DSO DSLR "Exposure Values"

[pete_81]

As a photographer with DSLR, I understand that I can take any typical lens and the brightness of the final image is essentially dependent on the "exposure triangle". For example, if I use a 70-200 lens set at 200mm, and do exposure for 1sec at f/2.8 and ISO 100, I could change the ISO to 400 and reduce exposure time to 0.25s (factor of 4 increase in ISO, factor of 4 decrease in exposure time) and the image will essentially be identical (allbeit with a minor amount of noise added and less camera shake!). Alternatively, I can take the aperture to f/5.6 (2 stops, or 4 times less light) with the ISO back to 100 and increasing exposure to 4sec, and other than increased DOF, the image will be the same (exposure wise).

Similarly, if I switch to a 50mm f/1.4 prime lens, I can set exposure to 0.25sec, ISO100 and f/1.4 (again 2 full stops, so x4 increase in light, hence the 1/4 exposure), and again have the same image brightness. Focal length doesn't change the brightness, only the field of view in this case.

So, purpose of this topic is to try to build up a bit of a database that would give users some idea on starting settings for when they're starting out. The real thing with imaging is getting exposure right, so as to not blow out stars too much (too long exposure) but obviously not have too short exposures, and I'm hoping this may help some folks (including myself!) with DSO photography.

I've had a go with M42 using f/8 ISO100 30sec which seemed to blow out the stars significantly, but still can give some colour info

Yet with f/8, ISO100, 10sec, there is obviously less saturation with the stars, but the nebula starts to become more in line with the noise levels of the exposure:

Yes yes, I know... Take many exposures, etc, then ensure we all take flats, darks, bias and any other possible calibration frames! Before getting to that though, what are some good starting settings for different DSO's? And on that, yes, I appreciate there's not going to be a 'set' of ideal parameters - bortle levels, atmospheric conditions, time of year (darker evenings in winter), % light-transmission through a narrowband filter, and many other factors will all play a part, but I'm looking to have some general 'ballpark' starting settings for exposure and ISO, so perhaps newcomers don't try taking 500 images of M1 at the same settings as M42! Clearly M1 needs to be minutes, horsehead would be several minutes, M42 the order of 10's of seconds, and M45 even shorter, for given f-ratio and ISO.

Then there's the large assumption of telescopes - they're essentially large lenses for cameras, so the larger telescopes working at f/5 only achieve a 'higher zoomed image' as the 'exposure' values depend on the f/#. Am I right on this?

So! TLDR! With some relation to magnitude of a DSO, what camera settings are folks using?! Perhaps also include some info if other features are used (eg filters)?

EG:
Images above, just from single light file

Nikon D7200 DSLR, Tamron 500mm f/8 mirror lens, no filters in imaging train, bortle 6 skies
M42, Magnitude 4, ISO100, f/8, 30sec - stars saturated, light pollution visible but can be processed out
M42, Magnitude 4, ISO100, f/8, 10sec - Nebulosity visible but many light frames required to get away from noise

 

So, by having a bit of luck in between clouds, 40x 15sec exposures and the usual DSS image requirements, I've produced my first guided astrophoto, M42, and hope some guide settings lead others to achieve the same!

 

[pete_81]

https://stargazerslounge.com/topic/373659-rosette-nebula-difficulty-seeing-in-live-view-and-in-post-processing-is-it-there/

Bortle 6, D5200, Evostar 72ED, f/5.8 420mm.

Rosette Nebula, Magnitude 9, ISO1000, f/5.8, P2 light pollution filter, 22sec. Visible following stretching (amcl posted this on above) but should try for longer exposures.

[Newforestgimp]

This was my second attempt at M42, From memory these were about 30 or so 240 second guided subs at ISO400 and many attempts at post processing, sadly I struggle with focus for some reason, think my eyes are lying to me, have been using APT Bhatinov Mask Focus aid since and seems better on last attempts.

Edited March 13, 2021 by Newforestgimp

[Soligor Rob]

Hey that's a fantastic Picture I would be well happy with that for a 2nd attempt, well done.

[Newforestgimp]

7 minutes ago, Soligor Rob said:

Hey that's a fantastic Picture I would be well happy with that for a 2nd attempt, well done.

Thank you, I recall nearly spitting my tea over the laptop when the raw images started coming through 🤯

[Soligor Rob]

8 hours ago, Newforestgimp said:

Thank you, I recall nearly spitting my tea over the laptop when the raw images started coming through 🤯

Hey you live in a special part of the country we have spent many a night in our motorhome around Lyndhurst, our son is getting married in Brockenhurst in August.

