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5mm EP and UK observing


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Hello there,

Just a quick query, on this forum many users advocate the use of 7 and/or 8 mm eyepieces over the use of a 5mm eyepiece when observing from within the UK, why is this? I have often read comments like, 'you won't get much use from a 5mm eyepiece in the Uk, go for the 8mm'.

Does increased magnification offer less clear and defined images?

Thank you in advance for any replies.

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Hi,

The focal length of any eyepiece will produce many difference magnifications depending on the scope it is used within. You ascertain the magnification produced by an eyepiece by diviiding the focal length of the telescope by the focal length of the eyepiece.

Scopes in the focal ratio of F5 (focal ratio of a scope is the aperture in mm divided by the focal length) are common and a 5mm eyepiece in an F5 scope is bordering the 1mm exit pupil threshold which is where the image at the eyepiece starts to degrade down to 0.5mm where it becomes unusable. Exit pupil is the shaft of light that comes out of the eyepiece after all is said and done. The higher the magnificationt he smaller the exit pupil. Exit pupil diameter is calculated by dividng the aperture of the scope in mm by the magnification produced by the eyepiece.

I think the problem you have found is that people have probably made statements out of context or have assumed that the reader has knowledge of the implied context in which their comment is made. Often people will respond subjectively to the poster asking the question and make suggestions based on their equipment which is often listed in the signature.

For example a 5mm eyepiece in a 80mm F5 refractor (which has 400mm focal length) would provide a 80x magnification. the same eyepiece in a 14" F10 SCT type telescope (3500m focal length) would provide a whopping 700x magnfication.

I hope that helps to make a bit more sense.

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I have a 9mm and 6mm EP , the 6mm rarely comes out to play except on the very rare occasions when the seeing is first class , and to be honest I get a better view then with the 9mm Barlowed.

These are 'budget' EP's so I can't comment on the top-end glass , though I suspect that poor seeing negates Naglers just as much ... :p

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A 5mm eyepiece makes sense with a telescope with a focal length less than 1m, as it gives a maximum of 200x. Once you start going above 1m focal length, however, you start going above 200x which is considered something you should only do on nights of exceptional seeing or if you are splitting double stars.

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I use my 4.7mm in my scopes all the time. All my scopes are F5 or faster.

If you have an F5 scope I would definitely have a 5mm eyepiece. I often find myself wanting a 4mm, and maybe a 3mm. The scope will always show the image, it doesn't magically stop working at a certain point. It depends how much you are willing to degrade an image by magnifying it. A very subjective thing. I think a 5mm in an F5 scope is a safe bet and pretty much required for any lunar/planetary viewing. 1mm exit pupil is perfect for these targets.

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I regularly use 5mm, 4mm and 3.5mm eyepieces with my scopes. The seeing conditions don't always co-operate but more often than not I find I can use 225x - 300x on the moon and planets to good effect.

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To add to everyones excellent replies. Magnification does dim the image so magnifying a dim image doesn't necessarily make it clearer hence the reason many faint fuzzies are seen better at low magnification

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That's a good point. Your target choice also has a bearing on the usage of an eyepiece. If you only wanted to look at galaxies and never at the moon or planets then a 5mm would see limited usage simply because these targets look better at lower magnifications.

I think as a general work horse though a 5mm is a great choice in an F5 scope.

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Am I correct in assuming this then; the more you magnify an image the more it degrades (telescope etc aside), so a 5mm Ep is more likely to 'degrade' an image than a 8mm Ep? Sometimes I get superb views with a 6mm and other times the image appears somewhat blurred, the telescope is collimated though.

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My 5mm is the best thing Iv ever had for lunar observing, almost feels like Im having a hot air baloon ride over the surface. Iv only had chance to test it on the moon and Saturn and both come out really nicely in it.

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Yes, the magnification does degrade an image in that it is stretching the light to make it bigger in the eyepiece. The light entering the scope is constant and how you manipulate it through mangification can only make it worse if you magnify it. Naturally there are sweet spots in these optical system and it stands that too little magnification is also a bad thing as the light not refined enough, this is when the exit pupil becomes too big to physically fit all the light into your retina. The iris of your eye is a limiting facotr as well in this system and as you age your ability to dilate your pupil will effect your observing too.

Now the second point you hinted at these is probably nothing whatsoever to do with the mangification and everything to do with the atmospheric disturbance caused by air moving. Our atmpsohere may appear invisible but it is very real and ever present and you must look through it to observe anything in outer space. Air currents, wind, moisture in the air etc all effect the view at the eyepiece and the worse the conditions the worse the viewing will be. the higher the magnification the more obvious this effect becomes. The accepted standard for high magnification observing in this country is 200x and that threshhold stands because of our atmpspheric conditions. In space you could go to 1000x magnfiicaiton with the right set up and have a perfect image.

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Thank you all for the replies, I have a much better understanding now, it explains why some nights my images are crystal clear then on other nights the same images aren't so clear and well defined, Saturn being a good example.

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I regularly use 5mm, 4mm and 3.5mm eyepieces with my scopes. The seeing conditions don't always co-operate but more often than not I find I can use 225x - 300x on the moon and planets to good effect.

I find my Barlow combo with 6 mm ( yields 217x ) to give rather pleasant views of Saturn too, when seeing conditions cooperate, and my Barlow is a cheap one, something in between would be nice. Already thinking about adding that 4 mm at some stage for something in between I can use without the Barlow. Though a BST is first on the list for my medium range.

Coming next week I hope in time for my birthday, hints at the misses all week :D

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I'm no rep for Televue but I really think every astronomer with a short focal length scope and who don't have to wear glasses should seriously consider getting a 2-4 or 3-6 Nagler zoom. They are not that expensive (£340) when you consider you will be getting good quality views from a well revered brand, magnifications spanning across several focal lengths (equates to £85 a focal length) including in-between and the faff free twist of a dial to match a magnification to suit the nights seeing conditions. No swapping out EP’s in the dark or missing a break in the clouds just enjoying the hobby while the UK weather allows it. Don’t get me wrong I personally feel there is no better a planetary EP than the BGO's but the amount of time you will sit waiting for the seeing to play ball to give you that one glimpse of perfection at any particular FL you could be matching the magnification in seconds with a zoom.

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I use a barlow to achieve my high power views with a Skyliner 250PX Dob. I don't have a 5mm equivalent at the moment. Often my 5.6mm is sharp and I feel it could take a bit more but 4mm is bit too much, so I think 5mm would be a good FL to have available for my (and your) scope.

5mm would get used sometimes but I can use my 7mm almost every night so if you could only afford one of them, the 7mm would be more useful. (which is why I have a 7mm but not a 5mm yet).

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A Skywatcher 200P f/5 with a 5mm eyepiece will give a 1mm exit pupil - that should be fine. The classic rule of thumb is that you don't want an exit pupil smaller than 0.5mm (i.e. you want a magnification no higher than 2 times aperture in mm). On my 12" f/4.9 my highest power eyepiece is 4mm focal length, giving exit pupil 0.8mm.

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I agree with comments above to a large extent. with a 1200mm focal length such as you have with the 200mm dob, a 5mm eyepiece will give 240x and this is readily usable on the moon and double stars, sometimes planets too.

broadly, the UK seeing, cooling and collimation affect the quality of the image in that order. the quality of the eyepiece and scope comes fourth and the experience of the observer is somewhere in the middle. the UK seeing both nationally (i.e. in the sky above the country) and locally (e.g. heat plumes from heating vents, rooftops and paving) affect things dramatically. seeing is how steady the air is on a given night. often slightly misty skies with high cloud have the best seeing and steadiest skies; good for high power on brighter objects like planets. very clear nights often have good transparency but poor seeing as the air is less steady great for low power views of galaxies etc. sometimes it's about picking your target.

I'd say that an 8mm and a 5mm would be good for your set up but personally I have tightly packed eyepieces at the top end having 6-3mm zoom, 7, 8, 9,10, 11, 12.5, 13, 15, 16, 20, 25, 26 and 32mm eyepieces; I genuinely use them all. the seeing varies so much that it's worth it to me although my scopes have focal lengths of 1200mm, and 1600mm x 2 so each mm of focal length of eyepieces makes a fair difference to magnification in the longer scopes.

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