Another attempt at the Pleiades

[MarkS]

This is one of my favorite astro-photography targets.  I think last night's version is my best version of it so far.

40x5min at ISO 800
H-alpha modified Canon 350D on Tak Epsilon 180ED with IR/UV filter.



Larger version (2/3 scaled) here:
http://www.markshelley.co.uk/Astronomy/2011/pleiades27112011.jpg

I think there is probably more faint stuff that can be pulled out of this but I didn't want to sacrifice the faint wispy bits of nebulosity.  I might give it another process.

Mark

[Rocket Pooch]

Hi Mark,

Good image, but where's the brown and outer nebula?

Chris

[MarkS]

Good image, but where's the brown and outer nebula?

I'm afraid they're swamped by background gradients and noise from the light pollution.  But there is just a hint of them if an extreme stretch is applied:



To image them properly would require a dark site or just possibly using a light pollution filter.

Mark

[Tony G]

Mark,

I think your processing skills have taken a turn for the worse, and in the last image, you have burnt out the centre of those stars......what has come over you???????????

Tony G

[Carole]

Beautiful Mark, and all those diffraction spikes.

with IR/UV filter

So are you saying having removed your IR filter to modify the camera, and have now put an IR filter back for this particular image?

Carole

[MarkS]

So are you saying having removed your IR filter to modify the camera, and have now put an IR filter back for this particular image?

Yes, I replaced the Canon internal filter with glass so I must use an external IR/UV filter instead, for all imaging.

[MarkS]

I'm afraid they're swamped by background gradients and noise from the light pollution.  But there is just a hint of them if an extreme stretch is applied:

Continuing the discussion, here's some further data.

[Start of Technical Waffle]
If I add all 40 subs I get a total background count of 24000 electrons per pixel (it is important to do these calcs in electrons and not ADUs).  Of this, approx 800 electrons is dark current given that I was using a 350D at an ambient temperature of 0C.  The standard deviation of the background noise is therefopre the square root of this i.e. approx 155 electrons (making the usual assumption of a Poisson distribution).

The very faint wispy bits measure (which are just beginning to appear in the stretched image) have an electron count of approx 220 electrons.

This means of signal-to-noise ratio of the wispy bits is 230/155 = 1.48  once the background is subtracted.  This is why the wispy bits hardly appear above the noise.

Suppose I use a CLS filter instead.  On previous experience this will reduce the background level by a factor of 10 which sounds quite impressive.  But this only reduces the standard deviation of the background noise by a factor of 3 (the square root of 10).

But since the wispy bits have a broadband spectrum and the CLS filter cuts out half the spectrum, the signal level of the wispy bits will be reduced by a factor of 2.

So adding a CLS filter will only improve the S/N ratio by a factor of 3/2 = 1.5  In other words, the CLS filter makes very little difference to S/N ratio when imaging a broadband object, at least in this neck of the woods.   However, it is very good at reducing background gradients - this is the main reason I tend to use a CLS filter - it makes the processing a lot easier.

Another thing that would help is to use a mono camera (with luminance filter) - it will receive 3 times as many electrons per pixel since it sums R, G and B.  So it receives 3x as much signal and 3x as much background.  The net effect of this is an improvement in S/N ratio of the square root of 3 i.e. 1.7

On the other hand, imaging at a dark site could reduce the background sky flux by a factor of 2, 4 or maybe 8 and this would directly improve the S/N ratio by a factor of 2, 4 or 8
[End of Technical Waffle]

So the main conclusion of all this waffle is that if you really want to image the secondary nebulosity and brown dust of Pleiades, you really need to be using a mono camera at a dark site.  Also, to prevent dark current becoming the main limitation, the mono camera must be cooled.

The other conclusion is that using a CLS filter makes very little difference to imaging broadband objects except that it substantially reduces background gradients.  It should be noted, however, that it is very effective for narrowband emission objects.

[doug]

     I agree with you, Mark.  The Pleaides is also one of my favorite star clusters...... and I think the diffraction spikes give the image an etherial quality.

     Doug.

[RobertM]

That's a lovely image Mark as always.  Your data holds so much more information - are you going to reprocess ?

I'm wondering about those corner gradients and why they didn't calibrate out.  It seems to me that without them you could display more of the background nebulosity and maybe some 'brown stuff' too.

Robert

[mickw]

Very nice Mark

[MarkS]

Your data holds so much more information - are you going to reprocess ?

Here we are then:


Larger version here:
http://www.markshelley.co.uk/Astronomy/2011/pleiades27112011v2.jpg

Mark

[RobertM]

That's much better, it's looks the business now

[Fay]

Beautiful M45 Mark. as ever

[Rocket Pooch]

Much better Mark.

[julian]

That looks much better Mark