no concentric circles in star test

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

On Sep 23, 2:25 pm, 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?

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.

On Sep 23, 5:28 pm, wrote:
On Sep 23, 2:25 pm, 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?

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.

Hi Bill,

Thanks for the tip. I wish I had known earlier about the magnification
issue. It would have saved me a lot of time. I read Star Testing
Astronomical Telescopes" by Suiter. I am getting the book.

I star tested tonight- Vega, Deneb, and Eltanin. I left the scope
outside for a few hours. There was a full moon, so the stars were not
too bright. The star test was excellent. I got about 4 or five
concentric circles, and in the dead center was actually a white dot.
The star looked like a bullseye.

This helps a lot. I was postponing buying a bigger scope because I was
afraid of not being able to handle the tweaking of the optics. It also
shows that I don't need a laser collimator which was not recommended
in an earlier post.

Clear Skies.
Bruce

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
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

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

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
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

On Sep 23, 6:25 pm, (Brian Tung) wrote:
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.


Hi Brian
I think it just needs to be stated that if one is defocusing to the
point
that they see the donut ( either the scondary shadow or the aperture
shadow, depending on direction of defocus ) they've over shot where
they are suppose to be. It is true that one need not have high
magnification to see defraction rings. Even in the donut, one can
see slight rings at the edges of the donut, it is just that there
isn't
enough information there to clearly see how much out the scope is.
Sometimes, it is hard to see the rings because of poor atmospherics.
In such cases it take a little more patients.
Dwight

wrote

Sometimes, it is hard to see the rings because of poor atmospherics.
In such cases it take a little more patients.

Only if there's a doctor in the house...

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
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!

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.