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Eyepieces - the very least you need.


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Thanks for this great thread TW. :)

On the info supplied and gleaned from other threads I've gone out and bought a 2x barlow (Celestron Ultima) and a 15mm Plossl to go with the 10 and 20mm that came with the scope.

Next month will be a 8mm Plossl, the following will be a 32mm and that hopefully should be enough. :o

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EP focal length and the selection is one thing, it would be good to answer the question "which is better 1.25in or 2in eyepieces?". Perhaps expand this into an "All you wanted to know about Eyepieces but were afraid to ask" type of post :)

I couldn't answer this authoritatively however I get the impression that the longer EP sizes (ie 30+mm) behave better with a 2". Is that the case?

Vixen A80Mf 910mm, f/11.4 comes with vixen no-name 6mm (151x) and 20mm (45x) EP. I found that the 20mm was good for an overall picture, the 6mm is good for getting into things but it's missing a ~12mm (75x). Additionally the f/11 and budget optics means that there's not much light getting to the smaller EPs (6mm) so I'm hoping the Baader Hyperon at 13mm (70x) with it's better optics will pass more light.

Not yet tried the 670mm f6.38. The vixen no-name 6mm (111x), 20mm (33x) and the Baader Hyperon 13mm (51x). As the optics are better quality I may get a barlow/EP to give ~175/200x.

edit: the Hyperon 13mm has just arrived.. I think the article should also make a point about the wieght of EPs!! It weighs a ton!

Edited by NickK
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EP focal length and the selection is one thing, it would be good to answer the questionn "All you wanted to know about Eyepieces but were afraid to ask" type o "which is better 1.25in or 2in eyepieces?". Perhaps expand this into af post :)

Perhaps this is the kind of guide you are talking about:

http://stargazerslounge.com/primers-tutorials/63184-primer-understanding-choosing-eyepieces.html

It also goes into the difference between 1.25" and 2" eyepieces.

HTH

Andrew

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  • 3 weeks later...

DOBSONIANS

To get a 5mm exit pupil out of a 250mm mirror, you will need an eyepiece that gives you 50x. This means a 25mm eyepiece. To get a 7mm exit pupil out of the same mirror means a magnification of 36, and a 35mm eyepiece.

Hi guys and i have spent alot of time reading this post, but i cannot work out the formula above?

I got to the 250mm (size of mirror or aperture) / 5mm (or what exit pupil i require) = 50x (which means an EP which gives this mag will give a exit pupil of 5mm) it is the next bit that throws me, why does it mean a 25mm eyepiece, where is the formula?

I am sure i am missing something, doh!

To work out what an eyepiece can do for my own scope i take the focal length of the scope and divide it with the focal length of the EP = Magnitude, so (1250/25=50x) and from this i can then work out what the exit pupil will be, still assuming we have the 250mm (10") F5 scope, i then use the aperture or mirror size of the scope and divide it with the mag of the EP (250/50=5mm)

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Magnification: focal length of scope divided by focal length of eyepiece

Exit pupil: focal length of eyepiece divided by focal ratio of scope = exit pupil diameter in mm.

Focal ratio: focal length of scope divided by diameter of primary mirror or objective lens = focal ratio, expressed as F/5, F/7 etc, etc

True field of view = Apparent field of view of eyepiece (eg: 50 degrees) divided by the magnification that it gives in a particular scope = the true field of view in degrees (how much sky is shown).

I think I've got those right :(

Hope it helps a bit !

Edited by John
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Perhaps this is the kind of guide you are talking about:

http://stargazerslounge.com/primers-tutorials/63184-primer-understanding-choosing-eyepieces.html

It also goes into the difference between 1.25" and 2" eyepieces.

HTH

Andrew

The difference is in the width of the field of view. For a plossl lens, you get the full FOV at up to a 24mm eyepiece, but above that you need a 2" eyepiece. At 32mm, your FOV is restricted to about 48º, no matter how great the FOV of the eyepiece may be. This makes a great difference in the Widefield eyepieces with FOVs of 60 to 100º.

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Hi guys and i have spent alot of time reading this post, but i cannot work out the formula above?

I got to the 250mm (size of mirror or aperture) / 5mm (or what exit pupil i require) = 50x (which means an EP which gives this mag will give a exit pupil of 5mm) it is the next bit that throws me, why does it mean a 25mm eyepiece, where is the formula?

I am sure i am missing something, doh!

To work out what an eyepiece can do for my own scope i take the focal length of the scope and divide it with the focal length of the EP = Magnitude, so (1250/25=50x) and from this i can then work out what the exit pupil will be, still assuming we have the 250mm (10") F5 scope, i then use the aperture or mirror size of the scope and divide it with the mag of the EP (250/50=5mm)

In any scope, the exit pupil will be whatever multiple of the f/number is represented by the focal length of the eyepiece. So for an f/5 scope, a 5mm eyepiece will give you an exit pupil of 1mm, and a 25mm eyepiece will give you an exit pupil of (focal ratio/ focal length of ep =) 5mm. This is independent of the magnification calculation.

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Thank you Warthog for the explanation and a superb post that has helped a whole lot , as it guided me to choose the correct EP's for my dob (Skywatcher Skliner 200P F5.91 fl1200mm) as i was about to buy an EP that would have taken me to 240x and even though the scope can take it as you say best to be around 200x on a dobsonian, so i went for wFOV EP's so i do not have to nudge the scope around excessively and good eye relief too.

Cheers Gaz

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  • 3 weeks later...

Yep, as above...... no pun intended,

Thank you for taking the time with this sticky WH. On your advice I've bought a barlow x2 for my F6. An Orion Shorty.

It cost me £34 from local tiny camera/scope shop in Plymouth. I went on brand and price.

Tell me it is a decent Barlow...... please........:D

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oh many thanks Andrew,

I have used it for first time last week, with my shipped EPs. I have taken onboard the fact that shipped EPs are not too good. My 25mm in the Shorty proved acceptable -to a newcomer like me - but the 10mm showed up as rubbish in terms of the exit pupil and the fuzzy, dimmer, but larger image.

But, I was viewing Saturn for the first time which more than made up for my cr*ppy 10mm.:D

I'm currently using WartHogs's EP sticky to guide me.

Thank you for replying - 10mm EP next! (I think)

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  • 3 weeks later...

Would someone please explain the name "plossl" to me? Is that a trade name or does it refer to something else?

There is a wide price range between lenses. Does the quality differ to the degree inferred by, what appears to be, the exorbitant prices?

Edited by OneSG
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Would someone please explain the name "plossl" to me? Is that a trade name or does it refer to something else?

There is a wide price range between lenses. Does the quality differ to the degree inferred by, what appears to be, the exorbitant prices?

The name Plossl refers to the design of the eyepiece and is named after G.S.Plossl who devised this design in 1860.

A true Plossl eyepiece is a four - element design consisting of two nearly identical pairs of lenses.

Many manufacturers now market five-to-seven-element Plossl variations carrying various trade names.

Compared with Orthoscopics the Plossl has a slightly wider field of view and works better on f/6 or faster scopes but has shorter eye relief.

HTH

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You get what you pay for with eyepieces but, for your scope, the TAL, Skywatcher, Meade or Celestron plossls will work pretty well.

First Light Optics have a good deal on Skywatcher Plossl's at the moment:

Skywatcher - Skywatcher SP Plossl eyepieces

John

Thanks for the link to the great deal on eyepieces. They will go well with my scope :)

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  • 4 weeks later...

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