Vendor eyepiece description

[Skinnypuppy71]

Just a wee question..when a vendor describes a eyepiece as suitable for scopes with a aperture of f6 and above....would that be faster or slower...in my mind as a amateur photographer above f6 would be faster...f5, f4 etc...or is the vendor using the actual numbers in ascending order (f8,f9,etc)for the average Joe whom might not know a "f" stop from a bus stop.lol.

[John]

With scopes, F/5 is considered "faster" than F/6 , F/9, F/15 etc.

 

 

 

[Star Struck]

I would ask the vendor to explain what they mean by that statement.

No such thing as a stupid question.

[Skinnypuppy71]

29 minutes ago, John said:

With scopes, F/5 is considered "faster" than F/6 , F/9, F/15 etc.

As is the same with camera lenses...

But when a seller says f6 and above.....faster or slower?   Numbers above f6 in ascending order will be slower...but in camera and lens terms above f6 would be referring to a faster lens....eg f2.8 if you get my drift.

 

 

 

 

[Ricochet]

Does the vendor use the word aperture or have you added that word? "f6" describes the focal ratio of the telescope or camera lens, not the aperture. I think the confusion is from having used camera lenses, which have a fixed focal length and an adjustable aperture. Adjusting the aperture also changes the focal ratio, as the focal ratio is focal length/aperture. For example if you have a 35mm camera lens with a 5mm aperture, then you have a focal ratio of f7. If you then increase the aperture to 7mm the focal ratio becomes f5, so the aperture has increased, but the focal ratio has decreased. So while the focal ratio of the camera lens has changed, it has only changed indirectly, because what is actively changed is the aperture (measured in mm), even if the camera scales are only marked as the ratio to the focal length.

With regards to eyepieces, if the phrase used is "works with telescopes f6 or above", it means telescopes with a "slower" focal ratio, that is f7,8,9 etc. The focal ratio describes the angle of light coming from the objective that the eyepiece must correct so that your eye can generate an image. The steeper that cone of light is, the more difficult it is to correct. That means that an eyepiece which is suitable for f5 telescopes will always also be suitable for f10 telescopes, but an eyepiece suitable for f10 telescopes may not be suitable for f5 telescopes.

Edited May 29, 2020 by Ricochet

[Skinnypuppy71]

That's what I needed to know..thanks Ricochet...and yes the seller flo did describe it as a focal ratio up to f6 and above...so  I've now bought a eyepiece that might not be suitable for my William optics z73..although it's not much out @f5.9.

Edited May 29, 2020 by Skinnypuppy71

[RT65CB-SWL]

Hi @Skinnypuppy71 and a belated 'Hi and welcome to SGL.'. 

+1 for the above by @Ricochet. 

Just to add, that is why catadioptrics, i.e. Mak's and SCT's have a 'slow' f/ratio, because the light beam is folded a few times into a short tube and a FF/FR helps to 'speed' them them up a bit if you are into astro-imaging.

[Skinnypuppy71]

Hi Philip...I've actually been a member for a couple of years now...but have been away from astro since selling my 150pds...but purchasing some bins last month and  getting some new gear in the next week or so is getting me back into it again.

[RT65CB-SWL]

Ok! '...welcome back' 😁

[Ricochet]

32 minutes ago, Skinnypuppy71 said:

That's what I needed to know..thanks Ricochet...and yes the seller flo did describe it as a focal ratio up to f6 and above...so  I've now bought a eyepiece that might not be suitable for my William optics z73..although it's not much out @f5.9.

If it is described as f6+ and your scope is f5.9 I wouldn't worry about it as the difference will be so small as to not matter.

[John]

Virtually all modern eyepieces are sharp in the central 50% of the field of view no matter how "fast" or "slow" the scope is.

It is in the outer 50% of the field that the distortions can start and the extent of those depend on how well corrected the eyepiece is and how "fast" the focal ratio of the scope is. Some of the distortion is generated by the scope optics and some by the eyepiece.

