Collimation of a Cassegrain

If you are constructing a Cassegrain telescope, you will have to collimate it in the final stages of construction. This means setting the two mirrors in line exactly on the axis of the focuser and at 90 degrees to it. Even if you buy a ready made telescope rather than constructing it yourself, you can expect to have to collimate it when you receive it.

You may also have to re-check your Cassegrain at intervals, especially after a large one has been moved. The first part of this page explains the adjustments you must build in to the telescope during construction so you can eventually collimate it properly.

Critical Dimensions

If you are constructing a telescope then you must stick closely to the dimensions provided by the optics supplier.
Or conversely, when ordering optics for an existing telescope, you must specify the critical dimensions to the optics manufacturer for him to build to.
The two critical values are the separation distance between the mirrors and the vertex back focal length.
Oldham Optical measures the values from the mirror surfaces as in the diagram. Most other suppliers probably measure from these surfaces as well, – but no harm is done by confirming where the measurements are taken from.
If you stray far from the dimensions specified, it is still possible to focus the system and get good images from a star on axis, but you will lose some of the performance off-axis. In particular, Ritchey Chretien coma free optics are designed for a fixed separation between the primary and secondary mirrors to operate properly.

If your telescope focuses by altering the separation between the two mirrors, then a few mm either side of the optimum value wont make any difference in practice. However, if planning to use something with a wide field (like a 35mm camera, or bigger), then design the focuser and camera fittings to keep close to the correct dimensions when using the camera. This may mean some compromise for direct viewing with eyepieces, but these involve narrower fields, so should not be a problem.
The telescope when assembled may enclose both the primary and also the secondary mirrors, so if practical try and sort out during the construction of the telescope a method of measuring the two dimensions easily.
Perhaps a hole might be provided through the mirror cell close to the focuser allowing the distance from the back of the telescope tube to the back of the primary mirror to be measured. The hole can be blanked off afterwards with a screw.
Perhaps the position of the secondary mirror could also be measured to some reference point on the outer telescope tube?
Primary Mirror Lateral Adjustments
Its not that common, but is covered here in case. It is possible to provide the primary mirror with proper lateral adjustments for “Up and Down” and “Side to side”.
These can be used to set the primary mirror on axis with the focuser. If the mirror is hung from a strap, then varying the length of the strap provides a simple “Up and Down” adjustment. Don’t worry if your telescope does not incorporate this feature, but if it has full or partial adjustment, be prepared to take advantage of it later. It will be used to centre the hole in the primary mirror over the focuser.

Primary Mirror Tilt Adjustments
What the primary mirror must always have is a means of tilting the mirror to set it to exactly 90 degrees to the axis.
Similar arrangements to a Newtonian are often used.
There are plenty of photographs and designs available of mirror cells for Newtonians, but they usually boil down to some form of three point suspension.
Usually each of the three legs has an adjustment. Varying the lengths of each leg allows the tilt of the mirror to be adjusted so as to set the mirror to exactly 90 degrees to the axis of the telescope.
The ability to tilt the mirror is needed in both X & Y directions, as the diagram adjacent done separately in 2D and 3D tries to show.

An example of how to do it is in the next photograph. The picture is of a big Newtonian, but the principle applies to a Cassegrain Primary mirror as well. The three points supporting the mirror can be seen.
Each has an adjustment by screwing or unscrewing the appropriate nuts which enable the mirror to be tilted as required so as to set it at exactly 90 degrees to the axis of the telescope system.
These adjustments are used to minimise coma in the final part of collimation.

Secondary Mirror Lateral Adjustments
All large Cassegrain telescopes should have a means of adjusting the secondary mirror physically “Up and Down” and “Side to Side” to set it exactly on the telescope axis. Small telescopes may just rely on the basic mechanical engineering to set the mirror on axis.
This is often (but not always), done with an adjustment on the spider that holds the mirror. This adjustment will be eventually used to centre the mirror on the axis of the telescope.
There are no apologies for using a Newtonian as an example. It’s exactly the same principle for a Cassegrain and good pictures of  one of David Lukehurst’s big Dobsonians are readily to hand.

