Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Why specify glass type if you don't specify optical accuracy?

  • Please log in to reply
228 replies to this topic

#101 bobhen

bobhen

    Aurora

  • *****
  • Posts: 4,804
  • Joined: 25 Jun 2005

Posted 13 January 2021 - 07:47 AM

In cars or telescopes, if "performance" is what one "values", having “more” of it (no matter if it is only one second faster to 60mph) will bring “more” enjoyment. If one feels uncomfortable with spending a certain amount on a car or telescope (for any reason) then having a value-priced telescope with more money in his or her pocket will bring that person enjoyment.

 

However, one’s enjoyment "does not" impact the fact that some cars are faster than others and some telescopes preform better than others.

 

Knowing the glass type (and other specifications) is important for those that are seeking performance. Indifference about glass type with more of a focus on price is what the value buyer seeks. Value telescope makers know this and they can get away with not offering any specifics about glass etc., as long as they offer a low price.

 

Bob



#102 Paul Hyndman

Paul Hyndman

    Apollo

  • -----
  • Posts: 1,285
  • Joined: 13 Jul 2004
  • Loc: Connecticut Shoreline USA

Posted 13 January 2021 - 07:10 PM

Two words: "discretionary spending". So long as the wolf isn't baying at the door and you've got the wherewithal to do so, why not indulge oneself? Life is a journey, enjoy the ride!

 

Regarding glass types: You don't need the recipe to appreciate a well made cake (and one of my favorite "chefs" is Rolando) winky.gif


  • Paul G likes this

#103 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 89,375
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 14 January 2021 - 08:51 AM

In cars or telescopes, if "performance" is what one "values", having “more” of it (no matter if it is only one second faster to 60mph) will bring “more” enjoyment. If one feels uncomfortable with spending a certain amount on a car or telescope (for any reason) then having a value-priced telescope with more money in his or her pocket will bring that person enjoyment.

 

However, one’s enjoyment "does not" impact the fact that some cars are faster than others and some telescopes preform better than others.

 

Knowing the glass type (and other specifications) is important for those that are seeking performance. Indifference about glass type with more of a focus on price is what the value buyer seeks. Value telescope makers know this and they can get away with not offering any specifics about glass etc., as long as they offer a low price.

 

Bob

 

I agree, what someone wants to spend their money on is not my business.  (In a car, performance to me means reliability, durability, efficiency, sufficient comfort, ability to haul telescopes.)

 

In my mind, it's the value telescope makers who benefit the most from specifying glass types as well as Strehl ratios.  You know Roland and Yuri will be using the good stuff.  

 

But the value buyer is usually choosing between several almost identical telescopes so knowing the particulars can be important. 

 

For example.. The Orion 130mm Eon:

 

https://www.telescop...pe/p/132454.uts

 

The Astro-tech 130EDT

 

https://www.astronom...riplet-ota.html

 

They are probably the same scope. 

 

At some point  Mike did say it uses FK-61.  Orion does not say but history says that if it's FPL-53, they tell you.

 

The Astro-tech is $1899, the Orion is $2499.. 

 

Jon


  • Daniel Mounsey likes this

#104 Daniel Mounsey

Daniel Mounsey

    Vendor (Woodland Hills)

  • *****
  • Vendors
  • Posts: 7,985
  • Joined: 12 Jun 2002

Posted 14 January 2021 - 08:56 AM

Jon, good points. I think the only problem is that most of the buyers who worry a great deal about these glass choices don't quite realize how good some of these affordable ED's actually are for the money you pay. They're actually not that bad. I think sometimes they get a bit too overly concerned as if they're gonna look through these things and be disappointed. 


  • Jon Isaacs and John Huntley like this

#105 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 89,375
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 14 January 2021 - 09:11 AM

Jon, good points. I think the only problem is that most of the buyers who worry a great deal about these glass choices don't quite realize how good some of these affordable ED's actually are for the money you pay. They're actually not that bad. I think sometimes they get a bit too overly concerned as if they're gonna look through these things and be disappointed. 

 

I agree 100%

 

I think the Astro-Tech 130 EDT is a heck of a scope for $1899. And just one nice scope.  20 years ago, it would have been unbelievable.  

 

The main thing is that value buyers are trying to understand what they are buying and trying to get the most for their dollar.  It looks to me like the Astro-Tech at $1899 and the Orion at $2499 are the same telescope.  I know which one I'd be buying. 

