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Main
Date: 14 Jun 2007 12:04:16
From: galwacco
Subject: Smaller hole in telescope's cover
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Hi, I am seriously new to the world of reflector telescope maned
astronomy.
I have one weird doubt that I just couldn't manage to find any answers
about that.
I just got my first reflector telescope! It's a 4" aperture,
equatorial, the point that made me really curious was the cover that
came along with the telescope.
Of course, a cover is to protect the telescope against dew and dust,
the thing is that the cover has one whole of about one inch and this
hole also comes with a cap! I wonder what is that whole for?
Am I supposed to use the telescope with that cap on but only with the
smaller whole opened?
Sorry if this Q is just too stupid, I couldn't help but just ask it!
My curiosity was killing me!
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Date:
From: Martin Brown
Subject: Re: Smaller hole in telescope's cover
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Date: 19 Jun 2007 02:37:26
From: Peter Webb
Subject: Re: Smaller hole in telescope's cover
|
Great answer - thanks.
Funny thing is I once wrote a paper on using Golumbs ladder to choose
frequencies for satellite uplinks to minimise harmonics from non-linear
amplifiers. The concept being triplets like f, f+k, f+3k don't have
cross-mod noise on active frequencies. So it was interesting seeing it pop
up in a related field but in a completely different question.
Also thanks for the lattice explanation.
I am digesting this and the other replies - thanks to all.
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Date: 17 Jun 2007 07:06:49
From: Helpful person
Subject: Re: Smaller hole in telescope's cover
|
On Jun 16, 10:37 pm, "Peter Webb"
<webbfam...@DIESPAMDIEoptusnet.com.au > wrote:
> "Chris L Peterson" <c...@alumni.caltech.edu> wrote in messagenews:g78573l8s4e1qej0u5ikqp8atp0rv29gc2@4ax.com...
>
> > On Fri, 15 Jun 2007 16:56:40 +1000, "Peter Webb"
> > <webbfam...@DIESPAMDIEoptusnet.com.au> wrote:
>
> >>Why do they make one hole with an area of (say) 1 square inch, instead of
> >>three smaller holes spaced as far apart as possible? That way you would
> >>get
> >>the full resolution of the telescope and reduced light, instead of reduced
> >>light and reduced resolution.
>
> > A circular aperture produces the smallest diffraction pattern. The more
> > complex diffraction produced by occluded or multiple apertures can cause
> > reduced contrast at a fine scale (the MTF is degraded). So while there
> > are special cases where unusual apertures are useful, as a general rule
> > circular is best.
>
> I know you are correct, because that is how telescopes are made (and
> telescope makers are not idiots), and you obviously know far more about this
> than I do.
>
> But I still don't understand why. As I don't have a telescope, I am reduced
> to theory (which is my interest anyway).
>
> Here are some statements, and I would like a true or false statement on
> each. Lets assume we have an 8" reflector, no spider, perfect world, etc:
>
> 1. An 8" telescope stopped down to 1" aperture has the same light gathering
> power and resolution as a 1" telescope of the same focal length, so you are
> chucking away 63/64th of the light and 7/8 of the angular resolution.
>
> 2. An 8" telescope with a cover with 1,000 evenly spread tiny holes adding
> to the same area as hole of 1" diameter chucks away 63/64th of the light,
> but none of the resolution, and indeed performs the same as an 8" telescope
> with a filter at the front-end which blocks 63/64ths of the light.
>
> 3. The images taken of an object taken with a single hole will be
> diffraction limited to the size of the hole, but if multiple holes are used
> will be diffraction limited to the spacing of the holes (not their
> diameter).
>
> 4. If the total amount of light is not an issue (as for example when viewing
> the moon or terrestrial objects), then the only other thing a larger
> telescope is giving you is angular resolution - which is thrown away when
> you "stop" down the aperture with a single hole.
