(Click on image for a larger image with more info)

14.5"f/5 Computer Controlled Alt/Az

My "main" scope these days (nights?) is a 14.5"f/5 Newtonian, on a computer controlled AltAz Mount with an image plane field de-rotator.. I rolled all the lessons learned from building and modifying my 10" f/6 newtonian into the 14" scope. 

A LONG time ago it started out as a "standard" dob mounted, sonotube configuration. The sonotube was held in a "cage" that allowed the entire OTA to rotate. Since the OTA was rather large and unwieldy, the sonotube was cut in half, with clamps to clamp it back together.

14" scope in Dob Mounted Sonotube config at TSP

An equatorial platform was eventually built for the scope. The platform allowed video photography, and piggyback 35mm photography. The Shoemaker-Levy impacts were monitored by large groups of guests with a 19" color monitor hooked to my 8mm camcorder rigged for afocal photography thru a 7mm Nagler. The disk of the planet filled the TV screen in living color with terrific details!!! (No, we did not see any impacts in real time, but did see great details of the previous impacts!)

Then, along came the ccd revolution! And with it, the need for more accurate tracking. Initially, I changed the tangent arm drive to a drive roller system for the platform. This allowed unguided exposures of 15 to 30 seconds at prime focus with my Cookbook 245 camera.

(Click on the image to see a larger image of the camera and slide with more info)

However, the time needed for the very accurate polar alignment needed for 30 second prime focus unguided shots was enough to keep me from setting up during weeknights. Thats when I decided to build a computer controlled Alt/Az Mount. Mel Bartels has a simply terrific design for such a mount, and has made available the software he wrote to control the mount! Click here to check out Mel's GREAT website.  In addition, Berthold Hamburger has set up a listserver for builders of Mel's system called "scope-drive".  Details of how to join the list server are on Mel's website.

For more detailed information on the Lessons Learned in converting the mount from a Dob on a Platform to a Computer Controlled Alt/Az, click here to go to the article I wrote on doing the project.

Now, setting up and aligning the mount takes 3-5 minutes and I can get up to one minute UNGUIDED shots. Normally, I stay at 30 to 60 second unguided shots since something always seems to happen during the longer shots (the wind blows, an airplane flies though the FOV, a car headlight shines on the scope, or I simply kick the thing and get "foot doubles" as I stumble around in the dark). There is another reason to stay with the shorter shots. I actually can saturate the ccd detector when I am using my focal reducer (converts the f/5 system to f/3.8). The sky glow in my suburban Houston (Clear Lake City) limiting magnitude is usually around 3 to 3.5, and at f/3.8 I cannot shoot for more than about 90 seconds! I have to admit, its really nice to not have TRACKING always be the limiting factor in my suburban imaging exposure durations!

An image plane de-rotation system is needed due to the field rotation that happens with an alt/az type of mount.  While software can de-rotate the ccd images, that will cause a small amount of image degradation.  In addition, depending on where in the sky the images are being taken, the field rotation will limit the exposure duration.  Click here for more information on field rotation 

(Click on the image to see a larger image of the image plane de-rotator with much more info)

The longer the exposure though, the more likely "something" will cause the tracking to not be perfect, which causes me to lose some of the images I will later stack.   For that reason I added an autoguider system.  A piggyback guidescope on an Alt/Az mount will suffer from field rotation just as the main scope's image plane does, and so will have to be rotated in sync with the image plane de-rotator.  To  test the concept I built a prototype guidescope de-rotator (using a "leaf hinge" type configuration) and mounted it on the Danciger 32"f/4. Click here for information on the Prototype Guidescope rotator.  The prototype proved the concept works really well and allows piggybacked guide scopes to be used with an Alt/Az mount!  The guidescope rotator for the 14 had to be built more compact for the smaller scope however.  So, initially, I used a type 4 Trott design Barn Door approach to rotate the  guidescope. It too seemed to work well!!  However, even the barn door configuration caused balance problems with everything piled on "top" of the OTA.  In addition, both the leaf hinge and the barn door configurations were limited in how much they could rotate, which made it more complicated to get their rotation axis limed up with the optical axis of the main scope.  The third configuration was one that allowed 360 degree rotation of the guidescope, and is mounted on the side of one of the altitude trunions.  In that location, the altitude balance is not affected, and the 360 deg rotation makes lining the rotation axis up with the main scope very easy.

Click on the photo for a larger view of the current configuration

Click HERE  for more information about Guidescope Rotators

The overall configuration of the scope is a double truss.  The truss geometry was actually derived from some folding chairs that allow a strong truss assembly, yet have no loose parts.  The double truss allows the altitude trunions to be about midway along the optical tube assembly (OTA), which allows me to have a lot of weight on the secondary cage (cameras, etc.), and still not have to use counterweights.

