Astrophotography Question

[ArchRival]

This is in regards to wide-view photography of the night sky.

Given that i want to capture as many stars as possible, is it better to take longer exposures at lower ISO or can i just make do with shorter exposures at higher ISO?

Another angle to this question is, is the ISO setting merely a gain in signal strength in the camera? If so, it should reason that for a given exposure time, an ISO 400 image should capture the same number of stars as an ISO 800 image, only dimmer.

Also, for images of stars, should a change in focal length (hence a change in F/ratio) at constant aperture affect its brightness in the image? A star, at least ideally, is a pinpoint on the sensor. Regardless of "zoom", the star should still be a pinpoint, the exposure not affected by focal length.

[roygoh]

My opinions are not based on experience. Just from a theoretical point of view.

Originally Posted by ArchRival This is in regards to wide-view photography of the night sky. Given that i want to capture as many stars as possible, is it better to take longer exposures at lower ISO or can i just make do with shorter exposures at higher ISO?

Your enemy is sensor noise. I don't think either extreme (very low ISO with very long exposure or very high ISO with lower exposure) would work the best. So I believe ISO 400~800 would be a good compromise.

Originally Posted by ArchRival Another angle to this question is, is the ISO setting merely a gain in signal strength in the camera? If so, it should reason that for a given exposure time, an ISO 400 image should capture the same number of stars as an ISO 800 image, only dimmer.

Seems true. But bear in mind that the image captured at ISO400 will technically have better S/N ratio, so if you boost the expoure in post processing on the ISO400 image you might end up with a picture that is close to the one captured at ISO 800.

Originally Posted by ArchRival Also, for images of stars, should a change in focal length (hence a change in F/ratio) at constant aperture affect its brightness in the image? A star, at least ideally, is a pinpoint on the sensor. Regardless of "zoom", the star should still be a pinpoint, the exposure not affected by focal length.

You are right. For astrophotography aperture is the most important factor. However, if you are shooting larger objects like nebulas I believe the focal length do matter. This brings up an interesting question. If you shoot a nebula then wider FOV will make the nebula brighter against the background of starts, and narrower FOV will make the nebua dimmer against the background of starts...

[mdzmr]

In my opinion, the biggest enemy is the surrounding environmental light pollution. need to have extremely dark surrounding for wide field.

Hello Weixing, where are you?

He is the best person to solve ur problem.

[ArchRival]

The reason for these questions is related the problem of light pollution.

If ISO has no effect on the number of stars captured, but exposure time does, then given our severely light-polluted conditions, is it not wise to use a lower ISO and use a longer exposure?

[mdzmr]

Here some of my actual pics haven't do any treatment yet.
Shot with 55mm f2 lens
WB: Cloud





I just realise that i missed to shoot at ISO800

[Learnphotography]

yr images show that yr surrounding is too polluted with light.
I read on astrophotography that learnt that keeping at most 30secs exposure is a sweetpoint before u get star trails. HOwever it still depends on yr location.
ISO 400-800 is just nice.

learn astrophotography: http://www.astropix.com/HTML/I_ASTROP/TOC_AP.HTM

[weixing]

Hi,

Originally Posted by ArchRival This is in regards to wide-view photography of the night sky. Given that i want to capture as many stars as possible, is it better to take longer exposures at lower ISO or can i just make do with shorter exposures at higher ISO? Another angle to this question is, is the ISO setting merely a gain in signal strength in the camera? If so, it should reason that for a given exposure time, an ISO 400 image should capture the same number of stars as an ISO 800 image, only dimmer.

This is a very difficult question to answer, because a lot of factors will affect it:
1) DSLR noise,
2) any banding at lower ISO,
3) light pollution,
4) weather condition and
5) mount tracking accuracy.

Generally, IMHO, if you are using Canon DSLR, you can do longer exposure at lower ISO. If using Nikon DSLR, it's best to do higher ISO at shorter exposure. And yes, ISO setting is a gain in signal strength and should capture the same amount of stars regardless of ISO.

