FAQ’s and Articles

Please find below some answers to FAQ’s (Frequently Asked Questions).

Placing an order

To place an order simply confirm your requirements in an email. No deposit is required. When the mirror is finished full payment will be requested by bank transfer prior to shipping. Please note that all bank charges for the transaction are the responsibly of the buyer. Please check exchange rates carefully to ensure the full amount in UK pounds is paid. Overseas buyers may need to pay import duty, please ensure that you are aware of this to avoid any nasty shocks.

Secondary mirrors

The secondary mirror size for any telescope is determined by a number of factors including the fully illuminated field of view required, the diameter of the telescope tube and the height of the focuser and whether or not a coma corrector is to be used. The secondary’s recommended by us should be suitable for most telescopes. We try to be flexible please let us know if you have a particular size requirement for your secondary mirror. Don’t get too obsessed about secondary mirror sizes, a few mm oversize will have no effect on the image seen at the eyepiece.

Mirror thickness

Mirrors between 18 and 24 inches in diameter generally have a finished thickness of about 44 mm, this can vary slightly. 14 and 16 inch mirrors can be between 40 and 45mm in thickness. 12 inch mirrors are available in 25 mm and 35mm thickness, the price list page shows prices for 25mm thickness, please enquire for 35mm thickness prices.

Mirror coatings

For both primary and secondary mirrors enhanced aluminium coatings are used. Coatings are  provided by Orion Optics, more details can be found here…. http://www.orionoptics.co.uk/OPTICS/opticalcoatingsh.html

Coma correctors

All paraboloidal telescope mirrors suffer from an off axis aberration called coma, as a result of this star images near the edge of the field of view look like little comets. When to employ a coma corrector is a sometimes a matter of personal choice, for mirrors faster than about F4 they are pretty much essential.  Between F4 and F5 is the area in which some observers have differing opinions as to whether they are necessary.  Mirrors slower than F5 do not generally need a coma corrector.

Scratches

The effect of scratches on a mirror is to scatter light resulting in a loss of contrast in the image. The amount of scattering produced by one or two scratches would certainly not be visible at the eyepiece. Dust on the surface of the mirror would have a far greater effect on the image as a result of light scattering. Most observers use mirrors with some degree of dust coverage without any perception of a degraded image. Some years ago an unfortunate incident occurred at the McDonald observatory in Texas which resulted in a handgun being discharged into the primary mirror of the 107 inch Harlan J Smith telescope. The resulting craters in the mirrors surface had very little effect on the performance of the telescope which continued in service. The point being that one or two scratches are of no significance in terms of optical performance.

It is not possible to avoid occasional cosmetic faults resulting from bubbles in the glass, pits and fine scratches. These are no more than cosmetic issues which will have no effect on the image formed by the telescope.

The majority of optical surface made by Nichol Optical are free from any cosmetic defects. Occasionally a surface may suffer from minor cosmetic issues which will in no way affect the performance of the optics.

Articles

Optical Standards

The Testing of Astronomical Telescope Optics

Thoughts on Focal Ratios