>
> 5. Radio telescopes use the same technique - multiple widely separated
> "holes" (antennae) instead of one big hole (or lots of little holes
> clustered together). So, for that matter, does Keck, but regrettably only on
> one axis.
>
> What am I missing?
>
> TIA
>
> Peter Webb
You do not understand how the aperture size and shape affect
resolution. Let me try and explain in as simple a manner as I can.
The MTF (Modulation Transfer Function) of a lens defines the response
of the lens to spatial frequencies. A lens is a low pass filter,
passing through low spatial frequency (in the image) and reducing the
contrast of high spatial frequencies. This can be described as a
graph of modulation versus spatial frequency.
The MTF curve can be derived by autocorrelation of the lens pupil.
Consider two pictures of the pupil (usually round) on top of each
other. Then translate one sideways. Calculate the shared area
between the two pupils. The translation is proportional to the image
spatial frequency (in the direction of the translation) and the shared
area is proportional to its MTF. (All this is without considering
aberrations.)
If the aperture is a funny shape or has obstructions, some of the
spatial frequencies will be reduced or missing. This may be different
for different direction in the image. It is a fairly easy exercise to
determine the MTF for any pupil shape you choose.
This is exactly analogous to a low pass filter in electronics, except
in optics we are dealing in two dimensions rather than one.
Please visit my web site at www.richardfisher.com
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Date:
From: Martin Brown
Subject: Re: Smaller hole in telescope's cover
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Date: 14 Jun 2007 20:07:59
From: Tater
Subject: Re: Smaller hole in telescope's cover
|
On Jun 14, 2:04 pm, galwacco <claudio.ferna...@gmail.com > wrote:
> Hi, I am seriously new to the world of reflector telescope maned
> astronomy.
> I have one weird doubt that I just couldn't manage to find any answers
> about that.
>
> I just got my first reflector telescope! It's a 4" aperture,
> equatorial, the point that made me really curious was the cover that
> came along with the telescope.
>
> Of course, a cover is to protect the telescope against dew and dust,
> the thing is that the cover has one whole of about one inch and this
> hole also comes with a cap! I wonder what is that whole for?
>
> Am I supposed to use the telescope with that cap on but only with the
> smaller whole opened?
>
> Sorry if this Q is just too stupid, I couldn't help but just ask it!
> My curiosity was killing me!
I have a similar scope
I've found that the contrast of the moon becomes much better, but it
could be my imagination.
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Date: 14 Jun 2007 23:01:22
From: David G. Nagel
Subject: Re: Smaller hole in telescope's cover
|
Tater wrote:
> On Jun 14, 2:04 pm, galwacco <claudio.ferna...@gmail.com> wrote:
>> Hi, I am seriously new to the world of reflector telescope maned
>> astronomy.
>> I have one weird doubt that I just couldn't manage to find any answers
>> about that.
>>
>> I just got my first reflector telescope! It's a 4" aperture,
>> equatorial, the point that made me really curious was the cover that
>> came along with the telescope.
>>
>> Of course, a cover is to protect the telescope against dew and dust,
>> the thing is that the cover has one whole of about one inch and this
>> hole also comes with a cap! I wonder what is that whole for?
>>
>> Am I supposed to use the telescope with that cap on but only with the
>> smaller whole opened?
>>
>> Sorry if this Q is just too stupid, I couldn't help but just ask it!
>> My curiosity was killing me!
>
> I have a similar scope
>
> I've found that the contrast of the moon becomes much better, but it
> could be my imagination.
>
Nope, you are not imagining things. The use of the cover with a hole in
it acts to reduce the massive amount of light reflected from the moon.
This decrease in light makes your eye work easier and does increase the
contrast.
Dave n
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Date: 15 Jun 2007 16:56:40
From: Peter Webb
Subject: Re: Smaller hole in telescope's cover
|
"David G. Nagel" <nagel@core.com > wrote in message
news:13743ujliil2762@corp.supernews.com...
