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ric_capucho
Vendor
Reged: 05/11/03
Posts: 57
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Hi All,
A reply to one of my threads in the forum next door got me thinking... just how much of an advantage would, say, a 4" ED refractor have over a 4" Mak?
I know, I know, I know that the refractor world is always discussing chromatic aberration, and the like, and barely glances at anything with a mirror unless it's a 2" diagonal. Meanwhile, Mak owners point and sneer at SCTs. Unless there's Newt about, in which case Maks and SCTs join forces against the common enemy.
But what exactly are we all paying for?
Soooooo, we have a multitude of experts here who really really can tell the difference. Let's hear it then: in layman terms, using say a good 4" Apo as the bench mark, how much bigger would an Achro have to be, to compare? Or a Newtonian? Let's ignore focal lengths, and rich fields versus planetaries versus fuzzies, huh?
i.e. something on the lines of...
4" Apo
5" ED
6" Achro
11" Mak
12" SCT
14" Newtonian
Come on then, gimme an idea! Oh, and before I get tarred, feathered, and dragged out of town tied by my feet to a wild horse, I'd like to say that this question is 99% in jest! The other 1% wants to know if I've spent my money wisely on a half-decent ED!
Ric
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Anonymous
Unregistered
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The easy answer is "If you're happy with the scope, and if you use the scope, then it's the right scope for you." No point on spending money on something that's only going to gather dust.
The problem with "Is X better than Y?" comparisons is that different telescopes serve different purposes. What you want to observe will dictate what kind of scope you should get. There isn't any good way to compare different optical trains. Some people will always claim that more aperture is better, and some will always claim that refractors give the best images. It's a matter of personal preference.
That being said, I'll make these general observations:
Newtonians will always be cheapest per inch of aperture.
Catadioptrics usually represent the best balance of aperture vs. focal length vs. scope size vs. optical quality.
Refractors are the most flexible, and at a given aperture an apo will show finer detail and provide better images than an achro.
Jeff
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**DONOTDELETE**
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Well the situation is complicated because tehre are alot of different factors that can effect the performance of a small telesope. An excellent 4" Newtonian will outperform a poor 4" Apo as an example.
Now assuming that the scopes are all of equal quality (and high quality at that) I would rate the scopes as follows.
1. Apo. Extremely versatile, able to handle high powers as well as low wide field views. Inch for inch the highest perfomance.
2. ED. Pretty much everything said about the Apo applies to the ED, assuming that the ED is not simply a marketing gimick. The best ones like Vixen's are very close to Apo performance. If one would rate an Apo like the Vixen Flourite as 9 out of 10 (the 10 out of 10 being reserved for scopes like the Traveler, TMB-105 and the NP-101) the Vixen ED would be a very solid 8 to 8.5.
3. MCT: The MCT is, when done right, capable of amazing performance. Maybe the equal of ED scopes were it not for the fact that to get the performance, they sacrifice maximum field of view. A questar provides a beautiful image provided you are willing to settle for a field of view several times smaller than what an APO or ED can provide.
4. Achromat: Achromats are capable of great performance, they either can make good wide field instruments or great planetary instruments, but not both. The longer f/ratio refractors (f/15) provide crisp high definition views of the planets, but at the expense of field of view. Faster achros (f/5) are great for lower power work but are not nearly as pleasing on planets as the above listed scopes.
5. SCT. Theoretically capable of the same performance of the Mak, in practice it is harder to make. No examples I know of below 5" are currently being made.
6. Newtonian. At smaller sizes, the geometry of the Optical path in a Newtonian does not favor high performance. If we were talking in larger sizes (6" and larger), an extremely well made Newtonian would be in front of the MCT, SCT and the Achromat and there are so few Apos and EDs in that size range that the Newtonian might as well be on top. The biggest hinderence of Newtonians is that only in larger sizes (>10") are people willing to pay the money that is required to get the most performance out of them.
Just my thoughts. Take them for what they are worth 
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Anonymous
Unregistered
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Dear all:
ED (extra-low dispersion) is not an optical design, it is a type of glass. Apochromatic, achromatic, Schmidt, and Maksutov telescopes could all use it. Now ED glass is usually reserved for the better scopes because of its cost, but I would find out it the scope is an apochromat or an achromat first. That would probably more of an issue.
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jrcrilly
Refractor wienie again
Reged: 04/30/03
Posts: 22464
Loc: NE Ohio
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Quote:
I would find out it the scope is an apochromat or an achromat first. That would probably more of an issue.
Hi, Wil.
And before we can do that, we'll need to agree on a definition of apochromat. I've been gone for a while, but apochromatic used to be commonly used to refer to a high-quality triplet. Meade calls their ED doublets apo's but I don't think many accept that. I don't consider my 80mm Ultrascan with an ED doublet an APO. My 101mm TV SDF Petzval-style scope seems to be considered an APO these days, but my Vixen Petzval-style is a "neo-achromat"???
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John C
Urban Observatory
Tele Vue Pronto
A&M/Astreya 76mm F/6 APO
TMB/LOMO 80mm F/7.5 APO
Tak FSQ-106N F/5 APO
Meade 152ED F/9 "APO"
152mm F/10 achromat
Tak CN-212 8" F/12 classical Cass/ F/4 Newt
Teeter 20" F/3.8 truss Newt w/ServoCat
LXD750, EM-200, CI-700
ST-10XME
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ric_capucho
Vendor
Reged: 05/11/03
Posts: 57
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Well, the thoughts above are all very very valid indeed.
However, what I'm interested in is not a price/performance comparison, but a simple what you get for your aperture comparison, regardless of cost. I think mchale made a good point that quality is a huge factor, so I guess we have to agree that each type should be considered a quailty example, if not the very best in its class. Jeff Verona also spotted that different telescope designs excel at different tasks. Hmm, I guess we'd better ignore that issue as well! :-) Wil's point about ED not being a true class of telescope seems to be fairly well accepted by all. However, I guess the likes of Vixen and Borg are busy trying to invent a middle ground between Apos and basic Achros. Be patient with me, Wil, and let this pseudo-class stand for now.
Next point I spotted myself: my comparisons are a mixture of totally different optical designs, (i.e. refractor versus SCT) and (if we're honest) different levels of refractor quality (achro versus Apo).
So, just how much extra aperture would a Mak need to catch up with a 4" Apo? An extra 0.5"? An extra 2"? What do you think? Ditto a standard Achro? Say, a decent SCT (Ok, point taken that there's no 4" SCT available out there, but let's imagine, huh?).
Like I said, this is very tongue in cheek, so please don't anyone get offended in any way.
Ric
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Tom Trusock
Reged: 02/26/02
Posts: 27405
Loc: Alternate Reality (TM)
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Ric,
I compared my N5 to my TV102 (it's on CN here somewhere....). I'd say that a 5" mak newt and a 6" mak cass would probably give a 4" apo a run for it's money optically - maybe even beat it depending on the target.
The 14" newt would slam them both (assuming quality optics). Heck my old 8" sct (which was a touch undercorrected) put my 4" apo to shame.
There are many many other considerations than simple optics. Portability, collimation (or lack of need for same), cool down time, price, size, weight, fuss factor, field size, etc...
That's why I long ago decided that the best scope for me was actually a combination of scopes. For me, this means: A small wide field refractor (APO) and a large(r) dob. Of course if I ever actually get my observatory built, I'll probably put a 12-14 inch sct in it.
While I'd love a 20+ inch dob, I seriously doubt I'd use it very often. It would just be too much fuss for my observing style.
Tom T.
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There are two theories to arguing with my wife. Neither one works.
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jrcrilly
Refractor wienie again
Reged: 04/30/03
Posts: 22464
Loc: NE Ohio
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Quote:
That's why I long ago decided that the best scope for me was actually a combination of scopes. For me, this means: A small wide field refractor (APO) and a large(r) dob. Of course if I ever actually get my observatory built, I'll probably put a 12-14 inch sct in it.
Hi, Tom.
I agree that a quality fast refractor and a larger reflector make the hot combo. I found the 16" Dob was too much work so I went to SCT's and am very satisfied. Direct comparos as to what will "beat" what require that the class of object be specified first. When I want a four degree field of view there's nothing except the refractor that'll do it; when I want to look deep the refractor can't possibly keep up.
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John C
Urban Observatory
Tele Vue Pronto
A&M/Astreya 76mm F/6 APO
TMB/LOMO 80mm F/7.5 APO
Tak FSQ-106N F/5 APO
Meade 152ED F/9 "APO"
152mm F/10 achromat
Tak CN-212 8" F/12 classical Cass/ F/4 Newt
Teeter 20" F/3.8 truss Newt w/ServoCat
LXD750, EM-200, CI-700
ST-10XME
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rboe
Numbfinger
Reged: 03/16/02
Posts: 39712
Loc: Phx, AZ
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John;
Since I keep my 16" dob on the patio, it's the second scope I will go with (first being the Pronto or my bino's) instead of the NS11.
The dob is already setup (and once I get the bigger wheels installed) and a breeze to get into action compared to the SCT; which involves several trips in and out for OTA, tripod and acc. My wife refuses to let me install a pier in the yard (will interfere with her agility stuff). So until I can do the roof thing or move - the NS11 will be a sometime use scope. Bummer because once it's set up it's a killer!
Some guys have their NS11 on dollies and just wheel them out the garage into the driveway. In my neighborhood this is ill advised (people and street light wise).
It's interesting to hear the different circumstances folks face in their hobby.
Ron
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Ron
NS11GPS
Pronto
16" dob
15X70 Obies
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jrcrilly
Refractor wienie again
Reged: 04/30/03
Posts: 22464
Loc: NE Ohio
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Hi, Ron.
Things change, too. I kept the Dob in the dining room and could roll it out on to the deck very easily until my Daughter went to London for her MBA and dumped her cats with me. They aren't outside cats (she was in NYC before) so I have to keep 'em in and it got to where I wasn't setting up because of the hassle with them scurrying out while I was moving the scope. I guess I should be glad; now that I think about it that was probably the reason I decided to add the observatory 
I hated to let the Dob go; it was quite a conversation piece in the dining room!
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John C
Urban Observatory
Tele Vue Pronto
A&M/Astreya 76mm F/6 APO
TMB/LOMO 80mm F/7.5 APO
Tak FSQ-106N F/5 APO
Meade 152ED F/9 "APO"
152mm F/10 achromat
Tak CN-212 8" F/12 classical Cass/ F/4 Newt
Teeter 20" F/3.8 truss Newt w/ServoCat
LXD750, EM-200, CI-700
ST-10XME
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Ron B[ee]
Tyro
Reged: 04/27/03
Posts: 4719
Loc: CA
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See this modern definition of an APO by one of the world best APO designer  .
http://voltaire.csun.edu/tmb/definition.html
Indeed, a doublet can be an APO depending on the quality of the special glass used.
Ron B[ee]
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5-inch Tele Vue NP127 APO 
4-inch Tele Vue TV-102 APO 
8-inch f/6 Discovery PDHQ Dob 
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Anonymous
Unregistered
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An achromat brings two wave lengths to one common point of focus. An apochromat brings three wave length of light to one common focus. They are not optical designs, but rather the result of an optical design. That's the theory, but in practice it is much more complex. By the time it becomes a physical reality, you have an almost but not quite the equivalent of what you designed. I read the article by Jeff Beck posted above and he seems to say the same thing.
The labels used with telescopes can only give you a hint of its performance, but until you look through it, you will never know. An optical system is just that. Every componant will influence the result - the eyepiece, the eye, the observer, the sky, the film, etc. Even optical tests will not give the whole story. The star test can be one of the most subjective ways to judge a system because of the varibles. MTF curves can be very misleading. It is possible for say Lens A to have a better MTF result than Lens B, but images from Lens B to look sharper. 
If you like the view, it's the telescope for you! 
And while I'm venting a spleen, what the hell is a "semi-APO"? It sound like being a little bit pregnant! That term is the worst kind of marketing I've heard in a long time. 
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Anonymous
Unregistered
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I've seen that term used quite frequently myself. Anyone know what the ad agencies mean by that?
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rboe
Numbfinger
Reged: 03/16/02
Posts: 39712
Loc: Phx, AZ
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What do you call a scope that performs better than your run of the mill achromat yet is not good enough to be a true apochromat? The use of the term "semi" has been used with success in other areas; eg. semi-sweet chocolate, semi-trailer: You can probably think of a few more.
Def. 3 inThe American Heritage College Dictionary defines semi as Resembling or having some of the characteristics of:
Certainly applies in this case or in the case of my Pronto.
I think they have a legitimate claim to use semi. From my reading of other reviews, other manufactures should have used the term semi instead of APO.
Thanks
Ron
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Ron
NS11GPS
Pronto
16" dob
15X70 Obies
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Tom Trusock
Reged: 02/26/02
Posts: 27405
Loc: Alternate Reality (TM)
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While many professional lensmakers eschew the term semi-apo,
I actually did see a semi-technical definition of the term at one time (having to do with the wavelengths brought to focus in the airy disk, it compared achromat correction to apo correction in wavelengths of light and used a value 1/2 between the two if I remember correctly). Basically it is used to mean color correction better than an achromat, but worse than a top of the line apo. No manufacturers that I am aware of use the term. Vendors occasionally do.
From what I've seen there is definatly a range of color correction - the NP101 is a little better than the TV102 which is a little better than the SDF, etc.
What I find interesting is how "APO" has changed in the past few years. A scope that was considered a top end APO 10-20 years ago often has more color than today's mid line APO's.
There is a TON of information in the SAA archives on groups.google.com if anyone is interested.
So, I have a handle on semi-apo - But what the heck is a NEO achromat or tak's new line of ortho apochromats?
Tom T.
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There are two theories to arguing with my wife. Neither one works.
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Anonymous
Unregistered
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I believe the neo-achromat is a three-element system, with a doublet up front and a correcter element in the rear to flatten the field. The ortho-apochromats are triplets designed to deliver a flat field. Not sure what the "ortho" stands for in that designation.
Jeff
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jrcrilly
Refractor wienie again
Reged: 04/30/03
Posts: 22464
Loc: NE Ohio
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Hi, Jeff.
Vixen's neo-achromats are a Petzval-style system, with a slow doublet at the front and a second doublet to flatten the field and to reduce the F ratio. Same sort of thing as the TV101 and NP101 without the exotic glass.
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John C
Urban Observatory
Tele Vue Pronto
A&M/Astreya 76mm F/6 APO
TMB/LOMO 80mm F/7.5 APO
Tak FSQ-106N F/5 APO
Meade 152ED F/9 "APO"
152mm F/10 achromat
Tak CN-212 8" F/12 classical Cass/ F/4 Newt
Teeter 20" F/3.8 truss Newt w/ServoCat
LXD750, EM-200, CI-700
ST-10XME
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Ron B[ee]
Tyro
Reged: 04/27/03
Posts: 4719
Loc: CA
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"Semi-Apo"? I used to  when I heard the terms "semi-pregnant" applies to it....until I read the book Telescope Optics by Rutten and Venrooij  ! Here the authors define an achromat as a refractor that has secondary spectrum (i.e. the difference between the green and red/blue line) as 1/2000 the focal length of the lens. They went on to define "semi-apochromat" as a design that achieve secondary spectrum of 1/4000 focal length.
I'll give you a very recent experience. I used to have a 4" f/9.8 C102-HD achromat. Its purple halo  eventually bothered me too much. I got a look at the 4" Vixen f/6.5 ED at a star party just this weekend on Jupiter. Having been using an APO for the past 2 years, I immediately saw purple halo around Jupiter at around 150x. Note that the halo was much less in size than through my C102-HD and the view of Jupiter was actually quite pleasing, remarkable when you think how short the Vixen is (a very, very nice refractor  by the way ). So the Vixen seems to have much better color correction than my old achromat, but fall short of the APO.
How then shall we call  such a refractor as the f/6.5 Vixen?
Ron B[ee]
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5-inch Tele Vue NP127 APO
4-inch Tele Vue TV-102 APO
8-inch f/6 Discovery PDHQ Dob
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rboe
Numbfinger
Reged: 03/16/02
Posts: 39712
Loc: Phx, AZ
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Magnum-achromat!
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Ron
NS11GPS
Pronto
16" dob
15X70 Obies
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Anonymous
Unregistered
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I had read the an archromat color corrects at two wavelengths and the apochromatics correct at three different wavelengths. Is this correct?
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