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Apos better than Newts?

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#351 timps

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Posted 21 July 2015 - 06:31 PM

There are so many mass produced "cheap" newtonians on the market now, I am sure that many people haven't even seen a good quality "long" newt.

I have one of those mass produced large aperture (14") newtonians and I have posted on here before, when comparing planetary views, how my little (4") apo is superior. How can a 4" apo show more planetary detail than a newtonian with 10" more aperture? Yes the newt is assembled correctly, yes it is collimated properly. yes the mirror is clean...and so on and so on. I have heard it all.

The simple fact is that my apo is a premium instrument. My newtonian (in quality terms) is not much more than a toy. One has to compare apples with apples. If the mirrors on my newt were manufactured to the same quality as the lens on my apo, the performance of the newt would be far superior, especially if it were practical to have the same focal length.

There seems to be more high quality affordable apos on the market than high quality newtonians. 


 

#352 charles genovese

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Posted 30 July 2015 - 11:25 PM

Ranger- I have a 10" f/4.5 Newt and a 10" f/6 newt both with perfect optics and yes I have a 5" APO and the Newts eat the APO. But wait- I also have an Orion 8" f/4.9 and 10"f/4.7- the optics are almost as good and yes they will eat your 4" APO and I paid about $250 for the 8 and $300 for the 10. Not sure where you got your 14 but if the problem is you mirror (and not thermal issues or mounting issues plus your local observing conditions) it can be refigured and recoated for probably 1/5th (or less) the cost of the APO and give spectacular views. (for local observing conditions consider, for instance, if you are observing on concrete the APO will have an advantage being off the ground- but if you wet down the concrete with water near sunset there will be a huge thermal difference)


Edited by charles genovese, 30 July 2015 - 11:28 PM.

 

#353 precaud

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Posted 31 July 2015 - 05:23 AM

Nature has been wetting down the concrete for me every afternoon/evening, but it hasn't helped  :(


 

#354 ed_turco

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Posted 31 July 2015 - 05:38 PM

As I've said before, my Definitive Newtonian scales up marvelously to a 12'' or a 16" f/6, even without the Barlow lens, with central obstructions going down to 13% or so, simply due to the geometry of larger Newtonians.   For those who objected to such DNR f/ratios causing observers to need a stepladder, I pose this question:   What kind of observatory would you need to house that 12" or 16"  APO? 

 

And one can only dream of having such an APO  this side of Heaven.

 

But best of all, the good news is that an ATM can actually build one of these things with his own hands.  And that is really something!

 

On a related note, there used to be a lot of that battle going on between APO and Newtonian owners.  At times, it became rather rancorous.  I know this because I visited the Refractor Forum over three years ago.  When I mentioned some of my bright ideas, I got rather beat up in the process by folks who had the money but not the knowledge it would really take to bring me down.

 

Instead, I was subjected to "belief" and "common knowledge" that in any scientific discussion becomes awfully close to superstition.  And three years later, it still does.

 

So I wrote my article.  I made very sure of my documentation; the article had rigor as is said in scientific circles and I built the telescope too.  And do you know what?  Not much of anything out of those APO owners at all.   Not a single objection from manufacturers so as to expose all the lies I might be telling.  No one promised to write an article to refute my contentions, with rigor or without.  Finally, I changed my signature, (Read below and see!)  and started contributing to the Refractor Forum, but never openly saying a word about what I did, or picking a fight with anyone.  In fact, my postings were genuinely appreciated.  Even by APO owners.

 

The idea of the debate raging on after all this will only happen when my article (and me) gets buried by time; however, some new wiseguy will come on the scene, cite the same or similar sources and put this baby to bed, hopefully for an even longer time.

 

In the meantime, I think I done good.

 

 

ed

After all this time, over 2 months, I've decided to demonstrate my claim that a DNR does scale up nicely.   I have a 12" f/6.25 in my garage, just a plain vanilla Newt with no bells, whistles or otherwise.

 

I'm asking my friends to lend a hand on this project as I am even more increasingly disabled.  I envision making an optical window 14" in diameter, 1/2" in thickness.  The optical glass I need will set me back $300 or so, hardly a major setback even if I am on Social Security!

