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FPL-53 vs FPL-51 glass

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#201 starcanoe

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Posted 29 March 2018 - 02:56 PM

The people here are talking APO/high end refractors. The limiting factor in terms of costs...even including mount considerations is aperture...not focal length. And even the aperture thing is a fairly hard limit....best ever 4 incher....pricey...5 incher...very pricey...6 incher OMG....7 incher only1 percenters need apply....8 plus call Bill Gates...

 

If you think the pro hi f ratio crowd here is arguing for a 3 inch f14 vs a 6 inch f7....well, I think you are not under the right impression.


Edited by starcanoe, 29 March 2018 - 03:20 PM.


#202 Jon Isaacs

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Posted 29 March 2018 - 02:57 PM

None of these eyepieces require any "struggling"....if anything getting some decent magnification with moderate focal length eyepieces is a Godsend. You aren't forced to buy fancy eyepieces to have an eye relief that doesn't require you to press your eye into the eyepiece to see...and the long focal ratio means a much simpler eyepiece works just as well as a complex one at faster f ratios (and the complex ones approach perfection abberation wise at slow f ratios).

 

 

The Godsend is called a Barlow lens..  

 

I'm glad this thread is back on topic.  ;)

 

Jon



#203 starcanoe

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Posted 29 March 2018 - 03:31 PM

But that's two more glass elements....one more glass to glass interface....and TWO whole more air to glass interfaces....not to mention the mechanical slop :)

 

PS. I think the mechanical slop is actually a significant (well slightly) problem that gets ignored.

 

Yeah, you can argue that a 6 inch f 8 apo is "good enough"....which is BTW pushing up on the limits of what the manufacturers can make "good enough"....and on that same token...I can argue that an 8 inch f6 with a coma corrector can equal or exceed that. Or heck for this kinda money a 10 inch f5 well made will kick both their butts.

 

This current debate is IMO about same aperture....vs various f ratios.

 

There is a reason that AP didn't start with maximum aperture minimum f ratio refractors...I wonder what that reason was...



#204 jay.i

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Posted 29 March 2018 - 03:44 PM

There is a reason that AP didn't start with maximum aperture minimum f ratio refractors...I wonder what that reason was...

There could be many reasons. If one was implying that they started with slower focal ratios because they're superior to faster focal ratios, I would say it's more likely that they started with slower focal ratios because it's easier to make a quality optic with slow focal ratios. Not necessarily because they're better. Use lower dispersion glass, spend more time polishing it, and bam, faster focal ratio with the same optical quality! (In theory, of course... don't shoot me!)



#205 starcanoe

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Posted 29 March 2018 - 04:04 PM

There could be many reasons. If one was implying that they started with slower focal ratios because they're superior to faster focal ratios, I would say it's more likely that they started with slower focal ratios because it's easier to make a quality optic with slow focal ratios. Not necessarily because they're better. Use lower dispersion glass, spend more time polishing it, and bam, faster focal ratio with the same optical quality! (In theory, of course... don't shoot me!)

Bingo.

 

At least the manufacturing part.

 

The theory part is the thing.

 

So, if in theory, what they manufacture is close enough to perfect....well what is close enough to perfect? Couldn't they make it a bit faster/bigger and still be "close enough"....oh no, that wouldn't be close enough.....well, maybe their close enough wasn't quite close enough to start with....

 

Zambuto....known for some of the best mirrors an ATM can buy...has a lower limit on the F ratio he will make (THAT is my understanding of the situation)....and he has stated (again MY understanding) that there is a higher f ratio where "the magic happens".

 

So, barring everything else..where on that f ratio spectrum do you think a Zambuto mirror is likely to go from darn good to OMG (well okay OMG to kill me now :)  ) ?

 

Keeping in mind Zambuto has to make a living doing this and not spend six months with his mad skills making one perfect mirror....which he could probably do.

 

The same thing applies to refractors...probably in spades because you are talking 4 to 6 surfaces....2 to 3 thicknesses....wedge on 2 to 3 elements...diameters on 2/3 elements....radius tolerances on 4 to 6 surfaces....the lens cell....AND variations on the glass depending on the which melt it is...and a probably a few other things....heck, it is probably a miracle that supper accurate lens assemblies can be made compared to a mirror element.


Edited by starcanoe, 29 March 2018 - 04:14 PM.


#206 Jon Isaacs

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Posted 29 March 2018 - 04:17 PM

There could be many reasons. If one was implying that they started with slower focal ratios because they're superior to faster focal ratios, I would say it's more likely that they started with slower focal ratios because it's easier to make a quality optic with slow focal ratios. Not necessarily because they're better. Use lower dispersion glass, spend more time polishing it, and bam, faster focal ratio with the same optical quality! (In theory, of course... don't shoot me!)

 

When Astro-Physics started , Roland was not the skilled optician and designer he became . The scopes were designed using short flint glasses, ED glasses were not available other than Fluorite.  The goal was affordable quality. 

 

Thomas Back's  A Brief History of Astro-Physics lens discusses all this .. 

 

http://www.csun.edu/...n/tmb/tmb1.html

 

I always like this section:

 

"Then the new era began. It was AstroFest 1990, and Roland brought his ultimate line of ED EDT triplets. I had my 6"f/12 setup next to the new 6.1" f/9 EDT, and at first glance cast on Saturn, I knew it was all over. This scope was a knockout, and in color correction and contrast, clearly beat my 6" f/12 Super Planetary. This was a prototype, was airspaced and not even coated."

 

Jon Isaacs



#207 Redbetter

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Posted 29 March 2018 - 04:26 PM

Have you done the math?

 

Lets assume a  6 inch f15 scope.

 

A 4 mm exit pupil....which given the rampant light pollution these...it about as low as you generally want to go most of the time....you need a 60mm eyepiece....they even make 80mm 3 inch format eyepieces these days....though a 50mm is probably what somebody on a budget is going to use.

 

In theory....your typical eye has about the same resolution as the scope when the exit pupil is 2mm.....which requires a 30mm eyepiece.

 

In practice....for a 6 inch scope with very good optics....in good seeing...most people seem to hover around a 1 mm exit pupil down to a half if they really like to magnify things....which requires something more like a 15 to 7 mm eyepiece.

 

None of these eyepieces require any "struggling"....if anything getting some decent magnification with moderate focal length eyepieces is a Godsend. You aren't forced to buy fancy eyepieces to have an eye relief that doesn't require you to press your eye into the eyepiece to see...and the long focal ratio means a much simpler eyepiece works just as well as a complex one at faster f ratios (and the complex ones approach perfection abberation wise at slow f ratios).

 

Or, I guess one could complain that very low fields of view/magnification are not possible with a high f ratio refractor....though IMO that is like complaining that a fancy convertible sports car is not suitable for hauling 20 bags of concrete mix or a few sheets of plywood....

 

PS... we (or at least most of us) aren't talking a 6 inch f7 vs a 3 inch f14. We are talking SIMILAR diameters and medium vs long focal lengths.

 

Have you done the math or actually considered the practical considerations that we have been bringing up?  Comparing apertures does matter, because a larger aperture apo/doublet is likely to provide as much or better detail with a more comfortable/less limiting exit pupil using the same mount and with better versatility than a long ratio apo/doublet.  The larger aperture will do so with a more user friendly exit pupil at planetary magnifications.  You can put on blinders and be academic about it, but then it doesn't apply to the real world discussion. 

 

For planetary magnification the problem is not with the eyepieces in my experience. It is the exit pupil.  Neither a longer ratio nor a Barlow change the exit pupil required to achieve a given magnification/image scale. 

 

If you want to go to lowest common denominator with the light pollution argument then you are essentially imposing an urban/suburban viewing restriction.  I guess if one is only interested in a backyard planetary scope that is true, but if one actually intends to view showpiece DSO's with a refractor then Bortle 3 or 4 skies are in order (and better if available.)  I have been able to find those sort of sites every place I have lived across the country.  I haven't lived in the northeast Megalopolis though. 

 

You have essentially made the argument for a planetary/lunar only scope that is used in the backyard.   But even that has problems.  The best planetary views I get are rarely in my suburban backyard other than a few good nights each year.  Instead they are at my dark sky sites, which suffer less from low altitude seeing problems, heat radiating from roof tops, furnace vents/AC condensers, and urban heat island impacts.  As a result at my dark sites more detail is visible on the planets because the seeing is better, and more moons are seen around Saturn.    On average a super duper f/15 apo with perfect figure is going to give me worse images in the backyard than the same aperture ED doublet less than an hour away in dark skies (nevermind a comparison with a somewhat larger aperture doublet.)  It isn't the fault of the instrument.

 

And when one is at a dark site with a refractor, the extra field of view and large exit pupils become desirable.  I have used up to nearly 8mm exit pupil in my 110mm f/7 and was rewarded with the best views I have had so far of Barnard's Loop.  One of the greatest strengths of refractors is the wide field views they can provide.  Being able to do that as well as provide a high quality planetary image is a real plus.   

 

I tend to employ refractors in 2 modes, high power and low power/wide field with little in between.  That plays to the strengths of the apo/ED doublets.  A long ratio removes one of the strengths.



#208 starcanoe

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Posted 29 March 2018 - 04:36 PM


I tend to employ refractors in 2 modes, high power and low power/wide field with little in between.  That plays to the strengths of the apo/ED doublets.  A long ratio removes one of the strengths.

 

Then why do you need one scope that is overkill for one and not quite perfection for the other?

 

There is a solution for that.



#209 Jon Isaacs

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Posted 29 March 2018 - 04:48 PM

I tend to employ refractors in 2 modes, high power and low power/wide field with little in between.  That plays to the strengths of the apo/ED doublets.  A long ratio removes one of the strengths.

 

waytogo.gif

 

The virtue of the apo is it's versatility.  One scope does is all just about as well as it can be done, within the limits of its aperture. 

 

Some how these threads take tangents and are side tracked in discussions that are irrelevant to the original topic. Most often , this digression involves planetary performance.  This thread is a good example. 

 

"An 80 mm F/7 to be used for EAA, is there a significant difference between FPL-51 or FPL-53?"