> Tater wrote:
>> On Jun 14, 2:04 pm, galwacco <claudio.ferna...@gmail.com> wrote:
>>> Hi, I am seriously new to the world of reflector telescope maned
>>> astronomy.
>>> I have one weird doubt that I just couldn't manage to find any answers
>>> about that.
>>>
>>> I just got my first reflector telescope! It's a 4" aperture,
>>> equatorial, the point that made me really curious was the cover that
>>> came along with the telescope.
>>>
>>> Of course, a cover is to protect the telescope against dew and dust,
>>> the thing is that the cover has one whole of about one inch and this
>>> hole also comes with a cap! I wonder what is that whole for?
>>>
>>> Am I supposed to use the telescope with that cap on but only with the
>>> smaller whole opened?
>>>
>>> Sorry if this Q is just too stupid, I couldn't help but just ask it!
>>> My curiosity was killing me!
>>
>> I have a similar scope
>>
>> I've found that the contrast of the moon becomes much better, but it
>> could be my imagination.
>>
> Nope, you are not imagining things. The use of the cover with a hole in it
> acts to reduce the massive amount of light reflected from the moon. This
> decrease in light makes your eye work easier and does increase the
> contrast.
>
> Dave n
Something I have always wondered about these holes (which are also useful
for terrestrial use of the scope).
Why do they make one hole with an area of (say) 1 square inch, instead of
three smaller holes spaced as far apart as possible? That way you would get
the full resolution of the telescope and reduced light, instead of reduced
light and reduced resolution.
Seems to make no sense to me ....
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Date: 15 Jun 2007 14:28:04
From: Chris L Peterson
Subject: Re: Smaller hole in telescope's cover
|
On Fri, 15 Jun 2007 16:56:40 +1000, "Peter Webb"
<webbfamily@DIESPAMDIEoptusnet.com.au > wrote:
>Why do they make one hole with an area of (say) 1 square inch, instead of
>three smaller holes spaced as far apart as possible? That way you would get
>the full resolution of the telescope and reduced light, instead of reduced
>light and reduced resolution.
A circular aperture produces the smallest diffraction pattern. The more
complex diffraction produced by occluded or multiple apertures can cause
reduced contrast at a fine scale (the MTF is degraded). So while there
are special cases where unusual apertures are useful, as a general rule
circular is best.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 17 Jun 2007 12:37:49
From: Peter Webb
Subject: Re: Smaller hole in telescope's cover
|
"Chris L Peterson" <clp@alumni.caltech.edu > wrote in message
news:g78573l8s4e1qej0u5ikqp8atp0rv29gc2@4ax.com...
> On Fri, 15 Jun 2007 16:56:40 +1000, "Peter Webb"
> <webbfamily@DIESPAMDIEoptusnet.com.au> wrote:
>
>>Why do they make one hole with an area of (say) 1 square inch, instead of
>>three smaller holes spaced as far apart as possible? That way you would
>>get
>>the full resolution of the telescope and reduced light, instead of reduced
>>light and reduced resolution.
>
> A circular aperture produces the smallest diffraction pattern. The more
> complex diffraction produced by occluded or multiple apertures can cause
> reduced contrast at a fine scale (the MTF is degraded). So while there
> are special cases where unusual apertures are useful, as a general rule
> circular is best.
>
I know you are correct, because that is how telescopes are made (and
telescope makers are not idiots), and you obviously know far more about this
than I do.
But I still don't understand why. As I don't have a telescope, I am reduced
to theory (which is my interest anyway).
Here are some statements, and I would like a true or false statement on
each. Lets assume we have an 8" reflector, no spider, perfect world, etc:
1. An 8" telescope stopped down to 1" aperture has the same light gathering
power and resolution as a 1" telescope of the same focal length, so you are
chucking away 63/64th of the light and 7/8 of the angular resolution.
2. An 8" telescope with a cover with 1,000 evenly spread tiny holes adding
to the same area as hole of 1" diameter chucks away 63/64th of the light,
but none of the resolution, and indeed performs the same as an 8" telescope
with a filter at the front-end which blocks 63/64ths of the light.
