#3622 Apr 12, 2001
Date: Wed, 11 Apr 2001 12:53:12 -0000
further to your last memo,I met Eddie T because we both had the same
problems with the G-11 but we pretty much worked them out.
Hopefully,some of those findings will help you out.
First,there is a lot of "techie" stuff that people do,mostly because they
can and then others follow suit who can't...that causes problems.
I'm not saying this is you but sometimes reading these posts,I've got to
shake my head as to why intuition is sometimes so askew.I've been guilty of
the same and when taking a "backward" step have found I was the problem.(but
don't admit this to my wife eh!)
But lets go back to the fundamentals and perhaps something might result that
can help you....plain and simple,the G-11 right out of the box will track
visually and pretty damn near perfectly.You cannot assemble it wrong as long
as the ra DSC moves when the ra axis does and ditto for dec.
Thousands are using these well and I for one have never monkeyed with the
secondary system alignment but my LXD 500 8" AND c-11" all caught
the objects within a 40mm meade EP and now 31mm nagler.
A couple things about the dsc's is to make certain they are programmed at
the proper tics (4096 I think it is).The manual is confusing because
it more or less implies 9000 setting can be used and would be more accurate
but that is only if you use an aftermarket encoder of that value.
Next,pull your polar scope and dump that skinny little flex washer.
Instead I use a rubber O ring that fits snug over the end.(O rings are made
to very accurate tolerances)Now when I snug it up,I can just turn it for
alignment but it does not move laterally or longitudinally.
While the polar scope is out,mount it on something like a camera tripod
or sawhorse.Focus and sight it in just like a rifle scope.Centre the
furthest object you can.If at twighlight use a star.
The next part is not meant to be condescending,but I'll go over polar
alignment just so there is a check list.
First,spot and confirm polaris.
Look thru the ra polar axis hole and centre the star.
Level your mount
Ensure polaris is recentered
Install polar scope.
***the value of the Losmandy polar scope is really the orientation of
inscribed constellations.If you cannot see the other 2 guide stars
because of seeing conditions and only see polaris,you will always be at
least .75 degrees away from alignment.You will also have to calculate in
what direction "off" is.
Assuming though that all three are visible in the polar scope,I find that
their proper alignment is in the middle of the 3 tic marks for each star.
Having done this and I can do it in about 2 minutes,you are polar aligned
period!Any finer is purely drift method and for long exposure
photography.You should not notice dec drift for hours.
So what else can go wrong?One thing is the DSC cpu.It operates in UT time.
I have tried everything but what works for me is the following protocol.
Set up the DSC with the current date in your local time.(don't go forward
just because its another day in England or change it after midnight)Enter
and then pick a locational star in the area of sky you want to view.Verify
its identity and then centre it in reticle ep under hi mag.(You cannot be
accurate in a normal viewing ep.)
Now,turn the scope to a close easy to find M object.Use the DSC to do this
because when you "zero" out and the object is not centered you'll understand
by how much.This is a timing issue between the algorithms in the DSC and
your actual location/time.This variance however should only be in RA and
pretty consistant.Centre the object.Then hit
MENU and REALIGN ON OBJECT.
Now all your objects should be much closer.
BY THE WAY....use 1 star alignment.(not 2 star)
Intuition tells you that 2 stars would be more accurate than 1 but thats not
true in this case.The accuracy comes from polar alignment and 1 star.
(using 2 throws eveything off)
I'm certain the Above will put you an object in a 9.25FOV using a 35mm
Now you can concentrate on the other big alignment factors of
diffraction,mirror flop and mirror slop.The other issue in setting this up
is to view with the ep straight thru if you can.This will ease your mind as
to the diagonal.The diaonal or oag can cause some big issues so do straight
thru first...be happy then try with the diagonal.Once happy,don't change its
orientation or you may have to realign on a new object.When you swing more
than 15 degrees or cross the meridian you may have to realign.
After getting this to a level where you are happy and you notice you are off
more than the "standard" when going to farther parts of the sky,try
orthogonting your axis.
The method in the Losmandy DSC manual works well,Just remember to move the
scope,physically (not the alt/az fine controls).Be patient and iterrate
until polaris is centred.Once done then recheck you scope for level and
polar align again.
Now there is one last consideration of pointing accuracy with the G-11.
The RA shaft/needle bearing connection.
Without a doubt,when you clutch,align then swing the scope,unclutch to
center and the reclutch you are going to have error.
The bearing/shaft machining tolerances and/or wear due to loading weight
play a big factor...not so much in guiding but in pointing for sure.
I ahve found there are shafts like mine that are really good..1.249"
Some though go down to 1.247.Thats a lot and it is magnified by as much as
double because being suspended at either end the load will pull to one side
at the top and force the shaft to the far side at the bottom.
This eccentricity is a big problem with pointing.
Good news though is its easy to fix most of it.Add the third bearing.
Do it in DEC and RA because the smae thing happens in both axis but is more
noticable in the DEC.
Good luck...I went thru all this stuff,2 stars etc and now I do the above
and after my first setting and realign I use only the DSC's.
To tell you how accurate,I have my f6.8pronto loaded with an st-7e.It is
side saddle with a TV 101.I centre my star in a TELRAD.The guide star
usually shows up in the focus mode of the st-7e.I centre it in the screen
and hit ENTER on my DSC.Then I use my tv 101 with a 31mm Nagler5
to finf an M class cluster.Centre it.It usually comes onscreen
somewhere.Centre it on the computer.Hit REALIGN ON OBJECT and for the rest
of the nite,never look into the TV101 again to realign.
I'm sure the fov on the st-7e is close or less than the 9.25.
let me know how it works out
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