 

I think there is.  I have two 80mm FPL-53 doublets,  F/6 and F/7. I have had FPL-51 doublets in similar configurations,  the color correction is significantly better with the 53 and I suspect that matters in EAA . In terms of focal ratio,  those EAA folks want fast focal ratios. 

 

Jon



#210 Redbetter

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Posted 29 March 2018 - 05:04 PM

Then why do you need one scope that is overkill for one and not quite perfection for the other?

 

There is a solution for that.

Not sure what your "solution" is, but I like to be able to use the effective range of a refractor from 0.5mm to ~7mm exit pupil.  The solution I use doesn't require a heavier scope and mount costing several times what I already have.  And if I wanted to go that route it would likely be 6" or so anyway.  Long ratio is not in the cards for me at such an aperture as it would be restricted even more to in town viewing with poorer seeing...and that doesn't even address what it would take to mount it properly.

 

The 80ED on a simple alt/az mount is often used as a "scout" for backyard seeing due to its portability.  (The 60ED is too little aperture/resolution to effectively evaluate seeing for larger scopes, but it has other uses.)  The 80ED and 60ED also work well for quick looks at planets/etc. low in elevation (dismal seeing) where I have to find an appropriate vantage point in the yard and sometimes move the tripod while viewing.  With respect to scouting for planets that are more reasonably placed, if the seeing is good or something interesting is visible, then the 110ED, 8", 10" or 20" scopes are brought out to play depending on conditions, and the time/effort I am willing to expend.  When the seeing is good enough that I set up the larger apertures, the small ones just end up sitting there other than some quick comparisons. 


Edited by Redbetter, 29 March 2018 - 11:14 PM.


#211 starcanoe

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Posted 29 March 2018 - 05:36 PM

Not sure what your "solution" is, but I like to be able to use the effective range of a refractor from 0.5mm to ~7mm exit pupil. arisons. 

 

So, you want one scope to do it all?

 

How about a something like a 6 inch f12 apo with a gazillionth wave optics and a color correction that makes a reflector go "dammm".

 

Couple that with say a 4 or 5 inch semi apo (or heck an achro at this level) with 1 wave optics (because at LOW powers none of this stuff makes a difference) for only the low power views.

 

There you go.



#212 daquad

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Posted 29 March 2018 - 06:58 PM

 

 

In theory....your typical eye has about the same resolution as the scope when the exit pupil is 2mm.....which requires a 30mm eyepiece.

 

 

Actually, the resolution of the telescope is reached at about a 1 mm exit pupil, because the effective aperture of the eye for most people doesn't equal its actual aperture until its actual aperture is 1 mm.  Most person's irises may not close to 1 mm, but at the telescope the exit pupil is the  eye's aperture.  For some people, especially the young, the effective aperture may equal the actual aperture at somewhat more than 1 mm.

 

Try to detect the Airy disc of a 2nd magnitude star (no flare) at a 2 mm exit pupil.  I think you will not see the Airy disc as a disc, but as a pinpoint.  In other words, the Airy disc is not resolved as such.   At 1 mm exit pupil the Airy disc is small, but discernable as such.  So to realize the full resolution of any telescope a maximum exit pupil of 1 mm or 25X/inch is required.  Smaller exit pupils will make the Airy disc more prominent.  

 

For double stars, an exit pupil of 0.5 mm or less is OK because you are simply trying to see the separation between to very high contrast objects.  For low contrast planetary detail, small exit pupils, < ~0.5 mm also mean dimmer images, making it more difficult to distinguish between low contrast features.



#213 starcanoe

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Posted 29 March 2018 - 07:08 PM

Actually, the resolution of the telescope is reached at about a 1 mm exit pupil, because the effective aperture of the eye for most people doesn't equal its actual aperture until its actual aperture is 1 mm.  Most person's irises may not close to 1 mm, but at the telescope the exit pupil is the  eye's aperture.  For some people, especially the young, the effective aperture may equal the actual aperture at somewhat more than 1 mm.

 

Try to detect the Airy disc of a 2nd magnitude star (no flare) at a 2 mm exit pupil.  I think you will not see the Airy disc as a disc, but as a pinpoint.  In other words, the Airy disc is not resolved as such.   At 1 mm exit pupil the Airy disc is small, but discernable as such.  So to realize the full resolution of any telescope a maximum exit pupil of 1 mm or 25X/inch is required.  Smaller exit pupils will make the Airy disc more prominent.  

 

For double stars, an exit pupil of 0.5 mm or less is OK because you are simply trying to see the separation between to very high contrast objects.  For low contrast planetary detail, small exit pupils, < ~0.5 mm also mean dimmer images, making it more difficult to distinguish between low contrast features.

 

 

Thanks for the input.

 

I am just working with round numbers...and I don't disagree with yours.....the point being in this thread (regarding my point specifically ) somewhere around 1mm exit pupil plus or minus a factor of X is where you can see what the scope can deliver....

 

Now maybe somebody with eagle eyes only needs 2 mm and somebody with...well...pigeon eyes :)....needs 0.5 mm....that is both true and probably needs yet another thread :)



#214 Peter Besenbruch

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Posted 29 March 2018 - 07:53 PM

But I guess Peter continues to stand by his stance that all these wonderful scopes were/are futile exercises...

