Re: Zhumell 10 Inch Dobsonian Reflector Telescope

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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