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Thread: 6" F/5 Achromatic Refractor Wide Field Eyepiece

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    Default 6" F/5 Achromatic Refractor Wide Field Eyepiece



    What will be the best wide field eyepiece for a 6" f/5 achromatic refractor. Read somewhere that a 30mm eyepiece is the highest power that should be used. Thanks

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    Default Re: 6" F/5 Achromatic Refractor Wide Field Eyepiece

    Depending on what your maximum dark adapted pupil is you could use a 35mm which would give you a 7mm exit pupil and the lowest useful power.
    Nebula filters start working well at around the 5mm exit pupil which with your telescope is a 25mm so do not neglect that area either.
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    Default Re: 6" F/5 Achromatic Refractor Wide Field Eyepiece

    Hello Refractordude,

    the longest focus EP for my 6" F/5 achro refractor is the 25mm W70 Antares (70° AFOV),
    and I screw on it the yellow filter to protect my dark adaptation.
    With the large exit pupils you take the risk of loosing the dark adaptation due to the brighter sky background.

    Before, I have been sometimes using the wide 32mm Aspheric Baader Hyperion, but always with a 2" nebular filter mounted in its nose. This is a fairly good, sort of Erfle wide eyepiece, but I have sold it because of its seldom use.

    On this refractor, the ultrawide (82°) EPs are helpful at the focus lengths around 15mm (3mm exit pupil),
    but even then, I'd say that the higher quality and more comfortable view 70° EPs are enough.
    As always, to each his/her own.

    Best,

    JG
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    Default Re: 6" F/5 Achromatic Refractor Wide Field Eyepiece

    [QUOTE=refractordude;1058645710]What will be the best wide field eyepiece for a 6" f/5 achromatic refractor. Read somewhere that a 30mm eyepiece is the highest power that should be used. Thanks

    Actually read that a 30mm eyepiece should be the lowest power.

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    Default Re: 6" F/5 Achromatic Refractor Wide Field Eyepiece

    Quote Originally Posted by refractordude View Post
    What will be the best wide field eyepiece for a 6" f/5 achromatic refractor. Read somewhere that a 30mm eyepiece is the highest power that should be used. Thanks

    Actually read that a 30mm eyepiece should be the lowest power.
    Folks who say that are assuming a 6mm exit pupil is all that can be handled. This isn't necessarily true. 5-7 mm is a good range for humans. So if your eye dilates to 7mm then one can go to a 35mm eyepiece. If your eye only goes to 5mm then 25mm is the max. It depends on your eyes as much as the scope.
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    Default Re: 6" F/5 Achromatic Refractor Wide Field Eyepiece

    Quote Originally Posted by not_Fritz_Argelander View Post
    Folks who say that are assuming a 6mm exit pupil is all that can be handled. This isn't necessarily true. 5-7 mm is a good range for humans. So if your eye dilates to 7mm then one can go to a 35mm eyepiece. If your eye only goes to 5mm then 25mm is the max. It depends on your eyes as much as the scope.
    I love the "humans" bit not_Fritz!

    Back to refractordude:

    Televue have a convenient calculator to show the different magnifications, exit pupils and FOV's of different eyepieces: Tele Vue Optics: Calculator
    I have found my ES 20mm 100° eyepiece gives a very nice wide view (about 2.6°) in my 150mm f5, but generally I use my 32mm Plossl (because the ES 20 is BIG and heavy!), which gives the maximum possible FOV for a 1 1/4" eyepiece: in this case around 2.1°.

    You can of course put in longer FL eyepieces that give larger than 7mm exit pupils. Refractors are OK with this as there is no central obstruction to block your view. You will get a wider FOV, but the problem as mentioned above is that some of the light can't make it into your dilated pupil, so the image will not be as bright as it could be. Mind you, this is what happens in day-time use of telescopes anyway, because our day-time pupils are more in the order of 1-2mm dilation...

    I have used both my 50mm plossl (FOV 3.3°, magnification 15x) and my 42mm GSO Superview (FOV 3.8°, mag. 23x), just for fun. The plossl gives a nice view, pretty sharp right to the edge. I had terrific views of the LMC and SMC through this! The Superview is not designed for f5 optics though! If I was non-human and had 10mm dilation the 50mm plossl would be great!

    Mind you, I have a friend who claimed to be able to see mag. 7.5 stars naked eye in a dark sky (SQM around 21.8), when I could only see just fainter than mag. 6. Assuming my dilation is 6mm and we have the same type of optical receptors and brains, that means his dilation must be in the order of 10mm. Hmmm. I had always thought he wasn't human...

    Bottom line: you can get a wider FOV with longer than 35mm eyepieces in an f5 scope, but, assuming you are human with a maximum eye dilation of 7mm you will lose some of the light. Realistically, a 31mm Nagler or 30mm ES 82° eyepiece giving exit pupils of around 6mm with a FOV of around 3-3.2° would give us the best wide-field that most of us could hope for. You could get a FOV of up to 3.5° with (say) a 55mm Plossl or 41mm Panoptic, but you will lose some of the light through-put.

    Happy viewing!

    - Dean

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    Default Re: 6" F/5 Achromatic Refractor Wide Field Eyepiece

    Quote Originally Posted by DeanD View Post
    I love the "humans" bit not_Fritz!
    I wasn't thinking of extraterrestrial aliens, I swear! The random thought popped of 'what kind of exit pupil could a blue whale handle or an orca?'

    Mind you, I have a friend who claimed to be able to see mag. 7.5 stars naked eye in a dark sky (SQM around 21.8), when I could only see just fainter than mag. 6. Assuming my dilation is 6mm and we have the same type of optical receptors and brains, that means his dilation must be in the order of 10mm. Hmmm. I had always thought he wasn't human...
    Actually.... The sensitivity of optical receptors varies more among humans than the dilation does. I can see to about 7.5 magnitude at a dark site. Used to make close to 7.8. Can't dilate quite as much. Now I'm not claiming you friend isn't an alien, but..."
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    Default Re: 6" F/5 Achromatic Refractor Wide Field Eyepiece

    Quote Originally Posted by not_Fritz_Argelander View Post
    I wasn't thinking of extraterrestrial aliens, I swear! The random thought popped of 'what kind of exit pupil could a blue whale handle or an orca?'

    Actually.... The sensitivity of optical receptors varies more among humans than the dilation does. I can see to about 7.5 magnitude at a dark site. Used to make close to 7.8. Can't dilate quite as much. Now I'm not claiming you friend isn't an alien, but..."
    ... either that or you are both aliens, and it is a conspiracy!

    Seriously though, do you know of any studies/data that relates optical receptor sensitivity for different individuals in terms like faintest magnitude star that is visible? For instance, I have never seen fainter than mag. 6.5 naked eye, and that was in a sky about as dark as you can get on planet earth (SGM ~ 22.1), and neither have most of my observing colleagues (apart from the alien...). If you can see mag. 7.5, that's a full magnitude or ~ 2 1/2 times fainter, which is a huge difference. You must really really like carrots!

    I like the idea of having eyes like an eagle though: I heard a professor of ornithology recently say that a wedge-tailed eagle (that's our Aussie version) could tell the colour of your eyes from 1 km away! Who needs telescopes with eyes like that!

    All the best,

    Dean

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    Default Re: 6" F/5 Achromatic Refractor Wide Field Eyepiece

    I have 6" F5 refractor and lowest useful power depends not just on your max pupil size, but as others stated on the sky condition. Even at bortle 4 skies of my dark site I seldomly go above 25mm, unless using filters. Still 30-36mm EP is nice to have. I recommend Baader Aspheric 31 or 36mm, 72 deg EPs

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    Default Re: 6" F/5 Achromatic Refractor Wide Field Eyepiece

    Quote Originally Posted by DeanD View Post
    ... either that or you are both aliens, and it is a conspiracy!
    Well my high school crush would tell you she was sure I was an alien, and she didn't have weird hair.

    Seriously though, do you know of any studies/data that relates optical receptor sensitivity for different individuals in terms like faintest magnitude star that is visible? For instance, I have never seen fainter than mag. 6.5 naked eye, and that was in a sky about as dark as you can get on planet earth (SGM ~ 22.1), and neither have most of my observing colleagues (apart from the alien...). If you can see mag. 7.5, that's a full magnitude or ~ 2 1/2 times fainter, which is a huge difference. You must really really like carrots!
    Carrots? I use a few but the remainder sit in the fridge until they mummify or rot.

    In terms of NELM there is a lot of variability. I'm just on my tablet so I'll try to follow with references tomorrow.

    Sedgwick has 7.93, Norton has 6.93, Baker has 6.03 all for young eyes.
    For older eyes S has 7.59, N has 6.59, B has 5.69.

    Heber Curtis could see to 8.1-8.5 mag so he's the champ.

    This variation is too large to be explainable by differences in dilation and, although I know of no studies specific to the question, has to be due to intrinsic sensitivity differences. These could be structural (like density of receptors) or chemical (production of greater density of photosensitive dyes). Based on my limited knowledge of biology and genetics I think the chemical explanation more likely. The concentration variations of various chemicals produced in the body due to genetics is big enough in general.

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