SBIG STX Beta Report

And in the red corner, weighing in at just over 10 lbs, is SBIG’s 16803 based CCD camera, the STX.

Figure 1

It is a good looking camera, and easy to manage, thanks to the two handles on the rear of the unit that sit on either side of the single fan (not three or four, as in other cameras). The STX also comes fitted with an Ethernet interface (not tested) and the typical autoguider port, a USB 2.0 and additional connections for accessories such as the CFW and the future AO. There is also a port for a new Remote Guide Head (RGH). It is unfortunate that the current RGH unit cannot be used on the STX, and this is because SBIG is using a better quality Kodak chip in the next iteration, so the architecture is different. The filter wheel is a sizeable beast, which houses 5 x 3mm x 65mm filters (Baader filters were used during the test period). The filters have to be this size in order to cover both the imaging and guiding CCDs. However, SBIG supplies an insert for the filter carousel that will allow customers to use the 50mm variants. By the way, with my optical system the G2V weights on these filters worked out to be 0.84:1:1.25.

I have always been very, very particular (and concerned) about how filter wheels attach to a camera body, being ever mindful about tilt, especially where fast focal ratios are concerned. No dovetails are used here, the filter wheel is bolted to the camera body in four places, and I could not insert a 0.08mm feeler gauge anywhere. During the test period, I took several hours of data on different objects; and, for the most part, I enjoyed round, sharp stars right to the edge of the FOV. However, the Takahashi FSQ106 is renowned for both its huge image circle and flat field. What I did notice, however, is that stars were slightly elongated on the guide chip. But you have to consider just how far this is off-axis given the size of the imaging CCD: add the weight of the camera and a bit of drawtube sag, and users might find issues there. The good news, though, is that I was able to use the clever little ‘Tracker Focus’ adjustment and, by doing so, those elongated stars tightened up nicely. Additionally, what helped in my situation is the combination of the FSQ 4” drawtube and a custom screw-fit adapter, which meant that my drawtube was only extended by a few mm. Guiding was near-perfect most of the time, but then again, I was imaging at 3.5 arsecs/pix on a Paramount ME.

Figure 2

Now, I have had a few filter wheels in my time, but SBIG has achieved something quite unique in my experience with this implementation. The method by which you install the filters is simple, very quick and extremely practical. Having placed the filters in their respective positions, SBIG now uses a thin aluminum plate that acts as a clamp. You simply place it on top of the filter carousel, screw it down in several places and the filters are fixed into place. Delrin washers, 2mm thick, are supplied -- which are placed on top of the filters, effectively acting as padding and a buffer between the clamp and the actual filters themselves. The filters were installed in just a couple of minutes. No more messing with tiny screws and washers.

On power up, the filter wheel homes using an optical sensor and from ambient, the camera cooled to -20C in two minutes, which was regulated very well indeed. Here in Omaha, Nebraska, I have been operating the camera in temperatures as low as 23 degrees Fahrenheit all night long, and have consistently used the camera at -25C (for convenience). Consequently, the camera cooler power has been cruising along at around 20%.

Driver installation is a little tricky, and users will have to carefully review the installation instructions. The new 64bit/32bit combined DriverChecker worked on both a VISTA 64bit box and an XP Pro 32bit box with no problems at all. However, on my observatory PC, I do not have native USB2.0 architecture, and use a USB2.0 PCI card. The drivers would not install on this box, so I had to revert to a pure 32bit version. This meant that I could not access the STX Firmware tab, a feature of the new 64bit driver. At the time of writing, this was still unresolved, but I fully expect it will be so before production units start to ship.

Figure 3

I characterized the camera for read noise, gain and Mean Dark Current using AIP4WIN2, but SBIG has made many improvements to the production models that invalidate the results this camera produced. Image download is, of course, very fast with USB2.0, and the camera has two download modes (fast and normal). There is a short delay as the camera digitizes the data before the download commences, which, given the 32Mb raw file size, takes just a few seconds. As previously mentioned, I do not have native USB 2.0 on my observatory PC, so I would fully expect download times to be even faster on those motherboards that do. The aforementioned delay only applies to a full-frame unbinned image, and the delay is obviously less when using subframes and/or binned images. This was common across CCDOPS, Maxim DL and CCDSOFT.

Residual image? What can I say? I have not found the need to pre-flash the CCD. I took a 30-second image through a luminance filter on a 70% moon, immediately followed by a 30-second dark frame, and there was nothing to be seen. I followed up with a 15-minute image on M31, again through a Luminance filter, and then took a one-minute dark frame – nothing could be seen. The preflash feature works with all programs (CCDOps, CCDSoft and Maxim) as it's a feature of the firmware in the camera. The only thing you cannot do is enable/disable it with CCDSoft or Maxim. When you configure it in CCDOps, the setting is retained in the STX Flash memory and remains effective until changed by the user.

Figure 4

At the time of writing, there was no update available for ‘The Sky 6’ to reflect an STX/Dual CCD FOVI (although, one could manually create his own) and the differential (artificial star) guiding was not available in the beta test unit. This was a real pity because I have been experiencing guiding issues clearly caused by differential flexure in my current set-up and it would have been a great test.

Conclusion

The filter wheel assembly, including installation of the actual filters, is a big plus for this camera. The STX also cools really quickly and does not appear to have any issues operating in very cold temperatures. SBIG has maintained the dual-ccd ability in this camera, which is great, but be aware of how far the chip is off-axis and how you can potentially resolve it with the ‘Tracker Focus’ mechanism. Residual image lag is supposed to be an issue with the 16803 chip, but I could not see any evidence of this. Overall, I think it is an excellent camera and I draw this conclusion from experienced use of more than one 16803-based camera. If you opt to buy Astrodon filters, the camera, CFW and filter package, it is going to cost you over $16,000 (I could not find the cost of the Baader filters).t

To view the full size image and additional images taken with the STX when they become available, visit Martin’s website:
http://www.martinpughastrophotography.id.au/STXBeta.htm