A quick Jellyfish nebula

[Daniel]

Hi All, here's a quick jellyfish nebula I got a few nights ago, only got an hour and 40 mins on it despite it being clear all night as I was experimenting with my 80ED that night and it looks as though my optics are out of alignment, getting huge amounts of coma on my images.

Anyway, this was taken once I switched back to the 120ED, as I said, it's 1.40 mins worth of 10 minute exposures taken at ISO 800 through an astronomik 13nm Ha filter, it's not great and definitely needs a re-look, but my first image on this, and looks promissing for an interesting nebula.

I've added a false colour version aswell

Daniel
:O)

[RobertM]

Very nice Daniel, focus is good and your camera is working really well for Ha.  Not sure but is that amp glow on the bottom right ?  This is definitely on my list of targets when it clears the trees later this year.

[Daniel]

There's quite a lot of nebulosity around that area, but that could very well be amp glow, since I was only able to do 4 darks for this image, that said, so far i've never noticed amp glow with the 40D.

[Mike]

Why are you doing 4 darks? Surely 1 at 10 mins would be sufficient or are you averaging them?

[MarkS]

Nice image Daniel - that's a difficult target.

If you think your optics are out of alignment, download the trial version of CCDInspector and run it on the images you have just made.  If it finds sufficient stars in the image it will give info about how far out you are.  Otherwise, take further images of a star filled area e.g. around Cassiopeia and use those.  Are you sure it's not just field curvature?

Mike, yes it is definitely best to average many darks because, strangely enough, subtracting a single dark actually adds noise to the image.

Here's why:
Light Frame = Light + Noise from light + Thermal background + Noise from thermal background
Dark Frame = Thermal background + Noise from thermal background

Subtracting dark frame from light frame gives: 
Light + Noise from light + Noise from thermal background (from light frame) + Noise from thermal background (from dark frame)
So the thermal background is subtracted but extra noise is added at the same time.

If you average many dark frames then you can minimise the "Noise from thermal background (from dark frame) " component

But since your own camera has a cooled CCD you don't need to worry about all this!

Mark

[Fay]

Daniel, that has been on my list since last year. I think it looks really good in mono.

Fay

[Mike]

...But since your own camera has a cooled CCD you don't need to worry about all this!...

Admittedly I rarely do darks (more out of laziness than anything else).

[Daniel]

I just read an article by Greg parker mentioning that Jellyfish works particulaly good in SII, may have to have a go at this. Next good night I get Im going to have anotheer crack at this, hopefully by then, I'll have the hyperstar too, that said, apparently using narrowband at F1.9 doesn't work too well, so may have to be a Colour image.

[MarkS]

...  apparently using narrowband at F1.9 doesn't work too well

Is that because the angle of the cone of light is too wide for the filter coatings to cope with?

[Daniel]

I think at f1.9 the light is actually shifted a few nm so by using narrowband filters your actually cutting out some of the band your trying to capture, apparently there are filters which are slightly wider bands that do work well at F1.9 but I don't fancy paying out for yet another set of filters just yet.