From my experience of using a wide range of different eyepieces in a wide range of scopes I would say that some of the vendors claims are a little optimistic so should be taken as a general guide rather than a precise forecast.

The difference between F/5.9 and F/6 is not significant with this in mind.

 

[Skinnypuppy71]

Ok thanks John..I didn't think the difference between f5.9 and f6 would matter..in the camera world it would be next to no difference in the brightness of the image . cheers for the help guy's it's much appreciated....meanwhile this is my wee list of goodies on order can't wait..I'm  afraid as soon as it gets here the sky's in currently sunny Scotland will have changed into the usual rain and cloud.....that's the way it goes eh...2 weeks of free clouds with a new scope.lol.

[Timebandit]

25 minutes ago, Skinnypuppy71 said:

Ok thanks John..I didn't think the difference between f5.9 and f6 would matter..in the camera world it would be next to no difference in the brightness of the image . cheers for the help guy's it's much appreciated....meanwhile this is my wee list of goodies on order can't wait..I'm  afraid as soon as it gets here the sky's in currently sunny Scotland will have changed into the usual rain and cloud.....that's the way it goes eh...2 weeks of free clouds with a new scope.lol.

 

 

You certainty are not doing things by half's ,are you ☺. That is going to be a large box of goodies to sort through. I can see lots more questions coming. Welcome to SGL , second time round😉🔭 

 

 

[Guest]

1 hour ago, Skinnypuppy71 said:

Hi Philip...I've actually been a member for a couple of years now...but have been away from astro since selling my 150pds...but purchasing some bins last month and  getting some new gear in the next week or so is getting me back into it again.

Welcome back... Hope you're getting some decent viewing weather in Ayrshire.😉

[Skinnypuppy71]

Weather is lovely today..about 24 Celsius on the west coast.

[Cornelius Varley]

1 hour ago, Skinnypuppy71 said:

Ok thanks John..I didn't think the difference between f5.9 and f6 would matter..in the camera world it would be next to no difference in the brightness of the image . cheers for the help guy's it's much appreciated....meanwhile this is my wee list of goodies on order can't wait..I'm  afraid as soon as it gets here the sky's in currently sunny Scotland will have changed into the usual rain and cloud.....that's the way it goes eh...2 weeks of free clouds with a new scope.lol.

Unless you are planning to use the telescope for terrestrial viewing as well as astronomy most of us would probably choose a 90degree diagonal instead of the 45 erecting prism diagonal.

[John]

9 minutes ago, Cornelius Varley said:

Unless you are planning to use the telescope for terrestrial viewing as well as astronomy most of us would probably choose a 90degree diagonal instead of the 45 erecting prism diagonal.

Good advice there.

 

 

[Skinnypuppy71]

Well we do have some badgers and a ospreys nest in the Galloway forest area..I'm not too far from.

[Skinnypuppy71]

I know the WO z73 won't be great for observing...but it'll do until I can put away the cash for a newt of some sort for that purpose...I thought I'd get the best apo I could afford at the moment for imaging  as opposed to try buying something of lesser quality and getting a newt at the same time.

Edited May 29, 2020 by Skinnypuppy71

[Guest]

4 hours ago, Skinnypuppy71 said:

Weather is lovely today..about 24 Celsius on the west coast.

It's is indeed, fae Sunny Saltcoats. 😉

[Skinnypuppy71]

Just doon  the road fae ye then.

[Guest]

1 hour ago, Skinnypuppy71 said:

Just doon  the road fae ye then.

Village Of The Damned aka Stevenston? 🤔🤣

[Skinnypuppy71]

Erm no....A wee bit further south ..lol..Ayr.

Edited May 29, 2020 by Skinnypuppy71

[Tenor Viol]

I know I've had to explain to photographers moving to astronomy that astronomers and photographers describe optical system in different ways. Photographers when describing a lens will use focal length (e.g. 105mm) and aperture (e.g. f/2.8). Astronomers use diameter of primary objective lens/mirror (e.g. 100mm or say 10") and focal ratio (e.g. f/6.3)