Defeated by Orthogonality

[rogerg]

I've been trying to understand orthogonality of my fork mount LX200 for the last couple of weeks. Trying to determine the best way to measure error and adjust it.

I'm feeling defeated.

There seems to be something stopping me from using each one of the methods I find on the web of checking & adjusting orthogonality. Also seems technically a bit beyond me when learning only from reading the info on the web. Have tried Barry's and others. Haven't got to the stage of actually being ready to try and adjust it yet.

Quite disappointing & frustrating. I'm quite sure my fork must not be orthogonal because after precise polar alignment my pointing is considerably poor (15' 48" over a movement of 60 degrees of Dec).

At a bit of a loss how to proceed right now. Hmm. :shrug::rolleyes:

[GrampianStars]

Get a hold of "Barry Gerdes" I'm told he's the "Guru"
for setting orthogonality for these Scopes. :thumbsup:
"Barrykgerdes" on here

[rogerg]

Thanks, but unfortunately his instrucitons is one of the sets of information I've been working from ... after this (http://www.iceinspace.com.au/forum/showthread.php?t=53386)thread got me started on adjusting the orthogonality.

Barry's instructions are quite detailed and I'm sure it works for him but I've just got stuck at this stage. With his I need to find a couple of pieces of equipment required to perform his method, and achieve a level of accuracy I am not usually capable of in DIY exploits :lol:

[Barrykgerdes]

Hi Rogerg

I agree that setting up orthogonality is a problem with the SCT telescopes. The method I use with the Meade telescopes requires skills that not everyone can use and workshop equipment that is normally available in a small machine shop.

The essentials of orthogonality are to first get the Dec Axis to be at rightangles to the azimuth axis by adjusting the height of the fork arms so that the orthoganality of the dec axis is within 30 arc seconds. this corresponds to a height variation of less than .05 of a mm. Very difficult to measure with accuracy let alone adjust.

The second step then is to get the Optical axis of the OTA orthogonal to the Dec Axis by adjusting the OTA in the tube saddles. This was quite easy when the OTA could be rotated right through the forks but extremely difficult with the OTA's that only rotate about 90 degrees.

I do all these adjustments with many variations to the method depending on the size and vintage of the telescopes.

I have been pressed into doing alignments for friends who bring their scopes to me with about four free hours of spare time and are able to assist in the heavywork. This was a free service but over the last year my stamina for this type of physical effort has deteriorated ( I'm now 75) and I can only advise while people do the work.

Regretfully Western Australia is too far away for a personal visit.

Barry

[rogerg]

Barry that's fine, really, I am extremely appreciative of your help so far, it's not the fault of your instructions that I'm stuck or your responsibility to provide any further assistance at all. It's just a shame that I'm currently stuck, but no fault of yours. I will probably ask you more questions about it when I manage to progress beyond my current sticking point. Right now I really don't know what to ask so am just thinking about how to tackle it.

Thanks again,

Roger.

[Barrykgerdes]

Hi Roger

What model telescope have you got. I can probably steer you in the right direction if you put your questions to me.

Barry

[bmitchell82]

roger if you want i can find my way upto your house to help you. not that i know much about it, but im generally very good at the tinkering of things to make them work. Let me know if you would like a hand. I also have access to a proper machine shop.

talk to you then

[rogerg]

I've got a classic 12" LX200.

I haven't worked out how to accurately recreate the scenario you have mapped out in Fig2 of your document for checking the fork height, or fully understood where errors would be introduced:

I have a metal pole 4ft tall (taller), I have clamps.
I don't have a circular guage but have calipers and have been trying to work out if I can use them instead. I have considered buying a circular guage but am not sure if the <$50 ones are accurate enough to not introduce more error.
I haven't worked out how to attach the guage or caliper such that it's securely held at an apprpriate height.
I haven't worked out how to make up an equivalent of the machined bolt shown in Fig 1 and Fig 2. I have wondered if just finding a bolt of suitable thread, with a hut to clamp the DEC axis, woud be sufficent but I don't think so due to varying height of the thread?

I guess I'm just having trouble working out how to make the whole contraption rigid enough that it makes the alignment better rather than worse too :)

Regarding the algnment of the OTA your Method 1 where you point it at a star seems the easiest to me, but I don't understand how it works with anything less than a star that is immediately and precisely overhead. I don't really understand if the method using diffuse light source and eyepiece to focus on the RA bearing is applicable for my 12" classic, and if it's more or less accurate than the star method.

