Sky Watcher Star Adventurer Portable EQ Mount

I purchased this portable mount in London from The Widescreen Centre in London (very close to Baker Street!) instead of lugging my old Vixen Polaris to the UK.  I got the "Astro-Photo Bundle" for £299.00 (about US$ 500) and am still waiting for the £40-odd VAT rebate, which would bring the price down to about US$ 400.  This mount is also available from Perseid in Malaysia for 1700 MYR, or about US$ 540.

The build quality is better than expected, with some nice touches such as the worm-and-sector drive for the altitude axis of the equatorial wedge, a design very reminiscent of the Astro-Physics Mach1 GTO, albeit with much slacker tolerances. There even is a ratchet on the altitude locking bolt (again, just like the AP). Here I have secured it via the 3/8" bolt to the aluminum tripod of the Vixen Polaris.  The mode dial and power switch is also visible here:



Right Ascension and Declination clutches are the large knurled plastic wheels.  On a less sturdy tripod, it is quite easy to knock off the polar alignment when tightening or loosening these clutches.


The declination slow-motion knob is visible in the photo below.  Note that the equatorial wedge, declination slow-motion, counterweight shaft, and counterweight, are all part of the "Astro-Photo bundle" and are separately priced if one purchases the base package.  On the other side of the Star Adventurer body are two electric RA slow motion switches (12X sidereal) to assist in centering objects, since tightening the RA clutch is a fiddly affair that tends to throw objects out of the field.


There is a fairly nice polar scope, although the reticle illuminator is attached to the far end of the polar scope bore, which is obstructed by the declination assembly.  Hence use of the illuminator requires removing the entire declination assembly (and any payload on top), which throws off the polar alignment.  The 4x AA batteries are under the top cover, and are supposed to last for up to 72 hours of tracking. Maybe that's with lithium batteries.


Below is a 100% crop of the area around Deneb, 2-minute exposures with a Canon 70-200mm f/2.8 lens at 200mm.  There is a bit of RA drift, and a much smaller DEC drift.  The RA drift is a combination of periodic error  and polar misalignment in altitude (as I only used the polar scope for alignment, this was in Lancashire in the UK).

Notice the nasty diffraction spikes on the bright star. A nifty trick I learned is to use a lens filter step-down ring to stop down the aperture (for tighter stars) while retaining nice round stars. If you use Canon L lenses, most of them have 77mm filter rings, so a 77mm to 67mm (or similar) step-down ring would be appropriate.


On returning to Singapore, I decided to load up the Stellarvue SV80ED, and a Meade DSI.  Widescreen hasn't received their shipment of counterweight shafts and counterweights yet, so the mount was severely unbalanced (no counterweights).


Using PHD2 I was able to measure a bit less than 2" RMS error in RA, after guiding.  The Star Adventurer has a standard ST4 guide port, but only can guide in RA, since the DEC is not motorized.



Unguided performance shows about 23" peak-to-peak periodic error, and there is no PEC.  The worm period is 10 minutes (144 teeth).  This implies that peak periodic error over a 2-minute period would be approximately 9" - so unguided exposures at 200mm and of 90-second duration should be possible at zero declination (the duration would be longer at higher declinations).

Guided performance should be acceptable at around 2" to 3" pixel scale, limited only by declination drift.  So a scope like the SV80ED would be a good choice if guided.  However, the mount is close to its limits with such a payload. A 200mm range camera lens or small refractor like a Takahashi FS60 is probably a better choice.



In summary, this mount is a far cry from my Mach1.  Although it looks better-built than other Chinese mounts, and some of the parts are CNC machined and not cast, the illusion of quality falls apart under close scrutiny.  That said, if you want something supremely portable for up to perhaps 300mm focal length, this mount will do very well.  It says something that the entire mount weighs not much more than a Mach1 Eagle half-pier (and also doesn't cost a lot more than said half-pier).

