Performance of AstroTrac TT320X-AG

It’s been a busy year. I bought this at NEAF back in April and have only just now been able to get out and give it a spin. The new feature, of course, is the autoguiding. With the original TT320, the specifications state a periodic error limit of 5 arcseconds per 5 minutes. As it works out, when using my Borg 45ED lens, the pixels in my Canon 350D are about 5 arcseconds wide. What I’ve seen is that I can, with great care, get exposures up to 4 minutes long, but some of them invariably show trailing and have to be thrown out. Getting good 1 minute exposures is easy, and getting good 2 minute exposures takes only a little work during setup. Beyond that takes a lot of work, and anything over 4 minutes is pretty much guaranteed to fail. Yes, I hear stories and testimonials for people whose gear works better than that, but I consider that just being lucky and getting a unit that performs better than the specification. Mine performs at the limit, but no better.

Well, the bad news is that it still takes a lot of work to get the mount properly polar aligned. But use of the AstroTrac pier and head have made it possible to get things aligned quite well, even if it doesn take time. The good news is that once aligned, the autoguider does it’s work quite well and I have a "successful" 10-minute exposure of the region near Eta Cygnii that shows round stars. That never happened with the original unit.

I called the test "successful" in quotes because of some limitations of my location. I’m in Brooklyn where the sky is never really dark, and I had a waning gibbous moon to contend with. Toss in a weather front that was slowly increasing the humidity during the evening so that by the time I ran the test, the moon was hazy looking and there was a lot of sky glow and it’s hard to do anything. To cut down on the effects of sky glow, I used a Lumicon H-alpha filter (which is really a deep-red filter) which allowed me to expose continuously for the 10 minutes. The picture is worthless except for showing round stars. Since that’s all I really needed to show, I consider this a success.