NGC 6405 (M6) - the butterfly cluster in Scorpius

I used telescope T13 at Siding Spring to take this 240 second image of NGC 6405 the Butterfly Cluster.

 North is up and East is to the left.

I solved the original fits file - this is the plate solution

The image has a plate scale of 3.67 arcseconds per pixel and is aligned to within 5 minutes of North. The sky coverage is just over 1.5 degrees by 1 degree 13 minutes. The cluster is 33 minutes square so it is slightly bigger than the full Moon.

 From Burnham he describes the cluster as being "some 5 degrees north of the tail of Scorpius and about 3 1/2  degrees northwest of the similar cluster M7"  

The bright orange star is the semi-regular variable BM Scorpii. I looked this up in the AAVSO VSX catalogue:

 

The last observation of this star logged at the AAVSO was on the 30th September 2014 and was given a Visual magnitude of 6.3

 I generated a light curve on the AAVSO site by requesting 1000 days of observations be plotted noting from Burnham that it had a long period of about 850 days. 

 I find it difficult to see a regular variation and realise why the AAVSO table above did not specify a value for the period.

Back to the cluster itself it is believed to be at a distance of 400 to 450 parsecs according to Burnham but the WEBDA catalogue gives a more up to date value of 687 parsecs. The diameter of the cluster is about 25 light years.