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Thread: Why clearer/sharper at the EDGE of EP?

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    Default Why clearer/sharper at the EDGE of EP?



    Last night was probably the best I have viewed Jupiter. GRS was clearly visible along with the bands. I was using my ES 8.8 (82*) and sometimes the 2x Barlow.

    I have a question regarding the views. Although the views were fantastic in the center of the EP, I was able to see MUCH more detail when the planet drifted off to the edge. That's when the detail really came through.
    The GRS and the bands showed amazing detail but better at the edge of the EP. Is that unusual?
    Bigzmey likes this.
    John...... Listentothestars.net
    Scopes: ES AR152 Achro, SW 80mm ED APO/SW NEQ6 Pro Mount, Orion XT10i Dobs
    Cameras: ASI1600mm-c, ASI120mc (guide), Canon T2i (550D) DSLR
    Filters: Astrodon Ha 5nm, ZWO LRGB, LPS D-1,
    EP's: ES 2" 82* 18mm, 24mm & 30mm, ES 8.8mm, 11mm.
    BINS: Garrett 12x60, Nikon 9x25
    Processing: SGP, BYEOS, DSS, As!2, Reg6, Star Tools, PS, PHD2, Stellarium, CdC, PIPP

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    Default Re: Why clearer/sharper at the EDGE of EP?

    We think of the primary's focal plane as being flat. It is not. It is perhaps more useful to think of it as being a focal surface. The focal surface is curved. The telescope's image in the focal surface with be curved.

    The reason we align the focuser axis, as reflected off the secondary, with the primary doughnut when collimating is such that the focal surface is normal (or square) to the axis of the focuser. This will ensure that the focal surface of the eyepiece is 'parallel' to the focal surface of the primary. This will enable us to focus equally, all round, on the telescope's image in the focal surface.

    A better corrected wide-angle eyepiece such as your 8.8mm 1.25" ES82° will add a curvature into its magnified image that is opposite to that induced by the primary into the telescope's image. The purpose behind this correction is an attempt to flatten the entire FOV so to say.

    We would expect that when the object is on axis, right in the center of the FOV, the focused image of that object would not be subject to any field curvature or any curvature correction.

    If the image of Jupiter at the edge of the FOV is better than when it is at the center of the FOV, this may be an indication that either:

    1) the focuser axis is not sufficiently well aligned with the doughnut resulting in minor, and unexpected, focusing issues at the center of the FOV.

    2) the optical axis of the primary is not sufficiently well aligned with the center of the focuser resulting in minor, and unexpected, coma issues at the center of the FOV.

    It might prove interesting to see whether your 2" eyepiece plus your 2" 2x Barlow gives you the same benefits at the edge of the FOV before adjusting your overall collimation.

    3) you may have just lucked out fortuitously in that the FC correction applied by the 8.8mm eyepiece just matches the FC induced by the primary identically. In other words, they cancel each other out perfectly at the edge of the FOV. Some would say by design, some would say by an incredible stroke of luck.

    I also use the 8.8mm ES82° in my 8" 1200mm f/6. On Thursday night of last week, I enjoyed exquisite views of Jupiter with truly excellent seeing. I do not recall a better image of Jupiter at the edge of the FOV. As expected, with a well collimated scope and these better corrected sub-premium eyepieces, the best image of Jupiter was on axis, right in the center of the FOV to the best of my recall. I'm sure had it been different, like in your situation, a flag would have been raised.

    (FWIW, my 6.7mm also delivered exceptional images of Jupiter on that same evening. The seeing 'gods' were being really kind to me. Just wish that I could have said the same of my pesky neighbor, 5 houses down, and his overkill backyard security spotlight. I am of the opinion that security light was previously employed in the service of an airline as the single headlight on a Boeing 747.)

    Out of curiosity, however, I will pay more attention to the image as Jupiter drifts across to the edge of the FOV of the 8.8mm the very next time I am out. If there's something to add, I will be happy to provide the extra feedback.

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    Default Re: Why clearer/sharper at the EDGE of EP?

    Thanks (as always) Phil. I'll experiment with the 2" EP's next chance I get. One other thing I thought of is that it COULD be my eye...
    I have Recurring Cornea Erosion and when I have a bad episode (which I did a few days ago), my vision in the effected eye goes blurry for about a week. I just notice while reading a piece of paper that, dead-on, was not quite as clear as off to the side. That may be a contributing factor also.
    I believe that when I look to the edge of an EP, I do move my eye and not my head...if that makes any sense.
    John...... Listentothestars.net
    Scopes: ES AR152 Achro, SW 80mm ED APO/SW NEQ6 Pro Mount, Orion XT10i Dobs
    Cameras: ASI1600mm-c, ASI120mc (guide), Canon T2i (550D) DSLR
    Filters: Astrodon Ha 5nm, ZWO LRGB, LPS D-1,
    EP's: ES 2" 82* 18mm, 24mm & 30mm, ES 8.8mm, 11mm.
    BINS: Garrett 12x60, Nikon 9x25
    Processing: SGP, BYEOS, DSS, As!2, Reg6, Star Tools, PS, PHD2, Stellarium, CdC, PIPP

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    Default Re: Why clearer/sharper at the EDGE of EP?

    I have enjoyed a few high-res images of Jupiter in my C8 and can attest that ES EPs produce very sharp images right to the edge. However, I never noticed any significant improvement when planet drifted from the FOV center to the edge. I use 6.7mm and 4.7mm mostly, I will pop 8.8mm next time.

    I see sometimes when Jupiter is drifting across FOV it seems to go in and out of focus, which is basically due to air movement.

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    Default Re: Why clearer/sharper at the EDGE of EP?

    Quote Originally Posted by Bigzmey View Post
    I have enjoyed a few high-res images of Jupiter in my C8 and can attest that ES EPs produce very sharp images right to the edge. However, I never noticed any significant improvement when planet drifted from the FOV center to the edge. I use 6.7mm and 4.7mm mostly, I will pop 8.8mm next time.

    I see sometimes when Jupiter is drifting across FOV it seems to go in and out of focus, which is basically due to air movement.
    I'm starting to think it's more and more my eye with my medical problem. I'll experiment with it more next session.
    John...... Listentothestars.net
    Scopes: ES AR152 Achro, SW 80mm ED APO/SW NEQ6 Pro Mount, Orion XT10i Dobs
    Cameras: ASI1600mm-c, ASI120mc (guide), Canon T2i (550D) DSLR
    Filters: Astrodon Ha 5nm, ZWO LRGB, LPS D-1,
    EP's: ES 2" 82* 18mm, 24mm & 30mm, ES 8.8mm, 11mm.
    BINS: Garrett 12x60, Nikon 9x25
    Processing: SGP, BYEOS, DSS, As!2, Reg6, Star Tools, PS, PHD2, Stellarium, CdC, PIPP

 

 

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