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dreamregent
sage
Reged: 04/06/09
Posts: 455
Loc: Clearwater, FL
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Ok, guys... I just finished my mirror and I'm finalizing my telescope plan. I've got an ES 14 and I want to make sure it comes to focus without having to lower my mirror when I use it. I did use it in a scope recently where the focuser tube didn't come out far enough to bring it to focus...I had to pull it out slightly with my hand. When seated in my JMI focuser, the bottom lens element sits about 1.44" inside my focuser draw tube. My draw tube has 1.7" of total travel and I assume that I need to reserve some of this distance for in-travel so I'm not sure where I need to place my focal plane. For this particular ep, should the focal plane be at (or near) that bottom lens or should it be further up inside this ep? I also intend to get an Antares 1.6x barlow...how much in travel do I need for it?
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Building a f5.24 10" Dob
in an octagonal wood tube
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Darenwh
Pooh-Bah
Reged: 05/11/06
Posts: 1224
Loc: Covington, GA
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Is this going to be a truss dob? If so, you can finish up the mirror box and upper cage then attach the mirror box to one end of a 2x6 and try the upper cage at different locations until you locate where all your eyepieces come to focus. Basically, stand the whole thing up pointed at polaris and try each eyepiece until you find how far the UTA has to be from the mirror box for everything to come to focus. Measure this distance and set your truss tubes up to match. That's it, your done.
I would go ahead and get the barlow also so you can insure all will come to focus with it also. If you find that you cannot get all eyepieces to focus then set it up needing more out focus to focus your remaining eyepieces and use an extension tube.
If it's not going to be a truss then mount the focuser w/the secondary on the 2x6 and then adjust the primary's position until all focus. This way you know how long the tube needs to be.
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Daren
Covington, GA
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dreamregent
sage
Reged: 04/06/09
Posts: 455
Loc: Clearwater, FL
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I understand what you're saying but I haven't got my secondary, yet, so I'm not able to do that experiment. Is there a way to calculate this beforehand? Maybe if someone knows roughly where the focal plane should be in relation to the field lens?
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Building a f5.24 10" Dob
in an octagonal wood tube
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dreamregent
sage
Reged: 04/06/09
Posts: 455
Loc: Clearwater, FL
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I originally posted this in the eyepiece forum since my question is in regard to this eyepiece specifically. However, I only got one response so I requested that the thread be moved here. Does anyone have anything else to add? Thanks in advance...
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Building a f5.24 10" Dob
in an octagonal wood tube
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joe colontonio
sage
Reged: 03/02/06
Posts: 206
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Bring it out to Starkey tonight and we will check it out.
Joe
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1276
Loc: Ottawa, Ontario, Canada
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You can use any old 'scope or lens to test the range of positions an eyepiece set requires in order to come to focus. It all comes down to the location of the eyepiece's shoulder with respect to the focal surface. Note that the shoulder is the 'ledge' on the eyepiece which stops further inward travel into the drawtube. (Incidentally, Televue provides this info for all their eyepieces.)
This testing can be conveniently performed on a bench indoors, with the objective focused on something across the room. Place a marker at the plane of focus and leave it in place during the testing. Then note the position the eyepiece shoulder with respect to the focal surface marker.
I'll stress that it's not necessary to use the 'scope you intend to observe with for this testing, nor is it necessary to focus on infinity (or a very distant target.) All that matters is that you form an image, by whatever means, so that you have a reasonably sharp reference to focus on. Whatever lens you use, its focal surface will be fixed in space just as for your bigger 'scope.
After all this, it's time to decide on where to position the focal surface with respect to the focuser's drawtube in the finished 'scope. Your eyepiece set just may require a larger range of travel the the focuser provides. Then you must decide if an extension tube would be useful, and/or whether some eyepieces could benefit from 'lifters' in order to better bring them into parfocality. You get the drift....
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
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