Thursday, July 15, 2010

Construction Details: Meade DS Motors on Vixen Great Polaris

I've written this documentation on how I mounted the Meade DS motors to my Vixen Great Polaris equatorial mount. My key constraint was that I do not have ready access to a variety of metal brackets, and I don't have much in the way of metal-working tools. All the metal-cutting for this project was done using a Dremel with a carbide cutoff wheel.

First: the dimensions of the Declination and Right Ascension mounting plates. These can be made of sheet aluminum of appropriate thickness, or, in my case, I used angle aluminum for extra stiffness.

Also, I re-did the Declination mounting plate by extending it opposite the motor so the plate would "grip" the dovetail saddle and be more secure against rotation, since it's secured by a single bolt.

Here is a detailed view of the Declination mounting plate:

The Declination mounting plate from the other side, showing how the motor is mounted (using ordinary bolts going through holes in the motor casing) and the 6mm shaft coupler. The DS motor shaft is more than 6mm in diameter, so I had to file it down a bit by chucking it in a drill and pressing a file against it while the drill was spinning.

Also, the shaft on the Great Polaris is slightly less than 6mm diameter, so I had to put in some flat tie-wire inside the coupler so that it wouldn't slip. Note that my couplers are friction-fit (the set-screw doesn't bear on the shaft but instead tightens down the entire coupler hole) and if you're using a set-screw coupler, you will not experience my troubles.

Here is the Right Ascension mounting plate. I had to put one-and-a-half thicknesses of 9mm plywood so that it was mounted just the right distance from the (original) RA motor mounting area, so that the DS motor's shaft aligned with the RA shaft. I got the half-thickness of plywood by splitting the plywood edgewise with a kitchen cleaver.

Another view of the Right Ascension mounting plate. I drilled a couple of holes in the Great Polaris itself in order to secure the plate. Since my GP is old and cost $200, I didn't feel too bad about putting two small holes in it.

Yet another view of the Right Ascension plate, showing the one-and-a-half thickness of 9mm plywood.

I used a plastic box that I found in an electronics shop to hold the connector panel. The box is screwed into my plywood 6" half-pier. The half-pier is needed because I live at a very low latitude (1 degree North) and the Vixen SX pier costs as much as my entire GP mount!

Sunday, July 11, 2010

Auto-Guiding with PhD

I constructed a Meade 505 compatible serial cable so that I could upgrade the firmware on my Autostar 497 hand controller. The pinout for this cable can be found on Mike Weasner's Meade Autostar Information page. I'm reproducing the schematic diagram here:

The only challenge was wiring the RJ12 (4P4C) connector; I had to buy a crimper specifically for it. Most crimpers cost $20 to $30, but I found a really cheap one for $3.00 but it cannot crimp RJ45 connectors (it's purely for telephone cables).

The black dongle above is a Targus PA088E USB-to-serial converter; then a short RS-232 cable; and finally my home-made Meade 505 cable.

I got the ASCOM Platform files from ASCOM Standards, and the drivers for "Meade LX200 Classic and Autostar #494, #495, and #497 (combined telescope/focuser, 5.0.3)" from the scope drivers page on the same site.

Everything worked great out of the box, although it took PhD quite some time to connect to the Autostar (I thought it was hung).

I used the Philips Toucam as the guider; a bit of a challenge because it's not very sensitive and the sensor is so small. I used my William-Optics Zenithstar 70ED as the guide scope, and there was no payload. This scope is f/6.2 so if I use a 50mm f/1.8 standard lens as my guider, I should have less troubles with the sensitivity (or lack thereof) of the Toucam. The wider field of the 50mm lens on the tiny ICX098BQ sensor will also make life a lot easier.

I got an RMS of 0.36 (which I've read is pretty good) out of the box! and no major RA errors to be had. There was a persistent error in DEC that would take PhD some time to correct, I believe this is due to my lack of polar alignment, so I have to do a bit of tweaking.

Still, very good results! now I see why over-mounting is so important; the Zenithstar is rock-solid on the Great Polaris, even with the big imbalance (the standard counterweight is too heavy).

So now all that's left for me is to find some dark-sky site and good weather!

Jupiter First Light with the C9.25

Last night was one of those rare nights where I could actually see Jupiter through the clouds. Of course most stars were completely invisible, but I'll take whatever I can get.

Here's the setup. I used a Philips Toucam webcam with my robotized Great Polaris, extra counterweights to balance the C9.25, and a jury-rigged (masking taped!) electric focuser. I was using QCFocus for focusing and capture, and Registax for stacking.

I had to add three 2.75-pound weights on the standard Vixen counterweight bar.

Here's a closer view of the extremely stone-age electric focuser; coupling the motor gear to the C9.25 focuser shaft was "implemented" with masking tape.

And the best image of Jupiter that I could get. This was at prime focus (no barlow lens) at f/10:

The morning after, I was able to improvise an extended counterweight bar, using a length of M16 threaded rod. With this extended shaft, I was able to dispense completely with the additional barbell weights. I'll buy one more and exercise my arms so as not to waste the weights.

I also improved the electric focuser with a mounting bracket that doesn't look like it came from a junk pile.

And here's the big scope and small scope. Our living room is a bigger nerd haven than Leonard and Sheldon's..

Friday, July 09, 2010

Poor Man's SkySensor 2000

Here's the Orion Great Polaris (re-branded Vixen) which I got for USD 200 plus shipping:

It has the following modifications:
  • three 2.75-lb weights to balance a C9.25 (actually still a bit top-heavy, but the clutches hold..)
  • homemade 6-inch plywood half-pier
  • wooden tripod from a Vixen Polaris (I recycled the HAL-100 on the Polaris)
  • Meade DS motors
  • Meade Autostar 497 patched to unofficial firmware 43GF for equatorial mount support on non-LXD55/75 mounts

All told, the modifications added up to around another USD 150 (more than half of that amount was the Autostar 497, not shown) plus I used my trusty Makita router to make the half-pier.

Still haven't tested how accurate Go-To's are, but it seems to track well in RA (but all this trouble for RA tracking!) I also will be able to auto-guide because I constructed a Meade 505-compatible cable to do the firmware update.

Tuesday, July 06, 2010

Poor Man's Vixen Great Polaris

In keeping with my "poor man" theme, I purchased a Vixen Great Polaris mount (without motors) for US $200 on ebay. This is a very low price for a GP mount; my Vixen Polaris is far too small to carry the Celestron C9.25 Schmidt-Cassegrain, so I needed something larger but didn't want to spend much money.

The GP I got (Orion-branded at that!) was cheap for a reason - it seemed to have been stored in somebody's garage for years. All the fasteners were frozen or rusted, and the polar scope was full of dirt. The worm gears were fine though, to my great surprise and delight.

I spent the past three weeks cleaning out the mount (still some rusty screws but those are just cosmetic), re-lubing it, and constructing another of my plywood half-pier specials.

My C9.25 (another ebay special) came with a Losmandy-style dovetail, and I didn't have a Vixen dovetail. The local astronomy dealer here in Singapore doesn't have a 17" Vixen dovetail either, so I'd have to order it again from the US and suffer another long wait.

Instead, I decided to make my own Vixen dovetail.

It's basically three strips of plywood epoxied together for strength (the laminated strips are stronger than a single plank of wood; besides I still had a lot of plywood left over from my latest dobsonian build). I screwed two stainless steel reinforcing straps from Daiso to each side of the plywood, where the Vixen saddle and retaining bolt hold on.

Primitive, but it works.. after a fashion.

Now this mount doesn't have any motors, and to go after Jupiter I need tracking. The Orion EQ-1M is on the Polaris (my grab-and-go, I just realized how much lighter it is after manhandling the Great Polaris plus C9.25 around) and I have some Meade motors lying around. I'll have to fabricate some brackets for the Meade motors sometime.