The Next Chapter
I spent nine glorious days in the desert. Through the miracle of solar power on the truck camper, and excellent 4G access so far away from civilization I was able to work during the day, and observe at night. A guy could get accustomed to this. I can't stay up too late because of work obligations, but some serious observing is getting done. Cruising the periphery of the California Nebula with an H-Beta filter kept me busy for awhile. Other highlights included checking out the tidal bridge from M110 to M31, and the Horsehead was downright easy with the combination of big optics and extremely dark skies.
New Moon contacted me again about the focuser board asking me to take measurements of the various holes on the board, because the focuser board that shipped with this telescope is a one of a kind piece. New Moon ships a standard focuser board for their f/3.3 to f/5 scopes, but mine is an f/3. I'm not sure how that's any different as it's the same focuser and same upper truss ring - so I expect the focuser board shouldn't be any different. Then I think to myself...if this focuser board is a one of a kind, wouldn't you check to see that it was made correctly before putting it on a telescope? That clearly didn't happen but, whatever.
In an effort to achieve the highest level of accuracy, I shipped the focuser board back to New Moon Telescopes upon my return home. When the new focuser board arrives, the angle is now more or less square. There's nothing wrong with it, but it's notable that it's not the same as the previous focuser board. The center of the focuser axis is 100mm from the top of the truss ring, instead of 103mm like the previous board. And the hole in the board for the optical path isn't centered with the holes for the Feathertouch mounting bolts. None of this affects the function of course, but its an indicator to me the quality control on such a large purchase as this isn't what I had expected it would be.
I mounted the focuser board, and went through the procedure to identify the 180º point on the virtual tube, and a point 100mm from the top of the truss ring. This was to identify the proper axis for the focuser. Putting a laser into the focuser showed that it was a little bit off....as was expected.
Some adjustment of the mounting plate solved the problem. The side of the focuser closest to the primary mirror was now raised up about 1mm in order to square the focuser.
In our discussion about collimation shift, Ryan said that the spider vanes weren't tight enough. When he tested the scope he said that he observed less than 1/32" movement from zenith to horizon. So, my 1/4" of observed movement from zenith to horizon was an 800% increase. I find it hard to believe that the spider became that loose during transport considering how tightly it was all wrapped in protective bubble wrap. Whatever the case, he said the vanes needed to be very, very tight.
I had concerns about the offset secondary holder, as it was just going to shift the mirror 0.4" while the spider remained in its current place. I question the sanity of moving that much mass off-axis when collimation shift is already a problem. In preparation for the offset secondary holder, I ensured that the spider itself was exactly centered in the truss ring and tightened the vanes very tight as had been instructured. And then I put the laser back in the focuser.
The focuser axis has shifted a full inch (25 mm). I'm not an optical genius nor am I a mechanical engineer but I'm pretty sure this is not a good sign. I’m also concerned that tightening the spider causes any change in the alignment of the focuser. It seems to me that the two should be unrelated, and it indicates that something in the structure is flexing. Installing the secondary is pointless if the focuser isn’t square to the axis. So at this point, I have a half assembled telescope with no clear path forward. I contacted Ryan with my experiences and waited for some guidance. It came quickly - he's quite responsive, and always has been.
In his response he tells me that I don't need to tighten them that much, to back them off some. That tightening them too much will flex the ring. That's probably okay, as the flex is the new shape - but they don't need to be that tight. He also says that the top bolt in the spider vane will need to be a little tighter as the mounting post flexes slightly - but I've probably noticed that. I am very concerned that things are flexing. I spent a good amount of time doing deep sky astrophotography at longer than normal focal lengths, and was reasonably good at it. It taught me an important lesson - flexure is bad. Always.
The offset secondary holder has arrived. The holes drilled in the secondary holder for the mounting screws don't line up with the shroud currently encasing my secondary mirror. Ryan has another shroud and holder made and they'll be shipped to me as soon as they're done.
My confidence is low that moving the weight of the secondary mirror this far away from the spider's center is going to benefit my issue with collimation drift. I loosened the vanes on the spider and ensured the focuser was hitting the proper point on the opposite side of the tube. I then ensured the spider was still centered in the tube and tightened the spider vanes right up to the point that tightening any further pulls the laser off-axis. Once I reach this point, it's surprising how far and quick the laser moves offline with just a slight turn of the screwdriver. I put the original non-offset secondary holder in and collimated the telescope with it pointed at about 60º altitude. Then I checked again for collimation drift. The results this time were better....but still nowhere near acceptable.
Here's the tublug at zenith
Here's the tublug at about 45º
And finally, the tublug at about 20º altitude
I'm pretty frustrated at this point. I contacted Ryan again with these results. I told him that I expect better than this - that my 15" Obsession has none of these issues and it was 1/4 of the cost. I've spent more time working on this New Moon Telescope in six weeks than I've spent working on that Obsession in 17 years. I don’t know how we resolve this, because to my eye it doesn’t look possible. If I tighten the spider to prevent the drift, the upper truss ring will warp and pull the focuser out of axial alignment. If I leave it alone, the weight of the secondary shifts collimation as the telescope is moved lower in altitude.
Ryan told me that the best way to collimate will be to put the secondary mirror in the telescope first, and then to align the focuser to the secondary. This is completely opposite to everything I've ever understood about Newtonian telescopes. I suggested that this was untrue - that the focuser should be aligned, and then the secondary placed to intersect that path in the proper place. I also said - "I hope we agree that if tightening the spider vanes more than they are causes the focuser to tilt, that's a bad thing. I think we both agree that tightening the spider vanes beyond a certain point does warp the upper truss ring and we want to avoid that regardless of how the rest of the telescope is aligned. A warped truss ring is bad."
If I have a telescope here where the spider vanes can't be tightened enough to prevent collimation drift *and* keep the focuser on axis without warping the upper truss ring, then I have a telescope that doesn't work. Astrosystems says that the spider vanes should have 70-100 pounds of tension. There's no way this ring holds its shape with that kind of tension placed on it.
In his response, Ryan told me that "The ring can flex some." I now fully realize that my idea of a proper telescope structure and New Moon's idea of a proper telescope structure have some fundamental differences. I pretty strongly said that "No, the ring can not have some flex." It doesn't flex in my 15" and I expect the same from this telescope.
The primary/secondary/focuser need to be at 90º angles to each other with a secondary mirror that doesn't move 1/8" (3.2 mm) as the telescope moves through altitude. Ryan's response is to say that I'm correct that the light path needs to be at a 90º angle, but wrong in that I can't accomplish that with a slight flex of the upper truss ring.
I give up. The third secondary holder has arrived. It's slightly too small for my non-standard 4.8" secondary. So it has to go back. I'm shipping back all three secondary holders, as the first one fits - its just not offset. So the state of affairs at the moment is that this premium-level dobsonian is not usable - there's no working holder for the secondary mirror.
Edited by Mike Wiles, 06 December 2018 - 03:24 PM.