ANTIQUE TULLEY & SONS
5-FOOT ACHROMATIC
Chris Lord
"Tulley & Sons, Islington, London c1830"
Tulley & Sons, 5-foot achromatic refractor 3.75-inch f/16 astronomical achromatic refractor on German
Equatorial. 11-inch finder, rack & pinion focuser with RAS drawtube, and five Huyghenian eyepieces with
Sun & Moon eyecaps, in mahogany case. Mahogany tripod.
TULLEY & SONS 5-FOOT ACHROMATIC
This undocumented Tulley is a very early example of an English telescope maker mounting an astronomical
achromatic refractor on a German style Equatorial. The mount design is taken from the existing Dollond
altazimuthal, Smeaton's block & the Tulley universal equatorial, and retains features from both, with
wormwheels close to the cg, and the Declination wormwheel in line with the polar axis. It maybe a German
style equatorial, but it is an English interpretation and execution. Fraunhofer's equatorial had wormwheels at
the ends of each axle. Unlike Tulley's universal equatorial in which the elevation of the polar axis can be
adjusted to the observer's latitude, the polar axis of this mounting is fixed at 51º.5, the latitude of London.
Charles Tulley (1761-1830) and his sons William (1789-1835) & Thomas (1791-1846) traded as Tulley & Sons
1826-1830. Prior to 1824 Charles Tulley signed his achromatics on the rackmount flange. Charles Tulley may
have been apprenticed to Peter Dollond or Benjamin Martin. His business flourished from 1799 - 1824
according to British Science Museum records. Grace's Guide maintain Charles Tulley bought Benjamin Martin's
business in 1782 after he was declared bankrupt, which I think unlikely as he would have only just served his
apprenticeship. King in his "History of the Telescope" p192, suggests he set up in business between 1775 and
1785. Tulley was born in 1761, and to whomsoever he was apprenticed, he would not have completed that
apprenticeship until he was 21, in 1783. It is possible he bought Martin's bankrupt business, but I can find no
record. Benjamin Martin's son, Joshua Lover Martin, patented a machine for drawing and plating brass and
copper tubes in 1782, the year of his father's death. The patent was not enforced, and Ramsden is known to
have bought such a machine. (See Anita McConnell, "Jesse Ramsden (1735-1800) London's Leading Scientific
Instrument Maker"). Charles Tulley reputedly purchased the late Benjamin Martin's tools in 1784.
Based upon the signature on the telescope tube, this particular 5-foot achromatic was probably made between
1826 & 1830. OG 3.8 inches clear aperture, 60-inches focal length.
OG cell front
OG cell rear
The objective is an air-spaced contact aplanatic achromatic doublet, a variation of the designs that Charles
Tulley made from 1799 until his demise. The front element is blown window crown with a pale green caste,
caused by iron contamination in the melt. It is free of bubbles. The rear element is French (Guinand) flint,
water white, with a few small bubbles. Close examination of the doublet in it's cell revealed no spacers. A
polariscope examination revealed no internal strains or straie. The doublet is held in it's cell by a push fit brass
ring that cannot be withdrawn. The cell screws into the telescope tube, the thread is 4".120 OD x 16TPI Whit.
There is no collimation adjustment.
Using a Fob Watch spherometer the first surface has a power of 1.5D, the fourth surface 0.0D. There is a
disparity between the surface radii provided by Oderic Vital (Mr. W. Bradbury) in his article published in the
1882 English Mechanic, scaled for a 60-inch focal length crown-flint doublet. The lens curves have been bent to
bring r4 flat whilst making r2=r3. There is no doubt as to the authenticity of this lens. The telescope, originally
owned by Lord John Wrottesley, was acquired at an auction of property remaining within Wrottesley Hall,
Tettenhall, Staffordshire. The implication of my measurements is Charles Tulley, during the quarter of a century
or so he was making achromatic doublets, had developed the ability to "bend" a doublet.
scale drawing of Tulley's lens bendings
(Bradbury, W., E.M. July 7,1882, No. 902, p393) refers to Tulley's table of curves having been published in the
letters section some time previously, but he does not provide a specific reference. He states Tulley used
"Ratcliffe Crown", which had a refractive index 1.528. The Guinand flint had a refractive index 1.5735. Ratio of
refraction, 1.74, Dispersive ratio, 0.65. Bradbury stresses that Tulley's ratio of refraction does not mean the
ratio of refractive powers of the glasses, but the ratio of the differences of the red and blue indices of each
glass. I have tabulated the curves of four doublets made by Peter Dollond & Charles Tulley and for modern
comparison a BK7 - F2 Fraunhofer doublet.
r1 = 37".313
r1 = 26".474
r2 = -21".191
r2 = -13".870
r3 = -21".598
r3 = -13".870
from spherometer measurements of Tulley lens c1830
r1 = 19".5
r2 = -16".45
r3 = -24".55
§ Based on South's Herschel aplanat - 1824.
