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GlennLeDrew
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Re: Can a decent sub F6 150mm Petzval be made? new [Re: Jon Isaacs]
      #5615617 - 01/10/13 02:12 PM

Jon,
While OT, if you want to scan vast nebulosities, think bino with filters. Two eyes do a lot here...


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Jon Isaacs
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Re: Can a decent sub F6 150mm Petzval be made? new [Re: GlennLeDrew]
      #5615929 - 01/10/13 05:34 PM

Quote:

Jon,
While OT, if you want to scan vast nebulosities, think bino with filters. Two eyes do a lot here...




Glenn:

I have considered and may consider it further. A while back I started a thread in the binocular forum about over and under binoculars with star diagonals. The usual problem is the eyepieces and the angled design. I know you built something based on a pair of Ethos eyepieces, I am still trying to figure out something that goes one-eyed with the 31mm Nagler.

I have an extra 2 inch focuser which needs needs $28 worth of bearings but with a shortened, short, short drawtube, should mate nicely with my 80mm F/3.75 UO finder. I am calculating an 8 degree TFoV at 10x with an 8.2 mm exit pupil and a 7.4 degree TFoV with a 7.5mm exit pupil (28mm Uwan)

Large aperture wide angle binos may be in the future but they would represent a serious investment in eyepieces and filters...

Jon


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Ajohn
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Re: Can a decent sub F6 150mm Petzval be made? new [Re: Jon Isaacs]
      #5615994 - 01/10/13 06:20 PM

A strict Petzval reduces colour aberrations not field curvature as was mentioned early on. The problem with the Petzval is distance between elements and maintaining it. There are flat field variants using less glass than that usually requires but spacings are even greater. The Tesar is reckoned to be the next step up and I believe is simpler but still has large element spacings. It's really a Cook with the rear element split.

I mentioned oil spaced triplets because I was thinking apo's.

If a near apo is wanted it's already been done after a fashion. The longer focus 6in Syntra refractor and the corrector lens that some one makes to go with it. That doesn't seem to have been very popular probably due to variations in the Syntra refractor which is just and old style achromat. Length problems again.

John


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Jon Isaacs
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Re: Can a decent sub F6 150mm Petzval be made? new [Re: Ajohn]
      #5616082 - 01/10/13 07:13 PM

Quote:

A strict Petzval reduces colour aberrations not field curvature as was mentioned early on




Interesting... It seems that the term Petzval has been associated with refractors with built in field flatteners such as the TeleVue NP (Nagler-Petzval), the Takahashi FSQ series and the Vixen 140mm NeoAchromat. I guess maybe it's a loose usage of the term but it seems to be commonly used at least in the amateur world.

So, was the original intention to build a 150mm F/6 Petzval for the improved color correction or was it in the more loose sense of the word to have a fast refractor with a flat field of view?

Jon


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Ajohn
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Re: Can a decent sub F6 150mm Petzval be made? new [Re: Jon Isaacs]
      #5616772 - 01/11/13 07:34 AM

You tell me Jon. I think it was an assumption that a Petzval would allow a bigger faster scope or at least a bigger scope. APO or achromat. I assumed the former but it may be the latter. Flat field? Don't know. I have seen a design for a Petzval type some where or the other that was flat field and an apo but only 100mm dia F8 and the "lens" was about 500mm long. Designed for imaging not visual use so based on pixel resolution.

Going on the Vixen scope it may be possible for an achromat. I think that the Takahashi 6in is the fastest apo available at that size, F7.2 and has a field flattener available. I think that the problem with say making that F6 would be the same as trying to make a 10in F7.2. The do make a 10in but it's F10. You can just buy one of those it seems.

John


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Jon Isaacs
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Re: Can a decent sub F6 150mm Petzval be made? new [Re: Ajohn]
      #5616801 - 01/11/13 08:01 AM

Quote:

I have seen a design for a Petzval type some where or the other that was flat field and an apo but only 100mm dia F8 and the "lens" was about 500mm long.




Both the Takahashi FSQs and the TeleVue NP series are reputed to be Petzvals, they are apos and have flat fields.. The NP stands for Nagler-Petzval... Apparently the used ED elements in both the "objective" as well as the rear section...

Jon


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Ajohn
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Re: Can a decent sub F6 150mm Petzval be made? new [Re: Jon Isaacs]
      #5617188 - 01/11/13 12:27 PM

That's a 106mm and if you look at this you can see what I mean about the lens length. I wonder if they stole the idea from the one I saw on the web? A modified Pertzval as is this one.

http://www.takahashi-europe.com/en/FSQ-106ED.optics.php

It's wouldn't just scale up to 6ins. One of the problems is that the diffraction spot is determined by the F ratio. The focal length of the 106 is 850mm. A 6in would have to be 1200mm so the ray aiming has to be that much better because the spot size has to be the same. Another problem with lenses that have that much space between the elements is temperature - distance changes. I think this is one of the reasons why the design has never really been popular but can be good for smaller lenses.

John
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Jon Isaacs
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Re: Can a decent sub F6 150mm Petzval be made? new [Re: Ajohn]
      #5617819 - 01/11/13 07:16 PM

Quote:

That's a 106mm and if you look at this you can see what I mean about the lens length. I wonder if they stole the idea from the one I saw on the web? A modified Pertzval as is this one.

http://www.takahashi-europe.com/en/FSQ-106ED.optics.php

It's wouldn't just scale up to 6ins. One of the problems is that the diffraction spot is determined by the F ratio. The focal length of the 106 is 850mm. A 6in would have to be 1200mm so the ray aiming has to be that much better because the spot size has to be the same. Another problem with lenses that have that much space between the elements is temperature - distance changes. I think this is one of the reasons why the design has never really been popular but can be good for smaller lenses.

John
-




TeleVue has been building modified Petzvals for about 30 years, I am not sure when Takahashi began making theirs...

"The "NP101" is a sixth generation TeleVue four element design. It follows the original TeleVue 5 inch "MPT" system introduced in 1982, the beautiful brass 4 inch aperture f5.5 "Renaissance" telescope introduced in 1984, the "Genesis" 4 inch f 5 telescope of 1988, and the "SDF" 4 inch f5.4 of 1993, and the "101" and "Renaissance 101" telescopes of 1996. Al Naglers patented four element designs are based on the original "Petzval" concept, this is an arrangement of refractive elements where a two element air spaced objective lens (housed at the front of the telescope) passes the light through to a second doublet lens positioned at the rear of the telescope and before the focuser. The doublet lens group usually functions to 1) reduce the effective focal length, and 2) reduce or eliminate curvature of field."

Company 7 NP-101

Makes the NP-127 and made a few TV-140s which were also modified Petzvals.

Jon


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Ajohn
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Re: Can a decent sub F6 150mm Petzval be made? new [Re: Jon Isaacs]
      #5618638 - 01/12/13 09:12 AM

Interesting Jon. You might have noticed that I have had an interest in telescopes for some time - even ones I couldn't afford. My understanding is that the Petzval when modified can only provide a flat field up to a certain size and only be severely splitting the rearmost group of lenses as per the Tak design. The flatness is a matter of degree so it only works out up to a certain size. TV mention visual use and also do a large field corrector for their pertzval scopes - why bother if it's truly flat. With visual use eye accommodation can take care of reasonable levels of field curvature as the eye automatically refocuses it. A sensor can't do that. This sort of aspect becomes very apparent as soon as some one takes a photo of something that uses the full field.

Might all be folk law but I suspect not.

John
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