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Advice please re: "fast" and "slow" telescopes


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I have a Skywatcher  127 SynScan AZ GOTO with a set of BST Starguiders eyepieces, 8mm, 12mm, 15mm and 18mm

I am basically new to astronomy although I bought the scope in 2015

I'm trying to educate myself as much as I can but at times things can get too technical

I often read about EPs or diagonals in reference to "slow" or "fast" scopes and it's not clear to me what that means and if my mak is fast or slow.

Thank you!

  

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I asked the same last year , it the F  number tells you the speed F6 and lower are fast scopes draw in more light quickly F9 to F6 medium to fast  F9 and above  F15 are slow , there is a lot more to it but that's basic was told for a really fast scope you need fast eyepiece or the image is poor at the edge of FOV  . I hope that's right but some one tell you better than I can 

You're I think is F11 

Edited by Neil H
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  •     Diameter of Primary Mirror: 127mm
  •     Telescope Focal Length: 1500mm (f/11.8)

Your telescope, at f/11.8 would be considered a slow scope. A telescope with the same aperture (127mm) but a shorter focal length - say 600mm - would be an f/4.7. That would be a "fast" scope. 

In this example, where the only change is the focal length of the scope, using the same eyepiece, the faster scope will provide a much wider field of view and a much smaller exit pupil. The magnification of the image will also be greater in the faster scope. So what's the advantage of your slower scope? It is much more tolerant of eyepiece construction. You can use eyepieces that cost significantly less than the premium "green letter" EP's. A slower doublet does not show color aberration as readily as a faster doublet and costs much less to build than a similar size triplet.

Your narrow field scope is well suited for planetary work.

Edited by wrvond
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It's a photography term originally; faster meaning lower focal length to aperture ratio, slower being higher ratio. It basically means that lower f- number makes a brighter image than a slower or longer lens (or mirror), which results in faster shutter speeds.

Another way to look at it is that faster makes a smaller image at point of focus and compresses the available light tighter. Slower optics magnify the image, or spread out the light, making it dimmer.

Eyepieces are often suited or designed for faster or slower optics. For instance, the much maligned Huygens is an atrocious performer in say a fast dobsonian at f4, but well be more tolerable in a long focus f15 refractor. 

I hope that kind of helps.

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The descriptions fast and slow refer to the focal ratio of the telescope, which we can find by:

focal ratio = focal length / aperture

These days we might say that f5 or lower is fast and f8 or higher is slow, so your maksutov is definitely slow. This is good news for you as it means most eyepieces will be "well corrected" when used with your telescope. A good example of this is your 18mm Starguider. In your telescope, a star will be a nice point of light in both the middle of the field of view and at the edge of the field, whereas used with my f6 telescope a star is a nice point of light in the middle of the field, but becomes more of a large D shape as the star is moved to the edge. The reason for this difference can be understood if we consider how the light is bent as it comes into the telescope. Light coming into the telescope all across the lens is bent so that it all meets at the focal point, as shown in the image below:

spacer.png

We can see in the images above that the angle of the light cone at the focal point (on the right) is much larger in the fast telescope example. It is much more difficult to correct this steep angle and this is why some eyepieces perform better in slow scopes than in fast ones.

Historically, the terms are taken from photography, where lenses are described by their focal length. In this case the focal ratio gives the size of the aperture when you know the focal ratio, so if you have a 200mm photographic lens, a ratio of 1/f5 is an aperture of 40mm and a ratio of 1/f10 means it has an aperture of 20mm. You can see that the 1/f5 lens, with its aperture of 40mm, must let in more light than the 1/f10 and so it can take a picture faster, hence the term "fast".

However, when we talk about telescopes we define them by their aperture, so a 200mm telescope that is f5 has a focal length of 1000mm and a 200mm telescope with a focal ratio of f10 has a focal length of 2000mm. In this case both telescopes have the same aperture, and so neither telescope gathers more light than the other, they just have different image scales photographically or magnifications when used with a specific eyepiece. The photographic meanings don't really apply here, but we still use the terms as a useful shorthand to group telescopes according to their focal ratio.

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Thank you Neil H, wrvond, reezeh and Ricochet for your very helpful replies.

It all makes much more sense now! 

It was particularly helpful to learn why some EPs are more suited to slow scopes like mine and some to faster ones; thanks rezeeh and Ricochet for pointing that out!

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The focal ratio as mentioned comes from the photographic world. It does have some relevance to scopes and eyepieces however>

To get a value you take the focal length and divide that by the aperture. So a 600mm focal length scope with an 80mm aperture is 600/80 = 7.5. The focal ratio therefore being f/7.5 It is possibly a strange depictation of the value, it always looks inverted to me.

Ignoring that that is how to get the value. Yours is 127mm aperture and around 1500mm focal length so you have 1500/127 = 11.8 or f/11.8.

Fast seems to be f/6 and numerically lesser so f/5 is "faster" then f/6 - like a lot it is backwards or inverted or something.

Medium is around f/8 to f/6.

Slow being in a general sense f/8 and what is often termed "upwards" as in f/10, f/12 etc.

The relevance is that a "faster" scope is usually more difficult to make, either refractor or reflector. More difficult means greater cost owing to time. And for an eyepiece a fast scope is harder for them to reproduce a good image. As "fast" usually means the focal length is shorter then the curvature at the image plane is greater - images created are not flat. The greater the curvature the more difficult it is for an eyepiece to handle. Again a good eyepiece costs more.

The inverse being that most eyepieces will operate well in a slow scope will, so you do not need to go searching out £300 eyepieces.

In the old days of photography the almost standard mid setting for a camera was f/8, it managed to cover the majority of situations, good depth of field, good depth of focus and sensible shutter speeds. It is likely that in a scope f/8 is probably the still the mid ground where the scope will do the majority of situations reasonably well.

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Just stumbled upon this  thread  and  it  is great! All I really knew  before  was  how to calculate  f ratio  and  that  a fast  scope (such as   my 4.7 Dob) demands  higher quality eyepieces  but  I never  really understood why.  These explanations plus  the  easy to grasp diagrams make  it so much easier to grasp. I think I finally get  it!  Thanks  to all of  you for  sharing  your  expertise.   SGL never ceases to impress me!

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