How do you measure success? Sales? Optical performance for the time? How do you measure the willful misreading of what I said? I have never said the long focus instruments were bad. I have said they offer too little to offset their disadvantages. If you are referring to triplets prior to the arrival of modern glass, there was more of a reason for "going long" back then.



#215 Peter Besenbruch

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Posted 29 March 2018 - 09:05 PM

But that's two more glass elements....one more glass to glass interface....and TWO whole more air to glass interfaces....not to mention the mechanical slop smile.gif

Actually, I have done a lot of viewing in the last year or two with an f15 scope. It's a Maksutov, not a refractor, but it gives a good sense of where the eyepieces fall. So, I have a 56mm for my OIII filter looking. I use a 40mm wide angle for wide field viewing, if "wide field" is just under a degree. Shorter f-ratios do allow for a wider field of view. For planetary viewing with my 120mm, f7.5, I use eyepiece focal lengths of 40mm (3°) down to 3.2mm (280x and the eyepiece has 5 elements in three groups), and they are all useful. Yes, it's fun to push higher, but I'll stick within the range of the useful. That kind of an eyepiece range is due do better glass, and moderate f-ratios.

 

7-8mm gives a good high power view in the Maksutov, though I have gone crazy and used 5mm on a number of occasions. So, what do I use for a 7mm? Is it my Meade RG ortho, my 7mm BST 58°, or a Barlowed 14mm ES 62°? That's right, it's the Barlow combo. How is it better? More comfortable, better contrast, more glass (8 elements in 5 groups).

 

This current debate is IMO about same aperture....vs various f ratios.

There is a reason that AP didn't start with maximum aperture minimum f ratio refractors...I wonder what that reason was...

Two things: I reject your definition of what the argument is. Shorter f-ratios allow the use of larger scopes on the same mount. That is a major advantage. Second, as others have said, Astrophysics gave up on the "Super Planetary" with the advent of ED glass. It simply wasn't needed.


Edited by Peter Besenbruch, 29 March 2018 - 09:29 PM.


#216 Redbetter

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Posted 30 March 2018 - 12:31 AM

So, you want one scope to do it all?

 

How about a something like a 6 inch f12 apo with a gazillionth wave optics and a color correction that makes a reflector go "dammm".

 

Couple that with say a 4 or 5 inch semi apo (or heck an achro at this level) with 1 wave optics (because at LOW powers none of this stuff makes a difference) for only the low power views.

 

There you go.

 

That might work for you, but I don't need it.  It doesn't address anything I mentioned above or in other posts.  The field is still narrower than it needs to be for a scope that will be limited to ~0.5 mm exit pupil for planetary, so what is the point?  I can already put on an off axis mask and have an unobstructed 8" f/12.5, so I don't feel like I am missing anything.  Thing of it is, I prefer the planetary detail I get at full aperture.  And then there are the DSO's, galaxies out to 16.5 v mag, etc.  The 20" is already making the trip to the dark sites, and one of the ED's sometimes tags along for wide field views or to see what a smaller aperture is capable of in dark skies on specific objects.  The narrowest of the ones I have has well over twice the field of view of a 6" f/12.  So I don't see the point of trying to carry around a long tube and bulky mount...well...just to carry around a long tube and bulky mount.

 

Plus at 6" the effective exit pupil limit for me for planetary is going to be somewhere near 0.5mm, around 300x.  The scope might be happy with more, but my eye is not happy with the smaller pupil.   In 10" or above things are just really starting to get rolling when the seeing permits 350x, and that is still a comfortable exit pupil of ~0.7mm.   The 20" is still idling at 350 to 500x if the seeing permits, and there is no concern about exit pupil limitations.

 

The great thing about the ED glass is that it allows shorter ratios, more convenient tubes, and wider fields of view while still being able to perform properly at 50x/inch.  So for a versatile scope at 6" I would want a ratio more toward the f/8 end of the scale...if in the market for one. 

 

As I have said several times recently, the one niche I see for a longer ratio apo with 6" aperture or so would be as a home observatory/backyard scope if one lived in an area of good seeing (and it would be nice if it was dark/semi-dark as well.)  That would eliminate much of the hassle factor and allow it to perform to its limits at high frequency.   Of course, that still is not a single refractor solution...which is why something with a shorter ratio and somewhat smaller aperture would still be necessary for what I like to do.  And it still doesn't address the fact that a larger Dob can still do the planetary better, as well as many other things the smaller aperture frac can't do.  But having a nice planetary refractor at home ready to go on an equatorial mount in good seeing would be valuable...if I had the seeing in the backyard to justify it. 



#217 Max Lattanzi

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Posted 10 April 2018 - 06:19 AM

This is classic hand waving, much like your reductio ad absurdum drivel.

I am sorry mate, but what you call "hand waving", in any class, from Primary up to University is considered plain "reasoning".
Just to name things appropriately.

 

Or maybe you also call it a "sleight of hand" when you do not follow a Chess strategy...?!

 

If someone is doing hand-waving here, it is certainly not your humble servant -- who has been trying to do exactly the opposite, through clear and open reasoning on factual data, not by simply re-stating assertions (like someone else keeps doing).

 

Do you remember the sharing of knowledge mentioned above...?!

Ponder, please.


Edited by Max Lattanzi, 10 April 2018 - 07:37 AM.


#218 Max Lattanzi

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Posted 10 April 2018 - 06:26 AM

I then proceeded to argue that glass like OK4, FPL53, 53, and 55 allow improved levels of correction at shorter f-ratios. Therefore these elements constitute better glass than FPL51. I have looked through all of the rest of your responses and that was basically the only meaningful point related to glass that was raised, and you missed it completely.

 

IF well-mated -- pls note the *if*--, YES, glasses like OK4, FPL53, FPL55, etc. do allow an improved *chromatic* correction (NOT correction of all the other aberrations) at shorter f/ratios than FPL51.

Pls also note I am here assuming similar quality grade of the glass (read: better go with a Grade A FPL51, than a lower grade FPL53 -- believe me...).

 

NOP, it is not "better" per se: it is better depending on the use.

I thought I made it clear to the OP almost a month ago (and several pages back) when we were solely discussing of FPL53/FPL51 glasses.

 

It is certainly better for photography, where you look for fast lenses.
Not better for, say, hi-res observing where you look for a slow optics that would allow a better correction of all the aberrations (not just the CA).

 

I spare you the nuisance to go back and check that post. Here it is:

 

<<Given you have a *proper* mating element (there is a short list to choose from) AND you increase the f/ratio of about 20% (roughly; depending on the mating), you have indistinguishable results, chromatically speaking. Being it a doublet or a triplet.

Of course, if your aim is photography, all things being equal, a FPL53 based objective (IF the FPL53 blank is a good/excellent one, and IF it is properly mated and executed), will be always preferable for it allows a faster f/ratio.

So, yes, in this case one may say that "FPL53 is better than FPL51". Your CCDs will indeed prefer slightly faster f/ratio better (=faster exposure times, more exposures, better s/n ratio, etc etc).

If your aim is instead visual (I mean hi-res hi-power observing; for low-power deep-sky observing everything is fine), increasing the f/ratio of about 20% (still with a good/excellent FPL51 blank, properly mated and executed) will give you a better instrument than its FPL53-based *shorter* counterpart.

So, in this case, one may instead say that "FPL51 is better than FPL53". Your visual train (eyepieces, etc. AND your eyes) will indeed like slightly slower f/ratio better.

As a rule of thumb, all things being equal, a 4" (say) FPL53-based triplet @f/6 will be *chromatically* as well corrected as a  FPL51-based triplet @ (roughly) f/7.5, which will be as well corrected as a FPL53-based doublet @ f/9, and/or a FPL51-based doublet at (roughly) f/11.

Chromatically, as said. All the other aberrations are reduced as much as you go longer.

So, if you do not need the fast f/ ratio for photography (or for ease of transportation) the humble FPL-51 f/11 doublet will be, surprisingly enough, the best of the four.
I have indeed some medium-focus FPL51-based refractors which are as white as pure water and perform close to optical perfection.
But are not good for deep-sky photography.

If instead you need a fast scope for photography, the FPL53-based triplet will be the best of the bunch.

As you may see, being "better" depends on what you need.
I hope the above helps in clarifying the actual difference -- if and how much that is significant or not, it's up to you.>>

 

 

It doesn't look like I missed the point...



#219 Max Lattanzi

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Posted 10 April 2018 - 06:48 AM

I will finish by restating what I have written above:

 

1) A medium focal length, ED refractor has considerable advantages over a long focal length one. It is possible to either use a lighter mount, or a larger size on a given mount. The larger size will show more. (...)

 

Peter, you are of course free to verbatim re-state your thoughts as much as you like.
The restating still does not transform them in universal binding Commandments.

 

Indeed, what you say it sounds true *to you* only for *you* have set *your* limit in *your own mind*.
Of course, being it your own limit, it is very true for you.

 

You see, I have a friend who absolutely hates standing at the telescope. Understandably enough: being a planetary and double-star observer his sessions at the eyepiece are several hours long.
Now, for him a Newton is a no-telescope, for he has to stand (or, anyhow, uncomfortably sit/stand on a ladder) to observe.

To him, say, a 20" Zambuto-powered Newton is worth nothing vis-à-vis, say, a 4-5" refractor or a C-8 or anything similar...

 

Would you agree? I guess not. For what it matters, I certainly would not.
BUT for him *this* is the truth. The Truth (capital T).

 

For him it is not a matter of price, or size, or weight. His key criterion is "I have to be comfortable when I observe". Full stop.
He is comfortable behind his refractors and the C8. Not so with the Newts (any brand, any size).

 

Is this a limit in the optics? Is the Newton a bad telescope? Not at all, of course. Rather the opposite.
But *for him* the Commandment is: "No Newton is a good telescope". Period.

 

Would you argue with this? Optically speaking of course yes.
But, as per his own comfortableness and choice, you simply cannot. Exactly for it is *his own* comfortableness and choice.

 

Given for granted his own legitimate preferences, what remains disconcerting is of course the capital T. Indeed.
Like every-time a capital letter is added to justify and reinforce what is nothing more than a (legittimate) personal preference.

 

 

Getting back to us (to you, actually), I wonder if you realise that you set a limit as well, i.e. your actual present mount (or the one you are ready to buy and/or you are comfortable with).
But this Limit (capital L) is not valid *per se*. It is just a limit you set for yourself.

 

Of course, this limit of yours could be shared by many others.

But for many others -- to me, for instance (and for many other like me) -- it simply does not apply

 

 

Mind you, it does not mean I do not have my own limit -- I do, of course (like anyone else).
Though it is not a Commandment, rather a mere practical usage limit.

 

In practice, when I am alone, my limit is trivially "What I can carry and mount all by myself".
And, according to my own limit, your statement is once more false.

Let me show you why.

 

As already mentioned above, this is my portable setup, i.e., this is the larger refractor I can mount all by myself: a TEC 200 f/9 on a Titan mount.

 

MAX_3566.1280.LQ.jpg

 

Actually this is an old picture I have at hand: now tripod and telescope are the same, but I've shifted first to an AP1200 (larger worm wheels both in RA and Dec) and then to an old HGM-200 (larger Dec worm wheel than AP1200).

Of course I could further improve the situation with an AP1600 or anything similar, but the HGM-200 has *to me* some advantages (that is of no utility to mention here); so, at present, I am fine with that.

Tomorrow, who knows.

 

My aim remains to improve the quality of my observing, which means the mount HAS to be transparent and let me observe at 600-800x with minimal (possibly zero) vibrations.

Thus I do not want to save weight on the mount, rather I wish to have it as big as possible -- of course within the limit of my own capabilities.

As per my own capabilities, for instance, an AP 3600 is out of the question -- out of *MY* question -- as per the portable setup I can load on the 4WD, bring with me at remote locations, mount and then unmount.

 

You might have noticed that, in all this mumbling, I am NOT talking at all about the telescope. That was clear already: the largest availableI can mount all by myself.

 

Now, in accordance with your statement, you would suggest that, to improve the stability even further I could shift to a shorter-focus "better-glass" instrument.
But, nop, that won't work. And I tell you why.

 

Yes, I could get myself even, say, a 200 f/7 (FPL53, CaF2, OK4, Unobtanium... whatever). Yes it's shorter an lighter. Maybe I'd save 5-6 kgs.
Who cares? I don't.

That is a better photographic instrument but visually it would be worst, with a lower level of correction of all possible aberrations, from CA to spherochromatism.
After I drive for hours and unload and mount the setup you have seen, sorry, I do not want to set for a lower level of correction, just to save a few kgs (that I know I can handle in any case).

 

Or, I could get a FPL55 or CaF2 f/8 version which could be just barely shorter, save me some 3-4 kgs, and would slightly improve the final correction of about 7% below 480 nanometers...
Have been there: when you also sum up the different behaviour of the whole optical system when getting down from f/9 to f/8, the gap is even reduced.
The difference in weight and torque is insignificant (both to me and to that mount).
Visually, the difference in the whites is like having # FFFFFF instead of # FFFFFD (using Hex Codes). Let's be serious...
And this is between CaF2 and a "lesser" (but extremely high quality) FPL51-flavoured objective.

 

Now, if instead of being f/9, that instrument would be f/11, there yes I would be ready to shift to a 20% longer instrument, which would be certainly a bit heavier (but still within my own capabilities), still properly manageable for my mounts, and have significant (to me) improvement on the final correction (i.e. not 5-7% but rather a more substantive 20-22%, which is 3-4 times more!). In that case, yes, *I* would be ready to accept the added hassle for the benefit.  But unfortunately, something like that is today not available and has to be custom-ordered (which can be done, of course, if top manufacturers would not be so busy in producing photographic instruments).

 

You see? The criterion I apply here is personal and clearly stated as such.

And, incidentally, it si exactly opposite to yours. Not to mention, mathematically and physically explained in details.

 

 

And if you suggest, instead a 9" f/7 -- i.e., in accordance with your statement, "shorter but larger" -- that has two huge limitations in this case that would make it not viable:

A. it cannot be done at a decent level of correction (i.e., in that diameter, short f/ ratio instruments have so many residual aberrations that pose actual manufacturing limits);
B. it would be at least 15 kgs heavier -- THAT I know I can still move around, but certainly CANNOT mount and unmount late at night all by myself (not just "comfortably", but "in safety").

 

So, you see, all this to plainly show you that your supposedly universal statement (that you keep restating restating restating) is not so universal in the end, but it varies according to the preferences/choices of each of us.

To me, for instance, does not apply for the reasons above.
Therefore it cannot be generalized.

 

It is as simple as it sounds.

And it requires no repeated statements using capital T, C, L or other letters to be explained.

Just plain reasoning.



#220 Max Lattanzi

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Posted 10 April 2018 - 06:55 AM

At the same size, a well executed medium focal ratio refractor will in practice show the same detail.

 

Hereunder a quote from the late Tom Back in an interview he gave back in 2006.

The quote also applies to your previous (re-re-re-restated) statement <<A medium focal length, ED refractor has considerable advantages over a long focal length one>.

 

 

Q: Regarding the new TMB 130-f/9 -- Is the improved color correction in the TMB 130-f/9 due to going from f/6 to f/9, or is a new glass and optical design combination being used?