Secondary Mirror Tilt Adjustments
If your telescope mirrors are figured for any Cassegrain that does not use a spherical secondary mirror, then you must have adjustments to adjust the tilt of the secondary mirror. This is also very useful but not quite as essential for a Dall Kirkham. A smaller scale arrangement to that used on the primary mirror may be used.
Although its not fashionable at the moment, – the best way to hold a Cassegrain secondary mirror is to manufacture it with a central hole. The mirror then fits over a post attached to the spider. The centre of the mirror is never used optically so there is no loss of light and there is no cell needed to hold the mirror which would otherwise increase the  obstruction ratio of the telescope. If you are thinking about building a big Cassegrain, – then talk to us about holding the secondary.

Tools For Collimating The Telescope
You now have a telescope constructed with all the adjustments. the mirrors have been put in place and it is ready for collimation. You have simple tools such as the right spanners, screwdrivers etc at hand to operate your adjustments.
There is one special tool you will need to make to assist in collimation. This is a pinhole. The focuser of the telescope must be fitted with a disc with a small pinhole in it of say 2mm diameter. The pinhole must be dead centre on the axis of the telescope.
The disc and pinhole must have arrangements to fit it to the focuser so that the pinhole is on the axis of the telescope. Where the focuser will take 1¼” eyepieces, or a reducer can be fitted to take such eyepieces, a standard plastic 35mm film canister can be converted to be a pinhole.
A 35mm canister happens to be a snug fit in the focuser. Cut off the bottom of the canister with a hacksaw and drill a 2mm hole dead centre in the lid. The device is now ready for action.
35mm Canisters are getting rare now digital cameras are taking over, but take a trip to any high street photographic developer such as Boots, and ask nicely at the counter and we would be surprised if you can’t get a hand-full for free.

There is a slightly more sophisticated version that makes collimating a Cassegrain slightly easier.
The picture adjacent is one of the more “sophisticated” type. This one is based on a 35mm film canister for the outer tube and the inner tube used to be the core of a sellotape roll.
It’s not designed to be telescopic! The translucent section allows light to penetrate the tube making it easier to adjust the tilt of the secondary mirror. The same thing could be achieved by cutting the 35mm film canister in half and sellotapeing the two halves back together, but with a gap between them.
Note you may have a very expensive telescope, but the point being made is that it does not need expensive equipment to collimate it. This class of device is good enough for the very best – and so suggest it’s probably good enough for you.
If you have a large focuser without a reducer, then you must make or engineer your own device before you can go any further.

Initial Set Up
The main part of the setting up procedure should be done inside a house or garage in good daylight. The telescope should not be set up looking out of a window or at any other bright light source as you will be looking through the tube from the area around the focal point using the pinhole.
You do not want to risk damaging the retina of your eye from inadvertently looking at bright lights!
Start by going through and loosening any lock nuts on the various adjustments.
Check the critical dimensions (again). These are the Vertex Back Focal Length & separation of the mirrors.
Check by simple measurements or a visual check that the hole in the primary mirror is central about the focuser bore. If there is any lateral adjustment on the primary mirror, use it to centralise the mirror on the focuser.
Check by eye and perhaps simple measurements from any fixed points on the OTA tube structure that both mirrors are set as close to 90 degrees to the axis as possible. This does not need to be exactly correct at this stage as it will be checked later.
If your telescope has lateral adjustments for the primary mirror, then adjust them by eye so that the hole in the primary mirror is centred over the focuser.

Setting The Optics On Axis
Fit the pinhole to the focuser and put your eye close to it. Rack the focuser in and out until you obtain a view through it as in the diagram opposite.
In the diagram the black area represents the inside of the focuser tube. The outer blue area represents the back of the primary mirror. The inner blue disc is the secondary mirror viewed through the hole in the primary.
Once you can see the hole in the primary mirror and the secondary mirror clearly, rack the focuser in and out so the hole in the primary appears only slightly bigger than the secondary mirror.
You are now ready to adjust the secondary onto the same axis as the focuser and the primary mirrorA warning first! All this can be easily done with the OTA lying on its side with the optical axis horizontal. Don’t perhaps be tempted to angle the tube upwards to get at the secondary mirror adjustments more easily. If you drop a spanner when the secondary is above the primary, it is likely to smash the primary mirror! So keep the tube flat such that if you do drop a tool, its not going to fall on anything expensive. Think through what you are going to do first.

