M20 "Trifid" Raises questions?

[LAW (Murphy)]

Hey everyone, I'm pretty happy with my results from M20 but as I'm trying to make sure I get the most out of my imaging sessions I've been reviewing the data and I'm confused. The 600 second exposure is showing less detail than the 300 second frame. I'm tempted to think that this is just a signal/noise ratio which will be equalised with stacking, but I'm also worried that I'm wasting my time with 10 minute frames when I could be stacking more (more useful) 5 minute frames?

I'm using a modded 450D (not known for their low noise ) which operates around 22-25 degrees, so I'm obviously pushing the limits at 10 minutes but I thought there would be a noticeable difference between the two exposures?

[alistairsam]

Hi Murphy,

Were the skies completely clear when you took these exposures?
the only time a 10min would have less than the 5 would be if there's a layer of thin clouds. i've seen it heaps of times.

as for ideal exposure length, yes, stacking more of shorter subs will help cause you would reject fewer subs and even if you do, their contribution to the signal would be small, so you lose less.
other major benefit with shorter subs is reducing light pollution.
i've found 4 minutes to be ideal even with an IDAS-LPS-P2 light pollution filter which is actually quite good.
8 or 10min subs look orange similar to yours. whereas 4 min subs are decent.
but if you use processing software light PI or startools, they can remove the light pollution very easily.

For CCD's, the ideal way is to calculate exposure length for your place based on the sky background level.
so take a 3 min exposure, measure the background ADU and use this site http://starizona.com/acb/ccd/calc_ideal.aspx.
there is also a formula you could use if you know certain parameters of your camera.

so best to do a few controlled tests. take 3,4,5 all the way to 10min exposures in 1 min increments, and flick through them yourself. the point at which there isn't much difference from the previous is where you'd stop. atleast this is what I'd do.
what is the focal ratio of your scope?


cheers
Alistair

[tlgerdes (Trevor)]

As Alistair said, where were you imaging from, if city based, light pollution could be swamping your signal. Also is your viewer doing some sort of display stretch with the image to display on your screen?

[rcheshire (Rowland)]

Working in Brissy the last 5 weeks, the sky is noticably brighter than Geelong, for instance. Lower iso, say 400 and 2.5 - 3 mins is probably ball park. As Alistair and Trevor say, more and less is best under polluted skies.

[RobF (Rob)]

Hi Murphy

A few comments, pretty much as others have said:

* Not much point shooting 10mins subs from suburbia - your signal to noise is limited by background light pollution - you really need to get away from Brissy if you're going to "let rip" with anything more than 5 min subs
* The 450D is quite capable to giving a nice result - here's an example from dark skies with an 8" Newt: M20
* Don't forget you need lots of subs to help cope with the noise, probably 2 to 3 times as many from the 'burbs vs dark sky
* Even then you won't get the faint blue regions if they're only as bright as your background sky....
* An MPCC would be a valuable addition and probably an IDAS filter too
* How are you focusing? Do you have a Bhatinov mask? You want it razor sharp with good collimation.
* 450D - remember in winter you will have noticeably less noise than summer - all the better to make the most of the bright nebs in the milky way! (Edit: I see you're cooled already!)

Its getting there!

Rob

[LAW (Murphy)]

Thanks guys, I didn't even think of the 'background' light pollution! I guess I'll be limited to 5 minutes in my backyard, still, I'm happy with that. I've got an MPCC in the mail I can't wait to be able to use the WHOLE sensor, and I do have a Bahtinov mask but I was lazy last night, I just used BEOS and the moon to get the focus close before moving to fainter targets.

I gave it another stack without the 10 minute frames and it seems to have taken the colour cast off the image without taking too much detail away.

[LewisM]

When I used to use DSLR's, the max sub was 5 minute at ISO600. Never varied from that, otherwise got FAR too much light pollution.