[Aavso-photometry] Bruce Gary's book and other material

[Wolfgang Renz]

Hi

If the tube saddle can hit the mount at all, its a limited and IMO
therefore bad GEM design.

With tripods its nearly always an issue that a longer tube setup
(e.g. long tube refractors, larger diameter "short" tube scopes,
long on-axis imaging  setups placed at the back of short tubed
scopes, but not just wide-field camera lens setup) can hit the
tripod for stars having an absolute declination between 90°
(the celestrial pole) and the absolute geographic latitude minus
about the opening angle of the tripod towards the celestrial pole.
So this gets especially bad for GEMs at lower absolute geogra-
phic latitudes. There even the counter weights of the GEMs might
be able to hit the tripod (when pointing the scope to hour angles
around 6h and 18h) if the absolute geographic latitude is smaller
than about the opening angle of the tripod towards the celestrial
pole. For declinations well outside this range, there should be no
hitting possible and therefore also no meridian flip unconditionally
required.
When using a vertical pole/pier, the first issue can occure for stars
having about an absolute declination between 90° (the celestrial
pole) and the absolute geographic latitude. The second issue should
just occure very close to the equator. The exact ranges will depend
on the width of the pole, on the width of the the counter weights and
on the actual offset of the mount towards the celestrial pole that is
defined by the design of its polar axis adjustment and the actual
geographic latitude.
But this can be avoided by using a Modified German Equatorial
Mount with a flexed pole/pier with a distance between the dec axis
and the inflection point is choosen large enough that the setup can
turn 24h in RA without hitting the pole/pier. The inflected part of the
pole/pier might not have to point exactly to the celestrial pole, if the
GEM's polar axis adjustment can still be used. But not all available
GEMs can be modified in a way to make them work that way (at
least not without spending some extra money for changing the polar
axis adjustment).
<http://upload.wikimedia.org/wikipedia/commons/3/3f/Forststernwarte_Jena_50cm-Cassegrain_1.jpg>
http://www.heavensgloryobservatory.com/ngt12-03web.jpg
http://www.heavensgloryobservatory.com/ngt12-04web.jpg
With these, one will have to stop continous tracking and imaging
just if external cables are wraping around the pole/pier too often
or internal cables get .

For a GEM solution in Antarctica at Dome C (-75°geographic lati-
tude) see e.g.:
http://www.company7.com/astrophy/graphics/ap900gtoant691702.jpg
(Note: That AP900 (spec: 19° to 68° latitude range) required an
 additional wedge to be able to get used there.)

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO



----- Original Message ----- 
From: "Bill Goff" <b-goff at sbcglobal.net>
To: "AAVSO Photometry" <aavso-photometry at mira.aavso.org>
Sent: Wednesday, May 14, 2008 11:28 PM
Subject: Re: [Aavso-photometry] Bruce Gary's book and other material


> Michael,
> The meridian overlap zone is determined by tube length and the amount  
> of offset between to polar axle and the tube itself.  On my mount, I  
> have only 7 degrees of movement past the meridian before the tube  
> hits the pier or the tube saddle hits the mount.
> 
> Bill
> 
> 
> 
> On May 14, 2008, at 11:31 AM, Michael Koppelman wrote:
> 
>> I've never understood those GEMs (like the Paramount) which force you
>> to move the scope the moment it hits the meridian. A GEM can traverse
>> the meridian and doesn't need to be moved for hours afterward. You can
>> start 3 hours on one side and end 3 hours on the other. GEMs do not
>> perform poorly at the zenith.
>>
>> I'm not religious about this mount versus that mount but I have had
>> great luck with GEMs.
>>
>> M.