 

Jon


  • Daniel Mounsey and Illinois like this

#106 PKDfan

PKDfan

    Mariner 2

  • -----
  • Posts: 253
  • Joined: 03 May 2019
  • Loc: Edmonton

Posted 14 January 2021 - 10:59 AM

Personally I don't understand why more people are'nt doing their homework more diligently. If a product is in the market for any length of tìme their will be reviews somewhere, you ought to be able to find a few especially since this is the digital age now supposedly?
No one is buying a new product I hope!
Unless that company has a great track record of course!

REALLY don't understand why everyone just does'nt buy a Synta proed optic? For a LINE of optical products there can be no better examples that I can think of for CONSISTENT optical excellence than the 'original' three!(80,100,120) they are legendary surely? and the 72 and 150 are only a hair behind.
For the elite who need to spend money on ultimate end products well then you'll be getting a Takahashi I guess. For the value minded, buying the proed is a fool proof purchase as your usually getting ~95%-98%+ of 99.8% perfection. Those ultimate details are seeing limited and with the fpl-53/bk7 combo for the 80,100,120 variants the contrast in the eyepiece will be superb! My 100 in recent performance at 270x was jaw dropping! Focus snap is resounding and star test is excellent. Its right around 1000$ now U.S.
If you desire that little bit more for at some absurd price please spend 3-4 times more heck do 10 times if you think that will help. I certainly won't stop you but for ME that ~3000$ would buy a killer Dob!

Btw you won't be getting 99.8% if you do buy that Tak anyway!



Clear skies & Good seeing

P.s. Do your homework!
P.p.s. fun fact! Ex-Takahashi optical designer created these products
P.p.p.s.(possible urban legend as I can't find source quote!)
P.p.p.p.s I am NOT a shill for Sky-Watcher just a fan!!🥰
Edit: clarity

Edited by PKDfan, 14 January 2021 - 11:08 AM.

  • LDW47 likes this

#107 LDW47

LDW47

    Fly Me to the Moon

  • *****
  • Posts: 6,311
  • Joined: 04 Mar 2012
  • Loc: North Bay,Ontario,Canada

Posted 14 January 2021 - 11:59 AM

Of course that is not how it works - with anything.  There is a law of diminishing returns.  That last 5% of performance and  quality can cost a lot.  To some it is worth it and to some it is not.  

Diminishing returns is a way of thinking, the theoretical and then theres the actual, the real world, I learned that well in my long career ! 



#108 City Kid

City Kid

    Soyuz

  • *****
  • Posts: 3,511
  • Joined: 06 May 2009
  • Loc: Northern Indiana

Posted 14 January 2021 - 12:51 PM

You are correct but I would like to know what the ‘every scrap of performance’ is for the cost ? Its a great question which I have always wondered about but never seen answered especially now that some of the important data is not part of it all ! Have you personally been able to tell by just looking through one ?

I think I understand what you're asking. Here's an example. I had an Orion XT10 for approx. ten years before swapping out the mirror for a Zambuto. During that ten year period with the original mirror I was never able to see more than a hint of spiral structure in M51 when at my normal dark site. For reference my normal dark site isn't really all that dark. Point being I'm not talking about black zone level dark where spiral structure jumps out at you in a 10". After putting in the Zambuto mirror I have been able to make out spiral structure in M51 more than once at that site although it still depends on the quality of the night. To some that might not be a big deal. To me it's huge.


  • Mitrovarr, bobhen, eros312 and 3 others like this

#109 junomike

junomike

    ISS

  • *****
  • Moderators
  • Posts: 20,888
  • Joined: 07 Sep 2009
  • Loc: Ontario

Posted 14 January 2021 - 01:16 PM

I think I understand what you're asking. Here's an example. I had an Orion XT10 for approx. ten years before swapping out the mirror for a Zambuto. During that ten year period with the original mirror I was never able to see more than a hint of spiral structure in M51 when at my normal dark site. For reference my normal dark site isn't really all that dark. Point being I'm not talking about black zone level dark where spiral structure jumps out at you in a 10". After putting in the Zambuto mirror I have been able to make out spiral structure in M51 more than once at that site although it still depends on the quality of the night. To some that might not be a big deal. To me it's huge.

Great example of " The last 5% - 10% Theory".

To some it's not worth it, they keep the OEM mirror.

To others (you) it is.

 

In the end everyone is right and more importantly everyone is happy.


  • doctordub and LDW47 like this

#110 LDW47

LDW47

    Fly Me to the Moon

  • *****
  • Posts: 6,311
  • Joined: 04 Mar 2012
  • Loc: North Bay,Ontario,Canada

Posted 14 January 2021 - 01:19 PM

Great example of " The last 5% - 10% Theory".

