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Equipment Discussions >> Refractors

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johnnyha
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Reged: 11/12/06

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Re: Istar 5" f/8R30, first light new [Re: Kevin Barker]
      #5678978 - 02/14/13 08:59 AM

Quote:

Also if they are not using special glass of some sort for the R30/35 etc lenses why are they using so much glass. They are truly very massive.




...so they can then market a normal lens set as the"special new lightweight version?


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t.r.
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Reged: 02/14/08

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Re: Istar 5" f/8R30, first light new [Re: johnnyha]
      #5678990 - 02/14/13 09:13 AM

Just FYI, Royce has had a variation on the achromat for some time using a Barium crown glass instead of Bak7. It is said to reduce secondary spectrum by about 15% and can be visually appreciated. The lens is also oil-spaced resulting in two air-to-glass surfaces. To me, this is a true "new" offering, even though it has been around a while and was actually conceived years ago by another well know optician. It is an improvement on the fraunhofer design, whilst preserving what a C-F correction provides. Interestingly enough, years ago I tried to commission Rob to make me a 7" F/12 C-e corrected achromat discussed earlier in this thread. He wouldn't do it. Said that it would be great at planetary, but nothing else and wouldn't want his name attached to it!!! He said that his design hit a sweet spot as a planetary scope at 7" F/12 with the Barium...I may have to revisit this.

http://www.rfroyce.com/refobs.htm


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tomharri
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Reged: 09/19/08

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Re: Istar 5" f/8R30, first light new [Re: t.r.]
      #5679046 - 02/14/13 09:47 AM

I don't care how they are made or what they are made from. That's all private information, company secrets. All I care about is how it performs. And the 5" R30 performs very well to my expectations and usage.

Sure a regular achromat 'may' be better under 200x, kind of hard to quantify what you are seeing sometimes. But where I spend the most time is over 200x, and the R30 is clearly better than an achromat here.

Yes the first versions are kind of weighty, but it looks to me as if they are addressing the issue with a thinner and even better performing 2nd version. Kinda like the motorcycle you are buying today is inferior to what's coming next year. Is this marketing? Buy the variation that best suits you, or sit on the fence and wait forever for the perfect 'one'. Don't denigrate the company for not replying to your every whim.


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t.r.
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Re: Istar 5" f/8R30, first light new [Re: tomharri]
      #5679330 - 02/14/13 12:13 PM

Quote:

Sure a regular achromat 'may' be better under 200x, kind of hard to quantify what you are seeing sometimes. But where I spend the most time is over 200x, and the R30 is clearly better than an achromat here.




THIS is a very valid point and one that begs for more reports to confirm or deny!!! If others can confirm this, then the R30 has its merits and truly does offer something "new" over the standard achromat...higher obtainable magnification without a reduction in low contrast planetary detail! I will say this, the design choice is counterintuitive to enhancing planetary detail, but alas, Istar is very good at thinking outside the box and offering new concepts with products like Raycorr, the TCR and these lenses.


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stanislas-jean
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Reged: 10/22/08

Re: Istar 5" f/8R30, first light new [Re: tomharri]
      #5680676 - 02/15/13 04:36 AM

If this design, visually well seen, helps to diminish the blue hapo at high power, at the opposite it should reinforce a red brown halo around brilliant planetary disks.
I own a 150mm F10 istar classic that works already very fine.
In the blue area, the documents brought by T.R. is intereesting for showing the lacks at the ends of the visual spectrum: blurring in deep blue and so in red channel for classic achromats.
The 150mm istar get against already some improvements where:
- in blue channel we can go from the W80A filter to the W38A filter,
- in red channel the W29 filter is very well workable.
I think the R30 design has less spherochromatism in red channel and globally the CA curve is shifted to the red channel portion.
This represents a good optimisation for the Halpha field where the spherochromatism is well improved.
Personnally this my feelings about.
I owned also a 1529 model antares refractor, 152mm and 990mm focus length, where the effects can be similar.
Improved in blue fields (the W80A filter well workable, the W38A too limited) with anyway the blue halo more confidential than a classic doublet, but in the red fields the red-brown halo is present. The front lens has also an aspheric surface on the outside part created by a soft different curvature (well accessible for making this evident at a ronchi and 13lp/mm grating). Also a matter of spherochromatism result.
Here a F6.5 the istar F10 being on a step above for contrast images.
But we are far from an apo.
The istar works well on Mars with 300-375x without filter, 200x on Saturn, Venus in daylight without blue halo at 150x, Jupiter at 150-165x. The same characteristics on the 1529 but with light coloured filters.
There was some istar doublets with lanthanum glass having say 25-30% less CA, where this is now? Was interresting with regards the costs and performance.
Stanislas-Jean


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mikey cee
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Re: Istar 5" f/8R30, first light new [Re: stanislas-jean]
      #5683247 - 02/16/13 11:08 AM

Well I just finished looking at Damian Peach's double transit of Jupiter. Judging from the pinkish orange of the GRS and GRS Jr. my 10" R/11 R30/35 leaves absolutely nothing to be desired in color or contrast. Glad I made the plunge into R30/35 land instead of waiting around for others to be the guinea pigs!! Mike

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