Borg 101 f/4
The Borg 101 ED
F/4 Astrograph
"It's a Borg – Resistance is Futile!"
By Craig Stark
If you could design your own refractor,
what would it be? An achromat? An apochromat? Something small for travel or something
larger to bring in the faint fuzzies and give
more resolution? Would it be in a normal tube
or an extra-short one to allow for binoviewing? Something geared for astrophotography
or more for visual use?
We all have different answers to these
questions and many of us will answer these
questions differently when asked at different
times. Unfortunately, when we buy a refractor,
we’re pretty well stuck with it. A focuser swap
is about all we can do and that’s not usually
easy or affordable. I know - I just sold an excellent scope because I wasn’t happy with the
focuser when it came to using it for astrophotography.
Into this quagmire, enter Borg Telescopes, sold in the USA by Astro Hutech
(, also known for their
IDAS filters. Borg takes a different approach
to telescope design than any other manufacturer. Borg telescopes are completely modular, made up entirely of interchangeable parts.
Want a shorter main tube? Order a shorter
main tube (or order both so you can have your
choice). Want to start with an achromat and
upgrade later to an APO? Order the achromat
objective initially and later order the APO version. This approach is wonderfully versatile,
but this versatility is what can put people off
of Borg telescopes. Put simply, when you
order a Borg, you’re designing your own telescope from a long list of parts.
Reviewed here is a Borg 101 ED f/4.1 astrograph. Or, more precisely, it is a: 2101 +
7135 + 7749 + 7835 + 7704 + 7920 + 7601
(2 of these) + 7522 + 7505 + CB101 + 7083
+ 7755. Imagine the following encounter at a
star party, “Hey what do you have there?”,
“Why it’s a 2101 + 7135 + 7749 + …” That’s
not going to go over too well or impress your
guest very much. Having gotten my feet wet
with the Borg system, I now simply reply, “It’s
a Borg – resistance is futile.”
What’s in the Box
Open up a new telescope and you expect
to see, well, a telescope. Open up your box
from Astro Hutech and what you see is,
Image 1
The box of boxes that make up a Borg telescope as you receive it. This doesn’t quite look
like a telescope yet. But, have no fear, this collection of parts is soon to transform into a
Image 2
The Borg 10 minutes later, fully assembled, looking like a telescope and
not a collection of parts.
well… boxes (Image 1). Yes, this is a telescope
and no, it’s not in the black case there. The
telescope comes as a collection of parts. Fortunately, Astro Hutech provides documentation and there are diagrams in there (and on
the website) to show just how everything can
fit together for the configuration or configurations chosen. About 10 minutes or so after
I took the first shot here of all the boxes, I took
this second shot of the fully assembled scope
(Image 2). Assembly was smooth and painless and everything fit together perfectly – a
testament to the consistent, precision machining and manufacturing of the parts. There
were no extra screws, no time spent scratching
my head, and no panicked calls to Ted at
Astro Hutech. It really couldn’t have been
much simpler.
What’s shown here is the scope I wanted,
configured the way I wanted it with the optics
I wanted, the reducer I wanted, the focuser I
Get Yours
wanted, and the camera mounting gear I
wanted. Nothing more, and nothing less.
Deciphering the Parts List:
How I “Built” My Borg
OK, so it went together easily and there
were no missing or extra parts. How did I get
here? First, I did my homework. (As a professor, I’m convinced this is one reason Borgs
aren’t wildly popular – nobody likes to do
homework…) The Astro Hutech website
shows a number of “standard configurations.”
These are basic templates or starting locations.
I do wish that they would list all the parts that
go into each scope they show. Coverage here
on the website is a bit spotty and, if improved,
would go a long way to making the decisions
easier. I started with the page on the “f/4 Astrographs” as I was in the market for something with a nice, wide field to mate with my
APS-sized camera (CCD Labs Q8-HR) for
Let us custom build you
the best, strongest, most
well constructed scope
transport case available!
very wide shots. In the PDF that describes the
dedicated reducer that makes the second heart
of the system (more on this later), you’ll see a
typical Borg system diagram like the one
shown in Image 3.
