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Eyepiece Sizes


daz

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Well, it looks as if the wife is going to cave in under relentless pressure, and let me have a new toy! :) It's our 10th W.A. this year, so it's going to cost me a packet anyway..... :)

After much advice given by many of you, it will be the Skywatcher 200 on the HEQ5 - but before I commit visa to keyboard, just one quick question..

The scope can take both 1.25 and 2" EP's (presumably via an adapter for the 1.25) - should I think about changing EP's to 2", or is this not important right now?

Ta

Darren

[P.S. The average marriage in the UK lasts 11 years, Great!! One more year to go........ :twisted: ]

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Congrats Daz, you'll love that setup! You can't beat a Newt for value for money, that scope will blow you away!

I'm not a big fan of 2" EP's - they are heavy, expensive and don't really give you much extra in the way of views until you get up to around 32mm-40mm, the reason being is the EP 'size (say 40mm = 1.5"??)' is greater than 1.25" so the barrrel of the EP tends to 'stop down' the image in 1.25" EPs once you get to 40mm.

So that leaves you looking at (maybe 32mm) 40mm and 52mm EPs, then you run into the problem of exit pupil, which (if you don't fancy looking it up) generally means that as we get older the ability of our pupil to dilate decreases. The upshot of this is that if the light focussed on the pupil by the eyepiece (its exit pupil) is larger than the pupil then the light is lost to the observer, that that tends to happen with larger focal length eyepieces. You''ll still get the widerfields, but no more detail (brightness), than you would with a smaller EP.

If you PM with your email, I'll send you a Excel sheet I wrote that gives the mag/True Field of View and exit pupil for any scope/EP setup.

...sorry about the ramble....... :)

Gaz

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Hi Darren,

It is certainly more convenient to have all your eyepieces the same diameter but personally, I think there is something magical about the views seen thru a good low-power 2” wide-field eyepiece that IMHO, makes owning one worth the inconvenience.  There is not a good reason for changing your existing med/high power eyepieces to 2". 

As for exit-pupils: You can find an exit-pupils diameter by dividing the focal length of the eyepiece by the telescopes focal ratio or alternatively, by dividing the telescope aperture (in mm) by the eyepiece magnification … as with binoculars.

With reflectors, Exit-pupils larger than 7mm (for those in their 20’s or about 5mm for the fortysomethings) do waste light, though primarily because the black spot in the exit-pupil caused by the secondary mirror obstruction becomes larger (not a problem for refractors). 

The smallest exit-pupil considered suitable for general observing is about 0.7mm, although for observing double stars and other special purposes some observers use 0.5mm or even less. With a 200mm scope, you’re more likely to be limited by atmospheric stability than by exit-pupil size. Unless you live in a location with extremely stable air, you’re unlikely to find magnifications much higher than 250X to 300X useful. Those translate to exit-pupils of 0.8mm to 0.66mm respectively. 

Hope that helps,

Steve  :)

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Hi Daz,

Can't add much to what the guys have already covered here, always nice to have the option of 1.25 & 2" though, and that scope gives you that option...

Congrats on buying a nice bit of kit, it will serve you well I'm sure of that.

Enjoy!.

Rob

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With reflectors, Exit-pupils larger than 7mm (for those in their 20’s or about 5mm for the fortysomethings) do waste light, though primarily because the black spot in the exit-pupil caused by the secondary mirror obstruction becomes larger (not a problem for refractors).

Steve, thats slightly mis-leading IMHO. It gives the impression that the 'light lost' is CAUSED by the secondary in relflectors and so isn't much of a problem with refractors.

If the EP exit pupil is larger than the observers pupil size then the 'extra' light is lost irrespective of whether the scope is a reflector or a refractor and the observer is effectively stopping down the aperture of his/her scope.

The 'black spot' you mention in reflectors is a symptom (not the cause) of this and is caused when the image of the secondary dominates the observers view because he/she is only able to see the centre most part of the image because their pupil size is too small for the whole image. The 'black spot' would not be visible if the observer were able to see the outermost portions of the image. The fact this symptom is only visible in reflectors doesn't mean that losing light in refractors doesn't happen, it just doesn't manifest itself in that way, you can lose the light (and hence image detail/brightness) and not realize it in such an obvious way as in a reflector.

I'm not saying don't get a 2" EP for widefield veiws, but you need to be very aware you could effectively be paying a lot of money for the privilege of making the aperture of your scope smaller. :)

Gaz

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Daz, you'll get to experience a 2" eyepiece first hand because the Explorer 200 with HEQ5 comes bundled with a 28mm 2" eyepiece (unless Synta have changed it). I'm with Gaz here. I think there's only room for one 2" eyepiece and that's a nice low power jobby. Absolutely nothing to be gained from having a medium power 2".

Russ

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I have a 30mm 2" Radian EP. I have to say that the veiws through this are truely amazing! The FOV is over 2 degree's and when viewing things like open clusters the veiws are just about the best you'll ever see....

I was visually observing M57 a month or two ago with the EP when I got it. I was amazed at just how crisp the image was (with slight flaring of the stars towards the edges) and was very pleased to be able to centre M57 and still have the two bright stars either side of M57 within the FOV.

But I have to agree that I don't really see any advantage to medium or high power 2" Ep's

Ant

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... I'm with Gaz here. I think there's only room for one 2" eyepiece  and that's a nice low power jobby. Absolutely nothing to be gained from having a medium power 2".