Anyway, anything can happen in a long exposure... aircraft/cloud/satellite/meteor come into the field of view, sudden strong wind shake your setup and etc. So usually, I'll do multiple shorter exposure (at least 1 minute) at slightly higher ISO and stack them together. This method got few advantages:
1) If cloud or aircraft trails appear in a few frames, I can just throw aways those frames and the total exposure time after stacking is still quite decent or if my total frames is a lot, I can still use those frames to improve my signal-to-noise ratio... those unwanted aircraft/cloud/satellite/meteor trails will usually "magically" disappear after the alignment and stacking process.

2) Lower noise as random noise will usually be reduce after stacking.

3) If you are not doing guiding, dust shadow will be reduce after stacking... usually no flat frame are required... unless you need very high quality images.

4) If you are not doing guiding, vignetting will also be reduce after stacking... usually no flat frame are required... unless you need very high quality images.

Originally Posted by ArchRival Also, for images of stars, should a change in focal length (hence a change in F/ratio) at constant aperture affect its brightness in the image? A star, at least ideally, is a pinpoint on the sensor. Regardless of "zoom", the star should still be a pinpoint, the exposure not affected by focal length.

Focal length only change the image scale (but star will still be a pinpoint) and does not affect the image brightness... the f-ratio and exposure time does.

By the way, for non-lunar/planetary astrophotography, shoot in RAW is basically a must as you need to do a lot of post processing.

Have a nice day.

[Daedalus Trent]

Thought i'd share this shot i took cos i'm interested in astrophotography too :


Exposure: 30
Aperture: f/5.0
Focal Length: 200 mm
ISO Speed: 200
Exposure Bias: -2/3 EV


Hardly got any light pollution in this shot cos the weather was really awesome that day. No clouds obscuring the sky at all and could see stars you rarely see under the cloud cover in SG

[mdzmr]

Originally Posted by Learnphotography yr images show that yr surrounding is too polluted with light. I read on astrophotography that learnt that keeping at most 30secs exposure is a sweetpoint before u get star trails. HOwever it still depends on yr location. ISO 400-800 is just nice. learn astrophotography: http://www.astropix.com/HTML/I_ASTROP/TOC_AP.HTM

Yap that the purpose of it how bad it is the pollution.

[weixing]

Hi,
Actually, you can still shoot decent astrophoto under our light polluted sky using normal DSLR. The key is to shoot in RAW, shoot multiple images to stack and expose each image long enough without saturation... make sure the brightest part of your image in the the histogram don't hit the right edge (leave a bit of space at the right). As long as the image don't hit the right edge, you still can get very decent image after processing even you can only "see light pollution" when view on the DSLR. I got one example of this... will show it here if I can found it.

By the way, if you look at Canon DSLR histogram (not sure about Nikon as I don't use Nikon DSLR), the raw value for the middle (8192 for 14-bit DSLR) of the output value is actually located at somewhere from the start of right most last section to the mid of right most last section on the Canon DSLR histogram.

Have a nice day.

[Learnphotography]

Originally Posted by weixing Hi, Actually, you can still shoot decent astrophoto under our light polluted sky using normal DSLR. The key is to shoot in RAW, shoot multiple images to stack and expose each image long enough without saturation... make sure the brightest part of your image in the the histogram don't hit the right edge (leave a bit of space at the right). As long as the image don't hit the right edge, you still can get very decent image after processing even you can only "see light pollution" when view on the DSLR. I got one example of this... will show it here if I can found it. By the way, if you look at Canon DSLR histogram (not sure about Nikon as I don't use Nikon DSLR), the raw value for the middle (8192 for 14-bit DSLR) of the output value is actually located at somewhere from the start of right most last section to the mid of right most last section on the Canon DSLR histogram. Have a nice day.

very good tips, thanks