Imaging open star clusters

[OzDave]

Having spent a fair amount of time imaging deep sky objects, I thought I might try an open star cluster for a change. If I want to make an LRGB image (possibly including some narrowband data too), how long should my subs be?

Take for example the double cluster. I would have thought that you wouldn't need long subs like for a deep sky object. But I wonder how short to go without running into noise issues etc. Would 5 minutes be ok for example (for broadband filters at least)? I guess narrowband would need longer.

How long would people recommend for a target like the double cluster which is just mainly stars?

In case it matters, I'd be imaging at 500mm or 1000mm FL with my QSI 683.

David

[kirkster501]

A good question and I look forward to the answer too; been pondering the same. I have heard you don't need luminance for Globular clusters. I wonder if that's the case for open clusters too?

[OzDave]

I got bored waiting for someone to reply so I went ahead and tried 10 minutes subs for RGB. This produced me some nice data for M13. I'm trying again on the Owl Cluster (NGC 457) tonight.

[ollypenrice]

Here at last! I wouldn't bother with luminace for clusters without background nebulosity. All it does is blow the stellar cores and reduce colour saturation. So if it really is just stars then RGB only and a couple of minutes per sub. You'll get stronger star colour that way.

However, when is a cluster just a cluster? Here's a simple example, the Coathanger. Nice things happening in the background...

Image credit shared with guest Rauno Paivinen.

But brace yourself, that pales into insignificance. Check out the Double Cluster by the incomparable Fabian Neyer...

http://www.starpoint...c884large1.html

Eek!!! When is a cluster not a cluster? You have to find out, I guess...

Olly

[OzDave]

Certainly an informative post Olly. Fabian's double cluster is exquisite! And the coat hanger is not bad either.

So I see what you're saying, quite often clusters have interesting background details that you want to pick up in addition to the nice star colours in the cluster itself. And I guess to do that, you have to know what is there and adjust your subs length accordingly. And I suppose in some case you'd want to shoot some short subs for star colour only and then longer ones to pick up the interesting background.

Out of interest, do you recall the capture details for the coat hanger? I'll have a look on Fabian's site for details about the double cluster, unless you happen to know?

Regards,

David

[JamesF]

Is it feasible (or even worthwhile) to mask the stars out of an LRGB image for the background nebulosity and then overlay RGB stars onto that?

James

[OzDave]

Is it feasible (or even worthwhile) to mask the stars out of an LRGB image for the background nebulosity and then overlay RGB stars onto that?

I expect it would be quite possible. It is quite a common technique when there is high dynamic range in the image, such the as core of M42. People take long subs for the bulk of the object and quite short subs to get a decent image of the core. Then the two sets of data are processed and overlaid and blended together to produce an image where the overall dynamic range has been effectively compressed. Quite a few tutorials on that floating about the net. I expect a similar technique would work with stars and probably be a lot easier too since the stars are typically easier to mask than a relatively diffuse area of nebulosity.

If you go the Fabian's site with the double cluster image, there is quite a good write up of how he put that together. Not really specific details, but if you read between the lines you can figure out how it happened.

[ollypenrice]

Is it feasible (or even worthwhile) to mask the stars out of an LRGB image for the background nebulosity and then overlay RGB stars onto that?

James

I do it the other way round, usuallly. I put the RGB stars underneath with the background adjusted to match the top layer and erase the LRGB cores. It can be better to do it the other way round in some cases.

The Coathanger just had 6X10 per colour and 4X10 luminance. It was a quickie. That's at F5 or, more importantly perhaps, at 3.5 arcsecs per pixel. More would have been nice.

Olly