Subs with a DSLR .. quantity or exposure ?

[+Graham]

This one has been bugging me for a while now using my 350 D.

 

Is it better to have 30 x 2 min subs per hour or 12 x 5 min subs per hour.

 

With the shorter subs you benefit from less noise and less guiding errors.

Short subs give you more subs with which you can stack out errors.

 

The longer subs give a greater chance to gather those elusive photons.

 

So good people your opinions and ideas on this subject are called for.

 

Quantity or Exposure.

[Brantuk]

I'm no imaging expert but from what I've read and heard around I think it's a good idea to have a combination of both. It depends on the target but using M42 as as an example, I imagine shorter subs would catch the trapezium and miss a lot of the nebulosiity, and the long subs would grab the nebulosity but over expose the trapezium. Then use software to combine and layer the bits you want to keep......

 

I think that's the principle used to get the core of M31 and the dust lanes into one picture - it's also how they get Jupiter and it's moons into a picture - but do check with someone like Pete Shah or Olly Penrice for better details of how to do it.

[+Graham]

Hi Kim.

Your points on the likes of M42 and M31 are totally valid and is in fact the normal method used for a very bright target.

I think I need to clarify my question.

I am asking the question in direct relation to Nebula's or faint Galaxies.

[glider]

Short subs = lower signal, limited noise.

 

Averaging inproves signal to nosie ratio by averaging down the noise.

 

Long subs = more signal, more nosie.

 

Averaging 'more' subs can get the noise back down to the level of short subs, with more signal.

 

Hence better signal to noise.

 

With skies in the UK, unless you are at a dark site then the reality is that the dreaded orange glow probably limits you to 5 to 10min subs and is by far the largest source of 'noise' in any case.

 

IMHO.

[+Graham]

Glider.

Point taken with the signal to noise ratio.

The trouble is with these short nights now upon us time is a limiting factor.

With only 3 to 4 hours of darkness is it better to spend the limited time shooting lots of shorter subs or fewer longer subs.

My point being by shooting fewer longer subs I am not going to get enough of them to bring the signal to noise ratio back down to an acceptable level, especially with the weather being what it is.

[andyboy1970]

The more subs you can get the better.

Sub length is constrained by several factors (DSLR), ISO, LP, ambient Temperature and brightness/dimmness of the object.

On these short nights it may be worth upping the ISO and shortening the sub length so you can snap more of them.

Thats my take on it.

[Antares_Paul]

I have finally settled for 600s for nebula and 300s for galaxies. ISO 800 dark sky 400 with Luna.

With 90 minutes of sky time I will go 9x600s or 18x300s.

 

With 360 minutes of sky time, I will shoot four targets 

 

I simply don't have the patience to put hours into one image and I guess my thing is deep sky 'snapshots'.

I've also decided a  CCD camera will not be purchased as a DSLR will suffice for my needs.

 

I'm not one for the easy route but I think you have to set a limit both time-wise and financially. I don't have abundant amounts of either ...

 

Regards.

[Perkil8r]

More subs wins no matter what length.

[+Graham]

More subs wins no matter what length.

 

Thank you all.

Mike I was hoping you would impart with an opinion on this as you use the same camera.

The 350 D is a brilliant astro camera but it does suffer badly from noise on long subs.

I have come to the conclusion my first test run should be at 120 seconds on ISO 800.

That is if these pesky clouds ever go away.

[Guest ollypenrice]

The situation with an uncooled camera is complicated. With a cooled camera it isn't. WIth a cooled camera long exposures blow short ones out of the water if you have the sky quality not to flood the sensor with light pollution. You do have to avoid blowing out the cores of stars and the cores of galaxies but this danger is grossly exaggerated. I routinely use 15 minute subs at F5 and 30 minute subs at F6.8 with cooled CCD. A soft stretch of these deep subs usually reveals that little has been burned out.

 

However, without cooling the camera's noise rises as the chip temperature rises and this limits you. Talking to DSLR users I get the feeling that the max is atound 8 minutes. Some improvise a bit of cooling using frozen physio gel packs etc.

 

I'm fairly confident that, under a half way decent sky, fewer 5 min subs will beat more 2 min subs. I don't image with a DSLR but I talk to those who do and this is my feeling. However, DSLRs burn out star colour very quickly so a set of shorts just for star colour might be an idea. Don't throw them all into DSS, though. Combine them carefully in Ps.

 

Olly

[Perkil8r]

I've not yet encountered really bad noise. My longest so far (in any qty) are 5 mins and with correct darks and suitable flats I haven't encountered too many issues. But (there always has to be a but...) I have never been happy with any image I have done that has had less than 60 subs.

 

And to add to what Olly has said, I never mix different sub lengths in DSS, I process all subs in groups and save the TIFF as such, then merge the different lengths in PS to get the desired effect.

Edited May 21, 2013 by Perkil8r

[+Graham]

Olly

Totally agree with the cooled bit. I can run any length of sub I want with the Atik 314 L,

 

My poor old Canon 350 D is another kettle of fish altogether.

 

Here I have to agree with Mike with regards the number of subs required. I always aim for a minimum of 100 subs so after weeding out the bad ones due to helicopters, aircraft and satellites I still have a decent number to stack. If not it gets impossible to develop a decent photo in PS.

 

I think an experiment is called for. 3 hours of 2 min subs vs 3 hours of 5 min subs.

 

That's if we ever get 2 clear nights to do it.

[Perkil8r]

I have managed to bring out more detail with shorter subs but more of them than with longer subs and the same combined time in the past. The more subs the smoother the image so it will stand far more pulling about in my limited experience.

[andyboy1970]

My best DSLR shots have been with 3 minute subs at ISO400.

As Mike said stack different length subs separately and combine in PS.