Is there an ideal exposure time?

[bilbs84]

So I'm somewhat, no, I'm pretty much entirely new to the world of astrophotography.  I bought a 60D a couple of months ago, and then after takin a stack of 3 second exposures with the kit lens of the milky way, I was somewhat hooked.  I bought myself an 85mm lens that was a world better than the kit lens that the camera came with, and then patiently waited for the astronomy stores near me to emerge from lockdown so that I could get a tracking mount.  I arrived home with my shiny new mount, set it up, then got everything out to get ready to give it a try that night, only the second hand 60D that I had decided it didn't want to work anymore.

Cut to now, and an ebay dispute over a 70D that took way to much effort to actually get, and an 80D that I bid on as a backup in case the 70D never arrived, and an Australian Spring that has had about 5 hours of cloud free nights, tonight, I finally managed to get a decent alignment using N.I.N.A. and there seem to be little to no clouds, finally, I'm taking decent subs.

So that got me thinking, what is my limit on exposure time, am I limited by the mount, or is there another limit that I need to take into consideration?

Im not in the worlds darkest skies, Bortle 5 is what is listed on lightpollutionmap.info  Does that play a role in exposure time.  Im using an EOS 80D, with 85mm f/1.8 lens, on a Star Adventurer pro.

 

Thanks.

[PeterCPC]

The best approach is to experiment with different settings. If you get trailing then reduce the exposure time. Everyones sky is a bit different.

[Clarkey]

At f1.8 in bortle 5 you are likely to find anything over a couple of minutes is likely to swamp the image with background light. Obviously this will depend on the iso setting and any filters. The mount will depend on how good your alignment is and the mount tracking. Although there are various way of calculating these things, my suggestion would be trial and error. Just set a number of exposure lengths to see what works best with your set up. I'm guessing something around the 1 minute mark would probably be good.

Edited December 4, 2021 by Clarkey

[vlaiv]

Choosing optimum sub length is balancing act.

On one side - you have incentive to get as long subs as possible, as stack of smaller number of longer subs has edge over large number of shorter subs - if they add up to same total time.

On the other side - here are things that make / let you use shorter subs:

- mount tracking - it is better to have shorter subs with round stars than longer subs with little streaks

- light pollution - different between long and short subs is only in read noise of your camera, or rather what is relation of read noise to other noise sources. If you are in light polluted area - LP noise quickly becomes dominating noise source and swamps read noise - when this happens - there is not much benefit in going with longer subs (there is still benefit but that benefit is very small and often not perceived by humans in the image)

- chance of ruined sub. Wind gusts? Cable snag? Passing airplanes / traffic / neighbors turning lights on/off, earthquakes? All of these favor shorter subs as it is better to loose 1 minute of imaging rather than 10 minutes of imaging in case something goes wrong.

There are some things that go in favor of long subs - like amount of data storage and processing capability. Do notice that some algorithms actually prefer having larger data set to work with - and that favors shorter subs.

All of this can be a bit daunting to take into account - and for that reason look into taking 30s-60s to start with - and test if your tracker is capable of good tracking at those times.

[JamesF]

The mechanical performance of the mount will obviously impose some sort of limit -- the better it tracks or can be guided, the longer exposure is possible.  My understanding is that there's also a "signal processing" type of limit as well, beyond which increasing the exposure time really doesn't usefully get you any more data.  From memory (ie, I could well be wrong ) it's the point at which the background noise from the sky swamps the read noise from the sensor.

Robin Glover (of SharpCap fame) made a very informative presentation a couple of years back that you may find helpful.  I've not used SharpCap for years, but I think there's support for doing some of the necessary calculations built into the software.  The presentation is here, amongst other places:

James

[JamesF]

Our posts crossed, but vlaiv appears to have confirmed what I posted, explaining it better in the process

James

[Clarkey]

40 minutes ago, JamesF said:

explaining it better in the process

I think Vlaiv explains most things better than us mere mortals!

[bilbs84]

First of all, thanks for the advice and feedback.  You've basically confirmed my own suspicions.

I remember when I shot my first test image with no tracker, stacking over 300 2.5 second exposures took forever, I understand that the processing can be time consuming, but it sucks when you only get 30 minutes or so of exposure for it.  For this reason, I thought I'd push the limits of the mount and setup to 45 seconds and see how things turned out.  I'm fairly pleased with the result, and next time, might see if it will handle a minute.  I used N.I.N.A's 3 point polar alignment to dial in the mount, I managed to get it to under 2' error.  While I'm sure that I could do better, that was about the limit that I could get with the camera taking 2 second exposures constantly while I was getting it dialed in.  Maybe next time I'll give the web cam feature of the 80D a crack and see if it can get enough exposure to plate solve with that.  At least that would cut down on the shutter count.

I was shooting the Large Magellanic Cloud on an 80D, with the EF 85mm f/1.8 lens.  Unfortunately the clouds rolled in only half an hour after I started imaging.  I ended up only being able to use 29 of the 40 frames that I shot due to clouds.  That being said, I'm pleased with the result, especially since I was able to get the polar alignment dialed in, and was able to get decent exposure times.  Hopefully we get some clear skies again soon, and I can gather some more exposures soon.