I also haven't worked out the simplist of things - what in the world I can use to wedge in under the forks to nudget the forks up by factions of a MM. Any metal I have lying around is at least 1mm in thickness, I have been wodnering what type of metal I could find which is sufficiently thin but strong enough to push in under the fork.

I hope you don't mind I have attached your fig1 and fig2 for clarity and to hopefully make the explanation clearer.

Thanks,
Roger.

[rogerg]

thanks for the offer brendan, I might take you up on that .. I'd like to progress my understanding a bit more first, and it would be good to have the appropriate equpiment/materials before you come so that I don't waste your time...

[Barrykgerdes]

Hi Roger

That's good. The function of those extension pieces is only something to measure against. My first effort used 1/4 bolts anout 3" long screwed into the holes. The extension pieces came later in an effort to minimise run out.

The dial guage kit I got from Hare and Forbes for abut $60 especially for the job. It has a magnetic attachment system .However I can also use it in other areas. The support for it came from materials I had hanging around. They just need to be rigid and stable.

The 12" Corrector end won't go through the base but at least it will swing right over the top so you will be able to establish two measuring points 180 deg apart.

The first thing to do is measure the run out on the two 3" bolts at points 180 deg apart to establish the mean height of the Dec shaft. This is where a lot of time can be spent as you need to do it many times to get repeatable results. This found will give the difference in height to about .01 mm with a bit of practice. Do all the adjustment on the lower side. I used a handfull of steel rulers and similar shimming devices. Measuring their thicknesses so that I can combine them in many forms to get total shimming in increments of about .02mm

I forced them into the space under the arm where it bolts onto the base till I cannot get any more in. I then loosen the four bolts (about 3 turns) that hold hold the arm. A top quality hex key drive on a wrench will be needed to get the bolts loose and later tight. I look for a shim piece about the thinkness (using a micrometer) that the arm needs to be raised then use a screwdriver or similar to prise the arm upwards to get the extra shim in then tighten everything up and go through the whole measureing process again to establish how much improvement (worse) I have made it.

From here it is a matter of patience and re-adjust/re-measure till You get the two arms within about .05 mm of each other (better if your patience is better than mine) this job can take a couple of hours to get satisfactory results but it is worth it in the long run.

This done you will have the dec pretty good. Next job is to do the OTA.

If you look at the back of the saddles you will see two theaded (4-40) holes. Get four Stainless steel screw about 3/4" long to fit these holes. I found suitable screws originally in old computer parts. DO NOT use these screws for direct adjustment the will sheer of and leave you with the terrible job of extracting the bits.

Now set the scope up in land mode and look at a TV aerial mast some distance away, centre it and turn the telescope on and read out the azimuth. Use the RA button to slew the scope exactly 180 degrees, swing the scope over and centre the TV mast. Rotate back and forth through 180 deg. Till you get repeatable results. With a bit of practice you will be able to measure the error in arc seconds.

Now look at the three bolts in each saddle that hold the OTA. They are in slotted holes. If you can figure out which way to move the OTA well and good. Otherwise look at the side that allows most movement both ways in the slotted holes. Mark this side so you can do all your adjustments from the same side. Point the OTA straight up and screw your 4-40 screws up against the hex holding bolts. Undo them two turns then loosen the three screws that hold the OTA in the saddle and let it drop onto the 4-40's. (May need some persuasion). Tighten everything up and do the measurements again on the TV aerial. This will establish if you have gone the right or wrong way. If the wrong way point OTA down and loosen the screws to move the OTA well into the rother direction. Screw the 4-40's up to the new position of the hex screws and go through the process again. Repeating as many time as necessary to make the TV aerial dead centre at each 180 degree point whichever way you drive it.

You are now an expert. Your Gotos should be within a couple of arc
minutes over the full range.

Barry

[Merlin66]

I replaced the Dec bearings in both my 10" and 12" Classic LX200 last year and was faced with a similar problem.

As per the LX200 Y! forum I used the stub extension shafts on the dec to get the forks aligned and found it after three or four goes, to be a real PITA.

