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Is the great f/ratio debate dated?

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#26 jjack's

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Posted 30 March 2022 - 03:50 AM

Could it be better to utilize a medium to long refractor and put on a reducer corrector for AP or a short corrected refractor ?


Edited by jjack's, 30 March 2022 - 03:52 AM.


#27 Wildetelescope

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Posted 30 March 2022 - 07:07 AM

All depends on what you want from the refractor. I do that with my tv102. But if I want to do something like the heart or North American nebula, a faster scope is how I go personally.

Jmd

[quote name="jjack's" post="11805432" timestamp="
Could it be better to utilize a medium to long refractor and put on a reducer corrector for AP or a short corrected refractor ?[/quote]

#28 bigbangbaby

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Posted 30 March 2022 - 07:16 AM

Wow, lots to unpack here.

 

Takeaways so far:

 

Slower refractors

 

Upsides:

Can use simpler, less expensive eyepieces.

Easier to achieve higher magnification

Easier to focus

Don't suffer field curvature to the extent faster ones do

 

Downsides:

Smaller field of view

Larger and harder to handle

 

Question:

Is it practical (for visual) to use a reducer to shorten focal length?

 

Faster refractors

 

Upsides:

Large field of view

Smaller and easier to handle

Can take high magnification without field curvature

 

Downsides:

Require more complex eyepieces to work optimally

Require higher power eyepieces or Barlows to reach high magnifications

Suffer field curvature

 

Questions:

Is a field flattener a practical option for fast refractors for visual? This would address the field curvature issue.

With quality Barlows up to 5X available, why not use lower power eyepieces for high magnifications? By adding another optical element, do Barlows noticeably compromise image quality?

 

Observation: The TV NP101 seems to strike a balance between fast and slow, which begs the question: If it is the Goldilocks optical design, why don't all premium scope makers offer one?

 

Larry


Edited by bigbangbaby, 30 March 2022 - 07:18 AM.


#29 jgraham

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Posted 30 March 2022 - 08:18 AM

"A well made, fast apochromat can do an excellent job at high magnifications and still provide low power, wide field views that are as perfect as perfect can be."

 

Exactly. I am amazed by the views through a modern Apo or ED. Something magical happens when all of the light goes where it is supposed to go.

 

However...

 

My experience has been that fast (and even not so fast) Apos and EDs can be a leetle bit finnicky, particularly in the larger apertures. They need to be well collimated and setting the sharpest focus can be a bit touchy. I often use a Bahtinov mask to make sure that I've set the best possible focus when it is critical and so I can quit fishing for it. :)

 

Still, modern Apos and quality EDs are wonderful.

 

However (again)...

 

I recently started using some very long achros; 60mm, 75mm, and 102mm f/15s (and I've got a 154mm f/15 in the works). These are simply amazing scopes! Tack sharp focus across the field, no discernable CA visually (I can pick up a tad on some images), flat fields, super easy to focus. To go 'wide' visually I use a long focal length eyepiece. My trusty old 56mm 2" Meade Super Plossl works great, and my 26mm 2" QX does a fine job with modest fields of view, 1.25" UWAs for high magnifications. Super sharp Airy disks, super sharp, circular diffraction  rings with very little energy in the first ring. Imaging-wise I found that even a 4/3 sensor does well with my 3" f/15 and I'm going to try an APS-c when I get a chance. The Plan for the 4" f/15 is to use a full-frame DSLR. The tricky both with these long achros is that they need a good mount. Fortunately, the Unitrons are absolutely perfect fully manual mounts and also perform well for basic imaging. If I get serious about imaging with them I'll move them over to my Atlas, but for now I'm having too much fun using them for visual and simple imaging/EAA using some pretty basic kit.

 

So much to see, so few clear nights...

 

Enjoy!


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#30 vahe

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Posted 30 March 2022 - 08:53 AM

 

Observation: The TV NP101 seems to strike a balance between fast and slow, which begs the question: If it is the Goldilocks optical design, why don't all premium scope makers offer one?