3. The images taken of an object taken with a single hole will be
diffraction limited to the size of the hole, but if multiple holes are used
will be diffraction limited to the spacing of the holes (not their
diameter).
4. If the total amount of light is not an issue (as for example when viewing
the moon or terrestrial objects), then the only other thing a larger
telescope is giving you is angular resolution - which is thrown away when
you "stop" down the aperture with a single hole.
5. Radio telescopes use the same technique - multiple widely separated
"holes" (antennae) instead of one big hole (or lots of little holes
clustered together). So, for that matter, does Keck, but regrettably only on
one axis.
What am I missing?
TIA
Peter Webb
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Date: 17 Jun 2007 16:23:59
From: Chris L Peterson
Subject: Re: Smaller hole in telescope's cover
|
On Sun, 17 Jun 2007 12:37:49 +1000, "Peter Webb"
<webbfamily@DIESPAMDIEoptusnet.com.au > wrote:
>1. An 8" telescope stopped down to 1" aperture has the same light gathering
>power and resolution as a 1" telescope of the same focal length, so you are
>chucking away 63/64th of the light and 7/8 of the angular resolution.
Yes. But you should keep in mind that in many cases the actual
resolution you can achieve with a telescope is far less than the
theoretical resolution of the telescope as determined by aperture. Many
things conspire to reduce resolution, such as atmospheric motion, the
inherent low resolution of the human eye at low light levels, etc.
>2. An 8" telescope with a cover with 1,000 evenly spread tiny holes adding
>to the same area as hole of 1" diameter chucks away 63/64th of the light,
>but none of the resolution, and indeed performs the same as an 8" telescope
>with a filter at the front-end which blocks 63/64ths of the light.
A telescope like this does not have the same resolution as one with an
8" unobstructed aperture. In fact, it will have really poor resolution
for most objects. The interference patterns produced by 1000 individual
circular apertures will overlap and interfere with each other, producing
a seriously degraded MTF. Except for the special case of objects with
spatial frequencies the same as that produced by the hole pattern
(assuming the hole spacing is regular), you will not have a high
resolution result. This is easy enough to test by making a mask like you
describe.
>3. The images taken of an object taken with a single hole will be
>diffraction limited to the size of the hole, but if multiple holes are used
>will be diffraction limited to the spacing of the holes (not their
>diameter).
A set of widely spaced holes will provide for higher resolution along
the axis of those holes than you would get from a single hole, but along
other axes the resolution won't be as high. When you combine these
asymmetric resolutions with diffraction, the result is not something
most people would be happy with, certainly not for "aesthetic" viewing.
>4. If the total amount of light is not an issue (as for example when viewing
>the moon or terrestrial objects), then the only other thing a larger
>telescope is giving you is angular resolution - which is thrown away when
>you "stop" down the aperture with a single hole.
That is true, but most people don't stop down their telescopes enough
for this to be significant. For instance, when viewing the Sun, it is
common for apertures larger than just a couple of inches to be seeing
limited anyway. I regularly stop my 12" scope down to 4" when viewing
the Sun, and have never detected any drop in resolution.
>5. Radio telescopes use the same technique - multiple widely separated
>"holes" (antennae) instead of one big hole (or lots of little holes
>clustered together). So, for that matter, does Keck, but regrettably only on
>one axis.
They do. But they also mathematically reconstruct the image, just as is
done with optical interferometers. If you simply looked at the raw radio
data, or put your eye at the focal plane of the Keck interferometer, you
wouldn't see anything that really looked like a normal telescopic view.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 17 Jun 2007 15:28:45
From: Guy Macon
Subject: Re: Smaller hole in telescope's cover
|
Peter Webb wrote:
>
>Chris L Peterson wrote...