I ended up using a U "frame" made from 18mm MDF which was 100mm wide and long enough to sit across the tube, above the knobs on the Dec arms. I then cut a couple of identical "legs" to allow them to sit on the knobs and allow the top of the frame to just sit on the OTA when it was horizontal.
Measuring each knob, I found them to be identical diameter, so that when the "legs" were resting on the top of each knob the frame should be horizontal ( if the base is set up flat and horizontal on the tripod)
A piece of 100mm x 18mm MFD was slipped through the handles on the Dec arms and clamped to the arms ( to keep them relatively aligned)

I then used a large spirit level on the U frame to check the adjustment of each Dec arm - I used a nylon door wedge and screwdriver to "lift" the Dec arms until the level showed a horizontal reading.
Re-do the 4 large Allen screws, rotate the mounting by 180 degrees and re-check. After a couple of trials I got it very level!

The GOTO is now easily able to find my target stars for the spectroscope.
I don't have any images of the set up, but still have the MDF frame etc If you need more info - drop me a PM and I'll give you the sizes etc I used.
IMHO Much easier than the stub shafts and dial indicator method.

[Barrykgerdes]

Hi Merlin

I follow your method OK and it should give reasonanble results particularly on some scopes I have fixed where you can actually see the difference in height. However measuring the orthogonality that way will not give the precision that I strive for.

Yes if you can get the dec pivots to about half a mm and the OTA square to this your gotos will be well inside a 26mm eyepiece anywhere. On my permanently polar mounted scope that had been set by my method I had gotos within +/- 2 arcminutes anywhere any time.

Barry

[Merlin66]

Barry,
I'm not saying that it's the "absolute" best method, but IMHO it's much easier for the average guy/ gal to use.....
The accuracy is dependent on the spirit level used and the amount of time spent on the "fiddle-fart" of moving the dec arm and re-tightening etc etc.
I can get the 12" ( polar aligned on a cut-down HD tripod) to find my target stars well within the 12mm cross hair eyepiece ( at f6.3) I use for initial setting of the spectroscope.
Works for me.

[GrampianStars]

I've used the aluminum walls from beer cans as shims :thumbsup:

[gary]

Dial gauges tend to be more convenient in these types of applications, because
unlike the calipers, they are sprung. As one then rotates the job one is measuring,
since the gauge exerts a tiny force back onto the job, the readout they provide
is continuous and it is easy to judge the high and low points.

Brendan kindly mentioned he had machine shop access, so perhaps he might
be able to lend you the use of a gauge for the day?

Best Regards

Gary

[kinetic]

Roger,
I know it's probably no help at all but I easily zeroed out most
of my orthagonal errors years ago using the z1,z2,z3 error
correction feature of the Bartels system.
For a heads up, at least to try and visualise exactly the beast you're
chasing might I suggest at least a read of some of his archives.
A good place to start is here: The Scope-Drive archives:
I have given the URL for the error correction topics:
http://ben.davies.net/scopemanual.htm#ERROR%20CORRECTION:
and
http://ben.davies.net/scopemanual.htm#ANALYSIS%20FILE:

As Mel suggests at the top of the forum posts, the best way to
prioritise the errors and deal with them are in the following order:
I tend to agree with his suggestions:


quote >>(with some editing by me for clarity)

1.gear backlash
2.gear periodic error correction (called PEC)

3.for stepper motors, physical variations in the quarterstep spacings over the sequence of windings (called QSC for QuarterStepCorrection) (N.A. to you Roger)
4.drift
5.guiding corrections
6.atmospheric refraction
7.precession

Orthagonal errors:

8.discrepancy between the optical and mechanical axis in the vertical axis (called Z3 or altitude offset)
9.axis misalignment (one side of the rocker is higher than the other) (called Z1 or axis misalignment)
10.discrepancy between the optical and mechanical axis in the horizontal axis (called Z2 or azimuth offset)

end quote

I have a log somewhere of my ortho corrections done years ago.
I , too, used aluminium shims :) If I can find anything useful in the journal
I will post it.
Hope this helps,

Steve

[Barrykgerdes]

Here are some practicalities of measurement

1. Using a spirit level with merlins method on a 12" telescope
The readout precision of the spirit level will be about .5mm over
the 500mm of the measurment centres. I can't read the bubble
any better than this. You could be lucky and your read error
could improve your accuracy or unlucky and reduce your accuracy.
This will equate at .5mm /500mm as about 3 arc minutes of
orthogonal error. This would in effect make most gotos within a 26mm eyepiece

2. Using the dial guage the precision improves greatly. You should
be able to measure with precision about .01 mm with an accuracy
of about .03 mm. over 600mm. This will equate to .03/600 or
about 10 arc seconds. This coupled with the acuracy of the OTA
being within 30 arc seconds with a visual alignment on the TV aerial
Will theoretically put your Gotos inside +/- 1 arc minute cover the
range.

