Pear Tree Observatory

First, the PTO is finally back online. Now the painful details:

The original diagnosis of the new mount having a personality problem was incorrect. The control box and hand paddle checked out fine and were returned unharmed. As expected, the mount still suffered from pointing problems; both in positioning and parking. After an additional session of phone diagnostics with George from Astro-Physics (thanks George) the RA gearbox was dismounted and sent back to the factory for inspection and repair. George also requested the GTOCP3 control box accompany the gearbox (its second trip back home). The return paperwork listed new spur and cluster gears as the fix. While they had the control panel the troops at AP ran modeling software on the repaired gearbox and loaded a new PEC model into it.

With the repaired and reprogrammed parts reinstalled, the mount was ready for testing. We finally got some clear skies to align and test it and the results are amazing. With the old mount I was able to reliably take an unguided 30 second exposure without any visible distortion in a star’s image. A guided 300 second image would routinely pass a 0.2 roundness test. Now I can take an unguided 300 second image where the stars routinely pass a 0.05 roundness test.

The old mount’s less than optimal performance is not entirely its fault. First, I had loaded the mount right to its advertised limit (and over time probably a little more). Also, the polar alignment was off slightly in elevation. I found making fine adjustments to the elevation axis very difficult. I’m pretty sure this was mostly due to the design of the adjusting mechanism dealing with the weight the mount was carrying.

Of course, the primary reason for the observatory upgrade was to increase the load capability of the mount for the new (refurbished) telescope.

Installing the new scope will be the final step in this PTO upgrade. Watch this space.

The floor tasks have been put on hold for more conducive weather conditions and to be ready for the occultation on the 12th.

The original configuration of the control wiring and plumbing through the floor has been restored. However, the wiring configuration at the mount has changed drastically. The AP1600 allows the cabling to the focusers and cameras to be routed through the mount. I’ve rolled the dice and also routed the cooling hoses through it as well. This routing scheme significantly reduces the chance of wiring or hoses snagging on something. The size of the Losmandy mount prevented through the mount cabling so it was all external.

One of the challenges of a German equatorial mount has always been cable routing. Over time I had come up with a fairly good configuration, but there were certain positions in the sky that could result in a temporary snag. These weren’t critical enough to damage equipment; they would just cause enough drag to upset the guiding and ruin an exposure. The snag would then release and most of the time the mount guiding would recover. Once in a while I would have to go into the dome room and adjust the way the cables were hanging. Hopefully, that tasking is over.

So, here is progress made in the list of upgrade tasks:

• The scope has been balanced. The mount is fairly stiff and with the current weight load nowhere near the advertised limit, balancing it was not as finicky as the Losmandy. I still have another counterweight in the box for the 12.5″ scope. It may not be enough.
• The new ASCOM driver has been installed and is operational. Since the polar alignment software that came with the mount required it, installing it was moved up in the list. So far, all the supporting software packages seem to be playing well with the new scope control system.
• I started the polar alignment process on the 6th of Jan. and was able to get most of the way through it before clouds moved in. Finished initial alignment on the 7th.
• Characterizing any periodic error has not been accomplished yet. All recommendations that I have found said to perform the mount alignment first.
• In order to accurately position the slit in the dome, the dome controller needs to know where the mount RA axis of rotation and Dec axis of rotation intersect. If that intersection is not at the center of the dome, the differences need to measured and entered into the mount driver. Obviously the new mount is at least 18″ higher in the dome with the pier spacer. The intersection of the mount RA and Dec axes are also not the same as the G-11. So, I had to restring the dome and measure the new offsets. That has been done.

The AP1600 has 4 predefined parking orientations. I always parked the G-11 in the traditional equatorial position that everyone thinks of; telescope pointed at the north celestial pole with the counterweight bar down. However, that position is not the preferred position for this mount. Park position #4 has the telescope parallel to the ground facing south and the counterweight bar parallel to the ground facing west. It is definitely unconventional looking but has a totally unanticipated benefit. With both axes parallel to the ground and with the addition of the 18″ pier spacer, the lowest point of any piece of equipment is 6’2″ above the floor. I can now walk around the dome room without fear of bumping the mount and having to re-do a mount alignment.

