Testing your telescope There are many telescopes on the market today which are advertised as '1/8th wave', '1/4 wave', 'diffraction limited', 'superb optics', 'razor sharp images', etc., etc.

What does it all mean?Basically these statements are made to make the telescope appear more attractive by quoting what appear to be standards which the telescope being offered will reach and also a few colourful adjectives thrown in which again, appear to make the telescope an attractive proposition as compared to the competition. They are all methods of giving you information to assist the sale of the telescope. Unfortunately, apart from a select few manufacturers, the correct method (s) of explaining a telescopes optical quality, is rarely used. Reason being, it would immediately show that some telescopes were way, way below the accepted minimum optical standard for an astronomical telescope.How can you tell if your telescope achieves this minimum standard, or even surpasses it. Does it need expensive equipment costing thousands of pounds? Do you need to be an optical genius with years of experience? Does it need a computer?The answer to all three is no. You can, with a relatively inexpensive piece of equipment and a little practice, be in a position to detect errors in your (or anyone else's) telescope which are capable of causing problems with the image quality. The tests are sensitive enough to see errors which are, in several cases, too small to affect any noticeable difference in the image quality. Eyepiece tests on stars are an excellent method of detecting the smallest of errors but, it needs quite a bit of experience to give a decision with confidence as to what, if anything, is amiss with your optics. The Ronchi test, using a grating of very fine lines is far easier to see and understand. A few minutes practice will enable you to see the overall situation with your optics.

What is it going to cost? Well, you can do it cheaply with a plastic printed set of Ronchi lines for about 10. Trouble with these is they are almost invariably too coarse to see anything other than glaring faults and, they are easily marked and become useless. Orion optics offer Ronchi fine lined gratings 250 lines per inch, on glass, mounted in an aluminium holder suitable to fit in all 31.7mm focus mounts. These cost 79 plus posting and packing, 4 to UK, overseas postage will vary depending on location. Order here by email or visit our online shop and order there, they are in the Accessories section listed under Miscellaneous.

How do you do it? The tests are carried out at night. Point your telescope at the Pole Star if you do not have an RA drive fitted. If you do have an RA drive fitted, any bright star will do. Position the star in the centre of your field of view with a low power eyepiece and then replace the eyepiece with your Ronchi band adaptor. Focus as you would normally and you will see black and white bands begin to appear. Adjust your focuser until about 3 or 4 bands are visible and ensure you are within the focal point not outside it.With a perfect telescope you should see perfectly straight black and white bands perfectly parallel to each other with no deformities at all. It is doubtful if this will be the first view you have unless you have an excellent telescope. Before you can carry out the test you must ensure your telescope has had ample time to cool properly or incorrect assessment will result. Before going any further, as I said earlier, this test can detect minute errors which, have no appreciable effect on images. However, some errors appear small on the test but have a marked effect on image quality, in particular 'turned down edge'. Do not get paranoid if small errors are present, if the images your telescope are producing are good and are satisfactory to you, you probably should not be reading this document anyway. On the other hand, if you are experiencing image problems you are virtually guaranteed to detect exactly why with the use of these Ronchi bands.

What will you see? Not easy to describe, so below are some diagrams and explanations of various faults. Combinations of these faults are usually evident in some 'scopes so, you need to evaluate what you are seeing as a total situation. Two images are shown for each fault. The left hand image is shown viewed inside focus, the right hand one is outside focus. The bands have been exaggerated in contrast to assist in clarity of the explanation. The drawings are shown for refractors but they apply exactly the same for any telescope, reflectors included but the central area will normally be invisible because of the central obstruction of the secondary mirror. Same applies to SCT's, Maks. etc.

What you could see What is the fault? Is it serious? Can it be rectified?

No fault. Excellent optics

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Astigmatism. Common fault, especially in refractors and Schmidt Cassegrains. Images may be reversed and irregular but the fault is obvious.

Very serious indeed.

In a word, no, not without spending a lot of money.

A hill in the centre. An under-corrected central area not seen in catadioptrics or most reflectors.

No, provided it is not too large in diameter or the difference in line widths is not too noticeable

Yes, but needs expert advice and skills.

A hole, overcorrected central area not seen in catadioptrics or most reflectors. Notice is it exactly opposite to a 'hill' above

No, provided it is not too large in diameter or the difference in line widths is not too noticeable

Very difficult. Unless a problem aspect, leave it alone.

General over correction. Very common fault in badly figured telescopes

If visible with 200 lines per inch Ronchi bands as seen on left, yes, it is a big problem. Stars will never focus properly and planets lack sharpness

Reflectors can be improved, refractors and catadioptrics, very expensive, often impossible.

General under correction. Common fault in badly figured telescopes, especially Newtonians and refractors

If visible with 200 lines per inch Ronchi bands as seen on left, yes, it is a big problem. Stars will never focus properly and planets lack sharpness

Reflectors can be improved quite readily, refractors and catadioptrics, very expensive, often impossible.

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Sunken ring. Overcorrected circular zonal ring. Present to some extent in many telescopes

Not serious unless the zone is pronounced and near the edge

Very difficult to remove.

Raised ring. Under corrected circular zone. Quite uncommon.

Not serious unless the zone is pronounced and near the edge

Reflectors are readily corrected, all else, very difficult.

Turned down edge. The image destroyer. Images sharpness is very low.

The edge of the optical system is turned down, overcorrected.

Very Serious problem. Images are affected badly if pronounced. Very common fault in low grade optics

If you have one, you have an almost non-removable error without very high rectification costs.

Turned up edge. Uncommon in reflectors, often visible in refractors and SCT's

Not quite as serious as TDE above but will still effect image quality if pronounced, over 2-3% of overall diameter of system

Reflectors can be rectified, anything else, costly to impossible.

Obviously there are faults in telescopes which are not shown above but by far the majority of faults in telescopes optics are shown. Combinations of the above can make a diagnosis difficult so, if you have a problem not shown, or a combination you can't decide on, email us with an explanation, a sketch always help. List the telescope you are using and as much information on the Ronchi test results. if you are unable to use a Ronchi test, just mail us full details of your problem, making sure to list make and model, and we will reply with hopefully some helpful information.

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