Quite an improvement!

A thing to remember also is that orthogonality is less of a problem in ALT/AZM telescopes than in a polar mounted telescope. Getting a true polar alignment is virtually impossible if the orthogonallity is more that a few arc seconds out when using the standard procedures. This is why a drift alignment always seems to give different results than the manual described method.

Barry

[CoolhandJo]

For the record Barry helped me enormously. I took my scope over to him and he fixed it in a couple of hours. Thanks again Barry. One thing I will add is that when Barry was adjsuting/checking for Orthagonality he discovered that one of the OTA screws was loose. So even if you did fix the orthagonality it would "slip" out again at a certain angle. ?he installed a couple of screws that prevented any slippage and all was well from there on in! I achieve goto accuracy now.

[gary]

It is also worth pointing out that when one performs detailed analysis of fork
mounts of this type, that RA-to-Dec non-orthogonality, what is referred to as NP
and the Hour Angle component of Dec-to-optical axis non-orthogonality, what is referred
to as CH, is sometimes only part of the story as far as systematic mechanical
error influencing the pointing error residual is concerned.

Specifically it is not uncommon to discover various eccentric bearing error
and associated run-out effects in both the RA and Dec axes that often have
magnitudes as great as the NP and CH errors. For example, on commercial
SCT's eccentric bearing errors with magnitudes of 300 to 900 arc seconds
are not uncommon. The readers luck may vary.

It should also be kept in mind that the effects of refraction mean that
there is no such thing as a true polar alignment. At 45 degrees from the
zenith, refraction can account for around 1 arc minute difference in elevation.
In turn, refraction is a function of local barometric pressure and temperature.
The pressure of course can be influenced by the observer's altitude above
sea level. The effects of refraction then mean that rather than being some
specific point in the sky to which one can align the mount's RA axis which
is then good for the whole sky, there is really different specific optimal points
at a specific moment in time when the scope is pointed at a given zenith distance.
In other words, the optimal polar axis keeps moving as the scope tracks.

For this reason, the drift test is by no means the 'gold standard' when it
comes to determining the optimal alignment point as it does not take into
account a whole range of phenomena, including non-orthogonalities,
eccentricities, flexures, refraction, etc.

[rogerg]

Thanks for your input .. I really appreciate having everyone's help and alternative methods.

I was at one stage attempting to use this method, and still might go back to it:

http://home.comcast.net/~jmmahony1/LX10/forkalign.html

That one appealed because (1) it uses stars for alignment, and in the past I've found mechanical adjustments best measured using stars to provide accurate reference points to the adjustments made. (2) it means the telescope can stay on the wedge, for at least one part.



Great tip! Thanks!



Thanks for the explanation, that helps.



Thanks for the tip. I should check all the screws everywhere on my LX200 are tight. My pointing used to be near on perfect and the scope hasn't left the observatory since those days (few years ago), but has been taken on/off the pier many times an had motors etc replaced many times due to lightning strike, so something must have got knocked or drifted then, or perhaps come lose.


Thanks to everyone else for your input also. I'm going to persue Barry's method some more this week when time permits.

Thanks,
Roger.

[rogerg]

Barry,

For this part in your procedure, when pointing at the teTV aerial (or whatever) does the telescope not have to be pointing at 0 degrees altitude? If it can be at about 20 degrees this might be possible for me.

Thanks,
Roger.

[Barrykgerdes]

No. You are only interested in the azimuth as read from the handbox in land mode. Although it is best if you keep the altitude as low as possible and the TV aerial at least 400 metres away (makes focus easier). Use the edge of the mast at high magnification and you will be able to position/re-position to a couple of arc seconds with some practice.

A little patience will give very accurate results.

