Very over exposed looking subs and particularly poor final image :(

[TiffsAndAstro]

hi there, hoping for some thoughts on the 'final image' ill post below (not a lot more than a quick stretch) and an example of my exposures.

Im thinking the answer is high thin cloud that i can't see with my eyes is reflecting light pollution? if this is the case - if i see a test exposure in future like my example below - just try another night?

30sec subs iso800 canon 600d/t3i (unmodified) and an old  pentax m 50mm f1.7 shot wide open because im a noob.

anyway result is here:

 

 

 

and an example sub here:

 

thanks in advance for any insights anyone can provide. 

 

 

[iantaylor2uk]

If it were me, I would up the f ratio to 2.5 or 3, and reduce the sub exposure time to 15 seconds. You could also try playing around with the ISO setting too. 

[TiffsAndAstro]

20 minutes ago, iantaylor2uk said:

If it were me, I would up the f ratio to 2.5 or 3, and reduce the sub exposure time to 15 seconds. You could also try playing around with the ISO setting too. 

hey ty for the fast reply. tbh im very new to this (if you can't tell) and im reluctant to  stop it down because it will reduce the signal and the light pollution. iso800 seems the best signal/noise ratio. in my focus tests i did drop iso to 100 and it looked 'normal' as i was expecting. i also dropped exposure time to 5 and 10 sec for checking focus and seemed ok then too.  But if i can only take 5 sec exposures i might as well bin my tracker.

in your experience, is this likely to be caused by light thin high cloud i can't see with my own eyes?

Edited February 21 by TiffsAndAstro

[iantaylor2uk]

I should have said the final result looked good to me, even though the subs looked over-exposed. 

f1.7 is very fast and even f2.5 is fast - I wouldn't worry about stopping it down a bit since you want to let less light through to stop it being over-exposed. 

[TiffsAndAstro]

4 minutes ago, iantaylor2uk said:

I should have said the final result looked good to me, even though the subs looked over-exposed. 

f1.7 is very fast and even f2.5 is fast - I wouldn't worry about stopping it down a bit since you want to let less light through to stop it being over-exposed. 

i think the result is awful, its the result of  103x30sec subs. i got a far better image with a pentax k-x with the same lens on a tripod stacking 100 x 5sec untracked subs. 

if its clear for a while this evening ill try playing more with fstop and iso values in test shots before setting it to run. ty again. 

[iantaylor2uk]

Sorry - I only read the post quickly and didn't realize the top photo was with a different camera

[ollypenrice]

Stopping the lens down will improve star shapes in the corners. The best way would probably be with an aperture mask or a set of filter rings, rather than the diaphragm which will introduce diffraction artifacts.

Olly

[mackiedlm]

Its very difficult to work out whats going on there. Can you make the un-processed stacked image and a couple of raw subs available in a shared file and we can have a look at it. Perhaps high thin cloud may not be helping but I doubt if thats the main problem.

30 secs and ISO 800 are going to be OK as a starting point with this camera/lens. (from my experience with a 700d which I think uses the same sensor). 

Looking at whats in your image theres a few obvious issues, chromatic abberation and mishaped stars moving out from centre being the most obvious.. The mishhaped stars will be helped by stopping down (As Ollie pointed out - preferably with step down rings.) It may help the CA a little too but there are other ways of helping that in processing. I also think you have overstretched the image blowing out the bright stars and even some of the less bright ones.

 

Also, when was this taken in relation to the moon - i.e how bright and how close?

 

Edited February 21 by mackiedlm

[TiffsAndAstro]

1 hour ago, iantaylor2uk said:

Sorry - I only read the post quickly and didn't realize the top photo was with a different camera

both photos are from same lens and camera. i previously had a pentax k-x and got

better results stacking 100 x 5 sec subs untracked. 

 

 

this is what i mean from the untracked stack  its not great but can see flame nebula and possibly start of horse head.

 

 

 

 

 

 

 

 

 

Edited February 21 by TiffsAndAstro

[TiffsAndAstro]

8 minutes ago, mackiedlm said:

Its very difficult to work out whats going on there. Can you make the un-processed stacked image and a couple of raw subs available in a shared file and we can have a look at it. Perhaps high thin cloud may not be helping but I doubt if thats the main problem.

30 secs and ISO 800 are going to be OK as a starting point with this camera/lens. (from my experience with a 700d which I think uses the same sensor).

 

Looking at whats in your image theres a few obvious issues, chromatic abberation and mishaped stars moving out from centre being the most obvious.. The mishhaped stars will be helped by stopping down (As Ollie pointed out - preferably with step down rings.) It may help the CA a little too but there are other ways of helping that in processing. I also think you have overstretched the image blowing out the bright stars and even some of the less bright ones.

