A bit confused about exposure

[smr]

Hi all

 

I think I have got myself a bit confused about exposure times and settings.

The other night i had a go at capturing the horsehead nebula with my unmodified dslr. 

I figured that i would go for as long as possible subs on the basis that more light = more data. So i went for 90 second at iso 640 as well as 60 second iso 800 subs. On both the histogram peak was past 50 percent and on the the next band along so say towards 75 percent. 

 

The resulting stacked image is very washed out looking and bright grey. But if i had gone with a lower iso and tried moving the histogram to a third from the left it wouldnt have been able to pick out the faint nebulosity would it?

 

Is it just matter of how ever long you expose for making sure the peak is a third or 25 percent away from the left hand side? Even if it means using an iso of 100 on a 60 second exposure?

[wxsatuser]

Sounds like you have a fairly bright sky, light pollution??

The idea of the 25% is to get out of the read noise, in theory, as long as the histogram is totally detached from the lefthandside it's good.
Generally upto 50% histogram will be ok.

Remember you are looking at a jpg histogram, the RAW will be much closer to the left.

 

[drjolo]

What DSLR and telescope do you use? If you get such histogram position in 60s subs at ISO800 I think it may be some significant light pollution - that can be the reason for washed out images. Horsehead is quite demanding, and under bright sky with unmodified DSLR it can be challenge. 

[Demonperformer]

I never managed to get anything worthwhile on HH under my LP until I got a modified DSLR (although I know some others have managed.)

[Tim]

When you say, "the other night", what was the date? We have been 'enjoying' one of the highest brightest moons of the year this last week.

With any camera, but especially a standard DSLR, the horsehead is a target best suited to dark moonless skies. It is only the faint hydrogen nebulosity surrounding the horsehead shape that makes it stand out, and standard dslrs are  not brilliant at picking up that deep red wavelength, making it hard to get any contrast against the dark nebula.

Did you manage to catch any of the flame nebula nearby? If you have any of that, usually showing as a coppery orange, you may just have a bit of contrast at the Horsehead too to help you pick it out.

All practice is good with astrophotography, but some targets are more suitable for nights that the moon imposes itself on.

[smr]

Thanks for the replies guys, the other night was not last night but the night before. Although I did try again last night and went for two minute subs just to experiment really and see how long I could go for unguided. As it turns out that's probably a bit too demanding, or at least with a 250mm focal length, then again it was a bit 50/50 over a dozen subs as to whether the stars were trailing or nice and round. I guess that must be the wind.

Yes I shoot from my back garden and there is quite a bit of LP I guess. I did manage to pull out the flame with some careful curve and level adjustment and I can then see a very faint outline of the horse head. The night before last when I shot the moon was half and not far away from the Nebula. Last night when I shot again it was quite a bit fuller but further away, I get your points about shooting with no moon and from a dark sky location though. I just wondered more if exposing each sub with the histogram peak 75 percent over was the reason why I couldn't extract much from the data at all really.

I always shoot RAW - does the histogram not accurately display for this then?

 

DSLR is a Canon 80D and the lens attached is a 55-250mm EFS Canon.

[kens]

57 minutes ago, smr said:

Hi all

 

I think I have got myself a bit confused about exposure times and settings.

The other night i had a go at capturing the horsehead nebula with my unmodified dslr. 

I figured that i would go for as long as possible subs on the basis that more light = more data. So i went for 90 second at iso 640 as well as 60 second iso 800 subs. On both the histogram peak was past 50 percent and on the the next band along so say towards 75 percent. 

 

The resulting stacked image is very washed out looking and bright grey. But if i had gone with a lower iso and tried moving the histogram to a third from the left it wouldnt have been able to pick out the faint nebulosity would it?

 

Is it just matter of how ever long you expose for making sure the peak is a third or 25 percent away from the left hand side? Even if it means using an iso of 100 on a 60 second exposure?

The light from your target and the sky glow add together. The faint nebulosity will be a little bit above the sky glow. You only need to expose enough to overcome the read noise of the camera (identified as the left hand edge of the histogram curve). Beyond that you are losing dynamic range in your images where the dynamic range is the highest possible signal divided by the sky background. Thats why light pollution is a problem.

[edjrgibbs]

4 minutes ago, smr said:

Thanks for the replies guys, the other night was not last night but the night before. Although I did try again last night and went for two minute subs just to experiment really and see how long I could go for unguided. As it turns out that's probably a bit too demanding, or at least with a 250mm focal length, then again it was a bit 50/50 over a dozen subs as to whether the stars were trailing or nice and round. I guess that must be the wind.

Yes I shoot from my back garden and there is quite a bit of LP I guess. I did manage to pull out the flame with some careful curve and level adjustment and I can then see a very faint outline of the horse head. The night before last when I shot the moon was half and not far away from the Nebula. Last night when I shot again it was quite a bit fuller but further away, I get your points about shooting with no moon and from a dark sky location though. I just wondered more if exposing each sub with the histogram peak 75 percent over was the reason why I couldn't extract much from the data at all really.

I always shoot RAW - does the histogram not accurately display for this then?

 

DSLR is a Canon 80D and the lens attached is a 55-250mm EFS Canon.

If you did it the night before last the moon was out and very bright. I've concluded there's absolutely no point doing any DSO imaging when the moon's up. Everything is just washed out and impossible to recover (at least with my processing skills). 

 

[wxsatuser]

4 minutes ago, smr said:

I always shoot RAW - does the histogram not accurately display for this then?

If we are talking about the camera histogram, it's a histogram of the camera jpg even if your shooting RAW.
If you look at the RAW histogram it will be further left.

[smr]

1 minute ago, wxsatuser said:

If we are talking about the camera histogram, it's a histogram of the camera jpg even if your shooting RAW.
If you look at the RAW histogram it will be further left.

In Lightroom/PS etc. do you mean ? The histogram on the back of the camera shows the proper read out for whichever shooting mode you're in though doesn't it? 

[smr]

11 minutes ago, edjrgibbs said:

If you did it the night before last the moon was out and very bright. I've concluded there's absolutely no point doing any DSO imaging when the moon's up. Everything is just washed out and impossible to recover (at least with my processing skills). 

 

Ah ok that would make sense. Are there any easier targets to image? Bearing in mind I don't really know how to star hop properly just yet, I can sort of guide myself to a rough location in the sky with Stellarium and the stars. I guess it would be a lot easier if I had a Go-To...

[wxsatuser]

7 minutes ago, smr said:

In Lightroom/PS etc. do you mean ? The histogram on the back of the camera shows the proper read out for whichever shooting mode you're in though doesn't it? 

No it's a histogram of a jpg all the time, it's processed in camera as such.

If you look at in the Canon software you can turn the linear setting on and you will see the histogram is tight up to the left, not at say 25%.

[wimvb]

54 minutes ago, smr said:

Ah ok that would make sense. Are there any easier targets to image? Bearing in mind I don't really know how to star hop properly just yet, I can sort of guide myself to a rough location in the sky with Stellarium and the stars. I guess it would be a lot easier if I had a Go-To...

Star clusters are ok since they are bright and easier to deal with in processing.

Imaging away from the moon (90 degrees or more in any direction) can help as well. But during a near full moon, just put your imaging rig aside, do some visual, and plan for the next imaging opportunity.

[Demonperformer]

1 hour ago, wimvb said:

put your imaging rig aside, do some visual, and plan for the next imaging opportunity.

unless you fancy forking out the best part of £1500 for a set of 3nm narrowband filters ... really

[wimvb]

35 minutes ago, Demonperformer said:

unless you fancy forking out the best part of £1500 for a set of 3nm narrowband filters ... really

With an unmodded dslr? Sure.

[Stub Mandrel]

3 hours ago, wxsatuser said:

No it's a histogram of a jpg all the time, it's processed in camera as such.

If you look at in the Canon software you can turn the linear setting on and you will see the histogram is tight up to the left, not at say 25%.

BOG!

That explains a lot. many times I have reduced my exposures because the histo was past 50% and the previews washed out :-(

Sometimes I do wish I could process my images so they look as good as the back of camera preview JPGS!

[edjrgibbs]

3 hours ago, smr said:

Ah ok that would make sense. Are there any easier targets to image? Bearing in mind I don't really know how to star hop properly just yet, I can sort of guide myself to a rough location in the sky with Stellarium and the stars. I guess it would be a lot easier if I had a Go-To...

The moon is about the only thing i've ever imaged with a DSLR when its out. There's a frustrating correlation between clear nights and the moon being out... I normally leave the DSLR inside and do narrowband but thats another 'journey'...