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Date: 23 Sep 2007 13:25:33
From: brucegooglegroups
Subject: no concentric circles in star test
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If a star test does not show any concentric circles, but the donut
hole is in the center- what does indicate about the collimation of a
Newtonian?
Thanks.
Bruce
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Date: 24 Sep 2007 02:00:58
From: das namen ist-
Subject: Re: no concentric circles in star test
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brucegooglegroups wrote:
> If a star test does not show any concentric circles, but the donut
> hole is in the center- what does indicate about the collimation of a
> Newtonian?
> Thanks.
> Bruce
Im gues its an f5 or below scope? If yes then too little magnification
to even show rings.... up the power or pick a tiny bright star and up
the power a bit. There is no "donnut hole test". Where did you
get that? There is a "star test". If the DONNUT HOLE/BLACK HOLE OF
SECONDARY? is in the middle, then in the middle
of what? Off by "a few degrees"? Sheesh! I didnt know DONNUT HOLES
coordinates could be measured to that degree, against
nothingness! So up the power a bit, pick a smaller brighter star,
try centering the DONNUT HOLE in the middle of the FROSTING and may then
you wills ee some rings. Point is: the
rings are there - SOMEWHERE!
This isnt DONNUT HOLE SCIENCE.
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Date: 23 Sep 2007 14:21:43
From: Brian Tung
Subject: Re: no concentric circles in star test
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Bruce wrote:
> If a star test does not show any concentric circles, but the donut
> hole is in the center- what does indicate about the collimation of a
> Newtonian?
It means the collimation is off by some amount less than a couple of
degrees. The donut hole "test" is pretty insensitive; the collimation
has to be way off before it's noticeably off-center.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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Date: 24 Sep 2007 01:56:19
From: das namen ist-
Subject: Re: no concentric circles in star test
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Brian Tung wrote:
> Bruce wrote:
> > If a star test does not show any concentric circles, but the donut
> > hole is in the center- what does indicate about the collimation of a
> > Newtonian?
>
> It means the collimation is off by some amount less than a couple of
> degrees. The donut hole "test" is pretty insensitive; the collimation
> has to be way off before it's noticeably off-center.
>
> --
> Brian Tung <brian@isi.edu>
> The Astronomy Corner at http://astro.isi.edu/
> Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
> The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
> My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
God with all these tags I feel miniscule even posting ....
There is no "donnut hole" test!
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Date: 23 Sep 2007 20:39:07
From: starburst
Subject: Re: no concentric circles in star test
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Brian Tung wrote:
> Bruce wrote:
>
>>If a star test does not show any concentric circles, but the donut
>>hole is in the center- what does indicate about the collimation of a
>>Newtonian?
>
>
> It means the collimation is off by some amount less than a couple of
> degrees. The donut hole "test" is pretty insensitive; the collimation
> has to be way off before it's noticeably off-center.
>
You know, Brian, I understand what you're saying, but the eye has a
really impressive ability to tell where the center of something actually
is. For a fast scope, or an SCT, especially testing at low
magnification, being off a little will have a profound impact on the
image. Slower scopes, though, collimate surprisingly well on the donut
approximation (or at least they have in my experience.)
Chris
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Date: 23 Sep 2007 18:25:27
From: Brian Tung
Subject: Re: no concentric circles in star test
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starburst wrote:
> You know, Brian, I understand what you're saying, but the eye has a
> really impressive ability to tell where the center of something actually
> is. For a fast scope, or an SCT, especially testing at low
> magnification, being off a little will have a profound impact on the
> image. Slower scopes, though, collimate surprisingly well on the donut
> approximation (or at least they have in my experience.)
I think your experience is not quite universal, though. It is evident,
from the number of people who have posted with this same general
question and then have the "Aha!" moment when they magnify on the
diffraction pattern, they don't see it in the donut. It's possible that
once they know what to look for, *then* they can go back and see it in
the donut, but if someone is just starting out, I'd have to recommend
that they look at the diffraction pattern at (say) 30x per inch,
defocused a few waves. It reduces the reliance on visual acuity and
makes the misalignment easier to see.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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Date: 23 Sep 2007 14:28:02
From:
Subject: Re: no concentric circles in star test
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On Sep 23, 2:25 pm, brucegooglegroups <brucegooglegro...@hotmail.com >
wrote:
> If a star test does not show any concentric circles, but the donut
> hole is in the center- what does indicate about the collimation of a
> Newtonian?
Hi Bruce,
For starters, I would guess that you're using too little
magnification. If I recall correctly from previous postings (correct
me if I'm wrong.) your highest magnification is (or was) 80x, and
you're using a 4.5 inch Newtonian. That works out to about 18x per
inch of aperture. Check out the following link:
http://www.tmboptical.com/itemsGrid.asp?cat_id=31
50x or slightly higher per inch of aperture is recommended! At
sufficiently high magnifications and with defocussing by the right
(much less than you've apparently been doing) amounts you *will* see
rings! Precise collimation cannot be ascertained at "low"
magnifications.
"Star Testing Astronomical Telescopes" by Suiter is an excellent
reference on star testing. I would highly recommend Suiter's book to
anyone who has a serious interest in the subject.
Bill Greer
To sketch is to see.
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