 

 

 

Here is a comment by one premium scope maker, Roland, regarding Petzval refractors. Rolands comments appeared in Yahoo Refractors group some years ago.

.

.
"Improved field curvature does nothing much for you visually. In a normal refractor the field curvature is not noticed in a typical eyepiece, and the edge aberrations in eyepieces usually dominate. The 25% improvement in color correction means that an F5 Petzval will look identical to an F6.3 normal doublet color wise, so there is practically no gain. An F5 Petzval will be the same length as an F7.5 normal doublet refractor, and that scope would have slightly better color correction, although the max field in a 2” eyepiece would be smatter.

.
In a doublet refractor the field would be fully illuminated over the entire 2” circle, and that is not possible in a Petzval (unless the secondary lens size is 4” diameter).

.
Most Petzval designs fully illuminate only the very center of the field, with gradual light dropoff over a 2’ field. The light drop off is so gradual that one does not notice it, but the result is that you are seeing deep sky objects at less than full aperture over some part of the field.

.
As far as imaging, where the flat field is an advantage, you would be better off with a full apo rater than an achromat. You can always add a field flattener or TC to the back of an apo to achieve the same result. Achros can be used for imaging, but they leave a talltale blue bloat around stars – something that can look pretty to some people, I’ll admit."

.

Vahe


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#31 StarAlert

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Posted 30 March 2022 - 09:13 AM

Wow, lots to unpack here.

 

Takeaways so far:

 

Slower refractors

 

Upsides:

Can use simpler, less expensive eyepieces.

Easier to achieve higher magnification

Easier to focus

Don't suffer field curvature to the extent faster ones do

 

Downsides:

Smaller field of view

Larger and harder to handle

 

Question:

Is it practical (for visual) to use a reducer to shorten focal length?

 

Faster refractors

 

Upsides:

Large field of view

Smaller and easier to handle

Can take high magnification without field curvature

 

Downsides:

Require more complex eyepieces to work optimally

Require higher power eyepieces or Barlows to reach high magnifications

Suffer field curvature

 

Questions:

Is a field flattener a practical option for fast refractors for visual? This would address the field curvature issue.

With quality Barlows up to 5X available, why not use lower power eyepieces for high magnifications? By adding another optical element, do Barlows noticeably compromise image quality?

 

Observation: The TV NP101 seems to strike a balance between fast and slow, which begs the question: If it is the Goldilocks optical design, why don't all premium scope makers offer one?

 

Larry

Another downside to faster refractors. They will cost you more.
CFF offers the 160 in f/6.5 and f/8. The f/8 is about $500 cheaper.
Catalin also offers the CFF200 in f/6.5 and f/8. The f/6.5 is about $1,750 more expensive than the f/8. 
 


Edited by StarAlert, 30 March 2022 - 09:13 AM.

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#32 25585

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Posted 30 March 2022 - 09:38 AM

The Tak FC-76 DCU with extender operates as a quad-lens scope at f/12.6. Its color color correction is insanely good, noticeably better than the barebone FC-76 DCU fluorite which is already excellent. The combo's out-of-this-world performance is a testiment to what can be accomplished when a slow focal ratio is combined with fluorite and figured and polished to world-class standards.

Ditto for FC100DL & 1.6× Extender = 4" F14.4 Fluoride. Perfection.


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#33 weis14

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Posted 30 March 2022 - 09:50 AM

Observation: The TV NP101 seems to strike a balance between fast and slow, which begs the question: If it is the Goldilocks optical design, why don't all premium scope makers offer one?

I have never owned a Petzval scope, but I've seen multiple people talk about the need to make sure they stay collimated and taking extra care not to bump them during shipping, etc.  This is likely due to the need to ensure precise collimation between the two optical groups.

 

My experience is that most doublets and triplets, regardless of focal length, do not have this concern.  There are some notorious exceptions, such as older large Meade APOs, but the general rule is that most triplets do not need collimation ever and are pretty rugged scopes.  None of the doublets or triplets I've owned over the last 20 years (WO 80mm Zenithstar, WO 90mm Megrez, AT92, TEC160FL, CFF160, AP 130GTX or AP Stowaway) have ever needed to be collimated despite being primarily a mobile observer and transporting them frequently down rough roads to dark observing locations.

 

For a manufacturer like TEC or A-P, it is probably a better bet to produce a triplet and a high quality flattener/reducer that can be bough separately (which is what most do).  This allows them to have the best of both worlds and allows purchasers the option of imaging at multiple focal lengths with one scope.


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#34 Space_Race_T.J.

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Posted 30 March 2022 - 09:51 AM

Larry,

 

Questions:

 

Is a field flattener a practical option for fast refractors for visual? This would address the field curvature issue.

With quality Barlows up to 5X available, why not use lower power eyepieces for high magnifications? By adding another optical element, do Barlows noticeably compromise image quality?

 

 

1. Yes members here have been using the TS-Optics TSFLAT2 field flattener for visual use for low power viewing to address the field curvature in fast refractors for years. I'm going to use that field flattener when my Astro-Tech AT92 F5.5 506mm focal length scope arrives later this year.

 

2. As John stated, field curvature is reduced when using higher and higher magnification eyepieces. You could use a barlow to get to higher magnifications, but it's not really needed unless that low power eyepiece is the only one you have.

 

T.J.


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#35 russell23

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Posted 30 March 2022 - 10:04 AM

I think the important thing is that all options are available:  short focal ratio, long focal ratio, Petzval.  So people can select what best fits their preferences. 


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#36 Space_Race_T.J.

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Posted 30 March 2022 - 10:05 AM

The curvature of the field depends on the radius of curvature of the objective, about one third the focal length, and the field stop diameter. As magnification is increased, field curvature quickly disappears. It's only at lower powers where it's visible.

 

Consider an 80mm F/6. With a 40 mm SWA, the difference between center focus and edge focus is about 1.5 mm. The curvature is very apparent.  With a 7 mm UWA, 70x, the difference between center and edge focus is about 0.08mm, the depth of focus at F/6 is 0.08 mm, the curvature is not visible.

 

With a fast scope, off-axis eyepiece astigmatism is an issue at all magnifications. Field flatteners and Petzval designs do not help with that. With a scope like the NP-101, F/5.4, it's corrected for field curvature but unless you use eyepieces like the Naglers etc, the eyepiece's off-axis aberrations will be quite visible.

 

Jon

Jon,

 

Thank you for that detailed explanation. I have a few more questions though.

 

Do all eyepieces have off-axis astigmatism. Or is that only a problem with certain eyepiece designs or say less expensive brands/models?

 

I'm going to use a 31 Nagler and a 21 Ethos with the TSFLAT2 in my upcoming AT92 F5.5 scope for low power wide-field viewing. Am I going to see off-axis astigmatism with those eyepieces?

 

Thank you,

 

T.J.



#37 russell23

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Posted 30 March 2022 - 10:35 AM

Jon,

 

Thank you for that detailed explanation. I have a few more questions though.

 

Do all eyepieces have off-axis astigmatism. Or is that only a problem with certain eyepiece designs or say less expensive brands/models?

 

I'm going to use a 31 Nagler and a 21 Ethos with the TSFLAT2 in my upcoming AT92 F5.5 scope for low power wide-field viewing. Am I going to see off-axis astigmatism with those eyepieces?

 

Thank you,

 

T.J.

My experience is that the performance varies with the eyepiece design.  For example, the 32mm University Konig works very well in my 102mm f/11 but is a mess of astigmatism at the edges in 102mm f/7 and 10” f/6.3 scopes. 

 

I look at it more about how the scope-eyepiece combination works.  That seems to be a combination of the scope focal ratio and the # of elements and design of the eyepiece, not to mention the observer’s eyes. 

 

The eyepiece tends to get more of the blame, but a huge amount of edge performance cleanup occurs with longer scope focal ratios.  The difference between a 102mm f/11 and a 102mm f/7 where eyepiece edge performance is concerned is quite astounding when you are talking about 4 element eyepieces or even eyepieces such as the longer focal length XW’s. 


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#38 alnitak22

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Posted 30 March 2022 - 01:22 PM

Here is a comment by one premium scope maker, Roland, regarding Petzval refractors. Rolands comments appeared in Yahoo Refractors group some years ago.

.

.
"Improved field curvature does nothing much for you visually. In a normal refractor the field curvature is not noticed in a typical eyepiece, and the edge aberrations in eyepieces usually dominate. The 25% improvement in color correction means that an F5 Petzval will look identical to an F6.3 normal doublet color wise, so there is practically no gain. An F5 Petzval will be the same length as an F7.5 normal doublet refractor, and that scope would have slightly better color correction, although the max field in a 2” eyepiece would be smatter.

.
In a doublet refractor the field would be fully illuminated over the entire 2” circle, and that is not possible in a Petzval (unless the secondary lens size is 4” diameter).

.
Most Petzval designs fully illuminate only the very center of the field, with gradual light dropoff over a 2’ field. The light drop off is so gradual that one does not notice it, but the result is that you are seeing deep sky objects at less than full aperture over some part of the field.

.
As far as imaging, where the flat field is an advantage, you would be better off with a full apo rater than an achromat. You can always add a field flattener or TC to the back of an apo to achieve the same result. Achros can be used for imaging, but they leave a talltale blue bloat around stars – something that can look pretty to some people, I’ll admit."

.

Vahe

How long ago is that quote by RC? Anyway, it’s misleading, particularly the part about an f/5 Petzval having identical color correction only to a normal f/6.3 doublet. That’s only true if the Petzval uses normal achromatic glasses. Since  the Genesis in 1988, the TV Petzvals all used exotic glasses. The current NP127 and 101 have the equivalent color correction across the entire visual spectrum of an f/105 achromat. Which is quite different than an f/6.3. I wonder if Jon Isaacs or any other NP101 owners would agree that their scope has the same color error as an f/6.3 achro? I think not.


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#39 bigbangbaby

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Posted 30 March 2022 - 08:02 PM

Larry,

 

Questions:

 

Is a field flattener a practical option for fast refractors for visual? This would address the field curvature issue.

With quality Barlows up to 5X available, why not use lower power eyepieces for high magnifications? By adding another optical element, do Barlows noticeably compromise image quality?

 

 

1. Yes members here have been using the TS-Optics TSFLAT2 field flattener for visual use for low power viewing to address the field curvature in fast refractors for years. I'm going to use that field flattener when my Astro-Tech AT92 F5.5 506mm focal length scope arrives later this year.

 

2. As John stated, field curvature is reduced when using higher and higher magnification eyepieces. You could use a barlow to get to higher magnifications, but it's not really needed unless that low power eyepiece is the only one you have.

 

T.J.

That's good to know about the TS-Optics flattener. I have a TMB-92SS, which is the same focal ratio as the AT92.


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#40 EJN

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Posted 30 March 2022 - 08:13 PM

Long focus refractors have the values of tradition.

 

default.jpg


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#41 barbie

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Posted 30 March 2022 - 08:41 PM

All of this discussion about field curvature, flatteners and Petzval designs makes me EXTREMELY glad that I'm no longer involved in astrophotography!! Been there, done that!! I have absolutely no desire to go back to it ever again!! I just enjoy what my scopes show me and and appreciate them for what they are and ignore the critics!! After having designed and built many telescopes and cameras and spent hours collecting photons on photographic emulsions and CCD imaging, I now let my eyes do the judging of an optic's qualities.


Edited by barbie, 30 March 2022 - 08:50 PM.

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#42 photoracer18

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Posted 30 March 2022 - 08:53 PM

In my experience, yes. It's much more finicky to focus; is much more sensitive to cooldown; puts much more stress on diagonals, binoviewers and eyepieces.  

 

Yes. This means a longer focal ratio apochromat can use a considerably simpler and cheaper design, and still achieve superb color correction.

 

Yes, all else equal.

 

No, usually not. 

 

The general consensus today is that they can, but most observers today have never observed with a truly long-focal ratio apochromat, so they have no basis to actually compare on and can only use extrapolation (IE "my fast scope can show this and that, which good for its aperture, so it must be a good scope"). 

 

That all said, it's generally hard to go wrong with a modern f/7 - 8 ED or apo. They are excellent scopes for general use, including very good planetary views. 

 

 

Clear skies!

Thomas, Denmark 

Only if its 3" and under. 75mm has good aberration control at F16 and longer. Anything larger needs to be much longer. And yes the faster the F-ratio the shorter the focus range. Which is why they invented dual speed focusers. Did not need them in the days of F15 achromats. I have a number of long ratio achromats up to a Carton 100mm F13. Have not used them in years. Don't really expect to now that I have a Stellarvue SVX127D. Sure its way more money but I know what it can do visually. It can take the place of all my other refractors since I don't image anymore. Cools down just as fast as an achromatic doublet for all intends and purposes. At one time I thought I was going to have to lay my hands on one of Fred's 130/12 APOMAX scopes that Don used to sell for my retirement. But this scope is much handier and has 2 generations better low dispersion glass than anything available then. Its not that I have not liked long achromats its just I no longer feel the need to have to deal with the mechanics. But that's just me. YRMV.


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#43 photoracer18

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Posted 30 March 2022 - 09:00 PM

Long focus refractors have the values of tradition.

 

default.jpg

This is 1880 state of the art technology. With all that entails. Definitely a work of art. For someone else. I would not make a vintage Model T my daily driver either. I have daily driven classic muscle cars but I would not do it today.


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#44 photoracer18

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Posted 30 March 2022 - 09:12 PM

"A well made, fast apochromat can do an excellent job at high magnifications and still provide low power, wide field views that are as perfect as perfect can be."

 

Exactly. I am amazed by the views through a modern Apo or ED. Something magical happens when all of the light goes where it is supposed to go.

 

However...

 

My experience has been that fast (and even not so fast) Apos and EDs can be a leetle bit finnicky, particularly in the larger apertures. They need to be well collimated and setting the sharpest focus can be a bit touchy. I often use a Bahtinov mask to make sure that I've set the best possible focus when it is critical and so I can quit fishing for it. smile.gif

 

Still, modern Apos and quality EDs are wonderful.

 

However (again)...

 

I recently started using some very long achros; 60mm, 75mm, and 102mm f/15s (and I've got a 154mm f/15 in the works). These are simply amazing scopes! Tack sharp focus across the field, no discernable CA visually (I can pick up a tad on some images), flat fields, super easy to focus. To go 'wide' visually I use a long focal length eyepiece. My trusty old 56mm 2" Meade Super Plossl works great, and my 26mm 2" QX does a fine job with modest fields of view, 1.25" UWAs for high magnifications. Super sharp Airy disks, super sharp, circular diffraction  rings with very little energy in the first ring. Imaging-wise I found that even a 4/3 sensor does well with my 3" f/15 and I'm going to try an APS-c when I get a chance. The Plan for the 4" f/15 is to use a full-frame DSLR. The tricky both with these long achros is that they need a good mount. Fortunately, the Unitrons are absolutely perfect fully manual mounts and also perform well for basic imaging. If I get serious about imaging with them I'll move them over to my Atlas, but for now I'm having too much fun using them for visual and simple imaging/EAA using some pretty basic kit.

 

So much to see, so few clear nights...

 

Enjoy!

Sorry but F15 is not long enough to remove abberrations from a 6" achromatic doublet. Sure it will be better than an F8 one but to be at the level of a 75mm F15 it needs to be around F28 or so if I remember correctly. Sorry I never got to look thru Dr. Green's 10" F34 made by Barry of D&G before he passed away. He invited me up to LI a couple of times after I bought his Jaegers 6" F15 but I never made it.



#45 EJN

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Posted 30 March 2022 - 11:16 PM

This is 1880 state of the art technology. 

 

Actually 1824. That is the Dorpat refractor, built by Joseph von Fraunhofer, and at the time the largest refractor in the world (9" aperture).


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#46 Jan-S

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Posted 31 March 2022 - 02:11 AM

Another downside to faster refractors. They will cost you more.
CFF offers the 160 in f/6.5 and f/8. The f/8 is about $500 cheaper.
Catalin also offers the CFF200 in f/6.5 and f/8. The f/6.5 is about $1,750 more expensive than the f/8. 
 

 

And one more: some filters require the light to pass through as close to straight/parallel as possible and hence will work not work well with short F-ratios. 

 

Jan


Edited by Jan-S, 31 March 2022 - 02:11 AM.


#47 Lagrange

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Posted 31 March 2022 - 04:46 AM

Wow, lots to unpack here.

 

Takeaways so far:

 

Slower refractors

 

Upsides:

Can use simpler, less expensive eyepieces.

Easier to achieve higher magnification

Easier to focus

Don't suffer field curvature to the extent faster ones do

 

Downsides:

Smaller field of view

Larger and harder to handle

 

Question:

Is it practical (for visual) to use a reducer to shorten focal length?

 

Faster refractors

 

Upsides:

Large field of view

Smaller and easier to handle

Can take high magnification without field curvature

 

Downsides:

Require more complex eyepieces to work optimally

Require higher power eyepieces or Barlows to reach high magnifications

Suffer field curvature

 

Questions:

Is a field flattener a practical option for fast refractors for visual? This would address the field curvature issue.

With quality Barlows up to 5X available, why not use lower power eyepieces for high magnifications? By adding another optical element, do Barlows noticeably compromise image quality?

 

Observation: The TV NP101 seems to strike a balance between fast and slow, which begs the question: If it is the Goldilocks optical design, why don't all premium scope makers offer one?

 

Larry

From what I've seen the answer is usually no. Flatteners will often work just fine visually and if the manufacturer's own model doesn't lend itself to visual use then it's not hard to find a third party product that will.

 

Reducers significantly cut the amount of back focus available and in most cases leave you with too little to permit the use of even a 1.25" diagonal so unless you fancy observing straight through they're not really an option.

 

That's a huge advantage for Petzval scopes which are designed from the outset to have enough back focus for imaging and visual.

 

 

How long ago is that quote by RC? Anyway, it’s misleading, particularly the part about an f/5 Petzval having identical color correction only to a normal f/6.3 doublet. That’s only true if the Petzval uses normal achromatic glasses. Since  the Genesis in 1988, the TV Petzvals all used exotic glasses. The current NP127 and 101 have the equivalent color correction across the entire visual spectrum of an f/105 achromat. Which is quite different than an f/6.3. I wonder if Jon Isaacs or any other NP101 owners would agree that their scope has the same color error as an f/6.3 achro? I think not.

If he's talking about f/5 scopes and achromats then he must be referring to the original Genesis. I also suspect that a lot of observers would take issue with the suggestion that field curvature isn't a problem for visual astronomy.

 

 

And one more: some filters require the light to pass through as close to straight/parallel as possible and hence will work not work well with short F-ratios. 

 

Jan

That tends to be a problem at very low focal ratios - f/4 and below so it's unlikely to be an issue with most refractors. If you were using an FSQ-130 with its reducer to give you an f/3 scope then it's something you'd need to consider, but anyone who can afford gear like that can afford to spend a bit extra on filters!


Edited by Lagrange, 31 March 2022 - 04:46 AM.


#48 Jon Isaacs

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Posted 31 March 2022 - 05:06 AM

Only if its 3" and under. 75mm has good aberration control at F16 and longer. Anything larger needs to be much longer. And yes the faster the F-ratio the shorter the focus range. Which is why they invented dual speed focusers. Did not need them in the days of F15 achromats. I have a number of long ratio achromats up to a Carton 100mm F13. Have not used them in years. Don't really expect to now that I have a Stellarvue SVX127D. Sure its way more money but I know what it can do visually. It can take the place of all my other refractors since I don't image anymore. Cools down just as fast as an achromatic doublet for all intends and purposes. At one time I thought I was going to have to lay my hands on one of Fred's 130/12 APOMAX scopes that Don used to sell for my retirement. But this scope is much handier and has 2 generations better low dispersion glass than anything available then. Its not that I have not liked long achromats its just I no longer feel the need to have to deal with the mechanics. But that's just me. YRMV.

 

I suspect you did not realize Thomas was discussing apochromats.

 

 

"In my experience, yes. It's much more finicky to focus; is much more sensitive to cooldown; puts much more stress on diagonals, binoviewers and eyepieces. 

 

Yes. This means a longer focal ratio apochromat can use a considerably simpler and cheaper design, and still achieve superb color correction."

 

Jon


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#49 Jan-S

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Posted 31 March 2022 - 05:13 AM

And one more: some filters require the light to pass through as close to straight/parallel as possible and hence will work not work well with short F-ratios. 

 

Jan

 

 

 

That tends to be a problem at very low focal ratios - f/4 and below so it's unlikely to be an issue with most refractors. If you were using an FSQ-130 with its reducer to give you an f/3 scope then it's something you'd need to consider, but anyone who can afford gear like that can afford to spend a bit extra on filters!

 

A rear-mounted h-alpha filter improves with higher F-ratios going up to F/50. With a very fast scope, you'd have to stop it down quite a bit to get there.

 

Beyond filters, ADCs have similar requirements.



#50 Jon Isaacs

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Posted 31 March 2022 - 05:32 AM

Here is a comment by one premium scope maker, Roland, regarding Petzval refractors. Rolands comments appeared in Yahoo Refractors group some years ago.

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"Improved field curvature does nothing much for you visually. In a normal refractor the field curvature is not noticed in a typical eyepiece, and the edge aberrations in eyepieces usually dominate. The 25% improvement in color correction means that an F5 Petzval will look identical to an F6.3 normal doublet color wise, so there is practically no gain. An F5 Petzval will be the same length as an F7.5 normal doublet refractor, and that scope would have slightly better color correction, although the max field in a 2” eyepiece would be smatter.

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In a doublet refractor the field would be fully illuminated over the entire 2” circle, and that is not possible in a Petzval (unless the secondary lens size is 4” diameter).

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Most Petzval designs fully illuminate only the very center of the field, with gradual light dropoff over a 2’ field. The light drop off is so gradual that one does not notice it, but the result is that you are seeing deep sky objects at less than full aperture over some part of the field.

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As far as imaging, where the flat field is an advantage, you would be better off with a full apo rater than an achromat. You can always add a field flattener or TC to the back of an apo to achieve the same result. Achros can be used for imaging, but they leave a talltale blue bloat around stars – something that can look pretty to some people, I’ll admit."

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Vahe

 

You need to provide a link. It's required by the TOS and in this case, it's important because I would like to read the context of Roland's comments.

 

It's clear he is not talking about modified Petzvals like the TeleVue NP series and the Takahashi FSQ series that use ED glasses in both doublets.

 

He's clearly not talking about eyepieces like the 31 mm Nagler, 41 mm Panoptic etc. These are not "normal" eyepieces and show essentially no aberrations in very fast scopes. The field curvature with the 31 mm Nagler of a 4 inch F/5 doublet is easy to see, in the NP-101, it's as perfect as can be.

 

I normally find Roland to be on the money. In this case, his comments seem to be addressing achromatic Petzvals used with Erfle's and the like rather than apochromatic "modified Petzvals" used with well corrected eyepieces. These days, one does not have to spend large sums to buy a well corrected ~30 mm 82° eyepiece. The $200 Astro-Tech 28 mm 82° provides excellent views in the NP-101.. 4.33° at 19x with a 5.2 mm exit pupil.. 

 

Do you have much experience with scopes like the NP series using eyepieces like the 31 mm Nagler?

 

Jon


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