Wide angle eyepiece

[alan potts]

Colly,

Welcome to the Star Gazers Lounge, If I were you I would get yourself a 32mm Plossl as John suggested, it will show you a wider field and as he said it's more pleasing than the 40mm which just shows the same amount of sky at a lower amgnification.

I have used the 32 mm Meade 4000 before and they a fairly good for the money, I have also used the Celestron and didn't like it. I do beliwve it was one that came with a scope, so maybe not the best. If you can afford the Televue you would be buying into top quality eyepieces, these are not cheap but as televues go they are. Second hand is always a good bet if your lucky to find what you what, when ever I want something it shows up went I have already bought a new one.

Alan.

[Colly]

Wow cheers guys looks like it is all swings and roundabouts.....or not, might if funds allow buy the 32 and the 40 and compare them cheers for all your input guys some great theory which is certainly helping a newbie like myself although I did get a little confused on some of it the rest seems to make sense...I think haha cheers for your help anyway and hopefully we get some clear skies soon

[ismangil]

Here is quite a nice article about contrast and magnification http://www.televue.com/engine/TV3b_page.asp?return=Advice&plain=TRUE&id=114 This matches my experience directly.

Which is why I invested in 24mm 68 deg eyepiece, as that is the widest FOV for 1.25 at the highest magnification.

[RikM]

Fortunately I don't understand any of the theory relating to optics, being a Biochemist rather than a Physicist, so I never speak from a theoretic point of view, only what I have directly experienced for myself

I have a 40mm TV Plossl. This works okay as a finder eyepiece in a 70/450 refractor and 127/1500 Mak-Cas. Giving a low magnification, the image is quite bright but in my location, this includes plenty of light pollution. With only a 40° apparent field of view, the image circle it projects is quite small. Imaging looking down a kitchen roll tube. I can't use it in either 150/750 or 254/1200 Newtonians because I see a big dark blob in the centre of the field. This is the shadow of the secondary. Given the OP's telescope f/ratio, I strongly doubt that this secondary shadow effect would be seen but a 40mm 40° 1.25" eyepiece would still give a small image circle to look at, with a bright sky background if they have any light pollution.

I also have a 30mm 60° aFOV eyepiece. This is a 2" barrel so wouldn't fit in the OP's telescope, but I include it just for comparison. A 32mm 50° Plossl with a 1.25" barrel would be broadly similar to this. Due to the shorter focal length, this eyepiece gives a higher magnification than the 40mm Plossl. This makes the background sky appear darker and, if they are sufficiently bright, seems to give improved views of deep sky objects, including nebulae and galaxies. The 30mm with its 60° aFOV actually shows a larger area of sky than the 40mm Plossl and at 60° rather than 40° the image circle you see in the eyepiece is larger. Imagine looking down a toilet roll tube rather than a kitchen roll tube.

I also have a 20mm 70° aFOV with a 1.25" barrel. The same progression continues. This has a larger apparent field of view again, so the image circle you see when looking through this eyepiece is larger than either of the other eyepieces. (cut your toilet roll tube in half, so it is shorter, and look through it again now )To my eye, this size image is perfect, it more or less completely fills my field of vision and without moving my eye around, I can just see the black fieldstop at the edges of my vision. The 20mm gives a higher magnification again, which further darkens the sky background and again, to my eyes, can enhance the view of a great many objects. As you go on increasing the magnification with shorter focal length eyepieces, you do reach a point where the target object is no longer bright enough to see.

I have tried 82° and 100° aFOV eyepieces but I don't like these as much as 68°-70°. I do like to be able to see the edge of the field...just. This aspect of eyepieces (the size of the circle you like to see, rather than the area of sky actually shown) is an intensely personal feeling. Some love being unable to see the edges of the field, to feel like you are floating in space with stars all around (and will pay handsomely for the experience ) It's not for me...gives me vertigo Others prefer an even smaller apparent field of view to me. A small sharp image surrounded by the inky blackness of the inside of the eyepiece barrel. Actually I quite like this sort of view for planetary images, when the small image circle concentrates your attention on the planet. It just somehow seems more natural to me that a low power wide-field view should be...wide

And the best thing is, all these points of view are equally valid

[Ajohn]

There is still some confusion about what comes out of the 32 or 42mm eyepieces really. If a truly valid comparison is needed there is only one way of doing it assuming both exit pupils fit the eye. The magnifications on this scope come out at 31.25 and 23.8.

