M51, longish exposure

[narrowbandpaul]

hi guys.

yes 46000DN is 3/4...asumming full well corresponds to 65000

so 40000DN is nearer 2/3, but whats 6000DN between friends!

Strictly speaking, as long as your sensor is working in the shot noise regime, you dont require flats for FPN, Fixed Pattern Noise purposes anyway...vignetting yes.

The onset of this FPN is 1/P^2, where P is the photoresponse nonuniformity

Typically for a ccd we can expect 3% or less...ie P=0.03

so the onset is 1000DN, 2500DN for 2% and 10000DN for 1%

if your signal from your object is above this, for a given P you need flats to remove the fixed pattern noise.

But sensor exhibiting sub percent P and this happens for amateur cameras out on the market, the onset isnt until about 40000DN.

Vignetting is the biggest source of FPN, and could cause p=0.05, which is awful

Basically you need to take flats in the FPN regime, so that your flats contain the info you want removed.

This can be an issue for shooting images in bright skies...the bright skies enter the FPN regime.

Narrowband imaging is less concerned with flats since signal levels are usually low.

But flats are a great idea, and I would still suggest taking them at 2/3-3/4 full well, just incase your FPN is very low (which is a good thing)

These high signals are seen when lunar/planetary imaging, so flats here are essential, and lots of em

But why remove FPN.

this noise scales linearly with signal, and hence the SNR is constant in this regime. So if your images are in the FPN regime then longer exposures dont yield higher SNR.

Infact Max SNR is 1/P

we need good flats, at 2/3-3/4 full well, and about 20 of them, or more, to remove...

1)FPN, typically 1-2%

2)Vignetting, could cause FPN to be nearer 5%

3)Dust bunnies

Thats my beliefs for flat fielding

Hope that helps

[RichieJarvis]

WOW...

That is an amazing image Olly - so deep, and so colourful....

Well done

PoW I say!

Richie

[philj]

Thats a stunner Olly. I am interested to see just what the Meade 127 can do, cogs are grinding in teh old thinkbox now:-)

Phil

[Taff]

Hi Olly' What a nice capture great details there.Mark

[George]

Thats a great M51 Olly detail is stunning

[ollypenrice]

Thanks to all for the kindness and helpful advice. I've acted on it where possible and a final (are they ever final?) version is here...

www.flickr.com/photos/lesgranges/3318706912/sizes/1/

Thanks again,

Olly.

[mcrossley]

That link doesn't seem to work Olly, do you mean this one?

http://www.flickr.com/photos/lesgranges/3318996374/

That looks just about perfect to me.

Mark

[ollypenrice]

Sorry Mark and thanks. Yes, that's it.

Olly.

[ngc2403]

Hey, Olly ur right it does seem to stop quickly. i think a good selection of flats would improve the image though at least it can make it worse so you might as well if you have the time give it a shot.

there is a fainter halo all round the galaxy that can be seen in the really long images by Gendler and the like, it might be noisy in ur image but, with the flats you know that you will be getting everything you can from the image you have.

i would say that it might even be better than taking another hour or so of data.

Anyway did you enjoy the rugby? it was a very good game thought i think Wales are still the best side this year

ally

[ngc2403]

am sorry i was told today that you are not infact french ,

anyway keep the images coming in cos they are lovely to see

[ollypenrice]

No, not at all French and, alas, at a loss to understand Rugby! There must be easier ways to butcher your ears!!

Olly