Contains:  Gear
New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes

New 20" Scope: A Portable Set Up for Testing

Description

Since the summer monsoons have mostly shut down imaging in New Mexico, here are some images of another astronomy project that I've been working on this summer. Back in March, I took delivery of a new 20" Planewave scope that I hope to eventually put in Chile. Having run a remote scope, I already knew a lot about how I wanted it to work, but the big problem was how to go about getting it configured, aligned, and tested before sending it to another hemisphere to live out its life in a remote observatory. One of the most important things about running a remote scope is to make sure that it's working well before putting in the field; otherwise, it can be a source of endless frustration.

My first goal with this new scope was to use the same ONAG guiding and focusing system that I use on the C14 in New Mexico. Everyone tells me that the Planewave systems are super stable and that they hold focus throughout a wide temperature range, but I couldn't find any real data to support that conclusion. Carbon fiber has to be done just right to achieve zero CTE and while the folks at Planewave may have done a perfect job, I like the idea of constant, real time focus monitoring and the ONAG system lets me do that.

I've also learned that being able to flip my C14 about the meridian increases the odds of finding a nice, bright guide star--even with a large sensor in the guide camera. Since the PW20 is on a L500 mount, there is no need for a meridian flip; but, the L500 also can only be used in one orientation as it tracks across the sky. So, I really wanted to add a rotator to the imaging train. The rotator also allows better framing for some objects. The thinnest focuser+rotator that I could find was the Optec Gemini system and all of the performance specs looked excellent so I placed an order.

The first big obstacle appeared when I sat down with all of the precise dimensions for the scope, the focuser, and the ONAG system, made some CAD drawings, and discovered that it wouldn't all fit into the 8.81" back working distance of the PW20. Wouldn't you know it! I managed to settle on the Planewave scope that has the LEAST amount of back working distance of any of the Planewave products. I determined that I was only short by about 0.25" and after some discussions with the Planewave engineers, I decided that I could get the whole thing to work if I simply redesigned the primary mirror retaining ring to replace the Planewave "True Lock" mounting system on the back of the scope with a Gemini dovetail. I redesigned the part and because one of the wall thickness dimensions had to get a bit thin, I had it made out of 7075 aluminum, which is considerably harder and "stronger" than the original 6061 part that came with the scope. The folks at Optec were kind enough to make this part for me and they did a phenomenal job! I still have to check to make sure that the camera can reach focus but I was pretty careful in doing the design so unless I goofed, it should work.

The next problem was how to balance the camera package. The L-series mounts have no gears so it is very important to careful balance everything. Since the camera assembly on the ONAG unit is offset and it's on a rotator, the only solution was to design a counter balance system. I carefully weighed and measured the CG location of all of the components and I designed the counter balance system shown here. Again the good folks at Optec made the parts and I was happy to see that it worked perfectly. The whole system balances precisely about the optical axis--just like the calculation says it should!

Next, I had to figure out how to mount the scope. At the observatory, a precisely made wedge bolts to a steel pier attached to concrete in the ground. The mount bolts to the wedge and the whole thing is mechanically very stable. When the scope first comes out of the shipping crates, there is no way to set it up to try it as an equatorial mount without a pier! So, I sat down to design a "rolling base." Since the wedge that I ordered with the scope is set up for Chile, I had to add a "compensator wedge" to the base to allow polar alignment in Oregon. The base is designed to place the center of gravity of the telescope equally spaced between three heavy-duty, lockable jack screws that lift the base off the wheels when the scope is in use. The whole thing is made of aluminum to keep the weight under control but also to make it easy to machine. Welded steel would have been incredibly strong but I would have needed a crane to move it! Welded aluminum would have been a lot simpler (with a LOT less labor) and less expensive, but I'm not very familiar with making welded assemblies so I designed it to be simply bolted together using gussets. I had all the gusset plates water-jet cut and I was very happy with how they turned out. The water-jet process worked well even on the 0.75" thick plates that required a square edge (which requires a special process.) Unfortunately, some of the parts made by traditional CNC through an online machining service didn't turn out to be very well made and it took a lot of effort to get them cleaned up. I had to hand drill and tap all of the holes so it was a LOT of work to build this thing. It looks like a portion of a truss from a bridge and it turned out to be pretty stout. The acid test will be to see how much it vibrates and how well it damps under the stars. I can already tell that it's not nearly as stable as my AP1600 (which is like a rock); but that doesn't answer the question: Is it good enough? The good thing is that it lets me set up and configure the scope--and that's its real purpose.

I've attached a few photos showing the whole thing going together. Fully assembled, my estimate is that the entire assembly (base+wedge+mount+scope) weighs somewhere between 600 and 700 lbs. It doesn't take long to realize that everything gets exponentially harder, heavier, and more expensive as the scope gets bigger, which is why I shied away from going for a 24" scope! (Hey, Curious George--now I can better appreciate what you went through with your 24" system). I've since been able to get everything mounted on the back of the scope and most of the wiring is done.

