EQ3 Mount Conversion

EQ3-2 Mount, modified for GOTO (May 2003)

This setup is a very nice economical portable GOTO system for visual observing. BUT it is only right to point out that the EQ3-2 mount, although sturdy, is not a High Precision instrument. Periodic Error is quite high at around 70 arc-secs peak to peak, and a little jumpy as well. Not really suitable for long focal length long exposure astrophotography. As a wide field camera mount, particularly piggybacked using the Stellarvue (or whatever telescope is on the mount) as a guide scope, it should serve well.

Added later (2005) Periodic Error Control is now available for the Autostar, although the training is lost unless the mount is PARKed. But this can reduce the periodic error to +/- 10 arc-seconds or better, so considerably improves the long exposure capabilities of the mount. And later versions of the Autostar firmware accept pulse guide commands which can give guiding down to around +/- 3 or 4 arc-seconds accuracy - well suitable for wide field imaging.

Added later again (2008). The #909 module originally was made available by Meade to allow 'ST4' type guiding on the LX90. Later Autostar firmware, particularly with 'patches' from Dick Seymour (see Roboscope Group files section - link to the Group at the bottom of this page) allows the use of this unit with the Autostar driving DS motors. Meade stopped supplying this unit in 2008, but a 'clone' was made available by Gene Chimahusky here. Particularly useful for users of SBIG self guiding cameras, or the recent crop of economical guide cameras with built in guide ports, for example this or this equivalent.

The EQ3-2 German Equatorial Mount is a mid sized mount, made in China, and supplied as standard with a wide range of budget telescopes, up to around 6" Newtonians and 4 1/2" Refractors. It is quite well made and sturdy, coming as standard with 360 degree gears on both axes and a small polar finder 'scope. Not driven as normally supplied, although drives are available.

I had bought one ('new' secondhand) to mount my Stellarvue AT1010, previously mounted on a camera tripod, and was considering fitting the standard drives. But having spotted an advertisement for the Meade DS motor upgrade kit with 497 Autostar (see foot of page for Autostar settings) at a very good price (little more than the standard drives), I decided to have a go at fitting it. I looked around other sites, notably Chris Erickson's, for ideas, and eventually (no longer having ready access to a lathe) decided to use the existing Meade gear shafts.

This shows the Meade gear removed from its taper, with the fixing screw.

A pity to cut sound worm shafts in two, but I could only use the taper parts!

The gear ready for assembling to the mount, with a flat filed in the shaft for the coupler fixing screw. The hexagon nut can be unscrewed, but I left it on the shaft - it acts as a convenient stop for the coupler.

For shaft couplers (with 6 mm bores) I used Maplin (UK) electrical connectors, part no. MX06, at £1.29 each (priced May 2003). They also supply shaft couplers no. WC72P at £3.99 each, complete with hex key, but the ones I bought were without the hex key. The 6 mm bore of the couplers fitted the end of the gear shaft on the mount, but I had to drill halfway through each one to 7 mm for the Meade shafts. Brass 'grabs' the drill, so to prevent the bit running too far, I clamped a 6 mm drill half way in.

With the couplers held in a vice, the job was straightforward. I also had to shorten the mount end of the couplers to get sufficient depth for the mount shaft. DEC shaft assembly shown.

NOTE (May 2013) Connector MX06 no longer available, but WC72P is still on the web site, still at £3.99!

Then it was simply a matter of fixing the motors in suitable positions. I was very fortunate in having to hand some offcuts of Aluminium angle which fitted the bill nicely, the longer part being exactly the right thickness to space the axis of the DEC motor away from the top plate of the mount.

So in the end, the only metalworking tools I needed were a Drill, Hacksaw, File and 6 mm tap (for the fixing holes on the mount). Don't forget the vice! Although many people doing this mod on other mounts had to extend the RA axis, there was just enough clearance on this mount. A straightforward job.

The motors are held on with Duct Tape for now - there is very little clearance inside the motor casings, and if I use screws they may touch the innards. At the moment the mount performs well, putting objects nicely within a 9 mm eyepiece.

Ready for Action!

A week or so later.....

Although the duct tape did a reasonable job, there was a little bit of movement between the motors and the mounting brackets. I took some measurements and realised that there was enough clearance inside the motor cases for low profile socket cap screws, particularly if I filed them down a bit. Although I stated above that the Aluminium was exactly the right thickness to space the DEC drive, in fact a 0.5 mm shim was needed for the best result - a piece of plastic was located and cut to size (actually a label from some annual flowers I'd just bought for the garden!).

So out with the drill and file again, some careful aligning, and the job is properly finished. (Note the hole in the fixing bracket for one of the case screws). Rock solid, and I hope a little more accurate! Although not visible in these photographs, I screwed the original motor case mounting ring retainers firmly onto the shaft exit holes to add a little strength.

The Aluminium plate sticking out of the side of the mount is a camera platform.

MUCH later (2008)

I've sold my AT1010, but now have three telescopes which can be used on this mount. My TMB 105, William Optics ZS66 SD and Solarview 50. So to ease swapping from one to another, instead of having to remove tube rings from the mount I have fitted a dovetail clamp to the mount, and with dovetails on all three telescopes it's a matter of seconds to change telescopes and rebalance. (As before, the plate sticking out to one side is a camera platform)

Later again (2010)

Before my trip to the Southern Hemisphere in 2010 I was concerned about locating the Southern Pole, and bought a replacement Polar Finder which has a reticle for both North and South poles. It worked a treat! I knew it would need illumination in the very dark skies, so fitted a power socket and little 3mm LED to the mount before travelling. The power is fed from the mount (12 volt) supply via a project box fitted with a switch and variable resistor to adjust the light output to match the star brightness. Once aligned I unplug the controller, so no untidy cabling when observing/imaging.