 

Then, I intend to use that Barlow I used for my 6".  Please note that even without the Barlow, due to the geometry involved in setting the size of the diagonal in a larger instrument, I can get by with a 2" diagonal, for a central obstruction of 16.7%, yielding a fully illuminated field of .75''.  With the barlow, the diagonal size goes down to 1.6" for a CO of 13.3% and the fully illuminated field becomes 1".  How this happens, I explained in my article.  This has as good numbers as my 6" DNR or better with an aperture of 12 inches.  Since the volume of my tube is so much larger, I envision using four of those fans  behind my mirror cell to "clear that foul air."

 

Some sticklers will notice that my central obstruction numbers don't seem to tally;  it just happens that my 12-incher is really 12" and not the customary 12.5."   I would imagine that this instrument would cost in the $1000 range, but not that much more.  And if the mount is held to an equatorial design, this 12" becomes an observatory class telescope. 

 

I wonder what a 12" apo would cost and whether anyone could afford it.

 

The numbers, folks, are even better for a 16" !   And it takes a 3 step step-stool to handle those pesky high altitude positions of the eyepiece that I hear complaints about.  Frankly,  I cannot envision a DNR larger that this size, as tempting as that might be.

 

 

 

ed

 

 

P.S  I'm writing this in an "edit"; the following day.  I've been kidded via email by some friends that my age is showing.  I guess they are right.  I mentioned an equatorial mount for these DNRs without thinking of the Dobsonian mounts with computers and stepper motors that follow the stars.  I've also mentioned Poncet-style mountings in some of my statements and I've been reminded that they are considered to be "old hat." 

 

Poor me.  :gramps:     Does anyone think I'm through?   :ohmy:    Don't count on it!     Just wait until I'm 90!!  :scared:   Oh yes, my doc tells me I'm gonna die if I keep up smoking my pipe.   :sleuth:    If I stop smoking, I don't die, right?


Edited by ed_turco, 01 August 2015 - 03:33 PM.

 

#355 gnowellsct

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Posted 13 August 2015 - 11:15 AM

There are so many mass produced "cheap" newtonians on the market now, I am sure that many people haven't even seen a good quality "long" newt.

I have one of those mass produced large aperture (14") newtonians and I have posted on here before, when comparing planetary views, how my little (4") apo is superior. How can a 4" apo show more planetary detail than a newtonian with 10" more aperture? Yes the newt is assembled correctly, yes it is collimated properly. yes the mirror is clean...and so on and so on. I have heard it all.

The simple fact is that my apo is a premium instrument. My newtonian (in quality terms) is not much more than a toy. One has to compare apples with apples. If the mirrors on my newt were manufactured to the same quality as the lens on my apo, the performance of the newt would be far superior, especially if it were practical to have the same focal length.

There seems to be more high quality affordable apos on the market than high quality newtonians. 

 

It sounds like this instrument isn't even diffraction limited.  The mirror should be sent out for evaluation.  Not for nothing do some outfits make money regrinding Chinese optics.   GN


 

#356 ed_turco

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Posted 14 August 2015 - 05:36 PM

 

There are so many mass produced "cheap" newtonians on the market now, I am sure that many people haven't even seen a good quality "long" newt.

I have one of those mass produced large aperture (14") newtonians and I have posted on here before, when comparing planetary views, how my little (4") apo is superior. How can a 4" apo show more planetary detail than a newtonian with 10" more aperture? Yes the newt is assembled correctly, yes it is collimated properly. yes the mirror is clean...and so on and so on. I have heard it all.

The simple fact is that my apo is a premium instrument. My newtonian (in quality terms) is not much more than a toy. One has to compare apples with apples. If the mirrors on my newt were manufactured to the same quality as the lens on my apo, the performance of the newt would be far superior, especially if it were practical to have the same focal length.

There seems to be more high quality affordable apos on the market than high quality newtonians. 

 

It sounds like this instrument isn't even diffraction limited.  The mirror should be sent out for evaluation.  Not for nothing do some outfits make money regrinding Chinese optics.   GN

 

When I started this whole affair, I knew that I would try to build a Newt of stunning accuracy, along with presenting my research.  My DNR if mass produced to my accuracy would cost twice what it would cost to get a lousy one.  Using actual numbers from an apo owner, a 4" DNR might run $500 while his apo ran him $4000.

 

Point! And game!

 

 

ed


 

#357 BillP

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Posted 26 August 2015 - 06:54 PM

Using actual numbers from an apo owner, a 4" DNR might run $500 while his apo ran him $4000.

 

Point! And game!

