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).
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. .....
At a bit of a loss how to proceed right now. Hmm.
Get a hold of "Barry Gerdes" I'm told he's the "Guru"
for setting orthogonality for these Scopes.
"Barrykgerdes" on here
Get a hold of "Barry Gerdes" I'm told he's the "Guru"
for setting orthogonality for these Scopes.
"Barrykgerdes" on here
Thanks, but unfortunately his instrucitons is one of the sets of information I've been working from ... after this 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
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.
Regretfully Western Australia is too far away for a personal visit.
Barry
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.
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.
What model telescope have you got. I can probably steer you in the right direction if you put your questions to me.
Barry
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.
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
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...
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.
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.
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.
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,
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.
I've used the aluminum walls from beer cans as shims
Quote:
Originally Posted by rogerg
I've got a classic 12" LX200.
........
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.
[*]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.
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?
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.ht...%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:
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,
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
Last edited by Barrykgerdes; 03-01-2010 at 07:33 PM.
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.
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.
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.
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.
Quote:
Originally Posted by GrampianStars
I've used the aluminum walls from beer cans as shims
Great tip! Thanks!
Quote:
Originally Posted by gary
Dial gauges tend to be more convenient in these types of applications, because
unlike the calipers, they are sprung....
Thanks for the explanation, that helps.
Quote:
Originally Posted by CoolhandJo
One thing I will add is that when Barry was adjsuting/checking for Orthagonality he discovered that one of the OTA screws was loose..
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.
Defeated by Orthogonality