Barry

[gary]

Hi Roger,

The following background may be helpful.

For the following, let us assume the mount is in an Alt/Az configuration
and is level.

Alt to Optical Axis non-perpendicularity (CA) provides a constant pointing
error residual in Az for all elevations (i.e. all values of Alt).

Az to Alt non-perpendicularity (NPAE) provides an error residual in Az that is zero
at zero Alt and its maximum value when the OTA is pointing at 90 degrees
in Alt (i.e. zero zenith distance).

In surveying on the original optical theodolites, surveyors speak in terms of
"left-face" and "right-face" readings. When you take a sighting, then rotate the
mount 180 degrees in Az and then swing the OTA back through the forks to
invert it and then re-acquire the target, you are taking a "left-face" and
"right-face" reading. Both CA and NPAE errors reverse their apparent
directions when you do this procedure.

However, to null out CA, pick a target with zero elevation. That is because
at any other elevation your error will be a function of both CA and NPAE.
Once you have nulled out CA, choose a target at a higher elevation
and attempt to null out NPAE.

If you study the last two animated graphics and the accompanying text on this web
page on our web site, it will give you further appreciation.
See http://www.wildcard-innovations.com.au/geometric_mount_errors.html
The material on this page forms part of some of the presentations I have given
at star parties on telescope pointing errors and analysis.

[Merlin66]

Barry,
As a matter of academic interest, how do you set up the fork mount level on the tripod/ bench when measuring the Dec heights?
I've been levelling the tripod with a spirit level then placing the fork mount on it.
I also adjust the OTA alignment with a spirit level across the top of the OTA when it's pointed vertically upwards. Rotate by 180 degs and re-check.

[supernova1965]

I was defeated just reading the name:eyepop:

[rogerg]

If I understand Barry's method correctly then the telescope doesn't have to be precisely level, because you rotate it around to measure both sides, so even if the measured side is significantly lower than the opposite side, when you rotate it in azmith to measure the opposite side that side will then be at the same level the other one was. Hard to explain :)

Roger.

[AndrewJ]

Gday Merlin



I also use a similar method to Barry
The base doesnt need to be level, as long as the base of the scope
and the support holding the micrometer doesn't move.
You need to do a minimum of 4 measurements to get a correct value
but multiples are better.
Basically, I set up my extensions then point my OTA vertical
I measure the max value for the tops of the extensions
by rotating the RA 180deg
I then rotate the OTA to vertical down ( ie i move dec by 180deg )
Then repeat the process
You then average the readings for each side to get the correct delta
This eliminates any need to get the base level, or the extensions perfectly concentric to the axles.

Andrew

[Barrykgerdes]

Hi

Gary has brought up a few factors concerning the finer points of measurements which are quite correct. I did not go into them because I thought the process was complicated enough without trying to explain these factors.

One of the things I did try to stress was the difference between precision and accuracy. Many of the readings can be quite precise but the accuracy will depend on the general condition of the accuracy of the build of the telescope and the measuring equipment.

Andrewj has done a lot of work on the LX200 especially PEC correction and he is definitely the guru in this area. He also did some calculations on goto errors on telescopes polar mounted with orthogonal errors of only two or three arc minutes and how this can produce quite large errors as the observed items get closer to the poles.

The discussion on orthogonallity can be very difficult to put forward in a short answer as there are so many little things that can be missed. For most people who use these fork mounted scopes if the axes can be made within about 5 arc minutes the Gotos will be quite reasonable and almost perfect over small slews. However I have seen some telescopes with orthogonal errors close to a degree out of the box. Meade got around a lot of this by producing SMT which measures the errors at a number of points and develops a correction algorithm.

Trying to put procedures into words on how to do complicated tasks that require great expertise is almost impossible. I can tell you of many instances that cost the government millions of dollars because they wanted to save money by out sourcing work based on procedures that I tried to write as concisely and correctly as possible.

Barry

[AndrewJ]

Gday Barry



Caveat here. SMT, from both code review and having a squizz at several sets of real word data doesnt work. Lots of bugs in how it saves the data.

Andrew

[Barrykgerdes]

Hi Andrew

Nice to hear from you again. I had hoped to catch up with you at the end of the month but my trip to Melbourne has been posponed.

I have never had anything to do with SMT. I don't believe in fixing errors by adding offset errors is any sort of a solution. I just mentioned it in passing.

