Re: 1200RPA question

Oct 2, 2017



#58751 Oct 2, 2017

Hi all,

So I've reinstalled my 1200GTO in our backyard observatory after long stint a remote observatory. A couple of weeks ago, for whatever reason, drift aligning using PemPro showed I was waaay (> 3 degrees) off in azimuth. The only way I could see to get close was to rotate my entire pier. I wasn't too happy with that as the mount was fully loaded. I was also puzzled as the pier's legs were resting on cutouts in our floor that helped me roughly polar align.

When I had everything off, I realized that the 1200RPA's set screws were pretty loose, and more importantly, the Azimuth Adjuster block was scewed way to the east. I needed to adjust west, so I moved the block accordingly.

Drive aligning again this past weekend was still a bummer. I was still off in azimuth after fully adjusting as far west as I could. I'm only 40 or so arc-minutes off now, so not that bad, but definitely not as close as I'd like.

So anyway, is there a way that I can shift that azimuth adjuster block a little bit more without having to take the entire mount off?



#58754 Oct 2, 2017

Eric,     I wonder if you .centered. the AP-1200  fork baselock screws to your RPA, during re-assembly.     Maybe the screws are all jammed to one side of therotation slot. If so, you might  .ease off. those 4 screws, and twist themount (sliding its base around the RPA surface),  until they are in thecenter of their slot holes. Or, if that.s not enough, rotate the fork base untilthe AP-1200 fork lockdown screws are against the opposite side of their slots.Perhaps your original setup was with the lock screws at the end of the slotto begin with, and now re-centered, the tripod foot positions no longer work asbefore. Then DO NOT FORGET to lock down the fork again.     Best done with a couple of friends to help provide someassurance, stability, and shoulder the assembled mount and scope,  maybeadd some muscle to jog the base around the few degrees you need to bring backyour original  RPA adjustment range. You may want to also confirm ... thatyour tripod legs remained on their original floor marks, during all thattwisting . so you won.t confuse the original polar alignment foot positions,after your next field trip return.     Besides, you don.t want to turn the fork, as required,only to discover that you also accidentally moved the tripod feet around in theprocess, and made the compensation now twice as big ... to the other side.     You should be able to recover that lost AZ degree angle,enough to give your RPA some adjustment room again. Don.t know how .long. thoseslots are, but I suspect maybe a generous extra (+/-) 5 or 10 degrees ?AP might want to confirm the fork slot rotation angle (with lock screwsinserted),  for all of us here. Hope this makes sense.Joe Z. From: eja24601@... [ap-gto]Sent: Monday, October 2, 2017 1:20 PMTo: ap-gto@yahoogroups.comSubject: [ap-gto] 1200RPA question


#58760 Oct 2, 2017

Eric,     I was suggesting that you should back off the AZadjuster screws entirely away from the post -  so you can jog the loosenedfork, until the lock screws are at the other extreme.  That should give youa few degrees.     Actually, the task is even easier than I firstthought.Measuring my AP-900 & RPA I see that the slots in that fork are 1.25.long. The 1/4-20 .lock screw. is 1/4. thick, so there is an ample 1. of freeslot space  left over to shift the fork left or right.     You may not even need much muscle to try this.It seems to me, that if you loosen all four lock screws, just a bit . youshould be able to .crank the AZ adjuster knobs with a hex wrench. . to push thefork away from the post. Use the AZ adjuster scew to jog the freed fork, so thatthe lock screws are no longer dead center . they don.t actually need to be.     Since the fork and RPA are no longer locked together,using a hex wrench you should easily see the fork slip around the RPAbeneath,  a bit,   until you see that  the slots are farenough  off centre, as to give you a bit more AZ shift with  normalRPA AZ adjustments. After locking the screws down again, you should have gottenquite a few degrees from this. ****    A quick calculation as an example of how much adjustmentis available.My AP-900 fork lock screws are about 8 inches apart (diametrically).This presents a circumference of DxPi =  8xPi = 25 inchesroughly.The .slot free space. remaining for a rotational shift adjustment, whichI measured roughly, is about  1. of arc.So, this comes to ...   1/25 x 360 = 14.4 degrees of availableadjustment relative to the RPA itself.

     You have .14 degrees. to play with,  shifting thefork relative to the RPA. That is far more degree change than you need to getthe block reasonably centered again for normal polar alignments, After that,once the fork is locked to the RPA again, the RPA itself has its own 14 degreerotation with the AZ left/right screws, relative to the new fork.s offset angleposition on it.     Seems all you need is to de-centre the fork relative tothe RPA, a bit. Should provide plenty of travel to accomplish that, and youshould be able to do all that without assistance. Maybe someone can check my logic . seems right at first glance.Joe Z.  From: eja24601@... [ap-gto]Sent: Monday, October 2, 2017 5:49 PMTo: ap-gto@yahoogroups.comSubject: Re: [ap-gto] 1200RPA question


#58995 Oct 15, 2017

Joe (and anyone interested),

I ended up "brute forcing" the PA adjustment. There are a couple of bolt heads on my RPA opposite from the PA post. They appear to be used to make sure the RA assembly is properly centered on the RPA. Unfortunately, this prevented any shifting of the RA forks in the slots like you suggested. I *could* have removed the bolt heads, but I figured they were there for a purpose and chose not to.

By "brute forcing" the adjustment, I discovered that I could twist the pier ever so slightly, and use the tension bars to hold it in place. It took several tries - taking the mount off the pier three times, then finally realizing that I could *carefully* twist the pier with all the hardware in place and the tension bars loosened up. I actually ended up overshooting too far west, but that was handled once I figured out the maneuver.

Credit to Roland for sharing his daytime polar alignment procedure - it made short work of doing the adjustments. I used a Sol Finder to make the course azimuth adjustments, and PemPro later in the evening to do the fine adjustments. I believe I was only about 10-20 arc-minutes off using the daytime polar procedure.