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Can a decent sub F6 150mm Petzval be made?

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#1 gatorengineer

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Posted 05 January 2013 - 11:04 PM

I love my TV127, but would also love another inch of aperature.... and I also love the feel of refractors (yes I own a lovely MN86)....

Is it possible with modern glass to get there. Lets start with visual, and worry about AP later....

#2 gatorengineer

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Posted 05 January 2013 - 11:08 PM

Ok, lets also limit it to either an achromatic or at most expensive R30 objective..... I want to be practical.....

#3 orlyandico

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Posted 05 January 2013 - 11:41 PM

the vixen NA140SS comes close to 6".. not sure if you'd consider it decent.

#4 Ajohn

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Posted 06 January 2013 - 04:12 PM

Why only an extra inch when you could have one of these

http://www.takahashi.../en/FET-300.php

One of the problems with the extra inch is glass problems. They get a lot more expensive as the size goes up and it isn't just related to the volume of glass.

Larger sizes also need more attention to keeping the parts that make up the lens in tolerance. Things just get worse and worse as the size goes up. In fact a best case design would need the lenses redesigning according to the glasses that actually arrived to make them.

Tak will do a 150mm ex stock I believe. :tonofbricks: Maybe a better make of scope too.

John
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#5 ed_turco

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Posted 06 January 2013 - 05:34 PM

Funny thing is twenty years ago I acquired an antique 6" f/6 Voightlander Petzval portrait lens. The focus was horrible, so, one by one, I refigured each lens to see how far I could go.

Initially, there was 2" of focus difference between the center of the lens and its edge, when assembled. I assumed that this was due to the lens being corrected for short distances, as in "portraits," but testing of the concave surfaces alone showed that the optical workers really didn't pay any attention at all to the figures on each surface.

So I tried to bring everything back to a sphere and the result, by autocollimation was better by a factor of 1/200 and allowed my to go to 150x when this telescope was completed. What an RFT! I think that now, if my hands were working, I could do even better.

In passing, the heat of polishing made the cemented front element start to part, and when I finished the parting, that Canada balsam smelled as fresh as when it was applied 130 years ago! I recemented when I was done.

Pretty good for a $50 lens!



Ed

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#6 gatorengineer

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Posted 06 January 2013 - 06:34 PM

I havent looked through one of the vixen 140NA's its a pretty maligned instrument.....

Ed, that is a beaut..... How was the color correction?

Televue made a 140, there are a few examples in existance, so I know it can be done, just wondering how practically.

#7 ed_turco

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Posted 06 January 2013 - 08:41 PM

The color correction was 2/3 normal, and at f/6 was pretty good for what it was. But it was only achromatic...

Ed

#8 Ajohn

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Posted 07 January 2013 - 05:40 AM

I would ask why Petzval. There is a brief summary of what that really means here
http://www.sinopt.co...les/petzval.htm

My understanding with these especially when a field flattening lens is added as mentioned that you finish up with horrendous tolerance problems to reach diffraction limited performance. Bit like camera lenses really. So many elements and spacings that they have no chance of coming out "correct".

You don't mention F ratio. That will also have a bearing on feasibility. I would imagine that a 6in F15 apo is possible without using modern glasses.

It is possible to buy lenses of all sorts of sizes see
http://lzos.ru/en/in...iew&id=20&It...

There are also threads on here for 6in APO's eg
http://www.cloudynig.../Number/5035371

Interesting to see the mention of the old Meade scope. I have the 5in myself. Many rate that one but not the 6in as it pushed the design too far. An example of what an inch can do even at F9.

John
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#9 ed_turco

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Posted 07 January 2013 - 03:17 PM

I didn't need field flattener. The radius of the curved focal surface was long enough to deal with by eye accomodation; all my eyepieces were Plossls. I imagine if I used those fancy-dancy ridiculous 120* eyepieces I might have a problem.

Another poster asked "Why Petzval?" Petzval lenses have 2/3 the secondary color of ordinary refractors. Not APO, but better.

#10 gatorengineer

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Posted 07 January 2013 - 07:38 PM

Going back to the other posters question, it seems that potentially the only way to get fast and wide in a refractor is a Petzval design.... Looking for f6 or less in a 6", was hoping that a zemax jock would have taken up the challenge....

#11 Jim Curry

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Posted 07 January 2013 - 09:36 PM

I'm not sure why the V140NA would be maligned any worse than your average achromat. Built like a tank, R&P focuser, f/5.7, 3+ degree fields, short tube, color correction to I'd guess f/8-9. Flat field and reasonable price. What's not to like?

Jim

#12 Mike I. Jones

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Posted 07 January 2013 - 11:21 PM

was hoping that a zemax jock would have taken up the challenge....



After 21 years of using it daily at work and at home, I guess I qualify as a ZEMAX "jock", but sometimes available time and likelihood of actually having someone make what you design makes you pick and choose these ATM hobby designs. Doing the after-hours work to design, determine glass blank availability, thermally optimize, tolerance, perform ghost and stray light analysis and first-order barrel design for a multi-element lens like the 150mm f/6 being discussed here that has 0% chance of ever being built is not in on my high shelf. A custom design would perform admirably, as well as any commercially available lens, if made within derived tolerances. However, making a one-time multi-element custom lens like this could run $15K to $20K, or even more, depending on the amount of work the eventual owner would be capable of doing him/herself.

Just my 20 millidollars,
Mike

#13 GlennLeDrew

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Posted 08 January 2013 - 01:19 AM

The question is... Is the instrument to be designed to be for relatively narrow field visual work, or wider field imaging? Rather different requirements, which make a world of difference.