The exit pupils are then the diameter of the scope divided by the mag, 3.52mm and 4.62mm. There may be a problem with the later one getting into some peoples eyes. If the scope has a 25% obstruction it would be 27.5mm diameter. I would hope they haven't gone bigger than that and 20% would be noticeably better on planets. The black hole in the exit pupil behaves in the same way as the exit pupil does with the diameter of the scope and magnification. So it's size will be in one case 27.5mm/31.25x = 0.88mm and in the other case 1.16 mm rounding up. So there is a 4.62mm beam with a 1.16mm hole in it or a 3.52mm beam with a 0.88mm hole in it.

This can then be converted into the actual square mms of light that gets into the eye - area of the exit pupil - area of black hole. That comes out at 9.731mm^2 - 0.608mm^2 = 9.123mm^2 for the 32mm eyepiece and 16.764mm^2 - 1.057mm^2 =15.707mm^2. So basically the 32mm eyepiece looses more light due to the obstruction than the 40mm. Just hope the sums are correct. Never done that one before and then there is my sometimes payful calculator. Probably down to my fingers. It's interesting as it shows there is some mileage in the old codgers technique of using large exit pupils for dim objects. It would be worse in practice due to the reduced image scale at lower mag. It represents a rather large loss of light. Fit's in with what I have seen with an over sized exit pupil too, brighter stars. There there is a loss due to the size of the pupil and a gain due to the reduced image scale.

What hasn't been covered in this is eye problems. It would be difficult to say what these might do. For instance if some ones pupils could only just open up sufficiently to clear the larger black hole in the 40mm view they would have all sorts of problems. The effect of eye defects can be masked by using smaller exit pupils in a similar way to how some hospital people quickly test some one's vision by getting them to look at the test card through smaller and smaller holes except that is extending the eyes depth of field and reducing the effect of certain lens defects that might be in the eye.

I'm personally not keen on ultra wide angle eyepieces due to kidney beaning due to the eye having to be in the right position especially at low mag. Each to his own but I doubt if I will buy any other than the ones I have.

I've already covered why otherwise we don't see the black hole but have now added why it might cause problems in some cases - so big it's nearly the same size as the eye's pupil at the time or even bigger. The hole that can be seen at low mag sometimes is down to something entirely different.

John

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[RikM]

My point isn't about exit pupil. I am talking about the image circle as seen in the eyepiece, apparent field of view. For a 'wide angle eyepiece', which is what the OP asked about, the 32mm appears wider, in that it projects a larger image circle with a darker background sky.

This is an approximation of how the two eyepieces would compare visually in the OP's 114/1000 telescope.

[username]

I've read a lot of comments aroundthe net where people complain about too much eye relief on 40mm plossls which is something to consider as well. Eye relief on plossl designs is around 0.7 times the eyepiece length so 32mm your eye would need to around 20-25mm away and 40mm getting near 30mm away.

Large eye relief can cause eye position problems which is something to consider. I would go for 32mm (plus 32mm are easier to sell second hand if you upgrade your eyepieces later!)

[Ajohn]

I think you will find the site that generates those shots neglects the black hole Rick. The web and books are full of partially true information. I think the one based round point sources at infinity and extended sources such as sky glow and of course many of the things we look at is one of the worst. My view is fine take notice of it and bear it in mind but don't take it as gospel find out what is best for your self as it will vary. One of the problems with many sources of information is that they have to provide something that is new. Some even take no notice of physics.

I had read that an exit pupil larger than the pupil of the eye so many times I began to think about it for myself because I wanted to achieve a lower magnification than I could obtain on a C8 with what I could buy at the time. I came to the conclusion that the statement "like stopping the scope down" was complete and utter Rubbish just on the basis that the main scope puts the image on the focal plain and also sets the detail in it. All the eyepiece does is magnify it and present it in a fashion that is suitable for the eye. When I ordered the long focal length eyepiece the supplier had supplied them to others as well for the same reason. I also used it on the 5in APO I bought and the seller tried it too. He decided to get one himself.

