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Date: 05 Jun 2007 04:48:36
From: rickharp
Subject: Zhumell 10 Inch Dobsonian Reflector Telescope
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i am a newb, just bought my first light bucket, and have used it for a
few nights. i feel it is not as clear as i would of suspected and was
wondering if it could be something that is not right. when looking at
say jupiters moons, each moon is not a dot but rather a boomerrang
shaped, which one might think is from movement, but its the same with
the fan on or off, and the base is sitting on a pretty serious
concrete patio. i've collminated the telescope like 8 times, all with
the same results. i am using the 1.25" lense. i am wondering if it
is a factory problem, or something else, of if this is common, and i
should get use to it. if anyone has any experience or thoughts, it
would be very appreciated. thanks
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Date: 05 Jun 2007 16:20:03
From: Jan Owen
Subject: Re: Zhumell 10 Inch Dobsonian Reflector Telescope
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"rickharp" <rick@dryerbox.com > wrote in message
news:1181018916.893645.226360@w5g2000hsg.googlegroups.com...
>i am a newb, just bought my first light bucket, and have used it for a
> few nights. i feel it is not as clear as i would of suspected and was
> wondering if it could be something that is not right. when looking at
> say jupiters moons, each moon is not a dot but rather a boomerrang
> shaped, which one might think is from movement, but its the same with
> the fan on or off, and the base is sitting on a pretty serious
> concrete patio. i've collminated the telescope like 8 times, all with
> the same results. i am using the 1.25" lense. i am wondering if it
> is a factory problem, or something else, of if this is common, and i
> should get use to it. if anyone has any experience or thoughts, it
> would be very appreciated. thanks
>
There are several significant missing details here...
How long after putting the scope outside were you trying to observe???
How, and with what aids, did you collimate the scope???
What eyepiece were you using, and at what magnification were you operating
at the time???
At what time during the evening (or more specifically, at the time of your
observation attempts, how far was Jupiter, and environs, above the
horizon???) were you observing... Said differently, the lower the object is
in the sky (the closer it is to the HORIZON at time of observation), the
more likely for there to be natural seeing constraints to clean views at the
eyepiece).
I have NO personal experience with this particular scope, so I won't attempt
any guesses where it's optics may be concerned...
After you've filled in some of the blanks I've outlined above, I'm sure
there are lots of folks here who will be able to help...
--
Jan Owen
To reach me directly, remove the Z, if one appears in my e-mail address...
Latitude: 33.6
Longitude: -112.3
http://community.webshots.com/user/janowen21
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Date: 05 Jun 2007 08:38:56
From: starburst
Subject: Re: Zhumell 10 Inch Dobsonian Reflector Telescope
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rickharp wrote:
> i am a newb, just bought my first light bucket, and have used it for a
> few nights. i feel it is not as clear as i would of suspected and was
> wondering if it could be something that is not right. when looking at
> say jupiters moons, each moon is not a dot but rather a boomerrang
> shaped, which one might think is from movement, but its the same with
> the fan on or off, and the base is sitting on a pretty serious
> concrete patio. i've collminated the telescope like 8 times, all with
> the same results. i am using the 1.25" lense. i am wondering if it
> is a factory problem, or something else, of if this is common, and i
> should get use to it. if anyone has any experience or thoughts, it
> would be very appreciated. thanks
>
Hi Rick -
When collimating faster scopes, it's necessary to really dial it in
perfectly. So when you're collimating, you need to use a high power
eyepiece combo (like around 300X or above), and once you've collimated
it out of focus you need to collimate in-focus as well, using the faint
diffraction rings around the star. The problem is that there are very
few nights (at least around here in upstate NY) that are steady enough
to do this. Usually you're just getting close.
If the collimation is in, then the next likely culprit is either
cool-down or seeing. Make sure to give your scope a couple of hours to
come to the same temp as the surrounding air. With my C8 I routinely
notice improvements after 2-3 hours. With a 10, which is thicker, expect
even longer cool-down times.
Get the scope off of the concrete patio, too. Those things absorb heat
all day and radiate it away at night, ruining seeing above them. Put the
scope out in the middle of the lawn, or take it to a grassy spot in a
park or out in the country.
Seeing can be a real bear and can often mask decent optics. Take a look
at your local clear sky clock and take a look when the forecast is for
excellent seeing. If the collimation is good and your scope's
performance is still poor, start suspecting the optics.
On a related note, eyepieces can also cause kidney-beaning on stars at
the edge of the field. Do your jovian moon boomerangs seem to change
shape from one side of the field to the other? Could be coma in the
primary (which is always present to some degree in fast newtonians), or
it could be a mediocre eyepiece. To get the most out of fast optical
systems you need to unbelt a little for good eyepieces. You might try to
get a university optics ortho on astromart. Quality is excellent and
they're not too expensive second hand. It will help you to isolate the
problem, and they're terrific planetary eyepieces for sharpness (though
the field of view seems a little narrow compared to the plossls that
dominate the market today).
People usually jump to the conclusion that their mirror is poor, but in
my experience almost all of today's optics will perform better than
seeing allows them to. In other words, the normal factor limiting
performance is the sky, not the scope. So don't worry just yet. There
are some bad scopes out there, but they are pretty few and far between.
