ISO, an alternative for longer exposures?

[Perkil8r]

I've just about reached the limit of improvement in detail on M42 with my set up, or at least as far as I can get with the camera set at ISO400. I don't have guiding and I know I am at the mounts very top limit as it's only an EQ5. Is increasing the ISO to say 800 or 1600 going to allow me to capture the fainter detail?

I suppose the question is, if I were to take a shot at 800 ISO of 30 secs, would this be effectively double ISO 400, so in effect a similar result to a 60 sec shot at 400?

At 30 secs I am just starting to get a tiny bit of star trails, not much, but just a tiny bit. At the minute there is no way I am upgrading the mount as I really do need to start saving up for the drop off in work at the end of this year (happens every year) and also for our holiday. Plus it seems silly to be upgrading if I can achieve more with what I have.

[Guest Eddy]

Definitely go for 800 ISO and if the camera is good at handling noise 1600 shouldn't be an issue but I don't think any higher would be beneficial but I could be wrong

[Perkil8r]

The camera seems very good on the noise front. I'm at just over an hours worth of data at 400, but now it seems to have reached a point where it's not getting any better, hence the thinking behind longer exp vs hight ISO. Is it equivelant to double the exp if I double the ISO? The only reason I ask is, I have 2.5 sec, 5 sec, 10 sec 10 of each, then 24 x 20 secs, and 90 x 30 secs. I'd like to go up in 10 sec steps or equivelant of. So if 800 is the same as twic the exp of 400, then I could do some at 20 secs, then 25 and 30, then shift up to 1600.

[Guest Eddy]

Yep as far as I'm aware it's essentially doubling the exposure, if I were you I'd go for 800 first as I think that's what everyone else uses

[Perkil8r]

I will do, I'm determined to prove to myself that I can do better without upgrading yet. lol

[glider]

I'm afraid that the doubling ISO on a DSLR doesn't double your exposure.

All it does is turn the gain of an amplifier up, which worse case adds more noise in the final image the higher it goes.

If you think about it, changing ISO makes no difference to how many photons are hitting the chip and it makes no difference to how many electrons the chip makes per photon.

It just stretches what you do have out over a wider range in the subsequent electronics and hence image.

Now on wet film changing ISO really did mean changing to a film which was more sensitive to light, so for the same expsoure time it did get darker. Unfortunately you can't change the chip on you DSLR so easily as a roll of film.

What I do is start at ISO1600 and shoot say 5min. Then if that gives me a histogram that is right the way across the screen I'd go to ISO800 and shoot just under 10min. Now I really have doubled my exposure time, twice as much light is hitting the chip, but without saturating the image, and on my camera ISO800 is quiet noise wise.

At least that's how I see it!

[glider]

That's the technical bit, having said all of that what your eye will see if you do what you suggest is a brighter image which I think is what you were originally asking.

[Perkil8r]

Will it improve my image? That's what I'm looking for. A way to bring out more detail. I can't really expect my set up to allow much more exp time wise, it's already at the limit I think. Failing that, do I just continue adding more and more data, will this improve on the surrounding area? As you say, in film cameras altering the film ISO would have done what I'm looking for, I was sort of hoping that somehow maybe the chip was "de sensitised" for lower ISO's lol.

[glider]

No, it won't improve your image. It might even make it worse because of the amplifier noise issue.

Getting more subs and averaging them will lower your noise level and improve your signal to noise ratio.

Getting longer subs will increase your signal and hence improve your signal to noise level.

There is a diminishing returns involved with averaging, and a practical limit with longer subs (in your case your tracking, in mine my level of light pollution)

Think total time on target. A few long subs, or a lot of shorter ones. You just have to be sure that the shorter ones are at least long enough to catch the faintest parts of the object you are imaging.

[Perkil8r]

Hmm...... can't decide what to do now then. What would you suggest given what I have so far? (image is in "m42 progession" in DSO's)

[Guest Eddy]

Surely changing from ISO 400 to 800 will bring out more detail?

[glider]

To tell you the truth the first images look better than the very last one.

In the last one you may have stretched too far in an attempt to bring out detail that isn't there, and hence just brought the under lying noise out in the image.

I'd get more time. I go for at least 3 hours total time on a target before I can feel confident stretching the image without risking showing up the noise.

That said, M42 has a ridiculous dynamic range to it, I'm impressed you haven't blown out the trapezium in trying to get the faint whispy stuff, that's what I always seem to do.

[glider]

Afraid not, only more time on target gets more detail.

[Perkil8r]

I'll have a go at processing it differently, I get bags of detail but little colour, or plenty of colour but not as much detail depending on which way I go with it. Maybe I'm trying to make too much colour. Would added data help bring the colour out in the detail I already have? If you follow?

[glider]

Yes.

Are you using DSS? DSS tends to 'loose' colour unless you increase the saturation slider, I use about 13%.

[Perkil8r]

I am using DSS and have been using the saturation at around 20%. Photoshop I am not very good with I know that, but I'm certainly trying lol. I intend to try a different tactic of blowing the core right out with curves etc, then stitch a good core into it. I have a link somewhere for a tutorial on how to mask etc. I think that might be an alternative to what I'm attempting which is flawed by either blowing the trapezium or losing some more delicate detail.

[glider]

Have a look at the tutorials on Astronomy Shed forum.

If you're not a member it's worth joining, you may need a few posts to see all of the threads.

I'm a mod on there, PM Dion the owner to get a boost.

[andyboy1970]

I use ISO 1600 for frame and focus as it does produce a brighter image (and a bit more washed out) then drop it down to ISO800 for the real work.

ISO800 produces cleaner (less noisy) images.

When I had a 300d I found ISO1600 useless for imaging as it started to show lots of noise and amp glow.

[Perkil8r]

So we have a hung jury then lol.

I think the only way to see if it's the right thing for me or not is to try it and see. The 350D has a lot less noise than the 300D so it might well hold out ok. One thing's for sure though, doesn't look like I'll get chance tonight due to high cloud rolling in, so it could well be winter before I get chance to add anything more. I'm going to try and see what I can do with the processing. I've seen tutorials on how to stack layers in PS so I think I need to try that to bring out some more of the outer detail then patch the core back together with the shorter exposures. Certainly should be plenty to keep me busy for a while lol.

In the meantime I will get my alignment better if I can and try some longer exposures on other targets to see what I can squeeze out of the eq5. I'm determined to prove a point to myself lol. I suspect it will end up with me buying an eq6 later this year lol.