VintageBigBlue.org

 

Polar alignment limitation


Apr 8, 2001

 


----------------------------

#3542 Apr 8, 2001

��Hello group'���� I have been trying to improve the accuracy of mypolar alignment to assist modeling with the Gemini. My first simple measure wasto try to align my polar scope reticle - sounded simple enough. So:������ Experiment 1: I followed the suggestion toalign the reticle in day light - Astrophysics instructions. I don't think youcan really find those small holes in the dark much less stick those .035wrenches in them. A 4 pane window about half a mile away was a perfect target.Shortening up the north tripod leg lowered the��polar axis enough. I tookoff the OTA and dec shaft to reduce flexure issues. I inserted the polar scopetight with the lock collar and centered the reticle on the target with thealt/az knobs.��When I rotated in RA the reticle center subtended an arc ofperhaps 30'. With the small set screws I adjusted the reticle to the center ofthat circle. After a few repeats it rotated perfectly about the target center.

������ It seemed all to easy! Then��I backedoff the RA clutch knob about 90 degrees and rechecked the reticle - it had movedabout 20'��off target. Retightening the Clutch returned the alignment.Unfortunately this means that the plane of the mounting rim on the clutch is notperpendicular to the polar axis of the mount. By adjusting the reticle I movedit's center to offset the eccentricity of the clutch nut. Even if the nut wasindexed to return to the same��position on the shaft, at different times anddates the alignment scope will have a different radial relationship with theclutch nut.������ Experiment 2: I mounted a dial gauge to theRA housing and measured the run out on the surface of the clutch knob that seatsthe polar scope flange. The longitudinal run out was .006" and this amount overthe 1.5" dia will give��eccentricity of 13.75'. This is close to what Iobserved on the reticle estimating Polaris at 44' from thepole.�������� There are few possibilitieshere.��Perhaps I'm the only one with an eccentrically threaded clutch nutand the only one with the problem. If this is the case you will find little runout, and your��RA axis and polar finder optical axis will coincide. However,if your system has any appreciable run out you will not be able adjust yourreticle��accurately.������ If you are working with the polar alignmentreticle issue please try this and post the results. If you are not having anyproblems I will order another clutch nut and remeasure. If this seems to becommon I will try to turn the polar shaft with the nut in place and true up thesurface. There is no reason it can't be brought to within .0005" runout.������ Do you have anythoughts?����Thanks,Ed������������������



----------------------------

#3552 Apr 9, 2001

Hi Ed,��I just wanted to confirm��that��what you are seeing isnot unique.�� I measured the runout on the face of my clutch nut��whereit��mates with the polar scope at about .007".�� It seems that whateverthe steps are in the machining process, an error is being introducedsomewhere.�� Perhaps the��way to surface the face of the clutch nutwould be to use a threaded shaft as a mandrel.�� As long as I�� have anindicator out to measure things, I am just going to square things up with asharp file.����Marty����----- Original Message -----From:Ed JoganicTo: Losmandy_users@yahoogroups.comSent: Sunday, April 08, 2001 11:22PMSubject: [Losmandy_users] Polar AlignmentLimitations

��Hello group'���� I have been trying to improve the accuracy ofmy polar alignment to assist modeling with the Gemini. My first simple measurewas to try to align my polar scope reticle - sounded simple enough.So:������ Experiment 1: I followed the suggestionto align the reticle in day light - Astrophysics instructions. I don't thinkyou can really find those small holes in the dark much less stick those .035wrenches in them. A 4 pane window about half a mile away was a perfect target.Shortening up the north tripod leg lowered the��polar axis enough. I tookoff the OTA and dec shaft to reduce flexure issues. I inserted the polar scopetight with the lock collar and centered the reticle on the target with thealt/az knobs.��When I rotated in RA the reticle center subtended an arc ofperhaps 30'. With the small set screws I adjusted the reticle to the center ofthat circle. After a few repeats it rotated perfectly about the target center.

