Eyepieces

[charmedkelly]

hey me again lol, well i just wondered something are there any lenses that give better detail then a 9.5mm

kelly

[ashenlight]

Hi Kelly

Do you have any barlows?

I own both a 2x and a 3x barlow, the 2x with double the magnification of the eyepiece you are using it with whilst the 3x will triple it. The 3x has proved a great purchase when it comes to observing planets, especially Saturn. Jupiter's low position atm means it suffers from atmospheric conditions and so can't take the higher mag the 3x barlows gives... but I'll be using it soon as it gets to a decent height

[charmedkelly]

Oh yeah i own a 2x barlow and a 3x barlow, the only reason i asked this question was because ive seen 7mm and 8mm and i just wondered if they gave better detail but im not too sure what do i know im only a amateur.

Kelly

[Doc]

What scope are you using Kelly is it your 150p

[charmedkelly]

well to be honest i use both my scopes,

dsos are much better with my dob but yeah i suppose u can give me ideas on both or even just the skywatcher

[The Warthog]

An eyepiece is part of an optical system that includes the objective, the secondary (if present) and the Credit Card company that you use to buy your stuff. With your f/5 scope, you would be well served by a 2x Barlow to double the power you are getting now, without overpowering the scope. A 7 or 7.5mm with that Barlow would give you all the power your scope can reasonaably handle. As your Dob is probably an f/6, you could use the same eps with it.

[Vulpecula]

The best "detail" you can usually get is by matching your eyepiece to the F-number of the scope.

(atmospheric conditions determining the upper magnification limits of course - no point trying to exceed what the sky will allow!)

So for an F6 scope - a 6mm eyepiece gives most detail, an F11 scope - an 11mm eyepiece and so on. This isn't magic, just a happy co-incidence.

You can get higher magnification than this, but things get very dark very quickly, and unless you've got very good optics (such as an APO or Maksutov scope) then very soon there's no more detail, just a bigger but blurry image.

Nice easy rule to remember :-)

[steve_bham]

The best "detail" you can usually get is by matching your eyepiece to the F-number of the scope.

(atmospheric conditions determining the upper magnification limits of course!)

So for an F6 scope - a 6mm eyepiece gives most detail, an F11 scope - an 11mm eyepiece and so on. This isn't magic, just a happy co-incidence.

You can get higher magnification than this, but things get very dark very quickly, and unless you've got very good optics (such as an APO or Maksutov scope) then there's no more detail, just a bigger but blurry image.

Nice easy rule to remember :-)

Ive got a F10 scope so in theory my 10mm would give me the most detail (which would make it 125x) but a 6mm (which is 208x) gives me much better magnified image and still very clear.

I thought the general rule of thumb was 50x for every inch of appature. (i.e. 5" scope would be 250x) with a maximum magnification of 300x on totally perfect skies.

[Vulpecula]

a 6mm (which is 208x) gives me much better magnified image and still very clear

"blurry" was perhaps too strong a word - with brighter objects you might still get a "clear" image - but the issue is whether there is more detail in the image - i.e. is anything new resolved in the image with increased magnification?

I thought the general rule of thumb was 50x for every inch of appature

Aaaarrgh!! pet peeve of mine!!

Telescopes are sold in mm these days, so why does everyone always revert to the "inch rule" for magnification? Surely "2x aperture in millimetres" is much easier to work out and remember? - it's the same answer (near enough)

Anyways - sorry - got distracted with mild rant the thing is, the 2xmm "rule" (or 50x" rule if you prefer ) is for the highest potential magnification, not for the best "detail".

1xmm (i.e. mm = F#) is the point from which the image brightness starts to drop off dramatically and the image starts to fall apart in terms of resolution of detail. How badly the image quality drops will vary enormously depending on the quality of the telescope optics.

[Vulpecula]

I've just noticed that what I've said is pretty much the same as brianb in response to a post of yours on July 1st: http://stargazerslounge.com/beginners-help-advice/82648-much-magnification.html

(except he uses a "25xinches" calculation instead of the simpler "eyepiece = FRatio" :-) )

[The Warthog]

It's also what I said in my article on ep selection. I favour about 2/3 of the theoretical maximum as the sweet spot for a scope's power. On a good night, or an average night when vieweing bright planets or the Moon, you might go all the way to the max.

[Harmartolos]

Hi Charmed Kelly,

I remember asking a question completely unrelated to this topic on here a little while ago and someone referred me to the following site.

Telescope Science: All About Telescope Aperture

The site is very useful but the calculator sets out the most of the information that I think you need.

Hope it helps.

[Vulpecula]

A bit weak on "Maximum Power" in that article!

- I don't know *anyone* who would claim that the "Maximum Theoretical Magnification" of a telescope is Diameter/1.58

That takes conservative estimating to new levels! :-)

I wonder where he got the curiously precise figure of 1.58 from?

Stranger still, on a later page he reverts to "50x diameter in inches" - well? which one is it?? :-)

I also have issues with minimum power being D/6

[The Warthog]

Much of his math is a waste of time. Why figure out the actual area of an objective when you can calculate the relative difference by simply squaring the diameter? Perhaps the maximum calculation is a typo for D*1.58, which at least makes some sense, The minimum power calculation delivers an exit pupil of 6mm, which is a decent average for low end exit pupil.