Finishing testing of the Lens

Residual Lateral Color and Astigmatism:

Once you've mounted your lens and correctly collimated it, it's time to perform the last tests. These will assess the state of the correction for lateral color and astigmatism. You must let your lens, cell, and tube reach thermal equilibrium. Turn on the fan, if you have one (you should), and walk away for an hour. When you come back check the collimation again and rack through focus. Look once more for residual spherical aberration. It may be helpful to point the scope at a yellowish star of moderate brightness and put in a yellow or orange eyepiece filter. Then perform the usual star test by racking in and out of focus to equal distances and looking at the diffraction pattern. It should look identical or nearly so. No strongly marked zones should be visible.

Now move in close to focus. Does the image stay round or become elliptical in outline? It should stay round. Go to best focus. Do you see a round Airy disk and round Fresnel rings encircling it? Or does the image look like a small cross. Scrutinize it carefully. If you do see an oval outline close to focus and the orientation of the major axis shifts by 90 degrees from inside focus to outside then there's cause for concern.

There are several possible reasons for this appearance. As mentioned previously, tube currents can be a problem. But if you have your fan on and it's been on for some time, then this cause is unlikely. Sometimes the external seeing, if the air is cold and the jet stream is blowing overhead, can produce a similar appearance. You should only make your star tests on nights of good seeing, when the diffraction pattern looks relatively quiet away from focus. Certainly if it looks turbulent and boiling, then a star test may become inconclusive or misleading.

It's also possible that your cell is pinching the lens. Another problem could be that the focuser is not squared with the tube. This will cause the eyepiece to be held askew to the beam from the objective. Eyepieces should be squarely presented to the optical axis of the main lens or mirror. I remember that when I first finished my 8" folded refractor, I got a persistently astigmatic image. It took weeks for me finally to realize that my focuser was askew to the folding flat. The system looked collimated when I set my eye at the back of the focuser, and the objective plus two fold flats probably were square to one another. But the eyepiece drawtube was not. Once I fixed that problem, magically the astigmatism went away. Eyepieces have astigmatism also and it is one of the big aberrations in them.

If you have made your lens carefully and heeded the precautions about turning the lenses in their support beds periodically, then the last thing that you should suspect is the quality of your lens. So try to eliminate all other possible sources of astigmatic images. But if nothing else seems amiss, then it could somehow be your lens. You can test this by turning the lens in its cell and seeing whether the major axis of the defocused star image follows suit. If it does, then you have a problem. Fortunately, you choose to make an achromat instead of another mirror! With a mirror you'd have little choice but to go back to polishing or even grinding. But with a lens, an old optician's trick may save the day. It is possible that if you rotate the crown and flint with respect to one another, the astigmatism of one lens may compensate the astigmatism of the other, if they both have it. Or at any rate, the total astigmatism may come down to an acceptable level.

The way to do this is to mark the present orientation of the lens elements with respect to one another, and then turn them 45 degrees or so. Then reassemble the lens, retest and see if the situation has improved. Keep going until you find the position of least aberration. One may hope that in a particular position the astigmatism will be neutralized fully. If you can find that position, then mark the side-walls of your lenses with a "V" shaped mark that extends across both elements. That relative position of the elements must always be maintained or recovered after periodic cleanings.

In exactly the same way, the fault of lateral color may be corrected if it rears its ugly head. Lateral color is also unlikely, so long as the elements have been edged accurately round and to the same size and dewedged correctly. If they have, then when you examine a star image at high power, you should not see different colors fringing one side of the Airy disk than the other. To check this accurately, make sure that you hold your eye square to the eyepiece's optical axis. Because if you don't, then your eye is making an oblique presentation to the optical axis and the color error is in your eye (which is totally non-achromat). Not even a reflector's image can withstand this. Your eye must be presented squarely to the eyepiece. Taylor discusses lateral color error in great detail in his book.

But assuming that your eye is looking squarely into the eyepiece, then you must also make sure that you have not selected a star at too low an altitude. Choose one near the zenith if possible, or close by. Stars at less than 45 degrees - even in a reflector - will show lateral color error. This is caused by the atmosphere and is known as "atmospheric dispersion." If your test star is only at a moderate altitude, then check to see if the red fringe is on the higher side closer to the zenith, and the blue fringe on the lower side closer to the horizon. If so, then in all likelihood you're seeing atmospheric dispersion.

But if the lateral color really seems connected to your lens, then as with astigmatism, you may turn the lens in the cell and see if the fringe pattern moves along with it. If it does and you can find no obvious cause (make sure the elements sit squarely on one another and that the spacers are not of different thicknesses or placed carelessly at different distances from the edge of the lens toward its center)--if no other cause will explain the lateral color, then the only resort is to rotate the lens elements with respect to one another and see if that diminishes the problem Certainly you made a construction error of some kind in the lens. Do the rotation just as for astigmatism.

Of course, if you have astigmatism AND lateral color, then you had might as well regrind the lens and learn your lesson to be more careful. It is probably hopeless to think that the same rotational position for the crown and flint will cure lateral color and astigmatism at the same time.

This finishes the testing of your lens. If it is now completely polished and has received a final cleaning and assembly and has checked out well for all its image aberrations, then CONGRATULATIONS! You have successfully completed a challenging and impressive optic, and are now ready for other lens projects, should you choose to undertake them. But certainly you should spend a lot of time getting to know and enjoying your achromat. Over time, it may become a dear friend as you learn to savor its fine and colorful character!

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