Some short exposures on a static tripod

[Demonperformer]

Having been unable to get the mount outside for imaging since the beginning of this year due to injury, I decided to end my drought a couple of nights ago by doing some test shots on a static tripod with my Canon DSLR + nifty-fifty. My garden is surrounded by (white) street lights and I decided to push matters still further by imaging an area near to the celestial equator (as that is where the stars move fastest), so headed towards Altair and imaged the area below (south) of it (Altair is the brightest star centre/top). The ISO was 800 and the lens was stopped down to f2.8.

I selected 9 exposure times (1, 2, 4, 6, 8, 10, 15, 20 & 30s) and took six images at each exposure. The results of each exposure were registered in DSS & then stacked, the final image also being registered. The aim of the exercise was to determine the "optimal" exposure time with that setup in my circumstances. The images were only recorded as JPEGs rather than RAW for reasons of size and as it was only a comparison test.

The results listed below give: sublength; #stars in the individual subs; FWHM in the individual subs; #stars in the stacked image

1s; 88-104; 3.11-3.14; 514
2s; 197-208; 3.16-3.23; 920
4s; 375-394; 3.47-3.54; 838
6s; 539-563; 3.70-3.78; 716
8s; 590-615; 4.00-4.07; 680
10s; 538-558; 4.22-4.28; 488
15s; 125-149; 3.75-3.98; 127 ... this showed major trailing in the stacked image so the longer subs are not included in the results.

Apart from the general rule-of-thumb that smaller numbers are better, I don't really understand the detail of FWHM sufficiently for those figures to mean a lot to me, but include them in case they mean something to others. Unsurprisingly, as exposure time increased, so did the FWHM figure - until the end. This coincided with the number of stars crashing, so I guess some sort of tipping-point was reached with trailing so that it rejected the worst examples as not being stars and so the remaining ones were slightly better?

What interested me was that the number of stars in the stacked images peaked at 2s and then began to decline, despite the number of stars in the individual subs continuing to rise to peak at 8s. I'm guessing this is some interaction between the increasing trailing & light-pollution kicking-in at different points of the increasing length of the subs.

I'm attaching the 6*2s image and have done a very quick magnitude check, finding quite a few stars below magnitude 10.0, the faintest so far is TYC 1062-2509-1, listed in CdC as 10.46.

Obviously, this is a tiny sample and much bigger tests would be needed to come to any definitive answers, but I share it for what it is worth. If nothing else, it demonstrates that with very basic equipment, one can experience the (all too common) scientific situation of experimentation posing more questions than it answers!

Thanks.

Edited September 12, 2022 by Demonperformer