the utmost level of performance
possible in a system of limited aperture.
First I would like to outline the decisionmaking process that led to the purchase of
my primary observing scope – an Orion
100-mm ST (f/6 short tube) achromat.
I will state up front – I have a bias towards refractors. This is not a mindless
prejudice against scopes with mirrors, but
a matter of personal taste and habit. (So,
if you are a mirror man or a CAT person,
don’t stop reading! You may be able to put
44 Astronomy TECHNOLOGY TODAY
some of this to use – at least with your
piggybacked RFT.) I cut my teeth on
giant binoculars, which are essentially
twin rich-field achromats acting in tandem. It’s not that I have anything against
reflectors or compound designs, but my
background experience never involved
any exposure to reflectors, so they were a
complete unknown to me. So I decided to
stick to the familiar – refractors.
Once that decision was made, I
quickly narrowed down the field of possibilities: achromat or apochromat. The former was within my budgetary limits, the
latter was not. Another easy decision
made. My last decision before purchase
was the allocation of my gear budget –
would I spread it thin and purchase 2 or 3
scopes, or would I put all of my eggs into
one basket and purchase a single scope?
Rather than have a few under-optimized
scope setups that I would be unhappy
with in one respect or another, I thought
I would instead use my limited budget to
attempt to “soup up” a single scope. Now
I was ready to shop for a purchase – a single, good, achromat.
Acquiring the OTA
While browsing through the online
classifieds for refractor deals, I was given a
dose of reality. Refractors, even achromats,
get expensive quickly as aperture increases.
Although I had decided to put my entire
budget into one scope, realistically I had
to leave myself some change from the purchase so I could also acquire the obligatory accessories - mount, diagonal, finder,
and eyepieces. Taking this into account, I
had about $200 to spend on an OTA and
still leave myself enough money for the
rest. This severely limited my options, and
the prospect of a $200 refractor was not
exciting to say the least. Two things kept
my project on track – a good mount fell
into my lap and a hot deal came my way
on an OTA. The mount was a surprise
eBay find. For less than $50, I scored a
GSO 2-Speed Crayford
1980’s Towa EQ-2 mount and wooden
tripod. The mount was originally rated for
an 80-mm, long-focus refractor, and was
built like a tank compared to some of the
more modern EQ-2’s I have seen. I figured it was stable enough to support a
100-mm scope and it was in mint condition. Within days, I was offered a nice
100-mm Orion ST f/6 achromat with
rings, carry case, finder and eyepiece for
$200 shipped to my doorstep.
Well, now I was in the game. The
600-mm focal length OTA balanced
nicely on the slightly-undersized Towa
EQ. I had a basic mirror diagonal
lying around (leftover from an old
department store scope), and coupled with the finder and eyepiece
that came with the package deal, I
now had a serviceable short tube refractor on a decent mount. The key
words here are “serviceable” and
“decent.” I could have stopped
here, and used the rest of the
money in my war chest for a used
6-inch Dob or fast 6-inch Newt
that would also fit on my mount.
To me, this little 4-inch tube was a
blank canvas, or that rusted old
heap of a car that a restorer can
breathe new life into.
Having done my homework on
fast achromats, I knew false color
would be an issue on bright objects
– so I purchased a Baader
FringeKiller filter to help tame the
CA. Speaking of false color and aberrations in general, I liked the f/6 Orion
100-mm better than some other options
on the market, which included similar
102-mm f/5 “short tube” achromats.
Everything I had learned while doing my
homework, indicated that f/4 and f/5 are
truly "fast" scopes and require well corrected eyepieces to extract optimum
views. F/6 resides in that borderline area
between the very fast f/4 scopes and relatively slow f/8 plus planetary scopes. I
liked f/6 because it offers a little more
magnification than faster scopes and a
wider potential true field of view than
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Astronomy TECHNOLOGY TODAY
45
OPTIMIZING THE ORION 100MM F/6 ACHRO
slower scopes – all while being relatively
forgiving on eyepieces. Where a fast f/4
Newt will make mincemeat of a bargainclass wide-field eyepiece, a slower f/6 can
yield acceptable results without breaking
the bankbook on premium eyepieces that
each cost more than the scope itself.
