Revelation/Sentinel/Orion Express - official upgrades

[OXO]

Do tell.....

:laugh:

[russ]

Gaz/James, in fairness and conceding some ground on this, William Optics themselves don't use the term semi-apo because they are not comfortable with it. They stick with Well Corrected Achromat for the Zenithstar.

As far as the Skywatcher ED goes. Well i've seen some agruments around the web. But basically I agree with the one that says any refractor uses FPL53 Low Expersion Glass and successfully eradicates all Chromatic Abberation is a full APO.

Well my ED100 ticks all the boxes.

This thread could be pages long

Russ

[GordonCopestake]

1. Controversial discussions, debates and even differences of opinions are acceptable. But all discussions should be polite, and not give cause for offense. Think twice before posting.

Cloudy Nights Refractor forum guidelines

[GazOC]

A lot of apo vs semi apo seems to get into a USA vs Chinese thing, next thing you know a 5 page thread has been cut down to 3 by the Mods and you can't follow the arguement anymore.

[OXO]

It's purely for Legal Reasons Gaz

[Ant]

William Optics themselves don't use the term semi-apo because they are not comfortable with it. They stick with Well Corrected Achromat for the Zenithstar.

Sorry Mate, thats not actually correct...

Check this out!

Ant

[russ]

Well spotted Ant. How can i talk my way out of that one?

Well, the scope you've shown is their Zenithstar 66SD. This scope, unlike the Sentinel/Revelation/Express 80, uses a 4 element lens system grouped in two pairs with one element of ED glass. Where as the others use a 2 element lens with no ED glass. If the 66SD was being marketed by Meade or Synta, it would have been given full Apo status. William Optics seem to play it safe and down play their scopes a bit.

Russ

[GazOC]

The Zenithstar 66SD is a "proper semi-apo"

[GordonCopestake]

I've come to the conclusion that APO means whatever the manufacturer wants it to mean. Your idea of what APO means and the manufacturers may differ but thats life.

[GazOC]

I've come to the conclusion that APO means whatever the manufacturer wants it to mean. Your idea of what APO means and the manufacturers may differ but thats life.

No, most good manufacturers state what their definition of apochromatic is scientifically. Scientifically, these Revelations/Sentinels are achromats, no "ifs or buts" about it.

The only grey area is with the "real" semi apos like the ZenithStar and the ED range. Some poeple call them apos, some call them semi apos, the introdution of these new incorrectly named achromats has just confused the matter futher.

[Naz]

I was led to believe that Achromatic was corrected for two wave lenghts and Apochromatic was three wave lenghts.

There again are they corrected to 1/2 or 1/4 or 1/8 wave? what are the tolerences used in manufacturing? we can all discuss this till the good old cows come home, but unless you can compare like for like, there is not really a definative answer.

So possibly "Semi Apo" means that they have corrected for two and a half wave lenghts, after all as you said Gordon, manufactures seam to be able to call thier equipment whatever they like, and without the ability to test the claims in a proper standard way, we will keep on discussing / argueing "that my dad is bigger than your dad"!

naz

[GazOC]

"Semi apo" is taken to mean "nearly brings 3 colours into focus."

Apo or not apo is not the issue with these Revelations/Sentinels, they are achromats in every sense of the word and not apochomats. It's not a manufacturers thing or a "my dads bigger than yours" argument It's a provable fact. The only room for debate is where do you place the ED scopes, now thats a "my dads bigger than yours" argument!

[GazOC]

For those who want a solid definition:

Thomas Back's (TMB) definition of apochromatic:

After designing, testing and selling many different apochromatic lenses I can state this: There is no "definite" line where a lens becomes "apochromatic" in the world of commercial apochromatic lenses...But any lens, be it a doublet, triplet, quad, air-spaced or Petzval, that has a peak visual null (~5550A - the green-yellow) with a Strehl ratio of .95 or better, coma corrected and is diffraction limited from C (red) to F (blue) with 1/4 wave OPD spherical or better, has good control of the violet g wavelength with no more than 1/2 wave OPD P-V spherical and optical spot sizes that concentrate the maximum amount of photons within the diffraction limit -- a result of the low spherical aberration, which can be seen with modern optical design programs, as the "spot rays" will be seen concentrated in the center of the spot, not evenly or worse, concentrated outside the center -- will satisfy the modern definition of "Apochromatism."

[russ]

After designing, testing and selling many different apochromatic lenses I can state this: There is no "definite" line where a lens becomes "apochromatic" in the world of commercial apochromatic lenses...But any lens, be it a doublet, triplet, quad, air-spaced or Petzval, that has a peak visual null (~5550A - the green-yellow) with a Strehl ratio of .95 or better, coma corrected and is diffraction limited from C (red) to F (blue) with 1/4 wave OPD spherical or better, has good control of the violet g wavelength with no more than 1/2 wave OPD P-V spherical and optical spot sizes that concentrate the maximum amount of photons within the diffraction limit -- a result of the low spherical aberration, which can be seen with modern optical design programs, as the "spot rays" will be seen concentrated in the center of the spot, not evenly or worse, concentrated outside the center -- will satisfy the modern definition of "Apochromatism."

A Skywatcher Startravel ticks all the right boxes then? :)

[GazOC]

:)

If you read it, it seems that even apos have trouble with violet.

[FLO]

Found this today:

http://www.cloudynights.com/item.php?item_id=1420

[GazOC]

WOW! Thats certainly comprehensive!!

[GazOC]

The difference in false colour is pretty staggering, it does look like he's shapened up the images better for the apo images?

[FLO]

Any semi-apo owners on the forum ?

:wave:

[FLO]

A few months ... why?

[GazOC]

Just can't remember the exact moment you got suckered into the small fracy, larger FOV jibberish.

Ouch!!

[FLO]

Just can't remember the exact moment you got suckered into the small fracy, larger FOV jibberish.

Damien,

I have been wondering why you do not have a quality short-tube wide-field refractor on your hire list … now I know.

Whilst I understand your preference for long focal length catadioptrics, I do not share your attitude towards other types.

Currently, I own a short-tube 80mm refractor, a 4” Maksutov and a 10” Dobsonian. Each gives a different viewing experience and all are enjoyable.

The short-tube refractor sees the most use, as it is quick and convenient to set up on a simple Alt-Az mount or camera tripod. Fitted with a good 2” wide-field eyepiece, the views as you pan along the Milky Way are breathtaking.

With respect, perhaps you should broaden your horizons – literally

[GazOC]

Do I detect a schism in the Meade camp?

[OXO]

I too like all the differant types of scopes for as Steve Mentions you do get a differant view with each one(Ok clouds at the moment)

James

[russ]

A short tube frac can do things an ETX or SCT just cannot do. Plus, even in the semi, the star images are far nicer, far tighter than an SCT.

But i love the SCT design too. Good planetary and great deepsky. Portable depending on model. Just that wretched cool down....really puts the brakes on the SCT. Especially when the weather only allows for quick views through broken cloud.

There's certainly room for both types in every astronomers arsenal.

Russ