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Thread: Secondary distance from primary

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    Default Re: Secondary distance from primary



    Quote Originally Posted by FelixG View Post
    I suspect my questions are pretty common. Is there a large post somewhere that I've missed, where I can find help?
    About the optical vs. OTA axis, check the Don Pensack sicky in the Reflectors forum.
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    Default Re: Secondary distance from primary

    Quote Originally Posted by FelixG View Post
    I'll be mounting my equipment soon. I believe the easiest way to do so is to start with mounting the focuser and the secondary at theoretical positions? Then use a friend who looks through the eyepiece as I'm moving the primary back and fourth to determine where it's to be mounted.
    You could do it that way.

    Be careful though when looking thru the eyepiece and make sure you're trying to focus on something at least 1 mile away (or more), like a utility pole. If you're focusing on a tree that's close by, you'll get it wrong.

    Clears,
    Joe
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    Default Re: Secondary distance from primary

    Yes, I could do all this for you and nail it, but, "Teach a man to fish...", etc.

    I am old fashioned when it comes to this, I know there are telescope calculation programs and web sites, but there is no substitute seeing your OTA physically laid out in front of you. It also will guarantee it will be right the first time.

    Always start with a scale drawing of your optical layout, full scale if possible. Don't guess, don't take the manufacturers specifications as written in stone. You have to know the exact focal length of the primary. Set up a crude optical bench with an illuminated target, place a frosted light behind a piece of white cardboard with a 1/2"/13mm hole in it and reflect the image back to it. Maybe add some kind of target, like cross hairs to the opening, to get the focus exact. This would require a mirror stand like used for focault testing, easily made from wood. Instead, you could have someone hold the primary on edge on a table top. Never balance it on its edge, unless you are wealthy and don't mind buying a new primary to replace the one that broke. Measure this distance exactly and divide by 2. Never use the sun to project its image onto something, unless you want to set it on fire.

    At this point use a pencil so you can erase and redo drawings. To keep things simple, do not add the secondary, find the shoulder distance of the field stop in an eyepiece. That is the location of the field stop in the eyepiece in relation to the shoulder on the outside. Check a couple to be sure. They are almost all close, not quite parfocal, but close. Add this to the drawing, positioning the field stop at the prime focus. Next, add the focuser to the drawing with it in the mid range of travel and position its measurements at the shoulder location of the eyepiece. Add in the outside diameter radius of the tube. Now you know exactly where the secondary goes.

    Now, to add even more potential confusion, a 1 1/4" to 2" adaptor will consume shoulder distance, be sure to add this discrepancy to your calculations if you are using a 2" focuser. It is typically 3/8"/10mm. If you use a low profile focuser like me be sure to make or get a parfocal ring for your 2" eyepieces so you don't run out of focusing range between the two sizes.

    The first time you lay out an OTA, do not double check everything, or triple, you run through it as many times as you feel comfortable with and take your time, that is why I suggest using a pencil. Try to build in some "wiggle room", even the largest telescopes have it to get the image exactly where it needs to be. Use slightly slotted holes for the primary cell, spider and focuser. Even the collimation adjustments in the mirror cell can provide some. You can use this procedure with a refractor, too, except you won't have the same wiggle room. Fortunately they have a lot more focuser travel so it is not needed.

 

 
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