Hope this helps, and good luck - Chris
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Date: 05 Jun 2007 09:18:07
From: Dennis Woos
Subject: Re: Zhumell 10 Inch Dobsonian Reflector Telescope
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"starburst" <nope@nospam.net > wrote in message
news:f43lgq$pul$1@geraldo.cc.utexas.edu...
> rickharp wrote:
>> i am a newb, just bought my first light bucket, and have used it for a
>> few nights. i feel it is not as clear as i would of suspected and was
>> wondering if it could be something that is not right. when looking at
>> say jupiters moons, each moon is not a dot but rather a boomerrang
>> shaped, which one might think is from movement, but its the same with
>> the fan on or off, and the base is sitting on a pretty serious
>> concrete patio. i've collminated the telescope like 8 times, all with
>> the same results. i am using the 1.25" lense. i am wondering if it
>> is a factory problem, or something else, of if this is common, and i
>> should get use to it. if anyone has any experience or thoughts, it
>> would be very appreciated. thanks
>>
>
> Hi Rick -
>
> When collimating faster scopes, it's necessary to really dial it in
> perfectly. So when you're collimating, you need to use a high power
> eyepiece combo (like around 300X or above), and once you've collimated it
> out of focus you need to collimate in-focus as well, using the faint
> diffraction rings around the star. The problem is that there are very few
> nights (at least around here in upstate NY) that are steady enough to do
> this. Usually you're just getting close.
I think this is overboard at this point, given that the poster says that the
moons of Jupiter look like "boomerangs". I think this scope is f/5, and one
could collimate it well enough by eye-balling it through the drawtube to get
better results than "boomerangs".
>
> If the collimation is in, then the next likely culprit is either cool-down
> or seeing. Make sure to give your scope a couple of hours to come to the
> same temp as the surrounding air. With my C8 I routinely notice
> improvements after 2-3 hours. With a 10, which is thicker, expect even
> longer cool-down times.
Our full-thiickness 10" dob (with a fan blowing across the mirror) is very
usable after an hour. I think 2-3 hours, or more, is not realistic unless
the scope is mis-constructed in some fundamental way. Also, "boomerangs"
doesn't sound like bad seeing to me.
Assuming your collimation is decent, the two things I would recommend are a
star test (see http://www.astunit.com/tutorials/startest.htm) and getting
some assistance from folks in your local astro club. In our club you would
find folks who would help you to diagnose the problem, even to the point of
bench-testing your mirror.
Dennis
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Date: 05 Jun 2007 20:17:04
From: starburst
Subject: Re: Zhumell 10 Inch Dobsonian Reflector Telescope
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Dennis Woos wrote:
>
>>If the collimation is in, then the next likely culprit is either cool-down
>>or seeing. Make sure to give your scope a couple of hours to come to the
>>same temp as the surrounding air. With my C8 I routinely notice
>>improvements after 2-3 hours. With a 10, which is thicker, expect even
>>longer cool-down times.
>
>
> Our full-thiickness 10" dob (with a fan blowing across the mirror) is very
> usable after an hour. I think 2-3 hours, or more, is not realistic unless
> the scope is mis-constructed in some fundamental way. Also, "boomerangs"
> doesn't sound like bad seeing to me.
>
I didn't say that the scope wasn't usable - I said I notice improvements
later on. My scope is almost always performing best in the middle of
the night when I finish up. YMMV.
To me, boomerangs sound a lot like lack of cool-down.
> Assuming your collimation is decent, the two things I would recommend are a
> star test (see http://www.astunit.com/tutorials/startest.htm) and getting
> some assistance from folks in your local astro club. In our club you would
> find folks who would help you to diagnose the problem, even to the point of
> bench-testing your mirror.
>
The chances of his mirror being grossly deficient, to the point that he
can't get decent images, is pretty remote, I think. Not impossible, but
very unlikely.
Chris
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Date: 05 Jun 2007 20:54:41
From: Brian Tung
Subject: Re: Zhumell 10 Inch Dobsonian Reflector Telescope
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starburst wrote:
> To me, boomerangs sound a lot like lack of cool-down.
It *could* be, but it could be substantial miscollimation, too. I'd
check miscollimation, first, since it's relatively fast to do.
--
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: 05 Jun 2007 23:32:23
From: Curtis Croulet
Subject: Re: Zhumell 10 Inch Dobsonian Reflector Telescope
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"Boomerangs" sounds like pinched optics somewhere to me. Where are these
scopes made -- China? A personal friend of mine makes optics commercially,
and he makes scopes (not many these days) that are sold under a name most
people here would recognize. He once told me he rejected an entire shipment
of Chinese-made diagonals because of their poor quality, but another company
was installing these same diagonals in telescopes that they sold. My friend
decided the cost savings wasn't worth the hit to his reputation, so he
started making his own diagonals, and that's the bulk of his business now.
--
Curtis Croulet
Temecula, California
33°27'59"N, 117°05'53"W
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Date: 05 Jun 2007 16:50:45
From: Edward Erbeck
Subject: Re: Zhumell 10 Inch Dobsonian Reflector Telescope
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> Curtis Croulet (in a small part) wrote:
> My friend decided the cost savings wasn't worth the hit to his reputation, so
> he started making his own diagonals, and that's the bulk of his business now.
So how's Terry doing these days? ;-)
Crazy Ed
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