What this shows is that if I choose the
101ED lens as my objective, I should mate
this with the 7135 tube and drawtube, the
7749 draw tube holder, the f/4 reducer (consisting of the 7704 front, and 7704 rear), my
choice of the 7835, 7837, or FTF-M57 focuser, and a camera adapter. Now, how do we
know things like the 7835 is the focuser (apart
from guessing based on its location?) Well, the
full Borg parts list is linked under “Telescope
components,” giving you a long list of part
numbers with prices, brief descriptions, and
links to product pages with more of a description. In addition, the full catalog (circa
2003) is available in PDF format, organized
by part type (with indices) and containing a
good bit of information on each part.
In my case, since I wasn’t going to use this
much with a DSLR, but with a CCD that
uses T-threads, the camera adapter wasn’t
needed. Their site listed a 7920 + 2x 7601 +
7522 as the correct choice for a Starlight
Xpress setup. That served as a starting (and in
my case ending) place for choosing the bits
and pieces needed to get my CCD the correct
distance from the reducer lens. A few other
bits and pieces (rings, finder mount, etc.) and
I was prepared.
Once I had my list of parts, I contacted
Ted Ishikawa at Astro Hutech. Since I live just
down the road a bit, I drove up to Astro
Hutech to meet him and make sure I was getting the right set of parts. For those not so
close, Ted is very responsive by e-mail and
phone. He’s also incredibly helpful and patient
and clearly wants you to end up with the system that is tailored to your needs. He never
tried to sell me extra bits and bobs and handled all of my questions well (I might add that
at the time, he had no idea I was contemplating writing a review.)
101ED Lens+f/4 Super Reducer
The Borg f/4 Astrographs are designed
around the f/4 Super Reducer. This is a two-
Image 3
The “system diagram” showing the components and options available when using the f/4 Super Reducer. Note, when not using the f/4
reducer, a wider array of options is available. Diagram used with permission from Hutech.
component assembly (one on each side of the
focuser) that forms a four-lens system (with
one element being of ED glass). This is designed to work with doublets such as the 77
EDII, 101 ED, and 125 ED Borg lenses such
that together, they form a very flat, well-corrected 6-element system. (Note, I am not
qualified to determine if this is a “true” Petzval, an “advanced” Petzval, or something just
Petzval-esque.) In my case, it turns the 101
f/6.3 objective (640 mm) from a standard
doublet capable of visual or photographic
work into an f/4.1 (410 mm) astrograph. Of
course, when I say it “turns it into” such a
system, one can turn it back easily enough by
just unscrewing a few parts from the focuser
and replacing the focuser – something that
would take just a few minutes.
The Super Reducer has a few twists. One
of them, a literal twist, as it gives you the ability to rotate your camera free of any focus
shift. It also has a filter slide that lets you slide
52-mm or 48-mm filters in and out of the
light path (extra slides are available to keep
your filters mounted). The only downside to
this system that I found was that it was not
wide enough to accept my Baader 2-inch filters. This issue is not unique to the Borg. The
Baader filters are thicker than most by a good
margin and also do not fit in the Astronomik
filter drawer system.
Borg is one of the few telescope manu-
facturers who put any performance data on
their site and, to my knowledge, only AstroPhysics consistently provides comparable
amounts of performance data (Canon, to
their credit, publishes MTF curves on their
lenses). For example, the current 101 f/4 system has the following published graphs (note,
there is an older objective + f/4 reducer set also
on the site, but one designed for 6x7 film),
shown in Image 4 shown on next page (color
added for clarity).
On the left we have plots of the
spherical aberration for four different wavelengths of light (often labeled the longitudinal aberration plot). This shows where
different colors reach focus (c=656 nm, d=589
Image 4
Spherical aberration (left), astigmatism (middle) and combined percent-illumination and
MTF data for the Borg 101 f/4. Diagram colorized and used with permission from Hutech.
nm, g=435 nm, F=486 nm). A perfect system
would have all lines cross at a single point. In
cases where they don’t, some colors will be in
focus and others will not. The result of this is
chromatic aberration. Visually, since we will
focus most likely on green light (where our
eyes are most sensitive), this will often lead to
the “purple halo” as violet light is not in focus.
This isn’t quite on level with the highest performing triplet Apos out there and is why
some reviews have noted a touch of chromatic
aberration on bright objects at high powers.
But, this is a 400-mm f/4 astrograph. In this
setup, you’re not using it as a high-magnification lunar or planetary scope, but as a widefield (“low power”) scope. The proof will be in
the pictures to see if this introduces enough
chromatic aberration to be picked up with
typical pixel sizes and targets.
In the middle, we see the astigmatism
plot (M and S refer to directions meriodonal
/ tangential and sagittal / radial). The astigmatism plot shows a lot of information here.
The bottom of the plot is the center of the
image and the top is at the edge of a 22-mm
image circle (i.e., 11 mm from the middle). A
perfect lens system would show two vertical
lines atop each other. If M and S diverge, we
will see misshapen, astigmatic stars. If they
track with each other, but deviate from 0, we
will see the softening as the focus point has
shifted. What we see here is that M and S are
effectively atop each other and that there is
only a minor deviation from vertical (approximately 0.1 mm). One could split the differ-
ence in focus and focus 5 mm from the center
and balance this out. But, the stars should remain round to the edge.
On the right we have a combined
%-illumination and MTF plot. The numbers
on the bottom and the red line show the
%-illumination at several circle-sizes (diameter). We see that for a 14-mm circle, the edge
is 93% illuminated and for my 23-mm x 15mm (27-mm diagonal) chip, I should expect
illumination around 80% (I measured 85%).
The MTF, or Modulation Transfer Function, describes how “sharp” a lens is. The form
here shows how much contrast there is in an
image of a fine a line grating (100 and 40 lines
/ mm) as a function of distance from the center (x-axis) and angle of the lines (M = oriented perpendicular to the line from the
middle of the image to a given point, S = oriented or parallel to that line). This shows us
just how much off-axis “softening” of the
image we can expect on high frequency (100
lines/mm, green lines) and low frequency (40
lines/mm, blue lines) components of the
image. This is a very nice curve, letting one
see that not much softening occurs out at 27
mm. One could go even beyond this before it
would become objectionable. Note, these are
extremely high-resolution targets. Canon is to
be applauded for publishing MTF curves like
these on their lenses, and they make very fine
optics, used by many for wide-field astrophotography. But, they chose 10 lines/mm as their
low frequency target and 30 lines/mm as their
high frequency target. If you attempt to compare curves, keep in mind that the Canon’s
high-resolution target is lower resolution than
the Borg’s low-resolution target.
In the Field
I’ve gotten the Borg out on a number of
occasions now and am pleased to report that
I am quite impressed. I have gone through at
least four refractors and three field flattener /
reducers in an effort to find something I
would be happy with and keep. The Borg is a
keeper. Here’s why.
Physical Characteristics
The Borg is about the same size and
weighs less than a very nice triplet 80-mm
scope I had. Yet, it gathers over 50% more
light. It weighs only 5.5 pounds, making it
easy on the mount and it fits inside a very
svelte case. Want to travel with it? Unscrew
the objective and put it in its nice bag and
everything fits in even a small carry-on without concern. Despite weighing next to nothing and having a main tube the size of many
80-mm scopes, the thing is solid. There is no
flex that I can find in it. In part this is due to
the fact that for photography, there is no need
to ever resort to a 2-inch or 1.25-inch eyepiece
barrel. Everything screws in (with a very
smooth feel). The one thing that does slide in
and out (the main tube’s drawtube) is clamped
in place by two screws at 120 degrees (with
the opposing side making the third contact
The helical focuser works like a charm.
