Long Exposure

[Narciso]

Hello everyone! I am new to Stargazers Lounge and I am hoping that I can get some help when it comes to imaging, perhaps even post-processing. I took off to Malibu, Ca. a few nights ago to process Orion and all of its nebulosity, which has been tough for me to image. My idea has been to do an extremely long exposure, but in doing so, I blew out the image. I actually took a 10 minute exposure at ISO 1600. My question is, what would be the best exposure time to capture nebulosity? Also, will bringing down the ISO to the low hundreds to prevent the image from blowing out, even with a hypothetical 5 minute exposure, diminish the amount of nebulosity that can be obtained? By nebulosity, I refer to the Horsehead Nebula, Barnard's Loop, etc. And a final question, is it better to get several shorter exposures of say a minute and then stacking them, or to get a single long exposure of an excess of 5 minutes? The equipment that I used: Canon Rebel SL1, Canon 24mm f/2.8 lens, and the iOptron SkyTracker. I know I also don't have the best equipment, so I'm hoping that maybe someone can let me know if that in itself can prevent me from obtaining better images. I thank you all in advance for your help and advice! 

[Galen Gilmore]

Although I'm not experienced with imaging, I know that you have to capture dark frames so you don't completely kill the image. By dark frames I mean an image of the same exposure time and ISO but with the lens cap on.

[Thalestris24]

Hi

Have a look in the widefield imaging section and see what, and how, others have done with similar equipment. If you suffer from light pollution and skyglow that will limit the length of individual exposures. Perhaps think in terms of taking many short exposures and stacking them. I wouldn't worry about taking darks. I'm not sure how sensitive the sl1/100d is to Halpha but you might consider getting it astro modified or of getting another second hand canon to get modified.

Louise

[alacant]

Hi. Starting point: focus on Sirius - compose on Orion - f4 - -ISO800- take 20 2 minute snaps. Unless you have a modified camera you'll get little if any nebulosity. HTH.

[Art Gecko]

If your image is totally blown out, take a look at your histogram, the data spikes might be all over on the right hand side? If this is so, use the dark slider on the 'levels' to reduce the range and bring the image out and make adjustments to the 'curves' to see what you've actually captured.. You might be surprised! There is a technique called ETTR or 'Expose To The Right' I've seen it used to minimise light pollution... You may have accidentally stumbled upon this way of imaging.

[ollypenrice]

M42 is one of only a very few objects which has too much dynamic range to be captured by a single exposure length. It's the only object I've ever imaged on which multiple sub exposure lengths were literally essential. That's a complication you can do without, as a beginner. I would try different sub lengths till you get a good result and then take lots of those.

Olly

[wimvb]

+1 for @ollypenrice's advice. Add to that the fact that lower iso will usually give you less noise and more dynamic range, I would suggest to try ISO 800 or 400 if your setup and conditions allow. Since you already managed 10 minutes exposures, lowering ISO would make sense. Also, your lens may perform better when stopped down to f/4 or so.

In other words: experiment.

[Thalestris24]

FWIW there's an Orion taken at 24mm to compare with here

Louise

[Alpollo]

21 hours ago, Narciso said:

Hello everyone! I am new to Stargazers Lounge and I am hoping that I can get some help when it comes to imaging, perhaps even post-processing. I took off to Malibu, Ca. a few nights ago to process Orion and all of its nebulosity, which has been tough for me to image. My idea has been to do an extremely long exposure, but in doing so, I blew out the image. I actually took a 10 minute exposure at ISO 1600. My question is, what would be the best exposure time to capture nebulosity? Also, will bringing down the ISO to the low hundreds to prevent the image from blowing out, even with a hypothetical 5 minute exposure, diminish the amount of nebulosity that can be obtained? By nebulosity, I refer to the Horsehead Nebula, Barnard's Loop, etc. And a final question, is it better to get several shorter exposures of say a minute and then stacking them, or to get a single long exposure of an excess of 5 minutes? The equipment that I used: Canon Rebel SL1, Canon 24mm f/2.8 lens, and the iOptron SkyTracker. I know I also don't have the best equipment, so I'm hoping that maybe someone can let me know if that in itself can prevent me from obtaining better images. I thank you all in advance for your help and advice! 

With regards to dim objects, 5 minutes would be better then 5x1 minutes in most cases.

Lowering your ISO will improve colour, but it can obviously still be overexposed at some point. ISO just trades quality for time, but its not a substitute for time.

I can't speculate a specific ISO and and exposure not knowing your tastes and sky conditions, but I generally expose until I get a 1/3rd histogram at the back of the camera. It is a good starting point as that is usually when the sky colour starts to get dominate. Have a look then lower your ISO if need be, not your exposure time unless you really have to.

Low and Slow, as much as you can stand from a practical point of view. I generally start a session at ISO 400 and only go higher if I can't track long enough for a 1/3rd histogram.

It'll have to be a judgement call on your part, weighing out keeping the core contained with rich colours vs seeing enough nebulosity in a single image.

The tactic generally employed for the all in one image is stacking 2 or 3 images etc. I would still approach these images the same way. It would preferable the use the lowest ISO possible for all three, ideally the brightest one would have the longest exposure, all of them taken at ISO 400 if possible. (Or whatever the sweet-spot is for your model of camera, mine happens to be 400.) This will give you good quality data for your efforts.

[Alien 13]

It is usually best to shoot with an ISO that gives unity gain or slightly above, this depends on camera model but on most newer Canons it is around 1000 ISO (check your camera model), if this still over exposes then drop the sub time to around 2-4 mins which is usually the best compromise between read noise and thermal noise because the sensor is getting hot.

Alan

[Narciso]

Wow, thank you all for your replies! This will all be so incredibly helpful! I have Spring Break coming up soon, so I'll have plenty of time to test out all methods that you've all outlined. Also great to see that there are others out there that are trying for the same target to get plenty of insight. I attached my 10 minute single exposure along with this, in case anyone wants to maybe take it and see what they can get out of it!  I'll hopefully have some images to show off for you all very soon!

IMG_7437.CR2

[bobro]

14 hours ago, Narciso said:

I attached my 10 minute single exposure along with this, in case anyone wants to maybe take it and see what they can get out of it!

Well, I did have a go with the image (helps improve my basic skills in GIMP 16 bit). It wasn't really blown out (Orion nebula aside), just very bright. Bringing the brightness down helped with that, plus a small amount of stretching and basic flattening to try and remove the vignetting. There's a lot to like about the image - Orion, Running Man, Flame and HorseHead (just!) nebulas, plus NGC2264. The colour of Betelgeuse is lovely. A few more subs and it could be a cracker.