Wednesday, December 28, 2011

Poor Man's Vixen Starbook

Here is how I butchered a NGC Sky Vector Digital Setting Circle (DSC) to connect it to a SkyWire.

SkySafari from Southern Stars is a great planetarium application for the Apple devices and now Android. It's the closest to the old Starry Night Backyard that I knew and loved a decade ago.

One of the neat tricks that SkySafari has is its ability to control most any telescope via a wireless dongle called a SkyFi, or a cheaper wired serial dongle called a SkyWire:

The SkyWire is basically a serial port that connects to the port on the bottom of every iPod, iPhone, and iPad out there.

I had already previously used the SkyWire to control my Celestron CGEM mount, but the CGEM is currently in the doghouse because its GoTo's are darn unreliable. I already had an NGC Sky Vector that I snagged for a hundred bucks off Astromart. The Sky Vector has an RJ-11 jack for a serial connector to a PC (or SkyWire) but I didn't have an RJ-11 plug (or crimper) handy. I did however have a DB-9 female connector, so I decided to swiss-cheese the Sky Vector casing.


After two runs with a 6mm drill bit:

And much later after much dremel-ing:

The Sky Vector PCB (note the little Tangent Instruments label) with its vacuum-fluorescent display and minimalistic four-button interface:

Contrary to my belief (which caused a lot of head-scratching and gnashing of teeth) the RJ-11 jack is soldered to both sides of the PCB, and the correct pins to solder to.. are not obvious. Only three wires are needed: RX, TX, and GND. And I got them all wrong.

The correct pinout according to Southern Stars. Use the third column ("Southern Stars SkyFi") and fifth column ("Celestron, Astromaster, others").

So basically it's DB9 pin 3 to RJ-11 pin 3; DB-9 pin 5 to RJ-11 pin 4 (this is the ground pin); and DB-9 pin 2 to RJ-11 pin 2. More information albeit with not as nice-looking diagrams.

After several bouts of soldering:

Buttoned it up..

and in action (telescope mount not shown):

I noticed that SkySafari couldn't talk to the Sky Vector initially (although I could using a USB serial adapter and RealTerm which is a good HyperTerminal replacement). Turns out my V 3.65 Sky Vector doesn't respond to queries about its maximum encoder resolution, I had to hard-code these into SkySafari.

Next stop.. real alignment. Weather's pretty bad so I just did a simulated/fake align to verify operation.

The nice thing about SkySafari is that you can sync on anything on the screen. Doesn't have to be from a small list of alignment stars.

Sunday, December 18, 2011

Wood Low-Latitude Wedge for AP600E

The Astro-Physics 600E equatorial mount is not designed to go as low as 1 degree Latitude, where I live. Unfortunately the 900WDGA low-latitude wedge for the AP900 is not compatible with the AP600, and the successor to the AP600E (the Mach1 GTO) doesn't need a wedge to reach low latitudes.

I had a company in Malaysia fabricate a low-latitude wedge for my CGEM, but as I am not traveling to Malaysia anytime soon, I could not get one made for my AP600. Instead, I decided to make one myself out of wood.

The parts were a shelf board from Home-Fix DIY Shop (S$11) and two 8" diameter wood chopping boards from Carrefour (S$7.50 each). I cut a couple of angled pieces from the shelf board, sanded them down to get them flat and consistent, then glued them to one of the chopping boards. I also put a flat piece in-between the angled pieces for strength.

After attaching the top chopping board (also with epoxy) I drilled three holes 120 degrees apart and screwed M6 threaded inserts into the holes.

This allowed me to use hex-head M6 bolts to attach the ADATRI to the top of the wedge.

Another view:

This is what it looks like in action:

The setup is still shaky, but I don't know if it's my wedge, or the aluminum surveyor tripod. I'm planning to purchase a Takahashi SE-S wood tripod locally although haven't gotten around to it. Hopefully that tripod in conjunction with my wooden wedge, will be stable enough.

Tuesday, December 06, 2011

How to take a proper flat

Flats are necessary to eliminate vignetting and dust spots on your camera or CCD sensor when doing astrophotography.

I wrote an entire essay all about flats for my Swinburne astronomy course, but that's neither here nor there.

Here's a primitive but fairly effective way of getting a "dome flat."

All you need is your entire imaging train (telescope, flattener and filters if required, and CCD camera or DSLR), a sheet of white paper, and your friendly laptop.

The key things to remember are:

1. The telescope must be right up against the laptop screen (which should be displaying a blank white page, like a blank PowerPoint slide)

2. You should put a sheet of paper against the laptop screen to diffuse the light a bit

3. The CCD integration time (exposure time) must be more than 2 seconds. The reason for this is that at short integration times, all sorts of weird visual artifacts show up on the laptop screen, kind of like the "rolling picture" of old-time TV's when you see them in movies

4. Take at least four flats, rotating the entire telescope by 90 degrees between flats. Your software (I use DeepSkyStacker) should average these four flats to create a master flat.

A picture is worth a thousand words: