Wednesday, August 18, 2010

Poor Man's SkySensor 2000 Re-Engineered

Lalai and I went to Punggai, Malaysia, last 7th - 9th August, with the SingAstro folks. The first day was a bust due to clouds, but the second day was quite OK. I just had a lot of problems with my Vixen Great Polaris with Autostar refit. The most annoying was the tendency of the Oldham couplers to slip, which caused loss of tracking. But the killer bug was the C9.25 OTA colliding with the motor bracket when pointed at Lyra (basically to the North-East, above a certain altitude).

Also, the Autostar hand pad was very unresponsive due to the well-documented wearing-out of the rubber keypad. These problems really got me thinking that I should be buying a Takahashi mount. Sure it's lots of money, but leisure time is valuable too and wasted doing field repairs. I guess I am a bit lucky; one of the folks who went to Punggai couldn't get his new Meade LS-6 working.

To avoid the mount collisions, I had to re-fabricate the brackets so that the motor doesn't stick out where the OTA can hit it. To do this, I had to use some pulleys. The problem with such a scheme is that the Meade DS motor shafts are not held in place by anything, they rely on being mounted straight-through to the couplers.

I found these "rod end bearings" from RS Online, which I'm using to fix the end of the motor shaft:

I used a 6mm bolt (with the head cut off) as replacement motor shaft. From left to right, the rod end bearing (self-aligning); timing belt pulley with two set-screws; and the Meade plastic gear.

Here's what it looks like on the motor:

I fabricated some new aluminum brackets for the motors, and screwed these brackets to plywood scrap.

Close-up of the RA axis:

and the DEC axis:

It ended up really easy to mount the rod end bearing to the aluminum plate, as the bearing had an M6 bolt as part of it; a simple wing-nut held it in place.

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.

Friday, June 04, 2010

Telescope Mount Hacking

I purchased a Vixen Polaris mount off Astromart about a month ago. With my only past experience a lightweight Chinese equatorial mount, I knew that the Vixen would be a significant improvement.

I had to make a minor modification to the mount so that it can be used at very low latitudes, since Singapore is pretty much on the Equator.

My solution was to remove the existing metal pad which the latitude adjustment screw bore on, and placed a Teflon pad on the screw tip. I used a drill bit to ream out a cup-shaped divot on the Teflon pad. The latitude adjustment screw rests on this divot, and the pressure of the Vixen polar casing on the pad ensures that it doesn't fall off.

I still have to rig a half-pier (the real Vixen ones cost $200, more than I paid for the entire mount and tripod). This will allow enough clearance for the counterweight so the mount can be fully used close to the Equator.

Like many of these twenty-year old Vixen mounts, mine was missing the tripod leg spreader tray. I still had some spare 9mm plywood circles from my on-hold Dobsonian project, so I used that.

I also had an Orion EQ-1M quartz-controlled RA motor drive from the mini-EQ that I bought last year; I had given up on using this motor on the EQ-1 because that mount has a lot of backlash on its worm gear.

A few days ago I had the bright idea of trying to use the EQ-1M on the Vixen Polaris. I was expecting that an additional metal bracket or adapter would be needed to mount the Orion motor on the Polaris. But to my surprise, it bolted right up to an existing bolt on the Vixen housing:

Absolutely no mechanical changes were necessary to hook up the EQ-1M to the Polaris; however since there is no clutch, the RA slow-motion knob is unusable with the EQ-1M attached. Not a bad trade-off.

The EQ-1M tracked as designed: which, while better than no tracking, was not so great either. The Orion EQ-1 has a 100-tooth worm wheel, while all of the Vixen light and medium-weight mounts (Polaris, Super Polaris, Great Polaris) use 144-tooth mounts. This meant that the EQ-1M turned too slowly to track for more then 2-3 minutes at a time.

I decided to open up the EQ-1M hand box and see how to adjust the speed. I knew that the EQ-1M uses a stepper motor and was hoping there was some potentiometer inside the hand box that I could adjust to change the speed.

Opening up the hand box was quite straightforward. The first step was to pry off the caps on the key switches. After which I removed the two small screws that hold the 6V power plug on.

I then removed the four screws on the top cover; this allowed the top cover to be pried off quite easily. There was a daughter-board (or if we want to sound complicated, a "mezzanine board") screwed into the main circuit board on stand-offs. This daughter board held the key switches, and also had to be removed.

