Oh drool, an ED120 f7.5 from skywatcher

[h0ughy (David)]

EVOSTAR-120ED1 PRO


4.75” (F/7.5) APOCHROMATIC REFRACTOR OTA

• Highest Practical Power (Potential): x300
• Objective Lens Diameter: 120mm
• Telescope Focal Length: 900mm (f/7.5)
• No Eyepieces Supplied
• No Finderscope Supplied
• No Diagonal Supplied
• Supplied with Tube Rings
• Dual-fit 1.25”/2” Crayford Focuser (Backlash-Free) Brushed-Aluminium Hubs & rubber grip rings
• Fully Multi-Coated Doublet Objective Lens
• Single FPL-53 ED Fluorite Glass Objective Element

EVOSTAR-120ED1 PRO OTA £ 0.00 TBA






EVOSTAR-120ED2 PRO


4.75” (F/7.5) APOCHROMATIC REFRACTOR OTA

• Magnifications with Eyepieces Supplied: x45 & x180
• Highest Practical Power (Potential): x300
• Objective Lens Diameter: 120mm
• Telescope Focal Length: 900mm (f/7.5)
• Eyepieces: 5mm & 20mm LET* (*Long eye relief with Twist-up eyecups)
• 9x50 Finderscope
• 2” Star Diagonal
• Supplied with Tube Rings
• Dual-fit 1.25”/2” Crayford Focuser (Backlash-Free) Brushed-Aluminium Hubs & rubber grip rings
• Fully Multi-Coated Doublet Objective Lens
• Single FPL-53 ED Fluorite Glass Objective Element

EVOSTAR-120ED2 PRO OTA £1499.00

http://www.widescreen-centre.co.uk/t...ct/edscope.php for the image.

now that would be a huge treat with the camera. I was going to get an ED100 but I think I will now wait. My ed80 Skywatcher that I picked up from myastroshop is extremely pleasing, so I can only wait and see what this might offer if they become available in Australia?

[h0ughy (David)]

No I do not suffer from apature fever (much)!

[Striker (Tony)]

Not bad Houghy......atleast it's better then the orion 100ED at F9.

F6 would be great...but F7.5 is not bad at that price.

[h0ughy (David)]

now i will note with the ED80, the mag stars that I have teased out of my shots have been down to mag 13.8 or there abouts. So with this scope it would be alot better.

[[1ponders] (Paul)]

should look nice in the quiver h0ughy

[mick pinner]

have you seen the Meade 127mm f/7.5 Apo Triplet at Bintel $2800.00?

[h0ughy (David)]

Originally Posted by mick pinner

have you seen the Meade 127mm f/7.5 Apo Triplet at Bintel $2800.00?

No but I have been told they only come with a R& P focuser and that there are no tube rings or mount bar!

EDIT: they do have a crayford style focuser, sorry!

[Starkler (Geoff)]

Originally Posted by Striker

Not bad Houghy......atleast it's better then the orion 100ED at F9.

F6 would be great...but F7.5 is not bad at that price.

Being of the same design, its going to be a hell of a lot more colourful than the ed80 or the ed100. To be as colour free, this 120mm needs to be something like f12

I dare say they are taking license using the term APOCHROMATIC to describe this scope.

[h0ughy (David)]

Originally Posted by [1ponders]

should look nice in the quiver h0ughy

only testing the waters the fact that I can image more with it is a drawcard

[h0ughy (David)]

Originally Posted by Starkler

Being of the same design, its going to be a hell of a lot more colourful than the ed80 or the ed100. To be as colour free, this 120mm needs to be something like f12

I dare say they are taking license using the term APOCHROMATIC to describe this scope.

ok so youre saying that this scope is rubbish?

[Starkler (Geoff)]

Originally Posted by h0ughy

ok so youre saying that this scope is rubbish?

Nope, Im just saying dont expect the same colour correction you would get with an ed80 at f7.5 or an ed100 at f9. Its probably a lovely wide field visual scope.

I looked for a source, but have read in a few places that equivalent colour correction demands a focal ratio that is 'c x aperture' , ie for increasing aperture, the focal ratio needs to also increase.

I think it was f-ratio = 4* aperture in inches for achromats for a certain criteria.

[h0ughy (David)]

Originally Posted by Starkler

Nope, Im just saying dont expect the same colour correction you would get with an ed80 at f7.5 or an ed100 at f9. Its probably a lovely wide field visual scope.

I looked for a source, but have read in a few places that equivalent colour correction demands a focal ratio that is 'c x aperture' , ie for increasing aperture, the focal ratio needs to also increase.

I think it was f-ratio = 4* aperture in inches for achromats for a certain criteria.

for the money they should be spot on, the ed80 is and its f 7.5?

[Starkler (Geoff)]

Just so you dont think Im going crazy

Start reading from about halfway down.

[h0ughy (David)]

Originally Posted by Starkler

Just so you dont think Im going crazy

Start reading from about halfway down.

