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I've already added a Moonlite and Chromacorr. While I like the Chromacorr, a good ED doublet would be superior. My C-corr is a bit of a bandaid-- it corrects the overcorrected stock lens, but not (close to) perfectly...

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The chromacor is no bandaid. It just offers a different set of trade offs and I find the economics of it compelling, especially with 8" and larger achromats with focal ratios F12 and slower.

I've found that when properly set up the chromacor has visually superior color correction to the doublet "APOs" I've used, over an approximate .05-.2 degree field of view, depending on the F stop/aperture combination used. Where it begins to suffer in comparison is off axis with lateral color. And even the LC can be mitaged to some degree if that of the eyepiece compliments the LC of the Chromacor. Further, I've found that I can also mitigate to a degree atmospheric dispersion color.

I've a rather good 4" F10 Celestron achromat that I recently tried my Chromcors on. Oh, my. It was essentially color perfect up to at least 200X, much better than what I remember of my 102 ED Meade, which was really very good. Also I've stopped my 11" F12 D&G down to 8.5" to yeild an F16 focal ratio. Another major "Oh my" with similar results.

That being said, I'd love to have a 6" F8 FPL53 or similar doublet. What a nice, user friendly package that could be.

Jeff

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Well, on my scope it's certainly a bandaid. But not because there's anything wrong with the product, but rather because a really good match is tough to get. Add to this the difficulty in putting a wave-front accuracy to the star test of an achro, and things get sketchy...

It's a good product, but, if you need under or overcorrection built in, as is common with these Synta's, a really good match can be tough-- especially in light of current C-corr availability.

Make no mistake: I'm not bashing the product. I'm saying that an approximately matched Chromacorr on a lens that probably has other issues (fairly common on these Syntas) is very much a compromise, and inferior to a truly well made lens, achro or ed.

Off-axis performance is a key issue for those of us who use un-driven mounts. My Orion ED100 and old Meade ED127 perform much better off-axis than my C6 and C-corr.

I think the C-corr is a great product, and I wish it was still easily available. But it's not, and Vallery appears to have no intent to begin making these again, and hasn't made them for quite some time.

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Kerry

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hi
i wrote to a contact in Synta, and asked the development department there look at the possibility of making a ED replacement lenscell, and gave them link to this thread, so they could check out what people want, and what other producers are looking into producing.
so it may be a good thing to write, and say what we want, and maybe what we are willing to pay for it.
synta have shown with their ED series that they can produce good lenscells, so maybe this would be something they would consider producing, if they feel the market is ready for it.

alfi

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Good move, alfi!

It never hurts to let a vendor know there's a demand for a particular product.
The worst they can say is, "Thanks for the suggestion, but no...".

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A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.

"Semper ubi sub ubi"

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Hi All,

when we do the cell , we make it a bit multipurpose , so we will offer with adapters of choice, either for old or new Synta or for 6" Meade Achromats , but also for ATM people to build here own tube , most flexicable

when I come back from LZOS at 30.october I shall have most news in my hands

thanks

Markus

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Yeah, you’re right. I thought that OK4’s Abbe number was in the low 80’s but it is actually 92.05 making it very close to FPL-53! So after some quick number crunching pitted with some guestimation I would say that these 152mm f/8 doublets should be only slightly on the high side of being color free. I would suspect that some color will be visible, especially at higher powers, but the color should be very well suppressed (I’m guessing at about 33% to 50% better than the Meade 152ED).

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To put this thread in perspective I think the new lens would be equivalent to a 6" Fluorite doublet which will have noticeable color. depending on mating element and any touch-up, it could be slightly better or slightly worse. But definitely in the neighborhood.

Below is a star test image from the bench on a TAK FS152 that I used to own. Lots of color. But at precise focus the image was excellent, just slightly warm looking. The 1/6 wave of Astig is not seen when the scope is in the field pointing up - its an artifact of the test stand.

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Thanks for the insights, Paul and for posting those images taken with your FS152.

Very good info.

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A few telescopes of dubious value.
Understanding wife, two curious cats and one sadly departed.

"Semper ubi sub ubi"

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