r1 = 15".2
r2 = -22".8
r3 = -20".86
abstracted from Bradbury's English Mechanic article, 1882
r1 = 16".8
r2 = -25".2
r3 = -22".57
r4 = -88".070
r4 = ∞
Fraunhofer BK7-F2 doublet aplanat
Tulley's new "bent" aplanat - no air gap
r4 = 29".18
Tulley's original aplanat - edge contact §
r4 = 41".72
Tulley's early achromat - edge contact
r4 = 55".846
Peter Dollond's achromat- edge contact*
* Dollond's Table of Curves, "Smith's Mechanic", 1825 - Ratcliff No.4 crown; Ravenscroft flint nD=1.576 nF=1.586 nC=1.5718.
ref: 'New Light on the Invention of the Achromatic Telescope Objective'; Notes and Records of the Royal Society of London, Vol.50, No.2, pp195-210.
Referring to the drawing, scaled to the same element thickness and effective focal length, the changes to
Tulley's "bendings" may appear trivial, but the implications are profound in our understanding of Tulley's
insight. His early achromat is not aplanatic, although nearly so. It is a development of Peter Dollond's and
Benjamin Martin's prescriptions in which the ratio of the crown curves are 3:2 & the flint 2:1, spherically
undercorrected, and requiring local "figuring" to improve definition. Neither Peter Dollond nor Benjamin Martin
solved the problem of arriving at a practical rule for calculating curves by which both dispersion and spherical
error could be corrected for both parallel and virgent rays. Tulley did, and in quite an ingenious way. His
method is fully described by Bradbury in the E.M. article cited. (see appendix)
It has been presumed until now that Charles Tulley produced some of the best achromatic doublets of the early
C19th largely by trial and error, making numerous crown and flint components, and trying one against the
other until a satisfactory combination was arrived at. Whereas this might be so in attempting to correct
astigmatism due to poor annealing or inhomogeneity in the glasses, it cannot be inferred from the practice that
Tulley was unable to compute a lens bending, otherwise the lens in the telescope that I have acquired, would
not exist.
The Tulley's took their lens making methods with them to their graves, nothing of any detailed methodology
was revealed throughout their lifetimes other than the odd snippet of correspondence. However Fraunhofer, his
German competitor, cannot be said to have been any less secretive, and so the assertion Fraunhofer was the
first optician to make an aplanatic achromat is open to question.
J.F.W. Herschel published an article in the Royal Society's Phil. Trans., XVII, March 22, 1821 pp222-267, "On the
aberrations of compound lenses and object glasses." with the intention of providing practical tables that
artisans like Charles Tulley could use. Tulley did make a lens to Herschel's prescription for Sir James South, who
was delighted with it, but there is no evidence Tulley subsequently followed John Herschel's table of curves.
(The Herschel achromat is a class of stigmatic aplanat, it does not obey the sine condition and is therefore not
strictly aplanatic in the Abbé sense - see appendix). An article published in the JHA, XII, 1982, pp206-8, by J.A.
Bennet, "The First Aplanatic Object Glass", describes the recovery of the 3.25-inch, 45-inch focal length
refractor made by Charles Tulley for Sir James South. The telescope was made in 1822, and according to
Bennet the telescope is signed 'TULLEY & Sons, Islington, London'. I have dated my 5-foot Tulley based on
Webster's trade directory of scientific instrument makers. But it could have been made earlier, although given
the German style mounting not before 1824. Nevertheless we are faced with the implication of this Tulley lens
having a flat fourth surface, and aberration corrections similar to Fraunhofer's.
It is far more difficult to figure the fourth surface flat, but Newton's fringes, produced when comparing the
polished surface to a test flat are more definitive. Tooling is also simpler since r2 = r3, and Newton's rings may
be used when testing the second against the third surface.
The little we do know about Charles Tulley's early working methods comes second hand via Bradbury. I have
worked through the calculation to derive the curves for Tulley's early achromat, based on a crown with curve
ratio 2:3, & flint with curve ratio 2:1, using Ratcliffe No. 4 crown, index 1.528 and Guinand No.5 light flint,
index 1.5735, ratio of refractive powers 1.524, ratio of dispersive powers 0.7127. The calculation for those
interested is in the appendix. For an OG focal length 60 inches:
r1 = 15".169
r2 = -22".754
r3 = -20.807
r4 = 41"614
which compares closely with the extrapolated values extracted from Tulley's table published in the English
Mechanic. I have followed the method described by Bradbury as being that used by Charles Tulley. However my
solutions were obtained using an electronic pocket calculator, not log tables and long hand arithmetic. One can
only surmise the amount of time and effort expended on this tedious algebraic calculation, and the even more
tedious process of bending the lens subsequently. One can only admire the man's pertinacity.