 

Also, when was this taken in relation to the moon - i.e how bright and how close?

 

hi there ty for all this. yes i can make those files available will update here once i find out how to do that

right now im not that bothered about ca in corners and stuff, i really wanted to test out long exposures as that's why i got a tracking mount in the first place.  What is most disappointing is i got a lot more of orion nebula and flame nebula stacking  untracked 5 sec  subs x 100

dss.TIF

[TiffsAndAstro]

_MG_5127.CR2_MG_5181.CR2_MG_5222.CR2

 

3 subs one near the start middle and end of the session. will have to delete the stacked result about as its like 900MB in size

[happy-kat]

that is an enormous tiff file, did you use drizzle when you stacked

[TiffsAndAstro]

Just now, happy-kat said:

that is an enormous tiff file, did you use drizzle when you stacked

lol yes i did

[TiffsAndAstro]

ok so there might be 2-3 hours of clear tonight, my plan is:

 

put my tripod roughly were it  was when i took my previous untracked shots (can shoot orion earlier as trees out the way)

stop down to f2.8 and do test shots at 5, 10, 20 and 30 sec. if it looks good at 30s sec i might try a test at 60 and 90 seconds too.

let it rip until clouds come over. though i may do a similar process for pleiades just before orion, as they come into view a little sooner

[Elp]

When you look at the overexposed image in software or your camera body what does the histogram look like?

F1.7 is too fast if you're imaging in an urban environment unless you're imaging at zenith, stop it down to near F3/4. Also ensure your lens hood is adequate to prevent any stray light entering the lens, this can also overexpose images and lead to internal light reflections.

Check you don't have exposure compensation on, turn off all lens assists and noise reductions.

50mm is also quite wide, any lights nearby will affect the exposures as will the moon as may light reflecting off roofs.

Edited February 21 by Elp

[TiffsAndAstro]

8 minutes ago, Elp said:

When you look at the overexposed image in software or your camera body what does the histogram look like?

F1.7 is too fast if you're imaging in an urban environment unless you're imaging at zenith, stop it down to near F3/4. Also ensure your lens hood is adequate to prevent any stray light entering the lens, this can also overexpose images and lead to internal light reflections.

Check you don't have exposure compensation on, turn off all lens assists and noise reductions.

btw the moon was out almost full. not within frame, but its obviously more powerful than i assumed.

I don't have a lens hood i do have one on a vintage tokina 200mm but want to do better at 50mm before getting too advanced

exposure compensation? i did change something a few weeks back which sounds a little like this - but i thought it was to make live view look more like the final exposure? apologies for being rather new at this and ty all for the help. 

just googled exposure compensation and im pretty sure i didn't mess with this as i shoot in manual and it can't be changed in manual?

after grabbing my camera, i think i may have unintentionally had this set to +3. using the roller i thought just changed shutter speed, even in manual mode, but it seems it does that AND exposure compensation? this might be a very good catch ty so much. it would explain a lot i think?

Edited February 21 by TiffsAndAstro

[mackiedlm]

So several things from your files;

How are you saving these subs - if I open them in Photoshop or Pixinsight they look already stretched (see below) - its a bright dusky sky rather than a black image with just some bright stars showing. 30 seconds at iso 800 would not give that type of image, even with bad LP. You need to make sure that you are saving them as totally raw with no processing happening either in camera or after you download them to your PC.

From those its easy to see that you had some significant clouds passing through - also some vegetation in the lower part of the earlier image. You could go through them and discard those with clouds but i think you'll find that a lot of them will be unusable.

Likewise, the DSS stack is looks stretched - I guess that could be because the subs were already stretched or possibly because you saved the stretched version that DSS throws up (its been a long time since i used DSS so struggling to remember exactly how it works.

Its difficult to tell on those images but i do think your focus is soft.

Before you do anything tonight you should look at how the raw images are being handled and saved. get that right before worrying about trying longer exposures.

 

Edited February 21 by mackiedlm

[mackiedlm]

So with a close, almost full moon and exposure compensation, I think thats a large part of your problem and explains why the images are so bright.

Turn that off exposure comp, check everything else and, if you have to image tonight, point as far away from the moon as you can (moon is 94% tonight and not far from orion) Imaging broadband tonight and at that speed and that FL is going to be extremely difficult (even futile) and many imagers, myself included, would not try unfiltered broadband in these circumstances.