You have to centre the secondary mirror in the hole in the primary as you see it through the pinhole. This is done by adjusting the secondary mirror lateral adjustments described earlier. It should be a fairly straightforward task to centre the secondary mirror in the view.
If your telescope has no lateral adjustments on the secondary, but has them on the primary, use these to line up the axis instead.
When you have finished, the focuser, the primary mirror and the secondary mirror are all set on the same axis.

Adjusting The Tilt On The Secondary Mirror
Look through the pinhole again and concentrate on the view of the secondary mirror. On the face of the secondary you will be able to see the reflection of the focuser entrance. If you have the more sophisticated pinhole that allows light in as a circular ring, then you will see a bright ring around the central axis, otherwise you will just see the black hole of the focuser.
Alternately, you should also be able to see the reflection of the primary mirror on the face of the secondary. This can be used instead.
Adjust the three Secondary mirror tilt adjustments as necessary to centre the reflection of the focuser and/or the primary mirror, on the centre of the secondary mirror. The diagram adjacent assumes you are looking for the reflection of the primary mirror, which is shown here as light blue.
The process of adjustment is fairly easy, but the first time you do it you will have to experiment to find out which leg moves the reflection the way you want it to go. Once you have seen how the reflection moves, you will quickly zero in on the adjustments that move the reflection on to the axis.
If you need a bit more help in starting:- first note the direction that the reflection is most off axis and choose the leg that is pointed nearest to that direction.
There are three legs spaced at 120 degrees, but since they can move either forwards or backwards each also covers the direction 180 degrees out of correspondence. So choose the leg that is either pointed nearest to the reflection or at 180 degrees from it.
When you have adjusted the first leg to a minimum, the reflection will almost certainly still be off centre, but it should be exactly lined up with one of the remaining two legs. Adjust that leg to centre the reflection dead on axis.
Do not adjust the remaining third leg. If after moving the second leg you cannot completely centre the reflection, return to the first leg you moved and adjust that. Repeat moving the two in turn until you are happy you have the reflection centred.
Some of you may be surprised that using the view of the reflection is sensitive enough to set the secondary mirror tilt accurately – Most of the coma will come from an inaccurately set primary mirror and that is adjusted later.

Check Critical Dimensions Again!

Once you are happy the reflection is correctly centred,  get out the tape measure again and measure the critical dimensions:
Separation distance between the mirrors and the vertex back focal length.The act of tilting the secondary mirror could have altered these dimensions slightly. If necessary, compensate by moving all three adjustments on the secondary mirror by exactly the same distance in the opposite direction. This will shift the secondary mirror axially to get the mirror separation right without affecting the tilt.
Tighten up any lock nuts.
You have now nominally finished the in-door set-up but it would be sensible to go back and run through the same procedure at least once to check everything is still OK. The next job is the star-test.
Adjusting the Tilt Of The Primary Mirror
This job is done while viewing the stars. You will be working outside at night and it’s going to be dark!
Take this section very steady and do one thing at a time.
Have a routine for putting down any tools in one place so you can find them again.
Think through all the motions and movements first.
If necessary, practice the actions in daylight first.
First, loosen any lock nuts on the primary mirror adjustments.
Set the telescope up with a high power eyepiece and focus it on a star in the centre of the field of view.
There are plenty to choose from!
Rack the focus backwards and forwards through the position of best focus and you should be able to see a disk and ring structure on either side of focus.
This is comprised of the Airy Disc and diffraction effects from the central obstruction of the Cassegrain. Study the ring system either side of focus.
If the primary mirror is tilted correctly, there will be no coma in the system and the ring and disk system will be concentric either side of focus.
If the primary is not set correctly, there will be coma in the system which will show up as the disk and ring structure not being concentric. It will be offset in one direction on one side of focus and shifted 180 degrees on the opposite side of focus.