Around the time I bought the violetreduction filter, I also bought an Orion
enhanced 97% diagonal (1.25-inch) to replace the dubious department store diagonal I'd been using. Within days, another
batch of accessories arrived, including a
GSO 8x50 RACI finderscope and mounting bracket. A night of low power viewing was rewarding, because it revealed to
me my next area of improvement - the focuser. The stock rack and pinion focuser
was adequate, but I did not want to settle
for “adequate”. So I ordered a GSO 2inch single speed Crayford focuser and a
6-point Stellarvue 50-mm finderscope
bracket to replace the Synta-style bracket
that came with the GSO finder. (I just
didn’t prefer the Synta bracket; some folks
46 Astronomy TECHNOLOGY TODAY
love them, some don’t).
Parade of Crayfords
The GSO Crayford was an easy install. Using a Phillips screw driver, I removed the screws holding the stock
focuser to the tube. The stock R&P focuser slid out easily and sold just as easily
to another online gear-head days later
(more money back in the war chest). An
adapter ring, also bought with the GSO
focuser, bolted into the OTA without
need of modification, and the GSO Crayford focuser bolted into the adapter/OTA.
The necessary adapter ring added several
millimeters to the focal length of the tube,
thus eating up some valuable focuser intravel. Some eyepieces, including the 2inch GSO 32-mm Superview and 2-inch
UO 32-mm Konig, would not come to
focus using a diagonal once the stock focuser has been replaced. Even considering
the loss of focuser in-travel, the Crayford
was a big improvement over the rack and
pinion unit. Focusing was much smoother
with no image shift or slop. Finding a
crisp focus point was also easier when
compared to the stock R&P. I was quite
pleased.
But, as happy as I was with the 2-inch
GSO single-speed Crayford, it was not up
to the heaviest loads I hung on it. When
using a 2-inch diagonal, 2-inch Barlow,
and larger 2-inch eyepieces, there was
some risk of slippage when the scope was
pointed near the zenith. I had to crank up
the focuser tension screw to compensate
and this interfered somewhat with the
otherwise smooth action of the focus. If it
sounds like I was being picky, remember
that my goal was to “optimize” the scope.
A deal came my way on a JMI DX2 Crayford that had originally graced a Stellarvue
refractor. So the GSO single-speed
was pulled out and sold to help finance
the JMI.
To make a long part of this story
short, the JMI was a high quality unit, but
possessed an even longer profile than the
GSO. It could handle the increased load
of heavy accessories with no problem, but
I lost even more focuser in-travel. Many
more of the eyepieces in my collection
failed to reach focus when combined with
a Barlow in the JMI. So, as much as I
liked the JMI Crayford, I was forced to
yank it out and sell it. It has been suggested that I could have simply shortened
the scope tube instead, but because that's
not easy to do while maintaining that the
new edge is absolutely square with the
tube and because I wanted to stick with
“bolt-on” modifications, I chose not to.
I had heard that the 2-inch GSO 2speed Crayford could handle a slightlyheavier load than the single speed version,
and was enticed by the prospect of 10:1
micro-focus, so the scope underwent another focuser swap. I later learned that
both models of the GSO Crayford are
rated at the same load capacity. Whether
this is true or not, I’m not sure. But, I can
say that the perceived slippage problem
was not experienced with the 2-speed version, and the action of the micro-focus
was very smooth, allowing me to more
easily achieve that critical focus point. It
was around this time that red warning
flags began to go up about my scope’s optics. The views through the eyepiece were
great, until one pushed the magnification
to around 55x. At that point, the image
fell apart – bright colored halos around
everything in the field and mushy images
that would not crisply focus.
What a difference
collimation makes
Something was dreadfully wrong. Either I had thrown off my scope’s collimation when changing out the focusers or it
had lost collimation at some point in the
past before I got my hands on it. Either
way, it was out of whack. I am no expert
on star testing, but star tests didn’t look
right even to my inexperienced eye.
