Telescope Reviews: Optical Quality ?

The C8 was slightly out of collimation.

In the first image, you see a little box at the bottom that has a line that says "Tilt" and it states that for the quality analysis the tilt was "Removed". This is why you see it in the star test, but in the "Energy Distribution" plot (third image) you see the first ring as even.

This is a really excellent C8 by the way.

For the TEC image, you can see in the star test that there is a bit of astigmatism. As I mentioned in my earlier post, this would be impossible to see on the in-focus image and it is such a small amount that it has on effect on the telescopes performance. The camera shows that there are areas that are slightly briger and slightly dimmer in the rings, an the intensity plot shows it, but for the eye to easily see it, the difference in brightness needs to be about 15%. In other words, for the eye to easily see it, the brightest section of the ring would have to be 15% brighther than the darkest section of the ring. Our eye's contrast sensitivity is not that great.

Tilt has again been removed for the AP during the processiong, so you see a very slighty brighter ring on one side in the star test, but he has removed the tilt to calculate the Strehl.

And yes, the Synta has world class optical quality. Not that it is an APO, but for what it is (an ED doublet) it would be meaningless to improve the quality past this point. Again, .95 and better is from a practical standpoint, optically perfect. I Don't think there is an observer alive that could tell a .95 Strehl scope from a perfect one of the same type on the in focus image. Sure you might see it on the star test, but that is what makes the star test so powerful. It shows every deviation from perfect.

The Ronchi test.. It is almost useless for amateurs becasue it is not very sensitive, though I have beena able to see turned edges with it, and you can see them in many of the tests on Rohrs site.

Rohr however, is using a double pass autocollimator. This vastly improves the sensitivity of the test.

And what the Ronchi gratings are doing is shoing the surface figure, and when there are colors, they are for different wavelenght of ligth.

In a perfect reflector, they would be perfectly straight.

In systems that contain lenses (like the SCT corrector) there is often some difference in focus between the different colors of light.

So, the Ronchi shows how much deviation from perfect the surface is. A rough or astigmatic surface will show distorted lines. A turned edge will show a slight bend at the ends of the lines... And so on.

This is a site that one can learn a hugh amount about optics.

For exmaple, someone took offense a while back when I said Mewlons did not seem to enjoy the very high quality that Tak Refractors enjoyed.

Someone basically said I was being silly (not those words, but implied as much), and yet the Mewlons Rohr tested varied in quality be quite a bit, with (as I recall) two out of three falling just out of the "Excellent" catagory.

He has also shown many examples where there was some kind of mechanical or alignment problem that once resolved, greatly improved the performance.

And there are dogs... Not just telescopes that are OK or passabe, but rally bad. There are a couple of Meades, and a Vixen that are total barkers. But I would never say all Meades or all Vixens were not as good as this or that. Clearly though, lemons get through from time to time.

And that is why it is so important for amateurs to be able to star test properly. It is not hard at all, but it is considerably more ivolved than simply racking the focuser in and out.. That tells very little. It is a series of tests and evalluations, each telling you about one specific kind of optical defect and if it is present, and if so, how bad it is.

By doing all of the tests and adding together the results, you can see how good (or bad) a particular sample is.

Most Celestron SCTs are actually quite good. It is very rare to see a lemon, and in fact, many fall into the excellent catagory.

Synta builds excellent scopes.

Zumbuto figures amazing mirrors.

Intes Micro builds consistently excellent scopes.

A few envings reading Astro-Foren and learing to intrepret the results will give you a far better insight into what is likely and what is unlikely when it comes to telescope quality and brands.