Barry

[rogerg]

OK, so I have made significant progress in my understanding of how to go about this orthogonality thing since starting this thread. I've now started to make something to do the job, working along the lines of Barry's method.

I bought a dial guage today and have made up a stand from metal I had in the shed to form the structure of what will hold the dial guage. See attached pictures. I'll put some bricks on the legs to hold it steady.

Could someone help me out with explanations of how to attach the dial guage to the pole in a way that I can adjust it to the correct height then lock it solid in place? :help:

The guage has 1 inch of travel so I do wonder if I should just bolt it on hard to the pole aiming to have the pin compressed in about 1/2" and not try to have the pin "just touching" the dec axis pins I'm measuring against?

You can see the dial guage has an attachment point at the back.

I've been trying to think of how to have a leaver arm which is attached to a threaded rod or the like. Or just attach the guage directly to a vertical threaded rod some how. :shrug:

Suggestions appreciated.

Thanks,
Roger.

[gary]

You might like to improvise yourself a couple of components. One is a clamp
that goes around the pole and can slide up and down before being fastened into the place.
The second is an articulated arm that is attached to the clamp at one end
and is fastened to the attachment point of the gauge using a machine screw
and wing nut.

For the bracket that goes up and down the pole, possibly something like
the arrangement on a TV aerial with a threaded U-bolt and a plate at the back.

For the articulated arm, a couple of pieces of narrow and thin
rectangular bracket is all you would need. Something along the lines of what
one use to find in a Mechano set. The two pieces of bracket could be
fastened together with a screw and wing nut so you can adjust the elbow to
move the dial up and down.

The first component with the bracket should provide for course adjustment in height
and the articulated arm should provide you fine adjustment.

Make sure to treat the indicator with care as it is a precision device.
Just allow the probe to touch the job gently and try and avoid the dial
being allowed to take a sudden very large deflection, both when making contact
with the job and when removing it.

[kinetic]

Roger, Gary,

Gary has just described exactly the bracket typical of a dial indicator
when used on a lathe. You can buy this type of bracket with a magnetic
base. I have one. I will try to dig out a pic of it or a link to one
on the net. Edit: This is one:
http://littlemachineshop.com/products/product_view.php?ProductID=1593

You can even make a right angle transfer arm for doing what you describe.
These are used for fine measurements too close to a lathe
chuck which would normally foul with the dial indicator itself.

Steve

[rogerg]

Ahhh... thank you very much both of you. It was something as simple/obvious as using clamps that I needed.

The articulated arm - I'm sure I've seen them in bunnings or dicksmith or something somewhere :question: ... tempting to have a look. When I asked in Toolmart about a holder today they told me one would cost about $100! :eyepop: .. so I didn't end up buying the guage or the holder there.

I'll have a think of how I can construct my own before going to buy one. Hmm. I have angle aluminium rods so surely can use lock nuts and the like to make something up. Concern is it not being rigid enough.

Thanks for the tips on how to handle the guage Steve.

[gary]

Hi Roger,

I have a magnetic holder very similar to the style of the one Steve
provided the link for and the holder came with the actual dial indicator itself.

When they are on a lathe and being used to check that a job is centered,
things are a little easier because they can just be magnetically clamped
on the lathe cross slide and then the cross slide can be wound ever so
gently in until the probe just touches the job. You will have to be a bit
more careful to avoid accidentally hitting the probe too hard with your
improvised jig.

Hare and Forbes have a branch n Belmont in Perth that supplies dial holders
if you don't go the DIY route.

[kinetic]

Pleasure Roger,

As well as the suppliers Gary mentions, there is also Gas-Weld.
I was in there today for work to get something....
Page 7 their latest catalogue/specials...$39 bucks..magnetic.
Can't beat that.

Steve

[rogerg]

When testing some of my equipment for the test and correction of orthogonality today I noted that, I'm quite sure, there's a noticeable difference in height from the front to the back of the fork. That is, if I wedge some metal between the RA base and the fork there is a noticeably tighter side, the side facing the control panel/south/down-hill when on wedge. If my measuring with my calipers is accurate the difference is about 1mm! The top of where the forks meet the RA base supports this, as it's not flat/same on both.

Hard to describe.