 

Edited February 21 by mackiedlm

[TiffsAndAstro]

22 minutes ago, mackiedlm said:

So several things from your files;

How are you saving these subs - if I open them in Photoshop or Pixinsight they look already stretched (see below) - its a bright dusky sky rather than a black image with just some bright stars showing. 30 seconds at iso 800 would not give that type of image, even with bad LP. You need to make sure that you are saving them as totally raw with no processing happening either in camera or after you download them to your PC.

From those its easy to see that you had some significant clouds passing through - also some vegetation in the lower part of the earlier image. You could go through them and discard those with clouds but i think you'll find that a lot of them will be unusable.

Likewise, the DSS stack is looks stretched - I guess that could be because the subs were already stretched or possibly because you saved the stretched version that DSS throws up (its been a long time since i used DSS so struggling to remember exactly how it works.

Its difficult to tell on those images but i do think your focus is soft.

Before you do anything tonight you should look at how the raw images are being handled and saved. get that right before worrying about trying longer exposures.

 

i think i had exposure compensation on +3. possibly. clear skies in maybe 4 hours will have a check then. I'm not intentionally doing anything to the .cr files except copy them across to my pc. i was intending to sort out the crap subs with cloud/trees/aircraft lights before stacking, but couldn't see much  ty for this very very useful post. 

im sure my focus was soft, i was hoping it was 'ok' enough to test longer exposures/my PA etc but i will get it right first. i find it tricky to do, but jupiter should be around and it can help with focus.

Edited February 21 by TiffsAndAstro

[TiffsAndAstro]

11 minutes ago, mackiedlm said:

So with a close, almost full moon and exposure compensation, I think thats a large part of your problem and explains why the images are so bright.

Turn that off exposure comp, check everything else and, if you have to image tonight, point as far away from the moon as you can (moon is 94% tonight and not far from orion) Imaging broadband tonight and at that speed and that FL is going to be extremely difficult (even futile) and many imagers, myself included, would not try unfiltered broadband in these circumstances.

 

with english weather i think i have to shoot what i can. if i ever get a clear night i could be more choosy   if nothing else, my basic set up and quick polar alignment skills are improving

trying to google if there's a menu option to disable exposure compensation. 

Edited February 21 by TiffsAndAstro

[Elp]

For a quick lens hood make it out of black paper or cardboard or even toilet roll/kitchen roll tube. A blacked out internal will help but for quick testing nothing wrong with DIY methods.

[TiffsAndAstro]

2 minutes ago, Elp said:

For a quick lens hood make it out of black paper or cardboard or even toilet roll/kitchen roll tube. A blacked out internal will help but for quick testing nothing wrong with DIY methods.

i will have a go ty for the suggestion lol

[mackiedlm]

Just now, TiffsAndAstro said:

with english weather i think i have to shoot what i can. if i ever get a clear night i could be more choosy   if nothing else, my basic set up and quick polar alignment skills are improving

I live in the west of Ireland so I think I probably trump you in terms of crappy weather🤣🤣. I've not had even a partly clear sky since Jan 5th.

Good luck tonight, for sure the pratice setting up and getting an image is always useful, but dont be too surprised if the outcome is similar. Although no exposure comp and Elps's DIY hood, may help some.

 

[TiffsAndAstro]

10 minutes ago, mackiedlm said:

I live in the west of Ireland so I think I probably trump you in terms of crappy weather🤣🤣. I've not had even a partly clear sky since Jan 5th.

Good luck tonight, for sure the pratice setting up and getting an image is always useful, but dont be too surprised if the outcome is similar. Although no exposure comp and Elps's DIY hood, may help some.

 

im actually pretty confident as im sure it was exposure compensation. i just need to be careful not to do it again.

i was using the roller wheel to adjust exposure times for test shots. ill do it via bulb and an intervalometer.

at least west of ireland has lower bortle to make up for the even worse weather. 

[Guest]

So a full moon and fast lens indeed will give you that result even at 30 sec.  As far as exposure compensation, as long as you shoot raw and stack the raw files it shouldn't matter.  That's only for jpegs or using the camera settings when you bring the file in say Photoshop or use the Adobe converter to make raw into tiff.  Only the iso affects the raw data.

So to correct myself.  If you shoot manual exposure compensation is ignored actually.  If you set the exposure and f stop manually yourself then ec has no effect.  EC is only used in modes where it can control aperture or exposure time like av or tv.

The brightness of the background will affect your dynamic range.  You were able to rescue a good bit of it but as pointed out, when the sky is this bright I would either stop down a bit or better yet reduce exposure or iso.  On some cameras you can shoot a lower iso and maintain the cameras best dynamic range.  I would try a combination of iso400 and stopping down a bit as suggested.

Best solution of course is to wait for the moon to get out of the way.  But that won't be for a while.

 

Edited February 21 by dciobota
Correction