The final issue is the control box. When I first went to DSW, I couldn't figure out where to put all the power supplies, the IP power switch, the network switch, the PC and all the other little things that are needed to run the telescope. So back then, I simply configured a large plastic storage bin to hold everything. It's weather proof and mouse proof and it has worked really well for the last three years. The one big problem is that the box is a bit cramped so on those rare occasions when I do have to work on something, I often have to take things apart (yes, everything is neatly bolted to the inside of the box--it's not just all dumped in there!). This time around I opted for an "official" networking cabinet made by Navepoint. Everything is bolted down so that in principle, the cabinet can be turned upside down if needed. All of the network cables, line power and DC power cords are connector-ized through the back panel so that nothing has to be opened to connect the box. There are only four USB cables that have to be run through an access hole to be directly connected to the PC. My first scope taught me that it is that it is VERY important to minimize connections and distance between the cameras and the PC. Don't ever use a hub for the cameras and keep USB2 wire length less than 5 m and USB3 cables less than 3 m--unless they are amplified. The limits on USB cable lengths are what ultimately determine the maximum separation between the box and the scope. Of course, there are some very clean ways to extend that distance (using fiber optic or Network converters) but that's not very important for this application. The box can just sit at the base of the telescope in the observatory.

I've attached the wiring diagrams for the system to show how everything is connected. A lot of folks do this stuff differently but this approach has worked pretty well on my scope in New Mexico. One important note: Ground loops are a key thing to avoid (which is why I have an isolator on the mount.)

The PC is positioned in the cabinet so that the front panel and all of the connectors are easily accessible. The cabinet cooling fans are thermostatically controlled to keep them from running all the time. Over time, things always get dirty in an observatory and fans just magnify the problem of dirt accumulation inside the box so the idea is to only run them when it's necessary. I haven't weighed it but the finished cabinet probably weighs nearly 100 lbs. One of the heaviest components is the UPS, which probably has enough capacity to run the scope for 20-30 minutes in the event of a power failure, but that's something that I still need to measure.

One of the biggest challenges with the control electronics was to figure out how to run it all on the 240VAC 50Hz line voltage that's standard in Chile. The biggest stumbling block was the UPS. A lot of data centers are converting to 240VAC so there are UPS units that can handle that voltage; but, they are VERY expensive and total overkill for this project. I found a few on the web, but I finally gave in and decided to just run the whole thing on 120VAC and to send a voltage converter with the scope to Chile. A nice 3,000 Watt regulated 240 to 120 voltage converter only costs about $100. In the event that I have to run on 240VAC (for whatever reason,) all of the supplies in my cabinet will run on 240VAC, but two of them are not auto-sensing so they would have to be manually switched. It would also mean removing the UPS from the circuit. My goal is to make the wiring and the whole electronics/control package plug and play so that it takes less than 10 minutes to install.

I'm still working on finishing the last details before the scope is ready to be rolled out under the sky. Hopefully it will work well enough that I can actually grab some images, which I'll post along the way. Finally, I want to thank the folks at Planewave for their advice, to my friend Gaston at IFI who supplied detailed dimensions for the ONAG, and especially to the folks at Optec who bent over backwards to help me to get my custom parts machined. The logo on the side of my scope says Planewave but in this case it deserves another nameplate that says, "Powered by Optec!". Thanks Jeff, and Lee for your outstanding work and service!

If you are interested in this project, stay tuned to this page because I'll try to post a few updated images as I get everything finished. And...as usual, I'm always happy to answer any questions. I can't wait to get this thing out under the night sky!

Rev S: Second night: I first homed the mount, then pointed at Polaris just by positioning the base. I discovered that by lowering just one lift jack and turning the nose castor to 90 degrees, I could grab the free wheel to achieve very fine control over azimuth angle while I watched Polaris through the Telerad finder that I've temporarily mounted on the scope. Then I could use the lift jacks to lift the remaining two wheels and to control altitude while keeping the scope pointed at Polaris. Getting the scope axis precisely pointed took less than 5 minutes. Then I started a sky model. The L500 flies through a 72 point model in only about 10 minutes! The mount slews VERY fast, the plate solves are done as soon as the image is downloaded while the mount is slewing to the next point so this process is really fast. The same effort takes many hours with my other mount so I was very impressed with the nice job that PW did with this software. The result showed that I was within a few minutes of the correct polar alignment and the pointing accuracy was superb. It pretty much dead-centered any object that I pulled up and it tracks perfectly. I ultimately settled on the Cocoon Nebula and experimented with unguided exposures of 60s, 180s, 240s, and 300s and the scope tracked perfectly producing FWHM stars ranging from 1.7" to 2.0". Most of the variation appeared to be within normal seeing fluctuations across the exposures. While I worked I noticed that focus drifted a small amount so it seems that my strategy of using real time focusing may be justified. I could have tested longer exposures and even tried for producing an image but I was trashed from two late nights so I shut it down a little after midnight. As I pulled out to head home, I noticed that the sky transparency was terrible from smoke coming from fires all over the state so I didn't feel too bad about bagging it early.