To some it's not worth it, they keep the OEM mirror.

To others (you) it is.

 

In the end everyone is right and more importantly everyone is happy.

That was always going to be the case as per your last sentence ! But it was a great discussion as it should be, a great way to hear others points of view. Thats what astronomy is all about, the many sides.



#111 LDW47

LDW47

    Fly Me to the Moon

  • *****
  • Posts: 6,311
  • Joined: 04 Mar 2012
  • Loc: North Bay,Ontario,Canada

Posted 14 January 2021 - 01:24 PM

I think I understand what you're asking. Here's an example. I had an Orion XT10 for approx. ten years before swapping out the mirror for a Zambuto. During that ten year period with the original mirror I was never able to see more than a hint of spiral structure in M51 when at my normal dark site. For reference my normal dark site isn't really all that dark. Point being I'm not talking about black zone level dark where spiral structure jumps out at you in a 10". After putting in the Zambuto mirror I have been able to make out spiral structure in M51 more than once at that site although it still depends on the quality of the night. To some that might not be a big deal. To me it's huge.

Thats the kind of things, experiences we all want to hear. As I mentioned we aren’t trying to condemn anyone, to criticize them, only to hear the justification for the spending of ...... over and above just pride of ownership. 



#112 bobhen

bobhen

    Aurora

  • *****
  • Posts: 4,804
  • Joined: 25 Jun 2005

Posted 14 January 2021 - 02:36 PM

I think I understand what you're asking. Here's an example. I had an Orion XT10 for approx. ten years before swapping out the mirror for a Zambuto. During that ten year period with the original mirror I was never able to see more than a hint of spiral structure in M51 when at my normal dark site. For reference my normal dark site isn't really all that dark. Point being I'm not talking about black zone level dark where spiral structure jumps out at you in a 10". After putting in the Zambuto mirror I have been able to make out spiral structure in M51 more than once at that site although it still depends on the quality of the night. To some that might not be a big deal. To me it's huge.

And HERE is a link to why the Zambuto optic is better and costs a little more. Contrast of course also applies to other designs; the better the optical figure, the higher the contrast. The same also applies to planetary details. 

 

“… and there is contrast, which allows one to see detail.”
– Carl Zambuto

 

Bob


  • SkyRanger likes this

#113 dan_1984

dan_1984

    Vostok 1

  • -----
  • Posts: 165
  • Joined: 05 Jun 2016
  • Loc: Europe

Posted 14 January 2021 - 02:55 PM

Let's not forget that the average 10" dob will eat most refractors for lunch, low or high end....and they're truly apochromatic by design....

Nope, just plain wrong. Spend some time with a big refractor, it will change your view.


  • 3 i Guy, doctordub and Tyson M like this

#114 Bomber Bob

Bomber Bob

    Hubble

  • *****
  • Posts: 18,594
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 14 January 2021 - 07:15 PM

I think I understand what you're asking. Here's an example. I had an Orion XT10 for approx. ten years before swapping out the mirror for a Zambuto. During that ten year period with the original mirror I was never able to see more than a hint of spiral structure in M51 when at my normal dark site. For reference my normal dark site isn't really all that dark. Point being I'm not talking about black zone level dark where spiral structure jumps out at you in a 10". After putting in the Zambuto mirror I have been able to make out spiral structure in M51 more than once at that site although it still depends on the quality of the night. To some that might not be a big deal. To me it's huge.

I understand, and in relation to this topic, I've seen the differences in glass types:  My 1960s Swift Model 838 is a 50mm F14 achromatic with Outstanding Optics.  In fact, its pattern was so close to perfect that it was my Reference Lens on my DPAC rig -- I compared all other achros to it.  Then, I got my 1984 Takahashi FC-50 F8, and it won out over the Swift -- vintage fluorite beat traditional achromatic.

 

IF Swift & Takahashi didn't publish the glasses used, I still would've bought these 2 fracs, based on their well-deserved reputations for quality.  Don't need a DPAC rig to see it, but having that objective [pun intended] test of objectives is a warm fuzzy for my star & sky tests.  Do respected makers churn-out sub-par optics?  I'm sure it's happened, but I haven't read any negative press on these 2 models...

 

In a perfect world, scope makers would provide lab test results for each scope.  My Question:  Would amateurs pay a bit extra $$$ to have that report?  From a marketing & bottom-line perspective, it's cheaper & easier to advertise the glasses used, and let potential buyers base their decisions on that information.


