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  1. #1
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    Question On-Axis vs Off-Axis



    Can anyone help me with this one? I recently contacted a vendor regarding some questions on a particular zoom eyepiece and they stated it would work fine with my fairly fast scope "on-axis but might have some trouble off-axis". I'm not sure what they mean.....how can my eyepiece be off axis?, off-axis to what? They were aware of what scope I have; aren't all eyepieces off-axis on reflectors? What am I missing hear? Am I that dense? Were they referring to 45 or 90 degree adapters that I would never use with a reflector? I could go back and ask them, but I guess I'd rather risk sounding like a total idiot with you guys and gals! Thanks for being patient, everything I learn takes me farther away from ignorance......ah the joys of gaining an education on new equipment to me, such is life.
    I SOLD MOST OF MY PAST ITEMS AND NOW HAVE: SkyWatcher 8" Collapsible Dob, 10 & 25mm Plossl's, Baader 8mm Eyepiece w/28mm tuning ring, TeleVu BLW-3125 3x Barlow, 13% Moon Filter, Celestron NexImage Burst Color Imager, EQ-Tech Tracking Platform. Eyes to the skies- enjoy and appreciate the multitude of hidden wonders. :
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  3. #2
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    Hmm... hard to say.

    There are three (actually more than three, but three main) types of off-axis scope designs; the Herschelian, the Schiefspiegler, and the half-mirror off-axis Newtonian. It would be rare that someone would have one of these scopes, so they're probably not referring to the instrument class.

    Standard Newtonian reflectors use eyepieces and secondary mirrors that are on-axis - that the secondary mirror center and eyepiece focal point share the same optical center and axis as the main mirror.

    It could be that the vendor was referring to off-axis error/aberration. To some extent, all Newtonians will exhibit some coma, some astigmatism, and won't likely be in perfect collimation. A zoom eyepiece can exacerbate these optical errors, otherwise intensifying something like coma at the field edges. Perhaps this is what the vendor was talking about?

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  5. #3
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    Quote Originally Posted by AustinPSD View Post
    Hmm... hard to say.

    It could be that the vendor was referring to off-axis error/aberration. To some extent, all Newtonians will exhibit some coma, some astigmatism, and won't likely be in perfect collimation. A zoom eyepiece can exacerbate these optical errors, otherwise intensifying something like coma at the field edges. Perhaps this is what the vendor was talking about?

    Just to add a little; in other words, the image produced by the eyepiece may be sharper in the center of the field of view than it is at the edge.
    SXINIAS

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  7. #4
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    Thumbs up

    Thanks to all for the replies. I see what you're saying. I may have to contact the vendor and ask them to reiterate. This was in regards to the Baader Hyperion 8-24 zoom and I was asking them if it would work well with my fairly fast f/4.7 scope....I asked Alpine Astronomical in Eagle Idaho.
    I SOLD MOST OF MY PAST ITEMS AND NOW HAVE: SkyWatcher 8" Collapsible Dob, 10 & 25mm Plossl's, Baader 8mm Eyepiece w/28mm tuning ring, TeleVu BLW-3125 3x Barlow, 13% Moon Filter, Celestron NexImage Burst Color Imager, EQ-Tech Tracking Platform. Eyes to the skies- enjoy and appreciate the multitude of hidden wonders. :
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    With the added context, this makes a bit more sense - they're referring to the off-axis view with regard to a fast reflector.

    Because the mirror curvature is more extreme for a fast reflector, coma can be exaggerated in a zoom eyepiece at the field edges. An f/4.7 instrument is on the edge, but probably no so much as to make the coma a problem. Instruments at f/4.2 usually need a coma corrector (Paracorr), especially when used with a zoom eyepiece to help reduce coma at the field edges.

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  10. #6
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    Quote Originally Posted by David I. View Post
    Thanks to all for the replies. I see what you're saying. I may have to contact the vendor and ask them to reiterate. This was in regards to the Baader Hyperion 8-24 zoom and I was asking them if it would work well with my fairly fast f/4.7 scope....I asked Alpine Astronomical in Eagle Idaho.
    From another forum thread comes this quote:

    I use it (Hyperion 8-24mm zoom) in my f4 Schmidt Newt & the edge of field isn't bad at all.
    "Edge of field" refers to the off-axis performance of the zoom, while on-axis is what you see at the center of the field when looking thru an eyepiece. Hope this helps.

    Best,

    Hernando

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  12. #7
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    Thumbs up

    Thanks AustinPSD. I agree with you. Before I received your reply I contacted the vendor and asked them to reiterate......I'm waiting for their reply. I'll post it upon receipt.
    I SOLD MOST OF MY PAST ITEMS AND NOW HAVE: SkyWatcher 8" Collapsible Dob, 10 & 25mm Plossl's, Baader 8mm Eyepiece w/28mm tuning ring, TeleVu BLW-3125 3x Barlow, 13% Moon Filter, Celestron NexImage Burst Color Imager, EQ-Tech Tracking Platform. Eyes to the skies- enjoy and appreciate the multitude of hidden wonders. :
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    Thumbs up

    Thanks Hernando, that does help.
    I SOLD MOST OF MY PAST ITEMS AND NOW HAVE: SkyWatcher 8" Collapsible Dob, 10 & 25mm Plossl's, Baader 8mm Eyepiece w/28mm tuning ring, TeleVu BLW-3125 3x Barlow, 13% Moon Filter, Celestron NexImage Burst Color Imager, EQ-Tech Tracking Platform. Eyes to the skies- enjoy and appreciate the multitude of hidden wonders. :
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    Thumbs up

    Thanks everyone for the replies. I received a reply from the vendor. Yes, they were referring to "edge of field" as off axis. Now I understand thanks to all who replied on AstronomyForum. When someone (vendor) makes a blanket statement to a literal minded optics novice such as myself it can be confusing. Education is wonderful.......now I know what the vendor was referring to. It's unfortunate he didn't use a simpler explanation in layman's terms the first time....sometimes certain people like to impress others with their knowledge but most of the time "good teachers" want the student to learn without feeding their own ego. Anyway, I do appreciate the vendor taking the time to reiterate....I'm not badmouthing the vendor but I do think the individual should have provided a simpler explanation in laymans terms the first time. I have to say it's possible he wasn't trying to feed his ego he may just be an expert that is a lousy teacher....some people have vast knowledge on a certain subject but aren't good at imparting that knowledge to others. Teaching and informing well is a gift and there are many that possess it, some that don't.
    I SOLD MOST OF MY PAST ITEMS AND NOW HAVE: SkyWatcher 8" Collapsible Dob, 10 & 25mm Plossl's, Baader 8mm Eyepiece w/28mm tuning ring, TeleVu BLW-3125 3x Barlow, 13% Moon Filter, Celestron NexImage Burst Color Imager, EQ-Tech Tracking Platform. Eyes to the skies- enjoy and appreciate the multitude of hidden wonders. :
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    Smile

    Hi David,

    Thanks for asking the question, I'm slowly learning the lingo too! At least you know what a fast scope is?

    Carol

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