Max magnification WFT 204 f/6

[ivan86]

Hello guys,

someone with some experience under sky with the WFT 204 f/6 can tell me please how much is the max magnification in dso?

I dont care about CA but i want know if the optic could reach a good focus at magnification like 100 / 120x on galaxy and nebula.

Thanks in advance
Best regards

Ivan Tardio

[bigsky]

Don't know about the 204, but those magnifications with my 150 Perseus would be pushing it quite a bit. I've got bigger Newtonians up to 17.5" for that kind of work, although I generally keep the magnification much lower than you describe. You could probably do it but the real beauty of a refractor is in lunar/planetary work and splitting binarys.

[Ales - iStar Optical]

Ivan,
maximum usable magnification with for example 127 F12 R35 lens would be 255x, maximum usable magnification with 150 F15 achromat would be 300x, some push it into nearly 400x without loosing detail and contrast. When you step up to 204 F12 R35, you can push the magnification up to 500x and with 250 F11 you can go up to 800x easy. But with super fast, short focal length doublets, you are limited to 0,25D (where D is diameter of the objective lens in millimeters). So with a common achromatic doublet in 204mm diameter (clear aperture.. ) you are limited to 51x magnification. SOME of these doublets allow you to use magnification up to 100x and even more but this is different lens to lens so safe bet is to calculate with 51x magnification. If you step up to 204 F8 R35 SLIM, than you limiting magnification would be around 0,7D which is 143X.. not bad.. with 204 F8,8 achromat it would be around same number. If you go with F10 system than the usable magnification will increase further. The maximum usable magnification in large fast doublets is all about how precisely is the surface/radii done. Slower doublets do not require such strict lolerances, but faster you go, they increase by a huge increments. So once you reach F6 or F5 the surface and radii must be near perfect, if not, the usable magnification is limited. We produce a very good quality optics, but tell everyone where the limits of our optics are.
Also, pushing magnification up on nebulas or galaxies wont do you any good. They are best observed with low magnification for example up to 50x with 200mm lens. Where you WANT to step up your magnification would be in case of observing binary stars (there you see really how good your lens is.. you check the resovling power of your lens on hard to resolve binary stars), or globular clusters (you do want to see those small individual stars in their complete beauty.. not to see a small fuzzy blob of dim light), planetary nebulas
most are dim but if you have lots of aperture and crank up the magnification, you see them up close...), etc..
I know decisions are hard. You may be better off with 204 R35 SLIM doublet, but those cost quite a bit more. With nice 204 F8 R35 SLIM you can observe with greater magnification, lesser chromatic aberration and substantially increased detail.. well, its all up to you. Do some more research and come back to pick the right Istar scope. We are producing the 204 F8 R35 SLIM as well as F12 version and 180 F8 R35 SLIM in F8 and F12 version. Due to be finished in about two months from now (for those interested). We produce only 5 each now so act fast guys. More will be produced by early summer.
Best regards,
Ales
ISTAR Optical

[ivan86]

Thanks Ales for your detailed answer.

The only other telescope i see that is enough portable for me is the 180 F8 R35 with a lenght of 144 cm but 2 cm less of aperture.

I would love to take a 204mm at f/8 or f/9 but is symply too long for me.

Ill take some days to think about it. The 204 f/6 is really the ideal solution for me but for experience under dark sky, even with my old 152mm refractor achromatic at f/6 i need about 100x to get some nice view of planetary nebula like M57 or M27 and i was able to get also nice detail on M81, M82, M51, M66, M65, M104 galaxy.

A 204 mm can resolve easy globular like M13, M3 and M5 because i could with my 152mm f/6 but 90/100x are fundamental.

Again thanks a lot Ales, ill update you as soon as possible as i wrote in private email.

Ivan

[jamesling1000]

Hi Ivan and ALL....

I just want to share my personal views, but this may not be correct as its my personal opinion without actual field test....

I would say if anyone who wants big aperture and portability, go for the R35 version.....

That is to say if the 8" F6 is R35 version, it should perform like F6 X 1.35 = F8.1, and if this is correct, then it should also perform near to the F8.8 std version that I am having now....
And 200X power shouldn't be a problem......

Similarly, if one is thinking of stopping down the 8" F6 to 6" aperture, which will make the scope become F8....
And the planetary view will perform like an 6" F8, I think is not much difference....
Simply when I received my Istar 8" F8.8, I did a lot of trial and error testing to it, by stopping it down to 6" F12...., with a cut-out mask....
And in the end, why I still use full aperture, coz the resolution of the telescope still depends a lot on the aperture....

i hope this little advice may help whoever going for bigger aperture to consider the R35 version....
And I do not regret getting the 8" IStar F8.8 earlier, without the R35, simply the scope is performing to my expectation.....
And I just want to know when the raycorr is out, what will my scope become....

REgards

James Ling

[ivan86]

Hi james,

I have not see the 204mm R35 f/6 in the website but only f/8 version, exist?

[jamesling1000]

Hi Ivan....

Ales can always design and build to the specification that you require...

Only delivery lead time may be the constraint, to build specially for you ...

And the best is more than one person is interested in this specification, he will be able to give a better pricing...., and start the build ....

But Ales is still the best person to answer your question on their plan to build 204 F6 R35 version.....

Regards

James Ling