Thankfully, the Orion 100-mm f/6 achromat has an adjustable lens cell, and it also
came with a collimating eyepiece to adjust
the alignment of optical path. However, I
OPTIMIZING THE ORION 100MM F/6 ACHRO
Synta style finder bracket as originally
mounted on the focuser - too close to the
eyepiece for me!
was in over my head and anxious to stop
tinkering with gear and start doing some
observing. So I packed up the scope and
shipped it to a friend who is very experienced with refractors and was an expert at
collimation of all kinds. I also shipped
him the 2-inch William Optics 97% diagonal that I had purchased to upgrade
the Orion 1.25-inch unit, so that he could
confirm accuracy of collimation of the diagonal as well.
Upon receiving the package, my
optical man immediately confirmed my
suspicions – the scope was badly miscollimated. Over a course of two clear nights,
he collimated the scope and put it
through a series of trials and star tests.
When he shipped it back to me, the little
achromat had new life in it. The improvement in performance was obvious.
Low power views were now fantastic and
very sharp. Stars were nice little round
beads and high power views were surprisingly good. Saturn’s Cassini division was
easy to see, and high power lunar views
were crisp and contrasty. Star tests were
textbook and the scope would now easily
achieve the 50x per inch magnification
optimum (and more on nights of good
seeing). Sure, there was still some color
around bright stars at higher powers as
well as on the lunar limb, but it was very
manageable using a good violet-reduction
filter. Using a 3x Barlow to squeeze more
magnification out of the short tube, I
could push 300x on the Moon and still
get a nice, usable image. Saturn and
Jupiter looked great up until about 250275x, depending on the seeing and conditions. Cassini is obvious on most nights,
the GRS could be detected on Jupiter, and
yet I could still get a 4-degree field at 20x
using a long focal length wide-field eyepiece. In fact, thanks to the medium f/6
optics, I could put in a 7-mm Nagler and
see the entire lunar disc at 87x and have
enough room left over in the field to observe occultations. Watching a crisp lunar
disc at 87x occult the Pleaides in a field
that is barely shy of one degree was a nice
affirmation that I had made the right
scope choice for me. The little 4-inch
achromat was now paying dividends. But
my quest to push this scope to the limits
of its design and optics was not over.
More changes were in order.
The third mount
is the charm
Around the same time I got the scope
back from its collimation tune-up, I also
“upgraded” my mount from the venera-
Astronomy TECHNOLOGY TODAY
47
OPTIMIZING THE ORION 100MM F/6 ACHRO
ble Towa EQ-2 to a modern EQ-3. This
added a greater measure of stability to my
setup, but the mount had some backlash
in the RA gear, and I was not happy with
the setting circles. So, before it had a
chance to start feeling at home, I sold the
EQ-3 and purchased an old Vixen Super
Polaris with illuminated polar finder. The
Vixen was a work of art: superbly made,
smooth as butter, and nice looking as well.
And, while the EQ-3 and Super Polaris
look similar in size and capacity, the Vixen
was certainly more robust. In fact, when
mounted on the Super Polaris, the Orion
achromat is truly “over-mounted” in every
sense of the word. I can sit outside in a
strong wind, and rack the focuser at 300x
and the view stays rock solid without any
distracting vibration.
Of course, some of this is due to the
custom hardwood tripod legs I ordered
from an old friend. The new legs are 2
inches thick of Maple and Walnut, nonadjustable with a fixed length and
spreader, and beautifully finished. I went
48 Astronomy TECHNOLOGY TODAY
with non-adjustable legs to increase stability and remove any flexure points.
With the eventual addition of a motor for
the RA axis, my mount and tripod issues
are permanently resolved. Now that the
little Orion scope was over-mounted on
an attractive customized rig, it was time
to focus on the scope itself again.
Finishing touches
After four focusers and three mounts,
the Orion 100-mm achromat was finally
approaching respectability. According to
my optical man (who has forgotten more
about scopes than I can ever hope to
know), this particular Orion scope was a
very good sample, with a well-corrected
lens group, textbook star tests, and as
much optical performance as one could
reasonably hope for in a $200 budgetclass achromat. With the addition of the
2-speed Crayford focuser, the scope was a
pleasure to use – focusing required just a
very soft touch without the need to rack
back and forth over the critical focus point
to nail it down. The Vixen Super Polaris
mount and custom hardwood legs provided a stout platform for the scope – able
to withstand strong wind gusts without
noticeable oscillations in the view. The 2inch WO diagonal yields very good views
and completed a renovated optical train
from objective to visual back.