Bradbury does not provide the refractive indices at the F & C lines, so I entered the equations for the ratio of
refractive and dispersive powers and ratio of refractions, as defined in Bradbury's E.M. article, into a
spreadsheet. The nature of Tulley's equations are easy enough to understand. The ratio of refractive powers,
meaning the ratio of the differences of the red and blue indices of each glass, is the ratio of the length of the
spectra produced by identical apex angle crown and flint prisms. The ratio of the dispersive powers is that of
the constringences, i.e. the ratio of the reciprocals of the crown and flint glass Abbé indices. The ratio of
refractions is that of the crown and flint D-line indices. The use Tulley makes of this ratio for calculating the
curves of the flint to counter the aberrations of the crown, as a multiplying factor raised to the power five and
rooted can only be some rule of thumb process. Bradbury remarks, "However correct it might be for the kinds
of glass used by Tulley, it is by no means certain that it would be equally so for modern glass."
The glasses Tulley was constrained to use do not exist nowadays. The closest I could find to Ratcliff No. 4 crown
is Schott N-K5, index 1.52249 Abbé index 59.48 (compared to 1.528), and to Guinand No. 5 flint Schott N-F2,
index 1.62004 Abbé index 38.87 (compared to 1.5735 - there is no glass in the Schott catalogue with an Abbé
index close to 39, with such a low D-line index).
I decided to factor the dispersions for Ratcliff No.4 & Guinand No.5 based on N-K5 & N-F2, i.e.
Crown Ratcliff No.4
F-line index 1.534178
D-line index 1.528
C-line index 1.525302
Flint Guinand No.5
F-line index 1.583068
D-line index 1.5735
C-line index 1.569542
the values of which gave ratio of refractive powers 1.524, ratio of dispersive powers 0.7128. Not knowing the
actual F and C-line indices makes it difficult to precisely reproduce Tulley's values.
The values I have derived give constringences:
Crown: 0.016811 (Abbé index 59.49 - compared to 60)
Flint: 0.023585 (Abbé index 42.40 - compared to 36)
Of course the ratio of dispersive powers is the ratio of the Abbé indices and if the Ratcliff No.4 crown had an
Abbé index ~60, then the implication is Guinand flint No.5 had an Abbé index ~43. A lens made to these values
would be spherically overcorrected and require local figuring to the third surface, assuming the fourth surface
remains flat.
I decided to ray trace the lens based on the "new" bent aplanatic prescription. The paraxial and marginal rays
were traced using the method described by James H. Wyld in Albert G. Ingalls', 'Amateur Telescope Making',
Vol.2 Ch.A.6, pp139-151-1996 edition. The equations were entered into a spreadsheet, at first on the
assumption r2=r3, r4 = ∞ & no air gap. The nominal values gave a slight spherical overcorrection, chromatic
correction within tolerance, but an unacceptable difference between the F-line & C-line focii (F-C=213 microinches - Rayleigh limit 11 micro-inches). Separating the lens edges by 30thou and deepening the third surface
by 37 wavelengths of yellow light left the spherical error within tolerance (Conrady criterion) whilst bringing
the F-line & C-line focii into coincidence, equidistant from the D-line focus (F-D=C-D=0.039801). Longitudinal
chromatic error was well within tolerance (0.067%fD), the paraxial correction being 1/16 wave at F-line and
1/10 wave at C-line and the marginal correction 1/11 wave at F and C-lines. Performance off axis deteriorates
noticeably, spherical correction is maintained to 0º.25, and is overcorrected at 0º.5. Longitudinal chromatic
correction and coma correction remain within tolerance. Correction at the 1/√2 zone is also excellent (0.071%
fD D-C & 0.055%fD D-F).
In order for this Tulley lens to perform as well as it does (by modern standards), Tulley would have had to
perform his final figuring to the third surface, testing it on fine print within his workshop, and finally star
testing. Offence against the sine condition was within tolerance for the D & C-lines but not the F-line. The
design is ostensibly coma free. The spreadsheet may be downloaded off my logbook page.
Using the spreadsheet I was able to compare the performance of the Tulley lens in my telescope, to a
hypothetical modern Fraunhofer, prescriptions taken for 3.8-inch f/16.