UPDATE: There is significant star streaking during a 30 sec exposure. I have spent the last two days trying different techniques to eliminate the streaking. So far, no joy.

I am taking advantage of the holiday season, the full moon and the weather forecast to stand down the PTO. I will be accomplishing several upgrades that I have been sitting on for some time now.

During my initial observatory planning I neglected to include the dome base ring in the pier height calculation. That left it 12″ shorter than I was wanting. The pier I have is made by ATS (Advanced Telescope Systems). I contacted them and had them make an 18 inch tall “spacer”. Placing it under the existing pier has raised the mount 6″ higher than I originally planned but it is still be below the level of the dome slot. The tree line surrounding the observatory prevents observing any closer to the horizon.

18″ extension in place.

The second update is to replace my mount. Currently, I use a Losmandy G-11. It has proven to be an excellent mount, even though I long ago exceeded the stated load capacity (60 lbs). Two years ago I purchased a 12.5″ Cave Astrola Newtonian telescope at an estate sale. There is no way the G-11 would be able to swing that size of telescope. So, after evaluating several mounts in the load range I wanted, I purchased an Astro-Physics 1600. The AP 1600’s instrument capacity (220 lbs) should handle any future scope that the observatory can handle (or that I can afford).

Losmandy G-11 vs. Astro-Physics 1600

I only want to change one major component at a time, so for now, I will change only the mount and will continue to use the 10″ scope. Only after the mount is fully operational and characterized will I upgrade the scope.

Even though I had to get my oldest son to help with the weight of the mount, physically installing it and the spacer was the easy part. The real effort will be updating the operating environment.

• First and foremost, will be balancing the scope with all the support equipment (camera, guide scope, etc.). The increased load capacity of the 1600 makes it much less sensitive to balance than the G-11 but still needs to be done to optimize the mount’s tracking accuracy.
• Next will be polar aligning the new mount. The Losmandy was fairly well aligned to the Earth’s axis of rotation. It wasn’t perfect but the small misalignment was easily handled by the guider. The 1600’s altitude adjustment mechanism appears to be easier to adjust than the G-11 which will make it easier to get an improved alignment.
• The new mount, of course, has a different control system. That means changing to a new software driver and establishing new calibration parameters. It also means an extensive testing period to make sure all the different software packages play well with the new mount.
• The new mount comes with software support to characterize the periodic error inherent in all geared systems. AP mounts are known for their very small periodic error, but the software tool will be run and the resultant model incorporated into the overall system to improve the tracking.
• Since the mount is positioned higher in the dome and is physically larger, I will need to change the parameters used in the dome driver to ensure the slot is positioned directly in front of the telescope.
• I will have to see how much the tree line now blocks my view of the sky. The planetarium software I use (TheSkyX) allows a local conditions overlay to portray the visible portion of the sky. My guess is the vertical displacement will not change the view enough to worry about, but I don’t want to find out later that it did.

The last update this go around will be to the floor in the dome room. One of my best ideas was to install a raised floor there. The floor is sectioned into 7 different panels. Two are ‘permanent’ and the rest are designed to be pulled up to maintain the control and power cables that run underneath. All the cables come up through the floor adjacent to the pier footing and are protected by 90° PVC fixtures. The problem is that the PVC fixtures are mounted to the floor panel that is to be raised. That means the cables that penetrate that floor panel must be either disconnected or very carefully positioned in order to get under the panel. The current arrangement has become very cumbersome and I find myself putting off maintenance just to avoid dealing with the floor.

The plan is to cut some new panels. The new sections adjacent to the pier footing will be just large enough for the cable access ports and will be permanently mounted. This will allow the mobile sections of the floor to be pulled up without having to disconnect any of the cables.