#14 Ajohn

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Posted 08 January 2013 - 10:26 AM

A cheaper option than a petzval is probably an oil spaced triplet. Using oil gets round the cementing problems with large objectives. There are all sorts about including an old design for a 6in that was rather long. The glasses aren't available on that one any more and weren't that exotic either. People who made these didn't always polish the interior lenses either as the oil nulls the surfaces out. Only left with 2 reflective glass surfaces as well which means that no coatings wouldn't have such a bad effect. There used to be lots of info about on the web about this one. I might even have an oslo file kicking about on an old machine.

John
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#15 Jim Romanski

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Posted 09 January 2013 - 01:30 PM

When I was last at NEAF I asked David and Al Nagler when we would see the NP152. I think Al's concern was maintaining mechanical stability in scopes laarger than 127mm. David thought that it would be very expensive. I told them I could guarantee at least one of the being sold. :jump:

#16 gatorengineer

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Posted 09 January 2013 - 04:00 PM

Theres no way an oil spaced large triplet is cheaper than a Petzval.... The 127 is achromatic (lets say it costs $300, and lets say you have an 90MM rear Doublet with good ""stuff" FPL 53 that would again be say $500......

A $150 Petzval, should be bigger objective but the corrector should be the same. Astrozap tried it for $1200 and apparently failed, the NA140SS is a 2k Scope, somebody should be able to do it in a Petzval F6 for $4k..... If you look at the 6" triplets out there at F8 for 8k.....

#17 Jon Isaacs

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Posted 09 January 2013 - 07:48 PM

A $150 Petzval, should be bigger objective but the corrector should be the same. Astrozap tried it for $1200 and apparently failed, the NA140SS is a 2k Scope, somebody should be able to do it in a Petzval F6 for $4k..... If you look at the 6" triplets out there at F8 for 8k.....



The NA140SS is an achromat and costs $2K and it's not a fancy scope mechanically. Here we are talking 4K for 150mm F6 Petzval.. achromat...

That's a lot of money just to get a flat field of view and one can get a similar aperture, focal length and field correction with an 8 inch F/4 Newtonian + Paracorr for a lot less cash.

My own interests go the other way. Petzvals correct field curvature in refractors. As the apertures increase, the focal lengths increase and field curvature becomes less of a problem because the eyepiece field stops do not increase...

What I would like to see is an 80mm F/5 with a focal reducer/field flattener maybe based on an 80mm F/7-F/7.5 ED doublet. A mini NP-101. I have been messing around with my William Optics FD and various achromatic doublets I have laying around. A 52mm with a 400mm focal length looks promising but it poses some problems implementing it mechanically. The good thing about the William Optics scope is that it uses a 2 inch extension that can be removed to provide the necessary inward focuser travel.

Jon

#18 Jim Curry

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Posted 09 January 2013 - 08:02 PM

"NA140SS is a 2k Scope"
You can get this in the 1,500-1,700 range new; 1,000 or so second hand.

#19 gatorengineer

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Posted 09 January 2013 - 08:07 PM

Jon, vixen describes the 140 as "The advanced 4-element objective lens, set in two groups, minimizes residual chromatic aberrations. With its unique Petzval like correcting lens at the rear, this Neo-Achromatic refractor telescope is specifically designed to provide improved color and a flat field, ideal for astrophotography.

The AT65 Quadsounds like the scope you are looking for a little small but... fast and flat.

#20 Jon Isaacs

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Posted 09 January 2013 - 11:01 PM

Jon, vixen describes the 140 as "The advanced 4-element objective lens, set in two groups, minimizes residual chromatic aberrations. With its unique Petzval like correcting lens at the rear, this Neo-Achromatic refractor telescope is specifically designed to provide improved color and a flat field, ideal for astrophotography.

The AT65 Quadsounds like the scope you are looking for a little small but... fast and flat.


It is my understanding that the 140 is an achromat and the "improved" color correction is due to the effect of using a focal reducer with a somewhat longer focal length achromat. The hope for a 6 inch ED Petzval for $4000 seems unrealistic since Markus Ludes has just barely managed to come up with a 6 inch ED scope at that price point.

As far as the AT-65 Quad is concerned, it's only 65mm, it has a 420mm focal length and it does have a 2 inch focuser but I believe that backfocus is a problem for 2 inch eyepieces. This project is centered on using the 31mm Nagler with it's 42mm field stop to achieve a 6 degree TFoV with an 80mm aperture. The 31mm Nagler requires a lot of back focus/inward focuser travel.

What I am hoping to do is duplicate my 80mm F/5 Achromat with it's 2 inch focuser with my 80mm F/7 apo + focal reducer. I have had some just amazing views with the 80mm F/5 out under dark skies, the step up from a 4.5degree TFoV to a 6 degree TFoV doesn't seem like a big deal but there are large chunks of nebulosity that are lost with the narrower field of view. Scanning Cygnus for the first time with a O-III filter, a 6mm exit pupil and a 6 degree TFoV was an unforgettable thrill. It's probably more practical just haul along another scope and mount and put up with the field curvature and enjoy the views..

There are lots of field flatteners/focal reducers for 80mm F/7 apos but they are not designed for visual use with a diagonal, it seems like there are issues with both clear aperture/vignetting and back focus..

Jon

#21 GlennLeDrew

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Posted 10 January 2013 - 02:12 PM

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

#22 Jon Isaacs

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Posted 10 January 2013 - 05:34 PM

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

#23 Ajohn

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Posted 10 January 2013 - 06:20 PM

:confused: 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

#24 Jon Isaacs

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Posted 10 January 2013 - 07:13 PM

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

#25 Ajohn

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Posted 11 January 2013 - 07:34 AM

:confused: 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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