r1 = 37".313
r1 = 26".474
r2 = -21".191
r2 = -13".870
FRAUNHOFER BK7-F2
r3 = -21".598
r3 = -13".870
LA'
LA'
LA'
OSC'
OSC'
OSC'
D-line
F-line
C-line
D-line
F-line
C-line
F-C
D-F
D-C
SECONDARY SPECTRUM
F/∆F
CORRECTION TO THIRD SURFACE (WAVELENGTHS
TULLEY'S NEW "BENT" APLANAT LA'
D-line
LA'
F-line
LA'
C-line
OSC'
D-line
OSC'
F-line
OSC'
C-line
F-C
D-F
D-C
SECONDARY SPECTRUM
F/∆F
CORRECTION TO THIRD SURFACE (WAVELENGTHS
r4 = -88".070
r4 = ∞
=-0.03208073617
=-0.02565262257
=-0.03439812188
=-0.00043562216
=-0.00041041429
=-0.00044410538
= 0.00000000002
=-0.03070011404
=-0.03070011402
=-1966
D-line) = -0.58
=-0.07742196052
=-0.09125296052
=-0.07249032206
= 0.0006430167
= 0.00671146537
= 0.00063296394
= 0.0000000015
=-0.03980135992
=-0.03980135842
=-1506
D-line) = 37
Fraunhofer BK7-F2 doublet aplanat
Tulley's new "bent" aplanat - no air gap
TOL=0.0935994097
TOL=0.0025
fD%=-0.0511351419
fD%=-0.05113514186
TOL=0.09195379934
TOL=0.0025
fD%=-0.06696608174
fD%=-0.06696607922
LA' = longitudinal spherical aberration
OSC' = offence against the sine condition (coma correction)
F-C; D-F; D-C are focal length differences assuming the image is focused at the D-line
fD% = percentage chromatic error (Rayleigh criterion = 0.05%)
Compared to a modern BK7-F2 Fraunhofer aplanatic doublet the Tulley bending stacks up well, the slightly
lower performance being due to the lower partial dispersions of Guinand No.5 & Schott F2 flint glass types. But
it would have been far less delicate a task for Tulley to figure the third surface of his lens than that required to
figure the third surface of the Fraunhofer, being only just over half a wave.
Optical glass production in England at the time this objective was made was unsatisfactory. Both optical crown
& flint was generally available only in small sized pieces. A Swiss bell founder named Pierre Louis Guinand had
between 1784-90 developed a method of stirring the melt with fireclay rods. The fireclay could withstand the
heat. More importantly it was porous and absorbed gas bubbles in the melt. By continually stirring as the melt
began to cool and become more viscous, it was possible to keep the mass thoroughly mixed. The glass, once
annealed was largely free of bubbles, threads of metals termed straie, and was uniformly transparent and
homogenous, i.e. of uniform refractive index. England at this time did not possess such technology. Guinand,
worked with Fraunhofer & his business partner Utzschneider between 1807-14, and his processes remained an
industrial secret. In England flint blocks suitable for cutting out a blank bigger than 4-inches were rare.
Guinand on the other hand could produce flint blanks up to 12-inches. English achromats used either plate
glass or window crowns for the front element. Charles Tulley had obtained a stock of French flint glass in 1821
and in 1822 a large Guinand blank from which he made a 6.8-inch lens.
Out of curiousity I ran the data on a hypothetical 3".8 f/16 edge contact Dollond doublet:
DOLLOND DOUBLET
LA'
LA'
LA'
OSC'
OSC'
OSC'
D-line
F-line
C-line
D-line
F-line
C-line
F-C
D-F
D-C
= 0.03772578548
= 0.0258832746
= 0.04251728106
= 0.00115822274
= 0.00119139686
= 0.00114509748
=-0.00000000005
=-0.02588355508
=-0.02588355513
SECONDARY SPECTRUM
F/∆F
=-2310
CORRECTION TO FOURTH SURFACE (WAVELENGTHS D-line) = 26553
TOL=0.09223410011
TOL=0.0025
fD%=-0.04351056009
fD%=-0.04351056017
Dollond did not make f/16 doublets, rather f/20-f/30. The reason I ran a ray trace was to compare a lens made
from Ratcliff crown and Ravenscroft flint with my Tulley lens. Theoretically this lens would be excellent. The
snag would be working the fourth surface a process that according to King's comments on Fraunhofer's
examination of a Dollond doublet led to it being spherically over-corrected. Compare the LA', OSC' & fD%
values to the modern Fraunhofer.
It is claimed Fraunhofer's objectives were superior to those of English telescope makers. Certainly comparing a
modern BK7-F2 Fraunhofer doublet to my Tulley OG supports the claim. But how would a Fraunhofer objective
stack up at the time given the limitation of crown and flint glasses available to Fraunhofer? The table below is
abstracted from Potter, "An Elementary Treatise on Optics", Vol.1, p167, 1851, as being the optical glasses
made by Fraunhofer after Guinand had left the firm's employ.
