Re: Zhumell 10 Inch Dobsonian Reflector Telescope


"starburst" <nope@xxxxxxxxxx> wrote in message
news:f43lgq$pul$1@xxxxxxxxxxxxxxxxxxxxxxxx
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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