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Eyepieces and focal lengths - differences?


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Hello again,

at the risk of hijacking my own previous thread.... I thought I should start a new one. I had reasonably disappointing views of Saturn last night - scope is new and also just bought a lovely zoom eyepiece from @recceranger (long story, rick I might pm you again about that (turns out I quite like that eyepiece!!)), but on trying to diagnose my poor experience (which, granted, is like asking my dog to diagnose why my washing machine doesn't always spin at the speed it says on the dial) I began to muse comments I've read about Maks like mine being more forgiving on budget eyepieces than, say small doublets.

my understanding (correct me if I'm wrong) is that, the longer the f ratio, the more tolerant it is of lower quality eyepieces? Is that right? And if so... how does that work??

niall 

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Correct! 

Think of light coming to focus in a system with a large aperture and short focal length (i.e. fast).  The "cone" drawn out has a large angle at the apex.

Now imagine a narrower aperture and a longer FL - the cone angle is much smaller, i.e. the instrument is slower (bigger focal ratio).

In simple terms, distortions are more likely in the former case because rays reaching the EP are at higher angles to the principal axis (main line running straight through the optics).  When rays are closer to being parallel to the principal axis (latter case), the distortions are reduced. 

HTH.

Doug.

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Your reasoning is correct eg: an F/10 scope is less demanding on eyepieces than an F/5 one. The reason is due to the steep angles of the light cone that the "faster" scope produces which then place more demands on the optics within the eyepiece to bring to a focus with minimal aberrations.

The centre of the field of view is usually pretty good in most scope / eyepiece combinations but it's as the field moves away from the central optical axis that the issues become gradually more apparent. With the less steep light cone of the slower scope the less expensive and sophisticated eyepiece designs can bring the image to focus largely without much in the way of aberrations.

Edit: Doug got there 1st !

 

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Hi Niall, faster scopes (ie f/5 and above) have steeper light curves, so are not so tolerent of ep's that are not adjusted to compensate for this.  Maks are considered as 'slow' ie f/10 and slower, therefore are more tolerent of cheaper ep's.  You should have no trouble with any ep's for your scope, it is tolerent of all types of quality.  Your understanding of longer focal ratio's is spot on. 
 

John & Doug, I thought I was first but can't type quick enough!

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Hi Niall. I have some EP's that don't perform well in my f5 short 'frac, but much better in all my other scopes with a higher f ratio. As all have explained this is down to mostly the angle the light comes into the lens, so the narrower the light path into the EP the better the EP should perform. I have a 2' 25mm ES EP that I was trying to sell a few months back due to poor performance in some of the lower f ratio scopes I was using back then. Now as most of the scopes I mainly use are f8 or above it performs a whole lot better,  so keeping the EP now for good. Your f11 mak should perform very well with most EP's, however the more expensive ones should still provide a better views over plossls etc if you are willing to shell out the extra cash. 

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The one that tends to get overlooked or ignored or forgotten is that the focal plane of the lens or mirror is not flat:

So any eyepiece hs to take that into account (or not) in order for a reasonable final image. On a slow scope that is less then for a fast scope.

FP.PNG

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  • 4 weeks later...
On 6/18/2017 at 06:50, cloudsweeper said:

Correct! 

Think of light coming to focus in a system with a large aperture and short focal length (i.e. fast).  The "cone" drawn out has a large angle at the apex.

Now imagine a narrower aperture and a longer FL - the cone angle is much smaller, i.e. the instrument is slower (bigger focal ratio).

In simple terms, distortions are more likely in the former case because rays reaching the EP are at higher angles to the principal axis (main line running straight through the optics).  When rays are closer to being parallel to the principal axis (latter case), the distortions are reduced. 

HTH.

Doug.

Haha Your Simple terms explanation went right over my head.... At  least at first :). Had to reread it slowly a couple times lol. So if i may do a little hi-jacking, I own a 12" lightbridge with f/5 focal ratio, what should i be looking for in terms of hard specs (Or brand names or whatever) when im ready to buy another eye piece?

Scope came with 4 eye pieces, but only the 26mm 2" that originally came with scope looks very clear/good. The others, 12 and 9mm (1.25") look horrible, but seller admited they were laying around his home and he just threw them in to sweeten the deal. Both say meade on them.

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4 hours ago, Prester John said:

Haha Your Simple terms explanation went right over my head.... At  least at first :). Had to reread it slowly a couple times lol. So if i may do a little hi-jacking, I own a 12" lightbridge with f/5 focal ratio, what should i be looking for in terms of hard specs (Or brand names or whatever) when im ready to buy another eye piece?

It depends on your budget but at f5 you need something quite well corrected. In terms of eyepiece lines I would look at:

  • BST Starguider or Celestron X-Cel LX 
  • Explore Scientific 68°, 82°, 92°, 100° or 120° 
  • Vixen SLV or SSW
  • Baader Mopheus
  • Pentax XW
  • Any Televue eyepiece

I'm a great believer in the concept of choosing eyepieces according to exit pupil and having a minimum spacing of 1.4x focal length between eyepieces so I suggest you read the great thread linked below.

Personally, I currently prefer to choose an exit pupil of 2.4mm over the standard suggestion of 2mm for DSOs and so I would suggest that your first eyepiece purchase should be in the 10-12mm range (10/5=2, 12/5=2.4). 

For planetary viewing your scope will reach its limits with an eyepiece in the 4-5mm region but the atmosphere is likely to get in the way before that. In the UK we typically advise ~200x as the atmospheric limit but this may differ for your location. 200x will be achieved in the lightbridge by using an eyepiece with a focal length of (1500/200 =) 7.5mm. Given that the atmosphere plays such a part in determining the best focal length to use many people choose a zoom eyepiece for planetary in order to dial in the best view for a particular night.

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1 hour ago, Ricochet said:

It depends on your budget but at f5 you need something quite well corrected. In terms of eyepiece lines I would look at:

  • BST Starguider or Celestron X-Cel LX 
  • Explore Scientific 68°, 82°, 92°, 100° or 120° 
  • Vixen SLV or SSW
  • Baader Mopheus
  • Pentax XW
  • Any Televue eyepiece

I'm a great believer in the concept of choosing eyepieces according to exit pupil and having a minimum spacing of 1.4x focal length between eyepieces so I suggest you read the great thread linked below.

Personally, I currently prefer to choose an exit pupil of 2.4mm over the standard suggestion of 2mm for DSOs and so I would suggest that your first eyepiece purchase should be in the 10-12mm range (10/5=2, 12/5=2.4). 

For planetary viewing your scope will reach its limits with an eyepiece in the 4-5mm region but the atmosphere is likely to get in the way before that. In the UK we typically advise ~200x as the atmospheric limit but this may differ for your location. 200x will be achieved in the lightbridge by using an eyepiece with a focal length of (1500/200 =) 7.5mm. Given that the atmosphere plays such a part in determining the best focal length to use many people choose a zoom eyepiece for planetary in order to dial in the best view for a particular night.

Wow great post. Much appreciated. Exactly the type of answer i was looking for. Ill read through the thread you posted asap. Cheers

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