From: Guy Brandenburg 
To:  AMaroney{at}sptimes.com
CC:  atm{at}shore.net
Reply-To: Guy Brandenburg 


Dobsonian telescopes do have a 'bug', or design flaw, that is commonly
known as 'Dobson's Hole'. Near the zenith is really and truly a pain in the
neck to observe. As AMaroney noted, it is easy to knock the whole scope
over when you are very close to the zenith and you want to aim it somewhere
else. I have learned that it helps to use two hands. Often you have to
rotate the base and THEN change the altitude of the tube - and even then it
gets tricky, because it sometimes feels like 'you can't get there from
here' as the old Vermont saying goes.

None of that has to do with the particular type of Teflon you are using,
nor even how heavy your eyepieces are. (Though, to be sure, if you don't
have enough friction on your altitude bearings, or if your altitude
bearings are too small, then the angle at which you are pointed will make a
large difference concerning whether the scope will stay in place or not.
Large bearings = good. Small bearings = bad. Unless you work in some sort
of extra frictional device.)

A German Equatorial Mount has different, though somewhat related problems.
And an English Yoke mount makes it impossible to see anything near the
pole.

I think the easiest solution is simply NOT to look at anything that is very
close to the zenith. Just wait an hour, and look at something else in the
meantime.

Guy Brandenburg

AMaroney{at}sptimes.com wrote:
>>I noted this a few weeks ago just after my 'First Light' (alas not
>
> kosher Ebony Star but definitely good grade Teflon).
>
>>Looking at Saturn was a joy whilst Jupiter near the zenith became a pain
>
> after a while.
>
> My $0.02...
>
> This was probably due less to the bearing material and more to the weight
> of the focuser, eyepiece and finder all being on one side of the bearing
> (trunion) axis.  The heavy side of the tube. with all the hardware, is
> going to want to drop down.  Ideally the weight of the scope should be
> balanced across the trunion axis.  Since this hardware has a similar
> weight regardless of the size of the scope it has more of an effect on a
> smaller scope.
>
> It seems to make sense that if the trunion were located at the midpoint of
> the balance (instead of midpoint of the tube), which would be towards the
> side of the focuser that the extra mirror weight opposite of it would make
> for a more stable Zenith position.  Has anyone confirmed this?
>

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