Jerry Pinter's construction notes

Thanks for your compliments and interest in my scope. First of all, I need to acknowledge and thank Mel Bartels for his generosity and patience in providing his system plans, software, and support. We had a daily e-mail dialog for about 6 months while we worked out all the bugs in my scope and even a few in his software. I had severe backlash (about half my CCD field of view) which rendered my slewing/finding ability useless, and after many unsuccessful iterations Mel came up with a great backlash compensation feature that completely removes any backlash no matter how large. Now my ol' light bucket is a pleasure to use and can slew, find, track and image with the big boys!

Before I answer your specific questions, I'm forwarding you a general description of my scope which addresses most of your questions:

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My scope is a Newtonian reflector with a 13.1" f/4.5 primary mirror and 3.1" secondary mirror. The optical tube is supported by an altitude-azimuthal ("Alt-Az") Dobsonian-type mounting. A 16"-diameter fiberglass tube is held by cork-lined plywood split-rings that gently squeeze the tube to hold it in place. The rings can be easily loosened to allow the tube to slide back and forth if necessary. The tube is counter-balanced by lead weights. The tube rings have two 8" circular wooden disks that serve as the altitude bearings, the 8" disks (one on each side of the scope) have an aluminum ring for the bearing surface. The tube/ring assembly sits in a rocker box (which is somewhat similar in design to Coulter or Orion dobs). Instead of teflon pads which were great for visual observing, I replaced these with 2 ball bearings on each side for the altitude axis. Thus the aluminum-surfaced 8" wood altitude disks sit directly on ball bearings. The result is an almost frictionless altitude axis (very sensitive to balance and thus practically unusable when the altitude drive is not engaged). On one of the 8" altitude disks, an 11" wooden disk (concentric with the 8" disks) is used as a pulley for the belt-driven altitude 200-step motor. The stepper motor has a 150:1 reducing gearbox with a 1" toothed pulley. Thus the altitude drive has a final reduction of 11x150x200. A clutch can quickly disengage the drive by loosening the belt for manual movement (the motor housing has an adjustable spring-loaded tension).

The original drive apparatus is from "Dob Driver II" by Tech2000 which sells their own dob drive system (I am using their motor housings with release-clutches, gearboxes, ball bearings, pulley, and toothed belt.) The azimuthal axis of the rockerbox is made from an axle-bolt with brass shoulders (no side-to-side slop). The box rotates on 2 ball bearings (again, no more teflon pads for very low friction). The third support is the 2" knurled wheel of the azimuthal stepper motor gearbox. A clutch can quickly disengage this wheel and let the rockerbox rotate on a teflon pad for manual movement. The ball bearings and motor wheel roll along a formica sheet bolted to the bottom of the box. Four rubber wheels underneath the box allow it to be rolled around. When I set up to observe, I drop three 5" carriage bolts through holes in the baseboard which form three legs that the mount stands on. This provides a stable base on unlevel ground. The base does not need to be leveled since the drive software automatically compensates for any tilt in the base. The tube/ring assembly simply lifts out of the rockerbox cradle, so it is easy to transport and set up. I have a Telrad finder and a cheap 60mm refractor (with a 12mm illuminated crosshair eyepiece) that I use for aligning alignment stars.

The scope is controlled by PC using Mel Bartels' software as explained on the Internet at http://www.efn.org/~mbartels. The software uses the popular 2-star alignment method which initializes the software to the sky without the need for polar alignment or leveling of the base. In the software I have object libraries of 250 bright stars for alignment and a 5000-object library of NGC and IC deep-sky objects. The scope can slew to objects from the libraries or to any input coordinate position or by using the handpaddle pushbuttons to manually slew. The scope automatically tracks wherever it is pointed and displays it's position in both RA/DEC and ALT/AZ coordinates.

The CCD camera is the Cookbook 245 with the Low Dark Current modifications in place. I have replaced the water-cooling system with a forced-air convection cooler built by my friend Kurt Mihalco of Coherent Systems. His cooler is nice and compact, and replaces the bulky plastic tubes, copper coil, bucket, and noisy pump. I also use Kurt's custom power supply for the camera, it runs off of a single 12V battery. I use this single 12V battery to power both laptops, the scope, and the camera (about 6 amps total). I use a 95 amp-hour marine "deep cycle" battery that provides power all night while I'm out in the desert or on Palomar Mountain where I usually observe.

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Q. What did you use for the az bearing(s)?? A flat metal plate like Mel?? If yes, please describe, along with where you found it and what cost.

