Impressions on the SBIG STX-16803

Updated: Nov 10, 2010

The camera packed in a solid pelican case is a very fine machined piece in anodized dark red and black. The use of Maxim DL as aquisition software requires an update to the version 5.07 and a driver update with the new SBIG 64 bit driver checker. Also a firmware update check of the camera is recommendable (driver checker). After connecting the camera to the PC it is cooperating with Maxim without any problem (universal SBIG driver). The size of the camera is similar to the STL series, but no filter wheel is included within the body and it is heavier (2.2 kg). The STX filter wheel adds another 2.4 kg with filters installed, so the camera-filter wheel combo is at 4.6 kg. As connection to the camera a threaded 70 mm adapter flange is foreseen, I exchanged it against my own 100 mm dovetail system, bolted with 4 screws to the camera body. The mechanical shutter operates between 3600 and 0.1 sec, no electronic shutter is possible for full frame CCD chips in opposite to the STL-11000. However the shutter is photometric, even at short exposures it evenly illuminates the whole area of the chip in opposite to the faster iris type shutters. To the right you see the generously sized CCD window with following components: 1: main imaging CCD KAF-16803, 2: focus adjustable pick-off prism with guide chip 3: heated cover glass (invisible) 4: textile shutter with window - operating horizontally At the connections side (USB, LAN, AUX, autoguider, remote guider, supply) behind the black cap there is the adjustment screw of the focus position for the guide chip available. (track focus)
The STX-16803 uses a 37 x 37 mm full frame antiblooming chip with microlenses. The quantum efficiency peaks at 550 nm with 60% and drops off to 46% at 656 nm (H-alpha). See also the angle depending quantum efficiency curve here. Even very fast systems at f/2.8 stay within the 10 degree light angle and so do not loose efficiency. The primary resolution of 4096 x 4096 pixel can be used in 1x1, 2x2, 3x3 and 4x4 binning mode to shorten readout time for previewing. However I always recommend the 1x1 binning mode for all frames. Readout is done without interrupting the guiding within 6 sec, the actual download takes 2 seconds in full resolution. The original power supply cable with extension from the AC adapter is quite flexible even in cold environment, I use a second custom cable set for direct 12 V field supply.
The power consumption of the camera is acceptable low without active cooling: approx. 1.8 A, peaking at 2.0 A during the STX filter wheel change. The dependance of the power consumption (current drain at 12V supply voltage) from cooling output and the cooling performance is shown in the graph at right. At 4 amps the cooling capacity is approx. 43 degree below ambient, at maximum cooling the current uptake is 8 amps. The cooling fan operates at fixed medium speed. At high cooling rates >80% the fan speeds up and gets quite noisy. A continuous regulation or setting of the cooling fan speed is missing so far. In comparison to the STL design the distance from the imaging chip to the chamber cover glass has been increased, this is a big advantage for avoiding icing on the cover glass. I never experienced ice on the chamber glass even at 100% humidity. Due to the effective camera design the cooling reaches 52 degree C below ambient at 100% output, at 80% output 50 degree C below ambient is possible. Lowest cooling setpoint is -35 degree C.
The down side from the increased distance between chip and chamber window is the big backfocus distance of 35 mm for the camera itself, the wheel adds another optical 29 mm for a total 64 mm from the flange of the STX filter wheel to the chip. Therefore the use of Canon or Nikon lenses is not possible when using any filter wheel. To the right side you see a Canon EF lens attached to the STX body directly using a custom EF bajonet, focus is reached. Tip: To have the same relative orientation of the guide chip to the main chip and to camera body as in the STL camera a 90 degree clockwise rotation in the camera setup options must be set.
The original STX filter wheel accepts 5 square 65x65mm filters that are held down by a thin baffle plate that should be tightened just gently. The wheel connects to the camera AUX connector with a 9 pin plug. The internal positioning of the wheel is done by an optical sensor and is very accurate. To the right side you see the open wheel with the Baader LRGB-Ha filter set installed. Alternatively a 7-position 50mm square filter carousel for the same wheel is available at SBIG. This is for people who do not intend to use the built-in self-guiding CCD and want a full compliment of LRGBHaOS filters.

Light test with the STX-16803/STX wheel/Baader filters

A 1000x1000 cropped 30 min dark frame at -25C setpoint in full resolution as zipped fits file you find:
here (953 kB)

Flat field frame Vignetting is not an issue with the generous size of the filter wheel housing 65x65 mm square filters. To the right you see a linear scaled flat using the 16 " cassegrain at f/10.
Background evaluation To evaluate the background for long exposures a 30 min dark frame at -25C is examined. A 100% size jpg version can be seen to the right. Numerous cosmics are hitting the chip in random pattern, so it is very essential to do careful data reject procedures (i.e. sigma reject) on mimimum 3 raw images per channel to eliminate these artefacts.
Dark noise evaluation To check the characteristics of the dark noise development over time a series dark exposures have been taken, each of them master bias subtracted, the analyzed in Maxim. The table at right shows the average values and the standard deviation (noise) both in ADA and e- for the full frame at 1x1 after a simple hot pixel filtering.
Bias frames A cropped bias frame at -25C is shown at right with the statistics of the whole frame. The bias pattern is very even, but still bias/dark subtraction helps to minimize noise to the lowest possible value.
Baader LRGB filters To produce 65x65 mm filters in high quality can be a challenge. The Baader LRGB filter design and characteristics you can see here. Highest optical quality of the 3 mm glass substrate, parfocal design, durable coating, resistant against scratches and humidity are the key issues of these filters. The typical weighting factors for RGB combination using the STX are R:G:B = 0.8:1:1.2. An important issue is the complete absence of halos from reflection around bright stars. To check the quality of the filters I did a brute exposure of 60 seconds at alpha-UMA (mag 1.8) for each RGB channel and combined the channels accordingly. The stretched result can be seen to the right side.
Same target at 60 sec exposure for the Baader L filter accordingly. The stretched result can be seen to the right side. Baader 65 mm LRGB and NB filter sets are available from Baader or from SBIG directly.
My first deep sky test shot at stormy conditions with this camera targeted at NGC 1977 at f/10 with the 16": just one single 10 min exposure for each LRGB channel combined - the result at right is proving the sensitivity of this camera.
A real life deep sky example is showing a crop on the bright foreground star SAO 10956 at mag 5.6 exposed at an exposure of 4h for RGB and 4h for H-alpha. Again the remaining irregularities are caused by the special shape of the secondary mirror cell. The absence of star halos can be confirmed for the whole area of these filters.

Conclusion

Pro
* State of the art large format, low noise, high QE CCD camera
* Excellent mechanical and electronical design, exact CCD plane
* Excellent stabilized cooling, no dew on chamber window
* Maxim DL 5 used as very reliable aquisition software
* High quality Baader filter sets available

Con
* heavy and bulky - demands very stable focuser and mount
* textile shutter with minimum exposure 0.1 sec - no sky flats possible
* high backfocus distance, for STX including wheel no camera lenses can be used
* no 8 pos 65 mm filter wheel available so far (Nov 2010)

Deep Sky references with the STX-16803 / Baader Filters

	h and Chi Persei Double Cluster (NGC 869/884) (Nov 04, 2010)
	Center of Cederblad 214 (SH2-171) (Oct 07, 2010)
	Details in the Butterfly nebula (IC1318) (Sep 13, 2010)
	Galaxy Pair in Leo (M 65, M66) (Apr 10, 2010)

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