Wide angle eyepiece

[Colly]

Hi all, still quite new to astronomy and I'm after some help please. I have a meade starnavigator 114

Aperture = 114mm

Focal Length = 1000mm

Focal Ration = f/8

Aluminum tube assembly

Rack-and-pinion focuser with 1.25"

Alt/Azimuth mount with locks on both axes

#494 Autostar Electronic Control System

I'm looking for an eyepiece so I can see large parts of the milkyway etc currently I have a 9mm and 25mm eyepiece and a Barlow could anyone recommend an eyepiece as I'm baffled by all the different ones out there, and also whether they would be any good on a scope like mine

[rory]

hi colly, your right theres a mass of glass out there. personally for the time being id simply look at a decent 32mm plossl. makes such as gso,meade 4000 or vixen, or televue (second hand) cheap but work well.

[Colly]

Thanks very much for your advice we've been looking through very cheap binoculars and have enjoyed the wide view you get do we're hoping to get similar results through the scope

[rory]

that wont happen colly, your binos show far more sky than the telescope will. a good 2" wide e/p can get something like 2 degrees in the fov, but binos usually get you between 5 to 6 degrees (ish) .

perhaps a more experienced member will giveyou some practical wide field options for your particular scope.

[Ajohn]

Celestron and Meade seem to do a 40mm 1 1/4 in fitting Plossl. You could go up to 50mm for a "bigger"view but I don't think you will find that in 1 1/4 fitting. Basically the longer the focal length of the eyepiece the lower the magnification but eventually the image wont fit in your eye.

Some one posted a link to this site on here recently. Prices make them rather popular. Do contact them and make sure that the series 5000 they show top left is a 1 1/4 fiting 40mm as there also seems to be a 2ins version about.

http://www.skystheli... eyepieces.html

John

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[Colly]

Cheers guys your help is much appreciated were going to invest in some binos but would like to get that in between view ( not up close not far away) if that makes sense I will give the guys a ring in the site ou posted as see if its the 1.25. Will post back my results when I can

Many thanks!

[John]

A 40mm plossl in the 1.25" fitting won't show any more sky than a 32mm and the 40mm view will appear narrower and, in my opinion, less pleasant to view through. A good 32mm plossl will show as wide a field of view as a 1.25" eyepiece can.

[Colly]

Cheers John so you think the 32mm meade plossi would be the best option?

[RikM]

A 32mm Plossl in your scope will give you a field of view a little over 1.5°, so three times the diameter of the full moon. Without a 2" focuser, that is about the widest possible view you can get.

[Colly]

Cheers Rik that makes a lot of sense( not very good at numbers) better at visualising things than math!

[Ajohn]

All fine but the 40mm will show more contrast so take your pick.

Actually I have used a 75mm plus plossl in 1 1/4 - bags of contrast, nothing at all wrong with the view but obviously it wouldn't all go through my pupil. This particular plossl will actually nearly cover a 2in field. With respect your comment about a "narrow view" isn't correct,

John

[rory]

blimey 75mm plossl . you learn something new every day.

[RikM]

All fine but the 40mm will show more contrast so take your pick.

Actually I have used a 75mm plus plossl in 1 1/4 - bags of contrast, nothing at all wrong with the view but obviously it wouldn't all go through my pupil. This particular plossl will actually nearly cover a 2in field. With respect your comment about a "narrow view" isn't correct,

John

I find I get reduced contrast at longer focal lengths. The larger exit pupil gives a brighter background sky and this washes out the view of extended objects.

[Josh Wilson]

If you go too low in magnification, you'll start to see the secondary mirror get into the image. This happens when the exit pupil of the scope is larger than your pupil. The lowest you can go on your scope with dark adapted eyes is about 18x. That would require a 55.5mm EP.

I'd suggest a 2" 38mm EP but you have a 1.25" focuser. You could always get an adapter though...

[John]

All fine but the 40mm will show more contrast so take your pick.

Actually I have used a 75mm plus plossl in 1 1/4 - bags of contrast, nothing at all wrong with the view but obviously it wouldn't all go through my pupil. This particular plossl will actually nearly cover a 2in field. With respect your comment about a "narrow view" isn't correct,

John

The apparent field of view of eyepieces (which dictates the true field of view you get) is restricted by the inside diameter of the barrel. With a 1.25" barrel the field stop that defines the edge of the field of view can be around 29mm in diameter. This equates to a max apparent field of view of 52 degrees at 32mm, 43 degrees at 40mm and so on down. With a 75mm focal length eyepiece in a 1.25" barrel the apparent field of view can only be 25 or so degrees - really tube like to look down. I had a 55mm 1.25" eyepiece and it was just like looking down a straw !

On contrast, a little additional magnification (ie: 32mm v's 40mm) actually darkens the background sky somewhat which makes low contrast objects easier to see. A 24mm wide field can show the same amount of sky but with more magnification again so you get a bit more of the sky darkening effect and contrast increase. It's all subtle but this effect does happen.

I'm not sure what you mean by "covers a 2in field" ?. Fields of view are usually expressed in degrees. 2" is an eyepiece barrel diameter format though but then it's not a 1.25" one !

[John]

....I'd suggest a 2" 38mm EP but you have a 1.25" focuser. You could always get an adapter though...

Unfortunately you will loose the benefits of the wide field of view that the 2" format can deliver as the adapter will "stop down" the eyepiece from 70 degrees to around 45 degrees.

[Ajohn]

I should have included a quote on this but I think people will know which post it refers too.

Strange effect that I haven't noticed. I have used the eyepiece I mentioned to centre things in the scope. I just look around the exit pupil by moving my head around. That wont be needed with a 32 or 40 mm eyepiece. It might with a 60mm on this scope.

The only thing I could think of that might cause that sort of thing is if the inside of the eyepiece barrel was shiny but in that case the same effect might happen on any focal length to a greater or lesser extent.

John

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[RikM]

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.

[Colly]

Wow cheers guys looks like its not a clear cut decision after all

[Mr Spock]

In my C9.25, the 42mm gives the widest field possible, but for deep sky the 22mm and 17mm give much better views due to background darkening

You will always get lower contrast and lighter backgrounds with very low powers regardless of scope size

[andrew63]

I don't think you can go to far wrong with a standard 32mm plossl as suggested - you can discuss the ins and outs and by the time you decide the summer will be here! I'm sure you'll enjoy the view.

andrew

[Carbon Brush]

Good advice from Andrew63 and you don't need to spend much.

For next year, you might like to think about a more expensive option. It all depends on how and where the hobby takes you.

If you go for a bigger scope. Say 6" or 8" mirror, then you will gather much more light, giving you the ability to pick up more of the dim objects in the milky way.

The dozens of stars in the eyepiece become hundreds on a clear night.

The bigger scope will probably have a 2" focusser, giving you more choices on eyepieces.

As an example I have a Skywatcher 200P. 8" mirror with the same 1000mm focal length as your Meade. So the scope has (about) 4x the light gather.

That means many more objects are bright enough to see, not a bigger view for a given eyepiece.

The 2" focusser allows a wide choice of eyepieces types.

But a bigger scope requires more setup effort, a heavier duty mount, etc, etc.

These can hurt the wallet.

Any new eyepieces for your current scope are of course transferable to any new scope.

Or worst case, you can sell on surplus eyepieces without too much loss.

Hope this assists more than confuses.

[perrin6]

To be honest a 32mm plossl probably wont be much wider a view than the 25mm eyepiece supplied with your scope, better to spend the money on binoculars such as 15 X 70's or towards a wide field telescope (ie a reflector with about F5 focal ratio) if you want decent wide-angle views.

[Ajohn]

I have read that it isn't possible to get all amateur astronomers to agree on anything. This make 2 subjects on this site that I wont comment on. There isn't any point much of what I have read is entirely contrary to my own personal experience. ie I didn't just read it on the web..

By 2in field by the way I really meant a 2in field. The lenses in this eyepiece are fitted in a body that has a 1 1/4 in barrel. If fitted in a lens body with a 2in barrel they would just about cover it with some deterioration to the extreme edge. It's actual focal length is more than 75mm actually because it gives an exit pupil of well over what a 75mm eyepiece would give on a C8 with a larger field of view via moving my eye from side to side of any "ordinary" eyepiece I could put in it. It was made to order by Poiser and he neglected to mark the focal length on it and I can't remember what we decided on re a much lower magnification and too large an exit pupil.

Perhaps this is best to put this another way - even though there is probably no point. The scope produced a real image that unfortunately we can't see so a a magnifier called an eyepiece is focused onto this real image to convert it to a virtual one suitable for our eyes - that just mean that in rough terms things look like they are at infinity. In practical terms the actual distance is generally a lot shorter. We could if we wished see the virtual image by placing a piece of tracing paper etc on it. The focal length of the scope just sets the scaling of the virtual image, longer focal length = bigger image for the same angle of view. It's gets dimmer as well because it's bigger, slower F ratios etc.

The amount of light in the virtual image is determined by the aperture of the scope. The focal length of the eyepiece sets the size of the exit pupil. It doesn't alter the amount of light collected by the scope at all. The scopes resolution doesn't change either. Once the exit pupil exceeds the size of the viewers pupil all that happens is that they can no longer see the entire view that is being captured by the eyepiece. There is no other reason for limiting it.

Taking this scope the focal length is 1000mm so a 32mm eyepiece gives a magnification of 31.25 and a 42mm gives 23.8. The exit pupil for the 32mm is 114mm/31.25=3.65mm and for the 42mm it's 4.62mm. I would hope that an observers pupil would open up to that level when viewing dim objects. Look at the moon for instance with either and the pupil would close down - anybody ever noticed that there field of vision changes as the pupil size changes - no. It shuts down to control the amount of light entering the eye. Some one might notice that peripheral vision gets a little woolly when it's very wide open but that's about it. This boils down to the fact that telescope pupil size limits are set to make sure that all of the light from the scope gets into the eye when viewing dim objects in situations where the pupil of the eye can open wide enough to view it. Things get rather complicated if the object is too bright for that but within limits the full view will still be available even though the eyes pupil has closed down to less than the one coming out of the eyepiece. Think about binoculars. Say 7x50 = 7mm exit pupil used in daylight. They don't suddenly start behaving like 7x25 or worse.

The exit pupil size sets brightness. In this case the 2 eyepieces give the same field of view. Area wise the 42mm eyepiece exit pupil is 63% bigger than the one produced by the 32mm. Both still go into the eye though so the net effect aught to be the same as the same amount of light is entering the eye. One eyepiece is likely to be working at it's full field of view though - the other one probably isn't. Optics always degrade as the field of view is increased that's why very wide angle eyepieces get far more expensive. That would push me towards the 42mm rather than the 32mm especially on cheaper eyepieces.

Seeing the central obstruction is a weird area. It's always there presented as a black hole in the rays forming the virtual image coming out of the eyepiece. The size of the hole obeys the same rules as the exit pupil from the eyepiece so if it's a 30% obstruction it's always 30% of the diameter of the exit pupil. The eye doesn't care about this as the information in the image is all over the entire exit pupil so the full image can still be formed when it's focused by the eye. In my view the problem of being able to see it is down to things called conjugate plains. This is the fact that a lens will produce images at distances dependent on how far the object is from it. The distances depend on the focal length of the lens. At some point an eyepiece will directly project an image of the obstruction into the eye. That's my theory as to part of the problem anyway as no one seems to really get to grips with this subject just mention it. I know I can't see the central obstruction with my rather long focal length eyepiece on a C8 but I can on a mac spotting scope. The other effect is more obvious. Taking 30% again so the hole is 30% of the diameter of the exit pupil. If that exceeds the size of the eye's pupil no light will get into the eye at all. In any obstructed system more light gets to where ever it is "used" as the obstruction gets smaller even at the focal plain of the scope. There are also diffraction effects which is where the often mentioned 20% limit comes from.

Higher magnifications dims the sky and allows lower magnitudes to be seen? That's based on diameter of scope setting limiting magnitude of point sources at infinity. Stars are the nearest thing we can get that approach that condition. The sky glow is an extended source so it doesn't apply as I have read some where more related to photography - diameter sets limiting magnitude focal ratio sets exposure time. I've even read that fast focal ratios aren't too good for astro photo's a the sky glow is captured too quickly. That was comparing F2 with F4 astro photographic systems though. In fact I think it also mentioned advantage in using F8. Then there is the older view that bigger exit pupils capture more light and are best for dim objects. There is probably some truth in all of them. There are several complications. Are nebulae clouds point sources - far from it. Are stars in a galaxy beyond the resolution of the scope point sources - no. Do we all looks that sky trying to see what limiting magnitude individual stars we can see - well I don't. Then there is the various richest field telescopes aimed at showing as many stars in a field as possible. This favours smaller scopes and larger exit pupils due to the distribution of star magnitudes in the sky. Well one version does probably biased towards the milky way because that will have a dramatic effect on the magnitude distribution in the night sky. There are a hell of a lot of stars in that direction. There are other ideas which in my view loose the plot.

More magnification reveals more detail but unfortunately the contrast in the newly apparent detail decreases. That's physics and there is no way of avoiding it. At the extreme where a Rayleigh's limit double star is being split at high magnification there is hardly any contrast difference at all. Things called Modulation Transfer Functions are used to show this. Often generic ones are shown that relate to the theoretical resolution of any optic where contrast reaches zero. This one relates to a specific telescope that I play ray tracing now and again. It might help people understand the generic ones that are usually shown as this one has actual resolution for a 6in scope.

And for a laugh one for a Newtonian where the axial resolution is spectacular and it's a 10in scope

The modulation axis which goes from 0 to 1 is the ratio of contrast in to contrast out of the optics.

If some one puts all of this together it's all swings and roundabouts confused even more by the fact that contrast has a distinct bearing on how fine a detail we can see as well. There isn't a distinct rule of thumb for any of this really. People shouldn't let these graphs put them off. We have to live with it.

Couple of links - 1st mentions the binocular exit pupil effect - very hard to find those. On the other hand it's not surprising that astro binocs have a habit of having higher magnification. Wide field eyepieces can cause some curious effects on these as well as they can on telescopes.

http://www.skyandtelescope.com/equipment/basics/3077091.html?page=4

Much more technical but great pages in many ways. This one shows what happens MTF wise with central obstructions. Also info on pupil size and mag. Good info on collimation too on there other pages. Also mentions light levels going down when pupils are too big in the usual rather miss leading way. Mentions the richest field telescope too but the 70-90mm aperture relates to refractors. A reflector needs to be bigger. The old figure is 100-110mm and a very small 2ndry mirror giving just enough clearance out of the tube to use an eyepiece and cover the full field of view. None on the market as far as I am aware 70-90 mm applies to binoculars too. I would rather buy a small quality refractor. Quality is likely to be a lot better at similar cost.

http://www.handprint.com/ASTRO/ae3.html

Just a final note I just stuck the super long focal length eyepiece in a 76mm F 600mm cat newtonian and also a 32mm plossl. No signs of the central obstruction with either. Mag about 8x or less and about 19x. One of these bought during a mad moment at a car boot years ago. Had to make a 1 1/4 adapter for it and align the optics properly even by bending the focusing tube.

http://www.ebay.co.uk/itm/Vintage-Boxed-Astral-450-Catadioptric-Telescope-/130672222747?nma=true&si=Mvh3AHU7QtyciNmvSWxpKdylJ%2B8%3D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557

Looking through a straw? Can see the obstruction? Seems a little strange and miss understood theory based to me.

But by all means buy either the 32 or the 42mm but don't make the choice for the wrong reason. If your observing site is light go for the 32. If it's dark and you buy the 42 I would be gob smacked if you had central obstruction problems unless it huge. I suggested the 42 to keep things simple and just made sure the exit pupil was not too large.

John

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[John]

I think many folks make their posts on here based on their personal experiences John. I've no doubt you do and I know I do too

Perhaps it's not surprising that, as we are all different, our experiences will differ and thats fine. It's all valid input