Why are short focul length scopes “hard on eyepieces”

[Mike_S]

Hi,

can anybody explain to me what this actually means please? I understand how to calculate magnification and how useful magnification is deterred by aperture and seeing conditions but is there another problem I am missing with short focul length scopes of both refractor and reflector type? Is it to do with field of view? Exit pupil? I know how to calculate that as well though. I’m really not sure what people mean when they say they’re “hard on eyepieces”. 

[Sunshine]

I think what the term refers to is the fact that with a scope of short focal length, you need eyepieces of smaller focal lengths, to achieve a certain power. For example, my Mak has a focal length of 1800mm, very long, so for me to achieve let’s say 150x power I would use a very common 12mm eyepiece.

to achieve that same power in a scope of 700mm you would need a very short 4.5mm eyepiece, and even smaller again for 200x. 

I’m sure you’ll hear other opinions from users with short scopes.

[Knighty2112]

It’s all due to how steep the light curve is coming into the EP. With longer focal length scope the angle is less severe than with a shorter focal length telescope. With the shorter focal lengths the EP has to work harder to keep the image free from aberrations etc, whereas it it is easier in the longer focal length scopes due to the shallower light path. The respective light paths plotted out basically below for first the long focal length, and then the shorter focal length scope underneath it. The steeper light path in the shorter focal length scope is easily apparent. HTH.  

[Mike_S]

Thanks. I’ve also read people saying that some large aperture, shorter focul length dobsonians (think it was F4.7 compared to F5.9) are not for beginners due to the fast nature which I don’t really understand either? Is this to do with eyepieces or other issues?

[Demonperformer]

So does that mean that, for the same area, the same eyepiece would work equally well on the short focal length scope?

[SIDO]

For the shorter focal length scope the mirror or lens has a steeper curve, few but not all cheaper eyepieces do well with the short focal lengths because over time it has not been their primary design criteria to deal with this steep curve but to use these specific designs in long focal length telescopes. Were as some eyepiece manufacturers have specifically designed eyepieces that work much better with short focal lengths and of course because they have a broader range of use as far as focal length they are more expensive. 40 years ago these sorts of short focal lengths were very much unheard of and this transition to more expensive eyepieces reflects their current popularity. Short focal lengths have become popular because of the introduction of dobsonian telescopes and the later introduction of Ed-Triplets and other apochromatic refractors wich in most cases are mostly short focal length scopes as well.  So imho yes Knighty2112 is correct ?

[Stu]

There might be two different things wrapped up in your question here.

I think Gus’s diagrams address focal ratio as much as focal length, because both vary in his example. To keep focal ratio the same in the diagrams, the aperture of the second one would have to reduce so that the light cones were of the same steepness.

The only effect that I know of which specifically affects scopes with short focal lengths (irrespective of speed/focal ratio) is field curvature. This is where the centre and edges of the field of view in the eyepiece tend to focus at different points ie you can’t get both centre and edges focussed at the same time. It is much more apparent with longer focal length eyepieces and with those with large apparent fields of view. Some people can accommodate this with their eyes, others can find it a problem, and quite annoying. In my experience it is short focal length refractors which show field curvature the most, doublets seem to be worse than triplets I think.

Focal ratio relates to the steepness of the light cone, being the relation between aperture and focal length. Note that you can have a large aperture scope with a long focal length which is very fast eg a 32” f2.5 scope is very fast but still has a focal length of 2032mm so should not suffer from field curvature. It will however show a lot of coma which will need a coma corrector, plus be very tough of eyepieces, particularly widefield ones where they will need to be very well corrected and designed to work in fast scopes. With a very fast scope, the eyepiece has to turn the light through much steeper angles and this can lead to the edges of the field of view showing a number of artefacts like astigmatism mean stars do not appear as points, but rather like seagulls.

So, faster scopes are harder on eyepieces in terms of edges correction, and short focal length scopes tend to show field curvature, which can actually either be improved or made worse depending on the eyepiece used.

[SIDO]

In an 8" F6 dobsonian with a 1200mm focal length a long focal length is exibited with a short focal ratio, so it is the ratio and not the length. 

[Stu]

10 minutes ago, SIDO said:

In an 8" F6 dobsonian with a 1200mm focal length a long focal length is exibited with a short focal ratio, so it is the ratio and not the length. 

What is the focal ratio and not the length? An f6 8” is neither fast nor short focal length so I am unclear of your point?

[alacant]

Hi

I think it's best to do a side by side comparison looking at stars. Take an affordable eyepiece in my 6" f8 and the views are superb. Now put it in a 6" f5 and compare; rubbish. If however you only ever look through the latter, you don't realise, so it doesn't matter (end of worst explanation ever!). But... Be fair. Compare only telescopes with equal apertures;)

[SIDO]

11 minutes ago, Stu said:

What is the focal ratio and not the length? An f6 8” is neither fast nor short focal length so I am unclear of your point?