 

A: It's an LZOS built SD triplet like the 130-f6, 100-f8, 152-f7.9, 175-f8 etc.

   Moving to f/9 from f/6 improves things several ways.
  

   1. The size of the color error around the airy disc shrinks, making it less noticeable.
   2. The linear size of the airy disc at the focal plane increases, making any surrounding color error less noticeable.
   3. Spherical aberration is better corrected across a wider range of wavelengths (spherochromatism).
   4. Eyepieces have an easier time with an f/9 light cone than f/6.
   5. Lower fifth order spherical aberration.
   6. Flatter field.
   7. Greater depth of focus.
   8. Higher average Strehl Ratio.
   9. Smoother micro-ripple from less figuring.

 

Q: Can you comment on selecting an f/6 versus f/9 of the same aperture?

 

A: (...) If most people had the choice, they will always pick the f/6 or fast lens over an f/9 or slow lens.
Imaging wide field objects, lowest power viewing, weight, length, and mount considerations favor the f/6 versions.
But there is no way to match the wavefront quality of the f/9 lens, with all things being equal, vs. the f/6 lens.
Adding a Barlow will not lower the mono or polychromatic aberrations.
It does improve eyepiece performance however, especially off-axis, as the eyepiece sees a f/10.8 light cone, even though the damage has been done in terms of spherochromatism, average Strehl ratio, and depth of focus.
(...)
What about the dedicated planetary observer? If you do a major part of your observing viewing the planets, the Moon, and double stars, the choice should be the f/9 lens, as long as the considerations of the mount and portability do not interfere with your choice.>>

 

Verbatim from the (sadly) late Tom Back. My own underlines.

 

I hope this -- once more (since I already tried hard to do my part as well) -- replies to your several-times-verbatim-restated assertions.

 

 

Incidentally, the thread was kindly re-opened by the Moderator to allow me to share this info.



#221 Max Lattanzi

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Posted 10 April 2018 - 07:01 AM

2) There is no practical difference between a well executed FPL53/Fluorite/OK4 lens. No, I don't buy the secret formula makes LZOS better argument. Neither do I accept that there are magic beans.

 

As above, same interview Tom Back gave in 2006:

 

 

Q: What glass types are used in TMB APOs?

 

A: (...) I looked at every available glass in the Russian Mashpriborintorg glass catalog and ran the best possible three glass combinations through the computer. Being that the design is air spaced, it gave addition degrees of freedom in the design to correct for coma, spherical, zonal spherical and spherochromatism.

 

The best SD glass (super low dispersion glass) was OK-4, which has an Abbe dispersion of 92+. But this alone does not assure that a lens will be color free. To achieve this goal, the partial dispersions of the SD element must closely be reversed by the partial dispersions of the two mating elements (Pa = Pb). Also, the two mating elements must not generate high internal powers (steep curves), as this increases the mono and polychromatic aberrations. This is where glass choice becomes important.
(...)
I found, using the glasses that are available from Russia and LZOS (LZOS is the source for high quality glass in Russia), the two other glass types that match with OK-4, for the best corrected apochromat, using LZOS' glass.
(...)
The results even surprised me, as the one glass (the last element) was a glass that is not available in the Ohara or Schott catalogs, unless you specify a very expensive custom melt. This glass matches up in the triplet so the violet dispersion is under superb control, as I'm sure anyone that owns a TMB apochromat can testify to.

I cannot give out the glass types, as they are proprietary, but I tell you this: The front element is crown glass, the center element OK-4 SD glass and the last element is a special crown element. The Zeiss APQ uses K-11 crown as the last element, and the Takahashi FCT uses K-3 crown as the last element. In all cases, these glasses are not used like normal crown glass, but as crown glass that acts more like flint glass.

 

 

I hope this makes sense to you.
As you may see, there are no magic beans. Just glass types and formulas you are not aware of.

 

I may just add that I have the whole optical prescription (i.e. all the three glass-types and their detailed spec sheets), and when you run them into the software they do exactly as Tom explained.
When translated into actual lenses, as well.

 

Hope this is helpful.



#222 Max Lattanzi

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Posted 10 April 2018 - 07:05 AM

As Richard already pointed out in his follow-up post to the above statement, Peter's claim above is of course wrong, as any student of ABC optics knows.

 

As an example, have a look at the spot diagrams and longitudinal aberration diagrams of the LZOS 130mm F/6  vs  LZOS 130mm F/9 triplets.

 

ALL observers who have compared those two scopes report that the F/9 version shows the cleaner image, easily seen in practice.

 

 

So you think Buchroeder, Back, Lichtenknecker, and other renowned opticians & observers, were mistaken in designing long-focus APOs (130mm F/12 APOMAX, 130mm F/9.2 LZOS, 110mm F/15 triplet, etc.) ?

 

BTW, there's a thread going on at the moment in the Cloudynights ATM forum about a german amateur building a 130mm F/15 APO. The lens is an immersion triplet, as invented by Wolfgang Busch in Ahrensburg about a decade before Roland Christen re-invented the same idea independently of Wolfgang:

 

https://www.cloudyni...e/#entry8486234

 

But I guess Peter continues to stand by his stance that all these wonderful scopes were/are futile exercises...

