first light, and some questions

First light for my 8" Dob. It came with 25mm (48x) and 10mm
(120x)Plossls. Deep space was very nice, but I don't think I'm getting
the sharpness I should be getting:

-in focus, with either eyepiece, stars don't show any diffraction
pattern or rings...only a disk which is slightly grainy/sparkly on the
bright stars at 120x (my cheapo 60mm Jason refractor shows textbook
diffraction patterns/rings). Look at the left star pattern in the
following link...this is what bright stars look like:

http://www.geocities.com/beerauth/dobstars.2jpg.jpg

-moving very slightly out of focus on either side of focus, I start to
get a hint of rings forming, but they're faint, indistinct, and broken

-moving more out of focus on either side, I get a nice round doughnut
(but no rings)

-Castor could split at 120x, though barely, and somewhat grainy (look
at the right star pattern in the link above)

-on Saturn, I had a very very difficult time seeing the Cassini
division

-slight blue and orange fringing on planets (maybe it's the
eyepieces?)

-in M42, the Trapezium resolved to 4 stars at 120x (don't know how
many an 8" should show)

Is something wrong, or is this about right for an 8" f/6 Dob? Skies
were very clear (I could see M37, M36, and M38 naked eye with averted
vision when Auriga was near zenith), though it was windy. I got the
same results using it slightly out of collimation as it came, and
again after collimating it almost perfectly.
I'm thinking it could be: the main mirror, the presumably cheap
Chinese-made eyepieces, or seeing conditions.

How well was the telescope acclimated to the surrounding air temperature?
And what was your weather like at the time? 8-inch telescopes are much more
atmosphere-limited than smaller ones.

How well was the telescope acclimated to the surrounding air temperature?
And what was your weather like at the time? 8-inch telescopes are much more
atmosphere-limited than smaller ones.

Is something wrong, or is this about right for an 8" f/6 Dob? Skies
were very clear (I could see M37, M36, and M38 naked eye with averted
vision when Auriga was near zenith), though it was windy.

I'm thinking it could be: the main mirror, the presumably cheap
Chinese-made eyepieces, or seeing conditions.

Good seeing means a stable atmosphere. Wind is normally an indication that the
atmosphere is not stable and the wind itself is disturbs the image. Consider
that the light that comes to your scope from a star passes through a column of
air 8 inches in diameter from your scope all the way to outer space. If that
air is turbulent then it will cause the path of the light to shift slightly as
it passes and rather than sharp stars you will get round stars.

Regarding the diffraction rings: The size of the rings in inversely
proportional to the diameter of the objective. So if those rings show nicely
in your 60mm scope at 125X, it will take over 400X to see them in your 8 inch
scope. It takes some very good seeing to see diffraction rings in a larger
scope.

At this point you should avoid the temptation to blame poor images on a bad
mirror, that temptation leads to frustration and confusion.

Rather take this time to understand about your scope and how to get the most
out of it possible. Sometimes that will mean accepting that the seeing is just
not good and choosing targets that do not require good seeing.

The three main factors in optimizing the performance of a refector a

1. Collimation. If the scope is not collimated properly the images will not
be sharp. One can go over board here but still, learning what stars look like
when the scope is properly collimated is important.

2. Cool down. The mirrors need to cool down as does the scope in general.
Thermal convection currents refract the light inside the telescope and cause
image problems. Moving a telescope from a warmer place to a cooler place
causes the mirrors to distort as they cool down. Because of the incredible
precision of the mirror, (1/8 the wave length of light), it only takes a small
change in temperature to distort the mirror sufficiently to seriously degrade
the images.

With the metal tube DOBs, just touching the scope with your hand will cause a
tube current that can be easily seen in an otherwise thermally stable scope.
It might take 30 seconds for the current to disappear. If you are using
higher magnifications, you may be moving the scope every 30 seconds so you will
never get a clean view.

The solution is to wear gloves.

Normally it takes an 8 inch Newtonian an hour or more to reach thermal
equillibrium, during the cool down period, the views will be rather poor but
still useable at low powers.