Eye relief and Plossl's? Take care we have super plossl's now and eyepieces that loosely follow the design - two achromatic doublets with there convex side facing each other. The originals are plano convex from drawings and identical as far as I am aware. That isn't always the case. If I find an eyepiece is Rubbish I take it apart. As the sw 127 mac I recently bought came with a set of what I would call Rubbish I will probably unhappily sell them.

Can't get involved in this area again as with one other subject the posts get too long and I only have so much time available - Also know that it can be a waste of time. Over and out.

John

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[RikM]

I really enjoy sketching The 'site that generated those shots' was Me using Photoshop based on what I have seen with my own eyes There's no black blob shown, because you won't see one in an f/8.8 telescope with those eyepieces. I've checked by sticking the 40mm Televue plossl in my youngest Son's 114/1000 Celestron Astromaster, no dark blob visible, unfortunately it is mostly cloudy here

Hopefully we haven't annoyed the OP to much

[michael.h.f.wilkinson]

I really enjoy sketching The 'site that generated those shots' was Me using Photoshop based on what I have seen with my own eyes There's no black blob shown, because you won't see one in an f/8.8 telescope with those eyepieces. I've checked by sticking the 40mm Televue plossl in my youngest Son's 114/1000 Celestron Astromaster, no dark blob visible, unfortunately it is mostly cloudy here

Hopefully we haven't annoyed the OP to much

You are absolutely right Rik. I used a 40mm Plossl in my F/8 Newtonian, and at a 5mm exit pupil you will not see the central obstruction, if your eye is dark adapted (pupil dilated to 5mm or more), whatever John says. When your pupil is not fully dilated (as in solar or lunar work), you may see the central obstruction. This kind of EP does not work well during daytime observing for the same reason.

I think John also missed the main point you made: Both EPs show the same amount of sky, but the former shows it at a higher mag, and with a smaller exit pupil.

The argument about stopping the scope down if the exit pupil is larger than the pupil of the eye of the observer is true, as can be shown by simple geometric optics.

[Ajohn]

One last post on this but I am guessing. Going back to conjugate plains, horrible name but very simple, I suspect that compound scopes are more prone to it as the obstruction is further away than it is with newtonians.

As to the PHOTO's the exposures would be different and they are stars not extended objects. There are so many stars in the milky way many of which wont be resolvable I wouldn't like to classify it as either.

As to the rest as I pointed out I am wasting my time because people can't get their head round what happens when an exit pupil gets to big for the pupil in the observers eye. All it changes is light levels which is nothing remotely like stopping the scope down. They should phone up Carl Zeiss and tell them that many of their binoculars are a waste of money as the exit pupils are too big. The black hole in the exit pupil of the eyepiece on a scope with an obstruction is only a problem when it significantly fills the observers pupil. The other case where a perfect image is produced apart from a black hole in the middle is easily seen with something like the small Zenith mac with too long a focal length eyepiece in it.

Seems they can't get their head round aberrations in eyepieces when their full theoretical field is used either.

As some one pointed out earlier. Like him I read, I hear and then I compare with my own experiences and for visual purposes some of the ideas kicking about do not make a lot of sense as I have seen that for myself time and time again. Lower mag gives better contrast on most things I have looked at even loose clusters and that is from both severely light polluted skies where I live and rather dark ones - where I go once a month.

Ahhhh I know what the problem is. Planets are extended objects so we have to be very careful how much mag we use on them.

LOL As I have said there is no point going on. If I read any more I might fall off my seat.

John

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[michael.h.f.wilkinson]

The black dot you see when looking through a scope with a large central obstruction with a pupil smaller than the exit pupil, the black blob in the centre is not caused by seeing the conjugate plane. If this were the case you would see a stationary, sharp blob, rather than the eye-watering out-of-focus wobbly thing you actually see. Rather, it is caused by a combination of several factors, the first being the fact that the exit pupil is annular. If you were able to hold your eye perfectly still and centred at the exit pupil, you would not see a dark blob in the centre at all (assuming no spherical aberration or curvature of the exit pupil itself). You would simply see a rather dimmer image than the aperture of the scope promised. You never hold your eye perfectly still, at the perfect position. When you are not on the ideal position (e.g. you are too far in), light coming along the optical axis is largely blocked by the central obstruction, as the small pupil sits in the "empty" bit of the ring-shaped exit pupil. Light from the edge of the FOV comes in at an angle, and your pupil may be sitting on the edge of the ring-shaped exit pupil for those rays.