Russ

.... If you re-read my post you will find that I did indeed say that:

I think there is something magical about the views seen thru a good low-power 2” wide-field eyepiece that IMHO, makes owning one worth the inconvenience.  There is not a good reason for changing your existing med/high power eyepieces to 2". 

Steve :)

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With reflectors, Exit-pupils larger than 7mm (for those in their 20’s or about 5mm for the fortysomethings) do waste light, though primarily because the black spot in the exit-pupil caused by the secondary mirror obstruction becomes larger (not a problem for refractors). 

Steve, thats slightly mis-leading IMHO. It gives the impression that the 'light lost' is CAUSED by the secondary in relflectors and so isn't much of a problem with refractors.

If the EP exit pupil is larger than the observers pupil size then the 'extra' light is lost irrespective of whether the scope is a reflector or a refractor and the observer is effectively stopping down the aperture of his/her scope...

Gaz

Thank you for your comments Gaz but I shall stick to my guns on this one. 

I agree that when using any telescope, light that falls outside the eyes pupil is wasted.  But, with a reflector, this is less significant than the light loss caused by the shadow spot from the secondary mirror, which grows in size as the as the magnification is reduced.  Refractors are effected by the former but not by the latter. 

Steve :)

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Sorry Steve, just re-read your post and you're absolutely right. That'll teach me to try and read/reply before leaving for work.

One of my best eyepieces was an Orion (USA) Optilux 32mm. Fantastic eyepiece that gave superb views in the 10" Meade. Never tried a 2" eyepiece since.

However the 2" focuser is essential for DSLR Imaging. So whether using 2" eyepieces or not, still crucial to make sure the scope is equipped with 2" focuser.

Russ

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However the 2" focuser is essential for DSLR Imaging. So whether using 2" eyepieces or not, still crucial to make sure the scope is equipped with 2" focuser.

Russ

There is another advantage to having a telescope that can handle 2" eyepieces that applies to refractors and SCTs: Generally, a two inch diagonal is almost always better quality than a 1.25".  A 2" will typically achieve 1/8 - 1/10th wave accuracy whereas the 1.25" will be around 1/4 because it is more difficult to cut/grind smaller mirrors. 

Consider, with an SCT about a third of its quality is tied up in the diagonal so, even if you don't intend using a 2" eyepiece, it is probably still worth upgrading the supplied 1.25" diagonal to 2". 

Steve :)

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With reflectors, Exit-pupils larger than 7mm (for those in their 20’s or about 5mm for the fortysomethings) do waste light, though primarily because the black spot in the exit-pupil caused by the secondary mirror obstruction becomes larger (not a problem for refractors).

Steve, thats slightly mis-leading IMHO. It gives the impression that the 'light lost' is CAUSED by the secondary in relflectors and so isn't much of a problem with refractors.

If the EP exit pupil is larger than the observers pupil size then the 'extra' light is lost irrespective of whether the scope is a reflector or a refractor and the observer is effectively stopping down the aperture of his/her scope...

Gaz

Thank you for your comments Gaz but I shall stick to my guns on this one.

I agree that when using any telescope, light that falls outside the eyes pupil is wasted. But, with a reflector, this is less significant than the light loss caused by the shadow spot from the secondary mirror, which grows in size as the as the magnification is reduced. Refractors are effected by the former but not by the latter.

Steve :)

Steve, I agree, but your oringinal post gave to impression (to me at least) that light loss wasn't much of a problem with refractors because they don't suffer from the dark spot caused by the secondary. I've re-read the part I quoted and it still seems that way (to me).

Also the fact you can see the secondary in a reflector, whether it's the main cause of light loss or not in refectors, is only present because light is falling outside the pupil and the observer is unable to see it, his view is dominated by the centre of a image.

oh well, splitting hairs maybe?

Gaz :)

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Steve, I agree, but your oringinal post gave to impression (to me at least) that light loss wasn't much of a problem with refractors because they don't suffer from the dark spot caused by the secondary. I've re-read the part I quoted and it still seems that way (to me).

Gaz :)

Hi Gaz,

My response must have been ambiguous ... Sorry.   

Steve :)

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...or me being thick :)

I think the main thing to rememeber with longer focal length eyepieces is that they are dependent on your eyes to a large extent. They may not always do what they say on the box, so to speak. If you intend using them purely for wide field viewing then great, but if intend using one for low magnifaction and teasing out detail in diffuse objects, then there comes a point where your eyes let you down and theres no point in using a bigger EP.

Gaz

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Personally, having tried a few Super Wide Angles (around 67 degrees angle of view) and Ultra Wide Angles (around 82 degrees), I have come to prefer the SWAs.  To my eye they are more comfortable because they show all of the actual field of view whereas the UWAs require you to move your head/eye to view the periphery of the image.  I would guess those wearing glasses would find them particularly difficult to use.  Also, the extra field of view requires extra correcting elements in the light path, all of which come with a penalty (unless you are prepared to throw large sums of money at the problem!). 

Currently, I am aiming for a collection of two or three simpler eyepieces - plossls or orthoscopics - and one low power 2" SWA.  (Meades 5000 28mm SWA is working its magic on me at the moment!).

Steve :)

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I do wear glasses normally but when observing I stick 'em on top of my head - I find it awkward to observe with them on, and it lets in extraneous light as well. The only down side is when I remove my eye from the EP and put my glasses back on, it takes a second for my eyes to re-focus.

As russ pointed out, I shall get the chance to try the 2" so I shall get to see for myself, but it seems the common concensus is one low-power 2".

Thanks to you all for your inputs

Darren

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