Parabolic or not?

[Craig20264]

Comparing the Sky-Watcher 130 and its brother the 130P.

The 130P whilst being more expensive and coming with fewer accessories boasts a Parabolic Primary Mirror.

Does this addition warrant the sixty or so pounds extra it's price tag demands, and what differance, if any, will I notice as a beginner in image quality.

Cheers

Craig

[Shibby]

Hi Craig,

Spherical mirrors are much cheaper to manufacture, however they tend to produce less sharp images, however I can't tell you how noticeable this would be on the relatively small aperture of a 130mm. I think it's more of an issue on larger apertures, but I have no experience of this to relate!

[brantuk]

I don't know the maths but I understand the parabolic mirror focuses the light better at a single point across all wavelengths. I guess that's what we are all trying to acheive with optics when we upgrade scopes and eyepieces. Depends what price we're prepared to put on it really.

Personally - I would pay the difference - but not on a 130 - maybe a 150 and upwards I would

[swamp thing]

Whether a mirror can get away with being spherical or needs to be parabolic is linked to the aperture and focal ratio.

The faster the optical system the more the effects of spherical aberration are noticed.

As aperture increases the longer the focal ratio of a spherical mirror has to be to avoid Spherical aberration.

At 5" aperture I believe focal ratio needs to be around f/10 to avoid the effects.

I personally would always avoid a spherical mirror if possible.

Regards Steve

[barkis]

A six inch mirror of sufficient focal length, may perform quite well with a sperical mirror. However, ythe shorter the focal length, or in other words the faster the mirror with regard to Imaging, the mirror must figured into a paraboloid, otherwise it will not be capable of bring all the light rays from an object at infinity to a true focus if left as a sphere.

A paraboloidal mirror, is one where successive zones of it's surface, have increasingly longer focus points out from it's centre to it's edge zone. The difference is quite small, but Oh so important.

Ron.

[Craig20264]

Whether a mirror can get away with being spherical or needs to be parabolic is linked to the aperture and focal ratio.

The faster the optical system the more the effects of spherical aberration are noticed.

As aperture increases the longer the focal ratio of a spherical mirror has to be to avoid Spherical aberration.

At 5" aperture I believe focal ratio needs to be around f/10 to avoid the effects.

I personally would always avoid a spherical mirror if possible.

Regards Steve

The Sky-watcher 130 has a focal length of 900 giving it a Focal Ratio of f/6.92

Would you consider this unacceptable even on a scope at the lower end of the market?

[umadog]

An f/7 focal ratio is not particularly large, even though it appears so by modern standards. I admit I haven't looked through one of these spherical mirrored scopes, but I wouldn't buy one on general principles: it's like knowingly buying a telescope with bad optics.

Figure: Parabolic shape eliminates spherical aberration

[hemihaggis]

go for the pds 200

[Capricorn]

At f/6.9 I would avoid a spherical.

It may well be passable and cause you no real problems. However one aspect is if you keep in this hobby if you come to sell it then it may be a bit of a problem. Even selling one to make space for a bigger scope has to be considered. I would say a 130P would be a saleable item whereas a 130 Spherical may be a bit more of a problem.

Will temper this with all manufacturers will make the focal length as small as they can get away with and so balance out aberations in the system against convenience, in this case convenience is a short scope.

A parabolic is better but will require more collimation, it being shorter. You cannot win. :evil6:

If the intention is to buy, use, learn and give it away:eek: then perhaps the spherical should be considered.

If this is a scope you intend to keep for some time then consider a greater outlay and the 150PL. At 150mm you will have a scope more capable and one that will last you quite some time (years). Problem now is I am saying wait, save and get something else, which isn't really fair.

Have you checked the used market?

Search for: UK astro buy and sell

You may find a suitable scope on there, usually sold when some has bought a scope with the intention of dojg astronomy or when they simply want a bigger scope.

[swamp thing]

The Sky-watcher 130 has a focal length of 900 giving it a Focal Ratio of f/6.92

Would you consider this unacceptable even on a scope at the lower end of the market?

If this model has a spherical mirror, I would stay well clear.

A 5" at f/7 is not slow enough to avoid the effects of spherical aberration.

The images it produces will be unacceptable.

This is a case of wait and save until you can afford one with a parabolic primary.

Regards Steve

[Craig20264]

Many thanks for your honest answers guys, it's better to have it warts and all, at least I can now make a more informed decision, taking into consideration the old equation Budget/Time = Impatience

Two ways of looking at it,

£135 gets me out there while I save for the next scope, and in all honesty is probably good enough for someone at my level of experience.

£200 pinches £60 off my next scope but gives me a better image, which at the end of the day is what we all strive for.

Think I'll go and lie down in a darkened room