3. Seeing. Collimation and cool down are under the control of the observer,
atmospheric stability is not. You might be able to travel to a location that
is more stable but normally, especially in the winter, the seeing may be bad as
storm fronts pass through. There are various ways that one can use to gauge
the quality of the seeing, defocusing on a bright star, looking for the twinkle
in stars.

But the main thing is that when you take that first look at a bright star and
it looks like a ball rather than a point, it probably means the seeing is bad.

Other pointers: Make sure that what your are viewing is well above the
horizon, 30 degrees or more is good. At low angles you are looking through
more atmosphere which means more turbulence and haze.

Saturn and Castor are both rising in the early evening so your first views are
likely going to rather poor.

----

Last night the seeing was quite poor from my back yard. No hope for seeing the
Cassini division, Rigel was just barely split, no hope for the E and F stars in
the trapezium, in general things were just not that crisp. Heck even the moon
was not sharp at 60X which is a bad sign. So, I just went looking around at
low powers, enjoyed M36-38 and their associated clusters, checked out M76 high
over head, the double cluster and the rest of the clusters in Casseiopia.....

---

I learned about collimation, cool down and seeing the hard way, from confusion
and frustration.

One night the stars would be beautiful and sharp, the next big balls of fuzz.
Stars that would split one night would not the next. I would tear the scope
apart to recollimate it, looking for some problem in the optics.

But some nights I would collimate the scope and the views would be horrible and
the next they would be great and yet I had not touched the scope.

Finally I realized just how important the atmospheric conditions are and that
there is really not much one can do about them.

-----

So, enjoy your scope as it is, learn about it and learn about they sky.

One night here in a bit, the air will be still, you will have put the scope out
early and when you take that first look at Saturn it will be sharp and crystal
clear, the Cassini division will stand out like a black ring, almost three
dimensional. The planet itself will show subtle coloration and the moons will
be bright sharp points. Later Jupiter will rise and you may see the shadow of
a moon as it transits, the belts showing subtle details under the eyepiece.

And it will seem like these views will last forever, that your scope is finally
working and you have finally gotten the collimation right on.

Enjoy it while it lasts, those are the special nights to die for.

The next night, the seeing may be gone and a quick look at Saturn will be
disappointing and you will wonder what has happened to the scope.

Of course, it has nothing to do with the scope.

So, this is what I offer, some lessons learned the hard way, don't doubt your
scope, learn about it and learn how to let it do its best.

Jon Isaacs

Is something wrong, or is this about right for an 8" f/6 Dob? Skies
were very clear (I could see M37, M36, and M38 naked eye with averted
vision when Auriga was near zenith), though it was windy.

I'm thinking it could be: the main mirror, the presumably cheap
Chinese-made eyepieces, or seeing conditions.

Good seeing means a stable atmosphere. Wind is normally an indication that the
atmosphere is not stable and the wind itself is disturbs the image. Consider
that the light that comes to your scope from a star passes through a column of
air 8 inches in diameter from your scope all the way to outer space. If that
air is turbulent then it will cause the path of the light to shift slightly as
it passes and rather than sharp stars you will get round stars.

Regarding the diffraction rings: The size of the rings in inversely
proportional to the diameter of the objective. So if those rings show nicely
in your 60mm scope at 125X, it will take over 400X to see them in your 8 inch
scope. It takes some very good seeing to see diffraction rings in a larger
scope.

At this point you should avoid the temptation to blame poor images on a bad
mirror, that temptation leads to frustration and confusion.

Rather take this time to understand about your scope and how to get the most
out of it possible. Sometimes that will mean accepting that the seeing is just
not good and choosing targets that do not require good seeing.

The three main factors in optimizing the performance of a refector a

1. Collimation. If the scope is not collimated properly the images will not
be sharp. One can go over board here but still, learning what stars look like
when the scope is properly collimated is important.

2. Cool down. The mirrors need to cool down as does the scope in general.
Thermal convection currents refract the light inside the telescope and cause
image problems. Moving a telescope from a warmer place to a cooler place
causes the mirrors to distort as they cool down. Because of the incredible
precision of the mirror, (1/8 the wave length of light), it only takes a small
change in temperature to distort the mirror sufficiently to seriously degrade
the images.