>
>> A circular aperture produces the smallest diffraction pattern. The more
>> complex diffraction produced by occluded or multiple apertures can cause
>> reduced contrast at a fine scale (the MTF is degraded). So while there
>> are special cases where unusual apertures are useful, as a general rule
>> circular is best.
>
>I know you are correct, because that is how telescopes are made (and
>telescope makers are not idiots), and you obviously know far more
>about this than I do.
>
>But I still don't understand why. As I don't have a telescope, I am reduced
>to theory (which is my interest anyway).
>
>Here are some statements, and I would like a true or false statement on
>each. Lets assume we have an 8" reflector, no spider, perfect world, etc:
I am going to give these a shot, but keep in mind that I am just an
Electronics Engineer with some optical experience (mostly UV lasers),
so please forgive me if I get it wrong and please pay attention if
someone corrects my errors.
>1. An 8" telescope stopped down to 1" aperture has the same light gathering
>power and resolution as a 1" telescope of the same focal length, so you are
>chucking away 63/64th of the light and 7/8 of the angular resolution.
It seems to me that it *is* a one inch telescope. Enclose the shaft
of light with a tube and cut away the portion of the mirror that is
in darkness, and you have a one inch off-axis reflector.
>2. An 8" telescope with a cover with 1,000 evenly spread tiny holes adding
>to the same area as hole of 1" diameter chucks away 63/64th of the light,
>but none of the resolution, and indeed performs the same as an 8" telescope
>with a filter at the front-end which blocks 63/64ths of the light.
I don't think so. The filter doesn't have any edge diffraction effects.
The holes do. Do a web search on "Hartman Screen" / "Hartman Mask",
then do a search on "Photon Sieve" for a taste of the kind of effects
that this can cause.
>3. The images taken of an object taken with a single hole will be
>diffraction limited to the size of the hole, but if multiple holes are used
>will be diffraction limited to the spacing of the holes (not their
>diameter).
Wouldn't the two holes, if small, would be a variation on the classic
double-slit experiment, and thus would look less like an image and
more like an interference pattern as the holes got smaller?
>4. If the total amount of light is not an issue (as for example when viewing
>the moon or terrestrial objects), then the only other thing a larger
>telescope is giving you is angular resolution - which is thrown away when
>you "stop" down the aperture with a single hole.
Ignoring central obstruction and spider, the single hole seems to me
to actually be a smaller telescope. Which means less light gathering
and lower maximum resolution.
>5. Radio telescopes use the same technique - multiple widely separated
>"holes" (antennae) instead of one big hole (or lots of little holes
>clustered together). So, for that matter, does Keck, but regrettably
>only on one axis.
I believe that the radio telescopes and Keck use aperture synthesis
by combining fourier transforms. Simply adding the two waveforms on
a retina, film, or CCD array loses information compared to aperture
synthesis. Also, doesn't Keck use the rotation of the Earth to get
a second dimension? Or am I mis-remembering and/or confusing it
with another system?
BTW, take a look at C.O.A.S.T. (Cambridge Optical Aperture Synthesis
Telescope) at http://www.mrao.cam.ac.uk/telescopes/coast/
>What am I missing?
Diffraction and scattering effects?
Guy Macon
<http://www.guymacon.com/ >
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Date: 16 Jun 2007 22:25:11
From: Starlord
Subject: Re: Smaller hole in telescope's cover
|
I have watched a good number of sci programs and on one I saw the 100inch
mirror was covered with a seet of thin sheet of plastic film that had first
had very dim faint fuzzies maped out on it and then they cut out the circle
that mapped the size and then it was moved to the mirror and punt on it,
while 96% of the photo was dark, they got more light from the faint fuzzies
(longer Exp.) and came up with they ones they had mapped out and the new
photo was 50 times better than the over all one that they used for the
mapping of the plastic.
I have a dob (10" F5) and I use the full scope only I use a moon filter to
cut down the moonlight.
