Ronchi [star] testing achromatic refractor objectives.

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Since purchasing my Ronchi test eyepiece I haven't used it much at all. Normally [?] I have always had a habit of knife-edge testing every telescope [at the focuser] to see the figure of the objective. The problem with refractor objectives [rather than reflectors] is chromatic aberration. This effectively blurs the Ronchi bands because the light focuses at different points along the axis depending on exact wavelength. This is why a green filter is often suggested for star testing achromats. 

The Ronchi test would similarly benefit from reducing the test light to a narrow band of wavelengths in the visible spectrum. With all the rest of the spectrum discarded, the achromat is then effectively tested [for spherical aberration] in green light. Green-yellow is the light to which the human eye is most sensitive. Which means that it is more easily able to reject [subjectively] dimmer colours. The colours still have much the same brightness but the human eye/brain system can ignore them relative to the preferentially brighter yellow-green. The human eye is least sensitive to violet so can usually tolerate a violet wash. The lenses of older observer's eyes naturally yellow with age with a further subjective reduction of the purple haze. Which is why older users often report a perfect view from a faster lens. Where younger users would complain bitterly about the violet. 

The real experts in lens testing, and presumably achromatic lens manufacturers, use a large, precision optical flat and a laser as a light source. The light passes from the laser through the lens and is then bounced back through the lens again via the optical flat beyond it. Thence back to the camera or tester's eye. This is known as double pass testing.

The major advantage of such testing is the increased visibility of lens errors since they are effectively doubled. AND, that the tester does not have to wait for a test star to show between thick clouds. NOR, do they have to worry about the "seeing conditions" on that rare, perfectly  clear night. 

Thermal agitation in the atmosphere between the light coming from the distant star is perturbed by the difference in refractive index with changing temperature. Twinkling of the stars is the usual sign of thermal agitation. Using a telescope when the stars are twinkling strongly can add a "boiling effect" superimposed on views of the Moon or planets. Naturally this usually spoils the view!

When I first tested my iStar 180mm f/12 R35 with my new Ronchi test eyepiece I saw strongly curved test bands near focus. At the time I falsely assumed that this was the result of using a white light source. [A bright star without filtration.] In other words, my blind loyalty to my purchase was making me completely blind to reality. Regardless of colour [frequency] the Ronchi bands should still remain perfectly straight in a spherically corrected lens. 

Each different colour [frequency] of light will have slightly different focal point. The closer to focus the broader the bands appear and the further apart. These straight Ronchi test lines will all overlap in a confusing picture. But, they should all remain straight if the lens is well corrected for spherical aberration. Which it should be of course if it is to lay claim to being of astronomical quality and "diffraction limited". Which is the minimum standard expected of any decent telescope objective.    

Months later, while reading a thread on Cloudy Nights, I noticed an image showing nice straight and dark Ronchi bands from an objective test. I then asked "the tester" which green filter he had used. Whereupon I discovered that he used a green laser, microscope objective and optical flat to achieve his remarkably sharp test images indoors.

He did not recommend Ronchi testing on stars because of the "seeing" problems and chromatic aberration problem. The downside of his efforts is having to afford or borrow a large optical flat for testing at full aperture. Not to mention the skill required in setting up the indoor test system and then capturing the results.

Very narrow passband, green optical filters are available. However, I discovered a Baader Solar Continuum filter had useful testing properties for my intended use. The SC filter is meant to clean up white light, solar surface images which can be improved further with an IR/UV cut filter. [According to a comprehensive test of various green filters for solar imaging.] Since I hoped to do some solar imaging myself I took a very deep breath and placed an order for both filters online in the 1.25" size.

 CCD moon and planets

Last night, I tested my little Vixen 90mm f/11 refactor on the Fullerscopes MkIII mounting using the Neumann 10 lpmm/ 250 lines per inch Ronchi test eyepiece. Arcturus was shining brightly, though rather low down to the West, just above the local trees. The low altitude should have saved me some neck strain. It didn't, so I added a Baader 45° terrestrial prism to bring the test view almost horizontal. Capturing what was clearly visible with my eye was quite another matter! Starlight, spread over a distant lens, is both dim and small. [No eyepiece magnification!] The compact digital camera needed full zoom to make the test image [of the bare objective] large enough.

The problem then was the dimness. Every time I pressed the shutter button the image simply vanished from the viewing screen. Fortunately I managed to capture several full aperture images more by luck than skill. When considerably brightened, and further enlarged by cropping, I had a fuzzy and colorful record of my Vixen lens quality. Seeing conditions and the prism diagonal seemed to have zero effect on the quality of the lens test. Though the erecting prism caused a wavy line of discontinuity through the middle of the last image, the quality of the bands looked no different from the direct view. You can judge the images here for yourself. It should be said that for optimum accuracy the lines should really be expanded to show only a few by drawing closer to the focus. Having more lines visible shows the general nature of the surface including any edge problems. 

It follows that testing the bigger iStar achromat should produce very similar results. The Ronchi test lines should be just as straight as the Vixen's. I will share the images here when I have them. It was too late last night after practicing with the Vixen. The sky was still not fully dark after 11pm.

I really need to make a simple camera alignment adapter to center the 'nose' of the camera lens over the Neumann Ronchi EP. I have made a similar [recycled] detergent bottle top adapter already for [normal] 1.25" Meade 4000 eyepieces, but it won't go over the Neumann Ronchi test EP.

One slight oddity is the lack of a filter thread on the Neumann Ronchi test EP. The 'body' is very short and thick walled. It seems an oversight not to have provided a standard thread for a 1.25" filter.

Click on any image for an enlargement.

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