  • Steve Allison likes this

#115 Petripher

Petripher

    Lift Off

  • -----
  • Posts: 14
  • Joined: 24 Oct 2020

Posted 14 January 2021 - 07:50 PM

 

P.s. Do your homework!
P.p.s. fun fact! Ex-Takahashi optical designer created these products
P.p.p.s.(possible urban legend as I can't find source quote!)
P.p.p.p.s I am NOT a shill for Sky-Watcher just a fan!!
Edit: clarity

I thought Mr. Kevin Legore was the lead designer for Skywatcher, as said in this webcast by Mr. Jeff Simons here https://youtu.be/Vwb_s6B-ozk?t=433. Also, one user asked "If Synta manufacturers many well known brands WHO does the design?" (https://youtu.be/Vwb_s6B-ozk?t=3145) and Mr Legore didin't infirm this.



#116 Bomber Bob

Bomber Bob

    Hubble

  • *****
  • Posts: 18,594
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 14 January 2021 - 08:29 PM

Sky-Watcher USA Product Specialist Kevin LeGore...  https://www.facebook...98950113616025/



#117 Daniel Mounsey

Daniel Mounsey

    Vendor (Woodland Hills)

  • *****
  • Vendors
  • Posts: 7,985
  • Joined: 12 Jun 2002

Posted 14 January 2021 - 09:13 PM

I feel like I've read this thread 17 times already in the past.  But it's nice and familiar and comforting.

 

Can someone please explain to me what "optical figure" is and how I can use it to evaluate a telescope as a consumer?

 

Thanks,

JK

 

The market has most newbies focused only on color correction. Just because you have better color correction doesn't mean the telescope will yield the best contrast. Optical figure encompasses all kinds of aberrations that annoy purists such as excessive zones, astigmatism, rough surfaces, turned edges, excessive under-correction, some amounts of over-correction etc. All of it amounts to spherical aberration, while others obsess over chromatic aberration. You want to know what it makes the images look like?


Edited by Daniel Mounsey, 14 January 2021 - 09:13 PM.

  • doctordub, John Huntley, payner and 3 others like this

#118 payner

payner

    Vanguard

  • *****
  • Posts: 2,276
  • Joined: 22 Mar 2007
  • Loc: Bluegrass & SW Appalachian Regions, Kentucky

Posted 14 January 2021 - 09:45 PM

Go to this topic thread, post #35, (https://www.cloudyni...2#entry10802718) and there's a good discussion (w/ photo comparison) of early telescope makers, contrasting (no pun intended, I think!) lenses between two preeminent optical craftsmen of the period. Optical figure and zones are the differences and how that affects the quality or fidelity of an object, especially where low contrast objects and features are observed.

 

Optical figure is required of any lens, or mirror, to produce an optic that is useful in critical observation. One can utilize any glass type--the best--and without high level optical figure the quality characteristics are hidden/lost.



#119 Mitrovarr

Mitrovarr

    Gemini

  • *****
  • Posts: 3,362
  • Joined: 12 Sep 2004
  • Loc: Boise, Idaho

Posted 14 January 2021 - 09:57 PM

The market has most newbies focused only on color correction. Just because you have better color correction doesn't mean the telescope will yield the best contrast. Optical figure encompasses all kinds of aberrations that annoy purists such as excessive zones, astigmatism, rough surfaces, turned edges, excessive under-correction, some amounts of over-correction etc. All of it amounts to spherical aberration, while others obsess over chromatic aberration. You want to know what it makes the images look like?

 

I think it's because the majority of refractors are purchased as astrographs, where chromatic aberration is a bigger deal.


  • Daniel Mounsey, Jon Isaacs and ATM57 like this

#120 John Fitzgerald

John Fitzgerald

    In Focus

  • *****
  • Posts: 9,015
  • Joined: 04 Jan 2004
  • Loc: ROR Obs. near Pettigrew, Arkansas

Posted 14 January 2021 - 10:29 PM

It's likely that the seeing at my place will never be good enough for anyone to be able to differentiate between .9 strehl and .98 Strehl.  I just don't have good seeing.  Maybe I'll move someday, but it's looking less likely every year.



#121 Mitrovarr

Mitrovarr

    Gemini

  • *****
  • Posts: 3,362
  • Joined: 12 Sep 2004
  • Loc: Boise, Idaho

Posted 14 January 2021 - 10:34 PM

The funny thing is, people will focus on things like a tiny bit of strehl or a better glass type and ignore that cheaper = potentially bigger, with all the advantages that brings. Doublets are also much lighter than triplets of the same size.