I am wondering if:
1) this is a problem, and by adjusting orthogonality with wedges of metal which will co-incidentally flatten/level this issue out, is a good thing

2) when I make adjustments I need to retain one side being higher than the other.

Another way to explain it, when mounted alt-az and telescope looking at 0 altitude, the side of the fork arms facing the eyepiece is higher off the RA base by about 1mm than the side of the fork arms on the corrector side.

Roger.

[Merlin66]

To hold the arms reasonably parallel I placed a length of timber ( about 25 x 150 x 500) through the handles and clamped it at top and bottom on each arm. If the bearings are machined concentric the arms should pretty well lie vertical. I ended up with the base of the fork arm parallel to the base drive unit....

AndrewJ-
With the 12" Lx200 I could get the necessary Dec rotation on the OTA (The safety stop limit the travel).................

[marki]

G'day all. watching this with great interest. Barry is it still possible to get a copy of your instructions to set up a scope? I used to have a copy but have lost it.

Mark

[rogerg]

Well, I have much to report...

On Sunday, after a week of preparation, I attempted to measure and adjust the orthogonality of my LX200. I had limited success but progress none the less. I have attached a bunch of pictures to show some of what went on.

It took me the whole morning and until 3pm in the afternoon to un-wire and dismantle the LX200 in the observatory to a state that I would be able to take it off the Pier, and collect all necesarry tools and pieces of equipment together on the patio where I would do the alignment along with my measuring 'jig' for the fork height, etc. Then at 4pm with the help of a friend I took the LX off the pier and put it on the tripod under the patio. Then from 4 until 7 we worked on measuring and adjusting the alignment.

I'm quite confident the measurements made of the fork height and the OTA alignment were accurate. The readings I was getting were very repeatable and stable thoughout the many measurements. I used a spreadsheet to record readings for the fork heights - Left up & down, Right up & down. The measurements were averaged and compared.

I followed this procedure:
1) measure fork height
2) measure OTA alignment
3) adjust OTA alignment
4) adjust fork height
5) measure fork height
6) measure OTA alignment

After step 1, measuring the forks I had 0.58mm difference in fork height. Not too bad I thought, surprisingly close really. I was expecting much more. I moved on to measure the OTA.

Measurement of the OTA was showing 24 arc minutes of error. Reasonably large amount I thought, but I don't have anything to guage it by to know how big or small that is.

Then I went to make an adjustment and here is where I ran in to problems. First up, it seems both my OTA and my forks are adjusted to the extremes of the directions they would need to be further adjusted in. The way it's all adjusted it looks to me like if it was all losened off and reset it could be brought in to better alignment but because one's at an extreme the other is. Just a gut feel, from looking at it, it's like it's all twisted but if straightened and started from scratch would be fine.

Anyway, I tried to make an adjustment to see if I could go any further or straighten anything.

I first tried adjusting both OTA and fork (one at a time) in the direction desired. I losened off the required screws and tried moving it - nothing, no movement at all. not an iota of movement (that I could tell). Odd I thought. In the case of the forks I could see the gap now present between fork arms and base, so surely I should have been able to move them? I tried in the other direction, in the direction that there was plenty of movement in the screw slots - still no budge. No matter what I tried, with quite heavy force, I couldn't move either the forks or the OTA adjustments in any direction, even when screws were losened. I stll don't know why.

However ... after trying all that and tightenning it all up again, I re-measured, and interestingly there was a difference. Not sure exactly when the movement occured, but I now had....

Fork error of 0.24mm in the opposite direction to what the 0.58 was previously, and an OTA error of 28 arc minutes.

So, if my measuring is correct, I now have less fork error but slightly more OTA error. In reality I think I changed very little and most of the difference measured between before and after is probably in error of measuring, although I am quite confident my measuring was quite accurate.

I wouldn't be surprised if the differece beteween before and after is purely due to me losening and re-tightening everything and in no way a consequence of my attempts to make adjustments when losened. And I wouldn't besurprised if it's changed slightly since lifting the beast back on to the wedge.

For now the LX200 is back up on the pier but not yet all fully set up again (permenant cables, cameras, focuser, etc).

Back on the pier some slews and visual work last night showed the goto accuracy was OK but not brilliant. I was glad to see I hadn't thrown it completely off, it is still in the relm of "somewhat like before". I haven't done any exhaustive testing, just moving around a few objects like M42, NGC 1365, M79, Eta, etc. And I haven't tweaked the polar alignment since putting it back on the wedge so that will be off somewhat after taking the scope off and putting it back on.

I'm letting it sit there for a few days while I consider if I will try anything more regarding orthogonality before I go to the trouble of re-attaching everything. I think chances are that I'll just re-attach everything and see how it goes, perhaps reserting to TPoint, which I have but prefer not to use.

Input, comments and constructive criticism of my technique greatly appreciated.

Roger.

PS.... so perhaps after all that I'm still defeated by orthogonality ;)

[AndrewJ]

Gday Roger

First comment i have is that using raw bolts like that may not give accurate ( correct ) results unless their shafts at the point of measurement are the exact same dia and they are perfectly circular. Tho your results appear quite good if repeatable.
Just for info, as you rotated the OTA, how much deviation did you get on the dial indicator??? Ie how concentric was the bolt to the DEC axle axis?

I use a set of match made trunnion extensions to eliminate this problem.
To do this, i turned up some alloy bar and machined a section in the middle
then parted this off in the middle.
Thus both trunnion ends are 99.99% identical in dia.
I then put the extensions into a collet chuck and faced off the ends that fit to the scope to ensure they were square.

Andrew

[rogerg]

I'll have to check my spreadsheet at home, but I am pretty sure when rotated 180 degrees the difference was in the order of about 0.3mm which I thought was quite large, but accurately accounted for by averaging the two values. You have an interesting point about the diameter of the bolts. I'll be interested to measure them with my calipers when next able, see if there's any measurable difference.


Thanks,
Roger.

[Barrykgerdes]

Regarding run out on the dec shaft measurements. I normally see at least .4 mm runout some are even more. I have also seen some as little as .1mm. The extension pieces I made are quite true and have accurate faces to mate with the Dec shaft. However the dec shafts themselves are usually the culprits. That is why you need to spend so much time determining the runout before you calculate the mean level. The older LX200 classics seemed to have less runout than the newer GPS models which use ballraces.

Barry

[gts055]

Hii Roger, have u been to the MAPUG website? There is an excellent and simple procedure for aligning forks and tube to attain orthogonality @ http://www.mapug-astronomy.net/AstroDesigns/MAPUG/OTA_Alignment/OTA-1.html
I used this method to realign a very early LX200 8" that only had provision for a single star alignment in its software. Pointing accuracy excellent after following this method. All the best Mark

[rogerg]

Hi Mark,

thanks for the tip. I did read through that method in quite some detail and thought I'd go with it for a while, but ended up having issues with a few aspects of it and then found Barry's. I think now my issue is not determining the alignment but being able to correct it, irrespective of how I measure it :rolleyes:

Thanks,
roger.

[AndrewJ]

Gday Barry



I didnt want to go into this level of detail earlier, as short trunnions and being able to rotate the OTA 180deg in DEC removes most problems.
However
The only absolutely correct way to measure the "forks" is to remove the OTA and both trunnion arms, then put a continuous precision bar in there.
The main reason for this is the trunnions arent actually colinear and can also flex as you rotate the OTA.
Once the forks are set correctly ( sans OTA ) both sides needs to be bolted and locked up then one side drilled and reamed for two tapered
pins. Then refit the ota and use the pins to reset the fork height before tensioning the fork base bolts.
Thats probably overkill for Roger, but it does make setting the height easy.
The new ACF forks are much nicer here as they have a horizontal join in the fork arms, hence can be adjusted by using simple shims.

But dont worry Roger, its all good fun ;)

Andrew

[rogerg]

:lol: my head went "pop" when I read Barry's last reply, still trying to digest his... and I'll just leave the "trunnions" well alone for now :lol:

[AndrewJ]

Gday Roger

Cant give up yet :D
I have attached a copy of my orthogonality checking doc.
It shows why the trunnions need to be a const dia and short as possible.

Ignore method 3 as i found it was too badly affected by flex in the trunnions. ( A good idea at the time, but True life beats theory every time )

Andrew

[Merlin66]

Roger,
I see from your images that you didn't put a clamp plate across the Dec arms ( to keep them roughly aligned and parallel).
IMHO this helps to stop them "twisting" relative to each other.