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Main
Date: 30 Oct 2006 07:42:44
From:
Subject: Locating secondary mirror on Newtonian
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Hello -- Can anyone tell me where the focal point of a telescope needs
to be in relation to the eyepiece in order for the image to be in focus
when looking through the eyepiece? Does it need to be at the rear
element of the eyepiece? Or somewhere inside the eypiece? Or
somewhere outside the eyepiece beyond the rear element?
The reason I ask is that I am trying to decide where to locate the
secondary mirror on a Newtonian telescope. If I put the secondary
mirror closer to the primary, the focal point will be further up the
focuser tube. However, I would like to put the secondary as far away
from the primary as possible, since then I can use a smaller secondary
and cause less obstruction of precious incoming light. So how far down
the focuser tube can I put the focus and still allow the eyepiece to
"reach" it?
(In case it matters, the main mirror is 13.1" diameter and has focal
length 59.5" (f/4.5). The tube diameter is 16".)
Any help appreciated,
Robert Enenkel
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Date: 30 Oct 2006 18:04:37
From: Mike
Subject: Re: Locating secondary mirror on Newtonian
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>
> (In case it matters, the main mirror is 13.1" diameter and has focal
> length 59.5" (f/4.5). The tube diameter is 16".)
You answered the question. Your FL is 59.5" . The EP magnifies the image
at the FP. The FP can be outside or inside the EP barrel but where it lies
is up to you
and your ability to measure.
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Date: 30 Oct 2006 08:29:57
From:
Subject: Re: Locating secondary mirror on Newtonian
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renenkel@hotmail.com wrote:
> Hello -- Can anyone tell me where the focal point of a telescope needs
> to be in relation to the eyepiece in order for the image to be in focus
> when looking through the eyepiece? Does it need to be at the rear
> element of the eyepiece? Or somewhere inside the eypiece? Or
> somewhere outside the eyepiece beyond the rear element?
>
> The reason I ask is that I am trying to decide where to locate the
> secondary mirror on a Newtonian telescope. If I put the secondary
> mirror closer to the primary, the focal point will be further up the
> focuser tube. However, I would like to put the secondary as far away
> from the primary as possible, since then I can use a smaller secondary
> and cause less obstruction of precious incoming light. So how far down
> the focuser tube can I put the focus and still allow the eyepiece to
> "reach" it?
>
> (In case it matters, the main mirror is 13.1" diameter and has focal
> length 59.5" (f/4.5). The tube diameter is 16".)
>
> Any help appreciated,
> Robert Enenkel
When looking into the bottom end of most eyepieces, you should see a
field stop (located a fraction of an inch outside the field lens) which
should end up at or near the scope's focal plane when the scope is in
focus.
Don't try to use too small a diagonal; sometimes the edges aren't good
and you usually want good illumination across the field of view anyway.
A 2.75- to 3-inch minor axis (20 -23 % obstruction) is probably OK
for a 13.1-inch scope for visual use. You will also need to know the
"racked-in" height of the focuser and allow for additional distance to
your camera's focal plane (assuming you want to try 35mm
astrophotography, you'll probably need a somewhat larger diagonal.)
Dale Keller's Newt program (shareware? or freeware?) will help you work
out the numbers.
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Date: 30 Oct 2006 11:13:27
From: Dennis Woos
Subject: Re: Locating secondary mirror on Newtonian
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<renenkel@hotmail.com > wrote in message
news:1162222964.635593.7290@m7g2000cwm.googlegroups.com...
> Hello -- Can anyone tell me where the focal point of a telescope needs
> to be in relation to the eyepiece in order for the image to be in focus
> when looking through the eyepiece? Does it need to be at the rear
> element of the eyepiece? Or somewhere inside the eypiece? Or
> somewhere outside the eyepiece beyond the rear element?
>
> The reason I ask is that I am trying to decide where to locate the
> secondary mirror on a Newtonian telescope. If I put the secondary
> mirror closer to the primary, the focal point will be further up the
> focuser tube. However, I would like to put the secondary as far away
> from the primary as possible, since then I can use a smaller secondary
> and cause less obstruction of precious incoming light. So how far down
> the focuser tube can I put the focus and still allow the eyepiece to
> "reach" it?
>
> (In case it matters, the main mirror is 13.1" diameter and has focal
> length 59.5" (f/4.5). The tube diameter is 16".)
>
> Any help appreciated,
> Robert Enenkel
>
There is software to help you calc this:
http://home.att.net/~dale.keller/atm/newtonians/newtsoft/newtsoft.htm
Also, you will find a lot more assistance here:
http://www.atmlist.net/
Finally, I always leave some extra length at the primary end of the tube,
and only cut this end and mount the primary until after I have tested the
configuration.
Dennis
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Date: 30 Oct 2006 11:50:31
From: Howard Lester
Subject: Re: Locating secondary mirror on Newtonian
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<renenkel@hotmail.com > wrote
> Hello -- Can anyone tell me where the focal point of a telescope needs
> to be in relation to the eyepiece in order for the image to be in focus
> when looking through the eyepiece?
As a general guideline, set the focal plane about a half inch above the top
of the focusing tube (at the point where it is fully racked in). One-half
inch of "in-travel" should accommodate all or most eyepieces. TeleVue shows
such specs of its eyepieces on the TV website. Some Naglers, I believe, may
require more in-travel than one-half inch.
Howard Lester
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Date: 31 Oct 2006 03:12:44
From:
Subject: Re: Locating secondary mirror on Newtonian
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Uncle Bob wrote:
> On Mon, 30 2006 08:29:57 -0800, wsnell01 wrote:
>
> > When looking into the bottom end of most eyepieces, you should see a
> > field stop (located a fraction of an inch outside the field lens) which
> > should end up at or near the scope's focal plane when the scope is in
> > focus.
> >
> > Don't try to use too small a diagonal; sometimes the edges aren't good
> > and you usually want good illumination across the field of view anyway.
> > A 2.75- to 3-inch minor axis (20 -23 % obstruction) is probably OK
> > for a 13.1-inch scope for visual use. You will also need to know the
> > "racked-in" height of the focuser and allow for additional distance to
> > your camera's focal plane (assuming you want to try 35mm
> > astrophotography, you'll probably need a somewhat larger diagonal.)
> >
> > Dale Keller's Newt program (shareware? or freeware?) will help you work
> > out the numbers.
>
> It is a good idea when building the OTA (if you're using a tube) to make a
> provision to mount the mirror cell ~3/4" closer to the secondary for
> astrophotography purposes. Just an extra set of mirror cell bolt holes a
> little farther up the tube will do.
> They used to do this on Meade and Cave newtonians. Handy.
>
> Clear Skies and good luck with your project.
> Uncle Bob
Perhaps even easier, in the long run, would be to use a low-profile
focuser with an extension tube (or a focuser with enough travel) and
choose a diagonal large enough to provide acceptable illumination for
the film or CCD.
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Date: 31 Oct 2006 15:46:26
From: Grantland
Subject: Re: 911 -- Conspiracy Fatigue (only a really fast Yugo)
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On Tue, 31 2006 13:12:10 GMT, Ricardo <NSPA@blueyonder.co.uk >
wrote:
>William Black wrote:
>> "Keith Willshaw" <keith@kwillshawnospam.demon.co.uk> wrote in message
>> news:ei5k1m$lb3$1$830fa795@news.demon.co.uk...
>>
>>>"Vince" <firelaw@firelaw.us> wrote in message
>>>news:iqOdnV-2aMHArNvYnZ2dnUVZ_s-dnZ2d@comcast.com...
>>>
>>>>jeffreysmidt@yahoo.com wrote:
>>>
>>>
>>>>>Even a better example. Take a Bear Killer arrow, with razor blade
>>>>>head and aluminum shaft and shoot it through a engine block.
>>>>
>>>>I assume you mean with a bow
>>>>in which case I kind of doubt it.
>>>>50 Ft pounds will not penetrate steel
>>>>
>>>
>>>While I wouldnt give you any chance of cracking an
>>>engine block tests with arrows using short bodkin
>>>heads fired by longbowmen have shown an
>>>ability to penetrate mild steel plate armour at
>>>ranges of around 60 m
>>
>>
>> 25 to 30 meters is the best I've seen done with a Yew bow.
>>
>> With a steel bow you'd do a lot better, but nobody, well, nobody sane
>> shoots them these days as they're far too dangerous.
>>
>
>If a lack of sanity is a requirement to use a steel bow, many
>contributors to this newsgroup seem eminently qualified to use one!
>
>Ricardo
I wonder how a bow of amorphous steel would perform.
http://www.liquidmetal.com/
Grantland
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Date: 01 Nov 2006 23:28:09
From:
Subject: Re: Locating secondary mirror on Newtonian
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Uncle Bob wrote:
> On Tue, 31 2006 03:12:44 -0800, wsnell01 wrote:
>
> >
> > Uncle Bob wrote:
> >> On Mon, 30 2006 08:29:57 -0800, wsnell01 wrote:
> >>
>
> >
> > Perhaps even easier, in the long run, would be to use a low-profile
> > focuser with an extension tube (or a focuser with enough travel) and
> > choose a diagonal large enough to provide acceptable illumination for
> > the film or CCD.
>
> I like that. Can you recommend a low profile focuser and extension tube
> combo in the 2" range?
> Regards,
> Uncle Bob
I have an old helical focuser that has a minimum height of 1-inch, but
the manufacturer is now out of business AFAIK.
If I were to replace it with something better, then a JMI focuser
(which William Mattil has already recommended) would be at the top of
my list, although I might consider an Astrosystems Phase 4 or a
Kineoptics Helical Crayford.
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Date: 08 Nov 2006 14:30:28
From:
Subject: Re: Newtonian focuser diameter to avoid vignetting
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renenkel@hotmail.com wrote:
> A big thank-you to you all for the valuable advice! You have saved me
> much grief. In particular, I found the NEWT software very useful. I
> was surprised at the results. With the data for my telescope (13.1"
> f/4.5 mirror, 16" O.D. tube) I found that I could not avoid vignetting
> at the focuser of the 75% ray with a 2" diameter focuser, even with a
> low-profile focuser (1.75" high) and the focal plane only 0.5" above
> the minimum focuser height.
