Long exposure astrophotos using dslr camera

[pericles]

Hello.
One part of my photo interests is astrophotography,which means long exposure pictures. The theory says (correct me if I am wrong ) that instead of taking a long exposure picture , let's say two minutes,it's better to take 4 pictures X 30 secs each( using stacking methods ) , that's because we get noise reduction and generally better pictures.
But what I don't understand is,that , how it's possible to get the same picture info, (so important from the dark sky )of 30 secs exposure even if we get it four times,when
2 min exposure means more light and info coming to the camera.
My opinion is that stacking methods of many short exposures can not replace a long one in the field of picture info .
Could someone tell if I am right or not please.

Thanks

Pericles

[bambleweeney]

It is indeed way better to take several short exposures. One long exposure will introduce a lot of noise into the image, enough to overwhelm some of the less bright celestial objects. Several short exposures will not have the same problem. Really with stacking you only need a few photons to 'light up' a pixel consistently across several frames for the stacking program to recognise this as a star (depending on your stacking software naturallly). You also need to take black frames at regular intervals during the process to account for any extra noise generated as the camera circuitry warms up.
Anyhow this is the basis of what I remember when I downloaded skystacker (or starstacker ... or something) because I had some shots planned that would have rocked the photographic world - unfortunately it just didn't run on my PC - at all. Google stacking software, the ones I looked at had quite interesting technical explanations of the whole concept.
One day I'll find a version that works because it is a fascinating area of photography - I remember seeing a wide field stacked shot of Orion a couple of years back - unbeleivable !!

[victor]

Dear all

Try the following software:

Startrails from:

http://www.startrails.de/html/software.html

Sample picture below. From 29 stacked images of 30 secs each.



Regards

Victor aka David

[bambleweeney]

Thanks David, I'll look into this. DeepSkyStacker was the name of the software I was trying to think of if I remember rightly which was quite highly rated - but didn't even want to start on my PC.

PS just had a quick look at this and it looks quite good at star trails - as its name suggests. Some of the other stacker programs calculate the skies rotation and overlay stars on top of each other. The result is that even objects invisible to the naked eye appear in the image. The shot of Orion that I mentioned earlier (wish I could find it now) taken with a stacking program reveals a mass of stars that I can't see even on the clearest nights and a huge gas cloud round the constellation that I didn't even know existed - but I'm no astronomer so I might be the only one this was news to

[bambleweeney]

Oh - nice shot by the way David, particluarly like the movement captured in the clouds

[pericles]

Thanks for your prompt reply Paul.
I have some stacking programms and using them with dark frames etc. as well.
But I am talking about deep sky objects such as long distance nebulae ,galaxies etc .which they need 20,30.40 min and more exposure.I am only focusing at this point ,how can we have an acceptable image of so dark objects , dividing it in multiple short exposures ?.The ammount of light I think is critical and the only way to get it , is long exposure

[bambleweeney]

My experience in this area is ... limited shall we say. However this discussion re-kindled my interest and I decided to give DeepSkyStacker another try. Looking at their site there are a lot photos of deep sky objects so first thought is that it does seem to work. I still think that a 30 minute exposure will generate enough noise to drown out faint objects, noise reduction will take out some of the noise and I would imaging most of the faint objects as well. I did read an article some time back - don't remember where now - where it was stated that several (possibly talking about 2-300) exposures were more efficient than one long exposure. Even if only 1 exposure in 10 or 20 picks up a faint object then surely stacking would be better than the noise from one long exposure. Interesting from a technical point of view. I remember seeing some scope specific digital 'cameras' that employed serious capture chips and liquid nitrogen or something equally exotic to keep the temperatures stable. (Keep that one in mind Nikon for the D4).
I don't know if you've read this :-
http://keithwiley.com/astroPhotograp...Stacking.shtml
but it's quite interesting and explains how an even below threshold pixel which would appear black on a single frame will produce an image when stacked. Sorry if you already knew this stuff - I just find it fascinating
I live in an area totally free of light pollution ... if only I had the means to buy a scope

[Shadowman]

Are you using a telescope to track the stars? If so, what kind?

[pericles]

Yes , I have a Celestron 8 inch cassegrain , computerized ,with external guiding . And one 2,5 inch skywatcher telescope for driving.
All these comes with necessary accesories and adaptors.

[Shadowman]

Yes , I have a Celestron 8 inch cassegrain , computerized ,with external guiding . And one 2,5 inch skywatcher telescope for driving.
All these comes with necessary accesories and adaptors.

One other question is how are you photographing the image, piggyback or through the telescope lens? With either method, stacking should be easily done if the telescope is tracking the object you are photographing. That way you should get nearly the same image in each photograph.

I am a newbie when it comes to astrophotography so this is about all I know about the tracking function. I just purchased my first telescope (Celestron 114 LCM) about a week ago and it took me a few days to figure out star alignment.