If I ever travel to Gran Canaria,  I will certainly not be able to bring the Mach1 along, but this mount will fit perfectly in check-in luggage.

I did manage to get about 30 minutes of the area around Deneb with the Star Adventurer from Lancashire, but I missed the Pelican Nebula by a small amount.  The wispy nebulosity next to Deneb that I thought was a DSO, turned out to be dirt on the DSLR sensor. D'oh!  image scale with the 200mm lens was about 6" per pixel.



ED Doublet Versus FPL-53 Triplet

I just received my (used, $350) Stellarvue SV80ED doublet refractor.  I'd sold off a bunch of off-axis guider stuff and my ST80 to buy it.  My main goal was to have a decent guide scope, but also a portable imaging telescope because the Astro-Tech AT90EDT is too heavy and bulky for air trips.  The hope is that the SV80ED plus the Vixen Polaris that I am refurbishing would make a decent portable astrophotography rig.

I had thought of buying a Takahashi FS-60C to serve as a posh guide scope and portable imaging rig, but the FS-60C is quite expensive and the accessories and adapters to actually make it usable add even more to the cost.  So I went for the used SV80ED even though I knew it would not have perfect color correction.

So how bad, exactly, is an f/7 ED (probably FPL-51) doublet?  to answer this question, I piggybacked the SV80ED on my AT90EDT, and transferred my DSLR (a Canon EOS 40D) from one tube to the other.  In both cases I used the Altair Astro Lightwave 0.6X reducer/flattener.

First off: Sirius.  Admittedly a tough test for an ED doublet.  SV80ED:



and here is the AT90EDT. 10-second exposure.


Second: M44, the Beehive Cluster.  This image is from the SV80ED:


and this image from the AT90EDT. Also a 10-second exposure. Because the AT90EDT is only f/6.7 the focal length of the two scopes are very similar (540mm and 600mm).


I was unable to see any violet fringing on the Moon at prime focus on the SV80ED, however. It was very instructive to do fine-focusing with the DSLR Live View at maximum magnification.  On the FPL-53 triplet the star remained pure white with beautiful diffraction rings inside or outside focus; while with the ED doublet I immediately saw color aberrations once out of focus.

So the quick answer: even a non-critical imager will quickly find fault in the SV80ED, namely the blue halo around bright stars.  For visual, or for deep space objects, I expect the ED doublet would perform perfectly adequately.

Arduino-based Motor Focuser Controller

Everybody it seems has written their own Arduino-based motor focuser controller, of which the most popular is probably the SGL Observatory Automation version. There also is ArduinoFocus, which ambitiously reverse-engineered the Robofocus protocol; the sirJolo ASCOM focuser; and the E.J. Holmes Arduino focuser.  A common characteristic of all these projects is that they provide their own ASCOM driver.  I only tried the SGL version; sadly the ASCOM driver is in an almost-unusable state for me (basically it doesn't work - Sequence Generator Pro can't drive the focuser in an inward direction, which is pretty bad). So I decided to implement my own Arduino focuser, but make it compatible with an existing protocol.  This would allow me to use one of the commercial focuser drivers and avoid the problem of building my own ASCOM driver.

After some thought I decided to implement the Moonlite (otherwise known as the Lacey or EasyFocus) protocol. Since both a stand-alone and ASCOM driver are supported - and the stand-alone program echoes all the serial commands - it would be easier to debug the protocol.

On the hardware side, I wanted a design that can be readily-assembled, i.e. no soldering required. To achieve this, I used the Adafruit Motor Shield (AFMotor), version 1.2 - this version is now discontinued by Lady Ada, but China-made clones can be purchased for $5.00 shipped on ebay. This shield uses the L293D H-bridge controller, which is only capable of 600mA. This means that driven steppers must necessarily be small, and with fairly high resistance (in other words hybrid steppers cannot be driven).

