M45, The Pleiades - What Can You Do With My Data?

[russellhq]

I'm kinda new to DSO imaging and had a go at M45 last night.

5 Darks, 5 flats and 5 lights @ ISO800 and 180s exposure.

I'm finding the processing a little tricky and not sure if it's just me or the data. Here is my first attempt at the data and I've attached the TIF if someone would like to have a try and compare.

[russellhq]

Now with link:

https://docs.google....YVF0RXN3VVpJbTg

Apologies for the size, it's 70MB!

[AndyUK]

Hi Russell - It's actually looking very good so far... I'd say you just need some more data so that you can stretch it to bring the nebulosity out more. It IS most definitely there, but as you stretch it to try and bring it out, so the noise starts to creep in. If you had (say) 20-30 x 180s, that would help significantly.

(Your flats though - They may not be totally working as you can see there's some vignetting in the two bottom corners...?)

[russellhq]

Thanks Andy. I cheated with this flats, using some from a previous session where the camera may not have been in the same place. I'll take some more tonight and see if that helps the corners.

[Jessun]

That's a very nice capture. I have never been able to get anything like yours - I take my hat off!

There's a lot in the image to drag out, and more subs would certainly bring noise down so you can stretch it a bit more.

I have no useful data of this beauty of my own so I enjoyed just giving yours a quick go. It's tricky as it's tempting to let it all go overboard in blue and here I think I went down that path a little bit, but the data is there for anyone with more skills!

[GlassWalker]

Here's my attempt at processing it. The faint stuff is rather close to the noise floor and I've never been great at separating them...

[russellhq]

Thanks all for having a go!

Glasswalker, when you say it's near the noise floor, does that mean I just need more subs to bring the signal away from the noise? Or should I lower the exposure time?

[GlassWalker]

More subs and/or longer subs should help.

[Ags]

Lowering the exposure time will bring the signal nearer the floor! Taking more subs will increase the gap.

[russellhq]

I was concerned the light pollution would start to take hold of the image if I went for longer exposures. Is there any way to tell if the light pollution is too much for a given exposure time? I could take test exposure but I wouldn't know what I was looking for.

[Ags]

Look at the histogram on the camera - you need to keep the peak of the histogram to the left. I'm getting a CLS filter to reduce the light pollution in my area.

Here is a thread showing the difference it can make:

[Ags]

About taking longer exposures.... let's say you decide to take 6000 1-second exposures. That won't work very well! So you decide to take 1 6000-second exposure... that doesn't work very well either. The best solution lies somewhere between the extremes... perhaps 20 300-second exposures might work, getting enough signal per exposure while getting enough exposures to reduce the noise. Judging the right number of exposures comes with experience, which is where I'm lacking :-)

[russellhq]

I use one of these filters to control LP:

http://www.firstlightoptics.com/light-pollution-reduction/baader-neodymium-filter.html

Are you saying, as long as the histogram stays away from the right hand side I should be OK?

[Ags]

In my limited experience, yes. I try to keep the histogram to no more than one third of the graph.

[AndyUK]

Are you saying, as long as the histogram stays away from the right hand side I should be OK?

Exactly that - Yes. When taking DSLR images, I used to try and get the RGB Histograms between 1/3 from the left and the centre of the graph. If the LP is poor and your exposures are fairly long, if the histogram peaks are towards the right (or even tight against it), then you'll need to reduce the exposures (or maybe decrease the ISO if using a DSLR - ISO 800 is usually considered optimum). However, as you have that Neodymium filter, you should be able to get away with much longer exposures - Again, my "default" exposures when using the DSLR (with an Astronomik EOS clip filter) were 600s, but I'd not suggest using such long exposures for M45, and to be honest I thought that 600s was really at the noise limit of my DSLR (the chip got REALLY hot in the summer (30C+) and the digital background noise was atrocious!)

Arguably you can NEVER have enough data (subs), but you can get to a point of diminishing returns, although in my experience this point depends on the object you're imaging - If it's a dark "dusty" nebuIa, then I would probably go for 8-10 hours + (but that's just me). However, my self-imposed guideline (with the DSLR), was to aim to get a minimum of between 20-30 subs of c. 300-600s

As an aside, processing of this object can be a bit tricky as if you're not careful, whilst you may get plenty of nebulosity, the stars can totally oversaturate, but you certainly need to capture enough photons (data / signal) to be able to stretch the stack to bring out that nebulosity whilst minimising the noise...

[Cloudwatcher]

Here's a quick lunchtime tweak.

You may think it's a stretch too far (thinks Xmas card) but does show that there is a lot of 'blue stuff ' waiting to be pulled out.

[russellhq]

Thanks cloudwatcher, I really like the blues in the image although as you say it may be a stretch too far, the background stars are looking over processed.

As others have said, more data needed!