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Bill Morgan
member
Reged: 01/18/09
Posts: 24
Loc: California
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I made a optical test bench for testing a long focuse refractor to determine the tube length. Because focal length is rather long and was it a challenge to find the best focuse. I recorded the length I needed, however, I thought about the length I used for In-focuse. I moved the focuser out to the scale number *1* position on the focuser and made my measurements. Later, I thought about this and figured that this isn't very much in travel (10mm?) I would rather not set everything up again and remeasure in my simple and crude test bench. So, to figure the focuser out travel, do I subtract from my tube measurement or add? I'm not sure of how the light cone moves in relation to the diagonal to the eyepiece. Is more in travel needed for High powered eyepieces? and I need to subtract/add another 10mm from my tube measurement?
Thank you,
Bill
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In the makings:
16 inch F/4.5
13.1 inch F/4.5
10 inch F/6
Two Jeagar's 4" refractor lenses
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rboe
Reged: 03/16/02
Posts: 45406
Loc: Phx, AZ
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Shoot, I was hoping for a response as I'll be facing the same thing when I get around to building my refractor.
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Ron
NS11GPS
Pronto
16" dob
15X70 Obies
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GPak
member
Reged: 08/25/09
Posts: 28
Loc: Tucson,AZ
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Boy, This it wierd. I was just getting on CN to start a thread with basically the same question. Friday, I traded a guy some "stuff" for a 150mm objective in a cell with a dew sheild atached. Through cruising the net I've determined that it's a Skywatcher Evostar. I had already figured out tht it's an f8 by using the old stick and cardboard trick, but I'm only guessing that the point of focus needs to be at the first piece of glass it hits in the eye piece. Can anyone confirm that?
Bill, I've got a set of eyepieces that are parfocal,so if I hit focus on one then they all should work. But Id like to try hooking my DSLR to it also. That requires quite a bit more intravel. As I'm typing this, I'm thinking what I might try and do is pick up a chunk of pvc pipe. Tremporarily attach the lens to one end and the focuser, diagonal and eyepice to the other. Then just keep cutting 1 inch pieces off til I get to focus with the focuser half way in/out. Then pull the diagonal and put the camera in and see if I can get that to focus. Not very scientific, but it might work!?
Gary
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Owen
professor emeritus
Reged: 06/21/07
Posts: 514
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The Focal length of the eyepiece is stated in mm - that is the distance before the first optic in the eyepiece 
Have fun
Owen
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Bill Morgan
member
Reged: 01/18/09
Posts: 24
Loc: California
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Ok, I will go ahead and remount the optics and re-measure for the tube length. I have have never used a barlow and want to be able to figure this into the set-up. My question now is this: when using a barlow and focusing, does the focuser move in or out to achieve focus?
Bill
--------------------
In the makings:
16 inch F/4.5
13.1 inch F/4.5
10 inch F/6
Two Jeagar's 4" refractor lenses
Edited by Bill Morgan (11/03/09 10:25 PM)
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