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 (Roger)]

Originally Posted by AndrewJ

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'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 (Mark)]

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/Astro...ent/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 (Roger)]

Originally Posted by 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/Astro...ent/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

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

Thanks,
roger.

[AndrewJ]

Gday Barry

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.

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 (Roger)]

Originally Posted by AndrewJ

But dont worry Roger, its all good fun

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

[AndrewJ]

Gday Roger

Cant give up yet
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 (Ken)]

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.