BTW, I got the finder scope from First Light Optics. This one (the EQ3-2 version).

The switch is centre off, and spring loaded off in one direction, so I can have it on permanently or blink it - this proved very useful for South Pole alignment on the faint Octans stars. Because the LED is rated around 3 volts, the 20 Kilohm resistor limits the current when turned to full brightness.

Update, 2014. Skywatcher now supply a Polar illuminator for this mount (and some others in their range) which clips into the polar scope exit hole. Again available from First Light Optics and many others world wide.

A pic of the reticle showing the Octans stars. It was necessary to rotate the RA axis to align the stars with the finder shape, but the spacing was accurate, and certainly worked.

The reticle uses three 5th mag and one 6th mag star including Sigma Octantis. I couldn't be sure of any of them naked eye, so had to move the mount around to find them. The pole is located approximately 2/3 of the way along a line drawn between Acrux and (easy to find) Beta Hydri, so with the mount altitude correctly set, it was surprisingly easy to spot the stars in the finder.

Here are actual photographs from my trip showing the wider field and close up asterism for the finder scope. And once you know where to look - easy-peasy!

Here is the reticle superimposed on the star field, much as it would appear through the finder (illuminated of course!) I've seen other reticle designs, but in my opinion this is the most straightforward for the Southern Pole - although the stars used are faint, the grouping is particularly noticeable when it comes into view in the finder. (The finder view is of course rotated 180 degrees from the sky view).

The above images were taken in July - the Southern Winter. If there in the Summer - January etc., the sky would be rotated 180 degrees, and the reticle alignment also rotated, looking more like this.

Autostar Settings

The worm ratios on the EQ3-2 mount are 65:1 on the DEC and 130:1 on the RA. These translate to Autostar ratios using Steve Bedair's method.

The multiplication factor for my DS motors is 0.02281483333 times no. of teeth on the worm gear, therefore:

DEC ratio =01.48296
RA ratio = 02.96592

The Ratio sign is dependent upon the motor fixing position (which side of the mount). In my case, both negative. The Autostar already had the LXD55 series in its database, so I didn't have to use Dick Seymour's patch mentioned on the other sites to accommodate the German Mount.

There is a Yahoo! group, Roboscope, dedicated to this sort of conversion.

Performance
Slew speeds are approximately 5 degrees/second in Declination and 2.5 degrees/second in Right Ascension. Not exactly sprinting, but perfectly adequate in use. The system acts 'intelligently', always avoiding cord wrap, and warning of any likelihood of clashing with the mount. GOTO's are most satisfactory, particularly when using the Autostar High Precision mode, with objects placed well within a 9 mm eyepiece on the Stellarvue 'scope (480 mm focal length), and tracking is very good.

Power usage is approximately 100 milliamps when tracking and 500 milliamps (1/2 amp) when slewing at full speed with both motors running. The supplied battery pack holder takes 10 AA size batteries. So with NiMH batteries of around 1500 mAH capacity, a long night's observing would be accommodated without recharging the batteries.

This shows the Meade gear removed from its taper, with the fixing screw.
A pity to cut sound worm shafts in two, but I could only use the taper parts!
The gear ready for assembling to the mount, with a flat filed in the shaft for the coupler fixing screw. The hexagon nut can be unscrewed, but I left it on the shaft - it acts as a convenient stop for the coupler.

Before my trip to the Southern Hemisphere in 2010 I was concerned about locating the Southern Pole, and bought a replacement Polar Finder which has a reticle for both North and South poles. It worked a treat! I knew it would need illumination in the very dark skies, so fitted a power socket and little 3mm LED to the mount before travelling. The power is fed from the mount (12 volt) supply via a project box fitted with a switch and variable resistor to adjust the light output to match the star brightness. Once aligned I unplug the controller, so no untidy cabling when observing/imaging. BTW, I got the finder scope from First Light Optics. This one (the EQ3-2 version). The switch is centre off, and spring loaded off in one direction, so I can have it on permanently or blink it - this proved very useful for South Pole alignment on the faint Octans stars. Because the LED is rated around 3 volts, the 20 Kilohm resistor limits the current when turned to full brightness. Update, 2014. Skywatcher now supply a Polar illuminator for this mount (and some others in their range) which clips into the polar scope exit hole. Again available from First Light Optics and many others world wide.
A pic of the reticle showing the Octans stars. It was necessary to rotate the RA axis to align the stars with the finder shape, but the spacing was accurate, and certainly worked. The reticle uses three 5th mag and one 6th mag star including Sigma Octantis. I couldn't be sure of any of them naked eye, so had to move the mount around to find them. The pole is located approximately 2/3 of the way along a line drawn between Acrux and (easy to find) Beta Hydri, so with the mount altitude correctly set, it was surprisingly easy to spot the stars in the finder.
Here are actual photographs from my trip showing the wider field and close up asterism for the finder scope. And once you know where to look - easy-peasy!
Here is the reticle superimposed on the star field, much as it would appear through the finder (illuminated of course!) I've seen other reticle designs, but in my opinion this is the most straightforward for the Southern Pole - although the stars used are faint, the grouping is particularly noticeable when it comes into view in the finder. (The finder view is of course rotated 180 degrees from the sky view).
The above images were taken in July - the Southern Winter. If there in the Summer - January etc., the sky would be rotated 180 degrees, and the reticle alignment also rotated, looking more like this.