>
> The only focuser larger than 2" diameter I could find was a 3" one by
> JMI that costs $500US, which is a lot of money for me. Is it really
> necessary to have such a large, expensive focuser? What would happen
> if I get a 2" one instead? I noticed that in the NEWT ray-trace
> picture, the yellow 75% ray just grazed the edge of the 2" focuser, so
> perhaps the vignetting would not be very noticable, at least for visual
> use?
>
> Alternatively, does anyone know of less expensive 3" focusers?
>
> Thanks again,
> Robert
I would stick with the 2-inch focuser. 2-inch eyepieces have a max
field stop of about 46mm anyway. For photography the vignetting would
probably be the least of your problems even assuming that it was very
noticeable. Try running the numbers again, with ever smaller
diagonals, until the 75%-vignetting-at-focuser problem disappears.
Then note that the 100% illuminated field has also become much smaller
as a result.
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Date: 08 Nov 2006 13:27:29
From:
Subject: Newtonian focuser diameter to avoid vignetting
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A big thank-you to you all for the valuable advice! You have saved me
much grief. In particular, I found the NEWT software very useful. I
was surprised at the results. With the data for my telescope (13.1"
f/4.5 mirror, 16" O.D. tube) I found that I could not avoid vignetting
at the focuser of the 75% ray with a 2" diameter focuser, even with a
low-profile focuser (1.75" high) and the focal plane only 0.5" above
the minimum focuser height.
The only focuser larger than 2" diameter I could find was a 3" one by
JMI that costs $500US, which is a lot of money for me. Is it really
necessary to have such a large, expensive focuser? What would happen
if I get a 2" one instead? I noticed that in the NEWT ray-trace
picture, the yellow 75% ray just grazed the edge of the 2" focuser, so
perhaps the vignetting would not be very noticable, at least for visual
use?
Alternatively, does anyone know of less expensive 3" focusers?
Thanks again,
Robert
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Date: 08 Nov 2006 19:20:55
From: Dennis Woos
Subject: Re: Newtonian focuser diameter to avoid vignetting
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<renenkel@hotmail.com > wrote in message
news:1163021249.623476.126020@i42g2000cwa.googlegroups.com...
>
> A big thank-you to you all for the valuable advice! You have saved me
> much grief. In particular, I found the NEWT software very useful. I
> was surprised at the results. With the data for my telescope (13.1"
> f/4.5 mirror, 16" O.D. tube) I found that I could not avoid vignetting
> at the focuser of the 75% ray with a 2" diameter focuser, even with a
> low-profile focuser (1.75" high) and the focal plane only 0.5" above
> the minimum focuser height.
>
> The only focuser larger than 2" diameter I could find was a 3" one by
> JMI that costs $500US, which is a lot of money for me. Is it really
> necessary to have such a large, expensive focuser? What would happen
> if I get a 2" one instead? I noticed that in the NEWT ray-trace
> picture, the yellow 75% ray just grazed the edge of the 2" focuser, so
> perhaps the vignetting would not be very noticable, at least for visual
> use?
>
> Alternatively, does anyone know of less expensive 3" focusers?
>
> Thanks again,
> Robert
>
What is the length of the minor axis of the secondary mirror? Try 2.6"
which is pretty standard for this configuration.
Dennis
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Date: 08 Nov 2006 22:27:24
From: William R. Mattil
Subject: Re: Newtonian focuser diameter to avoid vignetting
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renenkel@hotmail.com wrote:
> A big thank-you to you all for the valuable advice! You have saved me
> much grief. In particular, I found the NEWT software very useful. I
> was surprised at the results. With the data for my telescope (13.1"
> f/4.5 mirror, 16" O.D. tube) I found that I could not avoid vignetting
> at the focuser of the 75% ray with a 2" diameter focuser, even with a
> low-profile focuser (1.75" high) and the focal plane only 0.5" above
> the minimum focuser height.
>
> The only focuser larger than 2" diameter I could find was a 3" one by
> JMI that costs $500US, which is a lot of money for me. Is it really
> necessary to have such a large, expensive focuser? What would happen
> if I get a 2" one instead? I noticed that in the NEWT ray-trace
> picture, the yellow 75% ray just grazed the edge of the 2" focuser, so
> perhaps the vignetting would not be very noticable, at least for visual
> use?
>
> Alternatively, does anyone know of less expensive 3" focusers?
>
>
Robert,
You can move things around a bit to improve the situation. For example,
if the 2 inch focuser vignettes slightly then make a 3 inch hole in the
tube and mount a riser with a 3 inch hole that is about an inch or
perhaps 2 tall and mount the focuser to that. Or perhaps even simpler,
move the primary mirror back an inch or so. Same net result. And you
might be able to use a smaller secondary if that's your thing.
Regards
Bill
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