Poor Man's On-Axis Guider

Everyone who doesn't own an Astro-Physics or Software Bisque mount, and everyone who needs to take >5 minute sub-exposures, needs to guide. And sooner or later, everyone discovers the joys of differential flexure in their guide scope setup. The cure for differential flexure, mirror flop, and a host of other ills is off-axis guiding. The main challenge with off-axis guiding is that the pick-off prism is getting light from the edges of the image circle, where there are aberrations galore. It is also pretty difficult to find guide stars using the tiny pick-off prism.

A company  called Innovations Foresight has developed a product called the On-Axis Guider.  This product uses a cold mirror (a first-surface mirror which is engineered to reflect visible light, and pass-through infrared light). In effect a cold mirror is a beam splitter that lets infrared light through.  This allows the guide camera to get light from the center of the image circle, where stars are not heavily aberrated.

There are several other similar projects:

• Joe Garlitz's pellicle mirror based system
• KZ Tanaka's page from 2003, which is probably the earliest such system

One distinct downside of the ONAG is its rather high cost - over $800.  Having used two different off-axis guiders with limited success, I decided to try my hand at building my own On-Axis Guider. As suggested on the atm_free Yahoo group, I started with the $100 Vixen Flip-Mirror Diagonal (model 2680).

This is a very well-built cast-aluminum unit with very little plastic in it.  To take it apart only requires two tools - a Philips screwdriver, and a 2mm Allen wrench.

The first step is to remove the 2" nosepiece, by removing the four screws.

The next step is to remove the knob and the little wheel inside which moves the mirror up and down. To do this, you will need to loosen the grub screw that holds the knob.

Once the knob and internal wheel are removed, the mirror can flip up and down unconstrained. To remove the mirror holder and mirror, you will need to loosen the two 2mm screws on opposite sides of the diagonal.

The stock Vixen mirror is 30mm x 35mm in size. This is large enough so you can use an APS-C (crop sensor DSLR) imaging chip with manageable vignetting. Obviously a smaller sensor like the 4/3-format SBIG 8300 and its competitors (e.g. QSI 583) will work even better.

There may be several methods of getting the mirror off the aluminum mirror holder, but I chose to boil it in a pot.

The mirror was pretty easy to pry off once the strange Chinese glue underneath had gotten soft from the boiling. Next step is to remove the strange Chinese glue. I used a sanding drum on a Dremel to do this. Don't get too enthusiastic. I used sandpaper to clean the raised portion of the mirror holder. Our overly-large cold mirror will sit on this raised portion, and we don't want to gouge it with the Dremel.

Afterwards, it is necessary to drill a hole through the mirror holder. I used a succession of bits (starting with a 2mm and ending with a 13mm) to make the hole. The aluminum is quite thick and you need to put the mirror holder in a vise of some sort so it doesn't grab the bit, spin around, and bloody your fingers.

I then used a step bit to enlarge the hole to 20mm.

The cold mirror I used is the Edmund Optical part number 64449, "High Performance Cold Mirror 45 Degree AOI, 35.0mm Square." Note that this is slightly larger than the original 30mm x 35mm mirror, so won't fit in the mirror holder.

The mirror has protective vinyl on both sides, and you need to remove the vinyl on the back side before adhering it to the aluminum. Make sure the aluminum is clean! also make sure you know which side of the mirror is the aluminized side! do not glue the aluminized side to the mirror holder!

An easy way to check which side is aluminized is to hold a pencil to the surface of the vinyl. If the pencil point touches its reflection, that is the aluminized side. If the pencil doesn't quite touch its reflection, you've got the correct side.

I attached the mirror to the mirror holder with ordinary silicone caulk. Same thing we use to attach Newtonians to secondary holders, so should sufficiently robust. A little too much caulking (which might stress the mirror and cause astigmatism). So less caulking is better.

Here we can see that the silicone glob was just a bit too large.

After letting the silicone dry for 12-24 hours, it's time to put everything back together again. Assembly is the reverse of disassembly.  I did not remove the protective vinyl on the cold mirror's front surface until after everything was re-assembled. This helps avoid nasty mishaps to the coating while screws and screwdrivers are in the general area. Make sure to tighten the two screws that form the hinge of the flip mirror, if they aren't tight enough, the mirror will jiggle back and forth.

I initially thought of drilling a hole in the side of the flip mirror housing, then tapping it with an M4 screw. This would allow the M4 screw to bear on the mirror holder and lock it down. But with the two side-screws tightened, the mirror doesn't budge. So it's good enough for now.

And after everything is put back together. Note that my focal reducer is behind the ONAG - this particular reducer (a William-Optics 0.8X model II) only has 55mm of back focus, so cannot be used in front of the ONAG. As a result, a large amount of in-focus is required. Note that the focuser is only racked out about 40mm - and this is on an Orion 100ED with a tube that was shortened by 4 inches. The net result is that the guide camera (here a Meade DSI) needs a 1.25" diagonal to reach focus.

Also note that the Vixen Flip-Mirror Diagonal has male T-threads on both the top and rear ports. As my reducer has a 2" nosepiece, I had to use a T-thread to 2" adapter from Blue Fireball. I had this around for my Vixen OAG anyway. Otherwise, the camera can thread directly to the Vixen Flip-Mirror Diagonal, at the expense of having no reduction.

It may also be possible to put a 0.63X SCT reducer on the front port (using an SCT thread to 2" adapter) and the camera on the top port. The SCT reducers have about 105mm to 110mm of back focus. I don't know if the Diagonal would provide enough back focus.

So does it work? here is Vega at 400% on the main camera (a QHY8). The out-of-focus stellar images looked like skateboards (!) so I was quite worried that maybe the cold mirror was pinched by too much silicone. But the in-focus image is nice and round. I think the skateboard-shaped out-of-focus stars are due to the strange shape of the aperture. I took several 10-minute exposures with the main camera, and stars looked pretty round to the corners. I do not have a flat to show yet, but the vignetting is manageable.

And here's what the guide camera (a Meade DSI Pro) was seeing at the same time (1-second exposure). The stellar image is definitely astigmatic (which is why we use a cold mirror, so that the reflected image is what goes to our main imager).

Is this DIY ONAG any better than an OAG?  I can't tell for sure right now, because I haven't tried it in dark skies.  With the terrible light pollution and clouds right now in Singapore, I needed 4-second exposures on the guide camera to pull in magnitude 5 stars. Pretty bad.  It does seem easier to get guide stars with this compared to my Vixen OAG - the OAG only illuminated about half of the DSI Pro chip, so I could only see half the stars.

I'm also planning to put a 0.5X 1.25" reducer on the DSI Pro.  This will improve brightness and probably reduce the out-focus distance required on the rear port. That would allow me to get rid of the 1.25" diagonal. Another interesting refinement is to add an X-Y guide star positioner to the rear port. Orion used to sell this, but apparently they have discontinued it.

Astro-Physics 600E QMD Go-To Conversion

Getting Started

Around my birthday last year, I was able to score an Astro-Physics 600E QMD (Quartz Micro Drive) mount from Astromart.  It is fifteen years old (or more!) and does not have any Go-To capability but was still a great birthday present! (and only cost as much as a typical leather handbag..)

After using the mount for some time (it does have Agilent axis encoders and so can be used in Push-To mode with a Digital Setting Circle) I decided that I wanted to convert it to Go-To. Initially my plan was to use an Arduino, but eventually I decided to use the Littlefoot Elegance Photo, a stepper motor controller developed in Germany. The other options were the FS-2 and Boxdoerfer (both also German), the Sidereal Technology servo controller, or the Losmandy Gemini II.

The SiTech controller costs about the same as the Littlefoot Elegance Photo, but finding proper servo motors is hard - and buying servo motors new is an expensive proposition. We can't use cheap Tamiya motors. The FS-2 is much more expensive than the Littlefoot and has a bad aura about it. Sigurd Boxdoerfer never answered my emails.

And the Gemini II controller is pretty expensive - add the cost of the AP 600E and the Gemini II, and I could have gotten an AP600E GTO (the Go-To version).

An additional, and important consideration is that the conversion must require no modifications to the mount, hence it should be 100% reversible in case I wanted to sell the mount and the buyer wanted it in an original state.

Opening Up the Mount

First step was to open up the mount. It is trivial to get the front plate off, they are held by four screws:

With the front plate off, the two stepper motors are visible. They are attached to a flat aluminum plate, which is fastened to the inside of the mount with four socket-head bolts. A long socket driver is a necessity for getting at those bolts!

The two motors are Nippon Pulse, 48 steps per revolution, with a 150:1 spur gearhead. These are incidentally almost identical to the MT-1 motors of the Vixen Great Polaris (which are the exact same model, but with a 120:1 gearhead). As an aside, look at the lot number on that motor! it was made in February 1981! and still works fine. And Astro-Physics still carries spares! one nice thing about stepper motors is that they are extremely reliable.

Notice the two transfer spur gears. These are 48-pitch Imperial gears with 64 teeth. An important gotcha: they have a 5mm bore (because the Nippon Pulse steppers have a 5mm shaft). A 1.27mm hex key is required to get the spur gears off the shaft.

You cannot buy a 48-pitch, 64-tooth Imperial gear with a 5mm bore! I have been told by Astro-Physics that these started out as 3/16-inch bore, and were bored out on a lathe. Resist the temptation to bore these out yourself - it is impossible to bore them out concentrically while maintaining perfect centering with a drill (even a drill press). If the bore is not centered, the mount's periodic error will worsen. I found out about this the hard way. Therefore, you should not buy any stepper motor with 6mm or 8mm shafts, because this will require modifying the spur gears.

The QMD motor control board uses an Ericsson PBD3517 stepper motor controller chip (actually two of them). This is an old device, and is not capable of micro-stepping. However it can half-step on the RA axis, which contributes to smoother tracking.

Keep the old motors, old motor mounting plate, and motor controller board, in case you want to restore the mount to its original glory.


Stepper Motor Selection

The problem with the existing Nippon Pulse steppers is twofold: first, they are unipolar steppers, and the Littlefoot is a two-phase stepper controller. Second, and far more important, the stock steppers have a 150:1 gearhead. Since steppers have a maximum rotational speed, the large gear reduction imposes a limit on how fast the mount can slew. For these particular motors, the maximum slew speed is around 20X sidereal, which is glacial. It would take 36 minutes to slew from horizon to horizon at that speed.

The only way to improve the slew speeds is to replace the motors. Specifically, motors with a lower gearhead reduction are needed.  After some trial and error, I discovered that you need to use Vexta PX243, PK243, PK245, or PK543 steppers (NEMA 17 size) with a spur (SG) gearbox. The common and dirt-cheap Vexta PK266 (which are NEMA 23 size) are too large to fit inside the mount.

Also important, the motors must be with spur gearbox (SG part number). Vexta steppers are also available with planetary and even Harmonic Drive gearheads. These are not usable because the gearhead output shaft is coaxial with the stepper shaft.  For a motor to fit in the AP 600E, it must have an offset output shaft like the original Nippon Pulse steppers.

Handily, the SG gearheads have a 5mm output shaft, so you can use the existing AP spur gears.

Vexta makes SG gearhead NEMA 17 motors with several gear ratios - 3.6:1, 7.2:1, 10:1, 18:1, and 36:1. The first gear ratio probably does not have enough torque, and based on Vexta's data sheet the maximum torque of the 18:1 and 36:1 gearheads is the same. Hence it makes no sense to buy motors with the 36:1 gearhead (which would suffer from painfully slow Go-To slews - although not as bad as the stock motors).

You can generally get the 7.2:1 or 10:1 motors on ebay for about $90 each. This is much lower than the new price of $220 (buying the motors new would make the Go-To conversion un-economic). However being a stubborn and cheap sort, I found the 18:1 motors for $33 each. The downside of using the 18:1 gearbox is that the maximum Go-To speed is reduced.

Ensure that the motors you get are rated at least 0.6 A, you can also get these in 0.4 A and I'm not sure if those have enough torque. Also ensure that the motors are 2-phase! a unipolar stepper (also called 5-phase) can be wired to work with the Littlefoot but the torque would be much reduced.


Stepper Motor Theory

A hybrid stepper like the Vexta PX243 and friends is limited to about 4000 half-steps per second at a high driving voltage (about 24 volts). Since these motors are 200 steps per revolution, that translates to a speed of (4000 / 2) / 200 = 10 revolutions per second.

Plenty, you say. But if you have a 10:1 gearhead reduction, the speed at the output shaft is only 1 revolution per second. Since the AP 600E has a 192-tooth worm wheel, it moves 6750 arc-seconds with one revolution of the worm.  So if our motor output shaft is rotating at 1 rps, then the mount is moving at 6750" / second - which is 450X sidereal.

Obviously with the 18:1 gearbox, the maximum speed would be 250X sidereal, and with the 36:1 gearbox, 125X sidereal.  All of these speeds are achievable - but there is a catch. To get the maximum motor speed, you need to drive the motor with a high-voltage input. Vexta publishes speed curves with their motors based on a 24V driving voltage, which is almost the best case.

The Littlefoot can take up to 30V DC input (and there is a 40V option, but it costs more).  But if you are driving the Littlefoot from 12V, which most people do, you will not get the maximum speed.

In my case, at 12V and with the 18:1 gearbox, the maximum speed I could get is 160X sidereal (as compared to a theoretical 250X). Above that speed, the stepper motor stalls. 160X sidereal Go-To is slow but usable - at 160X a horizon-to-horizon slew takes 4.5 minutes. By comparison, the original Takahashi Temma II (a comparable stepper motor controller) can slew at 350X at 12V, and 700X at 24V, while the Temma II Jr can only slew at 120X at 12V and 240X at 24V - the same physics is involved.

Incidentally the "new style Temma II" can slew at 700X at 12V. I suspect Takahashi is using an internal DC-DC converter to increase the driving voltage to the steppers.

You might wonder, why not use the 3.6:1 gearhead, that would give 1250X Go-To speed, in theory. The problem is that the gearhead is also a torque multiplier, and it takes lots of torque to turn the worm. The lower the ratio, the lower the torque. And since we are limited to a NEMA 17 motor, the torque we can get from the motor is also limited (larger motors produce more torque - but we can't put a larger motor inside the AP 600E).


Fabricating the Motor Mount

The existing AP motor mounting plate cannot be re-used, because the Nippon Pulse motors do not have the same bolt circle as the NEMA 17 steppers.

A new mounting plate can be constructed from 3mm thick aluminum plate. Take the original mounting plate and trace an outline of all its holes on the new aluminum plate.

The next step is to drill pilot holes. Use a nail (or better, a punch) to create starting holes in the aluminum sheet, so that the drill bit doesn't wander around. Don't drill out the holes with your largest bit, start with a small bit like a 2.5mm and slowly go up. This keeps the holes nice and round. Unfortunately the two large holes for the motor gearhead shafts are 18mm in diameter - and it's pretty hard to get an 18mm diameter bit. You would need a step bit (also pictured below, it looks like the devil's little ice cream cone) to drill the large holes.

Note that I have not yet cut out the mounting plate - it's much easier to drill holes in a large piece of metal than in a small piece of metal (that tends to catch the drill bit, spin around like a circular saw, and mangle your hands).

After drilling with the devil's ice cream cone. A bit of oil on the step bit and on the work will make the drilling much faster and quieter. Any oil will do, I used cooking oil.

Notice that there is a sort of raised "shelf" around the gearhead output shaft, and this shelf is 18mm in diameter. This is why the motor shaft hole in our mounting plate must be 18mm in diameter.

The bolt circle for NEMA 17 motors is 1.220" or about 31.5mm, we need to drill four mounting holes for each motor so that we can attach the motors to the plate. We use M3 bolts to attach the motors to the plate.

After all that drilling, we can attach the motors to the motor plate.

Now for the most annoying part of this activity: the new motors are much larger than the Nippon Pulse motors, so our new assembly won't fit inside the AP 600E.  We need to cut the motor plate in two:

The spur gears have been re-installed - the hub (the little metal protrusion where the grub screw threads in) is facing outward. Make sure not to install the spur gear with the hub inward, as the teeth will hit the insides of the mount and the motor will stall.

Motor Installation

We can then install the Declination motor. Do not install the Right Ascension motor first, you will not have enough clearance to install the Declination motor. Note that the Declination motor is not true inside the mount, it's a bit crooked.

This is because the motor is square, while the original motor was round. The square motor hits this large metal flange inside the mount that secures the declination axis (at the top-left, 11 o'clock position). Luckily there still is enough clearance to install the motor, and the spur gear (barely) meshes with the gear on the worm.

You need to remove the grub screw on the opposite side of the mount and check the engagement of the motor spur gear with the worm spur gear. The engagement must not be too tight, otherwise the motor will stall. My motors were double-shaft and had these nice knobs on the shaft, so it was very easy to turn the knobs with my fingers to check if the motor was turning smoothly.

