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Unlike a Newtonian reflector, a refractor uses only a very small portion of the diagonal to form any one star image. At f10 a refractor only uses about 0.2" inches of a 2" format diagonal for any one star. You need to have a nasty diagonal to see a significant error on a spot that small. Even at f5, the surface area of the diagonal that contributes to a single star image is less than a 1/2" diameter. Unless the mirror is very poor indeed, it won't affect your testing.

dan

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Unlike a Newtonian reflector, a refractor uses only a very small portion of the diagonal to form any one star image. At f10 a refractor only uses about 0.2" inches of a 2" format diagonal for any one star. You need to have a nasty diagonal to see a significant error on a spot that small. Even at f5, the surface area of the diagonal that contributes to a single star image is less than a 1/2" diameter. Unless the mirror is very poor indeed, it won't affect your testing.

dan

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Dan:

True, but the diagonal still could be miscollimated, some are surprisingly bad. This is particularly true of inexpensive mirror diagonals but I had a 1.25 inch TeleVue/Vixeen that was quite bad.

I would test it with the diagonal and if something looks fishy, try it without.

Jon

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Jon,

I agree that a diagonal can be poorly assembled or miscollimated but it is surprising how little this actually affects the image. You need a really gross diagonal error to significantly affect the star test.

Assuming 2" from the mirror surface to the focal plane, a one degree tilt in the mirror will displace the image at the eyepiece 0.0349". That's not very much and it can be difficult to maintain the star image on axis within that tolerance without a tracking mount. Even a very poor diagonal with a 2 degree tilt displaces the image only 0.070" or 1.778mm. Of course, this can be easily seen as off axis in an eyepiece that covers a 10mm diameter field.

If one attempted a star test with the image displaced by a tilted diagonal one would simply re-aim the scope to centre the target in the eyepiece. With a 600mm focal length scope the 0.0349" displacement of a one degree diagonal error, corresponds to 0.085 degrees off-axis for the image from the objective. That is a very small misalignment and I seriously doubt it would appreciably affect the image in a star test. The angular displacement is proportionally less in a longer focal length scope and it would be even more difficult/impossible to see any affect on the star test.

Perhaps there are others who are considerably more experienced than I am but I remain skeptical when I see reports of how much a refractor can be improved by minor adjustments to collimation errors in diagonals and focusers.

dan

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Dan,

I have always collimated refractors that have a non-collimatable cell by adjusting the focuser. One example is my Orion ST-80 guidescope. It was way out of collimation and I simple capped the tube, inserted my cheshire and repositioned the focuser until the collimation was perfect. A mis-collimated diagonal will definitely throw your overall collimation out on a doublet refractor. Try it--it is easy to test and see.

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Dan

All reasonable points. Still. I have seen st-80 diagonals that were misaligned enough that a laser hit the tube wall or that an eyepiece vignetted.

Jon

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