������ It seemed all to easy! Then��I backedoff the RA clutch knob about 90 degrees and rechecked the reticle - it hadmoved about 20'��off target. Retightening the Clutch returned thealignment. Unfortunately this means that the plane of the mounting rim on theclutch is not perpendicular to the polar axis of the mount. By adjusting thereticle I moved it's center to offset the eccentricity of the clutch nut. Evenif the nut was indexed to return to the same��position on the shaft, atdifferent times and dates the alignment scope will have a different radialrelationship with the clutch nut.������ Experiment 2: I mounted a dial gauge tothe RA housing and measured the run out on the surface of the clutch knob thatseats the polar scope flange. The longitudinal run out was .006" and thisamount over the 1.5" dia will give��eccentricity of 13.75'. This is closeto what I observed on the reticle estimating Polaris at 44' from thepole.�������� There are few possibilitieshere.��Perhaps I'm the only one with an eccentrically threaded clutch nutand the only one with the problem. If this is the case you will find littlerun out, and your��RA axis and polar finder optical axis will coincide.However, if your system has any appreciable run out you will not be ableadjust your reticle��accurately.������ If you are working with the polaralignment reticle issue please try this and post the results. If you are nothaving any problems I will order another clutch nut and remeasure. If thisseems to be common I will try to turn the polar shaft with the nut in placeand true up the surface. There is no reason it can't be brought to within.0005" run out.������ Do you have anythoughts?����Thanks,Ed������������������

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----------------------------

#3553 Apr 9, 2001

Hmmmm...is there an easy fix here? I mean, I understand that "in

theory" we would like polar scope alignment at any RA clutch pressure.

But "in theory" this is a job that is done only once a night. What

about making up your mind to polar align ALWAYS at maximum clutch

tightness, then loosen to suit your taste afterward?



In other words, have you tried loosening-tightening, say, 10 times in

succession, to see if you RELIABLY return to true?



It is also possible that there is some "default" tension that the

factory uses for setting the polar scopes and that the reason that

when they get to us they're out of alignment is due to the fact that

we've all got different weight scopes and use different levels of RA

tension. If that is so, we might want to know what this default

tension level is.



greg nowell













--- In Losmandy_users@y..., "Marty Niemi" mniemi000@a...> wrote:

> Hi Ed,

>

> I just wanted to confirm that what you are seeing is not unique. I

measured the runout on the face of my clutch nut where it mates with

the polar scope at about .007". It seems that whatever the steps are

in the machining process, an error is being introduced somewhere.

Perhaps the way to surface the face of the clutch nut would be to use

a threaded shaft as a mandrel. As long as I have an indicator out to

measure things, I am just going to square things up with a sharp file.

>

> Marty

>

> ----- Original Message -----

> From: Ed Joganic

> To: Losmandy_users@y...

> Sent: Sunday, April 08, 2001 11:22 PM

> Subject: [Losmandy_users] Polar Alignment Limitations

>

>

> Hello group'

> I have been trying to improve the accuracy of my polar

alignment to assist modeling with the Gemini. My first simple measure

was to try to align my polar scope reticle - sounded simple enough.

So:

> Experiment 1: I followed the suggestion to align the reticle

in day light - Astrophysics instructions. I don't think you can really

find those small holes in the dark much less stick those .035 wrenches

in them. A 4 pane window about half a mile away was a perfect target.

Shortening up the north tripod leg lowered the polar axis enough. I

took off the OTA and dec shaft to reduce flexure issues. I inserted

the polar scope tight with the lock collar and centered the reticle on

the target with the alt/az knobs. When I rotated in RA the reticle

center subtended an arc of perhaps 30'. With the small set screws I

adjusted the reticle to the center of that circle. After a few repeats

it rotated perfectly about the target center.

> It seemed all to easy! Then I backed off the RA clutch knob

about 90 degrees and rechecked the reticle - it had moved about 20'

off target. Retightening the Clutch returned the alignment.

Unfortunately this means that the plane of the mounting rim on the

clutch is not perpendicular to the polar axis of the mount. By

adjusting the reticle I moved it's center to offset the eccentricity

of the clutch nut. Even if the nut was indexed to return to the same

position on the shaft, at different times and dates the alignment

scope will have a different radial relationship with the clutch nut.