After the Declination motor is installed, spur gear meshed nicely, and turning smoothly, it's time to install the Right Ascension motor. Same procedure applies.

Face Plate Fabrication

We also need to make a new face plate for the mount, as the existing AP face plate is not suitable (and it would be sacrilege to modify it). The face plate is 136mm x 60mm in size, made of 4mm thick aluminum plate. In our case, we can use the same 3mm plate used for the motor mount.

Simply cut the aluminum to size and trace the four screw holes in the corners from the original plate. For the motor connectors, I decided to use the same type of 4-pin "military style" connectors that the Littlefoot already uses. These are the same type of connectors used in the Gary Bennett mods for the Celestron CGE, and are very solid connectors. They require quite large holes for mounting. Again, the step bit is very useful for drilling these large holes.

Wiring up the motors to the screw connectors is beyond the scope of this article. The pinout of the Littlefoot motor connectors is in the manual, and determining the A, A', B, and B' connections on the 2-phase stepper motors is a simple exercise.

Testing

After wiring up the motor connectors, you must then test the mount.  Configuring the motor power and gear ratios is discussed very clearly in the Littlefoot Elegance Photo manual, so I will not repeat it here. Basically you can define a custom speed for the 2X, 8X, and 32X speeds (three positions of the right toggle switch). You can set a maximum speed of 640X, but the motor will stall (hum but not rotate) above a certain limit.

You want to use the highest reliable speed - meaning the motor should turn and not lose stepper counts. This is also a good time to check that your spur gear meshing is not causing the motor to stall.  Unlike servo mounts like the GTO, steppers are open-loop and the controller cannot detect if the stepper has stalled. So it's quite important not to be too aggressive with the speed settings.

In my case, 160X was the maximum reliable slew speed. I replaced my 12V power supply with a 19V, 5A laptop supply - but the slew speed did not improve. According to Rajiva (the Littlefoot Elegance Photo designer) you really need to use 24V or higher to get a significant improvement over 12V.


Conclusion

After putting everything back together: it doesn't look too DIY and should be quite reliable. No motors dangling off external brackets. I could even stencil in some custom lettering or decals on the front plate. The hand controller is quite strange as it has two toggle switches and four direction switches - no keypad! so navigating it is a bit inconvenient at first but the interface is quite intuitive and very efficient. German..

The hand controller does not do Go-To out of the box (it can only Go-To a specified RA/DEC) but it has an SD card slot. The Littlefoot comes with a DVD containing the necessary software to write the catalogs to an SD card. You can also create your own custom catalogs, alignment star lists, etc.

The Littlefoot does not have any Go-To alignment procedure. You simply point it at something (either using the hand controller direction buttons, or you can push the mount manually, if you have encoders like I do). Once pointed and centered, you can sync the controller to that object, and you are aligned.

Of course if your polar alignment is off, Go-To's will be inaccurate. But if for example you do a meridian flip, you just need to point (again, manually or with the direction keys) at a known bright star or DSO in that area, sync, and Go-To's in the neighborhood will be accurate. It's quite convenient because if you lose power, lose alignment, etc. you don't need to go through a long alignment routine.

With cables on the Littlefoot: RA and DEC motors, power (also the robust screw-type connector), extension port (hidden, and not used); encoder port for Lumicon-style DSC's (this I use), serial/USB port, and LAN.

And the other side of the controller: power switch, video port (for Mallincam or similar video cameras), DSLR shutter control (for Canon cameras), Focus port (compatible with Robofocus stepper-based focusers), ST-4 guide port, and hand controller port.

Next Steps

I'm current hacking on this software called Aspect which is used to manage the Periodic Error Correction (PEC) table of the Littlefoot. Unfortunately the older versions of this controller (i.e. the original Littlefoot, the Littlefoot Vpower, and  the MCU Control) only had 256 PEC cells, while the Littlefoot Elegance Photo has 1024 PEC cells. Aspect currently cannot handle 1024 PEC cells, but I have made a lot of progress in fixing that.

Currently I also have a set of Perl scripts that I'm using to manage the PEC table from the command-line. So far I have gotten the native PE of about 15" peak-to-peak down to 5" peak-to-peak which is quite good.

For my birthday this year - I am looking for an AP 900QMD or a Takahashi NJP (non Go-To versions). The AP 900QMD costs less than half of an AP 900GTO, and can be converted to Go-To using this same procedure. I have yet to find one at the right price, however.

Arduino Bluetooth Digital Setting Circles for Telescope

Edit: Google Code has gone, latest source code is here.

I decided to build an Arduino digital setting circle for my mount because it's really hard to buy Tangent Instruments boxes cheaply, and the Argo Navis (the Lexus of telescope digital setting circles) is quite expensive. Besides it seemed like a cool project.

I used an Arduino Uno, which cost $32 (Singapore dollars)



an Arduino Proto Shield ($5), as well as some headers ($2)



I got an RJ-45 jack from Sim Lim Tower and soldered it to the Proto Shield. Obviously four (4) digital pins from the Arduino are needed to read the two encoder channels. A lesson painfully learned: do not use pins 0 and 1 (the serial RX and TX pins) for anything else!





Jim's Mobile (JMI) has some good documentation on the pinout of the RJ-45 encoder connector. By maintaining compatibility with the JMI/Tangent Instruments encoder cable, I ensured that my Arduino DSC is compatible with all the telescope encoder setups out there.

I used Mike Fulbright's Arduino DSC source code, although his code didn't implement 4X quadrature decoding, only 2X which results in half the encoder resolution.

In order to get 4X quadrature, you need to hook interrupts to both channels of the encoder. Since there are two encoders (RA and DEC), you need four (4) interrupts - but the basic Atmega328p CPU on the Arduino only has two interrupts. To get my four interrupts, I used the PinChangeInt library, which provides multiple virtual interrupts.

A side-effect of this choice of library is that my code won't compile on the Digilent Max32 which is a 32-bit Arduino-compatible board. I had bought this board because I wanted to make a full GoTo controller and thought I needed more RAM (the Arduino Uno only has 2K of RAM and an 8-bit processor, the Max32 has 512K of RAM and a 32-bit processor).

I also soldered a $9.95 "linvor" Bluetooth serial adapter from ebay to the Arduino proto shield and (breaking my earlier rule) wired the Bluetooth RX to the Arduino TX pin, and the Bluetooth TX to the Arduino RX pin.





A good overview of how to use these cheap Bluetooth dongles with the Arduino is here. Note that it is necessary to disconnect the proto shield from the Arduino Uno board when uploading code to the board, since the programming circuit and the Bluetooth board fight each other for control of the Arduino's serial pins (pins 0 and 1).

I then powered the Arduino DSC from a 12V supply and connected it to the RA and DEC encoders on my mount. Actually I also put a DFRobot LCD Keypad Shield on it so I could eyeball the RA and DEC encoder readings. Although for the "production" version this is completely unnecessary because the encoder counts are polled by a PC. Pairing the "linvor" Bluetooth dongle with the PC is trivial and beyond the scope of this article, but suffice it to say that the Arduino DSC was accessible on COM4 on my PC. I was able to verify its operation using Putty (RealTerm didn't work and persistently disconnected the Bluetooth connection, avoid!)

I used Dave Ek's ASCOM driver for digital setting circles and Cartes du Ciel planetarium software.



Basically, you configure Cartes du Ciel (or any Windows planetarium software that supports ASCOM) to connect to the telescope, select the Dave Ek ASCOM driver, select COM4 (or whatever COM port the Arduino DSC gets assigned by the Bluetooth subsystem) and align.

After a successful alignment, when you move the telescope mount around and the encoders change readings, Cartes du Ciel will update its on-screen reticle to correctly reflect where the telescope is pointing. Cool!

This DSC should also work with SkySafari for Android which supports Tangent Instruments-type encoders over Bluetooth. I'll test this out when I get my hands on an Android 2.2 device.

The nice thing about this DSC (as compared to say Dave Ek's DSC circuit) is that the amount of soldering and PCB-etching required is close to nil: all you need is an RJ-45 jack, the Bluetooth dongle, an Arduino Uno, a Proto Shield, and the headers.

The Arduino Uno at about $25 USD is so darn cheap that it doesn't make sense to knock yourself out etching your own PCB and soldering tiny parts to it. This DSC does require some soldering - the RJ-45 jack, headers, and Bluetooth dongle - but that's quite minimal. And the total cost is well under $100, under $50 even. Most folks want $150 plus for their old Tangent Instruments boxes, so as long as you have a laptop already or Android device, you're golden. And I always have a laptop with my mount anyway, since I use the laptop for image capture and guiding.