The Siamese Twins Galaxies--an experiment in integration times in Bortle 8/9 skies

 

The Siamese Twins Galaxies (NGC 4567 and NGC 4568) are merging galaxies with a band of strong star formation where they overlap. They are a difficult target for the RASA 8/Mallincam DS10C combination because they have a very small angular diameter in the scope’s FOV.  The astrometric images show a number of interesting galaxies and other objects.

I decided that these objects provided an opportunity to examine the effects of increasing exposure times on the quality of an image under Bortle 8/9 skies. Unfortunately, cloudy weather prevented me from completing a full series of images, and by the time skies cleared, the imaged field was behind the roofline of the house by the time skies became dark enough to image.

The imaging system was: RASA 8, DS10C, Optolong L-Pro filter. Imaging time was 7 seconds per integration at NINA Gain 1000. No binning. Short integration times with zero to low gain are essential under bright skies. Longer integrations than 7 seconds result in the histogram peak moving off the graph to the right. The 3 image sets I managed to capture had total integration times of 12 minutes, 42 minutes, and 102 minutes. The comparisons below show differences in selected, heavily cropped objects in the frame at different integration times. All images were captured in NINA, and processed identically in Siril, with noise reduction in Topaz and background extraction in Astroflat.

NGC 4567/8 12 min

NGC 4567/8 42 min

NGC 4567/8 102 min

The images show improvement in color saturation and balance, as well as fine detail in the galaxies (limited by the resolution). The difference between the 42 minute integration and the 102 minute integration is surprisingly small.

The differences for M 58, also in the field, are more apparent.

M 58 12 min

M 58 42 min

M58 102 min

The difference in detail is significant in this object and it could probably have benefitted from an even longer integration time. At 12 min integration, it is a featureless blob. At 102 min, detail is emerging in the spiral arms of the galaxy.

Regarding how deep exposure time takes the image in terms of magniturde, there is surprisingly little difference between the 12 minute integration and the 102 minute integration in terms of astrometry.

Here are the two astrometric images.

Astrometry for 12 min integration

Astrometry for 102 min integration

The fields are inverted and not precisely aligned, but the number of faint objects identified appears to be very much the same in each image. I suspect this similarity is due to the limiting magnitude of the bright skies under which I took these images.

The takeaway is that, in terms of capturing fine detail, longer total integration time is essential, even in very bright skies. Fainter objects do not seem to benefit as much as the brighter objects, but that is to be expected under these conditions. The limiting magnitude in both images seems to be about 16-17. I have managed to reach mag 21 on moonless nights under clear skies with a 40 min integration.