As described above, I use a piece of a formica countertop with a "pebbly" surface. It was a bit of a challenge to get this as flat as possible, which is important for slewing accuracy. I ended up using numerous bolts to squeeze the formica down to the plywood underneath, and then "tuning" the bolts to get it as flat as possible. A rigid flat metal plate would be a big improvement, I'm sure, and some day this would be a good upgrade for me.


Q. Did you have any problems with the getting the taper ground on the az bearing drive rod??? How did you calculate the taper?

As described above, the Dob Driver hardware uses a 2" diameter knurled metal wheel to drive the AZ axis. The knurled wheel rolls along the formica.


Q. Did you make your altitude bearings from wood, surfaced with aluminum??? If not, what?

Yes, but I didn't make them. They are original Coulter ALT bearings.


Q. What do you use for the motors/gearboxes (this is probably our toughest problem). Where did you get them? Gear ratios, etc? Costs? Are you happy with the periodic errors and backlash they have??? Details on the motors, etc..... (steps, voltages, etc). How about the drive rods themselves??? Bearings for them, etc......

As described above, most drive hardware is from the Dob Driver. I replaced the Hurst motors with Jameco steppers (200 steps/rev) for $10 each. I run them at 24V in order to get the slew speed up to about 1.3 degrees/second. These motors fit onto the Hurst gearboxes fairly easily. The gearboxes have backlash, but this is compensated by the software. I am now purposely keeping my ALT axis slightly off-balance which removes all the ALT backlash. The AZ backlash is about 7 arcminutes and is removed by the software.


Q. Clutches....what do you do for clutches for the drive rods?????

I use the Dob Driver clutches which are easily engaged and released by hand.


Q. I see a ccd camera on your picture of your scope.....How long of exposures can you make??? (unguided) without image smearing?

All the pictures on my Web page are single 30-sec exposures. I have taken some nice 60-sec shots, but I don't get consistently good results like I do for 30-sec shots. This is consistent with what Richard Berry reports, as he recommends "track and stack" multiple short exposures.


Q. What construction "Lessons Learned" do you have?????

By far the best advice is to make sure all your bearing surfaces and axis are flat, perpendicular, concentric, or whatever they are supposed to be! Any errors will result in slewing and tracking problems. After spending a lot of time tweaking my mount to remove sloppiness during original construction (I never dreamed I would be using a Dob to do astroimaging!), I still have to work within the limits of the scope's capability. I can't do long across-the-sky slews and expect to land on a faint object in the middle of my CCD chip. Instead, I choose an area of the sky and pick my initialization stars maybe 30 or 40 degrees apart max. Then when I want to slew to a faint object, I'll first slew to the nearest "alignment star" (I have over 250 bright stars in the database, so they cover the whole sky). After centering the alignment star, I "reset" and then slew to the object. This keeps the slew less than 15 degrees or so, and I will nail the object every time on my CCD chip (which has about a 15 arcmin field).


Q. Do you get much backlash with a belt drive??? (Yes, I know Mel's software addresses backlash)

Yes, I'm sure there is some small amount of belt stretching, as the belt has different tension on one side of the pulley from the other (easily demonstrated by plucking the belt like a guitar string). The belt backlash is probably very small, but even if it were large it would hopefully be consistent and easily removed by the compensation software. A bigger concern with belts is probably non-uniform stretching during tracking which would introduce image smearing. With my scope, construction errors in the alt axis (not being perfectly concentric, etc) are the primary source of error and probably not the belt.


Q. The third support is the 2" knurled wheel of the azimuthal stepper motor gearbox. A clutch can quickly disengage this wheel and let the rockerbox rotate on a teflon pad for manual movement.

How wide is the knurled wheel? Any appreciable width would introduce a tracking error due to the differences in the ID and OD described by the width of the wheel, I think thats why Mel suggests using a tapered drive roller?????
The knurled wheel is about 0.25" wide. Ideally, I should have tilted the AZ motor/gearbox/wheel housing a bit so that the edge of wheel rolls on the formica. Although I must admit that I didn't do this purposely, the wheel height is a bit higher than the two ball bearing heights, so the wheel does tend to ride only on the inside diameter (I can see this by looking at the worn track in the formica, it's a narrow line and not 0.25" wide).


Q. The scope is controlled by PC using Mel Bartels' software as explained on the Internet at http://www.efn.org/~mbartels. The software uses the popular 2-star alignment method which initializes the software to the sky without the need for polar alignment or leveling of the base.

Don't you have to initially tell the scope where the ax axis is aimed (I think Mel's software actually looks for an initialization point with the tube at 90 deg to "vertical" (vertical being aimed parallel with the azimuth bearing bolt, not necessarilly actual vertical)

Yes, the software needs to know the initial ALT angle w.r.t. the base. I have a wooden dowel rod cut to the exact length where the scope tube is parallel to the base, so I always start in this position (ALT=0). However, the software does not need to know what the AZ starting position is, this is determined by the software after initializing on 2 stars. The software calculates the AZ angle along with the observer's effective latitude and longitude.


