SBIG ST-i Autoguider

A box in shipping and receiving with my name on it. A much awaited box. The new SBIG ST-i autoguider!

For those who are uninitiated into the mysteries of astrophotography, an autoguider is the secret to taking hours of exposures without having the manually correct the telescope position constantly. For almost two centuries, from the first time a camera was attached to a telescope, through the invention of auto-guiders, guiding was a supremely tedious task. The photographer would spend hours on end, peering through an eyepiece watching a single star, if the position of the star started to drift, he would press a button to correct the position of the telescope during the exposure. This was necessary to achieve any sort of long exposure, as the telescope, no matter how precisely made, would drift a little during the night, leaving streaks in place of pinpoint stars.

I have done this, it is no fun at all.

Then came the autoguider. A small digital camera that could take a picture of the star, then check the image for drift in the star’s position and, if necessary, send a command to the telescope to correct the position automatically. The first commercially available autoguider appeared in 1989, the Santa Barbara Instrument Group model ST-4. This little device revolutionized astrophotography, allowing far longer exposures with much less effort and much better precision.

SBIG ST-i Autoguider
The SBIG ST-i with the optional accessory kit including mount and lens
It is an ST-4 I have been using to guide my photographs for over a decade. Any of the long exposure astrophotos seen here on DarkerView have used this venerable little device. I have had some issues with it as of late. Trouble acquiring stars, struggling with the cryptic displays to lock onto a star, the unit does require a great deal of experience and intuition to return good results. Modern autoguiders are so much easier to use with far better performance. I have been looking at the advertisements with envious eyes for some time now.

No more, I have broken down and purchased the new ST-i from SBIG.

The newest, latest and greatest, just released model.

Unpacking the box I am happy with what I see. Everything looks good.

It does seem like a small device for the $595, smaller than most of my eyepeices. Fit and finish looks good. A nice small package that will be easy to mount to any of my telescopes.

I setup the software and drivers on the laptop with no issues, simply following the provided instructions. The SBIG software for the camera , CCDOps ran first time, connecting to the camera and taking a frame. You can hear the soft click of the mechanical shutter in the camera. The bias frames look quite nice, a smooth field of salt and pepper noise with no gradients or other artifacts.

Attaching the lens from the accessory kit I take a few images of wood-grain on the kitchen cabinets across from the table I am working at. Another task will be to properly evaluate the imaging performance of the camera. It does have a decent CCD in it, the specifications indicate a proper 16-bit A/D system. A real photon transfer test will reveal if that system lives up to the specifications. A subject for another post!

Wrong Size
An improper fit of a button head screw into the mounting plate on the SBIG ST-i accesory kit
I did find one problem. Someone in engineering or purchasing screwed up when setting up the accessory kit. The screws meant to secure the rings to the mounting plate do not fit into the provided recessed holes. The head is notable larger than the machined recess. This leaves the head of the screw above the surface preventing proper mounting of the whole assembly.

A Better Fit
A proper fit after modification on the SBIG-ST-i mounting kit
Not to be deterred in setting up the camera I quickly set about a solution… I clamped the screw into the chuck of my cordless drill and removed the outer edge of the head with a file. A minute or two per screw reduced the head diameter enough to fit into the recesses. The remainder of the assembly went well. The mounting appears to be very solid, with little chance for flexure and the resulting image issues that can cause.

The package offers a nice solution, on paper at least. With the included 100mm lens the camera should provide a 165 x 123 arcminute field of view, or a 2.7 x 2.0 degree field of view. The literature promises sub-arcsecond guiding accuracy with this setup, with the ability to use 7th magnitude stars with one second exposures. These are claims that will have to be checked as well. I intend to do a proper job of verifying these numbers, I expect to get many years of service out of the guider. The new camera has quite an act to follow, replacing my classic ST-4!

I also hope to be able to guide on the Sun for the Transit of Venus. I will be trying the included software as well as Dave Solar System Recorder in the coming week. I wonder if either package will be able to guide on a large, non-stellar object like the Sun, perhaps with a shorter focal length lens.

This leaves only one real question… How well does it work? Unfortunately that will have to wait for another day. In true astronomical tradition, the receipt of new astro toys invariably occurs when the weather precludes their use. In other words… I have clouds!

A writeup of the operation of the new autoguider will be another post, when the skies provide me a chance to use it.

Author: Andrew

An electrical engineer, amateur astronomer, and diver, living and working on the island of Hawaiʻi.

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