(Click on the image to see a larger image of the truss assemblies with more info)

The altitude and azimuth motors are both 6v, 1.2 amp, 200 step (1.8 deg/step) unipolar steppers that I run at 12v and control the wattage to be within their rated limits via Mel's software.  Actually, they draw about 200 ma each while tracking, and only rise to their rated wattage (600 ma at 12v) while high speed slewing.  They turn a 360 tooth, 4 inch diameter worm gear (originally made for Byers CamTrack units.  The clutches are a similar design to how a GEM clutches its RA gear by pinching the gear between a pressure plate and a hub locked to the shaft, except they are not set up to slip, only to lock, or unlock with a 1/4 turn of a thumbscrew as one of the pressure adjust screws in the pressure plate.

Altitude Drive Motor	Azimuth Drive Motor

(Click on either image to see larger images of the drive gears with more info)

Recently I modified the mountings for the motors from the "hard" mounting seen in the photo's above, to a "soft" mounting.  The reason was to reduce the noise (a "ringing" sound) that the motors make, that gets amplified by the structure of the mount.  Click here for info on the "soft" mount, and a .wav file that demonstrates the rather dramatic reduction in noise.

The two Altitude Trunions are laminated up from two 24 inch diameter, 3/4 inch thick pieces of plywood,  cut round with a router, and then cut into two half circles with a table saw.  The edges are covered with 22 gage stainless steel for durability.

(Click on the images to see a large view of the images and more information about each trunion)

Keeping the Optical Tube Assembly (OTA) balanced is VERY important for obtaining maximum performance.  To more easily do this,  there is a clamp on weight (two 2.5# weights bolted together to a clamp) that can be positioned on any of the truss tubes to "trim" the balance of the OTA.  The balance of the OTA was deigned to not require the weights when a CCD camera is inserted into the focuser, so it is normally only needed for visual work (except when I borrow one of those HUGE Naglers!! <grin>)

(Click on the image for a larger view of the adjustable counterweight assembly)

Mel's software allows using encoders for keeping track of the mount's position.  When tracking, or when slewing to different targets, the encoders are not mandatory, since the software keeps track of the number of steps sent to the motors to maintain knowledge of the mount's position.  However, for manual slews, you can de-clutch the motors and manually move the scope, then re-clutch.  The encoders keep track of where things have been moved for this situation.  They also provide a backup capability in case the scope is bumped and the altitude or azimuth axes "skid" on their rollers.  If a threshold is exceeded, the actual position is updated to what is reported by the encoders.  This is really helpful at star parties when folks grab the scope accidentally, or kids chin themselves on the truss tubes!! <grin>.  If the scope is in track mode when its bumped, it will see the fact its had an uncommanded change in position, and move itself back to where it was aimed before it was bumped!!!!

The altitude encoder attaches to the "left" altitude trunion in a conventional way, with a bracket holding the encoder and the encoder axle inserted into a socket on the altitude axis.  The azimuth encoder is offset however, to allow the azimuth axle to be hollow to allow the electrical lines to the azimuth motor to be run through it.

(Click on the image to see a larger image of the Az encoder with more info)

One downside to using encoders and an old laptop to run the scope.exe control software is that the only external serial port on the laptop is used to access the encoder interface box.   This prevents using a second laptop running a planetarium program from taking advantage of the LX-200 interface protocol supported by scope.exe to control the telescope.  However, in a collaborative effort by Ben Davies, Joshua Titus, Dave Lane (Earth Centered Universe) a technique to use the internal modems in the laptops was developed.  These modems use com2, and allow the LX-200 commands to be exchanged via com2/modem, while com1 is used for the encoder interface.   A link to explain how all this works can be accessed by clicking here.

The telescope and mount were not built to be lightweight since they are primarily used for photography, and I have learned that "reasonable" mass provides a certain amount of stability.  In order to move the scope easily however, I have removable wheelbarrow handles as shown in the photo below:

(Click on the image to see a larger image of the wheelbarrow handles with more info)

Even though Mel's excellent software has the capability to measure, and then compensate for any imperfections in the construction of the mount, the better things are initially lined up mechanically, the better the overall performance.  Andy Saulietis showed me a really remarkable way to measure and adjust the alignment of the scope's axes with respect to the mount.  The accuracy is in arc seconds and the setup is deceptively simple.  The technique is called a "spin alignment".   

For a detailed description and photos of the Spin Alignment for an AltAz Mount, click here.

While the overall goal of the scope is to provide an excellent ccd imaging platform, capable of meeting the rigors of suburban, weeknight, imaging (portable, quick setup, able to image in a light polluted environment, able to FIND targets not able to be seen visually due to the light pollution, etc.) when used for visual observing, the performance FAR surpasses what is needed.  The capabilities in the software, such as GOTO , and Grand Tour, and Scroll Tour are simply amazing.

The system appears to be quite complicated, with computers and motors and complex looking struts, etc., but each sub system is actually quite straightforward (except maybe for the software, but Mel Bartels takes care of that!! <grin>).  This has been, and continues to be a wonderful combined ATM and CCD project.  The interaction with folks on the web is probably the high point of building the scope, both in giving and in receiving help.  And each new computer driven scope that gets built adds to the body of ATM experience that gets shared!

Waiting for dark at a Johnson Space Center Astronomical Society Star Party at Fort McKavett, TX

Set up beside the 32"f/4 telescope at the Danciger Observatory, Danciger, TX.  

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