Because my focuser is a Feathertouch, it uses a Starizona Microtouch MSM20 motor, which is a 300-step per revolution motor (with 48ppr and an internal 6.25:1 gearbox) giving 6.5 microns per step.  One benefit of the Microtouch motor is that unlike the Robofocus (which uses a DB-9 connector), the Microtouch uses a 6P6C (RJ-12) connector. It is very easy to purchase an RJ12 patch cable from any number of sources.  Then cut the RJ12 in the middle and bring out the correct wires.



During prolonged movements (i.e. a homing command), the L293D chip on the motor shield gets fairly hot; more so because I implemented full stepping with double-coil excitation to increase the torque (note therefore that my focuser does not implement half-stepping).  To alleviate overheating, it is a good idea to attach a heat sink to the L293D, I used an Aavid Thermalloy part number 580200B00000G which clips on top and under the L293D. You must make sure to buy an Adafruit Motor Shield (or clone) where the L293D is socketed, otherwise it would be impossible to attach this particular heat sink. Although the motor shield has two L293D's only one is used and therefore only one heat sink is necessary.



A final modification is required: most ASCOM drivers, when connecting the the focuser, immediately send a query or interrogate string. The problem with the Arduino is that when software connects to its serial port, the DTR line is pulled low, which resets the Arduino (this is a feature, necessary to implement programming the Atmega chip). It takes about 2 seconds for the Arduino to reset.  This becomes an issue because the ASCOM driver times out when it doesn't receive a response quickly enough.

The work-around is to prevent the Arduino from resetting when DTR is pulled low.  There are several ways of doing this, but on the Arduino Uno the easiest way is to connect a capacitor between RESET and GND. When the Arduino powers on, the capacitor is a short circuit, and the Arduino is stuck in a reset state. However once the capacitor charges up, RESET is now high, and if the capacitor is large enough, DTR pulling low (and pulling RESET low) will no longer work, because the capacitor provides enough voltage that DTR cannot pull RESET low anymore.  Alternatively you can omit the capacitor; the focuser can still be driven by the Moonlite stand-alone program, but not by the ASCOM focuser driver.


The side effect of the capacitor is that you cannot program the Arduino anymore! (more information in the last link above). In my case, I soldered the capacitor to the RESET and GND lines on the motor shield. Hence, you must detach the shield from the Arduino in order to upload the Moonlite focuser program; after the sketch has been uploaded into the Arduino, power everything down and re-attach the motor shield (with its capacitor).

So the total Bill of Materials consists of the following (aside from the Microtouch motor):
  • Arduino Uno or similar
  • RJ12 6P6C cable
  • Adafruit Motor Shield version 1.2 or clone
  • Aavid Thermalloy heat sink
  • capacitor, any value from 10uF to 100uF will work
Assembly is extremely simple: solder the capacitor to the motor shield; cut the RJ-12 cable, strip the four colored wires that are not black or white; tin these wires, and screw them into the left (M1/M2) motor connection block on the Adafruit motor shield.  Then upload the Moonlite sketch into the Arduino, unplug the USB, attach the motor shield to the Arduino, and you're all set.



In my tests, the Arduino plus shield can rack the Feathertouch focuser in and out under USB power alone (no need for a 12V power source on the barrel plug or on the auxiliary power connector on the motor shield). Results might vary depending on the torque required from the focuser.



Source code with some fixes for :GI# and compatibility with INDI; tested and working with both the stand-alone Moonlite focuser program, and the ASCOM 64-bit Windows driver loaded from inside Sequence Generator Pro. Note that this program does not implement speed changes (the AccelStepper library implements ramping, which makes speed changes un-necessary).

Selecting the Moonlite DRO Focuser Driver, and selecting the Arduino focuser. This program also does not implement temperature compensation, it just returns a hard-wired temperature. Also note that the speed setting, full step / half-step settings and temperature compensation commands are also ignored. I have not tested what happens if half-stepping is set or if temperature compensation is enabled.


Exercising the focuser through Sequence Generator Pro's Focus Control window.