Oldham Optical Astro Page

We can supply a large range of sizes in our standard product line of astronomical mirrors and elliptical flats from 150mm (6.25"), to 762mm (30"), diameter, with a PV 1/4 Wavefront accuracy, (PV 1/8th wave on surface.) These can be supplied in plate glass, or in low expansion glass. Our most common low expansion glass used are "Suprax" from Schott in Germany, and Pyrex from Corning in the USA. We can obtain supplies of other glasses as required.

Advanced Optics Telescope

Newtonian Telescope Optics.

Folded Newtonian Telescope Optics.

Cassegrain Telescope Optics.

Gregorian Telescope Optics.

Schmidt Camera Optics

Schmidt Cassegrain Optics

Nasmyth Telescope Optics

Companar Camera Optics

Most of our standard range up to 16" is kept in stock awaiting aluminising, - If not, dispatch is usually within 6 to 8 weeks of placing a order.

Most of the optical elements made at our site are for Newtonian telescopes. This type is the most commonly used of all astronomical telescopes. This is due to the relative ease of construction, portability, insensitivity to optical alignment, and low purchase cost compared to other telescopes with similar apertures.

This telescope design requires a primary mirror placed at the bottom of the telescope tube, plus a relatively small elliptically shaped flat mirror which is placed near the top of the tube to deflect the image out to the side of the telescope tube. The Dobsonian design is a Newtonian telescope that is very popular and often recommended as an amateurs first telescope. Most 300-400mm designs easily fit in the boot of a small car, and we have seen carefully designed 600mm units fit in as well.

The main drawback of the Newtonian apart from its length is coma aberration at the edge of the field of view. This becomes progressively worse in faster F/ ratios. If you are planning to use a Newtonian Telescope for Astrophotography with a 35mm camera, then you might want to stick to F/8 or slower? - Please see the Photographic Page for more details.

Faster Focal ratios are still very suitable for direct viewing and our Price List shows most mirrors as faster than F/6. This is where we perceive most of the amateur market to be at the moment.

We can and do produce mirrors with longer focal lengths which may be more suitable for prime focus astrophotography? - or mirrors over 30" Diameter - or PV 1/10 Wavefront if required. Please contact us for details.

If you want a Newtonian with high magnification, - then you need a long focal length. However the eyepiece is always near the top of the telescope tube in a Newtonian Telescope and if you are operating at high elevations, you will need to stand on something to get to the eyepiece.

Depending on what you arrange to stand on, and remembering you are going to be doing this on a dark night - there has got to be a possibility of falling off and injuring yourself. If this could be a worry for you there is an alternative that will keep your feet firmly on the ground, - the "Folded Newtonian".

This introduces a circular optical flat at the top of the telescope tube to reflect the light back down to the elliptical flat at a convenient height. This version of the Newtonian works well with larger primary mirrors slower than F/4 and focal lengths in the 8-16' range. Note that as well as lowering the height of the eyepiece, the total length of the telescope tube is reduced which may be significant if you are building a dome to enclose it.

A disadvantage is the extra obscuration caused by the circular flat, - but this may be a sensible price to pay for ease of use and safety. The extra Optical Flat is not as expensive as a secondary mirror for a Cassegrain telescope.

This Telescope gives a longer effective focal length and higher magnification to the Newtonian for a given Telescope length or eye piece size. This is made possible by the light being folded back down the telescope tube. This makes for a more compact telescope. Cassegrain Telescope

This design has two curved reflecting mirrors. The concave primary mirror has a hole at its centre, and is placed at the bottom of the telescope tube. A smaller convex secondary mirror is placed near the top of the telescope.

The secondary mirror reflects the light from the primary mirror back down the tube through the hole in the primary, to the focus point behind.

There are various types of Cassegrain systems characterised by the curves used on the primary and secondary mirrors. The original type is now simply known as the "Classical Cassegrain". It used a parabolic primary mirror with a hyperbolic secondary mirror. It offers no optical advantage over the later types except that it can be turned into a Newtonian by replacing the secondary mirror with an elliptical flat. There is very little demand for this type today.

