ED vs APO - in layman's terms

[Basementboy]

Late comment here – found this thread really interesting, particularly as I've been looking at those Altair ED doublets that are apparently rebranded Tecnosky scopes (had no idea).

Does anyone have any thoughts about the difference between FPL51 and FPL53? The price jump is substantial and I'm not sure it's worth it (I won't be imaging to any great extent)

And, corrollary, are any of these actually in stock anywhere? Or do we need to wait for the slow boat from China?

[KP82]

3 hours ago, Basementboy said:

Late comment here – found this thread really interesting, particularly as I've been looking at those Altair ED doublets that are apparently rebranded Tecnosky scopes (had no idea).

Does anyone have any thoughts about the difference between FPL51 and FPL53? The price jump is substantial and I'm not sure it's worth it (I won't be imaging to any great extent)

And, corrollary, are any of these actually in stock anywhere? Or do we need to wait for the slow boat from China?

Tecnosky is another one of the rebranders just like Altair and TS Optics. These guys along with Astro-tech, Stellarvue (used to be) and William Optics (sometimes) all source from the same factories in China but with each of their own minimum specifications.

It's very difficult to describe the actual difference between FPL51 and 53 in words except for listing out their dispersion characteristics measured in Abbe number (51 = 81.54, 53 = 94.66). The performance of the final product lens also depends on many other factors including the figuring of each lens element and the type of mating elements. But generally speaking a good fpl51 doublet at f/7 or slower should be pretty much indistinguishable from fpl53 doublet at low to medium high mag. It's only at extremely high mag (usually used for splitting tight doubles) where the difference can be seen. For anything faster than f/7, fpl53 or a triplet is more desirable.

[mikeDnight]

I imagine the FPL51 doublet would perform roughly equivelant to the old Vixen ED's such as the Vixen 102ED F6.5. That scope, and in fact all the old Vixen ED's I've ever used are fantastic scopes, and made superb visual instruments. I'd be very content with one anyway!   The FPL53 doubles are better able to control chromatic abberation,  and very close to the Takahashi FS series of doublet fluorite refractors in terms of CA.  FPL53 is among several glasses that can be thought of as Fluorite glass, so it can produce pure, virtually colour free views.  But as has already been pointed out in previous posts, its the quality of the lens polish and figure that is more important than glass types used, so you could easily have an old achromat with a top class figure outperform an apochromat who's figure is not so good, yet costing thousands of pounds more. 

Edited February 21, 2021 by mikeDnight

[Marc1964]

The most difficult part of this debate, coming at this from the position of the rank newbie amateur is the importance of the fundamental design. As has been stated already a good doublet with seemingly inferior glass and a lower cost can outperform a poor design that uses the best materials.

The difficulty as I see it is knowing the designs well enough to make an informed choice.

Also, what I do not think has been mentioned so far is that at some point you are going to get diminishing returns and the difference between certain designs etc. may let you see things differently, not necessarily better. Where I live, at the low, budget end of the market a relatively small increase in spend will (ceteris paribus) produce a qualitatively better image (in principal) but at the top end you seem to have to spend significantly more to get a more marginal improvement. 

Edited February 21, 2021 by Marc1964
Clarity and removing duplication of the word 'also'.

[mikeDnight]

It can be difficult to know who makes good scopes. Cost can be an indicator to some extent, and of course certain brand names such as Astro Physics, Takahashi, LZOS are among some highly respected names that pretty much guarantee a truly top class scope. Vixen is another brand name that to me indicates top end performance. But there are what some might consider lower end manufacturers, that produce telescopes that are genuinely not that far behind. The Skywatcher brand offers some superb ED and Apochromatic refractors that would satisfy the most serious observer or imager. 

[alacant]

 

On 09/01/2021 at 18:24, MaJiC79 said:

Apochromatic

Hi

I think perhaps the only true apochromatic telescopes are reflectors, where all wavelengths are brought to focus at the same point. Anything where light passes through glass is gonna be a compromise, athough I'm sure you can get close.

Cheers

 

Edited February 21, 2021 by alacant

[Basementboy]

4 hours ago, KP82 said:

But generally speaking a good fpl51 doublet at f/7 or slower should be pretty much indistinguishable from fpl53 doublet at low to medium high mag.

Good to know. Crucial actually. Thanks

[johninderby]

Going from personal  experience you certainly can tell the difference between an FPL51 and FPL53 doublet at longer than F7. The FPL53  is noticeably sharper with better contrast. Not that the FPL51 is bad at all it just that side by side you would see the difference. 

Of course quality of the optics also plays a part but if the quality is equal there is that bit of a difference.

[John]

6 minutes ago, johninderby said:

Going from personal  experience you certainly can tell the difference between an FPL51 and FPL53 doublet at longer than F7. The FPL53  is noticeably sharper with better contrast. Not that the FPL51 is bad at all it just that side by side you would see the difference. 

Of course quality of the optics also plays a part but if the quality is equal there is that bit of a difference.

Yes, that matches my experiences as well.

 

[Carl Au]

On 10/01/2021 at 17:08, paulastro said:

I've, as John with the 80ED, found the Skywatcher series of Evostar pro ED refractors to be of excellent optical quality and probably by far the best value for money.  In this range I've owned several examples of the SW 80ED, the SW 100ED and SW 120ED and they have all been fine telescopes.

I have also recently bought a Tecnosky 102ED F7 FPL53 Doublet.  I bought it in preferance to the SW 100mm ED, which is F9, as I paticularly wanted an F7 focal ratio.  I've found this to be excellent both optic wise and in build quality - at least as good or better than SW in every way.

I will say I have no other experience of Tecnosky telescopes so I can't comment on any other telescope in the Tecnosky range.

TS and Altair Astro sell their own branded version of the 102mm Tecnosky I have, and people have said good things about them, but I've not used any of them.

 

 

What he said: For excellent ED refractors you won't have to selll a kidnesy for I would start looking here: https://www.teleskop-express.de/shop/index.php/language/en/manufacturers_id/143/filter_id/222/astro/-TS Teleskope.html or here for some slightly better ones: https://www.teleskop-express.de/shop/index.php/cat/c287_TS-SD-APO-Refractors.html these are the same as the Technosky or Altair Astro scopes like these ones: https://www.altairastro.com/starwave-102ed-r-fpl53-refractor-459-p.asp or this one: https://www.altairastro.com/starwave-80ed-r-ed-doublet-refractor-telescope-466-p.asp

The SW ED 80, 100 and 120 are excellent scopes, but the build quality is at the lower end with some reporting dodgy focusers. Before the current Tak craze for 4 -  8 thousand pound telescopes everyone was spending 350 quid on Moonlight focuser up-grades to fit on these scopes, I was one of them...

You do not, I repeat you do not need to spend thousands of pounds to get a decent refractor... 

Edited February 21, 2021 by Carl Au

[Highburymark]

My SW Equinox 80ED was a great scope - but was not a full apochromat. On tough targets, it would show some false colour. But it was F/6.25. In comparison, F/7 TV85 clears up almost all false colour and F/7.4 Tak FC100 is colour free in focus. 

[John]

11 minutes ago, Highburymark said:

My SW Equinox 80ED was a great scope - but was not a full apochromat. On tough targets, it would show some false colour. But it was F/6.25. In comparison, F/7 TV85 clears up almost all false colour and F/7.4 Tak FC100 is colour free in focus. 

I guess you need a decent triplet to get rid of false colour more or less completely at F/6.3 ?

 

[Highburymark]

8 hours ago, John said:

I guess you need a decent triplet to get rid of false colour more or less completely at F/6.3 ?

 

Yes - it seems so John

[Louis D]

9 hours ago, John said:

I guess you need a decent triplet to get rid of false colour more or less completely at F/6.3 ?

 

It also depends on the aperture as I pointed out with that achromat chart by f-ratio and aperture in one of these achro/ED/APO threads.  Similar to achromats, an f/6.3 50mm with an FPL-53 doublet would probably be easily as color free as an f/6.3 FPL-53 triplet at 100mm, perhaps better.  The next question becomes, at what aperture would an FPL-51 doublet look as good as a much larger FPL-53 triplet.  I know of no equivalent chart for comparing FPL-51/53 doublets/triplets to each other across various combinations of f-ratio and aperture.

I think it is informative that no premium frac companies make 150mm f/5 scopes, despite Skywatcher/Celestron making an achromat in this size/f-ratio.  I'd like to know how good an FPL-53 triplet at this size/f-ratio could be.  750mm is a very manageable focal length on an alt-az mount.

[mikeDnight]

If someone who was new to the hobby and had their heart set on owning a 4" ED refractor, I'd say buy the one that fits snuggly into your budget and forget about false colour. Just enjoy the telescope! Most ED's don't show much in the way of false colour anyway, and those that do show only a tiny amount, generally around the edge of very bright objects, or when the scope is out of focus. Keep it focused and you'll not really see it, you'll just see the razor sharp view that refractors are famous for.   Another thing I'd mention as an aside, is that there's always going to be a better or bigger telescope, so be happy with whatever you end up with, and don't get sidetracked chasing after the wind - use it to the full! 😊

[Zermelo]

Another factor was discussed not long ago, in a thread I can't locate:  with air-spaced triplets in particular, the correct positioning of the elements in the cell, to very fine tolerances, can have a significant effect on performance. Hence some retailers will arrange for bench testing of scopes and adjustment where necessary before dispatch.

