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Thread: Useful Eyepiece Focal Lengths

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    Default Useful Eyepiece Focal Lengths



    Low power eyepiece:
    Since about the maximum extent that ones pupil can dilate in the dark is around 7mm here is an easy way to find out the longest possible focal length eyepiece that might be useful. Remember that a fully dilated pupil will reveal any defects that may be present in your eye. Just like with an eyepiece the view at the edge of your vision will not be as good as on axis. If you take the numerical value of your telescopes focal ratio, say for example F/6.25, and multiply that by 7 you will get the maximum focal length eyepiece that will work given dark skies. That works out to be 43.75mm for the F/6.25 telescope. Any focal length around that but close will be the optimum lowest magnification. Don't worry if you can only find or afford a 36mm eyepiece as it will be close enough.
    Now here come the ifs, ands, and buts.
    If you are not a youngster, say 20 years old or younger, then your fully dilated pupil's diameter may not be 7mm. It may be as low as 4mm or 5mm.
    And if your sky is not DARK then your pupil will be somewhat smaller than 7mm.
    But it never hurts to experiment to see what works for you.
    Okay so I fibbed. I could only come up with one if, and, and but.
    _______________________________________________

    Your telescopes maximum theoretical magnification is determined by its aperture. But, unfortunately there always seems to be a but, we function far better breathing air while we observe. The Earth's generally unsteady atmosphere limits us to a maximum useful magnification. The atmospheric steadiness varies from place to place and changes with time. Astronomers call this the "seeing". The larger the telescope the more it is affected by the seeing. A small telescope may be able to provide razor sharp views at 60x or more per inch of aperture. A very large telescope placed right next to the small telescope might have difficulty showing a similarly sharp steady image at 30x or less per inch of aperture. The larger telescope will always reveal more detail however. The greater detail shown will be more evident on short time scales. At a certain point of rapid image variability our eyes and brain cannot cope and then it is time to go to a lower magnification.
    _______________________________________________

    High power eyepiece:
    For many of us the most detail seen through a telescope occurs when we are using an eyepiece that provides an exit pupil of around 2mm. Some people claim that they get better results at 1mm and even .5mm exit pupils. It depends upon the observers eyes, experience, seeing, and type of object viewed. I personally think that these last mentioned individuals are the ones who design the department store boxes for telescopes and microscopes.
    So for the same F/6.25 telescope just multiply by 2, 1, and .5 times the numerical value of the focal ratio. That gives eyepiece focal lengths of 12.5mm, 6.25mm, and 3.125mm. Again anything close and affordable will be sufficient.
    Here comes another but.
    Many affordable short focal length eyepieces have very little eye relief. That means that you are going to have to cram your eyeball close to the eye lens to see anything. If you need to wear glasses for astigmatism you will require greater eye relief. Limited eye relief is uncomfortable and can lead to eyestrain, head aches, and facial skin frozen to the eyepiece in the winter in cold climates. So to obtain high power one may be better off to use a barlow lens. A barlow makes the telescope act as if its F-ratio is a higher number and preserves the eye relief of the eyepiece used. Get a quality barlow and try to avoid ones with very high amplification values. Also get one that will not result in duplicate focal lengths.

    _______________________________________________

    How many eyepieces do I need?:
    Remember I'm saying NEED, not want. Three to five eyepieces or three eyepieces with a barlow will work fine and provide plenty of enjoyment for many years. The spread between eyepiece focal lengths can be greater as the focal length increases. If you end up with multiple telescopes with varying focal ratios your eyepiece collection will grow. Just like with clothes. A shirt, pair of pants, and shoes to match or a dress, hat, and purse to match.
    As I don't have a F/6.25 scope my eyepiece range will be different. For me using an F/4.5 telescope before and a F/5 telescope now my eyepieces are as follows. A 32mm, 20mm, 13mm, and a 9mm. I used to have a 4.8mm but sold it after it pretty much sat in the eyepiece case for 25 years.

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  3. #2
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    Great post Ted
    Joel

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    Great info, Ted, and at a level that we can all understand. I think it would make a great Sticky.

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    Well, you might not see it Sticky here, but usually great post find their way to be part of the Sticky on the Helpful Links For Beginners Thread.

    Quote Originally Posted by RussL View Post
    Great info, Ted, and at a level that we can all understand. I think it would make a great Sticky.
    Joel

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    Quote Originally Posted by Carpioso View Post
    Well, you might not see it Sticky here, but usually great post find their way to be part of the Sticky on the Helpful Links For Beginners Thread.
    Done, both here and in "Helpful Links" in beginners

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    Great info, This will definatly help when I decide to get more ep's.
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    Default further advice?

    Quote Originally Posted by star drop View Post
    Low power eyepiece:
    Since about the maximum extent that ones pupil can dilate in the dark is around 7mm here is an easy way to find out the longest possible focal length eyepiece that might be useful. Remember that a fully dilated pupil will reveal any defects that may be present in your eye. Just like with an eyepiece the view at the edge of your vision will not be as good as on axis. If you take the numerical value of your telescopes focal ratio, say for example F/6.25, and multiply that by 7 you will get the maximum focal length eyepiece that will work given dark skies. That works out to be 43.75mm for the F/6.25 telescope. Any focal length around that but close will be the optimum lowest magnification. Don't worry if you can only find or afford a 36mm eyepiece as it will be close enough.
    Now here come the ifs, ands, and buts.
    If you are not a youngster, say 20 years old or younger, then your fully dilated pupil's diameter may not be 7mm. It may be as low as 4mm or 5mm.
    And if your sky is not DARK then your pupil will be somewhat smaller than 7mm.
    But it never hurts to experiment to see what works for you.
    Okay so I fibbed. I could only come up with one if, and, and but.
    _______________________________________________

    Your telescopes maximum theoretical magnification is determined by its aperture. But, unfortunately there always seems to be a but, we function far better breathing air while we observe. The Earth's generally unsteady atmosphere limits us to a maximum useful magnification. The atmospheric steadiness varies from place to place and changes with time. Astronomers call this the "seeing". The larger the telescope the more it is affected by the seeing. A small telescope may be able to provide razor sharp views at 60x or more per inch of aperture. A very large telescope placed right next to the small telescope might have difficulty showing a similarly sharp steady image at 30x or less per inch of aperture. The larger telescope will always reveal more detail however. The greater detail shown will be more evident on short time scales. At a certain point of rapid image variability our eyes and brain cannot cope and then it is time to go to a lower magnification.
    _______________________________________________

    High power eyepiece:
    For many of us the most detail seen through a telescope occurs when we are using an eyepiece that provides an exit pupil of around 2mm. Some people claim that they get better results at 1mm and even .5mm exit pupils. It depends upon the observers eyes, experience, seeing, and type of object viewed. I personally think that these last mentioned individuals are the ones who design the department store boxes for telescopes and microscopes.
    So for the same F/6.25 telescope just multiply by 2, 1, and .5 times the numerical value of the focal ratio. That gives eyepiece focal lengths of 12.5mm, 6.25mm, and 3.125mm. Again anything close and affordable will be sufficient.
    Here comes another but.
    Many affordable short focal length eyepieces have very little eye relief. That means that you are going to have to cram your eyeball close to the eye lens to see anything. If you need to wear glasses for astigmatism you will require greater eye relief. Limited eye relief is uncomfortable and can lead to eyestrain, head aches, and facial skin frozen to the eyepiece in the winter in cold climates. So to obtain high power one may be better off to use a barlow lens. A barlow makes the telescope act as if its F-ratio is a higher number and preserves the eye relief of the eyepiece used. Get a quality barlow and try to avoid ones with very high amplification values. Also get one that will not result in duplicate focal lengths.

    _______________________________________________

    How many eyepieces do I need?:
    Remember I'm saying NEED, not want. Three to five eyepieces or three eyepieces with a barlow will work fine and provide plenty of enjoyment for many years. The spread between eyepiece focal lengths can be greater as the focal length increases. If you end up with multiple telescopes with varying focal ratios your eyepiece collection will grow. Just like with clothes. A shirt, pair of pants, and shoes to match or a dress, hat, and purse to match.
    As I don't have a F/6.25 scope my eyepiece range will be different. For me using an F/4.5 telescope before and a F/5 telescope now my eyepieces are as follows. A 32mm, 20mm, 13mm, and a 9mm. I used to have a 4.8mm but sold it after it pretty much sat in the eyepiece case for 25 years.
    hello - can you advise me with a f12 1800mm 6inch mak the skywatcher pro 150? i want to know which widefield to get and higher power i also have a focal reducer x0.5 and its a bit weird.... can you advise please? i wondered about using baader hyperions but i do fancy a high mag spacewalk type eyepiece... thanks peter

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    Quote Originally Posted by star drop View Post
    Some people claim that they get better results at 1mm and even .5mm exit pupils. It depends upon the observers eyes, experience, seeing, and type of object viewed. I personally think that these last mentioned individuals are the ones who design the department store boxes for telescopes and microscopes.
    This comment is perjorative. They're hardly "department store box" designers - NASAs own FAQ says 1mm exit-pupil is fine!

    see Magnification and Telescopes

    Any moderate-quality or better telescope should effortlessly manage sharp image rendition with a 1mm exit pupil when using a quality eyepiece - if it's within the limitations of the sky.

    Beyond that, it depends on the scope. Brightness will drop (not a problem on the planets), and floaty-bits in your eyes are more obtrusive, but a top-class APO refractor telescope can manage excellent results with a 0.5mm exit pupil.

    A low-cost F5 reflector however, will just render a fuzzy blog at that point.

    More importantly, on most planets - when using reasonably-sized telescopes - you need to go well below a 2mm exit pupil just to get a size that's worth looking at.

    With a good telescope/eyepiece combination it's not a problem.
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    As a newbie who just spent two nights squinting through a set of eyepieces I purchased as a kit with my new Celestron NexStar 8SE, I need some advice. I need some magnification and some wide angle eyepieces, but I'm finding the Plossl variety aren't providing the eye relief I need. As was commented on another post, I'm finding I must plant my eyeball on the lens in order to see anything. I've looked around and noticed a variety of other eyepieces, Naglers, etc. Are there types that provide better eye relief at higher magnifications (say, 3mm to 9mm)?
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    Yes. On some designs eye relief is a funtion of the ep mm and can determine the fl breadth of the line(eye relief getting shorter with decreasing mm) - along with the field stop, but er is a variable the designer can play with in designs with more elements than a plossel or ortho. Now that you have a desired mag range (mm) for your eps just look at what the eye relief is rated for that brand.

    Another solution can be to get an inexpensive ep at double the mm and use a 2x barlow, but there are tradeoffs. Here is a not bad sort of waterfront coverage that includes info wrt many of the designs out there..

    Generally, look at your budget and try to find whatever will fit. A side benefit of many of the newer wider angle ep's is that they have an increased eye relief, but it varies by manufacturer and design. A lot of 'non-eyeglass wearers' / that don't like the tight designs like about 12mm as a minimum, so that may be a reasonable starting point?
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