The Lone Sidewalk Astronomer of Rosamond
Telescope Buyers FAQ
http://home.inreach.com/starlord
Sidewalk Astronomy
www.sidewalkastronomy.info
AD World
http://www.adworld.netfirms.com/
"Peter Webb" <webbfamily@DIESPAMDIEoptusnet.com.au > wrote in message
news:46749e3f$0$29665$afc38c87@news.optusnet.com.au...
>
> "Chris L Peterson" <clp@alumni.caltech.edu> wrote in message
> news:g78573l8s4e1qej0u5ikqp8atp0rv29gc2@4ax.com...
>> On Fri, 15 Jun 2007 16:56:40 +1000, "Peter Webb"
>> <webbfamily@DIESPAMDIEoptusnet.com.au> wrote:
>>
>>>Why do they make one hole with an area of (say) 1 square inch, instead of
>>>three smaller holes spaced as far apart as possible? That way you would
>>>get
>>>the full resolution of the telescope and reduced light, instead of
>>>reduced
>>>light and reduced resolution.
>>
>> A circular aperture produces the smallest diffraction pattern. The more
>> complex diffraction produced by occluded or multiple apertures can cause
>> reduced contrast at a fine scale (the MTF is degraded). So while there
>> are special cases where unusual apertures are useful, as a general rule
>> circular is best.
>>
>
> I know you are correct, because that is how telescopes are made (and
> telescope makers are not idiots), and you obviously know far more about
> this than I do.
>
> But I still don't understand why. As I don't have a telescope, I am
> reduced to theory (which is my interest anyway).
>
> Here are some statements, and I would like a true or false statement on
> each. Lets assume we have an 8" reflector, no spider, perfect world, etc:
>
> 1. An 8" telescope stopped down to 1" aperture has the same light
> gathering power and resolution as a 1" telescope of the same focal length,
> so you are chucking away 63/64th of the light and 7/8 of the angular
> resolution.
>
> 2. An 8" telescope with a cover with 1,000 evenly spread tiny holes adding
> to the same area as hole of 1" diameter chucks away 63/64th of the light,
> but none of the resolution, and indeed performs the same as an 8"
> telescope with a filter at the front-end which blocks 63/64ths of the
> light.
>
> 3. The images taken of an object taken with a single hole will be
> diffraction limited to the size of the hole, but if multiple holes are
> used will be diffraction limited to the spacing of the holes (not their
> diameter).
>
> 4. If the total amount of light is not an issue (as for example when
> viewing the moon or terrestrial objects), then the only other thing a
> larger telescope is giving you is angular resolution - which is thrown
> away when you "stop" down the aperture with a single hole.
>
> 5. Radio telescopes use the same technique - multiple widely separated
> "holes" (antennae) instead of one big hole (or lots of little holes
> clustered together). So, for that matter, does Keck, but regrettably only
> on one axis.
>
> What am I missing?
>
> TIA
>
>
> Peter Webb
>
|
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Date: 17 Jun 2007 21:39:10
From: Peter Webb
Subject: Re: Smaller hole in telescope's cover
|
Actually, I think there may be another reason that my idea wouldn't work. If
you have two or more holes, then presumably the holes have to be exactly the
same distance from the primary in order for the signals to be correlated.
This would require the cover to be optically flat, meaning that an
inexpensive cover wouldn't work.
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Date: 17 Jun 2007 20:21:22
From: Peter Webb
Subject: Re: Smaller hole in telescope's cover
|
"Starlord" <starlord@sidewalkastronomy.info > wrote in message
news:WKmdnRuZX7_uVOnbnZ2dnUVZ_tGvnZ2d@inreach.com...
>I have watched a good number of sci programs and on one I saw the 100inch
>mirror was covered with a seet of thin sheet of plastic film that had first
>had very dim faint fuzzies maped out on it and then they cut out the circle
>that mapped the size and then it was moved to the mirror and punt on it,
>while 96% of the photo was dark, they got more light from the faint fuzzies
>(longer Exp.) and came up with they ones they had mapped out and the new
>photo was 50 times better than the over all one that they used for the
>mapping of the plastic.
