Quote:
Originally Posted by 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. |
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