"Fast" Telescopes

[Bostonteax]

I have a SW F4 500mm Refractor and I have read (on here I think) that because my scope is "fast" I may need to buy more expensive eyepieces. What does this mean, and how does one define expensive?  I've just bought Celestron Omni 32mm and it seems very good. Now thinking about a 8mm or 5mm.  Need to make sure I get something suitable for scope. Any advice would be great.

Thanks

 

 

 

t

[Daniel-K]

not really you just a coma corrector 

[todd8137]

Hi and welcome from me , great Qs take a look at this and input you scopes  details 

http://www.skyandtelescope.com/observing/celestial-objects-to-watch/skyandtelescope-coms-scope-calculator/

pat

[laudropb]

This just means that a fast scope is usually more demanding on the eyepieces you use, especially on shorter focal length eye pieces. With cheaper eyepieces you will find that the image becomes distorted towards the edges of the field of view. Eye pieces vary in price from just a few 10s of pounds to many hundreds of pounds. You would be better to avoid the very cheap ones.

[RikM]

The 'F4' is what defines your telescope as 'fast', although I'm not aware of any SW f/4 refractors  The F ratio is found by dividing the focal length (500mm) by the aperture, which I suspect in your telescope is 102mm. That gives f/4.9. There are no actual agreed definitions for this but 'fast' is generally anything below about f/6, medium is f/6 to about f/8 and 'slow' is anything higher than that. The terms are carry overs from photography and related to the shutter speed needed to correctly expose an image at each focal ration. It is still a valid meaning for astrophotography, but in observing there is no direct significance to the terms.

What is important for observing are the consequences of the focal ratio. A 'fast' focal ratio, short fat telescope, produces a light cone with a steeper angle, so any eyepiece has to 'work harder' to form an image. Longer / slower focal ratio telescopes produce light cones with smaller angles, light rays closer to parallel, so there is less difference between the edge and centre of the image. The eyepiece doesn't have to be so well corrected at the edge.

As you might have assumed from the above, it is the edge that suffers most in fast telescopes, and the wider the apparent field of view of the eyepiece, the more exaggerated this effect. The stars at the edge of the field will look like seagulls rather than pinpoints. Eyepieces can be made to correct this effect, but it needs complex designs, very accurate machining and exotic types of glass. That costs money.

You don't have to spend a fortune if you don't want a superwide apparent field of view (the size of the circle of sky you see in the eyepiece). As you say, your Plossl gives a satisfying view. That will have an aFOV of about 50°. Some eyepiece give 70-85° or even more and these eyepieces start to get expensive if you want them to work well in a 'fast' telescope. You can spend £40 and expect to see some funny star shapes at the edge, or you can spend £200+ and get nice sharp stars accord the whole field. There is of course a whole spectrum of eyepieces in between and you may or may not be bothered by the effects on the star shapes. It is entirely up to you how you want to approach it. 

You can get really excellent views using Plossl eyepieces from reputable manufacturers.

[ollypenrice]

We have a large F4.1 Newtonian and find that the difference between premium and basic EPs is enormous. This is particularly important in the case of widefields where edge of field distortions are unpleasant in Plossls whereas in TeleVue Ethos, Nagler and Radian things seem fine. I've never felt the need for a coma corrector and far more experienced visual observers than I have always said that they didn't, either.

In our slower optical systems (F10 SCT, F7 refractors) the 'TeleVue advanatge' is vastly less imortant.

I dare say, though, that off axis performance is not very important if you are buying an EP for high power planetary observing. In a Dob it can be nice to have a good clean wide field, though, because you do less nudging.

Olly

[Bostonteax]

RiKM thanks- I think it's a F5 scope. That might explain your head scratching ! Thanks to all for comments . Can't afford to spend a lot of money on eye pieces . BST star guide get great reviews so may opt for 1 of those .

[triton1]

Yup I agree with the above for the money you can't go wrong with a BST starguider I still have one

[Stu]

23 hours ago, Daniel-K said:

not really you just a coma corrector 

A coma corrector corrects only coma, not the other abberations you get with eyepieces not designed for fast scopes.