 

Not quite.  $4000 for a 4" Apo?!  My TSA does not even cost that much!  Heck, you can get a wonderful performing Sky-Watcher ProED 100mm f/9 Doublet APO for $750.  You keep the Newt, I'll take the Apo, now we are both happy :lol:


 

#358 Scanning4Comets

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Posted 28 August 2015 - 04:55 AM

 

 

Apos?  Why bother?  

 

 

It's status, plain and simple!  Show up at a star party with an AP or Takahashi, especially of 5 inches or more, and people will line up to see M13 looking badly because the telescope is not big enough to show the stars in globulars well.  Two kinds of people think money can beat physics.  People getting face lifts and some backyard astronomers.

 

Those apos are very nice scopes.  I love them and have one.  But there is something perverse going on with peoples' desires for them.....

 

Glen

 

Star parties? Oh...I guess it's all about "status"....

 

:roflmao:


 

#359 Ian Robinson

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Posted 20 September 2015 - 12:34 AM

IMO size of light collector with a good figure is always going to be better.

 

APOs are nice, but really not much bang for the $.


 

#360 Asbytec

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Posted 20 September 2015 - 09:09 AM

Ed, I offer my heartfelt gratitude for building the DNR and the fascinating discussion that spawned from it.

In your article, you wrote one line that I think tells it all,

"In my discussion, I argue that only one additional element, related to aperture determines optical performance – the quality of a stellar diffraction image."


And somewhere above, you said,

"all else follows."


That would include light scatter and such.

I have often argued, the diffraction limit is nothing to sneeze at. By that, I guess we mean something better than the equivalent of 1/4 P-V primary spherical aberration giving an operational peak intensity of a star image of about 0.8 or better, or about 80% of the 84% possible light in the central disc and the resulting intensity distribution.

The true performance of a refractor, outside the appeal of the FOV and the reason for "tight" star images, is they tend to operate at their Strehl ratio, be it polychrmoatic or otherwise. It seems the true strength of optical performance of a Newtonian is it's closely parabolic primary. This is the perfect form for focusing light rays from optical infinity. The bane of the Newtonian is the obstruction, and of the APO refractor some residual higher order SA and a very slightly caustic focus.

Both designs, including your DNR, should have a very high Strehl ratio. Let's say they are equal, both at around 0.98. The 13.7% obstruction still imparts some additional diffraction and light into the rings thus reducing the operational peak intensity of the optical train. Without doing the math, the peak intensity is not going to be 0.98 as the APO is. Not anymore. It'll be something less, say around mid 0.90s thereabouts. What this boils down to is not so much the performance of the DNR being as good as a premium APO, but likely observationally undetectable in the field. In otherwords the difficulty discerning between 0.98 and mid 0.90 peak intensities.

This seems to be in line with the school of thought that an ~20% or less obstructed aperture is very hard to distinguish. That is, thanks to both your skill with pushing glass, scaling the obstruction (adding the Barlow was a neat idea), and tackling other gremlins, to my mind, you succeeded - with equal aperture where many of us cannot.

No doubt an operating peak intensity (distribution) of about 0.90, or better, is amazing, especially with a smooth and highly corrected primary mirror. Still, how do those images compare to ones that are at least diffraction limited at 0.80 peak intensity on the focal plane?

My own 6" scope is only about 1/6th P-V of primary spherical (pretty typical of advertised stats and consistent with the star test.) And I have seen some amazing things in mild tropical climates (ambient cooling) with laminar air mass (great seeing) and nearly perfectly collimated thanks to the excellent seeing. Albedo on Ganymede at 70x per inch, soft contrast detail on Jupiter and Mars (including detail in the NPC), and a tiny crater form on Plato's dark floor at an angular diameter near the Sparrow limit (~0.72" arc), or about the size of a middling magnitude spurious disc. (How in the heck is that possible?) The tightest star I've split is 7 Tau at about 0.75" arc, conservatively a tad less that Dawes, and have elongated stars below 0.5" arc. I have observed the 'apparent' elongation of Io (when near Europa for reference), and to date have bagged 7 craterlets on Plato's floor. Sirius B has been pretty difficult and mostly elusive, though. Still, got it once (along with about 10 diffraction rings around the primary LOL.) .