- John

PS. We've had a string of at least 100 perfectly clear nights in central Oregon this summer so I want to apologize in advance to all of the imagers in Oregon who will be shut down by the cloudy weather that is sure to happen as soon as I'm ready to roll this thing out! :))))))

Comments

Author

jhayes_tucson
John Hayes
License: None (All rights reserved)
334320
Like

Revisions

  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    Original
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    B
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    C
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    D
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    E
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    F
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    G
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    H
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    I
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    J
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    K
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    L
  • Final
    New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    N
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    O
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    P
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    Q
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    R
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    S
  • New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes
    T

B

Description: My friend Andrew was kind enough to offer help to get the wedge and mount up and bolted onto the base. Andrew owns and runs a computer and phone repair store so we were both careful to wear our masks as we worked together. This all happened before I bought the shop-crane.

C

Description: A 2,000 lb shop crane from Harbor Freight made short work of getting the scope on the mount. This was a nerve wracking operation but it was very stable and pretty easy. Just be careful!

D

Description: Since the L500 mount requires precise balance and because I am using a rotator, the ONAG assembly had to be precisely counter balanced. Here is what the assembly looked like before it went on the scope.

E

Description: The rear of the telescope with everything mounted and almost everything wired. It looks a bit haphazard but the components are distributed around the circumference to achieve balance about the centerline. The scope turned out to be perfectly balanced in all orientations.

F

Description: Here's a view of the 20" scope and control cabinet taken during the frequency tuning process required for the direct drive motors. Once tuned, the mount moves the telescope without making any noise. It is completely silent--and a little spooky!

G

Description: Here's a close up view of the ONAG guider on the Gemini Focuser. It's hard to tell in the photo, but the counter-weight sits far enough out that it can rotate without any snags or interference.

H

Description: This is the 9U 450mm Navepoint rack that holds the power supplies, UPS, IP power switch, and PC needed to run the telescope remotely. It is incomplete in this photo. The two loose power supplies will go on a rack panel and another blank panel will cover the other exposed shelf. The network switch is also missing.

I

Description: Assembling the base required carefully cutting the rectangular aluminum tubes and then drilling and threading the holes to attach the gussets. There are about 200 bolts holding the assembly together. It turned out to be a BIG job to get this thing built!

J

Description: This is the wiring diagram for the lower part of the scope. A goal is to allow various components on the scope to be individually controlled. This also keeps certain power sources separate to minimize potential electrical interference. Each camera has it's own power source and thermal control can be used only when needed.

K

Description: This is the power distribution wiring diagram for the OTA. The Focus Boss is actually the Gemini controller.

L

Description: This is the wiring diagram for the OTA data lines.

N

Description: Another view of the scope with the control box (under construction.)

O

Description: I used panel pass through connectors for everything except for the USB cables. This makes it very easy connect and disconnect the control cabinet. The USB cables simply pass through an access panel on the top to connect directly to the PC. It's not as elegant as a panel pass-through but it minimizes connections to the cameras, which is critical. All cables will be in cable sheath to discourage rodent damage.

P

Description: I used SendCutSend.com to laser cut a panel to mount the two Powerwerx power supplies and it came out really nice. The network switch is installed and there's one space left for the flat panel controller, which is on the way. Once that's installed a panel cover hides the power bricks and supplies above the power supplies. External antennas will make the system useable on my local network so it's close to finished!

Q

Description: First light under the stars showed that the scope focused almost exactly where the design said that it should (within the manufacturing tolerances of the components). I made a laser cut B-mask for the 20" and this is an image of a focused star at focus. The guide camera also focused perfectly! It's always satisfying when things come out just like the design says it should!

R

Description: It was windy, cold, and turbulent for the first night under the stars. The main goal was to check focus and get an initial assessment of image quality. This is the analysis of a 5 second exposure of a random star field in the Milky Way. FWHM measured a bit under 1.2 arc-seconds and the field performance looks good. The wind was 7-8 kts so I couldn't get any exposures longer than about 20 seconds with good looking stars. That will have to wait for another session.

S

Description: After aligning the mount and doing a sky model, pointing and tracking were superb! This is a stretched raw Lum-sub from a 5 minute unguided exposure on my second night of testing. The FWHM is right at 2.0", which is roughly consistent with the seeing conditions. An earlier 60 second unguided sub achieved 1.7" FWHM. Clearly, the roll out base works pretty well with the L500. The winds were calm, which always helps with a bigger scope like this.

T

Description: Here's the B-mask that I had laser cut to fit the 20". I used Ponoko and they have a size limitation so I had to bolt two halves together. I've since discovered that SendCutSend can easily handle this size and that they are at least 1/3 the cost of Ponoko! I painted the mask glossy white on the front so that I could more easily see it in the dark (I've nearly stepped on other masks many times!) A B-mask is essential for very accurately co-focusing the guide camera with the main camera and for setting color offsets. The L-mount doesn't even seem to notice the ~4 lbs on the front of the scope from this mask.

Histogram

New 20" Scope:  A Portable Set Up for Testing, 



    
        

            John Hayes

In these public groups

Cloudy Nights