There only remained a couple of
minor niggling issues to improve. First, I
am not a fan of the Synta-style finder
bracket and I am not happy with the
mounting point for the finder base that
sits on the body of the GSO Crayford focuser. It places the finder too close to my
head while observing and it frequently got
bumped and knocked out of alignment
during the course of an evening’s observing. To remedy this, I bought a third 100mm scope ring. I attached the finder and
finder mount to the third tube ring and
attached this about half-way down the
tube, out of my way and closer to the objective. With the finder mounted on its
own independent tube ring, I am free to
OPTIMIZING THE ORION 100MM F/6 ACHRO
change the configuration of the finder in
respect to the scope.
Next on the list was a roll of adhesive
backed felt liner. I used the black felt to
flock the inside of the dewshield which I
thought was a little too reflective, despite
being flat black from the factory. The felt
is much less reflective and eats up off-axis
reflections. Once the finder was located in
a more comfortable position, I went out
and found an illuminator for the finder
crosshairs. A pet peeve of mine is not
being able to discern the crosshairs of my
finder against a dark background sky – it
makes lining up targets a major pain. I
didn’t want to purchase an entirely new
finder, and the available aftermarket illuminators were relatively expensive. So I
contracted out the job of building an illuminator to a certain managing editor of a
certain astronomy gear magazine. He
built me a professional-looking illuminator unit with a rheostat to dim the LED
which sat at the end of a pigtail cord running from the illuminator housing. I attached the housing to the DEC-motor
bracket on the Vixen mount (I don’t foresee using a DEC motor any time soon)
and drilled a small hole in the eyepiece
barrel of the finderscope, inserted the
LED and voila – illuminated reticle finderscope. No more frustration trying to
center targets on the crosshairs.
Observing with the
finished scope
After much tweaking, I finally had
myself a scope that could perform to my
tastes. With a violet-reduction filter, the
4-inch achromat could easily attain 50x
per inch on any target I cared to observe.
Images are pleasing, stars are crisp little
beads, and the TFOV of the scope using a
budget wide-field 42-mm eyepiece was
over 4 degrees. Views of star fields in
Cygnus are stunning. M24, the Sagittarius
star cloud, is a thing of even greater wonder in its full context with surrounding
sky. Even planets, long considered the
bane of fast achromats, are fair game when
using a filter to
tame the color.
Numerous cloud
bands are visible on
Jupiter. I have seen
the GRS (or as I
call it, the “GTD”
– the great tan dot)
and I have watched
shadow transits of
the Jovian moons –
albeit on a small
image scale. Using
a 7-mm Nagler and
Meade 3x Telenegative Barlow for
257x yields a surprisingly
sharp
image, despite the
fact that such magnifications are generally regarded as
“stupid” in a fast
achromat.
On our bright
and
forgiving
neighbor,
the
Moon, I have successfully pushed
300x on nights of
excellent seeing –
granted, the image
was going dim and Evolving scope on the EQ-3 mount and fixed height, wood tripod.
soft at that point,
negligible, and one can start viewing imbut not to the point of being considered
mediately after setting up without waiting
“useless magnification” – I was seeing
an hour or more to achieve thermal equimore detail due to the increased image
librium with the outside temperature.
scale. I have seen Venus and Saturn in the
This experience has further cemented me
same field of view during a recent close
into the small refractor camp so much so
pass – something not easily achieved in
that I have turned down some spectacular
slower scopes.
deals on much larger scopes of different
But I think the best attribute about
designs including a 10-inch SCT that was
this scope, besides its surprisingly good
offered to me for next to nothing. The
performance, is its light weight and ease
old saying, “the best scope is the one you
of use. At 6 pounds bare, and just under
use,” has proven to be true in my case, and
9 pounds fully-loaded (with finder, diagI have gotten a lot of use out of this humonal, eyepiece, etc), this scope can be
ble 4-inch achromat. Now, pardon me
fielded with a variety of mounts that are
while grab-n-go!
well within the budget of the most frugal
Clear dark skies to all!
astronomer. Cool-down time is almost
Astronomy TECHNOLOGY TODAY
49