[Carl Au]

On 22/02/2021 at 10:15, mikeDnight said:

If someone who was new to the hobby and had their heart set on owning a 4" ED refractor, I'd say buy the one that fits snuggly into your budget and forget about false colour. Just enjoy the telescope! Most ED's don't show much in the way of false colour anyway, and those that do show only a tiny amount, generally around the edge of very bright objects, or when the scope is out of focus. Keep it focused and you'll not really see it, you'll just see the razor sharp view that refractors are famous for.   Another thing I'd mention as an aside, is that there's always going to be a better or bigger telescope, so be happy with whatever you end up with, and don't get sidetracked chasing after the wind - use it to the full! 😊

I completely agree, it's getting to the point where some of the discussions which seem to be increasingly advocating scopes with premium brand names with primium prices tags to match are going to start putting people off buying a refractor at all. Not everyone can afford to spend 6 grand on 4 inch refractor. One thing is clear. It certainly does not mean, regardless of what folks might say that everying which isn't a Tak, Japanese or what have you must by extention be a little naff, it simply isn't the case. It's a forum, people are entitled to thier views and if bragging rights about who has spent the most money on gear are important to them then who I am to argue, but please does it always have to end in "well you need a Tak' as the go to advise when some talks about CA or what have you, It has to put people off. It's getting like astro top trumps in here, it never used to be like this in SGL. 

 

 

Edited February 28, 2021 by Carl Au

[mikeDnight]

8 minutes ago, Carl Au said:

I completely agree, it's getting to the point where some of the discussions which seem to be increasingly advocating scopes with premium brand names with primium prices tags to match are going to start putting people off buying a refractor at all. Not everyone can afford to spend 6 grand on 4 inch refractorand it certainly does not mean that everying which isn't a Tak or what have you must by extention be a little naff, it simply isn't the case. It's a forum, people are entitled to thier views and if bragging rights about who has spent the most money on gear are important to them then who I am to argue, but please does it always have to end in "well you need a Tak' as the go to advise when some talks about CA or what have you, It has to put people off. It's getting like astro top trumps in here, it never used to be like this in SGL. 

 

 

I definitely understand this, and perhaps ive been somewhat instrumental in emphasising my love of Tak.  Twenty years ago Takahashi were one of a very small number of manufacturers offering scopes at the pinnacle of performance.  Today, although Tak are still up there, other manufacturers have greatly closed the gap , so that the real world difference is often barely noticeable. I think this has been brought about by the availability of glasses such as FPL53, which is very close performance wise to Fluorite crystal. I've observed with my Tak alongside others using SW ED's, AstroTech and others, and in all honesty the difference is barely discernable. And most tellingly, I've enjoyed using those other scopes equally as much as I have my own, and I'm sure other Tak owners have similar experiences to tell.  

[ScouseSpaceCadet]

36 minutes ago, Carl Au said:

I completely agree, it's getting to the point where some of the discussions which seem to be increasingly advocating scopes with premium brand names with primium prices tags to match are going to start putting people off buying a refractor at all. Not everyone can afford to spend 6 grand on 4 inch refractorand it certainly does not mean that everying which isn't a Tak or what have you must by extention be a little naff, it simply isn't the case. It's a forum, people are entitled to thier views and if bragging rights about who has spent the most money on gear are important to them then who I am to argue, but please does it always have to end in "well you need a Tak' as the go to advise when some talks about CA or what have you, It has to put people off. It's getting like astro top trumps in here, it never used to be like this in SGL. 

 

 

Unfortunately that pervading attitude is prominent in other hobbyist forums. Buy what you can afford, don't skint yourself, get into debt or think beginner scopes are not good enough. For instance, a cheap 150mm reflector and a selection of midrange eyepieces can keep you busy for years.

Remember, many of those who have been in the hobby a long time have traded up to reach the scopes or mounts they really fancy, having often reached a point were mortgages are paid off and kids grown up.

There's plenty too who have gone crackers on a first telescope, spending several hundred or thousands on kit and it ends up gathering dust in the shed.

 

Edited February 22, 2021 by ScouseSpaceCadet

[vlaiv]

10 hours ago, Louis D said:

I think it is informative that no premium frac companies make 150mm f/5 scopes, despite Skywatcher/Celestron making an achromat in this size/f-ratio.  I'd like to know how good an FPL-53 triplet at this size/f-ratio could be.  750mm is a very manageable focal length on an alt-az mount.

Maybe best option for such scope would be sort of modified Petzval design.

There are FPL-53 doublets that 6" and F/8 and have excellent performance - like SW 150ED (not sure which glass that one uses) or TS Photoline 150 F/8 doublet (advertised as FPL53 + lanthanum element). There is also fast F/5 doublet in "quadruplet" configuration: Bresser 127 and 152 F/5 scopes have modified Petzval configuration 2+2 elements.

I guess it is just a matter of time before someone makes decent 6", color free, flat field F/5 ED quadruplet scope

 

[vlaiv]

Just an observation that could be relevant to this discussion.