The disc and ring structure will probably not be exactly as depicted. What you see will be dependant on the telescope itself, diameter, focal ratio, obstruction ratio, eyepiece etc. The central disc may appear larger or just be a central dot. There may be more rings visible than shown. What you are looking for is any basic disc/central dot and ring structure that you can see is concentric either side of focus.
If you can’t initially see a ring structure, try reverting to a wide angle eyepiece and look for uneven brightness in the disk either side of focus.
Coma will be removed by adjusting the tilt of the primary mirror. Note the axis of the coma. In the diagram above it is running top left to bottom right. Choose the leg of the Primary that is closest to the 60 degree sector of the axis of the coma. Adjust it one way to see if it improves it. If it makes it worse, move it back the other way and adjust it backwards and forwards as necessary to reduce the coma to a minimum.
When you do reach a minimum position from adjusting one leg, – the axis of the remaining coma should be lined up exactly with one of the two other legs. Adjust this one to remove the remaining coma.
If adjusting the second leg does not completely remove the coma, return to the first leg and re-adjust that nearer to a minimum. Repeat with the first and second leg as necessary to completely remove the coma. Do not adjust the third leg.
Tighten any lock nuts when you have finished and check for coma again.
Once you have removed the coma, the primary mirror will be accurately set at 90 degrees to the axis. So take the opportunity to do a bit of observing – Have a coffee or perhaps something that comes in a glass and is a bit stronger! There is still a job to do in the morning, but that can wait.

The Morning After?

It doesn’t really have to be the next morning afterwards of course, but it is a job for the house or garage in good daylight. Do yet another check of the critical dimensions: Separation distance between the mirrors and the vertex back focal length.
The act of tilting the primary mirror could have altered these dimensions slightly.
If necessary, compensate by moving ALL three adjustments on the primary mirror by exactly the same distance in the opposite direction. This will shift the primary mirror axially to get the mirror separation right without affecting the tilt.
You will want to check for coma again when you next start observing, but it’s unlikely to need any tweaking and you will be an expert at the adjustments by now.
Perhaps its time for yet another glass of something a bit stronger?


Oldham Optical usually coats its mirrors with Aluminium as the reflective layer over-coated with a protective layer of Silica 1/2λ thick. This arrangement is usually known as “Protected Aluminium”. We believe this combination is the best compromise for general purpose astronomical mirrors. There are other coatings that can be used. This page is intended to give some details of the different coatings with the underlying principles behind them and the manufacturing process.

Metallic Reflectors.

Possible metallic reflectors for astronomical mirrors include Aluminium (Al), Silver (Ag) and Gold (Au). The relative performance of each material across the visible spectrum is shown in the adjacent diagram.
It can be seen that Gold is a poor reflector for astronomical purposes as its reflectivity drops off sharply towards the blue end of the spectrum, but if you were designing an infra red telescope, it would be the best of the three metals to have.
Silver is a fairly good reflector over the whole visible band and is better than Aluminium at wavelengths longer than 500nm. It does tarnish quickly if the surface is unprotected and this rapidly reduces the reflectivity. It can be given an overcoat of Silica (Silicon Dioxide), which does slow but not completely stop tarnishing. The remaining big drawback with Silver is that it is a lot more expensive than aluminium.
Aluminium has good general performance as a reflector over the visible spectrum and it is very cheap compared to the others. Considering how many Aluminium products you will have in the home, it may surprise you that bare Aluminium rapidly oxidises on contact with air. But the Aluminium Oxide coating formed is transparent and nearly impervious after a short depth has formed. The oxide layer will slowly thicken over time but the mirror is generally usable for over a year before the declining reflectivity requires it to be re-coated.
On balance, comparing costs and performance of Aluminium against Silver and Gold, it is generally accepted that Aluminium is the best choice for general purpose astronomical use.
The optimum thickness of Aluminium is in the range 75-100nm. This is the compromise area where the thickness is sufficient to cover the glass and avoid leaving partial transparency, yet not thick enough for the coating process used to develop significant unevenness in the layer.
A general rule is that the thickness of the reflecting layer should not vary by more than 1/100λ else it will affect optical performance. The normal coating process (described later) is aimed at maintaining thickness within 5%. So a thickness of 100nm is about the limit that would cause 5nm or 1/100λ unevenness.
Bare Aluminium is extremely fragile. It is easily marked and scratched. This does not stop bare Aluminium mirrors being used in large observatories. They prefer the bare surface because of the flatter reflectivity response across the full infra red to ultra violet spectrum. Observatories usually have their own vacuum chamber for easy re-coating at between 12 and 24 month intervals.
Most users who do not have such ready access to a coating chamber prefer that the bare Aluminium is given an overcoat to protect the surface to delay oxidation and protect the surface from scratching. The most common coating used for the visible spectrum is “Silica”. Chemically this is Silicon Dioxide (SiO2). Another fairly common coating is Magnesium Fluoride (MgF2), which is better for the ultra violet part of the spectrum.
Any overcoat does reduce the reflectivity but makes an Aluminium mirror usable for say over 10 years without re-coating. In the case of the Silica overcoat it can either be applied extremely thinly just sufficient to seal the Aluminium away from the air – in which case the mirror reflectivity is about 4% lower than that of bare Aluminium throughout the spectrum. More usually the coating  is applied to a depth equivalent to 1/2λ in the middle of the visible spectrum. Using Silica as the overcoat, then after taking the value of refractive index (1.46), into account, this works out to be a thickness of about 190nm. The effect of this particular thickness is to boost reflectivity in the middle of the visible band nearly back to the bare Aluminium value, but it does tail off at each end of the visible spectrum.
An Aluminium mirror over coated as above is known as “Protected Aluminium”.
Usually the overcoat of silica or MgF on a “Protected Aluminium” mirror is 1/2λ, – but check the thickness if it is not made clear. It may mean the very  thin coating just sufficient to seal the Aluminium in.

Dielectric Reflectors.
Instead of one good reflecting layer, a mirror can be produced using a stack of partial reflecting layers in series. The materials used are usually referred to as “Dielectric” materials and the resulting mirror is known as a “Dielectric” Mirror.
The principle is that when light attempts to travel from one medium to another, a portion of the light is reflected. If a stack of alternating layers are laid down on top of each other then the arrangement produces a number of reflective layers in series one behind the other.
Each reflects a percentage of the light reaching it. This makes the multilayer combination into an extremely good reflector. Usually two materials with sharp differences in their refractive index are used and the thickness of each layer is related to 1/4λ thick. This thickness causes the reflections from all the layers to re-combine in phase.

Lots of different materials can be used for the layers. Many metal oxides can be used as well as many metallic fluorides and Sulphides. Two materials often used are Zinc Sulphide (ZnS), and Magnesium Fluoride (MgF), with refractive indexes of 2.32 and 1.38 respectively.
Both layers are usually arranged to be 1/4λ thick to light.
The different refractive indexes means the Zinc Sulphide layers are physically thinner than the Magnesium Fluoride layers in proportion to the inverse of their refractive index value.
While these coatings can be designed to give reflectivity’s greater than 99.9% over a very narrow range of wavelengths they cannot offer a broad response across the visible spectrum. If a reflector for a single frequency laser is required, then a dielectric mirror will often be the best solution, – but this sort of reflector is not practical as a general purpose astronomical mirror across the visible spectrum.
However it is possible to overcoat a metallic reflecting layer, – usually Aluminium – with a small number of dielectric layers and using this principle “enhance” the performance of the aluminium. This is what is generally termed an “Enhanced Aluminium” mirror. Perhaps 2-6 dielectric layers are involved on top of the Aluminium.
While the dielectric mirror example given suggests two materials are used with thicknesses directly related to 1/4λ thick. More than two materials may be used and its not necessary to stick with thicknesses directly related to 1/4λ, – although the effective difference between the different material layers must still end up being 1/4λ different for the process to work. The specific materials and thicknesses used in the dielectric layers are usually proprietary to the coating manufacturers.
If you are considering an enhanced aluminium mirror, then clarify with the supplier what materials are being used. It will be useful to know this when the mirror coating comes to the end of its useful life and it is necessary to strip off the coatings. Some materials are more difficult to remove and result in the mirror figure being damaged..

The (maximum) improvement claimed for enhanced over protect Aluminum is about 5%. An example performance graph of Enhanced verses protected Aluminium and with bare Aluminium as a reference is adjacent. We suggest this 5%  may apply for 4-6 layer systems and a reduced number of layers give less
We are aware that one “enhanced Aluminium” mirror recipe is only one extra layer. It comprises the standard “protected” Aluminium coating of 0.5λ Silicon Dioxide followed by a single layer 0.25λ thick of Tantalum Pentoxide. While we have not seen any performance figures for this recipe, we do not expect it to perform as well as 4-6 layers, but it is still marketed as “enhanced”. It will certainly give some improvement, but perhaps not as great as 5%? However as it is only one extra layer it should be cheap to apply.
A Cassegrain telescope involves two mirrors. So the extra light delivered to the focus will be about 10% greater where two enhanced Aluminium mirrors are used compared to protected Aluminium.
A Newtonian also has two mirrors, the Primary and the elliptical flat. Be aware that the same coating as used on the primary will not work on the elliptical flat which is operated at 45 degrees. Flats require different thicknesses of each layer to account for the 45 degree angle of incidence. Even then there may be problems from partial polarisation due to the angle. If the partial polarisation reduces the light reflected by 5%, – you have wasted your money paying for an enhanced aluminium coating! Protected Aluminium does not present the same level of problem.

Coating Methods
Apart from depositing a layer of Silver, which can be deposited chemically, most coating involves a vacuum chamber. A very high vacuum is required and there are usually two pumps involved. The first is a rotary vane pump or similar type  that removes the bulk of the air from the chamber sufficient for a traditional oil diffusion pump to be brought into circuit between the chamber and the rotary pump to go on and produce the extremely high vacuum required. The diffusion pump may be replaced in more modern systems with a very high speed turbine pump that can achieve matching or even higher vacuums.

The main method used in the coating process is to directly heat the coating material to a high temperature in the vacuum and evaporate it. A tungsten heating element in close proximity heats the coating material.
If it is Aluminium being used as the coating, it is common to directly wrap Aluminium wire round the Tungsten heater element.
Because of the low pressure, the coating material changes directly from its solid form to a gas with no intermediate liquid phase. The gas then condenses on any cooler objects nearby which will include the mirror.
After coating with say Aluminium then if an overcoat is needed, a second heat source nearby loaded with the coating material is switched on and evaporates the coating in the same manner. If a series of layers for a dielectric coating are to be applied, each may have its own heater which will operate in turn.
The “traditional” vacuum chamber used for coating suspends the mirror over the heater with the distance between the mirror and the heater equal to the radius of curvature of the mirror. Since the heated material evaporates evenly in all directions, this geometry automatically results in a very even thickness of the coating over the full mirror surface. There is no risk of the mirror figure altering due to a varying thickness between centre and edge of the mirror.

However for a lot of big mirrors, this would need an enormous vacuum chamber! Common modifications are to change the geometry of the vacuum chamber to use a series of heaters across the full area of the mirror. Another chamber design maintains a single heater but sites it towards one side of the chamber while rotating the mirror during the coating process. This allows the rotation to even out the coating.
Each technique is aimed at evening out the coating delivered to the mirror surface with the aim to keep the coating thickness within 5%. If the deposited Aluminium layer was 100nm thick, the unevenness would work out to be about 5nm which is about 1/100λ and is small enough not to worry about.
Oldham Optical have often tested examples of mirrors before and after coating. We have never detected any measurable difference in the mirror figure before and after. So our experience is that unevenness is not a problem.

Modifications on the basic process.
The principle behind all methods is to separate individual atoms of the coating material and deposit them in a controlled manner as an even layer on the surface of the mirror.
For very large mirrors, where the problems of raising the mirror in the chamber are excessive, the mirror is located at the bottom of the chamber and the heater elements arranged at the top. the only disadvantage of this is that any dust in the chamber or any loose material will end up on the mirror surface. The picture adjacent is the vacuum chamber at the Air Force Research Laboratory’s Starfire Optical Range at Kirtland Air Force Base. It is capable of Aluminising a 3.5m mirror.
Instead of a tungsten element, an electron gun can be used to heat the coating material. This allows materials to be used with melting points above that of Tungsten. The electron beam can easily be deflected into adjacent crucibles to apply the overcoats.
After the vacuum in the chamber is established but before coating starts, there may be some preliminary processes. Although the mirror surface will have been well cleaned before being put in the chamber, an electrical discharge is often passed through the chamber first. This excites remaining residual atoms in the chamber. The atoms speed up and bounce round the chamber. They have the effect of sandblasting the inside of the chamber surface and the mirror. This gives it a final clean.
The evaporation process of some materials can be spoiled by atoms of oxygen and other atmospheric gasses that have adhered to the sides of the chamber and have managed to resist the initial vacuum. Most of the unwanted gasses can be released before coating starts by heating the chamber. Conversely during the coating phase, some dielectric materials actually prefer that a small amount of Oxygen is deliberately introduced during the coating process. This helps adhesion of the coating for these materials. Sometimes ionisation is maintained within the chamber which helps the coating atoms pack correctly on the surface.
An alternate technique to evaporation is called “Sputtering”. This method operates at a lower vacuum. Trace gasses are deliberately introduced to the chamber. These trace gasses are energised by an electric arc from electrodes sited close to the coating material, or by radio frequency energy from a small aerial.
The arc or the radio signal energise the trace gasses to move at extremely high speed within the chamber. They bounce off the walls and the nearby coating material. On striking the coating material they break off individual atoms. These loose coating atoms eventually deposit themselves on the mirror in an even layer.
Finally, some chambers may be arranged where instead of the mirror lying horizontally, it is turned through 90 degrees and hangs vertically.

