Mirror Care


From the moment a newly aluminised mirror is unwrapped, it will inevitably start to accumulate a fine layer of dust. Although this dust detracts from the pristine appearance of the mirror it will have little effect on performance unless the mirror becomes very dirty.

The main effect of the dust is to cause a slight loss of image contrast due to the light scattering effect of any dust or grime. It will also slightly reduce the reflectivity of the surface.

It is best to take every reasonable effort to stop the mirror from getting dirty in the first place by keeping it properly covered when not in use and storing the telescope in a clean environment. A clean Chamois leather can be used to cover the main mirror of a Newtonian. It should be kept warm whilst the telescope is in use and then replaced over the mirror when it is put away. Based on experience, this will protect the mirror and will not harm the aluminium coating even if it is directly in contact with it, so long as it is dry.

It is also advisable to avoid taking a cold telescope into a warm room as this is likely to result in condensation forming on the mirrors. If the telescope can be stored somewhere that is at outside temperature, it will also mean that when the telescope is taken outside to be used, it does not have to cool down before it will give steady images.

Dust can be carefully flicked off with a clean lens brush or blown off with a clean air duster. Care should be exercised when using aerosol duster as they can blow out liquid propellant which will mark the surface (although this will not cause any permanent damage).

Aluminised mirrors should never be rubbed with anything in an effort to clean off grime. This will inevitably scratch the coating. This sort of scratching, consisting of many fine parallel scratches, acts like a diffraction grating and will give rise to clearly perceptible bands of light radiating from stellar images.

If the mirror has become very dirty, a careful washing may be desirable. However, a new aluminium coating remains quite soft for some months and any cleaning should be avoided at this time. Over time, a very thin, hard, transparent layer of aluminium oxide forms and this protects the underlying metallic aluminium.

The mirror can be carefully immersed in lukewarm slightly soapy water and allowed to soak for a few minutes. It can then be wiped under the surface of the water very carefully with cotton wool. It should then be carefully rinsed with lukewarm water followed by a final rinse with lukewarm distilled water. It should not be rinsed with cold water as, in an extreme case, it could crack due to thermal shock. It should then be stood on edge and any drops of water remaining on the aluminised surface should be picked off with the corner of a piece of tissue. If done carefully, this cleaning process will clean the mirror very effectively and without harming the coating.



Telescopes for Different Types of Observing

There are many different types of astronomical observing that can be undertaken and no one telescope is ideally suited to every task.

Two common types observing that most amateur astronomers will wish to do are high-resolution lunar/planetary/double-star observing and then "deep-sky" observing.

Whilst a variety of telescopes will permit both types of observing, a single telescope is not going to be optimised for both of these contrasting functions.

Most of the telescopes that I have made have been Newtonian Reflectors and so I will confine myself to thoughts about the design of this type of telescope in relation to the type of observing to be undertaken.

Lunar/Planetary Observing

The objects of this type of observing that are capable of revealing fine detail are mostly fairly bright - the Moon, Venus, Mars, Jupiter & Saturn and double stars. For this reason, a large aperture is not required. The main requirement is to construct a telescope that will deliver the highest possible resolution and contrast. A strong limiting factor here is the atmosphere and in England, it would be a very exceptional night where the atmosphere allowed detail finer than Żarc-second to be resolved. This resolution is within the range of an 8 to 10-inch telescope. Also, a larger telescope can gather sufficient light for the image to be dazzling, particularly with the Moon but also with the bright planets.

Some desirable features of an ideal Lunar/Planetary telescope:

1.  Aperture in the 8 to 12-inches region. This gives ample light-gathering power and the theoretical resolution of the telescope equals or exceeds anything that the atmosphere will allow on anything other than a very rare and exceptional UK night.

2.  Focal ratio of f/6 upwards. Preferably f/8 or even f/10 This gives the advantage of satisfactory fully-illuminated field whilst still having a relatively small secondary mirror and thus reducing the adverse effects from diffraction around the secondary mirror. This is also helped if a low-profile focuser is used so that the focal plane is as close to the main optical axis as conveniently possible.

A further advantage of the longer focal length is the availability of higher magnifications with relatively longer focal length eyepieces with a correspondingly greater eye-relief and larger eye lens diameter. Also, eyepieces often work better with a less severely converging cone of light from a longer focal-ratio telescope. These factors are not so much of an issue now with many modern designs of eyepiece that are designed to function with fast optical systems but these eyepieces are very expensive and often very heavy.

3.  Other details such as thorough blackening of any parts of the telescope in or near the light-path to avoid stray light reaching the focal plane, lining the tube with black felt etc. will noticeably improve the image contrast.

"Deep-Sky" Observing

Deep-sky observing, where the objects are generally of a faint and fuzzy nature require above all a telescope that will gather as much light as possible to bring these objects, galaxies, nebulae,  (To be continued . . .)

27th November 2015