With the metal tube DOBs, just touching the scope with your hand will cause a
tube current that can be easily seen in an otherwise thermally stable scope.
It might take 30 seconds for the current to disappear. If you are using
higher magnifications, you may be moving the scope every 30 seconds so you will
never get a clean view.

The solution is to wear gloves.

Normally it takes an 8 inch Newtonian an hour or more to reach thermal
equillibrium, during the cool down period, the views will be rather poor but
still useable at low powers.

3. Seeing. Collimation and cool down are under the control of the observer,
atmospheric stability is not. You might be able to travel to a location that
is more stable but normally, especially in the winter, the seeing may be bad as
storm fronts pass through. There are various ways that one can use to gauge
the quality of the seeing, defocusing on a bright star, looking for the twinkle
in stars.

But the main thing is that when you take that first look at a bright star and
it looks like a ball rather than a point, it probably means the seeing is bad.

Other pointers: Make sure that what your are viewing is well above the
horizon, 30 degrees or more is good. At low angles you are looking through
more atmosphere which means more turbulence and haze.

Saturn and Castor are both rising in the early evening so your first views are
likely going to rather poor.

----

Last night the seeing was quite poor from my back yard. No hope for seeing the
Cassini division, Rigel was just barely split, no hope for the E and F stars in
the trapezium, in general things were just not that crisp. Heck even the moon
was not sharp at 60X which is a bad sign. So, I just went looking around at
low powers, enjoyed M36-38 and their associated clusters, checked out M76 high
over head, the double cluster and the rest of the clusters in Casseiopia.....

---

I learned about collimation, cool down and seeing the hard way, from confusion
and frustration.

One night the stars would be beautiful and sharp, the next big balls of fuzz.
Stars that would split one night would not the next. I would tear the scope
apart to recollimate it, looking for some problem in the optics.

But some nights I would collimate the scope and the views would be horrible and
the next they would be great and yet I had not touched the scope.

Finally I realized just how important the atmospheric conditions are and that
there is really not much one can do about them.

-----

So, enjoy your scope as it is, learn about it and learn about they sky.

One night here in a bit, the air will be still, you will have put the scope out
early and when you take that first look at Saturn it will be sharp and crystal
clear, the Cassini division will stand out like a black ring, almost three
dimensional. The planet itself will show subtle coloration and the moons will
be bright sharp points. Later Jupiter will rise and you may see the shadow of
a moon as it transits, the belts showing subtle details under the eyepiece.

And it will seem like these views will last forever, that your scope is finally
working and you have finally gotten the collimation right on.

Enjoy it while it lasts, those are the special nights to die for.

The next night, the seeing may be gone and a quick look at Saturn will be
disappointing and you will wonder what has happened to the scope.

Of course, it has nothing to do with the scope.

So, this is what I offer, some lessons learned the hard way, don't doubt your
scope, learn about it and learn how to let it do its best.

Jon Isaacs

On 27 Dec 2003 05:09:24 -0800, (Patrick) wrote:

First light for my 8" Dob. It came with 25mm (48x) and 10mm
(120x)Plossls. Deep space was very nice, but I don't think I'm getting
the sharpness I should be getting:

-in focus, with either eyepiece, stars don't show any diffraction
pattern or rings...only a disk which is slightly grainy/sparkly on the
bright stars at 120x (my cheapo 60mm Jason refractor shows textbook
diffraction patterns/rings).

The size of the Airy pattern grows smaller as aperture increases, so
don't expect to see a textbook pattern with any 200mm 'scope unless
the seeing is spectacular. OTOH, its _easy_ to see a pretty pattern
in a 60mm instrument of decent quality.

snip of target descriptions

Is something wrong, or is this about right for an 8" f/6 Dob? Skies
were very clear (I could see M37, M36, and M38 naked eye with averted
vision when Auriga was near zenith), though it was windy.

Clue

I got the
same results using it slightly out of collimation as it came, and
again after collimating it almost perfectly.
I'm thinking it could be: the main mirror, the presumably cheap
Chinese-made eyepieces, or seeing conditions.

I'd bet a bunch on the conditions. Keep on tryin' (remember POCO?).


