- GPDX Goto
- 20cm Newton
- 32cm cassegrain
|Click on the thumbnails to see a larger picture
A few remarks first:
To build telescopes and other astronomical things, one needs a great variety of materials. I talk about raw iron, all sorts of tubing, plates and rods in aluminium, stainless steel and other materials. Besides that, one also needs lots of screws, bolts and nuts, threated rods, washers etc. Of course the easiest thing to do, is to buy everything in the DIY or metal shops. But that eats much money, and besides that, you often buy too much. If you need a 10cm threated rod, you'll have to buy one full meter. If you want a piece of aluminim plate about the size of a pc screen, you'll have to buy 2 square meters. If you need 6 special stainless steel nuts and bolts, you will have to buy a package of 50 or 100.... etc. Therefore this golden advice. Pay a visit to a scrapyard. Some scrapdealers are specialized in dismantling old industrial machinery. It's incredible what you can find there. And most important, sometimes you can buy some very expensive parts at a very low price.
There I found all my stepper motors, the bearing spindles and the housing of my Bartels unit. Even the housing of my old cookbook camera was once a dirty piece of aluminium pipe.
MODIFYING A VIXEN GPDX INTO A GOTO MOUNT
The problem is that the originally Vixen steppers/gearboxes have a reduction of 200x
The GP worm has 144 teeth. That makes a total reduction of 28800:1.
That is too high for a goto at a reasonable speed. So i had to replace both steppers.
My friend Johan Coussens had two steppers/gearboxes wich had a reduction of 40 x
Thanks Johan for giving them to me ;-)
With some aluminum profiles and a few nuts and bolts, i made two supports for the motors and with flexible bellows couplings, they were fixed to the drive shafts of RA and DEC.
The old DMD-3 hand controller had to be replaced too.
Of course i went for nothing else then Mel Bartels' wonderfull scopedrive.
I wanted to make the electronics for the Bartels drive as small as possible.
Therefore i simplyfied the electronic circuit, so it would fit in a small metal box.
This is the electrical schematic;
The input 5 volt supply is taken from the PS2 mouse-port of the DOS (Bartels) PC.
This port can deliver enough current for the input IC.
The 24 volt stepper supply, and the DC-DC converter are both small 'powerpack' supplies.
The power FET's do not need heatsinks, because the motorcurrent is only a few hundred milliamps.
The isolation between input and output circuitry is done with two quadruple optocouplers CNY74-4.
All the parts can be bought in every local electronic store, or at Farnell and RS.
A 20cm GOTO NEWTON TELECOPE
Since i am not much of a mirror grinder, i purchased the main mirror at "Lichtenknecker optics" in Hasselt (Belgium) together with the secondary mirror
The primary mirror is a 20cm. I am very pleased with the optical quality. The mirror gives razorsharp images.
The telescope is driven by a goto steppermotor system. It's the system disigned by Mel Bartels. It works great, and is rather easy to build. Everything can be found on the internet, complete with building plans and free software. Of course one has to have some notices of electronics and soldering. The Bartels control unit is getting his (DOS)commands from a small pentium-II laptop. But even an older machine can do the job. A planetary program is running on another pc, wich is connected via the serial port with the Bartels machine. All I have to do is one initialisation on a certain star and that's it. Simply a mouseclick on a wanted object, and the scope slews at it.
Here are a few pictures of my Bartels control unit, with a detail of the powerstage for the steppermotors.
A very good system for driving a telescope is of course a Beyers worm. But these things are very expensive en besides that, it is almost impossible for an amateur with poor machinery and not enough mechanical skill, to construct an absolutely play-free (max. 0.002mm) drive. Despite this drawback, there is a system, when it is build with some care, it can compete with a Beyers gear. That system is called "the polar disk". The principle is simple. Fix a great perfectly round disk to the polar axle, and let it rest on two bearings. One of the bearings is driven by the RA motor. The other bearing is only supporting.
In fact it's the principle of the declination drive of a motorized Dobson telescope, but placed at an angle equal to your latitude.
My target was to make such an accurate polardisk as possible. The ideal is a full metal disk. But with the machinery I have that was unfeasible. For that, one has to have a milling machine, wich I do not have. So I had to invent another solution. I decided to make the disk in MDF plate 2cm thick. MDF is a beautiful material to work with. It is cheap, you can get it everywhere, It is easy to work in with wood tools, it doesn't deform, and it doesn't shrink or expand due to temperature changes. The only disadvantage is the fact that it is much less strong than metal. Therefore I tightend a stainless steel ribbon around my polar disk.
First I had to cut the disk out of a plate with a jig-saw fastened to a strip of metal who was fixed at a centre point. I made it a few mm too wide. The exact diameter of my polar disk is 720mm. To mount the disk to the polar axle, I used an aluminum flange with a bore of 50mm. A friend made one for me, because my lathe is too small.
The flange is exactly centrically fixed to the polar disk with 8 bolts. To prevent the disk from bending, I bolted also 8 metal profiles on it, as you can see in the pictures. One profile is made shorter, because there is a hole in the disk for looking through with the polar finder.
Now it was time for the most accurate job, namely, making the disk perfectly round. For this I made a sort of milling machine with the base of an old newton, and a Bosch router.
The stainless steel ribbon wich I tightend around the disk came from a scrapyard. I found one who fitted exactly.(2cm wide and 2mm thick). Both tips of the ribbon are pulled together with a M6 threated rod and two nuts.
