VintageBigBlue.org

 

re:polar alignment/G-11 protocol


Apr 12, 2001

 


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#3622 Apr 12, 2001

Date: Wed, 11 Apr 2001 12:53:12 -0000



Hi Greg



further to your last memo,I met Eddie T because we both had the same

magnitude of

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.

Align polaris.

***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

almost everytime.

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



JImmy









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