Tracker testing (1)

My equipment is a Canon 5D3 with a 70-200mm L USM F4 lens at 200mm, ISO 800.

I started with 30 sec exposure.  Until these were looking good there wasn't any point in trying any longer exposures.

Initial results were disappointing, and inconsistent which made it difficult to understand exactly what was going wrong. There was a mixture of issues:

1. star images streaked inconsistently in the tracking direction
2. star image movement in other directions, sometimes in a sort of horseshoe shape
3. the whole star field drifting in the tracking direction from image to image

(3) is fairly clearly that the tracking speed is too slow (or fast, depending on the direction of drift).

(1) would also appear to be a tracking speed issue, although the inconsistency of the streaking made it difficult to understand quite what was going on.

(2) would seem to show movement in some other direction: the "horseshoe" shape wasn't always exactly that shape, and often with different orientation. Nevertheless it suggested some sort of oscillating movement, not random like a shaky tripod.

30 sec exposure of the Pleiades showing "horseshoe" errors

I concluded that there were two main issues:

The first that there was something wrong with the stepper motor, which could account for problems (1) & (3). I was already suspicious that it was not turning consistently. I also had tried tweaking the speedfactor setting on the Photon to fix issue (3), and had varied it by up to 20% before the star field stopped drifting (and even then it wasn't completely consistent). I had difficulty believing that my calculations for the code could be this much wrong. I decided there was something broken or slipping in the internal gear train so I replaced it (£10 for a pack of 5 on eBay!). It turned out that there are a number of variants of this ubiquitous 28BYJ-48 motor, and my original was one of the unusual ones, with a different gear ratio. This went some way to explaining some of the problems I'd had.

Secondly I decided that issue (2) was down to the play in the gimbal axes. I'd assumed that the quality hinges would prevent any lateral movement of the plates, but this appeared to be what I was seeing, driven by some oscillation in the gimbals. I'd crudely made them of a steel rod (actually a sawn-off nail!), so I replaced these with screws that fitted into the gimbal blocks tightly but still allowed free movement.

These two changes improved things significantly, with most of the issues disappearing.

30 sec exp. Pleiades: no tracking and tracked

 

120 sec Orion Nebula

Stars still don't render as perfect points, so more investigation needed. The 120 sec image of the Orion Nebula is encouraging. Testing is painfully slow because of the scarcity of clear nights.

Notes on Imaging Sessions

I estimate my night sky in Leamington Spa is a Bortle  class 7.

I record images directly onto my Linux computer via a 5m USB cable, using gphoto2 to control the camera. It means that, once I'm set up, I can stay inside in the warm :)