The South Cave
taming of the Newtonian Secondary Mirror
Challenging the Best Refractors
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starts back in April 2006 when the black plastic front cover of my 8”
Wise Newtonian telescope had developed a crack, so I moulded a new
one over the telescope in glass fibre matting. On 9th
October 2008, I began using a new secondary mirror that I had fitted
because I had damaged the previous one having applied wrong
corrections for rotation, which had become apparent during
night of 27-28th November 2008, I set out to
double star STT 155 positioned 1/2?
south east of ?
Geminorium. I increased the magnification to 360 x to reveal the mag.
‘B’ star and was surprised that I was seeing a triple. The magnitude
7.3 ‘A’ component had a fainter companion
with a measured separation of 2.5” to 2.8”. I had made a
later this seemed to be confirmed, but it wasn’t there on 1-2nd
December 2008 or again on the 6-7 December 2008. I re-collimated
using a laser collimator but the star didn’t appear again. It
was a ‘birdie’ with its wings folded.
the secondary between then and 19th April 2009
improvement, in terms of shape, but was never permanent, so I began
to think of the crosshairs in sight tubes and Cheshire Eyepieces for
a gunsight. I am no shooter, but gunsights on a barrel only 6”
long? These can’t be much better than a collimated laser. Then
on 17-18th May 2009 I recorded, ‘Odd, but
far better by aligning a simple peephole with crosshairs pencilled on
the fibreglass front cover!
been a spell when the 114mm Bresser Newtonian telescope was
performing better at 3 to 6 arcsec separation. Then with its own
Collimating Front Cover it too benefited from my invention by showing
the bright binary Castor very distinctly separated.
June 2009 I’d recorded, ‘?
Ophiuchus no ovals. Little primary adjustment straight from
storage!’ At last, no more all night collimating. It had
certainly been like that in variable cloud.
0.6” separation split quite well at 300 x. In the 8 inch
Newtonian, now 99.9% OK.
September 2009 in A2 seeing and limiting magnitude 5.6 on 72 Pegasi,
first definite split of this attractive orange close binary of 5, 5.4
and separation 0.53”. Dawes Limit 0.58”.
October 2009, I rotated the 4 ½” Bresser’s
secondary by 0.7?
produced a very one sided out of focus star at 250 x. See Figure 4.
On reducing the rotation alignment error to 0, ?
Pegasi became very concentric.
October 2009. STF 449, mags.8.3 and 11 by 6.3 arc “ apart.
Beyond the limit now as regards confirming separation with a reticle
eyepiece, but I was still able to estimate agreement with the WDS
List and look forward to challenging the best refractors.
October 2009 at 1000 hrs. Sunspot return occurred observing through
the Bresser in the form of a large black tadpole with a distinct
curly long broken tail. Such detail on the Sun I have never seen
A New Beginning
with my invention
and invention described here, ends secondary mirror collimating
difficulties for amateurs at last. You simply look through the
special peephole at the reflection of its perimeter, the primary
mirror centre doughnut and the crosshairs of the translucent
Collimating Front Cover. Then if their centres are displaced
sideways, rotate the secondary mirror until the primary doughnut and
peephole are on the vertical axis.
The slightest rotation of the mirror is
detrimental to the fields of view and magnifications that have become
affordable. Slewing it unawares with adjuster bolts 2 and 3, Figure
2A, never quite gets rid of any oval slightly de-focussed stars and
birdies. I found myself reporting a false new close binary and
becoming not able to split double stars to their Dawes Limit and
lunar resolution half of what it could be. On 17-18th
2009, I stumbled on the solution described.
and 2A show the secondary mirror out of gunsight alignment and you
can see it is corrected solely by rotation of its whole body and that
it is definitely secondary mirror error .
Collimating Front Cover, Figure 1, in the full South Cave Collimator
System, is for correcting error  only. When used
conjunction with the other two aids described above, alignment is
seen much more easily and errors highlighted. Time then spent on
slewing, elevating and shunting the secondary mirror is reduced
because there’s no trying this or that, nor repetition once
it’s done spoiling every observing night trying to eliminate 1”
to 3” of arc deformities.