Later types of Cassegrain are named after their inventors/engineers. The two main types we supply are the Dall-Kirkham and the Ritchey-Chretien.

The Dall-Kirkham has a spherical secondary with an under-corrected paraboloid primary which makes it slightly easier to manufacture. It is quite suitable for direct viewing with small fields of view, - but it does exhibit significant coma and wider fields are impractical.

If you want a telescope for direct viewing with a really long focal length but still keep a relatively short tube, then a Dall Kirkham of around F/20-25 offers a practical solution with a low obstruction ratio.

The Ritchey-Chretien has overcorrected paraboloid (hyperbolic), primary and secondary and has the major advantage of being coma free. This type constitutes most of our Cassegrain production. Since it is coma free, - it has a wide field and can be used equally well for direct viewing, 35mm or even medium format photography.

Some texts on telescope making suggest it is extremely difficult to make and test a Ritchey-Chretien? - It's true it is a little bit harder, - but it's not that much harder and it's quite practical to do!

We can manufacture all species of Cassegrain, but our first choice would always be a Ritchey-Chretien, - If it's good enough for the Hubble, - we suggest it's probably good enough for you?

(Oldham Optical guarantee our Cassegrain mirrors will not need a visit from the Space Shuttle to make them work properly!)

Although we do not build many Gregorian telescopes, this is shown for completeness. Both the Newtonian and the Cassegrain have the drawback that they invert the image and are not very useful for terrestrial viewing without some external arrangement to re-invert the image. The Gregorian produces an erect image directly so is better suited to terrestrial viewing. Gregorian Telescope

In construction it is very similar to the Cassegrain, except that the secondary mirror is concave and is placed beyond the primary's focal point.

This has the disadvantage of making the tube length a little longer than a pure Cassegrain, but it this feature that causes a second inversion of the image, making the telescope suitable for terrestrial viewing.

If you need a short powerful telescope for terrestrial use then it makes very good sense to consider the Gregorian as a possible option.

We have made a few sets of Schmidt Camera optics for observatories, - but they are very specialised tools, and certainly not something used by many Amateurs. They are described here as it is the logical place to introduce the Schmidt Corrector plate.

If you want to take pictures using large film formats, - typically covering an angle of view of 3-7 degrees, then you might need a Schmidt Camera. This has a spherical primary mirror and a special lens placed in front of the mirror called a "Schmidt Corrector plate".

The camera was developed by and is named after Bernhard Schmidt, who apart from working out the necessary mathematics involved also developed a simple way of making the corrector plate. We still produce the corrector plates in the same manner he recommended, - please see the Manufacture page for details.

The corrector plate is unusual, - unlike nearly all other astronomical lenses and mirrors, it does not have a single smooth curve. This curve reverses at least once over the disk, and there are several apparently completely different looking shapes of corrector that will do the job for any one mirror.

Without the corrector the spherical mirror would suffer from spherical aberration, and would be useless as a camera. The classical position for the corrector is at the centre of curvature of the mirror which is twice the focal length. At this point it can completely correct all coma in the system allowing the large field necessary for large format photography.

At first sight this would mean an extremely long "Telescope tube" as it would be nearly twice the length of an equivalent Newtonian Telescope. However the mirror is spherical and it is easy to produce in fast sizes. Faster than F/2 is easy at 20" Diameter, and we happily produce F/1 mirrors at 12" Diameter. So potentially a 20" Diameter Schmidt Camera can be less than 80" long.

A Schmidt Camera is not a telescope and cannot be looked through, - it will need an auxiliary finder scope fitting in order to aim it.

The film position is within the "telescope" tube where it is difficult to access. Although it is possible to install a CCD Camera in place of the film, - no relatively cheap CCD's are made that are big enough to take advantage of the large field available.

Finally, the field is curved, and it may be necessary to bend the film to match the field curvature, or provide an extra field flattening lens.

A Schmidt Camera is a very specialised tool, and you are seriously considering one, you are advised to talk with us before doing any detailed planning. Note that If you only intend to take pictures using 35mm film, - then we will probably recommend you stick with a Newtonian or Cassegrain telescope.