Many readers have bad visions of helical focusers as they imagine the camera rotating
during focus. The Borg helical focusers don’t
rotate the camera / eyepiece (one very low-cost
one, the #4317 does, but this is the exception). Think of a SLR camera lens (before
auto-focus or think of the zoom on your autofocus DSLR). You rotate a ring and it changes
focus. The “standard” model I chose has had
no issue with camera load and it’s almost impossible (if not impossible) to get it to move
by any means other than rotating the ring.
What’s more is that it has no backlash I can
detect at all. Oh, and it has index marks every
80 microns. So, as you’re evaluating focus at
various positions, you can return to the same
spot within 80 microns (0.08 mm) just by
using the index marks.
If you feel you must have a Crayford style
focuser or the ability to connect to your
computer for auto-focus, Astro Hutech offers
a Feather Touch model that can be used
instead, screwing straight into the 57-mm
threads on the drawtube. While I love a
Feather Touch focuser (I had one on one of
my various small refractors I’ve gone through),
for my purposes the helical was the better
Images: On and Off-axis
Sharpness, Color, Etc.
There’s not much point to good
mechanics in a scope if the optics don’t hold
up under scrutiny. Sure, it looks pretty and
feels nice, but if the scope doesn’t have it
where the photons hit the silica, it’s not of
much use. I’m pleased to report the Borg
passes here with flying colors (and really only
colors that are meant to be there). While not
as bright as Venus or the Moon, few would
argue that M45 isn’t bright. In Image 5
shown on next page, we have some bright stars
Image 5
Crop of the core of M45 showing performance in the center of the field on a target
with wide dynamic range. Stars are crisp
with no obvious issues. Reflections around
brighter stars are off of the CCD coverslip,
not off of anything in the Borg.
amidst a sea of blue reflection nebulosity (with
some nice bright ones on a dark background).
It’s a non-trivial target for scope and camera
alike and presents a good test of real-world control over chromatic aberration and contrast.
The images in Image 6 were taken on a
CCD Labs Q8-HR (DSLR sized chip) using
Nebulosity, guided on a Takahashi EM-10
guided using PHD Guiding, an 8x50 finderscope cum-guide scope, and a Fishcamp
Starfish. A total of 120 minutes (40 frames at
3 minutes each) were used. Bad Pixel Mapping
was used in lieu of darks and processing was
done in Nebulosity, PhotoShop, and PixInsight. No noise reduction, smoothing, or anything “local” was done and nothing was done
that would specifically enhance or obscure
chromatic aberration. First up, we have a crop
of the core shown in Image 5. If I am truly obsessive I can just barely pick out a ring of violet
around the stars on the black background. I
have to be looking hard for it and zoom way
in, and then I can only see it on a few stars. I
can see the reflections off of the Sony ICX453’s
cover glass on the Sisters here. The fact that the
reflection halos aren’t centered on the stars indicates that my CCD isn’t perfectly square. Despite this, the stars are nicely round.
Round stars in the center aren’t tough to
come by. What is tough is keeping them round
as you move way out in the corners of a goodsized chip. While not the biggest chips out
there, DSLR-sized (APS-sized) chips put a lot
more demand on your optics off-axis performance than more modest sized chips. They
often expose flaws you never knew you had.
How does it do at the corners? Here, we have
full-size crops from all four corners in Image 6.
Stars are round with no noticeable aberrations.
We’re sharp to the edge.
While sharp to the edge, we’re not 100%
illuminated to the edge. I recorded a drop to
about 85% illumination at the corners of the
frame. This was easily taken out with a flat
frame (real or artificial).