How much better can it get? Is a 6" APO gonna beat that or match it? (I mean, sure, no doubt small bright low contrast planetary detail is probably close to a 4" APO, by all accounts, but resolution is right up there with the big boys.) Point being, even in a modest fairly well corrected, smooth optic, the universe drops our jaw. So long as it is at least diffraction limited on the focal plane, I am guessing, in terms of seeing, thermal equilibrium, figure, and obstruction - you're not gonna be disappointed. I have not been.

"I have a 12" f/6.25 in my garage, just a plain vanilla Newt with no bells, whistles or otherwise."


So do most other folks. What I am driving at is, to my knowledge, there is only one DNR in existence. A few of us may, one day, make one. So, how can the rest of us schmucks (and I mean that with endearment, myself included :) ) optimize our Run-of-the-mill Newtonian Reflectors (RNRs) or current scope to obtain refractor like images short of grabbing an additional 2 inches of aperture? To me, since none of us has a DNR, this seems to be part and parcel to the debate above. How can we make our Dob deliver the goods, or do we even have to? I think we should, but it may not even have to be a premium optic.

It may just have to be good enough, say the diffraction limit or better - on the focal plane - which includes the obstruction and all other gremlins. Because, as you said above, it all boils down to the diffraction image. The diffraction image, on the focal plane, simply has to be good enough. Yea?

The answer seems to be, short of grinding an optical window, to do many of the things you've done to include flocking, extended light shielding, tackling thermal gremlins, get the best optic within reach, optimize the obstruction, collimate, and pray for an excellent night of superb seeing. Doing so stands the best chance of getting the diffraction image up to spec for some not so disappointing views.

My avatar is one of my better Jovian observations, colors a little more saturated that real world. That's 1/6th PV primary SA, excellent observing conditions, and patient and determined observation. And a narrow FOV, the last remaining gremlin. :)

Edited by Asbytec, 20 September 2015 - 11:11 PM.

 

#361 ed_turco

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Posted 21 September 2015 - 11:25 AM

Asbytec: "So do most other folks. What I am driving at is, to my knowledge, there is only one DNR in existence. A few of us may, one day, make one. So, how can the rest of us schmucks (and I mean that with endearment, myself included :) ) optimize our Run-of-the-mill Newtonian Reflectors (RNRs) or current scope to obtain refractor like images short of grabbing an additional 2 inches of aperture? To me, since none of us has a DNR, this seems to be part and parcel to the debate above. How can we make our Dob deliver the goods, or do we even have to? I think we should, but it may not even have to be a premium optic."

 

The fact of the matter is this; I have shown you the way to have it all, provided you are willing to learn how to grind and polish.  While it is true that this can be a task the first time around, second mirrors have a propensity to be great improvement over over first attempts.  An optical window is an easy job after that second mirror.   Ask Jean Texereau!

 

True, it costs money and time, but what is a hobby for, but to **** away those very things.  (Asterisks provided by me, not the CN software.)  And with experience, any amateur telescope maker can make his own DNR.   My DNR is the first, but only the first; others will certainly follow.

 

By the way, one doesn't need to worry about the Strehl Radio of the Dark Bucket.  :)

 

 

ed


Edited by ed_turco, 21 September 2015 - 11:35 AM.

 

#362 ed_turco

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Posted 21 September 2015 - 12:24 PM

 

Using actual numbers from an apo owner, a 4" DNR might run $500 while his apo ran him $4000.

 

Point! And game!

 

Not quite.  $4000 for a 4" Apo?!  My TSA does not even cost that much!  Heck, you can get a wonderful performing Sky-Watcher ProED 100mm f/9 Doublet APO for $750.  You keep the Newt, I'll take the Apo, now we are both happy :lol:

 

But does that inexpensive apo have premium optics?  If you want to compare an apo with a DNR, your apo had better come with premium optics, because any equal aperture DNR built according to the suggestions of my article is going to cream your $500 apo.  The reason is this -- just because a telescope is an apo doesn't guarantee that it will be inherently better than anything else on the market.  An apo is subject to the same optical laws as any other telescope. Great telescopes demand great optics; if they do not, you have a cheap apo; that is all that you have.

 

 

 

ed

 

Hint:  Stick with that $4000 apo; you are going to need it.


 

#363 Pinbout

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Posted 21 September 2015 - 01:40 PM

 

But does that inexpensive apo have premium optics?

 

ed doublets can come with nice figures.

 

here's a dbl pass test against a flat.