TA, science is wonderful isn't it

[janoskiss (Steve H)]

The mismatch between the focal planes of different colours scales up with size the scope but the wavelength of light stays the same. So say 1/4-wave at 80mm aperture becomes 3/8 wave at 120mm. That's why the larger aperture needs greater f-ratio to maintain colour correction. But maybe the ED120 is more than just a scaled up ED80.

[h0ughy (David)]

Originally Posted by janoskiss

The mismatch between the focal planes of different colours scales up with size the scope but the wavelength of light stays the same. So say 1/4-wave at 80mm aperture becomes 3/8 wave at 120mm. That's why the larger aperture needs greater f-ratio to maintain colour correction. But maybe the ED120 is more than just a scaled up ED80.

so how would the scopes optics counteract that colour problem?

[ausastronomer (John Bambury)]

Originally Posted by Starkler

Being of the same design, its going to be a hell of a lot more colourful than the ed80 or the ed100. To be as colour free, this 120mm needs to be something like f12

I dare say they are taking license using the term APOCHROMATIC to describe this scope.

Geoff,

Your 100% correct here. A reasonable telescope it might be, a True APO it won't be.

In terms of Achromatic refractors (2 element crown and flint) there are 2 similar formulas for calculating the necessary focal length or focal ratio to elminate residual chromatic aberration for a given aperture.

John Sidgwick in his book Amateur Astronomers Handbook indicates the formula for focal length to be 2.88 x aperture (in inches) squared.

FL = 2.88D²

For an 80mm achromat (3.15") this gives a focal length of 28.58" or 725mm (F9).

For a 120mm achromat (4.75") this gives a focal length of 65" or 1651mm (F13.75)

In the book Telescope Optics by Rooten and Venrooij, they are even more conservative and suggest the following formula.

F/Ratio = Aperture (in mm) X .122

Using this more conservative formula an 80mm achromat needs an F-Ratio of F9.75

and a 120mm achromat needs an F-Ratio of F14.65

Regardless of which formula you use, its easy to see an F7.5 80mm scope is closer to the threshold and will have less chromatic aberration than a F7.5 120mm scope. Consequently the introduction of a single ED glass element in the 80mm F7.5 scope effectively solves the problem. Eliminating chromatic aberration in a 120mm refractor is significantly more difficult and will usually require the introduction of a flourite element or a 3rd lens element , ie a triplet, or a 4th lens element, a Petzval. viz, a Tak FSQ 106 or TV NP101.

The ED 120mm will be a nice scope and infinitely better than a 120mm F7.5 achromat but it won't be nirvana. That costs a lot more money with 5" of aperture.

CS-John B

[janoskiss (Steve H)]

Originally Posted by ausastronomer

...
A reasonable telescope it might be, a True APO it won't be

Don't be so sure.

Originally Posted by ausastronomer

...
Eliminating chromatic aberration in a 120mm refractor is significantly more difficult and will usually require the introduction of a flourite element or a 3rd lens element...

The page linked in Houghy's first post states that the ED120 does use a fluorite element.

Get one Houghy!!

[ausastronomer (John Bambury)]

Originally Posted by ausastronomer

Geoff,

Your 100% correct here. A reasonable telescope it might be, a True APO it won't be.

Originally Posted by janoskiss

Don't be so sure.

Of course your right, WTFWIK. I am still on "L" plates.

Originally Posted by ausastronomer

Eliminating chromatic aberration in a 120mm refractor is significantly more difficult and will usually require the introduction of a flourite element or a 3rd lens element...

Originally Posted by janoskiss

The page linked in Houghy's first post states that the ED120 does use a fluorite element.

So you think because Synta have stuck a flourite lens element in it, that now qualifies it as an APO ? My wifes Hyundai has Continental tyres on it just like the guy next doors Mercedes. That similarity alone doesn't make my wife's Hyundai a Mercedes. A bit more to making a 5" APO than sticking a flourite lens element in it.

The scope may well turn out to be superb, but I am a little sceptical. I am guessing its likely to be pretty good and a good level above the 5" achromats, but still a notch behind top grade 5" APO's like the Takahashi TOA 130 or the Televue NP 127, or even a good 5" Flourite doublet like the TAK FS-128. You don't get silk purses out of sows ears. I am just warning prospective buyers that producing a 5"/F7.5 APO is a LOT more difficult, and particularly when cost constrained, than it is to produce an 80mm/F7.5 APO. Hence buyers should approach it with an open mind as its performance "could" be a little down on what they may be expecting. The day will come when the Chinese produce a good 5" APO, I don't know that they are quite there yet.

Try before you buy if possible.

CS-John B

[Starkler (Geoff)]

Originally Posted by janoskiss

Don't be so sure.
The page linked in Houghy's first post states that the ED120 does use a fluorite element.
Get one Houghy!!

Says all the same stuff for the ed80 and ed100 also. They are all Skywatcher using the same lens materials and the same laws of optics apply.