Let's talk R30 Anastigmats

[Mike]

If I said I had two telescopes set up side by side and one was an ED doublet APO and the other was a classic achromatic doublet, would it be believable to say the ED scope showed 70% less color (chromatic aberration) than the classic?

Let's say a Televue 102 F8 (a common ED APO doublet) and an Istar classic achromatic 100 F8. What do you think? Let's start by a simple yes or no is it possible?

Mike

[gord]

Hi Mike,

Thanks for the invite! And taking the time to discuss this more, it's an interesting subject.

So, to answer your question, I'm going to say no.

Let me preface that a bit before explaining why. It's the 70% part that I (myself) find a bit confusing. I'm assuming you are meaning that for whatever amount of color defocus the achromat has, the apo/ED will have 70% less?

I find an easier way to look at it is with the absolute numbers, that is the amount of defocus for each of the different colors. This is the common LA, or longitudinal aberration graphs that are used to describe the color errors in refractors. They basically use the green focus point (ie. the focus length) as the reference and then measure the focus points of all the other colors relative to that. Usually this is limited to just the red and blue (sometimes yellow) for visual cases since those are the most important ones.

BTW, I'm not giving Mike an education here, just setting the context for my answer and this might be useful to some readers as I know it isn't well understood.

So, getting back to my answer, I'm saying no. Below are a couple of examples from Astro-Foren (Mr. Rohr's site) to illustrate why. One is the TV scope in question, and the other is a 100 F10 achro (a TAL it appears) as I was just looking for something close to your example. This one is a little longer so will be a bit better than your example, but is ok to illustrate.

In the TV, the max defocus of the primary colors is in red at 0.13mm (relative to green). Looking at the achro example, the most defocus is also seen in red in that one, but it's at 0.645mm.

You can see that the amount of distance that the red is out of focus is significantly more than 70%, although as I mentioned above, I get a little confused sometimes as to which one is being compared to relatively. Assuming it's 70% less than the achromat, then the TV is _way_ better than that! 70% would be something like 0.45mm by my calculation.

You have to consider the relative point of the blue channel as well, but in both of these cases the blue is out there as well (0.08mm in the TV and 0.345mm in the achro). As can be seen, the color correction in the ED is much better (ie. the colors focus much closer to the same point longitudinally.

www.microsofttranslator.com/bv.aspx?from=de&to=en&a=http%3A%2F%2Fwww.astro-foren.de%2Fshowthread.php%3Fp%3D31662%23post31662

www.microsofttranslator.com/bv.aspx?from=de&to=en&a=http%3A%2F%2Fwww.astro-foren.de%2Fshowthread.php%3Fp%3D27322%23post27322

Thanks,

-Gord

[gord]

Hi Mike,

I was thinking about this more going to bed last night, re: viewing things as a percentage and where there can be confusion. It's that whole is "by" or "to" way of looking at things.

Thinking about it more, I believe what you were saying is the ED will be reduced "by" 70%, not "to" 70%. The real numbers in the example I should are it's more like 80%, or said another way, the ED has only 20% of the color defocus of the achro.

But as you can see, it's possible that things get confused and it's better to stick to real numbers.

There is another way of looking at it as well. The color error is sometimes expressed as a percentage (or fraction) of the focal length of the objective. I believe this maybe be even considered a better way of actually looking at things because it's common relative to the design, not so much the instance.

To expand on that a bit more, a larger achromat would have more defocus than a smaller one. So just by the numbers alone, it's worse. This is true that it is worse in absolute terms, but relative to the design, they will be the same when expressed relative to the FL.

From the reading I have done, traditional (balanced) achromats are typically in the 1/1800th of the FL of defocus range. The example I showed above is showing around 1/1550th (1000/0.645). I'm guessing it's somewhat affected by that one not being a completely balance (red & blue common focus) setup.

To illustrate the point of a different size having a different amount of absolute defocus, but same relative amount, here is a D&G 6" F12 and a Chinese(?) 6" F15 achro:

www.microsofttranslator.com/bv.aspx?from=de&to=en&a=http%3A%2F%2Fwww.astro-foren.de%2Fshowthread.php%3Fp%3D30106%23post30106

www.microsofttranslator.com/bv.aspx?from=de&to=en&a=http%3A%2F%2Fwww.astro-foren.de%2Fshowthread.php%3Fp%3D33650%23post33650

The D&G is basically balanced on red/blue and has 0.855mm of defocus so shows a bit better at 1/2100th error. The other F15 one has the red hanging out a little more at 1.2mm for a result of about 1/1800th so right around what is traditional. But you can see how just changing the balance a little bit will have an effect on the amount of defocus error relative to the focal length. Also, there may be some weighting formulas applied in some cases as I see Rohr has another value that he uses (but I don't speak German!).

Going back to the TV ED doublet though, you can see how much better the defocus error gets when looked at the same way. Only 1/6769th of the focal length! The triplets take that even farther.

Clear skies,
-Gord

[Mike]

Gord, you are waaaaay over analizing this. Let's keep it simple and let me show you a simple point. You don't think if I had a TV 102 mounted side by side with a common achromatic of the same aperture and focal length that the APO, TV102, wouldn't show less false color to your eye?

[gord]

Hi Mike,

You've probably figured out by now that I can be long winded!

So, simple answer to will you see less color is, yes IF (a combination of these are satisfied):

-the power is high enough so that the blur is separate from the airy disk
-the star is bright enough
-the individual persons eye is sensitive enough to the color
-somewhat depends on the color shift of the specific achromat
-how is the transparency
-etc.

Speaking for myself, yes, it is obvious to my eye I find.

Clear skies,
-Gord