Checking Your Telescope and the 'Figure' of its mirror. The easiest way to check the figure or curve of the main mirror is to make use of the existing telescope. An experienced mirror maker can use a high power eyepiece in the telescope and miss focus the telescope eyepiece inside and outside of focus. The out of focus image of a star will appear the same a certain distance inside of focus as it does the same distance outside of focus. The atmosphere must be steady to get a good reading. You may as well try this test yourself since the test requires no test equipment. Next we might want to try to isolate telescope optical problems. Rotating the eyepiece in the eyepiece holder will isolate some eyepiece problems. Or try the eyepiece in another telescope. While the eyepiece is out of the eyepiece tube roll up a piece of printer paper and put it into the eyepiece tube. Is the eyepiece tube directed toward the diagonal? Problems of the diagonal may be due to misalignment of the diagonal or warping of the diagonal by its mounting. Some problems in the main mirror may be isolated by rotating the main or primary mirror in its mirror cell. Also on Dobson mirror cells tip the telescope up and spank the telescope on its bottom to seat the mirror on the mirror cell adjusting pads. Having the telescope now in operating condition we can return again to check for the proper optical correction of the primary mirror. Most errors in the primary mirror are symmetrical around the mirrors center. While mirror polishing the mirror is rotated every few polishing strokes in order to maintain its symmetry. Reading a mirror using the eyepiece is a bit challenging. With a bit more equipment reading a mirror is easier.
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Checking the figure of the primary mirror using the suns
reflection from a power pole insulator and a Ronchi grating.

 There is a more intuitive method for checking the figure of the primary mirror. To use this method we will need to obtain a 100 line per inch Ronchi grating. The Ronchi grating is merely a piece of glass or film with equally spaced black lines on it. The Ronchi grating appears gray from a distance but inspection under a magnifying glass reveals clear glass between the black lines. The ronchi grating is available from Orion, Telescope and Binocular or by mail order from Edmund Scientific Company. Remove the eyepiece from the telescope and place in a safe place. Find a cap that will just slip over the eyepiece tube. Possibly the lid from a gallon can. Glue the ronchi grating to the cap with a little RTV silicon sealant. On a sunny morning find a shady place for the telescope and find a distant telephone pole. Use the telescope finder to point the telescope at the suns glint from a ceramic or glass insulator on the telephone pole. CAUTION, do not point the telescope in the direction of the sun. With your head a foot or so from the eyepiece tube look down the eyepiece tube and center the power pole insulator behind the reflection of the telescope diagonal in the primary mirror. Now install the Ronchi grating on the eyepiece tube and place your eye as it would be while looking through the telescope eyepiece. Look at the primary mirror reflected in the diagonal. Move your head up and down and left and right until the entire mirror appears to light up from the light coming from the insulator. You should see perfectly straight black lines that appear to be on the mirror surface. Use the telescope focus to move the grating in and out. The lines become bigger and smaller but always stay straight. If the ronchi grating is exactly at the focus the mirror will appear all dark or all light depending whether the focused light strikes a black line in the grating or passes between two lines. The slightest vibration of even 100th of an inch will cause this to change as will the slightest refraction caused by heat waves. Move the Ronchi grating toward the tube using the focus control until five dark lines are visible, apparently on the mirror surface. This location for the Ronchi grating ( inside of focus ) is the setup used at the Chabot Space and Science Center, Telescope Makers Work Shop. If the lines are not straight the mirror is too close to the telephone pole insulator or there is an imperfection of the surface of the primary mirror. If the curve of the mirror were a sphere instead of a paraboloid the lines would be closer together at the center of the mirror and farther apart at the edge of the mirror. This is because the ronchi grating would be nearer the focal point of the mirror edge when inside of focus. Mirror makers call this condition 'under correction' since in the past it has been customary to make the mirror spherical first then 'correct' the figure to a paraboloid. In another common aberration the edge of the mirror focuses too distant from the mirror. The ronchi lines are straight as they should be except at the very edge of the mirror where they narrow. Mirror makers call this condition 'turned edge'. This explanation of the Ronchi test should be sufficient for you to find if you have a good telescope, optically at least. Recoating the primary mirror will not change its optical figure. If you need or want to refigure the primary mirror you will need additional information. For more information about the Ronchi grating try the at Stellafane website.  Willmann-Bell, Inc. will have needed materials.

For more information on mirror making I recommend my 'Build your own Telescope' program. The online version is at Build your own Astronomical Telescope. There is more to mirror making than we can fit on this webpage. 

Update, November 20, 2016