Often I hear phrase: such and such 4" APO can soak up magnification and image does not break down at x300 or x400. I always wondered about that. Can't be that my 4" Mak is so much poorer than these premium scopes? Price difference does suggest that it might be the case.

Then I realized important thing - telescope and eyepiece are not only optical elements. We have another lens in optical chain - our eyes.

Theory suggests that 20/20 vision needs at max x1.2 magnification per millimeter of aperture. 4" telescope really needs only ~ x120 times for someone with average vision to show all there is to be seen. Adding magnification only makes things easier to see as it makes them bigger. At some point, things are big enough for blur to be obvious - let's say that is "breaking" point of telescope.

I've also realized that about 1/4 people actually have worse vision than 20/20. This is with inclusion of corrective measures - glasses or surgery. A Quarter of us simply don't see as sharp as rest. There is very small portion of population that has sharper than 20/20 vision. For example, if person has 20/10 vision - they would only need x60 magnification to see all there is to be seen with 4" telescope.

It also means that for them - image is going to "break down" sooner than for the rest of us.

Eyesight starts to deteriorate with age and we tend to loose sharpness after age of 60 - regardless of glasses we might be wearing. It is not uncommon that young people can read distant signs without trouble (sharp vision) while elderly struggle even with glasses on.

For someone that does not have sharp vision - image will not start to break up until very high magnification - it is much like needing larger font on your computer to see it clearly - to someone with sharp vision, that font will be too large but you'll feel very comfortable with that font size as smaller fonts strain your eyes.

In light of all of that - I came to realization. We should not easily "recommend maximum useful magnification" - as that is very personal thing and depends on visual acuity of person in question. Some might find that 4" gives sharpest views at about x120 while others will happily use x250 for relaxed observing. It further suggests that we should not take "image breakdown" as criterion of scope performance as that is heavily dependent on visual acuity of observer regardless of actual differences between scopes. Third and most interesting thing - someone with poor eyesight might characterize performance of good and average scope as being the same - or scope of smaller and scope of large aperture - because they need much more magnification before they start noticing the difference.

[John]

1 hour ago, Carl Au said:

..... It's getting like astro top trumps in here, it never used to be like this in SGL. 

 

 

You make a very valid point there Carl.

We really should spend more time discussing how folks can get the best out of what they already have rather than fuelling a continual "race to the top".

 

 

[Louis D]

I think there's also the point that Chinese manufacturers seem to put more effort into getting a near perfect figure on their ED and APO scopes as compared to their achro scopes.  My ST80 has bucket loads of spherical aberration in addition to chromatic aberration.  So much so, that it makes it nearly useless for terrestrial or astro viewing.  I'm sure an ST80 could be made with as fine a figure as my FPL-51 and FPL-53 based scopes, but at what cost?  Would people be willing to pay a premium for a premium figured fast achromat?

[vlaiv]

2 minutes ago, Louis D said:

I think there's also the point that Chinese manufacturers seem to put more effort into getting a near perfect figure on their ED and APO scopes as compared to their achro scopes.  My ST80 has bucket loads of spherical aberration in addition to chromatic aberration.  So much so, that it makes it nearly useless for terrestrial or astro viewing.  I'm sure an ST80 could be made with as fine a figure as my FPL-51 and FPL-53 based scopes, but at what cost?  Would people be willing to pay a premium for a premium figured fast achromat?

I think that you can actually attempt to make it better yourself for minimal cost.

It is about spacing between two lens. Maybe try changing thickness of spacers (replacing them) +/- 10% and see what sort of results you get that way.

[Highburymark]

2 hours ago, Carl Au said:

I completely agree, it's getting to the point where some of the discussions which seem to be increasingly advocating scopes with premium brand names with primium prices tags to match are going to start putting people off buying a refractor at all. Not everyone can afford to spend 6 grand on 4 inch refractorand it certainly does not mean that everying which isn't a Tak or what have you must by extention be a little naff, it simply isn't the case. It's a forum, people are entitled to thier views and if bragging rights about who has spent the most money on gear are important to them then who I am to argue, but please does it always have to end in "well you need a Tak' as the go to advise when some talks about CA or what have you, It has to put people off. It's getting like astro top trumps in here, it never used to be like this in SGL. 

 

 

I agree totally, though I guess in a thread titled ‘ED v Apo’ there is bound to be discussion about the small differences between refractors at the top of the market. For what it’s worth, not once did false colour spoil the views through my Equinox 80ED. I think I’ve said elsewhere that I saw 60 odd Messiers through that little scope. Sliding dew shield so easy to carry on aircraft. Relatively light, so very happy on a £100 camera tripod and basic alt az mount. Cools down (or heats up) to ambient temperature very quickly. Pretty good focuser. Just a great all rounder.