Practical limits on exposure time?

[Hoges]

I'd setup last night to take 6 x 5 minute exposures on Centaurus A - I got one done and then the clouds rolled in. Looking at last night's photo made me wonder - more time = more signal = less noise. So, what practical limits are in place for DSLR (or astrocams) length of exposure. My Centaurus A photo is 330 seconds at iso 1600 and I can see the light pollution is starting to make it's presence felt there, so I feel I'm approaching the practical limit in this part of the sky. But, given a nice dark sky and if the autoguiding is good, why not 10 minutes? 15? 20? The one obvious thing that strikes me is that you wouldn't want to be 18 minutes into a 20 minute exposure and have Elon Musk's Sky Hoons turn up and party all over your exposure!

[Startrek]

Interesting topic
I believe there is always a limit and a point of diminishing returns due to our atmosphere and the physics and maths ( SNR ) involved in capturing and processing a decent image.
Have a look at this presentation by Dr Craig Stark ( inventor of PHD guiding ) which may help answer your question
Thanks for posting

https://m.youtube.com/watch?v=EO4QFb3ydNM

[Todo43]

Hi John,

This is a hotly debated topic. I believe you are using a DSLR? When I used a DSLR, I found that after about 5mins of exposure time, there was so much noise, it wasn't even worth processing the images. What you said about the dark skies, however, is an interesting point. You would have less light pollution, therefore more desired light would enter your sensor. Therefore allowing for a better SNR in your image.

Lachlan

[Startrek]

Here’s another excellent article on SNR

https://jonrista.com/the-astrophotographers-guide/astrophotography-basics/snr/

More time , more signal means more noise not less noise ( even under darker skies ) as noise comes in various forms ( not just from skyglow or Moon glare)

[JA]

Hi Hoges,

If you take a look at the exposure histogram for your image either on your DSLR whilst imaging or reviewing or even in Photoshop, like I did (see below), you will see that the exposure histogram is at about 17% say ~1/5 of full scale. That suggests that there is more than enough exposure latitude to increase the exposure in to the the approx 1/4, 1/3 possibly even up to 1/2 of full scale. Take a test image to verify that you are not clipping any highlights, but something in the 1/4 to 1/3 of full scale is usually a good target exposure, especially (depending on DSLR) at appropriate possibly lower ISO settings to help maximise dynamic range which can help retain star colour.

This is sometimes referred to as ETTR (expose to the right) in general photography to improve noise particularly in the shadows. The exposure even if brighter, can simply be adjusted via the blackpoint to suit. Of course as you already recognised longer exposures have disadvantages as they are challenged by clouds, planes, satellites, etc... and well as challenge the guiding/mount to be more perfect.

I like your image. You must be in a reasonably dark area to get that for 330 seconds.

Best
JA

[Hoges]

Many thanks guys. Some interesting info there. Craig made quite a few points I hadn't considered. I also came across this which made for an interesting read: https://www.galactic-hunter.com/post/short-vs-long-exposures-for-astrophotography-galactic-experiment-3 I noted the differences between the 60s exposures v the 15 minute exposures were not as dramatic as I had imagined. I also had not considered the % of the histogram as a measure - cheers JA. My camera (Pentax K50 DSLR) will most likely perform a little differently depending on temperature. It does it's own dark frame subtraction - if I take a 5 minute shot, I have to wait 3 min 50s extra for the noise reduction to do it's thing but it does seem quite effective. Certainly WAY better than the original Pentax 1stD I had several years ago. At least now I've got some autoguiding rigged up on the AZEQ6, I will be able to experiment with some longer exposures and see what results I get out of the Pentax. (which will eventually get replaced with a ZWO294MC or similar - my knees are gonna thank me for that!!)

[Hoges]

Geez! I thought staring at the reticle for 15 minutes was bad enough while trying to do PEC training....which never quite worked. I absolutely agree about the experimenting - I'm almost an expert at what doesn't work and how NOT to do stuff - probably all part of the journey.

[Startrek]

Alex,
I agree totally with experimenting, I’ve done that from the very beginning with both visual observing with my dobs and various eye pieces, Barlow’s and powermates etc... my early DSLR AP days with different ISO and exposures and recently with cooled Cmos cameras. However in regard to AP and cooled Cmos cameras I do review the technical data ( charts ) of camera first and get an understanding of its performance plus look at reviews from other folk who have the same camera.In saying that I agree totally that not one shoe fits all especially using various types of scopes with different focal lengths and apertures.
Using my newts and the 2600 I found that opening the throttle on the Gain ( higher dynamic range) Gain 0 and Gain 100 work best at both sites ( Bortle 8 Sydney and Bortle 3 South Coast)
When my 10” newt is set up in Dome down south around June / July there will be no need for experimental imaging , it’s Gain 0 all the way .....
Cheers
Martin

[Bassnut]

The darker the sky the longer the exposure required to bury read noise (on a CCD anyway). Over the years ive increased subs to a standard 1 hr each. OK, a stable system is helpful and risk of rejects is higher, but I found, with accurate darks, this is optimum for me (others here do this too). Can go very deep with enough subs.

[Rerouter]

In Summary:
The reality is weird as its per pixel, not per subject,

Say for stars, after a few seconds your usually clipping unless your shooting HDR to recover them, this means longer exposures generally loose signal on bright stars, the longer you shoot, the more you loose, however apart from clusters people usually don't care enough on blowing out bright stars,

For nebula, its faint, but there is a limit where even it can start clipping, past that point those pixels start loosing signal

For both cases, sky brightness starts eating up the dark point of the image, meaning the second you clip, your loosing signal

Getting into other parameters, there is tracking accuracy, seeing, guiding accuracy, atmospheric attenuation, that can work to smear detail over nearby pixels

There are technical issues, e.g. how long between needing to refocus, how long your FOV doesnt have any satellites, planes or birds cross, and what your lowest ISO is

To that end lowering ISO only really makes sense if you gain dynamic range by doing so, you generally aim for the knee where dynamic range starts to drop as that will be where you can capture the highest SNR per unit time,

On the camera there is all kinds of fun non linear noise sources, some that increase with time like shot noise and dark current, and others like dead pixels, not usually enough to lower SNR, but they factor in like the sky brightness chewing away at the black point

All of these in a way chew away at how much contrast you can capture in a single image and put a upper limit on how long you can image for,