Category Archives: Astronomy

Problems with long exposures

Following the previous post I’ve been asked (a) “Why 30 second exposures?” and (b) “Why a stack of five?”. Good questions. The answers are (a) because after that the tracking on my mount was starting to show star trails and (b) to try to enhance the signal to noise (NB I should also point out that I removed a dark frame from each image too – this mainly takes out the hot pixels and general noise).

In future experiments I will be endeavouring to improve the tracking of the equatorial mount, before perhaps resorting to some form of auto-guidance to feed back tracking corrections. Today’s post is to give an illustration of the single shot imagery for different exposure durations with the setup that I had on Saturday. The following five images (NB without dark frame removal – spot the hot pixels) are single exposures at five different exposure times to illustrate the problems.

124 seconds
JDC_300D_2011_0226_212714 124 seconds

63 seconds
JDC_300D_2011_0226_212031 63 seconds

30 seconds
JDC_300D_2011_0226_211901 30 seconds

15 seconds
JDC_300D_2011_0226_203608 15 seconds

8 seconds
JDC_300D_2011_0226_203242 8 seconds