 

Like, my Skywatcher 150ED cost less than a 100mm Takahashi. Care to guess which one would deliver more planetary detail?


  • Jon Isaacs and ATM57 like this

#122 Daniel Mounsey

Daniel Mounsey

    Vendor (Woodland Hills)

  • *****
  • Vendors
  • Posts: 7,985
  • Joined: 12 Jun 2002

Posted 15 January 2021 - 12:18 AM

I think it's because the majority of refractors are purchased as astrographs, where chromatic aberration is a bigger deal.

 

That's correct which is why I said in a different thread that imagers are the reason people know less about optics, not the other way around. This is also why imaging and observing are black&white issues and should not be mixed in the same discussions without some clarity. If anyone wants to skip to 3:50 of this video you can remove false color with the click of a button.

 

https://www.youtube....h?v=meZ5ozVmVTo

 

My favorite astrophotographer of all time was this guy.

 

https://exhibit-arch...nard/index.html


Edited by Daniel Mounsey, 15 January 2021 - 12:23 AM.

  • eros312 likes this

#123 bobhen

bobhen

    Aurora

  • *****
  • Posts: 4,804
  • Joined: 25 Jun 2005

Posted 15 January 2021 - 08:38 AM

The funny thing is, people will focus on things like a tiny bit of strehl or a better glass type and ignore that cheaper = potentially bigger, with all the advantages that brings. Doublets are also much lighter than triplets of the same size.

 

Like, my Skywatcher 150ED cost less than a 100mm Takahashi. Care to guess which one would deliver more planetary detail?

People buy 102mm class apos knowing full well that larger scopes will outperform them. People buy them because of their advantages NOT their disadvantages.

 

Here are a couple of posts from the thread: Most and Least used refractor, why in each case?.

 

Stowaway/Traveler most, 175 EDF least. Size.
Paul G – post 25

 

Same here, Gus!
Paul Hyndman – post 28 (referring to Paul G’s post)

 

Same, due to size: Traveler most, 140EDF least.

Doug D – post 32

 

Bob


Edited by bobhen, 15 January 2021 - 08:39 AM.

  • Daniel Mounsey, Paul G, Paul Hyndman and 2 others like this

#124 JKAstro

JKAstro

    Ranger 4

  • *****
  • Posts: 309
  • Joined: 11 Aug 2006

Posted 15 January 2021 - 12:16 PM

The market has most newbies focused only on color correction. Just because you have better color correction doesn't mean the telescope will yield the best contrast. Optical figure encompasses all kinds of aberrations that annoy purists such as excessive zones, astigmatism, rough surfaces, turned edges, excessive under-correction, some amounts of over-correction etc. All of it amounts to spherical aberration, while others obsess over chromatic aberration. You want to know what it makes the images look like?

Thank you, this is helpful.  In other words, the optical figure relates to how well the design is executed.  And one can test the optical figure using a variety of tests: star test, DPAC, Strehl, who knows what else.

 

But it isn't helpful for a consumer's decision making process before a telescope is purchased.  And this is ultimately the problem, there is no way to objectively convey to a consumer what the difference is between scopes under consideration for purchase.

 

To use an extreme example, Vixen still sells an achromat that is priced above the Astro Tech 4 inch using ED.  Even if the Vixen was built to perfection, I would guess the AT with its ED glass would still outperform the achromat even if the figure had imperfections obvious to an experienced observer.

 

So maybe a designation of ED glass is enough? But then again, I recall advocating for a 140mm FPL-51 refractor elsewhere in this forum because I was more concerned about aperture than perfect correction.  It was argued that no one would buy such a scope because APM using FPL-53 existed. But perhaps if no one knew the glass type then they would? 

 

I somewhat think this is what happened with the Evostar 150.  The APM 152 uses FPL-51 and Lanthanum, perhaps the Evostar used a less costly mating element and dared people to tell the difference and no one can.

 

JK


  • Jon Isaacs likes this

#125 Mitrovarr

Mitrovarr

    Gemini

  • *****
  • Posts: 3,362
  • Joined: 12 Sep 2004
  • Loc: Boise, Idaho

Posted 15 January 2021 - 01:05 PM

I feel like the Evostar must have something better than FPL-51 and a regular mating element. There is not much CA at all. It's detectable on things like Rigel or Vega, and that's about all I can say about it.

My personal guess, although I don't have any evidence or anything, is that's its using either FPL-55 or some odd glass that isn't well known.


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics