Is 5" the new 4" ?

[Macavity]

...but the effect is subtle & likely to be swamped by other factors.

I think that is the point worth underlining. E.G. Computer simulations of these many effects are an interesting study.

But most images appear far superior to what I actually SEE. I happily aknowledge my "observer limitations" therein.

[coliea]

Yes, most astronomers consider a doublet with an FPL-51 or an FPL-53 element to be an APO. Telescopes like the Zenithstar 66, Skywatcher ED80 and Megrez 90 are all good examples.

HTH

Steve, IMO most astronomers would be wrong then. Calling most of these refractors "APOs" is simple marketing, nothing more. EDs maybe - APOs nope.

That's one reason I admire Stellarvue. They appear to have business ethics and don't mislabel their products.

An old argument I know, but I don't like perpetuating a myth.

[mike bacanin]

Hi John,

The 4 inch refractor is a great very portable scope for pleasing views of the moon and planets, however the 5 inch does give a noticeable increase in resolution of detail, and to me is the best size/weight compromise for a refractor in typical uk skies.

i owned the tak FS 102 and whilst a lovely refractor simply does not match the FS 128 i now have.

problem is the big jump in price beyween the 2.

but at least there are some well priced 120 scopes now available.

Mike

[johninderby]

While some of the newer scopes don't meet the old technical definition of an APO the important point is that they perform like an APO and aren't too expensive, which is what counts to most observers.

What was that old saying "If it looks like a duck, walks like a duck and quacks like a duck then it is a duck"

John

[brianb]

however the 5 inch does give a noticeable increase in resolution of detail, and to me is the best size/weight compromise for a refractor in typical uk skies.

OK, let me put it this way. My main scope is a CPC1100 but I use a WO FLT 110 /EZTouch as a "grab & go" & IMO it's marginal for this ... there is quite a difference between a 4" and a 5" in terms of bulk & weight and carrying the scope fully assembled on its mount points this up. I bought the WO FLT 110 together with an HEQ5 Pro with the intention of using it for imaging but discovered that the mount wasn't up to the job.... It's a great wee scope - WAAAYYY ahead of the ED "semi apo" doublets - but it wasn't good value for money for me. As a serious "do everything" scope going from 110mm to 130mm wouldn't make enough difference to be significant.

I'm now considering ditching the refractor & using a 6" SCT as a "grab & go" instead. The HEQ5 has found a niche for itself supporting solar scopes, so that's a keeper!

[brianb]

While some of the newer scopes don't meet the old technical definition of an APO the important point is that they perform like an APO and aren't too expensive

Visually, maybe - the longer focus scopes like the Skywatcher ED100 are fairly good, especially for the price if you must have a refractor. But the ED doublet "semi-apos" have much, much more "blue bloat" than apos, or reflectors, which becomes obvious when imaging.... just try focusing a planet through a parfocal colour seperation filter set.

[johninderby]

Please note that I said for "most observers". Serious imagers would have a different set of requirements.

John

[John]

Steve, IMO most astronomers would be wrong then. Calling most of these refractors "APOs" is simple marketing, nothing more. EDs maybe - APOs nope.

That's one reason I admire Stellarvue. They appear to have business ethics and don't mislabel their products.

An old argument I know, but I don't like perpetuating a myth.

Well thanks for putting all of us straight on that !.

John

[johninderby]

Before the "APO wars" break out why can't we all be happy that these new doublets offer great performance at an incredibly low price and are bringing APO levels of performance within reach of most observers. The serious imagers will have more stringent requirements, but these doublets are good enough for most observers.

John

[brianb]

Before the "APO wars" break out why can't we all be happy that these new doublets offer great performance at an incredibly low price and are bringing APO levels of performance within reach of most observers.

Wars, sorry, discussions ... everyone is entitled to their own opinion & for many, cost is a major factor. But APO levels of performance are available at costs below those of even a non-ED achromatic refractor if you're prepared to accept a Newtonian reflector on an easily portable, inutitive altaz mount (Dobsonian). Even if you can't afford grade A optics, a 6" f/8 Dob will knock the spots off any 5" refractor - however expensive - even in terms of showing faint detail on planets; the extra aperture makes a lot more difference than the small central obstruction or the extra quality that going from bargain basement to top of the range will buy you. Sure you've got to collimate it, and Newts have got a bad name recently through many manufacturers making them too short .... f/5 is hypercritical for collimation, f/6 not too bad, f/8 hardly an issue at all. In terms of light grasp, with super high reflectivity coatings there simply isn't a contest any more. Incidentally the modern mirror coatings last almost indefinitely even in salty coastal air, whereas the older aluminimum films needed renewing every couple of years; that was a major point against reflectors but again the "rules have changed".

Small refractors are convenient for imaging because of the large amount of focus adjustment available compared with Newtonians. But they need field flatteners for wide field imaging, and for visual work the performance you can get by switching to a reflector at the same price makes the "traditional telescope" look very expensive.

[FLO]

Calling most of these refractors "APOs" is simple marketing, nothing more. EDs maybe - APOs nope.

Unfortunately there is no industry standard definition for what is, or is not, an apochromatic telescope. Those definitions that do exist are mostly from manufactures and others with a vested interest. Needless to say those producing expensive/elite brands set the bar higher than those offering more affordable telescopes. Ultimately it is for the end-user to decide what is apochromatic.

Personally, I do consider the likes of the ZS66, the Megrez 90 and Synta's ED80 to be apochromatic, but I would not dream of forcing that definition on others!

[johninderby]

Like most observers I have both refractors and reflectors, and each have their own disadvantages and advantages and limitations. You have to factor in things such as, portabilty, colimation, central obstruction, coma, seeing conditions etc etc and so there simply isn't one "best scope" that can do everything all the time. It's a case of the right tool for the job.

John

[FLO]

That's another myth that modern coatings have laid to rest. Modern reflectors with UHTC/Hilux/XLT type coatings almost certainly have a higher light gathering capacity than a refractor with the same nominal aperture, despite the central obstruction...

Whilst I agree that modern glass, coating, materials and manufacture have reduced the differences between telescope types, it is wrong to suggest that they are a myth or that they no longer exist. There is still sufficient difference between say a Catadioptric, refractor and a Newtonian for us to generalise when commenting about performance.

I do think you are right to point out that seeing conditions and user experience play a large role.

HTH

<edit> I see John was quicker off the mark

[brianb]

Unfortunately there is no industry standard definition for what is, or is not, an apochromatic telescope.

I'm sorry to pick nits, and ones that are unimportant to many users, but apochromatic design has always been defined as having three wavelengths focused at the same point. Triplets can (but don't necessarily) have this attribute, doublets can't. The term "apochromatic" is being misused by manufacturers when it is applied to ED doublets, however good they are (and some of them really are very good indeed).

However, freedom from false colour is not the only attribute that an objective should have. Correction of spherical aberration is another, this is not often done with short focus instruments, when aspheric surfaces are required. Flat field & freedom from coma & astigmatism are also attributes which an ideal telescope should have. For a small field, they may be unimportant, but they can easily become critical when used with large sensor cameras. Camera lenses tend to have lots of elements for a reason ... for instance the Canon 300mm f/2.8L IS lens uses no less than 17 elements in 13 groups!

And it's not entirely free of aberrations....

The Canon 400mm f/5.6L lens may be more akin to small "apo" refractors in its aperture and focal ratio but it still uses 7 elements in 6 groups to achieve a flat, relatively coma & astigmatism free field across a 36mm x 24mm field.

[FLO]

This is starting to feel like a cross between Cloudy Nights and DPreview

You are clearly passionate about this Brian and I respect that.

[adamsp123]

Out of curiosity is ANY telescope a pure Apochromatic?

[FLO]

Good question.

Newtonian fans will remind us that mirrors reflect light so do not separate light into its indivudual components, therefore it cannot produce colour-fringing so can be called a 'true' apochromat. Refracters use lenses that refract light so have a tendency to act like a prism, hence the colour fringing APO enthusiasts find so offensive. Through the use of exotic ED glass combined with a crown (equally as important) lens element colour fringing can be minimised but I doubt it can be completely eliminated.

This is a complex subject that can be approached from a number of directions and is often the source of lively debate.

HTH

[brianb]

Out of curiosity is ANY telescope a pure Apochromatic?

A Newtonian or pure two mirror Cassegrain type scope, used for imaging at prime focus, is completely free of chromatic aberration. When used visually, you will tend to have chromatic aberration from the eyepiece and the eye itself.

But a "pure apochromat" needs to have a three element objective, computed to bring three specific wavelengths to the same focus - this can be optimised for visual (with some spread at the blue/violet end) or photovisual (same focus for blue sensitive photographic film but more spread in the red-green part of the spectrum - though still less than an achromatic doublet, even one using exotic glass).

[mike bacanin]

It is true that the newt and cass systems are totally fee of chromatic abberation.

However, some eyes are more sensitive to slight colour in well corrected refractors, some not.

I personally cannot see colour on the edge of the lunar limb in my Tak FS 128 Fluorite doublet, it is pure white, others have reported very slight colour in similar high end scopes.

Again i'd like to stress the importance of seeing. As some of you know, i have had the privilege of owning the world's finest Sct, the Tak TSC-225.

But it just could not deal with my average seeing. The 128 outperforms it.

It would be a different story if the 225 was in excellent seeing.

Only an individual can determine through actual use if a scope is right for the job.

Mike

[adamsp123]

Out of curiosity is ANY telescope a pure Apochromatic?

In the case of refractors are there any, absolutely and totally CA corrected for all visible wavelengths, ones out there?

Ok I well understand that mirrors are Apochromatic, maybe I should ask is there ANY telescope free from all types of abberation / distortions?

If yes where is it and how much ?

[Cloudwatcher]

Well,what an interesting thread........... mind you,as far as I can tell the original question remains unanswered!

Does anyone know if "5 inchers" outsell "4 inchers" ............ might give us a clue?

[GazOC]

There was an original question???

[Cloudwatcher]

There was an original question???

:)

[FLO]

Yes, 5" telescopes do outsell 4" telescopes, at least they do at FLO.

[Cloudwatcher]

Thanks,Steve.

There you go John............ your suspicions confirmed.