This is the area in which firms like Meade and Celestron do a lot of business. It is said in the Schmidt Camera section that the corrector plate must be at the centre of curvature for optimum correction, - It can be brought closer, but coma is introduced.

However if Cassegrain telescope optics are substituted for the spherical mirror, then the mirror surfaces can be specially figured to counteract the coma and most of the other aberrations. This arrangement is known as a Schmidt Cassegrain telescope.

Note that in spite of having a corrector, - the telescope will not have the wide field of a Schmidt Camera. The field is comparable to that of any other Cassegrain telescope.

The presence of the corrector does theoretically allow extra degrees of freedom to cancel out aberrations compared with a Ritchey Chretien, which has only the two surfaces. However, the corrector plate introduces some new aberrations of its own.

In sizes up to about 12" Diameter, it is a very portable and attractive item. The corrector plate seals off the telescope tube and keeps dust from settling on the primary mirror and if the position of the corrector plate is close to the secondary mirror, then it can be used as a support instead of needing a separate spider. So issues other than pure optical performance are involved in the popularity of this telescope.

This is a derivative of the Cassegrain and is a very clever and practical telescope design that is in our opinion very under rated and under used. The Folded Newtonian was introduced to lower the height of the eyepiece. Nasmith Telescope Optics

With a Cassegrain, the converse can be a problem with the eyepiece sited too low for comfortable viewing?

The focal point of a Cassegrain is behind the primary mirror and if you intend viewing at high elevations, then the tube of the telescope would have to be supported high off the ground. It is of course possible to fit a 90 Degree bend on the focuser of a standard Cassegrain for use at high elevations, but if the telescope is above say 12" Diameter, you will still need a strong, tall and relatively expensive mounting.

So if you are considering a Cassegrain of 12-24" or above, - then why not consider having the 90 Degree bend built into the design at the start? - This is the design credited to James Nasmyth. It is a Cassegrain optical system with an extra elliptical flat from a Newtonian bending the light

out the side of the tube. The extra cost of the elliptical flat is not significant when the cost of the other two mirrors is considered.

The Nasmyth opens the possibility of using a much simpler and lower mount, as illustrated by the very basic design on the right which shows the focuser carried on the top of the tube.

The primary mirror does not need a central hole for optical purposes, but it is often there to provide a mounting point for the elliptical flat. If the elliptical flat is towards the primary end of the tube, there are no extra optical obstructions and as an extra bonus, the baffling needed in standard Cassegrains is not required. Stray light has a lot more difficulty in reaching the focal surface with this design.

Very large professional telescopes often have a "Nasmyth focus" deliberately aligned on a hollow axis of the telescope mounting so that the eyepiece and viewing position are always maintained in the same position and at the same height. This is illustrated by the original 20" Telescope built by James Nasmyth in 1845.

Some dictionaries suggest the feature of the hollow axis and fixed viewing position marks the "true" Nasmyth Telescope? - but there is no generally accepted name for the simpler version?

Note that a modern Nasmyth Telescope would have the same tube length as any modern Cassegrain or Schmidt Cassegrain. It would certainly not have the long tube of the original 1845 version illustrated on the left, - which looks more like it belongs to the Artillery than an Astronomer!

If expense is very little object and you are worried about the residual curved field of a Cassegrain for taking very big pictures, - and the internal position and curved field of a Schmidt is not for you? - then the Companar camera may be of interest.

The Companar Camera was proposed by Klaas Compaan and is a derivative of the Maksutov-Cassegrain. Like the Maksutov, this telescope uses two spherical mirrors and a meniscus lens to correct for spherical aberration.

Where it differs from the Maksutov is having a second lens in advance of the Maksutov meniscus. This is an aspheric plate. It is the aspheric plate that makes the design special. It turns the Maksutov design into a superb camera with virtually no aberrations, a potentially massive field, and the film is flat and outside the camera tube.

Anyone with an interest in the design might care to look in Rutten and Van Venrooij's book, "Telescope Optics", in the chapter discussing Maksutov-Cassegrains.