 

 

Quite right...!



#223 Max Lattanzi

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Posted 10 April 2018 - 07:11 AM

That said, ,the best 130 performer is probably the TOA130 wich has not any spherochromatism and still a short enough focal ratio.

 

Well... the TOA-130 is certainly an excellent telescope and, short of the extraordinary FSQ-130ED, probably the best out-of-the-box photographic 130 APO currently in production.

 

From here to say that "has not any spherochromatism" -- if that were true it would be either a mirror or a *much* slower APO -- the step is a long one...
As well as to imply that it cannot be further improved (or that is the best 130 out there).

 

Why do you think Takahashi came out with a specific Extender TOA for planetary view to make it f/12.3...?!
Have you seen the difference at the eyepiece?

 

And, given that difference, don't you think that, if the TOA-130 were a *native* f/13 instrument, it would have been even better?
Make the TOA130 a native f/13 and at that point you shall truly challenge in spherochromatism any mirror out there and have visible performances close to theoretical, which cannot be further improved.

 

Stuff to ponder about!

Cheers!



#224 Max Lattanzi

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Posted 10 April 2018 - 07:26 AM

A lot of this thread reminds me of debates in audiophile forums (...)

 

 

The above audiophile-flavoured quotes from Tom Back are also for you, my dear.

 

For the rest, it is plain obvious that you repeatedly try to set yourself as the benchmark for Science:

 

A.      When you are presented with empirical evidences which differ from yours, those become "subjective anecdotal experiences" (not yours, of course -- yours are Objective Truth, it goes without saying);
B.      When you are presented with figures, graph and well grounded theories (well grounded for are those producing actual telescopes, not mere arm-chair bla bla bla), then you wonder whether those hard data produce any difference actually "perceptible by human vision";
C=A. When you are presented with empirical evidences by very many -- but you -- that those differences are indeed (clearly) perceptible, then those becomes again merely "subjective anecdotal experiences" for they differ from yours...
(...)

 

I wonder whether you grasp the sterile circularity of such a (pseudo)logical short-circuit, which is void of any actual potentiality for cognition and growth of knowledge (not even a heuristic one).

And you call this Scientific Method...?!

 

So, according to you, Roland Christen was enlightened by Science when designed your beloved 130GTX f/6.3; but then he got suddenly blinded by "pure dogma" when he designed the 130 f/8.3...?!
According to you, Tom Back was enlightened by Science when designed the 130 f/6; but then he got suddenly blinded by "pure dogma" when he designed the 130 f/9.25...?!
According to you, Pal Gyulai was enlightened by Science when designed the 105 f/6 and f/7; but then he got suddenly blinded by "pure dogma" when he designed the 105 f/8.7 and f/10...?!
According to you, Karnapp and Prudenz were enlightened by Science when they designed the 100APQ f/6.3; but then they got suddenly blinded by "pure dogma" when they designed the 100APQ f/10?
(...I can go on but I guess the point is clear...)

 

Again, you call this a Scientific approach? You call this Science...?!?!

 

Mate, if this is your idea of Science, I hope you won't mind if I'd go on having my scientific instruments made by the blinded-by-dogma Gentlemen above.

 

And not by you.



#225 Max Lattanzi

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Posted 10 April 2018 - 07:36 AM

When Astro-Physics started , Roland was not the skilled optician and designer he became . The scopes were designed using short flint glasses, ED glasses were not available other than Fluorite.  The goal was affordable quality. 

 

Thomas Back's  A Brief History of Astro-Physics lens discusses all this .. 

 

http://www.csun.edu/...n/tmb/tmb1.html

 

I always like this section:

 

"Then the new era began. It was AstroFest 1990, and Roland brought his ultimate line of ED EDT triplets. I had my 6"f/12 setup next to the new 6.1" f/9 EDT, and at first glance cast on Saturn, I knew it was all over. This scope was a knockout, and in color correction and contrast, clearly beat my 6" f/12 Super Planetary. This was a prototype, was airspaced and not even coated."

 

Jon Isaacs

 

Jon, actually Tom's beautiful story does not mention (it is implicit, due to the dates, but then overlooked by readers) that his own Superplanetary was an early one -- there were indeed many iterations of such a telescope that Roland used as a sort of lab.

 

I had the luck (and pleasure) to use for long years what apparently is the last 152 f/12 Superplanetary produced (1993-4) -- Tom's quote does not apply there.

 

That specimen was tested at length against two 155EDF, a 152EDT, and another early 152SP (and many other telescopes) being clearly better than all the above.

It took an AP180EDT (refigured specifically for me at an extremely high wavefront, and FPL53-doped) stopped down at 6" f/10.7 to see a slight improvement on that stock pre-interferometer last 152 SP f/12.  Chapeau...!

 

 

So, I would be a bit cautious in generalizing on the base of an old story, without contextualizing the dates.

 

This being said, Tom's paper is truly enjoyable indeed, although sometimes not fully accurate.
Simply because his actual knowledge of internal AP cuisine was limited (for obvious reasons).
Roland mentioned this in more than one occasion.

 

Hope it helps. Cheers!


Edited by Max Lattanzi, 10 April 2018 - 07:44 AM.



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