Calculating Best Exposure time?

Hey all,
I have a general question to anyone really if any one’s up for it.

So hypothetically, I have a sequence ready for execution tonight of object ‘X’ and I’m going to take 30 Lights.

How do you go about working out what the best exposure time will be for your Subs ???

So I have one theory although it is an uneducated one, for a given exposure time, Take a ‘frame and focus’ single image and stick your cursor over the brightest star in the image and make sure it is close to saturation but not saturated ??

At the same time, ensure that if light pollution is present in the image then it is not too bright as to render the subs useless. How would you recognise when light pollution IS too bright meaning you will have to lower your exposure time to compensate.

So what’s your recipe for determining the optimum exposure time for your subs using your setup ???

I’m using an F4 Newt and a QHY8L

Regards
Paul

Depends on what you want to capture. If it’s nice star colours etc. then you need short exposures. But if you want good signal on nebulae, you need loooong ones using narrowband filter. Typically I try out a 600 sec exposure on a nebula and see if the signal gets seperated well from the left side on a chromatogram. In the end I do an HDR combination with short exposed stars.

I generally judge my exposure time by using the background ADU measure in image stats…I try to get below 700 adu…

Ray

Vincent,
Narrow band is not an option for me as im on OSC, nebula’s and galaxies are my main goal but other things are always a target. So on a bright star like the big bright blue’ish one at the foot of M42, will it loose colour in the core once it passes full well or does it usually begin to happen before that ?? HDR combination is certainly an interesting tool in the box, obviously a must for objects like M42 due to the real bright part at the center of the nebula.

libraryman2,
That’s an interesting theory for sure, if you were to be imaging a faint object I assume you would break your rule though ??

Cheers
Paul

Yes, you loose colour on stars very fast, they simply fill the well very fast. M42 is challenging to actually get very nice and a typical target for HDR. Not sure how many people us it, but if possible I always take a series of 30 second exposures or so for the star-field, and maybe some exposures in between to get the bright core resolved, then another longer one for the faint parts.

Paul,
You mention that narrow band is not an option for your one shot color set up. I have had great success using Astronomik’s 12nm Ha, OIII and Si EOS clip on filters with a modified Cannon DSLR camera. Typically using 600 sec exposures for the filters with about 3/4 of that for the one color shot RGB.
Jimmy