Enhanced Aluminium v Protected Aluminium.
While an enhanced Aluminium mirror will reflect up to 5% more light, there are factors in the manufacturing process that are more critical than for the simpler protected Aluminium mirror. Oldham Optical believe they are significant. The more complex process and different materials may mean the enhanced mirror has a shorter life before it needs recoating. And when it does need re-coating, its more likely to need re-figuring as well.
For the protected Aluminium mirror, the thickness of the aluminium coating is not critical. It is a reflecting layer. It is only necessary to deposit sufficient Aluminium to ensure a full cover. Any value in the 75-100nm band gives sufficient cover without causing excessive roughness of the surface.
If it is a simple chamber as described above, with the mirror at roughly its radius of curvature from the heater then the simplest way to work out what thickness of Aluminium will be achieved on the mirror is to assume the Aluminium when heated will vaporise in every direction evenly as an expanding sphere. Calculate the ratio of the area of the mirror to the area of the sphere that has the same radius as the gap between mirror and heater. From this ratio work out the weight or volume of Aluminium that must be put into the chamber to give say an 85nm thickness.
Neither is a Silica overcoat thickness particularly critical. It is aimed at being 190nm thick, but errors just move the peak response slightly up or down the spectrum. So production of a “protected Aluminium” mirror is not very critical.
By comparison the thicknesses of the dielectric layers used in the enhanced mirror must have very tight tolerances to get the improved performance. There are a greater number of layers to lay down. For these more critical layers the vacuum chamber may need fitting with equipment that continually measures the reflectance of the surface as the thickness of a layer builds up. When a peak in reflection is noted, the process can be stopped and move on to the next layer.
An alternate testing method is that once the coating is finished the mirror is removed and subjected to tests of its reflectivity. It will be rejected if it does not met the standard set. However since astronomical mirrors are usually large and will not easily fit in the sort of machines commonly available for testing reflectivity, small samples placed inside the chamber with the mirror may be tested instead of the mirror itself. They should have received the same coating thicknesses.
If a mirror with dielectric layers has to be rejected, it may be a major set-back. Depending on the dielectric materials used, the coating may not be removable without damaging the mirrors figure. The mirror may end up being sent back for re-figuring and testing before the next attempt to re-coat. If you are obtaining an enhanced Aluminium mirror then you are advised to find out what materials are being applied. So you will be aware of the consequences and costs when it subsequently has to be re-coated.
Even when the layers are laid down to the correct thickness, the dielectric mirror will on average have a shorter life. The evaporation process leaves stresses in the thin materials. The different coefficients of thermal expansion of the materials cause more stresses  between the layers from temperature changes during its life. While Aluminium has a coefficient of thermal expansion that is considerably different from glass, it sticks extremely well to glass and successfully withstands the normal tensile and compressive stresses of its daily life fairly well without cracking or delaminating. Silica and Magnesium Fluoride when used as single overcoats have similar characteristics.
Some other materials used in the dielectric layers are not as good in this respect. They do not resist equivalent stresses as well. Exceeding the material tensile stress limit causes a series of tiny cracks to appear over the mirror surface. This is known as “crazing”. Exceeding the compressive stress limit causes flakes of one layer to delaminate. Increasing the number of layers increases the chance of a stress problem appearing within a certain time. While coaters strive to improve their technique, It is suggested that an Enhanced Aluminium mirror, (with more layers), is likely to need re-coating more often than a protected Aluminium mirror.