Fraunhofer's glass types:
Crown
B
No.13
1.524312
No.9
1.525832
M
1.554774
Flint
No.3
1.602042
No.30
1.623570
No.23
1.626564
No.13
1.627749
C
D
1.525299 1.527982
1.526849 1.529587
1.555935 1.559075
E
1.531372
1.533005
1.563150
F
1.534337
1.536052
1.566741
G
1.539908
1.541657
1.573535
H
1.544684
1.546566
1.579470
Vd
58.42
57.55
51.74
1.603800
1.625477
1.628451
1.629681
1.614532
1.637356
1.640544
1.642024
1.620042
1.643466
1.646780
1.648260
1.630772
1.655406
1.658849
1.660285
1.640373
1.666072
1.669680
1.671062
37.46
35.05
34.57
34.18
1.608494
1.630585
1.633666
1.635036
The older crown and flint glass, No.13 & No.3 are similar to the glass available to Charles Tulley.
r1 = 21".829
r1 = 26".474
r2 = -23".881
r2 = -13".870
FRAUNHOFER
CROWN No.13 FLINT No.3
r3 = -20".948
r3 = -13".870
LA'
LA'
LA'
OSC'
OSC'
OSC'
D-line
F-line
C-line
D-line
F-line
C-line
F-C
D-F
D-C
SECONDARY SPECTRUM
F/∆F
CORRECTION TO THIRD SURFACE (WAVELENGTHS
TULLEY'S NEW "BENT" APLANAT LA'
D-line
LA'
F-line
LA'
C-line
OSC'
D-line
OSC'
F-line
OSC'
C-line
F-C
D-F
D-C
SECONDARY SPECTRUM
F/∆F
CORRECTION TO THIRD SURFACE (WAVELENGTHS
r4 = 1025".51
r4 = ∞
= 0.12222360363
= 0.13779138001
= 0.11629743034
=-0.00126615415
=-0.00121454944
=-0.00128523456
=-0.0000000003
=-0.02236310803
=-0.02236310833
=-2668
D-line) = 1213
=-0.07742196052
=-0.09125296052
=-0.07249032206
= 0.0006430167
= 0.00671146537
= 0.00063296394
= 0.0000000015
=-0.03980135992
=-0.03980135842
=-1506
D-line) = 37
Fraunhofer c1812 doublet aplanat
Tulley's new "bent" aplanat - no air gap
TOL=0.09214202563
TOL=0.0025
fD%=-0.03764951979
fD%=-0.0376495203
TOL=0.09195379934
TOL=0.0025
fD%=-0.06696608174
fD%=-0.06696607922
A Fraunhofer doublet made from his early crown and Guinand flint glass performs less well than the Tulley lens
in the telescope in my collection. It is within the OSC' & fD% but outside marginal LA' tolerances. Fraunhofer's
earliest doublet, and Tulley's both meet the sine condition. Later, No.13 Crown combined with No.13 Flint,
enabled a markedly improved performance.
r1 = 34.177
r2 = -21.360
FRAUNHOFER
CROWN No.13 FLINT No.13
r3 = -21.360
LA'
LA'
LA'
OSC'
OSC'
OSC'
r4 = -101.494
Fraunhofer c1827 doublet aplanat
D-line
F-line
C-line
D-line
F-line
C-line
F-C
D-F
D-C
=-0.02997183061
=-0.02043651305
=-0.03361046194
=-0.0015417853
=-0.00148671654
=-0.0015621508
= 0.00000000003
=-0.02129714891
=-0.02129714888
SECONDARY SPECTRUM
F/∆F
=-2809
CORRECTION TO THIRD SURFACE (WAVELENGTHS D-line) = 980
TOL=0.09232687642
TOL=0.0025
fD%=-0.03576766767
fD%=-0.03576766761
A coma corrected object glass has the property u1sinU'4=u'4sinU1 (following Conrady's nomenclature) where u
& u' are paraxial ray angles in radians and U & U' marginal ray angles in degrees. The prime signifies the
emergent ray, in this case from the fourth (last) surface, and the integer suffix the surface number, 1 being the
first. For a 3.8-inch doublet, 60 inches focal length, and field angle 0º.5:
D-line correction for OSC'
Fraunhofer BK7-F2
u1sinU'4 = 0.0002646279
u'4sinU1 = 0.00055595204 ratio 2.1:1 being close to the Herschel condition.
Fraunhofer CROWN No.13 FLINT No.3 c1812
u1sinU'4 = 0.00026638083
u'4sinU1 = 0.00026727514 ratio 1.0034:1 i.e. corrected for the sine condition.
Fraunhofer CROWN No.13 FLINT No.13 c1827
u1sinU'4 = 0.00026637117
u'4sinU1 = 0.000266651 ratio 1.0011:1 i.e. corrected for the sine condition.
Tulley lens in the telescope in my collection:
u1sinU'4 = 0.00026675316
u'4sinU1 = 0.00026693394 ratio 1.0007:1, i.e. corrected for the sine condition.
Tulley early lens
u1sinU'4 = 0.00026580125
u'4sinU1 = 0.00026483739 ratio 1.0036:1, i.e. corrected for the sine condition.
Dollond
u1sinU'4 = 0.00026615176
u'4sinU1 = 0.00026568644 ratio 1.0018:1, i.e. corrected for the sine condition.
Coming across a Tulley OG with a flat rear surface has ramifications as to who first made an aplanatic doublet. I
regard my finding remarkable. Hitherto there has been no evidence Charles Tulley produced such a lens.
HOUR ANGLE & DECLINATION SLOW MOTIONS & SETTING CIRCLES
The Declination & Hour Angle slow motion wormwheels are both 5".75 OD x 10TPI straight cut, and the worms
0".75OD x 10TPI Whitworth screw cut thread form, probably adopted from Maudslay. The HA setting circle is
engraved in 8 minute intervals, 2 hours corresponding to 15 teeth. The DEC setting circle is engraved in 2º
intervals, 10º corresponding to 5 teeth. The divisions on the HA are worn down due to excessive polishing. The
wheels and dividing would probably have been done on a Ramsden style screw cutting treadle lathe, as
described in "Making Scientific Instruments in the Industrial Revolution" by A.D. Morrison-Low, 2007,
pp188-191.
Each worm is dog-clutched, and free to swing away from the wormwheel when a taper peg is withdrawn. When
the worm is engaged by pushing the taper peg fully home, pressure can be applied ensuring engagement is
free of backlash, an ingenious arrangement.
The HA slow motion maybe controlled using a wooden rod connected to the worm via a Hooke's joint, that can
be turned by the observer at the eyepiece. The Dec slow motion is controlled by an indexed hand wheel divided
into 10 intervals, each interval corresponding to a 1/10 of 2º or 12arcmin. There is no Dec control rod, the
telescope has to be pointed using either the 11-inch finder, or by Dec & HA, on the presumption once the object
is centred in the eyepiece, no Dec adjustment is necessary.
The setting circle indexes are fairly crude, merely brass pointers. There are no axle clamps. In order to fit the
telescope onto the saddle plate, the wormwheels have to be engaged with the Dec axle directed to the north
meridian.
The brasswork of the mount comprises stamped plate bearing surface imperfections typical of the process and
severe pitting corrosion caused by atmospheric pollution. In all likelihood the head and wheels would have been
subcontracted to a clockmaker, to be assembled by the Tulley's in their Islington workshop at 4 Terret's Court,
Islington. Pearson describes the Tulley Universal Equatorial in his "An Introduction to Practical Astronomy" Vol.
2. 1829, so this mount may have been a prototype.
The tube, 3".846 OD, is split into two parts by a 3".5 OD x 10TPI Whit screw thread; the OG end, and the rack
end with the finder and steady rods, each 31".25 long, combined length 62".5, and the pivot point 22" forward
of the rackmount flange.. The tube has to be screwed together, and dropped onto the saddle plate. A pair of
screws in the rack end tube section have to be allowed to drop through the saddle plate, and the tube fastened
with terminals. Because the assembled tube is heavy (26lbsf) and the stand tall (6') this is an awkward
procedure for one person.
FIELD ILLUMINATOR
tube aperture cover slide
tube aperture
perpendicular view of tube aperture & diaphragm
oblique view of tube aperture & diaphragm
0
The diaphragm is elliptical and inclined at 45º with a 2-inch minor axis perforation such that it presents a 2-inch
aperture to the incoming light path. The diaphragm is made of sheet brass, and at some stage has been polished.
Pearson describes it has having been gilded matt gold, traces of which remain.
An unusual feature of the telescope is an aperture in the side exposed by sliding a curved cover plate around
the tube. Within this section of the tube is an internal diaphragm of polished brass tilted at 45º, facing the
eyepiece. The diaphragm has an elliptical hole 2" minor axis, cut into it, the shape being such that when viewed
from the OG it appears circular.
This feature is described in "An Introduction to Practical Astronomy" Vol. 2, 1829, by Rev. William Pearson,
pp125-126, and Plates IV & V, as 'Mr. Tulley's New Illuminator'. An accessory missing from the case, is a thin
frame that clipped into the hole and supported a gimballed Argand lamp. Lamplight was reflected off the matt
gilded brass diaphragm and shone into the eyepiece. Its purpose was to illuminate the field so micrometer
wires could be seen in silhouette, when making PA & Sep measures of double stars. This appears to be an
original feature of the tube and not a home made addition. I can find no examples of this feature on any Tulley
held in museum collections, and only Pearson refers to it. It is not mentioned by William Kitchiner in his,
"Economy of the Eyes" Part II, 1825, or Thomas Dick in his, "The Practical Astronomer" Part II, 1856. Pearson
attributes it to the "Younger Tulley" meaning William. I have photographed pages 122-126 from the copy of
Pearson's tome held at Chetham's Library, Manchester, and placed them for reference on my logbook page as a
zip archive. (©CUP)
DESIGN COMMENTS
The mounting by later C19th engineering standards is crude, it displays a limited understanding of dynamics.
The bearing surfaces are non-existent, the saddle plate is too far overhung. The axles are too thin, allowing
excessive flexure. Only the stabilising rods keep the telescope tube held stiffly. The setting circles, being
engraved on the face of the wormwheels, are awkward to read. However it has to be born in mind that the
craftsmen who designed and built this telescope worked in a tradition established by the Dollonds, Ramsden &
the Adams'. The refractor, although large by the standards of the first quarter of the C19th, was intended to be
portable. The use of telescopic steady rods was a common feature of portable telescopes at this time. William
Simms describes a similar mounting in his work, "The Achromatic Telescope" published in 1852, and Watson &
Sons were still using a steady rod on their "Century" telescope almost a hundred years later.
Fraunhöfer's equatorial was a radical departure from the designs of English telescope makers. In use it is
evident the Tulley's did not appreciate that in order for a German Equatorial to operate as intended the
telescope tube has to be balanced about both the polar and Declination axles.
The incorporation of steady rods and impact bar, a feature of the altazimuthal mounted achromatics of the late
C18th & early C19th indicates a faltering evolution of thought as to how best to give the telescope equatorial
motions to facilitate following a celestial object in order to make a detailed observation. Steady rods only
function correctly when they are either in tension or compression, and the spring collars are set just so.
Altazimuthal achromatics I have used are OG end heavy, placing the steady rods in tension. If the telescope
were in balance about the altitude axis, and the spring collars set so as to barely resist extension or
contraction, the slightest breeze would shift the telescope. Hence this telescope was mounted deliberately out
of balance about the Declination axis. The field illuminator would have been fastened onto the rack end of the
tube behind the pivot point, so the cg would have been slightly further forward, but the Argand lamp would
have had to weigh 10lbsf to bring the tube into balance, and the engraving in Pearson's book depicts a
lightweight affair, so the tube must have been left intentionally OG end heavy.
ENGINEERING DETAILS
The Dec axle terminates in a 5/8-11 screw thread based on the Maudslay screw form later adopted by
Whitworth. The mount did not come into my hands with the original c'wt, and I had to fabricate a c'wt system
weighing 24lbsf in order to balance the telescope about the polar axle.
The longitudinal tube cg was out by 5.25-inches making the tube OG end heavy by 124 in-lbs. There appeared
to be no provision as such for a tube c'wt, and I was left with the impression the imbalance was to be
accommodated by the pair of telescopic steady rods stowed alongside the rack end of the tube. These could be
extended and connected to brackets affixed to each tripod leg. However the action of the steady rods interfered
with the action of the RA & DEC slow motions because their frictional resistances reacted in different and
varying planes to the fixed polar and equatorial planes of the mount.
I decided it would be best to bring the telescope tube into balance, but there was no obvious feature on the
rack end of the tube intended to hold a c'wt and I did not wish to alter the appearance of the telescope.
Telescopes of this era with radial rack & pinion focusers have an empty space between the pinion location plate
and the tube end flange. Whatever I did to the tube to bring it into balance about the Dec axle, had to be
undoable. I did not wish to permanently modify the telescope. This cavity would be ideal for filling with lead
shot. But the rack & pinion mechanism would have to be partitioned to prevent lead shot damaging the rack. I
cut out cheek pieces from foamboard and araldited them to the inside of the brass tube. These were a snug fit
against the racktube. Once the remaining cavity was filled with lead shot an annular capping piece was offered
against the upper end of the cheek pieces. This would seal off the rack & pinion from the rest of the cavity and
prevent any shot reaching it.
The cavity was 3.5-inches ID by 2.5 inches deep, the racktube 1.55-inches OD, hence the net volume of the
cavity was 24.6 cubic inches. The section screened off was an arc of 30º or 1/12 the volume, leaving 22.6 cubic
inches. I calculated the packing volume of lead shot to be nearly 2:1, and cast lead weighs 0.41 lbs/cu.in. The
ballast would weight 4.6lbsf. Using a spring balance pulling the tube down at the rack flange between 4 & 5 lbsf
was need to counterbalance the OG turning moment.
My thinking was, once the telescope was in balance about both axles, the steady rods would not be necessary,
except perhaps in a stiff breeze. The telescope could then be adjusted to follow in RA using the slow motion
rod, without the reaction of the steady rods trying to force the tube in a different direction.
Filling the cavity with lead shot was awkward. It proved difficult to prevent a few balls of shot getting into the
rack & pinion mechanism. I eventually succeeded by maintaining pressure on the racktube against the cheek
pieces whilst pouring in the shot. The ends of the cheek pieces I sealed with double sided tape. Even so, once
the capping piece was firmly pushed into place, I could feel the rack jam because a ball of shot had found its
way into the rack cavity. The imbalance was however corrected. The next task was to find a sure fire way of
keeping the shot away from the rack & pinion. It occurred to me that if the shot was placed in a zip sealed
polythene bag, it could be carefully manipulated into the cavity. The only alternative would be to cast a lead
weight in the shape of the cavity, which would entail making an accurate mould and the cast weight somehow
located as not to interfere with the rack. This solution seemed to me to present just as many difficulties.
The bag option seemed the simplest, so I tried it, and it worked, although I had to spoon a little of the shot out
to accommodate the zip seal. The ballast bag shifted the cg back 5.75 inches, just 0.5 inches behind the pivot
point, leaving the tube slightly eyepiece end heavy. But the moment was reduced from plus 124 in-lbs to minus
12 in-lbs. When I screwed my brass star diagonal into the d'tube the cg shifted back a further 1/4 inch, leaving
a minus 18 in-lbs moment. Checked for the smooth operation of the rack, repeatedly, and there were no signs
of jamming. The beauty of this solution is that the bag can easily be removed, and one cannot tell it is there
just by looking.
Next task was to polish the steady rods to try and free them off. I used good old Brasso, reassembled them and
finished off with Renaissance Wax. However they were still fairly stiff. The spring collars are tight, which is as it
ought to be, but the friction load would be too great to enable the slow motions to operate effectively so I freed
of the spring plates using a jeweller's screwdriver, to leave the tubes free sliding.
The telescope lacked a dew shield, which I had rolled and silver soldered from a 12-inch square sheet of 20swg
engraver's yellow brass. This weighed 1lb:6ozs, and shifted the cg forward, so the tube became ever so slightly
OG end heavy by 61 in-lbs. A c'wt was made from 1" AF brass hex bar, slotted to fit on the impact bar between
the steady rods. This exerted a moment of 25 in-lbs, leaving the tube slightly OG end heavy. But this allows for
eyepieces and accessories weighing up to 1lb:8ozs.
Hexbar c'wt slotted onto impact bar between steady rods.
Brings the telescope tube into balance about the pivot point.
Dew shield rolled and seam soldered from yellow engraver's brass, 20SWG.
APPENDIX
Extract from William Kitchiner's "The Economy of the Eyes" Part II.
APLANATISM: THE TULLEY, FRAUNHOFER & HERSCHEL ACHROMAT
According to King, it occurred to Herschel that an objective could be so designed that it could be tested on
some object inside the workshop and yet used on distant objects. The resulting so-called Herschel condition
which clearly does not permit the simultaneous correction for coma, i.e. that
requires that
- the Abbé sine condition. Conrady notes that Fraunhofer's prescription departed from the form
demanded by the sine condition in the direction of the Herschel condition, and as Fraunhofer was a particularly
precise and painstaking worker, it seems practically certain that he must have anticipated Herschel's
suggestion; this is the more probable because it is known that Fraunhofer's favourite tests consisted in
examining everyday objects, like pages of print, at comparatively short distances. The empirical study of coma
in microscope objectives was first undertaken in England by Joseph Jackson Lister in the late 1820's, and then
theoretically for both telescope and microscope objectives by Richard Potter in the late 1840's. It was they who
coined the term, coma. See Richard Potter, "An Elementary Treatise on Optics, Vol. 2, Ch.V "On Achromatic
and Aplanatic Combinations", Art.62 pp163-165.
The lens in the Tulley 5-foot achromatic in my collection has a prescription different to that of Herschel's, and
Fraunhofer's, yet meets the same proximate coma correction. Fraunhofer was the first optician to design and
make a coma free achromatic doublet for the Koenigsberg Heliometer, 1824. However the Utzschneider &
Fraunhofer refractors I have seen have object glasses whose prescription anticipated Herschel's. Tulley made a
lens corrected for the Herschel condition in 1822, and the Tulley objective in my possession, although having a
different prescription to Herschel's, is a bending of it. Both Tulley's and Fraunhofer's prescriptions are close to a
fully coma corrected objective meeting the Abbé sine condition, but not truly aplanatic. At the focal ratios
typical of astronomical achromatic doublets of this era, visually the residual coma would be hard to detect.
r1
r1
r1
r1
=
=
=
=
37".313
26".474
33".579
41".135
r2
r2
r2
r2
=
=
=
=
-21".191
-13".870
-22".386
-11".922
* Based on South's Herschel aplanat - 1824
r3
r3
r3
r3
=
=
=
=
-21".598
-13".870
-22".162
-12".165
r4
r4
r4
r4
=
=
=
=
-88".070
∞
-100".529
72".463
Fraunhofer BK7-F2 doublet aplanat
Tulley's new "bent" aplanat - no air gap
Littrow centre contact
Herschel's prescription - edge contact*
ENGLISH MECHANIC ARTICLE by ODERIC VITAL (W. BRADBURY)