Image ISO and exposure for M13

[bluemaxroe]

Hi, I am looking at the weather on Saturday and thinking of trying to image m13.

What I would like to ask is: what is the best exposure length for an f7.5 ed80 scope.

The camera is a canon 1000d, I have a choice of modified or unmodified.

I will be using a 2" ed Barlow and a CLS clip filter as well.

Any advice welcomed. I am not sure but with my current setup image length is not really an issue. I should be able to achieve 10 min +

Thanks in advance, I wouldn't normally ask but I find myself under the stars so little now, that when I am I don't really want to be playing with exposure lengths before I start the main set of exposures.....and I wouldn't mind doing some darks before hand before it gets dark enough for the lights.

[Marky1973]

Hi there - firstly I am still learning, so please take this as the ramblings of an amateur, but I was playing with M13 last night and suspect we might need to be taking sets of subs at different exposure lengths to ensure we can capture the outer stars (longer exposures) and also resolve detail in the core without blowing that out.

This is a stack of 12 x 5 minute subs, with a very quick histogram stretch in Photoshop

As you can see the core is blown out already - this is heavily cropped mind - taken with an ED70 and a Canon 1100d with a 2" skywatcher LPF

When I get another chance I am probably going to try some shorter subs - maybe 3 minutes and even under 60 seconds to see what detail I can resolve in the core. It is always going to be difficult because of the amount of cropping required but longer is not always the best option when looking at brighter objects.

But, as I say, I'm no expert, so I am sure someone will come along to give you a better insight! 

Good luck!

[wxsatuser]

Use the histogram to give you an idea of the correct exposure.

Basically you want the histogram peak somewhere in the region of 20 to 40%.

I normally go for around 25%.......like the image below.

[lensman57]

Hi, I am looking at the weather on Saturday and thinking of trying to image m13.

What I would like to ask is: what is the best exposure length for an f7.5 ed80 scope.

The camera is a canon 1000d, I have a choice of modified or unmodified.

I will be using a 2" ed Barlow and a CLS clip filter as well.

Any advice welcomed. I am not sure but with my current setup image length is not really an issue. I should be able to achieve 10 min +

Thanks in advance, I wouldn't normally ask but I find myself under the stars so little now, that when I am I don't really want to be playing with exposure lengths before I start the main set of exposures.....and I wouldn't mind doing some darks before hand before it gets dark enough for the lights.

Lets start from the end of your post. Clip filter is fine, but ED Barlow? What is this in aid of? You are already going to use a scope on the slow side for AP ( @F7.5 ) and you don't need to make it any worse. It looks as if you are still in the daytime snapping mode. Ditch the Barlow and give the subs atleast 300s exposure. The use of a Modded camera for star field photography is not strictly necessary . The modded camera has increased sensitivity only to Ha wavebands and these are mostly in emission nebulae so there is no real advantage to using it in my opinion. With enough subs, about 30 or more you should be able to capture the outer stars as well as the dense core.

Set the ISO to either 400 or 800.

A.G

[brrttpaul]

Jumping on the bandwagon how do you manage if you havnt got live histogram? i.e nikon camera

[Tinker1947]

Jumping on the bandwagon how do you manage if you havnt got live histogram? i.e nikon camera

My D5200 has a histogram........

[wxsatuser]

Jumping on the bandwagon how do you manage if you havnt got live histogram? i.e nikon camera

I would have thought you have the histogram somewhere.

I don't have a Nikon so can't help.

[brrttpaul]

I have a histogram but am I right in thinking Canons have live histogram? pretty sure Nikons dont have live histograms. (before you have taken the shot) , maybe I dreamt it somewhere but sure i read it could be wrong though

[wxsatuser]

My 60Da has a live histogram in Liveview.

When in Liveview cycle the info button and it eventually appears.

Personally I would'nt use that to judge an exposure.

Depending on the sky conditions I would be looking for a minimum of 300sec subs or more.

You can use less if you want but still get the histogram upto at least 20%.

Here at my obsy when it's dark and the streetlights are on I know that at f/4 and ISO 1600 on any

of my lenses or scope I will get a histogram peak around 25%.

When the lights go out the histogram peak drops so the exposure can be increased to bring it back to 25 %.

When you first start imaging with a dslr, experiment, use the first exposures of the night to get the histogram to at least 25%

Once you find the exposure length for 25% start your imaging run.

Do this whatever your sky conditions, wether you have LP or not or if you have an LP filter or not.

BTW......it's not a figure set in stone so don't be to ocd about it, just get the histo somewhere between 20 to 40%