There were two integrated circuits on the main circuit board: a 74LS05 hex inverter, and an Atmel 89C2051-24 micro-controller. The Atmel had a 3.58MHz clock crystal. It was obvious that a dedicated stepper motor controller chip was not being used; instead, the Atmel was creating the stepping waveforms in software.

It immediately occurred to me that I could "speed up" the stepping waveform by making the CPU run faster. A quick check online showed that the Atmel is an Intel 8051-compatible chip, and can run at up to 24MHz.

The best-practice method of creating precise timing delays on an 8051 uses a countdown timer. Much like the 8253 in the original IBM PC! and crucially, the countdown timer decrements on every machine cycle, which is the main CPU clock, divided by 12.

So it was obvious that, assuming Orion used the best-practice for generating the waveforms, that the stepper frequency was directly related to the CPU clock, which is controlled by the 3.58MHz crystal.

Since the EQ-1M was designed for a 100-tooth worm gear, and the Vixen has a 144-tooth gear, the CPU would need to be sped up by 44% (144 / 100). This meant I would need a 5.15MHz crystal (3.58MHz x 1.44). No such crystal exists off-the-shelf, although Citizen (makers of the Eco-Drive watch, among other things..) has a 5.33MHz part. It's not generally available though (28-day lead time at Mouser) so I decided to go with a 5MHz crystal.

I had a very good experience with RS Online Singapore. I've purchased electronics parts from RS Philippines before, and they've always offered free delivery, but RS Philippines only takes bank deposits, and delivery takes a few days.

Anyway, I was able to buy a 5MHz crystal from RS Singapore; I put in the order at 2:00 a.m. and by 4:00 p.m. same day I had my crystals. I actually ordered six of them (5MHz, 6MHz, and 8MHz) because I wanted to try different speeds in case the 5MHz crystal didn't work out. Also the crystals cost $1 to $2 a pop, and I wasn't sure if RS would deliver a $2 part.

To replace the existing 3.58MHz crystal, the main circuit board had to be removed from the EQ-1M case. This required unscrewing three more small screws that went into the plastic. There were two machine bolts that held the stepper motor cable to the board; I removed these too but it turns out that step is un-necessary.

Un-soldering the existing crystal and attaching the 5MHz one was quite straightforward for anyone who's taken EE 12. The new crystal was much larger, so I had to bend it a bit so it wouldn't touch the keypad daughter board.

I also put a swatch of masking tape on the new crystal so it didn't short out any of the connectors on the underside of the daughter board.

Putting the handbox back together was straightforward; I decided against testing the EQ-1M dis-assembled, because taking it apart again would be quite easy anyway.

And the result: I had expected the mount to keep an object (Lunar crater) in the field for about 15 minutes at 215X, as opposed to 3 minutes un-modded, and 1 minute with no tracking. To my great surprise, the mount kept the Lunar crater in the FOV for much longer than I expected. I don't really know how long it could keep tracking, because I stopped timing after 40 minutes.

Overall, a great success!

And the end-result: my grab-and-go setup.

Sunday, March 07, 2010

Do Not Buy!! Chinese RF-155 / F-155 O-Ring Flash Adapter

Bought one of these gadgets at a store in Plaza Low Yat for 180 MYR ($54). There is a store in Singapore which sells these for much less, S$ 60 ($43). This is also sold in the rest of the world under the Phottix brand.

Now you may think this gadget looks very similar to the $300 Ray Flash, or the $200 Orbis Ring Flash adapter. Well, it does.. but it is far, far, far less efficient. I guess the factor-of-four price difference does count for something.. although honestly I don't see the value in a $300 ring flash adapter when you can buy a Sigma EM-140 DG ring flash for $380.

In my informal testing, you lose about six stops!!! of flash power. From 1/32 power at f/4.0 on my Pentax AF-360FGZ, I had to go to full power, and my target was still under-exposed. Of course the target was about 2 - 3 meters away, so if using this gadget for macro, then you'll have plenty of flash power.

But six stops of flash power loss is crippling. Overall, had I known how much light loss this thing has, I wouldn't have bought it, $50 price notwithstanding. I think there are LED ring lights out there that produce more light.

This guy reports only a two-stop power loss.