ZWO ASI2600mm-pro review

This is just a brief placeholder review.  Gary Walker loaned me his ASI2600mm-pro camera, but it has a non-functional sensor temperature readout and so can't regulate the sensor temperature.  He is returning this camera for repair, but will be sending me a second ASI2600 for testing.  When I get that one, I'll do a more extensive test.

The ZWO ASI2600mm-pro uses the same IMX571 sensor as does the QHY268M.  The differences are how the unit is packaged, and what parameters are available for the user to control.  The 2600 has a very similar look and feel as their earlier 183/1600/etc. cameras - red anodizing, cylindrical (90mm diameter), with a 2-port USB2 hub for accessories.  It uses USB3 to communicate with the host computer, and requires a 12V 3A power supply (not supplied) for the TEC.  Like with the QHY camera, there is a tilt ring on front to make sure the sensor is perpendicular to the optic axis, and dew heater resistors near the entrance window.   The back focus is 17.5mm, including the tilt ring.  A female M42x0.75 thread on the faceplate attaches the camera to the telescope or filter wheel.

The driver only gives access to one of the multitude of readout modes for this sensor (QHY gives 4).  However, the mode ZWO enables appears to be the same as the "High Gain Mode 1" of QHY.  It enables selection of gain and offset for that mode.  This readout mode is what I prefer in any case.  For largest dynamic range, setting the gain to 0 and an offset in the 10 range should be chosen.  Like on the QHY268M, there is a changeover from the low-gain amplifier to a high-gain amplifier, in this case around a gain setting of 100.  I only tested this camera at a gain of 0.

At a gain of 0 and offset 10, the system yields 0.77e-/ADU of system gain, and a read noise of 2.80e-.

When binning, QHY sums the native pixels to give the superpixel value.  Since pixels are limited to 16-bit size, this means that each native pixel of a 2x2 bin on the QHY would be limited to a count of 16383 before saturating the superpixel.  This means that is a 1x1 bias level is 500, a 2x2 bias level is 2000.  For ZWO, native pixels are (I think) put in 12bit readout mode, summed, and then scaled to fit the 0-65535 range.  This means that if a 1x1 bias level is 500, a 2x2 bias level is also 500.  This preserves the full well depth of each native pixel, at the loss of some dynamic range.  It may be that the 2x2 mode is done in true 16bit mode and then divided by 4.  The documentation is not clear, but I should be able to figure this out before the updated post.  A better solution for both QHY and ZWO is to image in 1x1 mode and then sum images after the fact in floating point before storage.  However, this complicates the acquisition step, so there is no simple solution.

Given that, binning the ASI2600 2x2 yields system gain of 3.08e-/ADU and read noise of 6.63e-.  The gain is exactly 4x the 1x1 value.  However, the system noise should be sqrt(4) = 2x the 1x1 value, or 5.60e-.  Instead, I measure a read noise of 6.63e- in 2x2 binning mode.  This is still very good (and less than almost all CCD read noise values), but larger than theory.  I need to study this difference!

I'll expand this post when the replacement camera is tested.  I'll also do a sky test with the replacement camera.  In general, the ZWO ASI2600mm-pro appears to be a good one to consider when upgrading from a smaller CCD setup.

Arne