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the new one's that APM has. In their case, they are a little different still than the IStar's since they both use ED glass as well. The Chinese one uses the older LaF glass, while I think the Russian one is using something like the FPL-53 glass. Because they use ED glass, they are better corrected to start with than an achromat.

However, they apparently are tuned as well to focus on reducing the traditional false color as well. Since they are starting out ahead of the achromat to begin with, they don't have as far to go, so aren't going to be as affected. But it's still not going to be as ideal (again for our Jupiter example) as a normally corrected ED.

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A quick note to the discussion on the refractor forum regards correction of our chinese and LZOS 152 doublet ED. both are not designed to reduce purple and increase red spotts. Both are correct designed, optimized for visual observations, tthe classic tradional metthod !!!!!!!!!!!!!

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Here's a reply from Markus, who as you know is no longer allowed to post in this forum, so he posted this in the vendors forum:

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A quick note to the discussion on the refractor forum regards correction of our chinese and LZOS 152 doublet ED. both are not designed to reduce purple and increase red spotts. Both are correct designed, optimized for visual observations, tthe classic tradional metthod !!!!!!!!!!!!!

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Comments anybody?

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That's right, Markus isn't allowed to participate here. I'll include a link to his thread over in Vendors so he can respond and we can relay his inputs. It's important to have his input.

APM 6" ED Thread in Vendors Forum

So I have a question for Markus. I went over the the APM site and looked at the detailed plots for these new doublets. The ones I found very interesting were the longitudinal SA color charts.

As I understand it, these show the relative defocus of a particular colour relative to a common reference (usually green as I understand it). If I look at the LZOS double plots, I see most of the lines stick pretty close together near the centre, but the two red ones hang out to the right side. As I understand it, this means there are a bunch of colors that are in-focus (or nearly so) and the red is out of focus. The opposite is that you could focus the red, but other colors would be out of focus. Is this correct?

If I compare it to the LZOS triplet, there is a point where they all meet.

How close am I on this?

Clear skies,

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Recently on the Istar scope club Ales K ( an part owner of istar) has released spot diagrams of 6 different 150 mm doublets and triplets. The red spot sizes are larger but not hugely larger than the blue spot sizes. Although the actual wavelength versus focus sketches are missing.

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I chiefly observe in seeing that is typically 6-8/10. The suggestion a scope can show the detail of the 10 inch processed pic continuously put forward surprises me.

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Valery
Was it a well controlled fair test ?

Same observer, night, eyepiece, mag, aperture etc on same object.

Or is what you are talking(comparison of four 5 inch refractors) about just an impression.

Kevin

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How close am I on this?

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Gord, the Chromacor can be "fiddly", but once adjusted properly It does work!
In my C6R w/o the CC, I would never turn the scope to Jupiter as It's just a purple mess!
With the CC however the purple is gone and the Image is very much Improved, however the AT111EDT is still in a league of It's own. The detail and contrast of the Triplet is just amazing.
I'm not sure If It's the glass used or ??? but the AT111EDT puts up a super rich view (contrast?) of Jupiter that I haven't seen in many other scopes. It's similar to the effect the TV Plossls have on Jupiter as well.

Also, as a side note, This past summer, A friend and I (Tank) were viewing with his RFT 6" F5 Refractor and decided to see how the CC would do with the newer "red shifted glass" (on Vega).
Answer......amazing! w/o the CC, Vega was a mess with the CA extending to about the size of a Full Moon (viewed with naked eye).
With the CC in place (no spacing), The CA was reduced to a size maybe 2X or 3X the size of the Star. An amount similar to what's seen in the older (Semi) Apo Triplets, or ED scopes. And this was a 6" F5 scope, so I'm sure the F5.9 or F6.5 version would fare better.

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Thanks for sharing this piece of info.

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In your opinion, where do you think a Chromacor equipped achro would fit? Or, based on what you were describing in terms of different scopes, do you think it could fit at many levels in the list based on how close of "fit" it is to the scope, and/or it's individual setup in the scope?

I've heard the comments about the need to be precise with the setup before and plenty of negative comments if it is off, and positive ones where it is on. So it seems like it's likely something like the latter I describe. I wonder where it could theoretically be at it's best?

It would be good to hear from more users.

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I can answer that with a definitive, "It depends".

From my direct experience, I'd say the CR6 with a well matched Chromacor I falls between the old Meade 6" F9 ED and the 6" F8 FPL53 doublet. With a well matched Chromacor II, it's a match for my APM 6" F8 triplet.

On axis.

The trade off when using the Chromacor is an increase in lateral color off axis. The degree and extent of the lateral color depends on the aperture, F stop and optimal spacing but generically, for a given aperture, as the F stop gets slower the spacing of the Chromacor in front of the focal plane increases and lateral color decreases.

But it's not that simple. As you move a given Chromacor forward for a slower F stop for a given lens native correction in green, the system tends to become over corrected in green.

Clear as mud now right?

I do disagree with Ales when he says "results may vary" when using the Raycor for "non-type design" achromats (R30s). Results WILL vary.

But the tweaking is part of the fun of this hobby (just like my audio hobby).

Jeff

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I can add one important thing: the optical designers in a past knew the optics and human eye work not any bit worser than todays designers. And I think they knew optics better.
All fundamental investigations in this field were done at that old good times. All classical telescopes were designed by masters and be sure, they experimented with different designs and chooses which work best.

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I'm sure you are correct. And if you and other opticians keep providing us info about optics, eventually consumers will really understand their scopes

Mark

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I guess I don't understand all the fuss going on here about a designers choice in balancing out the red and blue spot sizes.

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I looked at spot diagram of TSA-102.

http://www.takahashi-europe.com/en/TSA-102.optics.spots.htm

TSA-102 has much tighter focus on the blue end but FS-102 is much tighter on the red end.
Does it mean FS-102 is really that much better on Jupiter or Mars?
I both owned FS-102 and TSA-102 but the answer was no for my case even though the view was so close.

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BTW Mike, when you used your Chromacor on the 6" F5, you were probably stopping down the aperture slightly with the nose of the Chromacor. Your color correction improvement may have been, in part, due to the stop down.

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Gord you say
"Again, there is no perfect design, so it makes sense if they aren't as good at planetary, they may be very well suited to something else."

It seems you have convinced yourself to this end. (the istar R30 design does not perform as well as an C F corrected achromat on planets) based on spot diagrams.

You have however not seen through an R30 scope with your own eyes and in fact are making assumptions based on what a vendor has said before about other scopes.

This (Valery's insinuation directed at the R30 Istar scopes) reminds me a bit of Newton dismissing Cassegrain's scope design a few centuries ago.

I think his thread is unfair in this respect to 2 vendors (Istar and APM). They are being dismissed theoretically without any concrete evidence.

No one has done a comparison with an istar R30 and a similar C F achromat.

I guess at least folks are not still claiming they use the same glass as normal F2 BAK 7 fraunhofer achromats.

Casting dispersions can be unfair when they are not supported by hard facts.



Kevin Barker

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Jeff, Do you think (from the Diagrams) that Markus's APM 150mm F8 ED (Chinese) Refractor would be better, equal, or worse than a C6R/CC1?

Mike

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