I guess the one other thing that I hadn't really thought of until I was looking through the images I shot of Orion a couple of weeks ago, is that the exposure may need to drop for really bright objects, the core was a bit over exposed.  So am I better off limiting the exposure time, or reducing ISO to keep the exposure time up, I don't quite understand entirely how read noise works, so still unsure if shorter exposures at higher ISO's vs longer exposures at lower ISO's

Anyway, thanks for taking the time to help point me in the right direction.

[vlaiv]

47 minutes ago, bilbs84 said:

I guess the one other thing that I hadn't really thought of until I was looking through the images I shot of Orion a couple of weeks ago, is that the exposure may need to drop for really bright objects, the core was a bit over exposed.  So am I better off limiting the exposure time, or reducing ISO to keep the exposure time up, I don't quite understand entirely how read noise works, so still unsure if shorter exposures at higher ISO's vs longer exposures at lower ISO's

First of all - that is very nice image. I appreciate it more since it is the object I don't get to see imaged often and of course - can't be seen visually from where I am.

ISO is just conversion factor - so it does not mean anything in terms of SNR - but it does affect read noise. Higher ISO means lower read noise, but just how much - that is something that must be measured.

Shorter subs + higher ISO vs Longer subs + lower ISO - is again matter of read noise being dominated by other noise sources and unless we know things like read noise value at different ISO settings and your LP levels - we can't really tell which is better.

What I can tell you is that there is another way to deal with over exposed regions - and that is to take several short exposures at the end of the session. You stack those in separate stack and then replace over exposed pixels with scaled pixels from short stack. It is a bit involved to do it by hand - but some don't really bother doing linear blending of data - they process both images and then blend in PhotoShop / Gimp by using layers / layer masks.

 

Edited December 5, 2021 by vlaiv

[ollypenrice]

Lovely LMC.

Orion (or, at least, M42,) is one of the very few objects which does require different exposure lengths because no camera has an adequate dynamic range for that. If you use Photoshop, paste the short exposure on top of the long, make a layer mask and copy the long onto it. Greatly exaggerate the contrasts on the mask and give it a massive Gaussian blur. I'm sure you'll find detailed tutorials out there but the one I normally recommend has been deleted.

Olly

[Alien 13]

11 hours ago, bilbs84 said:

First of all, thanks for the advice and feedback.  You've basically confirmed my own suspicions.

I remember when I shot my first test image with no tracker, stacking over 300 2.5 second exposures took forever, I understand that the processing can be time consuming, but it sucks when you only get 30 minutes or so of exposure for it.  For this reason, I thought I'd push the limits of the mount and setup to 45 seconds and see how things turned out.  I'm fairly pleased with the result, and next time, might see if it will handle a minute.  I used N.I.N.A's 3 point polar alignment to dial in the mount, I managed to get it to under 2' error.  While I'm sure that I could do better, that was about the limit that I could get with the camera taking 2 second exposures constantly while I was getting it dialed in.  Maybe next time I'll give the web cam feature of the 80D a crack and see if it can get enough exposure to plate solve with that.  At least that would cut down on the shutter count.

I was shooting the Large Magellanic Cloud on an 80D, with the EF 85mm f/1.8 lens.  Unfortunately the clouds rolled in only half an hour after I started imaging.  I ended up only being able to use 29 of the 40 frames that I shot due to clouds.  That being said, I'm pleased with the result, especially since I was able to get the polar alignment dialed in, and was able to get decent exposure times.  Hopefully we get some clear skies again soon, and I can gather some more exposures soon.

I guess the one other thing that I hadn't really thought of until I was looking through the images I shot of Orion a couple of weeks ago, is that the exposure may need to drop for really bright objects, the core was a bit over exposed.  So am I better off limiting the exposure time, or reducing ISO to keep the exposure time up, I don't quite understand entirely how read noise works, so still unsure if shorter exposures at higher ISO's vs longer exposures at lower ISO's

Anyway, thanks for taking the time to help point me in the right direction.

The 80D is one of the few/only camera that Canon made that is more or less ISO invariant so you can use it with an ISO of 200 or less if you want with no noise penalty if you brighten it up in post processing.

Alan

[wongataa]

12 hours ago, Alien 13 said:

The 80D is one of the few/only camera that Canon made that is more or less ISO invariant so you can use it with an ISO of 200 or less if you want with no noise penalty if you brighten it up in post processing.

Alan

The 80D appears to be pretty ISO invariant from ISO 200 up using whole stop steps

https://www.photonstophotos.net/Charts/PDR.htm#Canon EOS 80D

But you can get signal to noise improvements at certain higher ISO settings

https://www.photonstophotos.net/Charts/PDR_Shadow.htm#Canon EOS 80D

In the end working out good exposure time will require a bit of experimentation with your kit and it will depend on what you are taking a picture of, the amount of light pollution, alignment accuracy, and tracking accuracy of your mount.