Modding the Mount
The HEQ5 pro is a good starter mount for the budding astrophotographer. It is reasonably heavy without being too heavy and it is a stable platform upon which to carry up to a 200mm telescope complete with camera, guide-scope and in my case a filter drawer. This puts it near enough on the limit of what Skywatcher say the mount will carry.
Skywatcher say the mount has a positioning accuracy of 1 arc minute which is close enough for my needs as combined with Stellarmate astrophotography tool I can usually get the object I wish to photograph in the middle of the frame. Tracking is another matter and as any HEQ5 pro user will tell you out of the box the HEQ5 pro can achieve anything between 30 and 60 seconds before stars get blurry. Add a guide-scope in combination with say PHD2 and the mount can achieve a fair degree of accuracy. Mine was around 3 to 5 arc seconds initially. I gave it a tune up to reduce the backlash which is a significant cause of guiding inaccuracy. This resulted in a guiding accuracy of 1 to 3 arc seconds. An improvement which was noted each night I used the mount.
The only way to get better was to upgrade the mount. So I ordered the popular Rowan Astronomy belt drive kit from First Light Optics and installed it. The following is a pictorial account of the installation.
The kit is well packed and contains a black and white set of instructions. I found it easier to download the Pdf version to my tablet and work from that as it was in colour.
This is what you get in the box all clearly packaged and labelled.
The RA and Dec belts each have their own sets of gears and idler wheel.
There is a bag of extra long allen headed bolts for refitting the cover along with two allen wrenches for the grub screws and the motor mounting screws.
In my case I also ordered the pinion extractor and it was well worth it. It also came with it's own allen wrench.
In fairness Rowan astronomy have done a good job with providing tools for the job the only items you will need in addition are a philips screwdriver, a flat bladed screwdriver, a steel rule, and a small round file.
On removing the covers this is what will greet you. A somewhat greasy set of gears and on the top the Dec shaft and the motherboard for the mount. It is taking up the backlash, necessary for gears to work, that causes the grinding sound, that HEQ5 users are familiar with as the motors start to move the mount. It is also this backlash which causes the tracking errors that no amount of tuning can ultimately remove.
The circuit board. At the top of the image is the Dec shaft. There are four plugs from left to right they are the RA motor connection, the polar illumination LED, the handset connection and finally the Dec connection.
Note the position of the wires when removing them and where they go around the board.
I worked on the RA motor first as it was the easiest to get out. Here it is ready to be stripped down for the new pinion and the idler wheel to be installed.
One thing which surprised me as a dabbler in electronics was how small the stepper motor was compared to one I had pulled out of an old Epson printer which was a NEMA 17 motor.
Remove the idler wheel by undoing the grub screws visible in the photo below the wheel and simply lifting the wheel out.
Then remove the mounting plate from the motor before attempting to extract the pinion.
A sound piece of advice in the instructions is to mark the position of the control wires before removing the motor mounting plate. Note the 'W' I added to the side of the mounting plate.
Pinion extraction halfway through. This is an easy enough task to accomplish needing a firm grip on the extractor whilst twisting the allen bolt to draw up the pinion. I have seen some folk suggest a plumbers pliers to hold the pinion extractor, but as long as you can tolerate a slightly uncomfortable grip over the slot of the pinion you don't need them.
Make sure you order the correct size of extractor. There are two. One is 3mm and the other is 4mm. I used the 4mm one after measuring the diameter of the hole in the pinion gear.
Although the instructions say the gap between the pinion and the motor should be 5.5mm Rowan Astronomy have machined the pinion well enough that it will fit without having to measure the gap as long as it is fully pushed down. Still best to check though.
I left one of the fixing bolts in place when I removed the mounting plate. Don't bother, the mounting plate will not go back on with the new pinion in place.
The idler wheel shaft was a tight fit and required the cleaning out of the hole for which I used a rat tail file gently twisted in the opposite direction to the cut of the file to remove surplus thread lock in the hole.
It pays to take some time to ensure the idler wheel and the pinion are in line. Then remove the large brass gear from the mount and install the new drive wheel. Again it helps to make sure this is aligned with the idler before tightening everything up and the belt runs without riding up on the drive wheel.
Here is the complete installation before closing up the side cover on the mount. Make sure you run the motors for at least one full revolution before closing up. I discovered I hadn't tightened the Dec pinion during this test and the RA motor was too tight. Make sure there is 2mm play under light finger pressure on the belt.
The first thing noticed during the motor test run was the loss of the grinding noise from the meshing of the pinion with the idler gear and the worm drive gear as backlash was taken up.
If you have thought about doing this upgrade but are unsure of your DIY skills then as long as you can undo and do up nuts and bolts then you should have little difficulty in doing this. The supplied instructions are nice and clear and the machining of the components ensure they go together well enough to fulfil their purpose.
All that remains now is to try out the mount under some clear skies and check on the improvement in guiding.
This modification has been in place for several months now and the tracking has definately improved. I was getting tracking accuracy of around 4 to 5 arc seconds and the modification reduced this to 1.5 to 2 arcseconds. However I thought there had to be more in it than just that and so I recorded one evenings tracking and entered the output on a spreadsheet when I did I noted a periodic error which concided with the stated periodice rror of the HEQ5 Pro. So I simply dialled this value in to the soft ware I use and the next night out I noted that guiding accuracy was now in the sub arcsecond zone. Some of my latest images have even been as low as 0.25 arc seconds.