>
> I have a dob (10" F5) and I use the full scope only I use a moon filter to
> cut down the moonlight.
>
Well, you can try it pretty easily. Grab some aluminium foil punch a few
dozen holes in it, and place it as a cap over the end, then try your moon
filter, then a piece of foil with a single 1" hole, so they all give the
same brightness level. Use you shortest eyepiece so you are diffraction
limited. Have a squizz at the moon, then use the same configs without the
moon filter to try and split a bright binary (say Sirius). I would love to
know what actually happens.
|
| | | | | | | | |
Date: 17 Jun 2007 08:53:42
From: Starlord
Subject: Re: Smaller hole in telescope's cover
|
That would NOT be the same, the cover they used was layed right on the
mirror itself, so theirs was 100inch across and they don't enclose that big
mirror in a metal tube, mine is a DOBSONIAN look that up if you don't know
what a Dob is, and my mirror is in a metal tube that mains the mirror has
screws holding it in there and because of the open area around the mirror
not only could they install it and watch it settle down, but with noone in
the same room the whole dome was dark and most dobs don't have fany tracking
on them either, they are some with point and ID watch your looking at, but
mine is total manual. Look at my sig lines, the top one stops anything like
that from happing on a dob.
The Lone Sidewalk Astronomer of Rosamond
Telescope Buyers FAQ
http://home.inreach.com/starlord
Sidewalk Astronomy
www.sidewalkastronomy.info
AD World
http://www.adworld.netfirms.com/
"Peter Webb" <webbfamily@DIESPAMDIEoptusnet.com.au > wrote in message
news:46750ae5$0$19153$afc38c87@news.optusnet.com.au...
>
>
> Well, you can try it pretty easily. Grab some aluminium foil punch a few
> dozen holes in it, and place it as a cap over the end, then try your moon
> filter, then a piece of foil with a single 1" hole, so they all give the
> same brightness level. Use you shortest eyepiece so you are diffraction
> limited. Have a squizz at the moon, then use the same configs without the
> moon filter to try and split a bright binary (say Sirius). I would love to
> know what actually happens.
>
>
|
| | | | |
Date: 15 Jun 2007 14:16:11
From: Andrew Smallshaw
Subject: Re: Smaller hole in telescope's cover
|
On 2007-06-15, Peter Webb <webbfamily@DIESPAMDIEoptusnet.com.au > wrote:
>
> Something I have always wondered about these holes (which are also useful
> for terrestrial use of the scope).
>
> Why do they make one hole with an area of (say) 1 square inch, instead of
> three smaller holes spaced as far apart as possible? That way you would get
> the full resolution of the telescope and reduced light, instead of reduced
> light and reduced resolution.
>
> Seems to make no sense to me ....
Possibly to get depth of field for terrestial use. I messed about
with my 5.1" newt a while back looking at a bird table perhaps 10"
across and 50' away. There were three distinct focus points - one
for the front edge of the table, one for the back edge and one in
the middle. That wouldn't be convenient at all for terrestial use.
Lower the effective apeture of the scope though, and the depth of
field will increase.
--
Andrew Smallshaw
andrews@sdf.lonestar.org
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Date: 15 Jun 2007 13:31:03
From:
Subject: Re: Smaller hole in telescope's cover
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Peter Webb wrote:
>Something I have always wondered about these holes (which are also useful
>for terrestrial use of the scope).
>
>Why do they make one hole with an area of (say) 1 square inch, instead of
>three smaller holes spaced as far apart as possible? That way you would get
>the full resolution of the telescope and reduced light, instead of reduced
>light and reduced resolution.
>
>Seems to make no sense to me ....
Imagine the central obstruction from the secondary mirror getting
a lot bigger and the rms becoming very thick. Note that contrast
loss is not linear with obstruction size; it is small from 0% to
20% obstruction, a bit worse at 25%, and really bad at 30%+.
