Exploring a ToupTek Astro AE676M monochrome multi-purpose astronomy camera. Part 1a: Fainter objects.

The reader is encouraged to re-read Part 1 before readin Part 1a as some changes were made, to Part 1 post publication.

Further to the serious caviat about fixed noise in previews of captured images of the bright object M17 with 60s exposures; this test involved imaging a fainter object using 3 minute exposures.

One and a half hours of total exposure were captured by AstroDMx Capture of NGC7822, the central part of the Question mark nebula; half an hour's worth of 3 minute exposures through each of H-alpha, O3 and S2 Altair 2" 7nm narrowband filters, plus calibration frames using an Aurora electroluminescent flat panel for the flats.

The data were stacked and calibrated in PixInsight and further processed in PixInsight, GraXpert, SetiAstroSuitePro and GIMP3.

Click on an image to get a closer view

On the face of it, a reasonable image was constructed in the Hubble palette

NGC7822 Hubble palette

However as in Part 1, the imaging experience was not a pleasant one. The previews of the images were dominated by large amounts of fixed noise which completely obscured fainter parts of the nebulosity. The brighter parts of the nebulosity could be seen through the Ha filter but nothing of significance could be seen in the previews through the O3 and S2 filters, even though there was information present, as calibration and stacking later showed. This is unlike anything we have seen with other cameras, cooled or uncooled. To be clear, this fixed noise is not created by any preview transform, it is present in the raw FITS data.

To illustrate this point we used ASI DeepStack to stack quickly the Ha data and the S2 data with and without dark-frame correction. The resulting 16 bit stacked images were only stretched, 20% Bg 3 sigma in GraXpert.

Stack without dark-frame correction

Stack with dark-frame correction

Just dark-frame correction made a lot of difference removing most if not all of the fixed noise.

The problem is illustrated by the following two animations:

Animation blinking between an uncalibrated stack of the Ha data and the dark-frame corrected stack

The uncorrected stack shows some Ha nebulosity but some of it is obscred by the fixed noise.

Animation blinking between an uncalibrated stack of the S2 data and the dark-frame corrected stack

The uncorrected stack shows virtually no S2 nebulosity while some faint nebulosity is revealed in the corrected stack.

The argument here is not that it it is impossible to obtain images of faint nebulosity. The point being made is that the imaging experience is atypical and very poor with the live view showing little if anything in the fainter S2 and O3 data.

We shall look further at other data that we captured in our tests, but will certainly be contacting Touptek for their comments and feedback.