Advice on imaging star clusters

[Padraic M]

Hi all, with lots of great star clusters coming around at this time of year, and not having imaged them before, is there any advice on what factors and techniques are important with clusters? 

I'm assuming that the key items to capture with clusters are:
- good sharp star shapes
- faithful star colour
- some way to make the cluster stars stand out against the general star background (highlighting the 'shape' of the cluster)

I've seen examples here and on AB that use long exposure times, just as you would with nebulae or galaxies, but this doesn't necessarily make sense to me as you can expose stars with short exposures, and many clusters don't have surrounding nebulosity or fine detail that requires a deep image.

I've included a sample taken over the last few nights of the Beehive M44. What strategy would be best to bring the cluster out better?
That was captured using an Esprit 80, ASI1600mm @ -20c, gain 139, offset 21, and 60-second exposures (40x for each of RGB), and processed in APP.

Some thoughts:
A lot of the cluster stars are saturated. I believe that this means that the main core of the star is white, and the actual star colour is only visible in the non-saturated halo. This implies that I should use either a shorter exposure time, or a lower gain - maybe even down to gain 0. Is there any science that says one approach is better than the other?
Many of the background stars seem to have a yellow core with a cyan or green halo. Does this imply that my overall colour balance is out?
If I use shorter exposure or lower gain, some of the background stars are likely to disappear. Maybe that's a good thing? Should I try layering two sets of images (or two different stretches) to have a primary layer of cluster stars, with a secondary layer of starfield stars?

 

 

[rnobleeddy]

Interested if you make any progress with this. I've been mainly imaging the tighter globular clusters for short periods between other targets, or at the end of the night, to see what works.

So far I seem to prefer stacks with lots of shorter exposures, primarily because it's easier to avoid overexposure. 

Time is always a scant resource, so I've tended to stick to collecting less data on clusters because they look OK with less data, but I may be doing them a disservice.

 

[EntropyTango]

The image looks very good to me

Any background star colours you are unhappy with, could simply be due to your refractor/flattener

rather than any imaging or processing issues. But really there are so many factors to consider.

Maybe a good comparison could be if you try imaging through an RC telescope as

this optical design should produce a flat and colour free image.

A direct comparison should show if it's your refractor or something else.

I have started using an RC6 for my imaging and found it free of false colours

Maybe you have a friend nearby who will let you borrow theirs and try it out