[Newforestgimp]

14 minutes ago, Soligor Rob said:

Hey you live in a special part of the country we have spent many a night in our motorhome around Lyndhurst, our son is getting married in Brockenhurst in August.

I know Lyndhurst and Brock, many miles cycled around those lanes, itching to get out in the wilds to some dark sky and see what I can do, but need to assemble all the power gear to make it doable with minimal effort.

[pete_81]

Wow Andy, @Newforestgimp, it was myself helping you out with the guide camera stuff and so pleased I did (although a little jealous of that image!) Yeah, my ISO100 and 30sec will never match up to the settings you used to get that, and I also remember the images downloading thinking how is that possible!

I've spent more time like yourself trying to understand post, and armed with Affinity Photo, I have also tried StarTools which is mentioned in the Rosette Nebula post above. Initially took some time to get into, but my image above in Affinity came to this one in StarTools along with this tutorial

Again nice image and good to see that all those evenings of stress are now just evenings of annoyance at the clouds like so many others interested in astro!

 

[pete_81]

M42, Magnitude 4, ISO400, f/5.9, 240sec; Shows very nice detail in nebula, a good setting. Canon DSLRs and 30-image-stack

 

[Newforestgimp]

1 hour ago, pete_81 said:

M42, Magnitude 4, ISO400, f/5.9, 240sec; Shows very nice detail in nebula, a good setting. Canon DSLRs and 30-image-stack

 

Thank you for the assistance, I wish every target was as straightforward to at least get a decent attempt at, been struggling with Rosette think I need to invest in a modified camera or a dedicated Astro cam. I will take a look at that tutorial see if I can get something ANYTHING out of startools !

messing about with APP currently on the 30 day trial, it’s very good

[pete_81]

Highly recommend APP, it's equivalent to Photoshop with all but the very highest level things, and a fraction of the cost, and even on offer at present! Yeah, the dedicated cam may be something I'm considering too but the lack of the clear nights (and light pollution locally) is making it not possible (yet!)

[jager945]

On 12/03/2021 at 04:00, pete_81 said:

I'm looking to have some general 'ballpark' starting settings for exposure and ISO

Hi,

ISO in the digital era is a rather confusing subject, particularly if you come from a daylight/terrestrial background.

For earth-based photography you can use the triangle rule-of-thumb, but once you get into low light photography where every photon counts, this falls apart rather quickly.

The "problem" is that you are dealing with one digital sensor that only has one, fixed sensitivity. To emulate other sensitivities, signal is either thrown away or signal is artificially boosted. Both of these operations are detrimental to your final signal.

For this reason, in astrophotography, you should find your sensor's native ISO, which is the ISO where you eat into your dynamic range the least, while not throwing away (or reshaping) any parts of the dynamic range.

This article by Chris can den Berge goes into good detail about what is going on exactly.

On the StarTools website, you can find a list if recommended ISOs settings that correspond most closely to native sensitivity of your sensor.

It does not list your particular D7200, but it appears an ISO of ~200 gives a linear response at highest bit-depth (and thus dynamic range).

With a fixed ISO, all that truly matters then, is the exposure time you can get away with without over-exposing your image and without tracking error creeping in. A good rule of thumb is aiming for a histogram peak (caused by background light) that is ~ at 1/3rd to the left of the start of your histogram. E.g. this leaves ~2/3rds of the dynamic range to describe true celestial signal.

You picked M42 as an example, which is actually one of the most troublesome objects, as it is one of the few objects where the core's brightness is such, that high dynamic range compositing may be warranted at long exposure times (but definitely not at short exposure times). That would be adding another layer of complexity.

Hope this helps!

[pete_81]

Thanks @jager945 And also for StarTools! Quite something on first playing! Just need to get more into post and set a work-flow. Don't all newcomers!

Yep, I'm aware of the ISO issue, but hadn't thought it so extreme to limit cameras to ISO200! But agreed that the main issue is exposure time (with photons-per-pixel or f/#). Also hadn't thought of the extremes of M42, but yeah, certainly saturated quickly at exposures where I'd have thought it would be OK.

Might have to resort to the crab for a bit and longer exposures on this, but hopefully that would give some indication on what I'm looking for with this post. Obviously, post is the main issue following this, but as a photographer during the day, my hope had been to 'get-it-right-in-camera' then tweak in post with the correct exposure settings.

Any tips on exposure times and f/# therefore appreciated! Viewers, take ISO values cautiously!

[pete_81]

M51, Whirlpool Galaxy, in Canes Venatici.
Mag 8.4

ISO 800, 5min exposures, f/7 (Astro Addict mentions "Omegon 102/714 APO" on his YT channel)
https://www.youtube.com/watch?v=o3Qf9aFw4Nc