140 Minute Exposure

[nighthound]

My first attempt was about an hour, this time 2 hours and ten minutes resolved more around the perimeter. Didn't think I'd need too much more time but maybe another hour or so, and then another. . . :-)

Shot with the Sky 90 II/G-11 • Canon 20D(unmodified) • 1600 ISO • 140 minutes: All subs between 2 and 3 min. unguided • IDAS LP Filter • Stiletto for focus.

The Rosette Nebula
http://i3.photobucket.com/albums/y67/Nighthd/Astrophotography/rosette140x.jpg

NH

[Ann]

What an awesome shot! Colour is amazing! One heck of a long exposure!!!

[stilllearning]

Great shot!! If these were unguided, how often do you need to manually adjust to keep it in line? If they are slightly misaligned does the software you stack with align them for you?

[Robert]

Incedible!! When one considers the size of each star, and the distances between them and from here to the nebula, this image becomes all the more awesome. Beautiful shot!!!

[nighthound]

Thanks Ann and Gary.

Gary, The reason these exposures were unguided was because I spent about an hour doing what's called the drift align method. This refines the initial rough polar alignment that I do when I set up. Not only must the mount track at the correct speed to counter the earth's rotation but the scope/mount must be pointed precisely so that the movement is not skewed or tilted to the plane of movement. Drift alignment essentially squares your rig up. There are two ways to guide, manually by keeping a chosen star centered in a cross hair-type reticle with the hand controller for the duration of exposure and the other is to guide automated with an autoguider unit that locks onto a chosen star in the FOV and corrects the mounts errors for you. Manually guiding requires a guide scope(for the reticle I mentioned), usually a small refractor piggy-backed on the mainscope which has the camera attached. You can manually guide without a guide scope using a flip-mirror in line with your camera that allows you to guide and simutaneously shoot through the incoming light path.

After I do a drift alignment I shoot test shots to see how long I can expose images before error starts to be visible. The better the alignment, the longer the exposures. I can get all I need at 2-3 minute sub exposures(at 1600 ISO, f/4.5), plus our skies are not dark enough to go much longer. Even with the light pollution filter there comes a point where the light pollution starts to win out over the good light I'm after. In the summer months I limit exposures to 2 minutes, much of the time at 800 ISO. The warm temperatures just drive the noise level too high. In the winter I can go longer with very good results.

Your question about image rotation is a great one. When shooting an object for a long period there are subtle rotations that are occuring as time goes by. There will be subtle differences in the angle of the object in the frame. When stacking the images you go through each frame manually in ImagePlus and select the same two stars at opposite sides in each frame. The software will align all frames using those two stars automatically. The result of stacking 30, 2 minute sub exposures for example results in a one hour of compiled exposure time with the added bonus of reduced noise levels. Really a great software program.

NH

[nighthound]

Incedible!! When one considers the size of each star, and the distances between them and from here to the nebula, this image becomes all the more awesome. Beautiful shot!!!

Amazing, isn't it Robert? Part of what drew me to this hobby was the mind boggling sizes and distances of all that's out there. The more you start learn the numbers the more incredible it all becomes. Many times an object's distance will make you say "whoa" and then you discover that the object is as large or larger across than the distance it is away - double"whoa"! The light that enters a telescope and camera in many cases is ancient. No one alive today will live long enough to see many of the most distant deep-sky objects as they actually look right this minute. We calculate long distances on this planet in thousands of miles, which is part of why we have such a hard time wrapping our minds around trillions and beyond.

NH

[stilllearning]

I can see I may not make it to deep space shots this winter. One is there still is much information for me to digest. Two there will be additional equipment needed to aid me in this venture. As we know that is extra $$.scared;

Are there advantages/disadvantages to taking more short exposures verse longer exposures when stacking your images? Say 30-60 seconds per. Is there a point of diminishing returns when it comes to stacking or is that determined by the software used?

[nighthound]

I can see I may not make it to deep space shots this winter. One is there still is much information for me to digest. Two there will be additional equipment needed to aid me in this venture. As we know that is extra $$.scared;

Are there advantages/disadvantages to taking more short exposures verse longer exposures when stacking your images? Say 30-60 seconds per. Is there a point of diminishing returns when it comes to stacking or is that determined by the software used?

The advantages to shooting shorter sub exposures is less noise per shot and a less accurate mount and alignment are required. If your skies are very light polluted there would be some advantage as well. To truly go deep and capture faint light however you must go long enough to get that light, at least faintly, in the exposures. If the signal's not there in the subs it won't be there in the stack either. There are so many objects and each has a different magnitude of brightness. The dimmest objects can be reserved for down the road as you improve your skill and upgrade equipment, if you choose to. There are plenty of bright objects to practice on, like Andromeda, The Great Orion Nebula, star clusters and of course the Moon and the brighter planets. Piggy back shooting is a lot of fun as well and is much more forgiving than shooting with the scope itself. I like widefield shots that show multiple objects.

This hobby can get costly but that depends on how far you want to take it. Your set up will allow you to get some very nice shots, some that will likely amaze you. Take it slow and keep it fun is my philosophy. Try not to be overwhelmed by all the technical info, I know there's a lot. It'll all start to click in no time at all.

NH

[Jytte]

Great to have someone like you in the forum to show these amazing photos. bowdown;

[stilllearning]

NH,
Thanks again for your help. I was an avid amateur astronomer from age 10 through 18. Lost track when I went to college in southern california and was confronted with air and light polution at night. I didn't have to deal with that out in the middle of Missouri while growing up. I guess I got spoiled thinking you could see the stars anywhere the same. Now I'm picking up where I left off and taking it several steps further. I will take it slow and my wife will help me take it slow.;) Otherwise I was ready to go get one of those Sky 90's.:D
Well hope you continue to stick around here. We need a resident astrophotgrapher to help us out now and then.

[nighthound]

You're very welcome Gary. Sounds like you've got a good plan and lot more knowledge than when I got started so I think you'll do extremely well. Once you start to see results it gets pretty exciting. The most difficult thing for me was trying to pace myself. I like quick results which means spending more $$ so I'd pull back and keep pushing the gear I have and work on processing, etc. In the long run it's the knowledge that you'll benefit from most. The high dollar gear doesn't operate on its own anymore than the lower dollar stuff does. Good luck and keep me posted as you get set up and under way.

NH

[nighthound]

Great to have someone like you in the forum to show these amazing photos. bowdown;

Thank you Jytte. It's great to be here and be able to share this fascinating hobby.

NH