At f/4, your focus is critical. On my Vixen
R200SS 8-inch f/4 Newt with a Baader
MPCC, I actually use the fact that off-axis
aberrations show up easily when out of focus to
help fine-tune my focus. I tried that trick here,
only to discover that, unfortunately, the Borg
101 doesn’t make odd-shaped stars in the corners when slightly out of focus. They’re just
slightly out of focus. Here, in Image 7, we see
the corner of a shot of the Rosette with the
whole frame just a touch out of focus.
There are many excellent refractors out
there these days at many different price points.
The Borg 101 ED f/4 has a lot in common
with other telescopes out there. It is portable
(though more so than any other 4-inch I’ve
seen), can put up very nice views (without the
f/4 reducer, it’s a fine visual scope), and should
set the owner up for years of enjoyment. It has
a number of things that set it apart from other
scopes, however. Most notable is that it isn’t
one scope. It’s whatever scope or set of scopes
you want it to be. If I were hopping on a plane
to Australia for a vacation, I’d love to bring a
scope with me. I’ve never seen Omega Centuri
and couldn’t pass up the chance. Were the
Borg purely an f/4 astrograph, I’d consider
something else. But, in a few minutes, it’s an
f/6.3 scope that is easily packed in carry-on. If
I have a camera here with a very different internal back-focus, it’s simply a matter of swapping out extension tubes to get it to the right
distance. Heck, if I decide that I want to be
able to go wider, I can swap in a 77-mm objective to get to 330 mm. Decide to put a tax
rebate to something bigger? I can swap in a
125-mm objective and a new tube, keeping
the rest of the scope. Headed backpacking and
don’t want to take the nice ED objective with
me? Swap in an achromat and consider it an
insurance policy.
The point here is that the Borg can be
anything you want and can change as your
needs or desires change. The versatility here is
almost limitless. No other scope / scope system
on the market offers this kind of versatility.
Versatility without performance would be
useless, but the Borg delivers here as well. On
axis and off, with a good-sized chip, the Borg
delivers clean stars and nice contrast. Its mechanics make it very well suited to astrophotography to boot.
So far, I’ve yet to say anything really negative about the Borg. Were there things I
would change if I could? Sure, but most are far
from reasonable (it’s not reasonable that it
should cost $20 for example). Here’s the small,
semi-reasonable, nit-pick list: 1) I wish the fo-
Image 6
Crops of the four corners of the same shot of M45 showing performane at the edges of an
APS-sized frame (27 mm diagonal). Stars are sharp and round with no obvious aberrations.
cuser had a bit more travel. The drawtube
setup works well to give you a rough focus, but
more travel is never a bad thing. 2) It still does
vignette a bit on the APS-sized chips, so I still
need flats. 3) It would be nice if the filter slide
held my Baader filters, but in my book, Baader
gets the blame here for making filters a lot
thicker than others. That’s the list in its entirety
and none of these are large issues or unexpected issues.
In closing, while one often doesn’t show
“first light” images, Figures 8, 9 and 10 (shown
in the online magazine version of this article at are the
very first images I took one with the Borg
(apart from one night of about 20 minutes of
quick test shots to determine the optimal set of
spacers for my camera). The fact that there
were no kinks to work out, no mechanicals to
rework, no duct tape to apply, etc. for it to
perform this well speaks volumes.
Image 7
(Corner_Slight_Defocus) Crop of a corner
of a shot of the Rosette Nebula taken with
a slight defocus. While stars are softer
than optimal and careful inspection can
show the defocus, there are no other
readily apparent aberrations.
Figure 8
Figure 9
Figure 10
Figures 8, 9 and 10
Sample full-field frames taken on the first night of imaging with the Borg.
Total exposure time for each was 2-3 hours on a CCD Labs Q8-HR. Other
equipment used: Takahashi EM-10 Temma, 8x50 finderscope converted into
guide scope, Fishcamp Starfish guide camera, Nebulosity image capture
software, and PHD Guiding.
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