[pericles]

Mostly through the telescope , but sometimes i am using piggyback as well . Yes I agree with you about stacking and I am using it a lot.
I realize all the benefits of stacking , but my main question remains . How a number of short exposures is equal with a single one of the same total duration (that's the books says ). The total amount of light we need for a specific exposure it doesn't added through stacking method .That is , for example , 5 X 20 secs exposures are equal to a single 1 min exposure regarding the amount of light . Is that right ? I think not.

[Shadowman]

Mostly through the telescope , but sometimes i am using piggyback as well . Yes I agree with you about stacking and I am using it a lot.
I realize all the benefits of stacking , but my main question remains . How a number of short exposures is equal with a single one of the same total duration (that's the books says ). The total amount of light we need for a specific exposure it doesn't added through stacking method .That is , for example , 5 X 20 secs exposures are equal to a single 1 min exposure regarding the amount of light . Is that right ? I think not.

5 x 20 second exposures wouldn't be equal. According to the author of a book I am reading, Digital Astrophotography by Stephen Seip, you should average two pair of images, then to average the respective images again. So you would want your two images to have similar exposures.

[Omykron]

As far as I know (and it is a little), what do you want to get is how you take the pictures and how much time do you expose the sensor to the (pale and dim) sky light.

if you want nice trailing lights of the stars in the sky, to show the earth rotation, you can stack images from 15, 30, 1' exposures. the stack will reduce sensor noise and make your sky looks awesome and add action to this very slow action. to do this, only play with F and mm.

if you want a bright, and easy to resolve constellations and to show the world the beautiful milk-way, than you do something (very) different (and interesting)).
You are going to need a high ISO, to compensate the pale and dim light from the sky. think very high ISO Values. And I really mean HIGH. and run from the city. countryside, away from light pollution. Wide and bright lenses increases the amount of light and color to the sky, and you can keep shutter speed as low as 5".

My opinion is that stacking methods of many short exposures can not replace a long one in the field of picture info .
Could someone tell if I am right or not please.

Your assumption is wrong for the second case, and true for the first. The level of detail for the first case is minimum, because all you are going to see is light trails. You wont see the Large Magellanic Cloud, or the coalsack nebula in the crux constellation.
With lower speed and high ISO, you add more light information, and with stacking and align, the point of interest will be align, and more light information from less brighter stars and nebulae (like the milk-way arms) will be brought to you, because the stack will add 2 levels of light to increase the total brightness. It is not rocket science.

This is what I get when stacked only 3 images + 1 dark frame with ISO 10900 (don't believe in the EXIF, used CHDK to override ISO) with 2.5 seconds from my SX30 IS (1/2.3" CCD Sensor).

[pericles]

If hypothetically (there is not such meter ) an exposure meter says that we need 30 min exposure for a specific dim object ( considering very good telescope guiding ).That means the sensor needs it in order to get the most info from it.If we divided the same picture taking let's say 6 exposures X 5 min and after using stacking method , we get better light , decreasing the noise and generally all the benefits of stacking method , but , this is the point which I insist , the INFORMATION we get , is not the same , as info ( details ) does'nt added with stacking method.This is the point which I am stacking too !!!!!!

[Shadowman]

Latest attempt through the telescope. As soon as the weather breaks I will open the window. I was recently in South Carolina and there wasn't a star in the sky. I would have thought this would have been an ideal area for astrophotography.

[bucketman]

hi all
heres one through my scope taken with a £20 webcam. About 1800 frames then stacked

[Shadowman]

hi all
heres one through my scope taken with a £20 webcam. About 1800 frames then stacked

I've read about this method, looks good.

[The Blue Boy]

@ Rob,

This is something that I'm starting to get interested in. Stacked using what program? Black frames? How many? What lens?????? What aperture?

So many questions to ask for someone (me) that has no clue!

I think we need a whole new thread devoted to this sort of photography.

[bucketman]

Hi Mark
First off its not a dlsr image. It was taken with a Celestron 6se scope. So about equal to 1500mm focal length. You then put a webcam where eyepiece goes and record a video about 90secs long. I used Sharpcap software. The video works out about 900 single frames. You then put it into another software programme Registax and this does the work. The one I've posted is 2 pics put together with Windows ICE. You could mess round with it in GIMP if you wanted. All software are free downloads. Just learning so sorry if this doesn't help much. I learnt it all from another forum, but feel free to ask ????????? and I will try and help
rob

[koolkat]

I also am interested in star trails. I've shot several Canon RAW single shots of an hour + time. Attached I hope is one I shot at 9:43PM F5 at 30mm using a Tamron 18x270 lens, manual settings for TV(shutter speed) of 3702 = 61.7 minutes.