 

gallery_106859_3508_111106.jpg


 

#364 Asbytec

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Posted 21 September 2015 - 08:09 PM

By the way, one doesn't need to worry about the Strehl Radio of the Dark Bucket.

 

 

That good news, thanks. :p

 

Okay, thanks for replying, Ed. Just wanted to ask the question about the diffraction image. In your thread years ago, I argued along side you that you could do it through tackling gremlins, smallish CO, and good figure.


 

#365 jouster

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Posted 05 October 2015 - 12:29 PM

As i always say  $ for $ you can't beat a super well made Newt with my very steady seeing and the kind of viewing i did at 600 to 1100x on the planets.  It would take a 10" or bigger APO to get the views i had with my 11 to 18" Zambuto and OMI optics.  I would hate to price out what a 10" APO would cost plus the cost of the mount needed to hold it.  

 

Chas

APM will seek you a 254mm apo for €59,900. After that, you'll need a mount. And, unless you're Hercules, you'll need a permanent site.


 

#366 Arief M.

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Posted 16 October 2015 - 12:10 AM

The best planetary telescope is the one you use the most...

 

aJjgbRU.jpg


 

#367 Asbytec

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Posted 16 October 2015 - 07:15 AM

The best planetary telescope is the one you use the most...

 

 

Sure, and the converse is also true. You use the scope the most because it is the best planetary scope. Once I saw what my own scope could do in good seeing and modest ambient, I use it almost nightly during the dry season. It has become my best planetary scope, and it get's used a lot because of it. I would probably use your scope a lot, too. :)


 

#368 KaStern

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Posted 22 October 2015 - 01:26 PM

Hello Norme,

 

 

Both designs, including your DNR, should have a very high Strehl ratio. Let's say they are equal, both at around 0.98.

 

there are only few apochromats that good.

My friend Kurt did test some of them in different wavelength

and calculated the weighted polychromatic strehl from his numbers.

Here is a link to it (sorry, in german):

http://www.astrotref...314&whichpage=1

 

Both longitudinal colour aberration and spherical aberration variation

over the different wavelength lead to a lowered strehl.

Some of the tested scopes did show bit of astigmatism and one did suffer from coma.

Astigmatism and coma are individual optic flaws

wich will not be present in a near-to-perfect sample.

 

However, you can see the strehl curves, the longitudinal colour aberration curves (= SWD)

and the number for weighted polychromatic strehl (both coma and astigmatism substracted)

you can find at the bottom on the right side of each coloured box (Bild 7 to Bild 14).

 

These polystrehl numbers vary from 0.82 Tak FSQ over several in the range from 0.86 to 0.91

to the best airspaced triplett apochromats in the range from 0.97 to 0.99 for the 3" Tak FCT.

 

Here is another link to a test of 3 samples of a russian 4"f/8 triplett apochromat.

They show sample variation, but all have a high polystrehl.

Bild 20 shows the strehl curves, and the strehl curve of Kurt's formerly owned

5"f/9 Meade ED Doublet, and another obviosly not well-made 130mm ED Apo.

 

In the 6"f/8 size like Ed's ATM Newt there are different tripletts like the 6"f/7,3 Takahashi TOA

with a very high polystrehl, the russian made TMB 6"f/8 with again high polystrehl,

but there are too doublets like the 6"f/9 meade and the new 6"f/8 doublets

wich range from 0.94 to 0.88 in theory, if all work as well as calculated,

wich is not allways the case, to say it in nice words.

 

What I want say is:

You can expect a high 0.98 polystrehl only from airspaced tripletts with an advanced ED glass

or from oilspaced tripletts with 3 glass types one of them an advanced ED glas.

 

If Ed did make a 0.99 strehl mirror and uses a perfect secondary with 15% linear obstruction

the encircled energy drops only from 0.99 to 0.94 wich is very near to a very good triplett apo.

 

If the aperture gets bigger the number for central obstruction gets even better.

On the other hand the polystrehl of an 10"f/8 triplett gets a bit lower.

 

One thing to remember is that the central obstruction leads to a bit smaller airy disc size.

 

http://www.telescope...obstruction.htm

 

This fact is the reason why the mtf-curve of a newt has this strange form,

lower at the middle frequencies but higher at the high frequencies.

