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MrCat

Eyepieces and wider field of view

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Hey folks,

I was delighted to be viewing the moon yesterday night, and delighted to be able to get such a high magnification with my scope as some had suggested it may not be capable of it.

I got down to using my 8mm eyepiece with a barlow 2x attachment making it 4mm, through which I could see just a small section of the moon but in great detail.

So I've still yet to get my head round how it all works and which mirror and lense is responsible for what so bear with me, but as you can probably gather I was wondering if another 'special' kind of eyepiece might give me that kind of magnification but with a wider field of view, I.E the whole of the moon (waterboys :) ) but at that super high magnification?

Or is that to do with the size of the tube/scope and the mirror at the back?

That's my main question but my extra waffle would be something along the lines of, does the back mirror size purely have to do with the amount of light it can collect and thus give you a better image of further objects, whereas the eyepieces provide the actual magnification?

I realise the latter waffle may be a bunch of very techy questions in one that I'll not likely understand the answer to unless you're really good at brief explanations to the layman but there it is :)

Cheers folks, and what a lovely clear sky tonight if you're new by me ;)

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The light gathering power and resolution of the scope is determined by the aperture ie the size of the mirror.

The magnification depends upon the focal length of the primary mirror plus the focal length of the eyepiece.

The field of view you end up with depends upon the apparent field of view of the eyepiece, and the magnification you are using

So....

Magnification = focal length of scope/focal length of eyepiece

True Field of View = Apparent Field of View/Magnification

Say you have a 200p with a 1200mm focal length. The 4mm would give you x300 which is pretty high. Assuming the eyepiece had a 50 degree afov, the actual field would be 0.16 degrees ie only enough to show you a small portion of the moon at high mag.

To achieve any better then you are into the realms of ES or Televue widefield eyepieces.

A 4.7mm Ethos SX for instance has a 110 degree afov so would give you x255 magnification and a 0.43 degree field of view, still not enough to fit the moon in completely.

To achieve that you would need to lower the magnification, either by using a longer focal length eyepiece or a shorter focal length scope.

A 6mm Ethos for example would give x200 with a 0.5 degree field, still not enough. An 8mm would give x150 with a 0.66 degree field, enough to fit the moon in with a little space around.

Choosing a different scope, say a 600mm focal length refractor reduces the mag so you could use a 3.7mm Ethos SX to give you x162 and 0.66 degree field.

To simply answer your question, even with the most expensive eyepieces around today, it's not possible to achieve magnifications well over x200 whilst still fitting the whole moon in. If you come down to x150 ish then it is possible, but expensive [emoji3]

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You need to work backwards:

The moon is half a degree.

To get it all in the eyepiece in use has the deliver therefore 1/2 degree view.

1/2 is nice and easy in a way.

Your eyepiece will have a defined Field of View, on Plossl's usually 50, on BST's it is 60, ES82 it is (strangely :grin: ) 82 degrees.

So to get 1/2 degree when looking through it the magnification has to be 2x the eyepiece FoV, so: Plossl = 100x, BST = 120x, ES82 = 160X.

So what magnification did you have with the barlow and 8mm ?

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The barlow and the 8mm eyepiece would be giving you 175x in your scope (I believe it has a 76mm mirror and a 700mm focal length ?).

Thats a bit over the maximum useful magnification for the scope but you can sometimes get away with that on the Moon.

To seen more of the Moon but still have a high magnification you will need a wide field eyepiece. These are available but they cost more than standard field eyepieces. Probably more than the scope has cost you so far as well.

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MrCat,

Any and all eyepieces that you might acquire for your present telescope can be used for telescopes that you may acquire in future.  Optically, eyepieces are fully the other half of the telescope.  One cannot be used without the other, and are as Punch and Judy; inseparable.  Therefore, think nothing of getting one better-quality eyepiece at a time, and so to build a truly fine set; over the weeks, months and years even.  Eyepieces are a constant; telescopes less so.  This 8mm would be an excellent higher-powered ocular, and with a respectable wide field...

http://www.skiesunlimited.co.uk/SLT2/telescope%20eyepieces.html...top, center.

The 60° eyepiece is also sold under various house-brands, and praised on both sides of the Atlantic.

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Ah cheers folks, I'm still busy (tidying and other boring stuff but am determined and will provide all calculations) to work out the maths for myself and will get back to you;

So what magnification did you have with the barlow and 8mm ?

I know it's already been pointed out but I have to do this :D

I'm liking the compatibility note, you do wonder when you get into a new zone like this how much you spend is going to be worthwhile in the future, and I'm all for future compatibility. Nice. 'Ark ark' of approval.

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On 29/10/2015 at 19:42, MrCat said:

Ahhhh haha let me work on this for mo :D

Brilliant reply lol don t know about you nebula but I haven’t got a clue what the other members are on about I’m new to this and was thinking of throwing my scope in the bin because I can’t get my head round all the tech stuff and I got a level maths a long  time ago ,like the other night I got Neptune in the scope although clear it was very small so how do you make it bigger just a simple answer would do don’t need all the equations thanks 

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Posted (edited)
On 20/05/2018 at 03:59, John2309 said:

Brilliant reply lol don t know about you nebula but I haven’t got a clue what the other members are on about I’m new to this and was thinking of throwing my scope in the bin because I can’t get my head round all the tech stuff and I got a level maths a long  time ago ,like the other night I got Neptune in the scope although clear it was very small so how do you make it bigger just a simple answer would do don’t need all the equations thanks 

Hi John,

1047px-Planets2013.svg.png

But that image does not show the enormous distances between each planet; only the sizes in relation to one another.  But you can see that Neptune and Uranus are a good bit smaller than Saturn and Jupiter.

Earth is about 150,000,000 kilometers from the Sun.  That distance is known as an "Astronomical Unit", or AU.  Saturn is 10 of those away from the Sun, but we get a good view of it through our telescopes because it's so large.  Uranus, being much smaller than Saturn mind you, is 20 AUs away; twice that distance. 

Neptune...:hiding:...is 30 AUs away from the Sun...4,500,000,000 kilometers; a staggering distance, three times that of Saturn's.  Given Neptune's relatively small size and enormous distance away, it makes it very, very difficult to observe it as large as we see Jupiter and Saturn, under high magnification, and with our relatively small telescopes.  I'm thinking that you'd need an observatory-class telescope to see Neptune as up close as you would like...

2006_winter_history_largestpieceofglass-

The mirror of that telescope is 5.1 meters in diameter.  Yours is 200mm, I gathered.  To see Neptune as large as possible with your telescope, the telescope must be collimated to near-perfection, the mount's motor must be turned on, tracking the planet, and with a 1.6mm eyepiece in the focusser...

https://www.firstlightoptics.com/vixen-eyepieces/vixen-hr-planetary-eyepieces.html

...and for 625x.  It wouldn't be impossible to accomplish.

Edited by Alan64

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On ‎20‎/‎05‎/‎2018 at 09:59, John2309 said:

Brilliant reply lol don t know about you nebula but I haven’t got a clue what the other members are on about I’m new to this and was thinking of throwing my scope in the bin because I can’t get my head round all the tech stuff and I got a level maths a long  time ago ,like the other night I got Neptune in the scope although clear it was very small so how do you make it bigger just a simple answer would do don’t need all the equations thanks 

Mostly the planets do appear much smaller in the eyepiece than folks new to observing think they might.

I guess we get used to seeing images and sketches where the planet has been enlarged considerably so we think that represents the visual view but, unfortunately, that is not the case.

Using massive magnifications (eg: 300x, 400x, 500x etc) by using very short focal length eyepieces or a barlow lens will boost the apparent size of the planetary disk but invariably this will be at the expense of detail. What you get is a slightly larger but washed out disk because of the limitations of the seeing conditions and scope optics.

Very occasionally the seeing conditions come together and very high magnifications deliver well but most of the time we have to be content with smaller images to get the crisp contrast and detail.

 

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On ‎23‎/‎05‎/‎2018 at 09:26, John said:

Very occasionally the seeing conditions come together and very high magnifications deliver well but most of the time we have to be content with smaller images to get the crisp contrast and detail.

I do not have John's depth of experience, but, over the last few weeks, Jupiter has been at its recent highest, just over 20 degrees altitude, due South at about midnight. My view in that direction is not bad when objects have risen above the trees, but I have struggled to get a good, crisp, view with magnifications above x120, with my 250mm Dob., 127mm Mak., or 120mm refractor, and with my selection of fixed and zoom eyepieces. In most cases, the limit was imposed by the atmospheric conditions causing a heat haze, with a view that I have seen described as "looking down at a coin at the bottom of a swimming pool". Once you have found the best compromise between clarity and magnification, you have to sit (much better than standing for long periods) looking through the eyepiece until you catch that brief moment when the "ripples" die down and you get that clear view.

At the turn of the year, I had a hunt for Uranus and Neptune. I would have struggled without good GoTo alignment. Both were visible, and, with magnifications of over x200; Uranus was a blue disc - significantly wider than adjacent stars, and no obvious contrasting detail; but Neptune looked very much like the adjacent stars, but with a hint of blue. The Synscan GoTo will help me to find them again (when in view), it's nice to know that they are there, and visible through modest telescopes; but if I want more detail, it is easier to go to NASA's web site. Voyager 2 sent back some superb views of Neptune, but when Voyager 1 was just beyond Neptune's orbit and was turned towards Earth, our lovely planet showed up as a pale blue dot under 1 pixel wide.

Geoff

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