Trifid (M20) and Lagoon (M8) Nebulae

[troypiggo]

Another attempt at 7x8min frames, but due to guiding problems only able to use 4 of them. Very frustrating.

Again, any tips appreciated.

http://piggo.com/%7Etroy/photos/2009/2009_04_25/trifid_lagoon.jpg

[Nighthound]

Still a beautiful image Troy. Lots of detail.

I'm curious, why 8 minute subs?

[troypiggo]

Thanks very much Steve. I was pushing the subs out to 8 mins because I'd read that for my camera, uncooled etc, that was about the max people were doing and I was under the impression that the longer the sub, the better to catch the faint stuff. I know you can just stack more short subs, but I also read that for really faint stuff that might be only emitting a few photons the longer the shutter speed the more chance of getting them. Think that was what the argument was. You reckon that's not as important?

I was shooting with some guys that were using DSI II CCD cameras. There were shooting galaxies and only using 30 sec and 1min subs but stacking heaps of them and having to process the hell to get good detail in the faint arms of the spirals. I told them if they lengthen their exposures they'll get better results because of the above logic. They didn't believe me, but one of them tried just 2 mins instead of 1 and I heard him go "wow". He said it was much easier to process with some of the longer subs. I dunno.

[Celestron]

Very nice Troy !

[troypiggo]

Thanks Ron :)

[A.S.I.G.N. Observatory]

Great widefield framed well Troy. Focus looks a little soft to me (looking at the stars). Yes, you are right about the exposure limits on the DSLR. Here in Canberra, it is about 5 minutes before the LP starts to wash out. In Sydney it is worse. 8 minutes must mean you have a pretty dark site no?

There is a bit of vignetting on the corners which should iron out with some flats. Colour is spot on mate.

[troypiggo]

Thanks Baz. I think the focus in the middle should be pretty good - used a Bahtinov mask. Pretty sure those softer stars around the outside are due to my scope's optical quality.

I was at a "new" dark site that I was invited to. Only about an hour out of Brisbane. Much closer than Leyburn where I went last time - about a 3hr drive there. So this site saves me around 4 hours driving. Significant if it's just an overnighter. I could probably even just drive home and not stay over if I wanted.

[Nighthound]

Thanks very much Steve. I was pushing the subs out to 8 mins because I'd read that for my camera, uncooled etc, that was about the max people were doing and I was under the impression that the longer the sub, the better to catch the faint stuff. I know you can just stack more short subs, but I also read that for really faint stuff that might be only emitting a few photons the longer the shutter speed the more chance of getting them. Think that was what the argument was. You reckon that's not as important?

I was shooting with some guys that were using DSI II CCD cameras. There were shooting galaxies and only using 30 sec and 1min subs but stacking heaps of them and having to process the hell to get good detail in the faint arms of the spirals. I told them if they lengthen their exposures they'll get better results because of the above logic. They didn't believe me, but one of them tried just 2 mins instead of 1 and I heard him go "wow". He said it was much easier to process with some of the longer subs. I dunno.

Well in theory yes, longer is better. Most definitely dark skies are a key factor in that theory. But also sensor sensitivity. With an astro-dedicated CCD there's a huge advantage from both low noise and sensitivity. With a DSLR there's a compromise in both aspects so that has to be considered. I find that at 5-6 minutes(especially in warm months) that noise becomes almost as much a negative as light pollution or sky glow. The combination of all of the above starts to minimize useful data per subexposure. I don't shoot dark frames(at some point I will) so the increased noise alone will require additional post-process removal which will compromise quality of my final image. I would venture to say that the difference in depth per sub between a 5 and 8 minute exposure with a DSLR would not be all that dramatic. TOTAL time is ultimately what is going to get you those details, and yes 30 second subs will never get you there since those short subs simply don't contain that data. Take a look at my Heart Nebula in my gallery, that's 5 hours total time shooting 4 minute subs. I know it doesn't compare to the images taken with an astro CCD, but for an unmodified DSLR that cost $600 and wasn't designed for this application, it's fairly deep.

My suggesting of shorter subs is twofold. One, to keep periodic or alignment error from trashing many of your subs and two, to keep your data quality at an optimal level for your mount, camera and sky conditions. If you can go 6-8 minutes and achieve that, that's great. I'd say go for what makes the entire process less of a pain that way the hobby remains fun, which is job one.

[troypiggo]

Thanks Steve. I see you points and will shorten those exposures down.

I didn't realise you don't take darks! Wow. Brave on the post processing front.

[Nighthound]

You're very welcome. I'd say if you did this well with 8 minute subs then 5 minutes each will definitely up your number of keepers. There's so much to get just right in this that you have to keep stacking(no pun intended) things in your favor any chance you get.

No darks and flats have resulted from me being ok with the results I get without them. I know to do this right I'll have to get around to playing by the rules eventually. :D

[chris.bailey]

Darks can add to the noise, especially if you dont take enough of them so if you can get away without them there is an argument not to. I don't but then my CCD has very low dark noise anyway.

[MidnightSun]

Ah yes, two of my favorites...well done. I'd have to agree with Nighthound about exp. length. I've found that 5-6 min. (un modded 350) is about all I can get without the noise or any ambient light starting to creep in. I have pretty dark skies with a slight light dome to the East from a town 20 miles away, and after 5-6 min. the subs tend to start getting a "reddish" tint.

[troypiggo]

Thanks for the tips guys. Hopefully you'll start seeing the fruits of your wonderful free advice in the coming months :)