David Lukehurst 22"ť f/3.61 Ultra-portable Telescope,
By Bart
Wide, September 11
th 2008.
I have fairly
intensively searched for a custom and premium telescope maker to craft
me a
telescope according to the desired specifications. I was looking for a
telescope with as large primary mirror as possible that would still fit
into my
car and that I would be able to manage without major effort alone as
well. That
meant 28 inches for the side of the bottom square of the collapsed
telescope
and 18 inches in height without the trusses.
I was tempted
by a 24 inch f/3.3 telescope, but the size and fear of coma lead me to
a
different decision. Although, in retrospect, I think that coma would
not be a
problem with such a telescope but portability would be unless it were a
rather
ultralight than ultraportable telescope. The costs might be soaring
high and
the investment itself might have been a bit more risky considering that
premium
telescope makers practically do not offer such big telescopes in such
design.
So, a 22
inch f/3.6 ultraportable telescope in the likes of Obsession's checked
and
tested ultracompact design is what I decided for; ultraportable in
terms of a
very collapsible telescope, not necessarily feather light but certainly
durable, usable and manageable. The only telescope maker among quite a
few I
asked, willing to craft such a telescope, was David Lukehurst from
Nottingham,
UK (
http://www.dobsonians.co.uk/).
David
was really friendly and accommodating. In a series of e-mails, we
discussed the
general design and specifics of the telescope. He was also helpful in
choosing
particular elements "“ for instance the possibility of using a
binoviewer. After
almost everything was laid out, I placed an official order by settling
the
first half of the amount (3,125 GBP "“ there were no VAT expenses).
The optics
were ordered at Nichol Optical managed by John Nichol in the UK (
http://www.nicholoptical.co.uk/).
John was also very friendly and helpful to deal with. In a matter of
two and a
half months, he finished the f/3.61 mirror out of Suprax (borosilicate)
to a
precision of 1/8 lambda P-V of surface accuracy, 1.8 inch thick. The
coating
process took somewhat longer but all in all, I think, the finished
mirror was
delivered to David fairly quickly compared to some delivery times of
other
premium mirror makers. Upon the arrival, the mirror was lacking coating
in 3
thin lines at the edge of the mirror, each a third of the circle apart.
John
assured me that the mirror is held in these three places during coating
and as
I agree with him, this will hardly contribute to the loss in light
gathering
power. When testing the optics, John applied the following optical
tests: Autocollimation
(double pass null test) against a Grubb-Parsons Master flat, Ross null
test using
an interferometrically certified compensating lens (Ceravolo optical
systems),
qualitative test. He figured the mirror to match or better computer
generated
image showing surface accuracy of 1/8 wavelength as he assures in his
certificate.
David then
finished the telescope in a matter of 2 weeks and in four months total
the
telescope was finished and deliverable.
The
telescope consists of following materials: steel, plywood, formica,
teflon,
laminate. The construction is perhaps more wooden than metal but in
essence a
balanced mixture of both. The telescope is well protected and seems to
be fully
weather resistant "“ apart from the mirrors of course. The primary
mirror rests
on an 18-point mirror cell and the collimation bolts underneath make
the
collimation really easy both for adjustment and locking. The tailgate
and the
mirror cell (with a 12 V cooling fan) form a strong construction
supporting the
mirror. When transporting the telescope 50 miles away, the optics is
tightly
held in its position which results in a very small miscollimation
afterwards "“
when the telescope is set up, collimator's laser beam hardly falls
outside the
center spot on the primary mirror and on the secondary the returning
beam
hardly deviates from the incoming one. The once called mirror box which
now
begs for a new name, is detachable from the rocker box. The thick 4.53
inch
minor axis secondary mirror rests on the secondary holder held by a 3
vane
spider. The UTA also carries the Telrad and the Moonlite CR2 focuser
both of
which indeed are a nice piece of equipment. I must add that I first
ordered a Moonlite
CR1 and when I raised concerns that such a fast mirror requires very
small travel
distances to achieve focus, David arranged for the upgrade from CR1 to
CR2 free
of charge. Thumbs up! The trusses, only 61 inches in length, are not
interconnected and come in three pairs. They are simple to mount and
are marked
for exact position, again to preserve collimation after having
reassembled the
telescope. The primary mirror is secured by a tightly fitting cover
which
safely rests over the mirror during transport or storage, not harming
the
optics on the top or on the side. The altitude bearings rest on teflon
pads and
are collapsible. The point of connection under the hinge is very smooth
and
when the telescope is moved in altitude, no obstruction is noticeable.
The
Rocker rests on a strong, flat tripod and again slides on teflon pads.
What remains
are the two baffles, one around the primary mirror and another one
opposite to
the focuser, covering the angle of 160 degrees.
The
telescope can be collapsed to a height of 18 inches, length of 30
inches and
width of 27.5 inches without the trusses and the UTA. I made some
adjustments
for the UTA to securely fit on top of the collapsed telescope for safe
transport and the entire collapsed telescope safely fits into the trunk
of my
Kia Cee'd which is not a big car at all. Since this is the first
ultraportable
telescope made by David, I think he will take care of this in his
future
telescopes and UTA-s will fit on top just splendidly. The trusses are
so short
that I can put them on the back seat or under it if somebody were to
sit in the
back as well. The entire telescope weighs 80 pounds, the primary mirror
alone
weighs 42 pounds! In principle, I can lift the mirror but I must admit,
it is
not all too pleasant. The eyepiece height at zenith is 76 inches. An
inch or
two could be chipped off to go to extremes but a small 10 inch step
stool does
its job and is even fun to use. In general, I am fully satisfied with
the dimensions.
Although the numbers seem small, this telescope is huge. Its sheer size
shocked
me when I first saw it, even though I was running around with a
measuring stick
several times prior to seeing the telescope.
I ordered
the telescope without a shroud and without wheelbarrow handles wishing
to
reserve some work and tweaking for myself out of joy. I made the shroud
out of
ripstop nylon, an elastic string for fastening to the UTA and a steel
wire
which I welded into a hoop to rest the shroud around the bottom of the
trusses
and to keep a nice round form while pulling it downwards with its
weight. The
shroud does not obstruct the light path. Instead of wheelbarrow
handles, I will
use a small collapsible trolley with additional forks for holding the
mirror
box.
The
performance:
There is a
fair amount of scepticism about fast optics and ultracompact telescopes
among
astronomers. People worry about how perfect the mirror has to be if it
is
supposed to be a f/3.6 paraboloid, how hard it is to grind such a
mirror and
how only a fistful of master opticians can produce a satisfactory
mirror like
this. In addition, there are prevailing tendencies that mirrors have to
be of
Strehl 0.92 or preferably more which considerably raises the costs of
the
optics. While on the one hand, obsession with absolutely perfect optics
seems
to prevail, people on the other hand also fear that collapsible metal
telescopes
cause ringing, that they are not stable in the wind, that the movements
are
problematic with collapsible altitude bearings, that such telescopes
tend to
miscollimate easily, etc.
When I
first used the telescope, I feared that I would regret ordering such a
fast
telescope with "śonly"ť 1/8 lambda surface accuracy mirror. Holding
the printed
images of optical errors revealed by star test in one hand and leaning
my eye
on the eyepiece offering the magnification of 420 x, I started focusing
and
defocusing on Vega. Unfortunately, my previous telescopes only revealed
fuzzy
blobs when trying to star test the optics. Because I did not know what
to
expect, I decided to repeat the star test a number of times before I
would make
any final conclusions. The mirror John has made is probably absolutely
great. Inside
and outside of focus, evenly pronounced and perfectly concentric
diffraction
rings were clearly noticeable and were comparable to the rings
designating a
perfect mirror. At 420 x, highest magnification through Paracorr which
does not
allow for the use of standard Barlow lenses, the images of stars were
sharp (to
note: seeing was good but not perfect). At the magnification of 300 x
in my old
12 inch telescope, I have so far on better nights not seen images of
stars as
sharp as with the 22 inch at 420 x. Nevertheless, I am yet to
experience what
the optics of this telescope are capable of in perfect or near perfect
conditions. When the Paracorr was properly set, the 26 mm eyepiece with
70-deg
apparent field of view revealed no coma at all. The flatness of the
field, to
note, was not particularly impressive but to compare with the flatness
revealed
by my old f/5 telescope in combination with the same eyepiece, the
situation
was the same. Only, f/5 showed more coma because I did not use the
Paracorr.
The
telescope is easy to use. One can either stick with one object and
track it for
a long time or jump from object to object like crazy; I can track
easily at
magnification 420 x, and yes, nothing is particularly slow about this
telescope
J Standing most of the time on my feet while
observing, I can as easily manage this telescope as I did my old 12
inch dobsonian
"“ the only difference being that movements are now slightly harder.
But the
longer optical tube compensates for manoeuvrability as the corrections
at its
tip are now more precise.
The only
problem, I have so far encountered with the telescope is that it does
not hold
collimation perfectly when moved in altitude "“ the miscollimation,
however, is
minute. It was only noticeable in the positions of collimator's laser
beam and
in the pattern of diffraction rings. When in focus, stars looked sharp
and
clean also in zenith "“ the only direction in which the telescope
displayed
miscollimation. Perhaps this is the only trade-off of this particular
telescope
design, but it might be that proper adjustments or tightening of
collimation
screws and bolts will do away with this problem in which case I will
withdraw
my observation.
To all of
you who are worried about going too fast or too compact, I hope this
review
gives some courage to decide for telescopes as large and portable,
perhaps even
as light as possible. Regarding the obsession with perfect optics: So
what if
magnification can only be pushed to 800 instead of theoretical 1100 on
that one
or two nights in a year for the majority of us who do not have the
luxury of
living under very dry and perfectly steady skies. In addition, the
difference
between 800 and 1100 is not so dramatic after all compared to the
dramatic gain
that the aperture offers, especially when going after thousands of
completely
different DSOs rather than after the details of a few planets.