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Date: 14 Jun 2007 12:30:40
From: galwacco
Subject: Re: Smaller hole in telescope's cover
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Thanks a lot!!
On 14 jun, 16:19, "Starlord" <starl...@sidewalkastronomy.info > wrote:
> One or two of the scope companys make that cover so a solor filter can be
> placed there and the cover put on and then they sun is viewed. But they fail
> to give good reasons why the filter is needed and no safty talk at all.
>
> Telescope Buyers FAQhttp://home.inreach.com/starlord
>
> --
> The Lone Sidewalk Astronomer of Rosamond
> Telescope Buyers FAQhttp://home.inreach.com/starlord
> Sidewalk Astronomywww.sidewalkastronomy.info
> AD Worldhttp://www.adworld.netfirms.com/
>
> "galwacco" <claudio.ferna...@gmail.com> wrote in message
>
> news:1181847856.736445.127240@z28g2000prd.googlegroups.com...
>
>
>
> > Hi, I am seriously new to the world of reflector telescope maned
> > astronomy.
> > I have one weird doubt that I just couldn't manage to find any answers
> > about that.
>
> > I just got my first reflector telescope! It's a 4" aperture,
> > equatorial, the point that made me really curious was the cover that
> > came along with the telescope.
>
> > Of course, a cover is to protect the telescope against dew and dust,
> > the thing is that the cover has one whole of about one inch and this
> > hole also comes with a cap! I wonder what is that whole for?
>
> > Am I supposed to use the telescope with that cap on but only with the
> > smaller whole opened?
>
> > Sorry if this Q is just too stupid, I couldn't help but just ask it!
> > My curiosity was killing me!- Ocultar texto entre aspas -
>
> - Mostrar texto entre aspas -
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Date: 14 Jun 2007 12:19:29
From: Starlord
Subject: Re: Smaller hole in telescope's cover
|
One or two of the scope companys make that cover so a solor filter can be
placed there and the cover put on and then they sun is viewed. But they fail
to give good reasons why the filter is needed and no safty talk at all.
Telescope Buyers FAQ
http://home.inreach.com/starlord
--
The Lone Sidewalk Astronomer of Rosamond
Telescope Buyers FAQ
http://home.inreach.com/starlord
Sidewalk Astronomy
www.sidewalkastronomy.info
AD World
http://www.adworld.netfirms.com/
"galwacco" <claudio.fernando@gmail.com > wrote in message
news:1181847856.736445.127240@z28g2000prd.googlegroups.com...
> Hi, I am seriously new to the world of reflector telescope maned
> astronomy.
> I have one weird doubt that I just couldn't manage to find any answers
> about that.
>
> I just got my first reflector telescope! It's a 4" aperture,
> equatorial, the point that made me really curious was the cover that
> came along with the telescope.
>
> Of course, a cover is to protect the telescope against dew and dust,
> the thing is that the cover has one whole of about one inch and this
> hole also comes with a cap! I wonder what is that whole for?
>
> Am I supposed to use the telescope with that cap on but only with the
> smaller whole opened?
>
> Sorry if this Q is just too stupid, I couldn't help but just ask it!
> My curiosity was killing me!
>
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Date: 16 Jun 2007 11:45:13
From: Thomas Womack
Subject: Re: Smaller hole in telescope's cover
|
In article <yJCdnZu1hIePC-zbnZ2dnUVZ_jednZ2d@inreach.com >,
Starlord <starlord@sidewalkastronomy.info > wrote:
>One or two of the scope companys make that cover so a solor filter can be
>placed there and the cover put on and then they sun is viewed. But they fail
>to give good reasons why the filter is needed and no safty talk at all.
With my 4" department-store Newtonian, I found at the time of the
transit of Venus that I could do eyepiece projection of the Sun with
the small hole open without a problem, but that if I opened the whole
aperture the image quality rapidly degraded, the tube filling with
smoke as the plastic back of the eyepiece caught fire.
Tom
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