Building A Cookbook Camera Autoguider

1. Why an autoguider?

I don't know whether it's due to the global warming or not, the weekend weather of my home town, Mobara city about 60 Km north-east of Tokyo, is bad.ﾊ It's not merely I feel it but there are statistics; in the total 3681 rainy daysﾊ from 1961 to 1997 around Tokyo area, the largest 574 days are Saturdays and it's 20%ﾊ bigger number compare to the least Wednesday's.ﾊ To ease the frustration every weekend, I decided to build something andﾊ prepare the starry night season.
At that time, I had problem in my exposure time. More than 2 minute exposures created elongated star images. Though I was not sure this was because my poor polar alignment or periodic error of my equatorial mount, I decided to build an autoguider to solve this problem.

2. Autoguider

This autoguider is designed to have the same interface as the one depicted in Richard Berry's "CCD Camera Cookbook".ﾊ The stepping motor control circuit is designed to fit with Vixen GP mount.

The principle of the operation is as follows;

(1) Track automatically a star by the CCD camera and software during one turn of the equatorial mount worm gear . All commands to the motor are stored during this process.

(2) Replay the stored command as a background job while taking a star image.
As mentioned above, the PC interface is the same as the Cookbook Camera's, you can use the same software for it.

3. SD-1

To drive GP mount, I already had an SD-1 controller. So I planed to add declination control circuit around it.ﾊ The SD-1 circuit I investigated is shown here though it may have mistakes. I actually found some improper designs in it. If you are bothered by unreliable switch operation, you may find the cause in it.

4. Design and Making the Autoguider

As mentioned ,ﾊ I added the declination control to the right ascension SD-1 portion. The declination controller uses right ascension oscillator commonly and separate countdown circuit with stepping motor driver are added. To maintain software interface, outdated non-programable UART is adopted.ﾊ To facilitate getting electronicﾊ parts, I will utilize microprocessor in my next design.

(1)Basic Operation

There are two right ascensionﾊ clock rates, one is sidereal (86164sec/day) and the other is King's rate(86200sec/day).ﾊ SD-1 utilizes 86164sec/day, as known by a 5.0469MHz crystal oscillator.ﾊ (Olav Callesen pointed out my mistake.)

The worm gear tooth count of GP mount is 144 and 1024x256 count down is performed by the SD-1 internal counter, it takes 11520 counts to rotate the worm gear one turn.

The autoguider send interrupt to PC every 60 counts and PC counts interrupts 192 times to obtain the worm gear period.ﾊ (11520=60x192)

(2)ﾊ Circuit Design

Basically, the circuit is consist of a crystal oscilator, counters, stepping motor drivers, logic for slow motion control and UART + RS232C interface. The right ascension motor is driven by modified SD-1, because it has improper design such as directly connecting CMOS outputs. The declination driver is different from RA in reverse rotation capability.

After some trials, I found out of order in stepping motor operation when I reverse the rotation. To prevent this, I added four diodes on each stepping motor driver. Though another four diodes are necessary for each stepping motor to prevent noise generation, this will do fairly well.

Schematic is shown here. Please keep in mind that I make no warrantyﾊ with respect to accuracy of the information or result of the usage.

5. Results

Honestly speaking, I did not expected the PEC (Periodic Error Correction) result so much. My first motivation was just the bad weather. What I've got is as follows.

M1 with PEC

M1 without PEC

By the same total integration time, big difference appears in exposing faint stars. Now I can extend the integration time from 2 minutes to 4 minutes. Total expense was only around $50 and I could get more than expected.
ﾊ

To be continued to next project - the most cost effective Digital Setting Circle with GP encoders, Ouranos Interface($90), and Guide 7.0 . Stay tuned!
ﾊ