Wayne Hoffman
33° 49" 17' N 117° 56" 41' W
"Don't Look Down"

http://home.pacbell.net/w6wlr/

On 27 Dec 2003 05:09:24 -0800, (Patrick) wrote:

First light for my 8" Dob. It came with 25mm (48x) and 10mm
(120x)Plossls. Deep space was very nice, but I don't think I'm getting
the sharpness I should be getting:

-in focus, with either eyepiece, stars don't show any diffraction
pattern or rings...only a disk which is slightly grainy/sparkly on the
bright stars at 120x (my cheapo 60mm Jason refractor shows textbook
diffraction patterns/rings).

The size of the Airy pattern grows smaller as aperture increases, so
don't expect to see a textbook pattern with any 200mm 'scope unless
the seeing is spectacular. OTOH, its _easy_ to see a pretty pattern
in a 60mm instrument of decent quality.

snip of target descriptions

Is something wrong, or is this about right for an 8" f/6 Dob? Skies
were very clear (I could see M37, M36, and M38 naked eye with averted
vision when Auriga was near zenith), though it was windy.

Clue

I got the
same results using it slightly out of collimation as it came, and
again after collimating it almost perfectly.
I'm thinking it could be: the main mirror, the presumably cheap
Chinese-made eyepieces, or seeing conditions.

I'd bet a bunch on the conditions. Keep on tryin' (remember POCO?).


Wayne Hoffman
33° 49" 17' N 117° 56" 41' W
"Don't Look Down"

http://home.pacbell.net/w6wlr/

In fact you mentioned wind. That means the air was unsteady. What you
describe is consistent with the atmospheric conditions.

In fact you mentioned wind. That means the air was unsteady. What you
describe is consistent with the atmospheric conditions.

Hi Patrick,
-in focus, with either eyepiece, stars don't show any diffraction
pattern or rings...only a disk which is slightly grainy/sparkly on the
bright stars at 120x (my cheapo 60mm Jason refractor shows textbook
diffraction patterns/rings). Look at the left star pattern in the
following link...this is what bright stars look like:

You need a lot more magnification with the 8". 300x is probably a minimum,

http://www.geocities.com/beerauth/dobstars.2jpg.jpg

Is the star on the right a double? If not, go inside and outside of focus.
If they both elongate but one is rotated 90 degrees, you have astigmatism.

-moving very slightly out of focus on either side of focus, I start to
get a hint of rings forming, but they're faint, indistinct, and broken

Yes, you need more magnification.

-moving more out of focus on either side, I get a nice round doughnut
(but no rings)

The hole in the doughnut is your secondary mirror. It means you went too
far.

-Castor could split at 120x, though barely, and somewhat grainy (look
at the right star pattern in the link above)

-on Saturn, I had a very very difficult time seeing the Cassini
division

Give it an hour to cooldown. You can also test cooldown by raising the
magnification and going out of focus. When you have rings, a tube current
will look like a little line rising out of the center and going through to
the top of the outer ring. (NOTE: "top" does not take into account image
rotation/flip)

-slight blue and orange fringing on planets (maybe it's the
eyepieces?)

Blue on one side and orange on the other? If so, that's atmospheric. Wait
until it is a little higher.

-in M42, the Trapezium resolved to 4 stars at 120x (don't know how
many an 8" should show)

It varies.

Is something wrong, or is this about right for an 8" f/6 Dob? Skies
were very clear (I could see M37, M36, and M38 naked eye with averted
vision when Auriga was near zenith), though it was windy. I got the
same results using it slightly out of collimation as it came, and
again after collimating it almost perfectly.
I'm thinking it could be: the main mirror, the presumably cheap
Chinese-made eyepieces, or seeing conditions.

I'd make sure it had cooldown time. Use higher magnification to check the
rings. Get collimation perfect. Enjoy more. Worry less.

You can also check at http://skyandtelescope.com/resources/organizations/ to
find an astro club near you. Take your scope to a public night and set it up
next to someone with a bunch of good eyepieces. Then tell him your scope is
new and ask if you can borrow some eyepieces to test it with. Most clubs
also have a couple of optics/star test gurus who will look and give you a
good opinion.

Enjoy!

Clear Skies

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/
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