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Loren Chang
member


Reged: 04/28/09

Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: MKV]
      #5540287 - 11/26/12 01:12 PM

Quote:

Hi Loren. My point was that if you add a third element to a paraboloidal mirror you end up with a Wynne-type corrector.




Hello Mladen,

You mean finally I'll get a Wynne corrector?


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Loren Chang]
      #5540606 - 11/26/12 04:22 PM

Hi Loren. What I meant to say is that, for a parabololoidal mirror, a high quality corrector at such fast focal ratios will most likely resemble the Wynne configration.

Paracorr's design is not public knowledge, but it is my understanding that it does not resemble the Wynne configuration. Rather I read somewhere that it consists of two sets of cemented achromats. Regardless, since it's a family secret of sorts, it is impossible to say how good it is on f/3 to f/3.5 paraboloids.

So far, all high performance correctors for very fast Newtonians resemble the Wynne corrector, having either three or four elements.

Mladen


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MKV
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Reged: 01/20/11

Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Loren Chang]
      #5541045 - 11/26/12 09:04 PM

Loren, I just found this site, which discusses the performance of three commercial Newotnian coma correctors, a Wynne, Paracorr, and RCC (Celestron's) brand.

The article is in French (thank you, Lord, for Google Tranalate!). The author presents imaging evidence of the performance and concludes that the Wynne and Paracorr are abot the same for f/3 to f/3.5, but that the Wynne has less vignetting and covers larger image frames. At the same time, he concludes that Paracorr is the most cost-effective product.

The plot thickens.

PS I would also like to add that if the Paracorr is indeed a set of two achromatic doublets then it beats any other corrector thus far in manufacturing as well. The Wynne requires very thin, deeply curved lenses that look more like potato chips! Not an easy thing for an amateur.

The Rosin requires a pretty strong hyperbolic primary and a couple of meniscus lenses, with slightly more agreeable thickness, but still n ot easy. One must never forget that an f/3 or f/3.5 hyperboloid is not an easy thing to make especially because many of the available tests presnet a problem with full illumination of optics - and the steeper the curves the tighter the tolerances!

So, at least one, rather well documented test seems to favor Paracorr as the corrector of choice for Newtonians.

Hopefully we didn't drift too much off topic.

Mladen

Edited by MKV (11/27/12 07:09 AM)


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Ed Jones
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Reged: 04/06/04

Loc: Sin-sin-atti
Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: MKV]
      #5541107 - 11/26/12 09:38 PM Attachment (46 downloads)

Most ATMs aren't into lens making. 8ere's an 8 inch F/4 hyperbolic using off-the-shelf lenses.

Edited by Ed Jones (11/26/12 09:45 PM)


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Loren Chang
member


Reged: 04/28/09

Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: wh48gs]
      #5541403 - 11/27/12 12:35 AM Attachment (46 downloads)

Quote:

But why use FPL51, if FPL53 is (I think) cheaper? Here's what it looks like.




Hello Vlad,

Why FPL51? The relative price of fpl51 to BSL7 is cheaper.

OHARA Glass

I do have several FPL53 designs. And you can even use ordinary glass. Here is OSLO file.


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cjc
sage


Reged: 10/15/10

Loc: Derbyshire, England
Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Loren Chang]
      #5541478 - 11/27/12 03:10 AM

As to Paracorr design, this is from the Optcorp page:

Paracorr (parabola corrector) uses 2 multi-coated, high index doublets, is completely color-free, center and edge, and installs like a Barlow. Coma is corrected so well, the diffraction limited field area of an f/4.5 Dob/Newt is increased 36 times!" -- Al Nagler

For spot sizes see:

http://www.televue.com/images/TV3_Images/Images_in_articles/Paracorr_2_chart.jpg


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MKV
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Reged: 01/20/11

Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: cjc]
      #5541591 - 11/27/12 07:23 AM

Thanks Chris. I guess that settles it. The phogoraphic evience I referenced to Loren seems to agree with Nagler's data. At under $500, as a simple add-on, it is both cheaper and more efficient than other correctors - and you can keep your existing parabolic mirror! There is no way a Rosin-type astrograph, or a Wynne-type corrector, can be manufactured at a comparable price.

As for Paracorr's vignetting, most folks with imaging devices today have digital processors of such size that worrying about vignetting hardly seems relevant.

Mladen


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wh48gs
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Reged: 03/02/07

Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Loren Chang]
      #5541634 - 11/27/12 08:04 AM

Loren,

Quote:

The relative price of fpl51 to BSL7 is cheaper.




You are right. Not long ago I saw the pressing price table, and only remembered FPL53 looking relatively cheap (obviously, processing adds quite a bit).

Yes, common glasses can be used (in the arrangement with the cemented positive doublet in front as well), but they always have the problem of not been able to correct for longitudinal and lateral color at the same time. When scaled to 10-inch f/3.5, and with minimized longitudinal aberration, the 406-8300nm spot at 1 degree off axis is spread over more than three green Airy disc diameters, laterally (not much less at 0.7 degrees). It is still within 0.025mm circle, but would probably result in elongated star shapes. The far red end is not that important, since CCD sensitivity drops toward it, but it is still there.

Vla


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MKV
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Reged: 01/20/11

Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Ed Jones]
      #5541649 - 11/27/12 08:17 AM

Ed, how can you have BK7 next to BK7, and shouldn't all the thicknesses be negative? Could you present this design in a standard tabular form, i.e. roc, space, medium, conic? Thanks.

Mladen


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Mike I. Jones
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Reged: 07/02/06

Loc: Fort Worth TX
Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: MKV]
      #5541724 - 11/27/12 09:31 AM Attachment (74 downloads)

Here's a 2-doublet coma corrector solution I messed around with over the weekend that probably works about as well as the Televue Paracorr, at least for f/4.5 mirrors and slower. I didn't try it with faster mirror focal ratios, but this at least shows it can be done with two doublets, for a usable field of view and over a wide spectral band.

Might be a good project to work further on with lower focal ratios. We could call it the Cloudynights Collective Coma Corrector!

Mike


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Mike I. Jones
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Reged: 07/02/06

Loc: Fort Worth TX
Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Mike I. Jones]
      #5541726 - 11/27/12 09:32 AM Attachment (39 downloads)

and here's the without/with performance using this coma corrector.

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cjc
sage


Reged: 10/15/10

Loc: Derbyshire, England
Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Mike I. Jones]
      #5541793 - 11/27/12 10:21 AM

It would be nice if Roger Ceragioli were to publish the specifications for the GSO corrector.

I note that the two lens Baader MPCC is due to be replaced possibly due to its spherical aberration of which its competitors are free. I wonder if the new one will have two doublets, like the Paracorr, GSO and the modified Ross-type design in Telescopes, Eyepieces, and Astrographs by Smith, Ceragioli and Berry. In passing that book says that it is not easy to produce an add on Newtonian corrector to fit in a 2" barrel that completely corrects coma. Their design does not fully correct coma but they say it is similar in performance to coma correcting modules available on the amateur telescope market. I am not sure if this includes the recent Paracorr II. Perfect coma correction is apparently possible with non-modular solutions such as the three-element Wynne corrector published in the same book.

I also note the availability of a coma correcting barlow from APM with published PSF, but no design. See: APM Coma Barlow.


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MKV
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Reged: 01/20/11

Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Mike I. Jones]
      #5541817 - 11/27/12 10:32 AM

I think you did a great job with this corrector, Mike. Mr. Nagler claims that Paracorr II does not exceed the Airy disk at f/4.5, and only at f/3.5 at distance greater than 15 mm off axis.

Without any public data there is no way to confirm this theoretically, and precision images taken with Paracorr II are not suitable for metrics.

PS Judging from the pitures of the Paracorr add-on, I'd say Nagler's specing between the doublets is a lot closer than yours. Maybe therein lies the answer.

Mladen

Edited by MKV (11/27/12 10:35 AM)


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Loren Chang
member


Reged: 04/28/09

Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Mike I. Jones]
      #5541858 - 11/27/12 10:56 AM

Quote:

We could call it the Cloudynights Collective Coma Corrector!




Hello Mike,

AKA C4?!


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Ed Jones
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Reged: 04/06/04

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Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: MKV]
      #5542664 - 11/27/12 07:17 PM Attachment (34 downloads)

Mladen,
Here is what this looks like. I do agree with you however that the Paracorr makes more sense. By the time you buy 4 coated lenses you could almost buy a Paracorr.


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cjc
sage


Reged: 10/15/10

Loc: Derbyshire, England
Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Ed Jones]
      #5543317 - 11/28/12 06:31 AM

Quote:

Mladen,
... I do agree with you however that the Paracorr makes more sense...




Or even an Astro Optik corrector, spots here: Newtonian 3" Wynne Corrector.


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MKV
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Reged: 01/20/11

Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Ed Jones]
      #5543361 - 11/28/12 07:40 AM

Quote:

Mladen,
Here is what this looks like. I do agree with you however that the Paracorr makes more sense. By the time you buy 4 coated lenses you could almost buy a Paracorr.



Thanks, Ed. Much obliged for the picture. I am not used to Zemax design presentation. I only now noticed that you have a diagonal mirror in that configuration, which would explain the change in sign.

Don't forget that, in addition to the cost of the lenses, there is also the cost of a precision machined lens cell and anodizing. I wonder if the Paracorr people took all that into account when they decided on the price of their product, i.e. to make it just not worth one's while to pursue a DIY approach. If they did, it's a smart marketing strategy!

Clearly, the cost, compactness and performance of the Paracorr is hard to beat, so if I were seeking corrector solutions I would be looking into two cemented doublet designs.

Mladen


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ed_turco
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Reged: 08/29/09

Loc: Lincoln, RI
Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: Mike I. Jones]
      #5543366 - 11/28/12 07:45 AM

Mike,

Golly!

Ed


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MKV
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Reged: 01/20/11

Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: cjc]
      #5543503 - 11/28/12 09:52 AM

Quote:

Or even an Astro Optik corrector, spots here: Newtonian 3" Wynne Corrector.



Chris, what's the price of AO's corrector? Paracorr 2 goes for about $450. The field coverage for both is the same (50 mm). The Wynne has a slight (but not significant or crucial) advantage in the zise of the fully illuminated field.

For vignetting to become a factor you'd need 60x60 mm CCD processors, something most amateurs can only dream of (a large format reconditioned Hasselblad camera goes for $20K!).

Regular SLR processors are 14x24 mm, well within Paracorr's fully illuminated field.

Also, the Wynne is a 3-inch corrector that doesn't fit into standard 2-inch focusers.

And the spots assume the limit of 20 microns, but today this old-fashioned (film grain) size limit has been surpassed as digital images are capable of resolution of detail smaller than 20 microns.

Mladen

Edited by MKV (11/28/12 09:54 AM)


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cjc
sage


Reged: 10/15/10

Loc: Derbyshire, England
Re: 10" f/3.5 Astrograph with Rosin focal reducer new [Re: MKV]
      #5543788 - 11/28/12 12:34 PM

Fromm APM the ASA 3" Wynne Corrector is 1,020 EUR including 19 % Tax, which would not be payable to the US, so somewhat more expensive than the Paracorr. However, it has the advantage of being a 0.95x reducer rather than the 1.15x Barlow of the Paracorr. There is a comprehensive range of ASA coma correctors including 2" models (the cheapest of which is 468 EUR), with various amounts of extension/reduction and a model for Schmidt-Cassegrains, see Reducers-Converter-Correctors. I wonder if it is an ASA corrector in the Orion UK AG astrograph Newtonians which I referenced earlier.

I do note that the spots are reference to 20 microns rather than perhaps 10 or even 5 which is common today and your comments about affordable cameras. However 3" focusers are becoming more common because 2" will inevitably vignette 50mm sensors and the widest eyepieces. I understand that ES has a 30mm, 100 degree, 3" eyepiece in testing...


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