I guess I consider f6 fast as opposed to f13.8 and 1200mm long as in comparison to a 500mm Newtonian with an f3.9 fr but more over realized the mistake made in my previous post and was agreeing with you. Isent f3.9 the bottom end of fast though unless we are talking camera lenses?

[Ben the Ignorant]

It's not the focal length as much as it's the focal ratio. Take a 100mm scope with a 1000mm focal length, the ratio is f/10, and the light rays from the edge are almost parallel to the light rays from the center; the eyepiece has to do almost the same job for both.

Now take a 100mm scope with a 500mm focal length, f/5. The light from the edge of the objective makes a large angle relative to the light that passes at the center. The eyepiece has to focus light from different directions at the same spot, that's what makes its job hard.

Eyepieces with more lenses, aspheric lenses and/or special glass can do it but they cost more.

Look up spot diagrams from objectives with different f/ratios, you'll see how star images are larger and less round at the edge with shorter f/ratios. They tend to go with shorter focal length in common telescopes but an f/4 900mm diameter scope has a 3600mm focal length, which is very long, however it has a "short" f/4 ratio.

And a 30mm f/10 scope has a very short 300mm focal length, but its f/10 cone of light is not hard on eyepieces, light rays are nearly parallel.

[Stu]

1 minute ago, SIDO said:

I guess I consider f6 fast as opposed to f13.8 and 1200mm long as in comparison to a 500mm Newtonian with an f3.9 fr but more over realized the mistake made in my previous post and was agreeing with you. Isent f3.9 the bottom end of fast though unless we are talking camera lenses?

There are some crazy big and fast scopes in the US mainly such as the quoted 32" (I think) f2.5 dob. The longest and slowest I've used was an f20 man with a focal length of 4000mm. So, let's say the range is roughly f2.5 to f20. Most people consider around f6 to f7 as neither fast nor slow. Below that is often considered fast, above it, slow.

My point was that short focal length, not fast focal ratio can result in field curvature. Secondly, fast focal ratios, not short focal lengths can result in edge correction issues with eyepieces not designed for the job.

[Ricochet]

4 hours ago, Knighty2112 said:

It’s all due to how steep the light curve is coming into the EP. With longer focal length scope the angle is less severe than with a shorter focal length telescope. With the shorter focal lengths the EP has to work harder to keep the image free from aberrations etc, whereas it it is easier in the longer focal length scopes due to the shallower light path. The respective light paths plotted out basically below for first the long focal length, and then the shorter focal length scope underneath it. The steeper light path in the shorter focal length scope is easily apparent. HTH.  

I like your explanation, but in your diagram the label "field of view" is incorrect. For each diagram the two light rays shown represent only the very centre of the image. The divergence of the rays after the focal point is simply where the image is no longer in focus. 

[SIDO]

1 hour ago, Stu said:

There are some crazy big and fast scopes in the US mainly such as the quoted 32" (I think) f2.5 dob. The longest and slowest I've used was an f20 man with a focal length of 4000mm. So, let's say the range is roughly f2.5 to f20. Most people consider around f6 to f7 as neither fast nor slow. Below that is often considered fast, above it, slow.

My point was that short focal length, not fast focal ratio can result in field curvature. Secondly, fast focal ratios, not short focal lengths can result in edge correction issues with eyepieces not designed for the job.

You hadda mention Crazy Big Scopes, this was on craigslist last month for $500 but you'll need a flatbed. The mirror is only 12 inches, the ad should be on the best of craigslist for astronomers if there was such a thing. At that unstated but apparent focal length and that short ladder I think it qualifies.

[Stu]

That looks quite slow, long focal length. This is more what is was thinking, very fast and large aperture/long focal length.

https://www.astromart.com/reviews/telescopes/newts/show/observing-with-webster-28-f27

[SIDO]

10 hours ago, Stu said:

That looks quite slow, long focal length. This is more what is was thinking, very fast and large aperture/long focal length.

https://www.astromart.com/reviews/telescopes/newts/show/observing-with-webster-28-f27

Awesome Stu, Thanks for sharing!

A 28" telescope with a 70" eyepiece

height at zenith?

[Waldemar]

I think the problem lies in the steepness of the lightcone, in other words the angle of entrance of the light in the eyepiece. So it is not just the focal length or aperture, but the focal ratio, the f value. As soon as you get below a certain ratio, say f4, the lightcone gets too steep for 'normal' ep's to cope with, because the lightpath through the glass in the centre will be a lot shorter than the lightpath through the glass at the edges.

I encounter the same with imaging with my RASA (f 2.2) when using NB filters, the shift in length of lightpath over the filter because of the very steep lightcone changes the FWHM so much, that my filters don't work properly anymore. That is why Baader produced a special set of filters for fast telescopes (< f3)

Exactly the same goes for ep's in fast telescopes, but because the lightpath through ep's is a lot longer than through filters, it will happen a bit sooner i.e. f4.5