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Newtonian Breechpiece and Plate Holder


This instrument consists of a number of roughly circular metal components on top of one another, all painted back, with a rectangular box at the top. Red and yellow wires are attached.

The base rings form an aperture and connection plate to a large telescope. Visible on the inside there are a pair of flapping shutters hinged on the outside and operated by a knob on the exterior of the instrument; behind this the plate box is visible. Above the attachment plates, there is a section of rungs which form an adjustable section; these pieces are not closely next to one another but instead ‘floating’ about half a millimeter apart, enabling adjustment. Around the exterior of these ‘floating’ rings, there are two points of adjustment, one on either side of the instrument, One of these consists of a knurled knob connected to a rod which runs beside the circular section and screws into a plate attached to an adjacent section of the ring. On the other side, there is another knurled knob; this one is attached to a knob with a scale around the circumference, which is attached to a screw rod that points directly towards the circular ring where it presses against a small circular plate. Below this, there is a spring that connects to an adjacent corner of a lower ring via a complicated mechanism which includes a small unmarked scale and an indicator arrow. There is a similar mechanism near the other knob.

The last section is the smallest in diameter and fixed to the rear of the instrument. It consists of a cylindrical section with a broader rim which is attached via heavy bolts to the rearmost floating ring. On either side of this, in the back portion, there a pair of rounded openings. One of these has a small tube with a lens in it pointing directly into the opening. This tube’s orientation is adjustable via a bracket that connects the instrument to a carriage that runs along a scale with a vernier attachment. The carriage’s movement is adjusted by a knurled knob.

On the rear of this piece, there is a rectangular box with a green sticker on it, reading “1”. A similar sticker is nearby. This box can be removed by turning a metal latch, revealing an internal box section where a photographic plate can be placed. On one side of this, there is a knob to adjust the plate holder’s position in one dimension.

Around the exterior of the instrument, there are wires. These emerge coiled together from the lens tube and then split into two pairs of red and yellow wires, one of which runs to a dial fixed to the rearmost plate. The other cable runs to a two-pronged plug.

Accession Number: 2019.ast.229

Alternative Name:

Newtonian Double Slide Plate Holder, Dark Slide Plate Holder, Newtonian Focus Breech Piece

Primary Materials: Metal: Iron Alloy, Plastic, Glass.


A label stored with the object reads: “Newtonian Double-sided Plate Holder
Sir Howard Grubb, Parsons and Company, Ltd., Newcastle-Upon-Tyne, UK
(some components are missing)”

Dimensions (cm): Height = 25, Width = 34, Length = 38.


This is the breechpiece and photographic plate assembly that was attached to the 74” telescope at the David Dunlap Observatory in order to take photographs of celestial objects, particularly variable stars in globular clusters, at the Newtonian focus of the telescope. The assembly consists of a plate holder to hold plates to be exposed (in the centre), tube for eyepiece and adjustment knobs to manually adjust the plate holder so that the telescope tracks stars correctly, and internal illumination for the crosshairs in this eyepiece. A second eyepiece, now missing, fitted into the plate holder to ensure the plate to be correctly centred on the target object.


Good: The surface paint of the instrument is chipped in places. This is especially true of the plate holder, where much of the paint is removed and the rest is chipped. On other parts of the instrument, the paint is only removed on the edges and corners of the instrument. The instrument is not very rusty; there is some rust on some components, including the plate that supports the scale and knob attached to the microscope, the spring attached to the knob on the floating section and some of the other screws and attachment components for the wires.

The adjustment knobs are in fair condition, although screws that were oiled are caked with dust in the oil. Plastic components are yellowed, although not cracked. Wire insulation is in good condition. Wires were added following original manufacture.

According to a label with the object, some components are missing. This includes eyepieces and other plate holders.

Associated Instruments:


Parsons & Company, Ltd., Newcastle-Upon-Tyne, UK

Date of Manufacture: c. 1930-1933


This breechpiece and plate holder was originally constructed for use with the David Dunlap Observatory’s 74″ telescope, where it was mounted in one of four orientations on the telescope’s tube when the telescope was in its Newtonian orientation. Following going out of use, possibly in 1971, it was likely stored at the Observatory until the Observatory’s sale in 2009, when it was moved to the University of Toronto’s Department of Astronomy & Astrophysics at the St George Campus. In 2017 it was moved to a new storage location in McLennan Physical Laboratories.

Additional Information and References:

Photographs of the plate holder dating from 1933 appear in the David Dunlap Observatory Scrapbook (pgs 114-115) where it is described as “Newtonian Breechpiece, with small Plate Holder and Guider Eye-piece. The breechpiece is atached [sic] to the telescope tube near its upper end. It is used when direct photographs of a celestial object are being taken.” More attachments are visible in the photographs on the next page. The photos reveal that the plateholder has been modified from its original configuration, but recognisable features remain.

The plateholder also appears on page 150 of the scrapbook which is pasted with pages from a document entitled, “74in Reflecting Telescope for the David Dunlap Memorial Observatory of Toronto University, Canada” published in 1934 (Reprint from ‘Engineering’ (March 9, 30, April 20, 1934). The apparatus is described on page 12 (picture appears pg 15): “The breech piece… comprises focussing [sic] gear and a plate holder with two guiding microscopes mounted on cross slides and operated by micrometer screws. Rotary motion is also fitted to correct for rotation of the field, which sometimes occurs at altitudes, probably due to refraction. The plate holders take plates of quarter-plate size and are interchangeable with a knife-edge focussing plate and with adapters for oculars.”

A third instance of the plateholder appears on page 162 of the scrapbook. This a photo captioned: “Mr. Cyril Young, Manager of Sir Howard Grubb, Parsons & Co. putting the dark slide into the Newtonian Breechpiece”.

Christine Clement, a University of Toronto Professor of Astronomy, used this plate holder in the 1960s, through to 1971 to photograph globular clusters as part of a program begun by Helen Sawyer Hogg. Viewing the images taken for the UTSIC catalogue entry, she has described the plate holder’s features and operation in some detail:
In the bottom image, it appears that the cover of the plate holder has been removed so that you can see inside. However, that cover must be closed when an exposure is being taken.  When the observer loads a photographic plate into the plate holder, this is the side of the plate holder that has to be opened – but it must be done in a darkroom—in total darkness.

There is a green marker on the bottom left of the plate holder.  It indicates a tab.  This tab is part of the other side of the plate holder. When the astronomer is ready to expose, she pulls on this tab, so that the other side of the plate holder slides open and allows the starlight to illuminate the plate.

There is an eyepiece… about 90 degrees in the clockwise direction from the tab.  This is the guiding eyepiece. During an exposure, the astronomer looks through this to “guide” the telescope to ensure it is tracking properly. Although the telescope moves at the sidereal rate to track stars, the motion is not precise enough and the guiding must be done to ensure the [stars] in the photograph will be round (and not trailed).

Before starting the exposure, the astronomer has to select a guide star. To do this, she needs to move the guiding eyepiece to a position where a suitable star can be centred on the crosshairs. Throughout the exposure this star must stay at the centre of the crosshairs. If it veers away, it can be moved back by using one of the knobs.  The crosshair wires must be illuminated—and this is the reason for the two-pronged plug. 

It seems that a couple of attachments for the camera system have not been included. There should be an [additional] eyepiece. Before the plate holder was inserted, one had to make sure the telescope was pointing at the right field in the sky—usually a globular cluster. There was an eyepiece that fit into the “plate holder” slot so that the appropriate object could be centred in the field of view. Another attachment was called a knife-edge. This was used for focusing the telescope.”
A photograph of the plate holder and its components, some of which are now missing, was included on page 23 in R.K. Young’s descriptive article about the David Dunlap Observatory, published in March 1937.

According to Richard Jarrell (1988, pg 133), the Globular Cluster work began and mainly undertaken by H.S. Hogg was, “the only work done at the Newtonian focus of the 74-inch [telescope, at the DDO] for many years.” An image taken in the observatory c. 1950 shows Hogg on the Newtonian focus viewing platform lowering plates from the platform to the floor level in a purse attached to a string. (University of Toronto Archives A1978 0041-027).
Thanks to Professor Christine Clement of the Department of Astronomy and Astrophysics for her assistance with this text.

Historical Notes: