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Thread: Optical stops

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    Devildadeo's Avatar
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    Default Optical stops



    So I'm waiting for my dob to cool right now. I have Capella completely out of focus so that I can check out the tube currents. I noticed that the defocused doughnut actually stops before the edge of the FOV. After that gap, you can see the reflection of the doughnut (greatly distorted) shining off of what I can only assume to be the interior of the EP within the internal air space. My question is, are all these things only visible because I am so far out of focus? Or are these reflections and whatnot degrading my view in focus without me realizing it? It is afterall an Orion Sirius Plossl.

    If any of you serious optics guys in here could let me know, that would be great. I really hadn't thought much about the inner workings of EPs until I saw this. So, if there also any go to links you guys know of were I could read up on optical theory or principles regarding lenses would also be appreciated.

    Thanks and clear skies,
    Rock
    Vixen Foresta 7x50, Nikon Action 10x50
    ETX70-AT w/Reflex Finder
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    Default

    It sounds like you are introducing some serious vignetting by intercepting the light cone far beyond the focal plane. Your test really doesn't mean anything regarding your scope/eyepiece performance. The best tests for eyepieces is the in focus image. Do star fields appear uniformly bright across the field of view? Are the moon's craters, Jupiter's cloud bands and Mars' south pole sharp and distinct. These are the Most important tests of your optical system. For grins, you may want to get Suiter's book on star testing. I spent a few years with it to help me determine how good my optics are. At the end of the day, it's what your eyes see at the eyepiece on a well focused image.

    Randy Roy

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    Default

    The views through the scope are everything I ever wanted for the money I spent thus far. Tonight I had a great view of Mars @ 240x which isn't always possible in my tight neighborhood, especially on a freezing cold night. The disc was very defined and sharp. The ice cap is not all that crisp but I've been thinking of that more as a brightness issue. Unfortunately the frost cut me off tonight. With the Sirius Plossls I have, the view is not great towards the edge of the field on dimmer stars. I've been trying to sketch M37 @ 120x for a couple months but keep losing track of my place everytime I nudge. I'm just figure that's what i get with a cheap EP.

    I'm interested in Suiter's book. I'll Google it. I just had a little time to kill waiting for the OTA to cool and started wondering how the EPs work in the first place.
    Vixen Foresta 7x50, Nikon Action 10x50
    ETX70-AT w/Reflex Finder
    XT8 (Tube Lined) w/9x50 RA illuminated and Rigel Quikfinder on TL Systems EQ Platform
    Explore Scientific 82 degree 30mm, 24mm, 18mm, 11mm
    Shorty Barlow / Zhummel 2" Barlow
    Burgess Planetary 8mm and a handful of Plossls and Kellners

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    Default

    Yes, the reflections you see should only be there when out of focus.

    There are two "stops" that you should consider. The aperture stop simply limits the fov. The second is a "field stop." That must occur at or near a focus. .

    If you could see the image at the focus of the primary and your scope were pointed at a well lit white wall, you would observe a circle within which the image is clear and equal brightness. Outside the circle the light falls off quickly. That circle is caused by the aperture stop.

    If I remember my design terms, the exit pupil is just the reimaging of the aperture stop.

    The ep is designed to be used with its focus coinciding with the primary focus. As you move it in and out, you are seeing the rays as they come into or go out of focus. It causes the eye to see a "virtual image" at infinity.

    To understand how this works, do a simple ray trace. Use a lens instead of a mirror for the primary. Draw a line (the axis) and place the lenses in position and mark the diameters with a vertical line. Mark the focii with points on the line.

    Light from a star enters as a bundle of parallel rays.

    A ray parallel to the axis will be refracted through the focus. A ray through the center will continue in a straight line. Where they come together is the image. Now mark the aperture and see that it limits the angle from which the rays may enter.

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