Open cluster NGC 6603

[Martin Meredith]

If you have a scope/camera combination with a relatively small FOV and type M24 into your goto controller, chances are you'll see something like the left hand image here (or indeed the right):

Messier 24 is around 2 square degrees so for my 0.45 x 0.35 degree FOV I'm only seeing a small part of it, and it is fortunate that the M24 goto in my case lands pretty much on the open cluster NGC 6603, seen here, located in the direction of the galactic centre within the Small Sagittarius Star Cloud (ie M24).

NGC 6603 contains mainly mag 14 and fainter stars. A 1998 study by Sagar & Griffiths analysed just over 3500 stars in the region and found around 54% members and a further 13% probable members, so 2000-2400 stars are likely to make up this grouping, the bulk of them with apparent mags 17-20. The same study suggests an age of half a billion years. The bright star is BD-18 4895, a mag 7.4 M2 type with a colour index of about 2.

Why two images? It turns out I looked at this almost exactly 2 years ago. I've observed a lot of OCs in the intervening time but this one stood out for its diagonal line of slightly-brighter stars.

I reloaded the older version and adjusted it to match the new one as closely as I could. They're remarkably consistent. For instance, the subtle pale yellow, peach and orange colours at the top end of the star diagonal are preserved in the two images. I did a crude blink comparison but couldn't spot any interlopers -- unfortunately, Pluto is at the other end of Sagittarius.

The main difference I think is slightly worse collimation in 2021 than 2019... The older image was taken with 5s subs while the newer one used 15s subs. What I find quite interesting is that very little colour data is required for the moderately-bright open clusters. The 2021 image on the left has a single 15s red filter and 45s in each of G and B (these were not deliberate mismatches but came from randomly-shuffling the subs to match the overall exposure in 2019). The only effect of using different exposures in the colour channels is a different quantity of colour noise in each, and for sufficiently bright subjects this isn't really of much concern, making OCs ideal subjects for EEVA-style observation.

cheers

Martin

 

[Mike JW]

Always enjoyable to see your cluster shots and the info.  Mine are never worth posting being in mono. I have checked to see if there are any doubles in this cluster - none shown in my star charts software.

Mike