It is the
end of trawling for advice and spoilt observing sessions because of
having suffered from:
centre mark on the primary mirror analogous to a foremost gunsight
being only halfway along a gun barrel.
secondary mirror being near the front of the main tube can be used to
eyeball concentricity with the perimeters of the tubes and primary
mirror reflections. However, this method is no longer good enough
except for trial and error specialists and professional experience. The
word to live is rotation, rotation, more than
concentricity of perimeters.
devices short enough to fit in the focus tube got going when black
was the only durable colour in plastics. They can be as worthwhile
an expenditure at high magnification as barking mad dogs and
Maskelyne’s lunars were for obtaining longitude at sea. The
problems I’ve had with a very well collimated laser collimator,
seen by a patent attorney, seem confirmed by patent application
US2007/0263284 A1 of Nov. 15 2007 which attempts to save that
I’d simply made a fibre glass front
cover over the rim 3 years ago to replace a black plastic one
cracked. Now it has a reticule consisting of two black fine lines
crossing at right angles. These turn it into the third
gunsight and ensure accurate collimation of the first of the four
errors of the secondary mirror, superior to anything betwixt it and
the eye. For existing telescopes, translucent white plastic panel
can be cut to fit within the front rim.
When accurately aligned on the primary axes,
fine spider wires are useful with a dull daylight sky. A white
plastic circular breadboard is capable of from having some merit for
this purpose to nothing better than. And they go in and out of
Into the focus tube at the other end goes a
canister with a peephole drilled through the centre with new drill
bits, starting with the smallest, enlarging only until you can see
through it properly towards the primary. Probably only those with a
depressed centre will do.
the peephole with Anita’s red glass paint removes any blandness
of the reflection and highlights against black. The 12mm coloured
decoration tape is mainly for stopping the canister dropping through.
Good placing lets more light in. This component isn’t new so
black cursor line from centre to edge on the inside face is, and
indicates whereabouts of the red perimeter if the primary centre
doughnut covers it. Curse and correct if you can line it up with
only one of the other two alignment aids. It comes from radar
greatest accuracy it is usually best that bland paper ring
re-inforcers on the primary mirror be down graded to masking tape for
a centre marking doughnut to be inscribed with a 21st century Tipex
pen. On the inside perimeter for 1200mm f. length and
probably on the outside for longer fl. instruments.
nearly dry remove the masking ring. A rough free edge is too far
away to be seen, and won’t arise for longer tubes if you Tipex
the paper ring instead.
view through the peephole to be like Fig.3, remove any central blob
with a plastic scraper, tested water rinseable solvent if necessary
on a cotton bud. White Tipex is changed into contrasting black by
the multiple reflections of the parallel reflecting mirror surfaces
when collimated, just like an Easy Tester - a metal peephole cap with
its inner face highly polished.
shows the unmagnified alignment achieved by rotation only on the 8”
Wise-Newtonian after using a combination of Mk.1 eyeballing of
perimeters, a plumb line, protractor and spirit level use and a laser
collimator beam reflecting straight back up itself, to be 2 mm
is easy were a disc on a rod rotates the secondary mirror. Secondly,
if the front basket or main tube bolt holes aren’t oval, you
can file them into shape. The 3rd choice is to loosen the centre
bolt half a turn, grasp the secondary and turn it. For Newtonian
telescopes fitted with a front corrector glass, ring keys can be made
from two drill bit ends and hardwood. Oval holes
easier with a decent instrument but no rod, which includes cheaper
ones if the manufacturer has been around for many years, selling
through retail astronomy shops.
shows the alignment achieved after correcting error .
Unmagnified it looks spot on and is very nice to see, but needs to be
only somewhere on the vertical axis at this stage.
Now it is worth correcting
remaining errors using a slightly defocused 2nd
star at a
magnification of at least 120 x. You usually need to ease the centre
bolt ¼ turn then slew, using absolutely the right screwdriver
bit or Allen key, then remove:
 Any lop side oval
appearance by slewing with adjuster bolts 2
and 3. Shunting hard out to the mounting plate first, then easing
back very equally can make quick work of this stage.
 Vertical or
horizontal ovals by elevating or depressing
bolt 1 and central.
Shunt in or out if  just causes flipping or only reduces
remaining ovals to ‘splashy’.
Proceed to primary mirror adjustment
using a star at 200 to
450 x to complete collimation. This occurs only when the
concentricity of de-focussed rings appearance is maintained all the
way in to focus by fine adjustment of the primary bolts that achieve
Full optical alignment has been achieved. With
reflectors shorter than f 6 there should be a
displacement between the peephole reflection and primary marker along
the vertical axis.
Please see the tabular Figure 4 for results
Cave Collimator System ends secondary mirror collimating difficulties
unless defamatory adjectives still describe the slightly defocused
star. Try shunting the primary boldly and if ineffective inform the
months into the invention and borrowing a friend’s Cheshire
Eyepiece and sight tube combination, I found it works in the same
manner but no better. In its instructions I came across rotation only
for the second time in 5 years of using Newtonians only in the last 6
months. Funny, but the manufacturer, Celestron, doesn’t mention
rotation in their instructions for recent Newtonian telescopes. They
state a hope that what is said will suffice. All set to change to
the translucent CFC and include the special peephole!
At f 6
the Cheshire shows an offset of 0.5mm as clearly as through my
invention, but at f 4.4 its peephole surround,
doughnut and cross hairs combine into one large unclear black blob.
With my £2 invention, and for a new telescope next to nothing,
clarity of each of the parts is fully maintained at f 4.4.
Difficulty with collimating ‘fast’ ‘scopes had been
news to me until the day of the Cheshire. I reckon it’s fixing
methods and weight and inertia of secondary mirrors more than f
ratios that are in charge here. Very useful to have around is
bottle of Capt. Tolly’s Creeping Crack Cure, but don’t
use sufficient to make things airtight.
using front glass corrector optics can consider reducing the area of
material used for clamping glass firmly. However, because error 
can be so easily forgotten about and in order for this suggestion to
be a good move, I recommend instructions for the use of both be kept
to correcting for rotation to highlight the wisdom of regular checks
to avoid, ‘ Surprise, surprise.’ Exception to rule is
when separation on the vertical axis looks much more than you are
likely to end up with for any offset needed which correction 
Why persist with bench methods when sub arc
collimating with a star can be achieved more easily than by methods
that after error  are no advance on Newton and there’s no
need? The benefit of magnification is more accessible now,
so use it. Get ready around sunset with a
and select a star. Estimate the altitude and azimuth then start
aiming before seeing.
Correcting error  without the Collimating
Front Cover may look
good enough. But optical illusion is a risk and may result in
correcting error  still with up to 1 mm of error
to be made
to do with by corkscrewing with bolts 2 and 3. And going by the
results from just 2 mm adrift, it is well worth making the cover or
having a translucent white inset mask cut at a plastics fabrications
works for £2 using a jig saw blade for wood. Remember the
If I were
to try doing without one of the three components it would be the
primary marker, but for assurance, I cannot imagine anyone not
eventually putting one on as prescribed.
the telescope along the car with the focus tube vertical can stop
rotation whilst travelling, as can smooth revolving on equatorial
mounts. Advantage dismantled Dobsonians.
arrival, or at any collimation check, an Easy Tester is the tool of
choice especially if there’s a grub screw for fine adjustment
of the eyepiece clamp. I have a spare one going for £20. These
last twelve months are about to become financially worthwhile!
© Peter R. Clark
England. Withdrawn patent application advertised
South Cave HU15 2DZ
East Yorks, England.
Figure 1. The Collimating Front Cover for new
telescopes, or as an inset
Mask for existing ones.
Figure 2. Primary centre doughnut displaced to
the right at f 6
and 1200 mm.
Figure 2A. Reason for the sideways displacement
and the arrow showing the way
to correct it.
Figure 3. Alignment of the three gunsight like
aids after rotation indicated
by Figure 2A.
Figure 4. Radial representation of gunsight error.
Figure 5. More Figure 2’s and 3’s, including
the one for f 4.4 and 500 mm.
Table 1. Log table list recording the system
checking several telescopes
Logbook of several telescopes checked with the invention.
whilst oblivious to rotation
after error 1 and all errors corrected
and limiting magnitude
8” Wise-Newtonian f/6
Ovals, distinct false
binary at 2 ½”
ß Delphi 0.6” split.
72 Peg 0.53” split. Confirm ALI not 952
4 ½” Bresser ‘Pluto’
Flaring of ξ Cephei
‘A’. ‘B’ OK.
4.5 6.4 7.9”
* in fine field |
Very concentric and
12” Dobsonian f/6
Large stars splashy.
Splitting OK. Round Airys
Shunting was the
20” Dobsonian f/4.1
No problems f/5 †
Owner’s claims agreed
4 ¼” Astroscan †
No problems f/4
8” gifted f/6
double images. Optrons.com
Return to dealer.
† = CFC on or improvised
PH-PC is the Peephole to primary separation
‘New Perspectives on Collimation’, p. 1-16, instrument
preparation. From the BAA.
‘Collimation and the Newtonian,’ Pensack.
www, is straight into rotation emphasis.
Norton’s Star Atlas 2004, p.38 is
clear on shunting.
‘Testing Optics’. www Cloudy Nights. For
1. Set up for checking rotation showing the special
peephole caps and front covers.
2. The Collimating Front Cover made for the