Wednesday, 15 October 2014

M6 "Butterfly Cluster" shot with ZWO ASI120MC on Celestron 130 mm Netwonian

This is the M6 "Butterfly Cluster" :

This is a 5-second exposure taken last night (14 October) with my ZWO ASI120MC astro-camera mounted on my 130 mm Newtonian telescope. The field of view is about 30 arc-minutes x 20 arc-minutes. (That is, the full Moon would just about fit in the long dimension of the image, but would more than cover the short dimension.)

The brightest star is V862 Scorpii at magnitude 6.76 (the slightly yellowish white star just below the centre of the image). The faintest visible stars are about Magnitude 11.5 - not bad for a 5 second exposure! The colours are all pretty accurate, when I compare with standard planetarium software.

Here is a B&W negative image, which can make it easier to pick out the fine detail:

One nice thing about shooting stars digitally (rather than on film as in the “bad old days”): 
For any given set of exposure settings, on a digital image there is a really nice linear relationship between the diameter of the star image and the apparent magnitude of the star. (When shooting time exposures on film, the relationship between the image diameter and the actual brightness of the star is much more complex.) This graph shows the relationship for a selected sample of stars on this particular image (bearing in mind I was only measuring the apparent diameters to about 0.5 mm accuracy when printed at A3 size). 

This means that with a few stars of known magnitude in frame, you can estimate the magnitude of all the other stars. I'll have to try this on a few variable stars and see if I can measure their variability, and do some “real science”!