Oh drool, an ED120 f7.5 from skywatcher

[h0ughy]

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/teledirect/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]

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

[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.

[h0ughy]

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]]

should look nice in the quiver h0ughy :P

[mick pinner]

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

[h0ughy]

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]

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 :eyepop:

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

[h0ughy]

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

[h0ughy]

ok so youre saying that this scope is rubbish?:shrug:

[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.

[h0ughy]

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

[Starkler]

Just so you dont think Im going crazy (http://whuyss.tripod.com/zeiss/presentation.html)

Start reading from about halfway down.

[h0ughy]

TA, science is wonderful isn't it;)

[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. ;)

[h0ughy]

so how would the scopes optics counteract that colour problem?:shrug:

[ausastronomer]

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]

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!! :D :tasdevil:

[ausastronomer]

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





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]

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.

[janoskiss]

You're right Geoff, they all specify the same fluorite glass.

John, I was not implying that these scopes would be as good as premium apos or that you are on "L" plates. I was merely pointing out that the scope appears to have the fluorite element you have mentioned as one of the ways of "eliminating chromatic aberration in a 120mm refractor". But I only read the info quickly and only on the ED120; did not realise the ED80 and ED100 were the same...

We can only wait and see what they are really like. I've seen them advertised overseas for a while but nothing closer to home yet.

[astroboy]

Hi Folk
Just to add my two bobs worth.
I have a 5"ED F6.4 Borg APO it only has two elements and in focus shows no colour on the Moon Jupiter etc where as the old TV Genesis did ( a little bit ).
Out of focus it shows lots of it , but well I prefer to look at stuff in focus anyway :) .
When CCD imaging I need to shift the focus very slightly between filters and for modded DSLR need to use a IR blocker.
Should it be called APO probably not , nice scope though.
Heres a pic taken with it and Borg flattener ( F5.4 ) and IR blocker sorry about the colour , needs a bit of tweeking .

Zane