I verified the roundness of the disk with a comparator, and found a maximum deviation of 0.07mm for the complete circumference. And a maximum deviation of 0.01mm over a distance of 20cm. the latter means more then 2 hours guiding with te RA motor. A little calculation gives a maximum declination error of 1 arcsec/hour due to variations in height of the polar axle, and less than 1 arcsec/hour due to diameter variations of the polar disk. Knowing that I can't make longer exposures than 10 minutes due to light pollution, the errors caused by the polar disk are absolutely neglectable. Especially with my cookbook camera wich has 19µm pixels. With this telescope one pixel is 5 arcsec. Of course these calculations are pure theoretical. The reality will be much worse. Because there are still a few things that cause periodic error, such as gearbox errors and the fact that the drive shaft is also not perfectly round. But the Bartels system has a solution for this problem. The Periodic Error Correction. (PEC)
The polar axle is made of steel and 50mm in diameter. The south bearing is a 50mm pillow-bloc. The base-frame is a welded construction in L-shaped iron profile.
The OTA is a truss type. The major part with the mirror cell is also made in MDFplate. The plates are joined with screws and waterproof glue. The truss is made of aluminum tubes of 16mm diameter.
This is how my newton finally looks like.
The declination drive is made with an aluminum disk with a diameter of 32 cm. The edge of the disk has a groove. In this groove runs a steel bicycle brake cable. The tips of the cable are attached to the nut of a bearing spindle. This bearing spindle is a high precision mechanism. The play is about 0.001mm.
Originally this spindle was the drive mechanism of an industrial scanner. Here you can see two pictures of the completed declination drive.
The finished telescope got of course a few layers of paint. Not only for a nice look, but also for protection against rust and moisture.
THE MAKING OF A 12.5 INCH REAL-CASSEGRAIN
The optics were bought second hand, and are genuine Coulter mirrors.
The primary is a 12.5 inch parabolical F/4 mirror.
The secondary is a hyperbolic 4 inch diameter mirror.
Both mirrors are designed for a PM-SM spacing of 36.6 inch.
Here you can see the main mirror. I turned the 3-point cell into a 6-point.
Eventually this optics were part of the great telescope who was once the flagship from the previous ORION astro-club in Roeselare (Flanders).
Sadly the club ceased to exist, and the telescope was bought by one of the former members.
After many years he decided to sell the telescope to me.
The mount was a MEADE DS-16. That is a heavy german equatorial with only a RA drive with a 220 volt AC synchronous motor.
DEC movement must be done by hand, with the aid of mechanical setting cercles. Fine adjustment was with a tangential arm and a screw.
This is a picture of a MEADE DS-16 on his mount, and a picture of the mount itself.
I wanted to transfer the mount into a goto. Of course with Mel Bartels' scope-drive system.
So i had to rebuild the mount completely.
I removed the tangential arm first.
Fortunately for me, both axles have the same diameter (1 1/2"). So i removed the wormdrive from the RA axle, and placed it on the DEC axle.
A major disadvantage was the fact that the DEC axle had bearings at the front, but a plastic bushing at the end, with a clamp and a screw as a sort of slip clutch.
So i turned a bearing flange on my lathe. I sawed off the clamp, and placed the bearing flange in its place. The old motor housing was bolted against the flange.
This delicate operation (for me) was a big succes, and the axle fits now perfectly in it. In this pic you can see the flange above the wormwheel.
The wormwheel has 100 teeth. The old synchronous motor was replaced by a SANYO DENKI steppermotor and a 10:1 gearbox.
This gives a total reduction of 1000 x. That is ideal for the Bartels system.
The worm was also a problem. It had no bearings, but brass bushings with a lot of radial play.
So i had to make a complete new bearingblock with ball bearings. That was also a delicate operation, but at the end it worked allright.
The RA drive became a steel belt drive. I had an extra ball bearing spindle from the scrapyard. Regarding the DEC system on the newton, i made a copy of this.
But i did not use a steel cable, but a steel band. The band goes round an aluminum wheel with a diameter of 23 cm.
This way the linear movement of the spindle is transferred into a circular movement.
Since a bearing spindle has almost no play at all (about 0,001 mm), this steel belt drive is very accurate with almost no backlash.
Pictures tell thousand times more than words
As you can see in the pictures, the removed clamp from the DEC axle, is mounted on the frontside of the wheel. That gave me a perfect working slip clutch.
No need to say that with this system, it is of course impossible to make a complete 360° turn in RA.
So when i observe, i make sure that the spindle is in its center position. I loose the clutch, and swing the scope by hand approximately near the wanted object.
Fine adjustment is done with the finderscope and the handpad. RA goto is possible within 45° left or right.
This works very well, and is a lot faster than some commercial goto scopes.
DEC goto is absolutely no problem since it is a wormwheel drive.
Here a picture of the completed DS-16 goto mount, with the Bartels scopedrive unit in front.
THE OPTCAL TUBE ASSEMBLY
Originally the tube was a 6mm thick fiber-resin one. Very heavy, full of scratches and unnecessary holes, and worn paint.
So i decided to make a complete new one with truss tubes.
First i made the spider cage in aluminum profiles. For this i needed help from a professional welder since i can't weld aluminum.
The secondary mirror holder.
The mainmirror box is made in high quality birch plywood.
A test to see if everything fits.
The backplate with the tree cooling fans.
The backside of the completed OTA. The focusser is a 2 inch crayford.
The completed telescope.