Removing The Old Mirror Coating
After say ten years or more a Protected Aluminium mirror will need re-coating. But first the old coating must be removed. Along with most other mirror makers, we use a chemical system to remove the old coating. In our case we normally use a combination of Hydrochloric acid with Copper Sulphate.
It only takes the chemical a few minutes to remove the coating and we wash all the chemical off as soon as all Aluminium has been removed. When you consider that for a Protected Aluminium mirror the acid has first had to eat through 190nM of Silica and then 85nM of Aluminium, – then not expecting the acid to carry on and eat away at the glass surface of the mirror underneath the aluminium is perhaps being naive?
If your mirror is 0.1λ “as measured at the focus” then it only takes an error of 25nM on the surface to take it out of tolerance. 25nM is a lot smaller than either the thickness of the Aluminium (85nM), or of the silica overcoat (190nM). If for instance there is some contamination that prevents the Silica and Aluminium being removed very evenly, then some parts of the mirror will become clear of aluminium well before the rest and the acid solution will continue and remove part of the mirror itself.
Remember that plate glass, or any low expansion glass, or virtually any other sort of glass used for mirrors can be described as “Silica with various impurities added”, and you can see that if the acid solution can remove a pure silica overcoat, it can also attack the surface of your mirror.
So the figure of the mirror can be affected before all Aluminium is finally removed. We do wash off the acid as soon as we can see the Aluminium has been dissolved and we seem to get away without needing to re-figure most of the time, – but it can’t be guaranteed. If there are extra layers, and more exotic materials are employed which are more difficult to remove, then its increasing the chance that re-figuring the mirror will be necessary after the coating has been removed.
Perhaps here is one reason large observatories prefer bare Aluminium? If the acid does not have 190nM of Silica to eat through first to get to the Aluminium, the Aluminium layer itself will be more evenly removed. This would minimise any effects to the underlying mirror surface.

Overall Performance Of Enhanced Over Protected Aluminium
(Or is the 5% extra light worth it?)

The mirror will be used either for direct viewing or for Astrophotography:
To make one stop difference in exposure time for Astrophotography, the difference would need to be about 50% rather than the 10% provided by a pair of mirrors. Oldham Optical suggest the improvement is hardly noticeable in practice. For direct viewing, then the light difference will hardly be noticeable to the naked eye either.
The enhanced Aluminium mirror costs more than protected Aluminium because of the greater number of layers and the increased precision necessary in the coating process. It will give up to 5% more light per mirror as a result.
But the Enhanced Aluminium coating has more layers. On balance, because of the increased number of layers the odds are that the enhanced coating will deteriorate a bit faster than the simpler protected Aluminium coating. When it comes to the end of its useful life and needs re-coating, then because of the increased difficulty in removing the coating,  the enhanced mirror is more likely to need re-figuring.
Other factors affecting the life of the mirror are environmental. If you can keep it in a dry atmosphere, it will last longer, If you are lucky enough to live in an area of low air pollution,  it will last longer. And if you can minimise temperature changes to avoid stress in the layers, – perhaps especially important for enhanced Aluminium mirrors, but also good for protected Aluminium, – it will also last longer.
If you really want more light to the eye, then perhaps the best way is a slightly bigger telescope with a larger primary mirror. Changing a 300mm diameter mirror for one of 325mm would give 20% extra light for example.
You may also get an improvement by changing your eyepiece? Choose a design that contains less glass elements. A glass air interface without an anti reflection coating loses 4% light. This does reduce to about 1% with a good anti reflection coating. But where you have a large number of separate glass elements within the eyepiece, the total losses could be over 10%.
Oldham Optical suggest that on balance the protected Aluminium mirror is the most cost effective for the amateur astronomer and that is what we supply. But if customers want an enhanced Aluminium mirror, Oldham Optical can supply a clean uncoated mirror for a third party to coat.
If you are considering an enhanced Aluminium mirror, then you should ask for full details of the enhanced coating. Ideally you will be supplied with a list showing each layer material and thickness. You should also ask for the expected life of the mirror before it needs re-coating. Plus the chances that the mirror will require re-figuring at the next re-coating.

And Finally – What Do You Think The Hubble Uses?
The Hubble mirrors are coated with Aluminium 100nm thick with a protective layer of Magnesium Fluoride 25nm thick. The use of MgF instead of Silica and at this thickness is to optimise performance in the ultra violet region.
Oldham Optical suggest the Hubble provides very good support for the use of protected Aluminium mirrors by terrestrial astronomers.