 

Perfect collimation + very good seeing + good optics = whow

 

Cheers, Karsten

 

 


 

#369 Asbytec

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Posted 22 October 2015 - 07:48 PM

Karsten, absolutely. Thank you for the detailed write up. I guess my point was aimed more at the obstruction effects which reduce the effective intensity distribution thus drag performance below the optical Strehl. Unobstructed scopes don't really suffer that problem and perform to their polychromatic and aberrant Strehl. But, Ed managed to get his obstruction small enough the diffraction effects are pretty much non observable. 

 

If Ed did make a 0.99 strehl mirror and uses a perfect secondary with 15% linear obstruction
the encircled energy drops only from 0.99 to 0.94 wich is very near to a very good triplett apo.

 

 

Sure. 

 

One thing to remember is that the central obstruction leads to a bit smaller airy disc size.

http://www.telescope...obstruction.htm

This fact is the reason why the mtf-curve of a newt has this strange form,
lower at the middle frequencies but higher at the high frequencies.

 

 

I have been trying to observe this effect splitting very tight stars. Not easy, but fun. Interesting too. 

 

Perfect collimation + very good seeing + good optics = whow

 

Even with my larger obstruction and less well corrected scope, I find this to be absolutely true. 


 

#370 KaStern

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Posted 02 November 2015 - 11:44 AM

Hello Norme,

 

sorry, I forgot to provide the link to Kurt's test of the 3 russian-made TMB apochromats: :
 

http://www.astrotref...&TOPIC_ID=98314

 

Best regards, Karsten


Edited by KaStern, 02 November 2015 - 11:51 AM.

 

#371 ed_turco

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Posted 03 November 2015 - 05:22 PM

There are so many mass produced "cheap" newtonians on the market now, I am sure that many people haven't even seen a good quality "long" newt.

I have one of those mass produced large aperture (14") newtonians and I have posted on here before, when comparing planetary views, how my little (4") apo is superior. How can a 4" apo show more planetary detail than a newtonian with 10" more aperture? Yes the newt is assembled correctly, yes it is collimated properly. yes the mirror is clean...and so on and so on. I have heard it all.

The simple fact is that my apo is a premium instrument. My newtonian (in quality terms) is not much more than a toy. One has to compare apples with apples. If the mirrors on my newt were manufactured to the same quality as the lens on my apo, the performance of the newt would be far superior, especially if it were practical to have the same focal length.

There seems to be more high quality affordable apos on the market than high quality newtonians. 

With all due respect, I did compare apples with apples.

 

And guaranteed, there will be smarter people than me who will delve into the matter and come up with my conclusion for better reasons, more fully documented reasons as to why this is so.

 

You may not wish to believe it; that is your privilege,  but my sources for my conclusion are impeccable.  I chose those who rank as giants in the field of telescope making, not some little putz like me. If you prefer not to believe the science of the matter, go argue with the likes of Richard Suiter.  As I have said before, I wouldn't dare and I mean it.

 

Why not disbelieve gravity while you are at it?  Science is science.

 

 

ed


 

#372 ed_turco

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Posted 03 November 2015 - 05:39 PM

Hello Norme,

 

 

Both designs, including your DNR, should have a very high Strehl ratio. Let's say they are equal, both at around 0.98.

 

there are only few apochromats that good.

My friend Kurt did test some of them in different wavelength

and calculated the weighted polychromatic strehl from his numbers.

Here is a link to it (sorry, in german):

http://www.astrotref...314&whichpage=1

 

Both longitudinal colour aberration and spherical aberration variation

over the different wavelength lead to a lowered strehl.

Some of the tested scopes did show bit of astigmatism and one did suffer from coma.

Astigmatism and coma are individual optic flaws

wich will not be present in a near-to-perfect sample.

 

However, you can see the strehl curves, the longitudinal colour aberration curves (= SWD)

and the number for weighted polychromatic strehl (both coma and astigmatism substracted)

you can find at the bottom on the right side of each coloured box (Bild 7 to Bild 14).

 

These polystrehl numbers vary from 0.82 Tak FSQ over several in the range from 0.86 to 0.91

to the best airspaced triplett apochromats in the range from 0.97 to 0.99 for the 3" Tak FCT.

 

Here is another link to a test of 3 samples of a russian 4"f/8 triplett apochromat.

They show sample variation, but all have a high polystrehl.

Bild 20 shows the strehl curves, and the strehl curve of Kurt's formerly owned

5"f/9 Meade ED Doublet, and another obviosly not well-made 130mm ED Apo.

 

In the 6"f/8 size like Ed's ATM Newt there are different tripletts like the 6"f/7,3 Takahashi TOA

with a very high polystrehl, the russian made TMB 6"f/8 with again high polystrehl,

but there are too doublets like the 6"f/9 meade and the new 6"f/8 doublets

wich range from 0.94 to 0.88 in theory, if all work as well as calculated,

wich is not allways the case, to say it in nice words.

 

What I want say is:

You can expect a high 0.98 polystrehl only from airspaced tripletts with an advanced ED glass

or from oilspaced tripletts with 3 glass types one of them an advanced ED glas.

 

If Ed did make a 0.99 strehl mirror and uses a perfect secondary with 15% linear obstruction

the encircled energy drops only from 0.99 to 0.94 wich is very near to a very good triplett apo.

 

If the aperture gets bigger the number for central obstruction gets even better.

On the other hand the polystrehl of an 10"f/8 triplett gets a bit lower.

 

One thing to remember is that the central obstruction leads to a bit smaller airy disc size.

 

http://www.telescope...obstruction.htm

 

This fact is the reason why the mtf-curve of a newt has this strange form,

lower at the middle frequencies but higher at the high frequencies.

 

Perfect collimation + very good seeing + good optics = whow

 

Cheers, Karsten

Karsten,

 

THANK YOU !!  Folks are always assuming that an apo design is perfect by virtue of being an apo; actually, an apo is just another design with interesting properties and none are perfect on paper or in execution.

 

Take an apo and enter any one of Suiter's aberrations to be tested and its performance falls to less than perfection and compares very well with a DNR, which is only a Newt put together with care as I made mine.*

 

Of course my telescope has errors and foibles like any apo design weith its own errors and foibles.  Taken this way,  there is no visible difference to the human eye between the two designs, and NO justification for the prices apo makers charge when performance is considered.    Why is it that apo owners brag of their 4" apos as mighty instruments while they call 4" Newts dinky, or worse.  Perhaps the pride of ownership enters the picture, but pride of ownership has no place in the evaluation of optical performance.

 

I'm sorry that most amateurs have experienced views in shoddy Newtonians.  But the fact is that an experienced amateur telescope maker can duplicate my results and build larger DNRs as pointed out above with better specs than mine.  I've heard of 16" Newtonians; where are the 16" apos?

 

I am too ill now to build that 16", but in my better days, I'd knock that job off in six months.

 

 

ed

 

 

 

*The beauty of Suiter's book is not only describing tests for aberrations but a checklist of what could go wrong in a Newtonian.  This is where I started my project.


Edited by ed_turco, 03 November 2015 - 05:43 PM.

 

#373 Deep13

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Posted 04 November 2015 - 12:31 AM

"APOs better than Newts?"

 

blah blah blah nuance blah blah specific purpose rah rah rah seeing yada yada cheap mirror blah blah collimation blah blah thermal control rah rah rah mount size blah

 

Even so, aperture rules, and the same money will get a much bigger Newt than APO refractor.

 

Inch for inch: yes.

 

Dollar (pound, Euro, Dinar etc.) for dollar: no.


 

#374 KaStern

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Posted 04 November 2015 - 11:58 AM

Hello Ed,

 

I definitely do not want to criticise your efforts to build the Ultimate-Newtonian,

but several times I thought "why not build it as a Maksutov-Newtonian?!"

It would share some of the advantages of your newt, but differ in some others:

 

The Maksutov-Corrctor does have power, whereas the plain glas plate 

of your newt do not have power. There would be some differences:

1) In the Mak-Newt you coiuld use a spherical mirror. The scope would have

a bit of spherical aberration and a bit longitudinal colour aberration.

At 6"f/8 both would be very low.

 

2) The benefit would be strongly reduced coma.

There would be only about 1/4 coma off-axis.

The low power widefield views would be much better than in a regular newt

without an additional coma-corrector.

 

3) Unfortunately the Mak-Corrector gets thicker and much heavier

when the aperture is increased.

 

Best regards, Karsten


Edited by KaStern, 04 November 2015 - 05:13 PM.

 

#375 ed_turco

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Posted 04 November 2015 - 03:39 PM

I like your thinking but what about central obstruction?  We have to deal with Suiter's 18% figure and it can be a tough one.  My window as an optical project is a lot easier than making a Mak lens.

 

How do we deal with that?

 

ed


 


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