[username]

May I just say with all due respect that if we want to discuss the physics deeply on this subject that a new thread is made. The OP is new to the hobby and the thread is getting frightening. The OP asked for wider fields in their scope. Like everything in this hobby there are quick answers and there are more rounded answers but I fail to see how the direction of this thread is helping a beginner who will in time, if they are interested and at their own pace, learn the science behind optics

[Ajohn]

The black dot you see when looking through a scope with a large central obstruction with a pupil smaller than the exit pupil, the black blob in the centre is not caused by seeing the conjugate plane. If this were the case you would see a stationary, sharp blob, rather than the eye-watering out-of-focus wobbly thing you actually see. Rather, it is caused by a combination of several factors, the first being the fact that the exit pupil is annular. If you were able to hold your eye perfectly still and centred at the exit pupil, you would not see a dark blob in the centre at all (assuming no spherical aberration or curvature of the exit pupil itself). You would simply see a rather dimmer image than the aperture of the scope promised. You never hold your eye perfectly still, at the perfect position. When you are not on the ideal position (e.g. you are too far in), light coming along the optical axis is largely blocked by the central obstruction, as the small pupil sits in the "empty" bit of the ring-shaped exit pupil. Light from the edge of the FOV comes in at an angle, and your pupil may be sitting on the edge of the ring-shaped exit pupil for those rays.

Now I am falling off my seat. The so called black blob is excruciatingly stationary in that small mac I mentioned. I think I am going to ask an experienced optical person to model it to see if I am right. There is always the possibility that some form of shadow could be seen due to eye accommodation problems. In other words at some size of black hole in the exit pupil a persons eye may choose some how to focus on that rather than the image. Hard to see how though. The other possible reason is eyepiece problems. Say the hole is large and the eyepiece actually "focuses" a black patch into the eye because the peripheral rays it should focus to form an entire image just doesn't happen or not well enough. Focuses a black hole isn't a good term to use it would be more a case of poor optical performance away from the centre of the eyepiece. That might also explain why I haven't had the problem. Most of my observing has been with early vixen plossl's or televue plossl's plus of course the long focus one. My more recent ED types have only been used on a refractor and only run to 25mm anyway. The long focus one is only using a tiny fraction of the field it can cover so even junk should work well.

The eye accommodation type problem is doubtful really. It's the sort of thing that shows up on microscopes and why people don't notice field curvature when they look around the view through an eyepiece on anything unless it's excessive. I converse with some one who spends a lot of time trying to interest kids in microscopes. As they are looking at something close they typically focus there eyes at about 6ins when focusing the microscope and that gives terrible images because they are designed to focus correctly when the eye is relaxed and focused at infinity, It can be a serious problem for many people to get over at low powers because the eye is very good at pulling the image into focus especially on stereo scopes which have a relatively great depth of field. Perhaps something similar can happen on a telescope.

John

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[John]

I don't know what this last post has to do with the original question. Can we keep the thread on topic please which was simply to ask for some advice on a decent wide angle eyepiece for his 114mm F/8 newtonian scope with a 1.25" focuser.

Thanks

[andrew63]

I've just woke up from a dream - is this the Cloudy Nights forum...

andrew

[Damo636]

I've just woke up from a dream - is this the Cloudy Nights forum...

andrew

I was just thing the same thing Andrew :-)

[Colly]

Ok it Looks like I've started a bit of a war here guys apologies for this, I am going to get the 32mm plossi first and then a 40mm to keep everyone happy! Haha cheers for all your help!!

[John]

Ok it Looks like I've started a bit of a war here guys apologies for this, I am going to get the 32mm plossi first and then a 40mm to keep everyone happy! Haha cheers for all your help!!

No need to apologise. Your question was perfectly reasonable and I hope some of the responses helped

[alan potts]

John,

This really has gone off the rails, I know I have been a little guilty of such before but I agree with you, nothing what so ever to do with the question.

Alan.