How much good does a good diagonal do?

I've been seeing a lot about 99% reflective dialectric coating diagonals from William Optics, Stellarvue, and others. How much of a difference would one of them make for lunar and planetary viewing with a typical entry level scope like my Observer 70?

It would make some difference. But is no substitute for aperture.

>>>I've been seeing a lot about 99% reflective dialectric coating diagonals from William Optics, Stellarvue, and others. How much of a difference would one of them make for lunar and planetary viewing with a typical entry level scope like my Observer 70?
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Lee:

My experience is that a premium diagonal would not make a big difference. For example, the other night I was observing Saturn with an Celestron C4-R, a 102mm F10 refractor that is a reasonable planetary scope. I was using the standard 1.25 inch Chinese mirror diagonal that came with it and getting good views of Jupiter at 143x and double stars at 200x and above.

I switched to a TeleVue Ever Bright 2 inch eyepiece later in the evening and the planetary/double star views seemed to be the same as with the simpler diagonal.

Some thoughts:

1. The difference in reflectivity is only about 10%, hardly noticeable and since the brightness decreases by the square of the magnification, the better diagonal would support only a 5% increase in magnification, hardly worth it.

2. At higher magnfications the amount of the diagonal that you are actually using is small so the flatness is only important over that small area, this is not hard to achieve.

3. Roland Christen, who has been selling dielectic diagonals for several years, has repeatedly said the reason to get a dielectric diagonal is that the surface is harder than a standard diagonal so that it can be cleaned without damage.

4. The cost of a good dielectric diagonal is a significant part of the cost of a better planetary scope and mount. A larger Newtonian such as an XT-6 or a SpaceProbe 130ST would be the way to go if you are interested in getting better planetary views without investing the big money.

5. Chinese diagonals can be out of collimation, not hugely important but it can make a difference, it is easy to fix. Of course my TeleVue/Vixen 1.25 inch diagonal is also out of collimation but it is not so easy to fix...

6. Buying a Dielectric diagonal with an eye to future would probably not be wise since it is most likely that in the future you would want a 2 inch diagonal rather than a 1.25 inch.

Bottom line: If you had a ED-100, then the investment of a fancy diagonal would probably be worth it. As it is, with an 70mm F10 scope your current diagonal is sufficient so I suggest saving your money with the idea of acquiring a larger scope that can provide better and brighter planetary views.

Best wishes,

Jon

Lee, I'm 100% behind what everybody else has said here...
I would NOT invest in a better diagonal at this time... as it won't make a difference.
Save your money for when you upgrade your scope, and then go for the bigger/better diagonal if needed...



Clear skies! 8)