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Old 13-10-2018, 08:59 PM
RyanJones
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Skywatchers 102mm refractor help

Hi all,

Today I purchased a skywatcher 102mm refractor from another IIS member. It didn't come with a diagonal or a dovetail so I have to get a couple of other bits and pieces to use it. In the mean time, I thought I'd throw an eye piece on the back of it and just hold it up to the moon and have a look. My problem is I can't get anywhere near focus with it with the focuser fully racked in and a 21mm eyepiece in it, the big white ball gets smaller as if it's moving towards focus but it doesn't even get close. At first I thought maybe it has to have the diagonal to get to the correct focal length to get focus but surely that would mean it would get closer to focus with the focuser racked all the way out ? I'm confused. This is my first refractor so I'm really flying blind. I'm I missing something in my head or on my OTA ?

Ryan
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Old 13-10-2018, 09:07 PM
Karlzburg (Karl)
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Sounds like you need an extension, try with your camera in it. I need 50mm extension for viewing and nothing with the camera attached.
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Old 13-10-2018, 09:10 PM
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mental4astro (Alexander)
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Ruan,

Just to check, did you try this by rucking out the draw tube, and if not enough out then pulling out the eyepiece? Worth a try just out of curiosity as I've never heard of a refractor not having enough in-focus. I also have a 4" f/5 refractor, and if you don't have a diagonal the draw tube needs to be rucked out a lot more to achieve focus, not in-travel, with an eyepiece. I know what you mean about the Moon getting "smaller", but the draw tube needs to be pulled out. Karl mentions an extension tube just for this purpose. A barlow with the lens removed does the same job.

Alex.
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Old 13-10-2018, 09:15 PM
raymo
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Yep, you need either an extension, or a diagonal.
raymo
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Old 13-10-2018, 10:15 PM
RyanJones
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Thank you Raymo, Alex and Karl,

Ok, extension tube it is. I'm assuming this is because of where the exit pupil is ? Is there anyway to work out what this length is or is it a case of as Alex mentioned, racking out the focuser then using a bit of trial and error ?
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Old 13-10-2018, 10:23 PM
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mental4astro (Alexander)
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Not exit pupil, but focal point of the objective lens.
Exit pupil is the ratio of the eyepiece focal length divided by the focal ratio of the scope. Different things altogether. You use exit pupil for different aspects in selecting eyepieces for a giiven scope and its focal ratio.

Trial and error. Even if you don't have an extension tube, if rucking out the draw tube isn't enough, you can always carefully pull out the eyepiece until you achieve focus. It's just an exercise in verification for now anyway.

Alex.
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Old 13-10-2018, 10:50 PM
Wilso (Darren)
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Ryan,

On the front objective lens of the telescope normally has the focal length printed on the ring. ie (816mm for f8 scope)
So if you measure from the lens 816mm with a tape that is the distance it should reach focus. So this is the distance to the eyepiece lens.

Hope this helps
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Old 13-10-2018, 10:50 PM
RyanJones
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Thank you Alex. That makes sense. I'll give it a go
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Old 13-10-2018, 10:53 PM
RyanJones
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Quote:
Originally Posted by Wilso View Post
Ryan,

On the front objective lens of the telescope normally has the focal length printed on the ring. ie (816mm for f8 scope)
So if you measure from the lens 816mm with a tape that is the distance it should reach focus. So this is the distance to the eyepiece lens.

Hope this helps
Thank you Darren, I'll get my tape out and measure it. When you say the lense of the eyepiece, I'm assuming that's the first lense at the bottom of the eyepiece yeah ?
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Old 13-10-2018, 11:10 PM
Wilso (Darren)
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It should be approximately where the lens meets the housing.
Objective lens lengths can vary slightly, but this should put you in the ball park!
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Old 13-10-2018, 11:14 PM
raymo
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You'd be much better off with a diagonal, unless you think you would enjoy laying on the ground when viewing anywhere near the zenith.
Even if you have a very tall tripod you will have to crane your neck back about 80 degrees, a most uncomfortable position.
raymo

Last edited by raymo; 13-10-2018 at 11:25 PM. Reason: more text
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Old 13-10-2018, 11:24 PM
Karlzburg (Karl)
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I think this scope will be for happy snaps Raymo.
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Old 13-10-2018, 11:28 PM
RyanJones
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Quote:
Originally Posted by Karlzburg View Post
I think this scope will be for happy snaps Raymo.
Correct
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Old 13-10-2018, 11:31 PM
RyanJones
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Quote:
Originally Posted by Wilso View Post
It should be approximately where the lens meets the housing.
Objective lens lengths can vary slightly, but this should put you in the ball park!
Now we are getting somewhere. Thank you Darren. I measured back from the objective 500mm and it is some 80mm beyond a fully racked out focuser. That explains it all. Thank you very much for your help, it is greatly appreciated.
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Old 13-10-2018, 11:32 PM
raymo
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Unless the camera has an articulated screen, it will still be quite a job lining
up on the target Karl; budget diagonals are quite cheap.
raymo
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Old 13-10-2018, 11:49 PM
RyanJones
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Quote:
Originally Posted by raymo View Post
Unless the camera has an articulated screen, it will still be quite a job lining
up on the target Karl; budget diagonals are quite cheap.
raymo
Thank you Raymo,

A diagonal is a given for visual and for setting my goto but I needed to understand why. My camera will be setup directly off the back of the focuser. Now I understand the focal length situation, I can now design my entire setup.
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