Q. The CCD camera is the Cookbook 245 with the Low Dark Current modifications in place. I have replaced the water-cooling system with a forced-air convection cooler built by my friend Kurt Mihalco of Coherent Systems. His cooler is nice and compact, and replaces the bulky plastic tubes, copper coil, bucket, and noisy pump.

Do you get enough of a temperature drop just using air cooling?? Is it stable??? The water really helps on both of these areas.... I live in Houston, and 80 deg nights are not uncommon....Air cooling will not drop the chip temp a whole lot I would not think.

I spent most of my time working on my scope, and not optimizing my camera. I've been quite amused reading all the CCD postings about people arguing over things like which metal alloy should be used for the wires connected to the chip to produce the best tradeoff in heat capacity vs. joule heating! I can only WISH (I think) that my set-up was optimized to the point where I need to rethink the alloy of wire I use. But your concern about air cooling is valid. For my 30-second exposures with the LDC mod, dark current is not a problem (I think the LDC mod reduces dark current by about 10X). But yes, the temperature does vary with the ambient, so I may have to take a couple dark frames thru the night. I try to keep the ref and reset levels the same for each image, which I believe helps the images and dark frames stay calibrated, but I may be wrong here (I need to brush up on CCD operation). Kurt was going to add a closed-loop system to the air cooler which would modify the Peltier current to keep the chip temp steady, but this feature is not yet available. For now I'm satisfied with the camera for my short exposures and enjoy the convenience of not dealing with the noisy pump, hoses, and bucket.

I also use Kurt's custom power supply for the camera, it runs off of a single 12V battery. I use this single 12V battery to power both laptops, the scope, and the camera (about 6 amps total). I use a 95 amp-hour marine "deep cycle" battery that provides power all night while I'm out in the desert or on Palomar Mountain where I usually observe.

As described above, most drive hardware is from the Dob Driver. I replaced the Hurst motors with Jameco steppers (200 steps/rev) for $10 each. I run them at 24V in order to get the slew speed up to about 1.3 degrees/second. These motors fit onto the Hurst gearboxes fairly easily. The gearboxes have backlash, but this is compensated by the software. I am now purposely keeping my ALT axis slightly off-balance which removes all the ALT backlash. The AZ backlash is about 7 arcminutes and is removed by the software.


Q. Excellent idea on the off-balance config to eliminate backlash in Alt! What voltage were the motors intended to be run at? When you slew, what rate do the motors step at??? Why the 24 volts?? Why not just up the stepping rate at the nominal voltage (12v??)

They're spec'd at 24V. If I did my math right, slewing at 1.3 deg/sec corresponds to about 1200 steps/sec. Steppers run faster at higher voltages, so running at 12V will cause the steppers to stall out at less than 1 deg/sec, too pokey for me. I could probably increase the voltage to 36V or more and get even faster slew speed, for example Mel talks about 2K or 4K steps/sec at overvoltages.

All the pictures on my Web page are single 30-sec exposures. I have taken some nice 60-sec shots, but I don't get consistently good results like I do for 30-sec shots. This is consistent with what Richard Berry reports, as he recommends "track and stack" multiple short exposures.


Q. hmmmmmm most of my shots are 30-60 sec images too usig my platform. I was hoping for a minimum of 60 sec, and perhaps 120 sec images reliabily.... so I could take 20 or so and add them together (I currently add my 30 sec shots as you mention). What is the predominant problem with not being able to go longer than 30 sec???

Good question! Imaging in the field with a closed-tube (non truss) dob requires almost windless conditions. Any gusts of wind will smear my images. Sometimes I hold up a sleeping bag during the 30 second exposure! If there's no wind, I get my best images. However, I rarely get drift-free images at 60 secs. I suppose its a combination of mount errors (my alt axis is not perfectly concentric so the radius changes slightly, and my AZ bearing surface is not perfectly flat so the scope will have up-and-down AZ movement instead of purely AZ). Also, the ALT belt may stretch as discussed above, and the gearboxes are inexpensive, not "precision" as Mel recommends. Taking all of that into account, I guess I should be pretty happy with my images, and I am!


Q. Thank you ever so much for taking the time to respond!!!!

You're very welcome. I'm glad to offer any information that will help you, and hope you have success with the project. It was challenging, fun, exciting, and extremely rewarding to convert my Dob into a scope that can perform alongside commercial scopes that cost 10X as much. Go for it!

Hope all of this helps, and have fun!

Jerry Pinter
jerryp@amcc.com