> Experiment 2: I mounted a dial gauge to the RA housing and

measured the run out on the surface of the clutch knob that seats the

polar scope flange. The longitudinal run out was .006" and this amount

over the 1.5" dia will give eccentricity of 13.75'. This is close to

what I observed on the reticle estimating Polaris at 44' from the

pole.

> There are few possibilities here. Perhaps I'm the only one

with an eccentrically threaded clutch nut and the only one with the

problem. If this is the case you will find little run out, and your RA

axis and polar finder optical axis will coincide. However, if your

system has any appreciable run out you will not be able adjust your

reticle accurately.

> If you are working with the polar alignment reticle issue

please try this and post the results. If you are not having any

problems I will order another clutch nut and remeasure. If this seems

to be common I will try to turn the polar shaft with the nut in place

and true up the surface. There is no reason it can't be brought to

within .0005" run out.

> Do you have any thoughts? Thanks, Ed

>

>

> Yahoo! Groups Sponsor

>

>

>

> To unsubscribe from this group, send an email to:

> Losmandy_users-unsubscribe@egroups.com

>

>

>

> Your use of Yahoo! Groups is subject to the Yahoo! Terms of

Service.



----------------------------

#3557 Apr 9, 2001

Hi Greg,



I think Ed's point does not relate so much to clutch pressure, but to the

fact that unless the surface that holds the polar scope is true and

perpendicular to the RA axis, no adjustment will will be repeatable if the

clutch knob is moved. Because I had a dial indicator handy, I trued the

face of my clutch knob that mates with the polar scope until it showed about

.001" runout. I then pointed the polar scope at a distant object and

adjusted the reticle screws to keep things centered through the maximum

rotation of the head in RA. The next test was to rotate the polar scope 90

degrees in relation to the clutch knob and check the alignment again. There

is still some additional offset being introduced, but is a much less than

before. I think that this is as good as this is going to get without a

redesign. The method chosen to locate the bore scope just permits too much

movement to ever get things dead nuts on. It is, after all, marketed as a

polar alignment aid and it will get you fairly close.



Marty

----- Original Message -----

From: gnowell@...>

To: Losmandy_users@yahoogroups.com>

Sent: Monday, April 09, 2001 12:05 PM

Subject: [Losmandy_users] Re: Polar Alignment Limitations





> Hmmmm...is there an easy fix here? I mean, I understand that "in

> theory" we would like polar scope alignment at any RA clutch pressure.

> But "in theory" this is a job that is done only once a night. What

> about making up your mind to polar align ALWAYS at maximum clutch

> tightness, then loosen to suit your taste afterward?

>

> In other words, have you tried loosening-tightening, say, 10 times in

> succession, to see if you RELIABLY return to true?

>

> It is also possible that there is some "default" tension that the

> factory uses for setting the polar scopes and that the reason that

> when they get to us they're out of alignment is due to the fact that

> we've all got different weight scopes and use different levels of RA

> tension. If that is so, we might want to know what this default

> tension level is.

>

> greg nowell

>

>

>

>

>

>

>

> --- In Losmandy_users@y..., "Marty Niemi" mniemi000@a...> wrote:

> > Hi Ed,

> >

> > I just wanted to confirm that what you are seeing is not unique. I

> measured the runout on the face of my clutch nut where it mates with

> the polar scope at about .007". It seems that whatever the steps are

> in the machining process, an error is being introduced somewhere.

> Perhaps the way to surface the face of the clutch nut would be to use

> a threaded shaft as a mandrel. As long as I have an indicator out to

> measure things, I am just going to square things up with a sharp file.

> >

> > Marty

> >

> > ----- Original Message -----

> > From: Ed Joganic

> > To: Losmandy_users@y...

> > Sent: Sunday, April 08, 2001 11:22 PM

> > Subject: [Losmandy_users] Polar Alignment Limitations

> >

> >

> > Hello group'

> > I have been trying to improve the accuracy of my polar

> alignment to assist modeling with the Gemini. My first simple measure

> was to try to align my polar scope reticle - sounded simple enough.

> So:

> > Experiment 1: I followed the suggestion to align the reticle

> in day light - Astrophysics instructions. I don't think you can really

> find those small holes in the dark much less stick those .035 wrenches

> in them. A 4 pane window about half a mile away was a perfect target.

> Shortening up the north tripod leg lowered the polar axis enough. I

> took off the OTA and dec shaft to reduce flexure issues. I inserted

> the polar scope tight with the lock collar and centered the reticle on

> the target with the alt/az knobs. When I rotated in RA the reticle

> center subtended an arc of perhaps 30'. With the small set screws I

> adjusted the reticle to the center of that circle. After a few repeats

> it rotated perfectly about the target center.

> > It seemed all to easy! Then I backed off the RA clutch knob

> about 90 degrees and rechecked the reticle - it had moved about 20'

> off target. Retightening the Clutch returned the alignment.

> Unfortunately this means that the plane of the mounting rim on the

> clutch is not perpendicular to the polar axis of the mount. By

> adjusting the reticle I moved it's center to offset the eccentricity

> of the clutch nut. Even if the nut was indexed to return to the same

> position on the shaft, at different times and dates the alignment

> scope will have a different radial relationship with the clutch nut.

> > Experiment 2: I mounted a dial gauge to the RA housing and

> measured the run out on the surface of the clutch knob that seats the

> polar scope flange. The longitudinal run out was .006" and this amount

> over the 1.5" dia will give eccentricity of 13.75'. This is close to

> what I observed on the reticle estimating Polaris at 44' from the

> pole.

> > There are few possibilities here. Perhaps I'm the only one

> with an eccentrically threaded clutch nut and the only one with the

> problem. If this is the case you will find little run out, and your RA

> axis and polar finder optical axis will coincide. However, if your

> system has any appreciable run out you will not be able adjust your

> reticle accurately.

> > If you are working with the polar alignment reticle issue

> please try this and post the results. If you are not having any

> problems I will order another clutch nut and remeasure. If this seems

> to be common I will try to turn the polar shaft with the nut in place

> and true up the surface. There is no reason it can't be brought to

> within .0005" run out.

> > Do you have any thoughts? Thanks, Ed

> >

> >

> > Yahoo! Groups Sponsor

> >

> >

> >

> > To unsubscribe from this group, send an email to:

> > Losmandy_users-unsubscribe@egroups.com

> >

> >

> >

> > Your use of Yahoo! Groups is subject to the Yahoo! Terms of

> Service.

>

>

> To unsubscribe from this group, send an email to:

> Losmandy_users-unsubscribe@egroups.com

>

>

>

> Your use of Yahoo! Groups is subject to docs.yahoo.com/info/terms/

>

>







----------------------------

#3558 Apr 9, 2001

Mr. Niemi:



I take your point, but isn't it the case that if one returns to the

maximum pressure point each time that this is a way of making sure he

has returned the clutch knob to the same angular orientation, thereby

assuring polar alignment *for that orientation or the knob* will be

restored??



Or what about putting a dot on the knob, like nail polish, and

counting in the number of full rotations you make for your "polar

alignment position," thereby insuring that the same angle is applied

each time you use it?



I do believe the original poster put the error margin, depending on

knob position, at 30 minutes of arc, which is half a degree! That's a

lotta error.



I perceive, Mr. Niemi, that you have a good deal of experience working

with metal. When I work with *wood* I make sure I have lots of scrap

on hand to cover up my blunders. Particularly with subtractive

processes like filing and drilling, where something once done cannot

be undone, I am not eager to learn shop basics on expensive cast

material, which, as a kicker, often takes months to get even when

you're willing to pay for it. For this kind of modification, people

who have tools and experience are at a distinct advantage over those

who don't.



This is why I'm looking for a 2nd best practical alternative to

modifying the metal as purchased, and also why, when I modify my

dovetails, I will pay someone else to do it.



I note that in theory were one to return the clutch knob to the SAME

position each time one might get, instead of pretty damn close, EXACT

return to correct collimation, but that is theory. I haven't had a

chance to check it out and won't until I get my #!%#! .035 allen

wrenches.



Meantime y'all can wish me luck tonight, I might have a chance to test

my OTA shimming. In theory I should be within 8 arc minutes already,

if parallax estimates posted yesteday were correct. That compares

extremely favorably to the 2 - 2.5 degrees error with which I started.



regards,



greg nowell



--- In Losmandy_users@y..., "Marty Niemi" mniemi000@a...> wrote:

> Hi Greg,

>

> I think Ed's point does not relate so much to clutch pressure, but

to the

> fact that unless the surface that holds the polar scope is true and

> perpendicular to the RA axis, no adjustment will will be repeatable

if the

> clutch knob is moved. Because I had a dial indicator handy, I trued

the

> face of my clutch knob that mates with the polar scope until it

showed about

> .001" runout. I then pointed the polar scope at a distant object

and

> adjusted the reticle screws to keep things centered through the

maximum

> rotation of the head in RA. The next test was to rotate the polar

scope 90

> degrees in relation to the clutch knob and check the alignment

again. There

> is still some additional offset being introduced, but is a much less

than

> before. I think that this is as good as this is going to get

without a

> redesign. The method chosen to locate the bore scope just permits

too much

> movement to ever get things dead nuts on. It is, after all,

marketed as a

> polar alignment aid and it will get you fairly close.

>

> Marty

>

> ----- Original Message -----

> From: gnowell@w...>

> To: Losmandy_users@y...>

> Sent: Monday, April 09, 2001 12:05 PM

> Subject: [Losmandy_users] Re: Polar Alignment Limitations

>

>

> > Hmmmm...is there an easy fix here? I mean, I understand that "in

> > theory" we would like polar scope alignment at any RA clutch

pressure.

> > But "in theory" this is a job that is done only once a night.

What

> > about making up your mind to polar align ALWAYS at maximum clutch

> > tightness, then loosen to suit your taste afterward?

> >

> > In other words, have you tried loosening-tightening, say, 10 times

in

> > succession, to see if you RELIABLY return to true?

> >

> > It is also possible that there is some "default" tension that the

> > factory uses for setting the polar scopes and that the reason that

> > when they get to us they're out of alignment is due to the fact

that

> > we've all got different weight scopes and use different levels of

RA

> > tension. If that is so, we might want to know what this default

> > tension level is.

> >

> > greg nowell

> >

> >

> >

> >

> >

> >

> >

> > --- In Losmandy_users@y..., "Marty Niemi" mniemi000@a...> wrote:

> > > Hi Ed,

> > >

> > > I just wanted to confirm that what you are seeing is not unique.

I

> > measured the runout on the face of my clutch nut where it mates

with

> > the polar scope at about .007". It seems that whatever the steps

are

> > in the machining process, an error is being introduced somewhere.

> > Perhaps the way to surface the face of the clutch nut would be to

use

> > a threaded shaft as a mandrel. As long as I have an indicator

out to

> > measure things, I am just going to square things up with a sharp

file.

> > >

> > > Marty

> > >

> > > ----- Original Message -----

> > > From: Ed Joganic

> > > To: Losmandy_users@y...

> > > Sent: Sunday, April 08, 2001 11:22 PM

> > > Subject: [Losmandy_users] Polar Alignment Limitations

> > >

> > >

> > > Hello group'

> > > I have been trying to improve the accuracy of my polar

> > alignment to assist modeling with the Gemini. My first simple

measure

> > was to try to align my polar scope reticle - sounded simple

enough.

> > So:

> > > Experiment 1: I followed the suggestion to align the

reticle

> > in day light - Astrophysics instructions. I don't think you can

really

> > find those small holes in the dark much less stick those .035

wrenches

> > in them. A 4 pane window about half a mile away was a perfect

target.

> > Shortening up the north tripod leg lowered the polar axis enough.

I

> > took off the OTA and dec shaft to reduce flexure issues. I

inserted

> > the polar scope tight with the lock collar and centered the

reticle on

> > the target with the alt/az knobs. When I rotated in RA the reticle

> > center subtended an arc of perhaps 30'. With the small set screws

I

> > adjusted the reticle to the center of that circle. After a few

repeats

> > it rotated perfectly about the target center.

> > > It seemed all to easy! Then I backed off the RA clutch

knob

> > about 90 degrees and rechecked the reticle - it had moved about

20'

> > off target. Retightening the Clutch returned the alignment.

> > Unfortunately this means that the plane of the mounting rim on the

> > clutch is not perpendicular to the polar axis of the mount. By

> > adjusting the reticle I moved it's center to offset the

eccentricity

> > of the clutch nut. Even if the nut was indexed to return to the

same

> > position on the shaft, at different times and dates the alignment

> > scope will have a different radial relationship with the clutch

nut.

> > > Experiment 2: I mounted a dial gauge to the RA housing and

> > measured the run out on the surface of the clutch knob that seats

the

> > polar scope flange. The longitudinal run out was .006" and this

amount

> > over the 1.5" dia will give eccentricity of 13.75'. This is close

to

> > what I observed on the reticle estimating Polaris at 44' from the

> > pole.

> > > There are few possibilities here. Perhaps I'm the only one

> > with an eccentrically threaded clutch nut and the only one with

the

> > problem. If this is the case you will find little run out, and

your RA

> > axis and polar finder optical axis will coincide. However, if your

> > system has any appreciable run out you will not be able adjust

your

> > reticle accurately.

> > > If you are working with the polar alignment reticle issue

> > please try this and post the results. If you are not having any

> > problems I will order another clutch nut and remeasure. If this

seems

> > to be common I will try to turn the polar shaft with the nut in

place

> > and true up the surface. There is no reason it can't be brought to

> > within .0005" run out.

> > > Do you have any thoughts? Thanks, Ed

> > >

> > >

> > > Yahoo! Groups Sponsor

> > >

> > >

> > >

> > > To unsubscribe from this group, send an email to:

> > > Losmandy_users-unsubscribe@egroups.com

> > >

> > >

> > >

> > > Your use of Yahoo! Groups is subject to the Yahoo! Terms of

> > Service.

> >

> >

> > To unsubscribe from this group, send an email to:

> > Losmandy_users-unsubscribe@egroups.com

> >

> >

> >

> > Your use of Yahoo! Groups is subject to

docs.yahoo.com/info/terms/

> >

> >







----------------------------

#3559 Apr 9, 2001

Ooops! Here's the crucial sentence (below) that didn't sink in first

time. It would appear that barring re-manufacturing the nut, as

suggested in the posts, the only solution is to guesstimate the

approximate angular orientation of delta Umi and Polaris at the time

one will do the set up and then fine-tune the polar scope accordingly.

This would be similar to fine-tuning the finder, which always needs to

be done, even though I have a quick release system that in theory

ought to keep everything from needing adjustment. But it is another

thing to do and one that is a good deal more difficult than adjusting

a finder.



greg nowell





--- In Losmandy_users@y..., "Ed Joganic" astron@p...> wrote:

Even if the nut was indexed to return to the same

position on the shaft, at different times and dates the alignment

scope will have a different radial relationship with the clutch nut.

>



----------------------------

#3578 Apr 10, 2001

In a message dated 4/9/01 11:07:55 AM EST, gnowell@... writes:

> What about making up your mind to polar align ALWAYS at

> maximum clutch tightness, then loosen to suit your taste afterward?



This is what I do always, AFTER making sure the mount is level.

(easier corrections when doing drift alignments)

I tighten the set screw on the polar scope after I have tightened the RA

clutch down as much as I can. Like you said, you only have to do

this once a night, and it's worked for me so far :)

This is what the manual says to do (IIRC) Also I always have the

counterweight shaft pointing at the ground, the DEC always turned

the same way. (to align hole with RA bore)



Allan



----------------------------

#3602 Apr 10, 2001

Hi group,������ ������ Greg and Marty, thanksfor the suggestions.��������Marty, you picked up on the problem.When the alignment scope is removed and then reinserted in a different rotatedposition - say 6hrs (or 3 mo.) later the error that had been��introduced bythe adjustment of the reticle��position will no longer cancel out thecorresponding error��produced by the canted clutch nut surface. As you bothsuggest I'll try to true up the nut for the smallest possible run out in theclutch tightened position.�������� For interest I measured the thickness ofthe clutch nut and found that the 2 opposite surfaces aren't parallel. Thicknessvaries by .005" around the circumference.��You would think that the maxthickness spot would correspond to the high point in the run out right?- itdoesn't.����������I think I can conclude that puttingthe stars exactly in those little gaps is sure fun - but - it's justfun.�������� Thanks, Ed����



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