Lights, exposure and ISO

[John Gill]

Hi guys,

A couple of questions

Which is the better option:

100 lights at a fixed ISO and exposure or 100 lights at various ISO's and various exposure times.

Would doing 200 - 300 lights give better results than 100 lights. I have seen awesome images done in 20 lights and also in 200 lights.

Thanks

John

[StargeezerTim]

Hi John... I'm not an expert but have a few thoughts. To get the best results you need to take calibration frames, and they are taken to match the lights (Darks are same exposure and iso, Bias are same iso, Flats are same iso and taken at the same time as the lights). So you are talking about groups of lights that will be stacked separately. DSS can do this and it will create a master flat, bias and Dark for each set of Lights before combining them.

You can also create more than one image of the same object, eg. one that underexposes to avoid blowing out the core of a galaxy or nebula and another that overexposes to bring out detail in the fainter regions, them combine them using layers in Photoshop or Gimp etc.

So i suppose it depends on why you do it. I have used different iso and exposure on an object on different imaging sessions then used groups in the stacking process, but I'm not sure you can just group a whole lot of lights with different parameters together, or if it would improve the image in any way if you could. I'm sure others with more experience than me will contribute and I would be interested to hear what they say as well.

As far as numbers of subs used, I think it depends on the total imaging time. 6 * 10 mins is the same as 60 * 1 min, though you would expect different depth of detail and noise levels. I have also been amazed at how some get brilliant images with very short exposures. A lot depends on light pollution as well. I don't think there are many hard and fast 'rules' in this and it is good to experiment and find out what works in your environment with your equipment. Cheers,  Tim. 

[RikM]

Stick to one ISO setting and get as many subs as you can of as long an exposure as you can. The reason for more subs is to improve the signal to noise ratio but you eventually reach a point of diminishing returns. Eg 30 subs is a good number but you need to go up to 60 to see a noticeable improvement from that, then 120 to improve further but by then the differences will be subtle at best.

[ollypenrice]

Can we just check that by 'lights' you are meaning images of the sky? I think you are, and this is the correct use of the term, but sometimes beginners use the term 'lights' to mean flats. An innocent mistake.

With rare exceptions, and in my experience they really are rare, only one value for data aquisition is optimal. Mixing sub lengths and ISOs is usually a red herring.  Make your subs as long as your skyglow allows. The shallow well depth of a DSLR might benefit from a short set of fast subs to obtain colour for your stellar cores. I don't know because I'm not a DSLR user but I strongly suspect this will be the case. Combining sub lengths will require some processing skills but they are not terribly difficult to master.

Olly

[John Gill]

Thanks for the info guys, I have been doing about 50 multi ISO and 30/60 sec exposures, then DSS and a bit of photoshop and a bit of

Astronomy tools. The scope is a Celestron 8" EdgeHD with a AVX mount and a Canon 600D. I normally do some darks and bias but no flats.

I am trying to get my images going from 'nice' to 'wow' but without "Photoshopping Everything" or blowing the budget on a baby Hubble.

I will try just a single ISO @ 60 sec and see what comes out tomorrow evening.

Thanks again