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Thread: Stacking images from alt-azimuth mount (image shift)

  1. #11
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    No, as long as you're Alt/Az, tracking or otherwise, you will get field rotation! You can still get field rotation with a GEM but, it is greatly reduced and proportunate to the accuracy of your polar alignment. Alt/Az is great and simple for visual observing but, for imaging, the exposures have to be kept very short and you'll have to stack a lot of them.
    Jenniferchristine has beautiful pictures from short exposures but, I bet a lot of stacked frames. It's a trade-off/compromise on how you want to get the images.
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  3. #12
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    With an Azimuth mount with tracking you are limited in exposure duration by field rotation. The length of time is dependent upon where in the sky the object is located... near the Zenith, very short .... seconds.... while near the horizon ... a minute or but a lot of atmosphere. When you stack the images you also adjust for field rotation. One thing to keep in mind, while it is true that say 200 ten second exposures is equal to one 2000 second exposure there are limits to how short the exposure can be .... dependent upon the brightness of the object and surrounding stars. Short exposures, especially if the surrounding stars are very dim, can be too underexposed for stacking programs to work.

    Trying to use a DOB would be an interesting experiment. I suspect that a high percentage of your images would be rejected by a stacking program as they would have too many flaws due to vibration, tracking error, image shift, etc. Keeping the DOB on target, vibration free, etc. during an exposure seems like a difficult task. Exposure rejection rates can be high even with a AZ computerized drive.
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    Going back to the original question (sorry) I've changed the settings in DSS and it's come up trumphs

    I know there different variables as to ISO settings but is there a favourite range that anyone uses particularly for galaxies or dso's? Only asking so next time I can get rid of some more noise because I had my 5d in a really high setting 6400 to help with the short exposures.

    Thanks Jennifer for the pic! Looks awesome and a big thanks to everyone's help (I love this forum you never fail me!)
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    At the risk of hijacking this thread, I am trying to do the same thing. I have an Orion XT12G (goto/tracking dob), a Nikon D40 DSLR and the SteadyPix Deluxe camera mount. I tried taking some a-focal images of M13 the other night. I set the ISO sensitivity to 1600 and used the 10s delay to trigger the shutter release to eliminate vibration. The exposures seemed to be about 10 seconds or so each. I've uploaded 5 images here:
    MobileMe Gallery

    They appear nearly completely black but I saved them after brightening up in Picassa here:
    MobileMe Gallery

    At any rate, attempting to stack the original (or brightened) images in DSS results in only 1 frame being usable?

    I don't understand that. Also, if I continue anyway I end up with an image that looks like an old black-and-white tv set on a non-broadcast channel (a bunch of grey "static").

    Thanks for any and all advice,

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    NO-- all alt az mounts will exhibit field rotation.
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    Quote Originally Posted by gbcarync View Post
    I set the ISO sensitivity to 1600 and used the 10s delay to trigger the shutter release to eliminate vibration. The exposures seemed to be about 10 seconds or so each.

    They appear nearly completely black but I saved them after brightening up in Picassa here:

    I don't understand that. Also, if I continue anyway I end up with an image that looks like an old black-and-white tv set on a non-broadcast channel (a bunch of grey "static").
    I would consider ten seconds at ISO 1600 to be underexposed, so I would expect them to be dark. I took some shots of M13 a couple of weeks ago, and my subs were 1 minute at ISO 1600. It seemed about right for prime focus at f/4.6.

    I don't know what effect afocal photography has on your effective aperture, but it seems to me that you are getting additional magnification from that configuration, which means less available light per pixel. Accordingly, you would need more exposure than mine.

    I too got a high-noise image out of DSS. However, it only took a moment of playing with the curves in Photoshop to fix that, and I ended up with an image that I am quite proud of.

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    In DSS if you drop your stars down to 2% -in the 'advanced' tag on the dialog box that comes up just before you stack, it may recognise more stars and give you a bigger stack.
    As Joe says, zenith gives huge field rotation very quickly -less than ten seconds. Under ten seconds exposures will give you a poor chance of stacking. It may well interpret dim stars as noise and discard them as stars.
    Pick your subjects at about 45 degrees along the east west line from your place. that will give you the best chace of getting a decent exposure - go for iso 800 and 24 seconds - unless your noise is very low in your camera - in which case you can use iso 1600 - winter is a good time for that!

    Use mirror lock up as well as delay - low vibration !
    Use raw pics in DSS if you can - there's more information in them
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  14. #18
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    Thanks Jennifer that's perfect!
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    Jennifer.

    Just getting into imaging..thks for the advice.

    Do you image in both alt az and with a wedge?

    Thks

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    Default Re: Stacking images from alt-azimuth mount (image shift)

    A motorized (tracking) dob will NOT eliminate or reduce field rotation because a dob is basically an AZ/EL mount and all AZ/EL mounts suffer from field rotation. An equatorial mount does avoid field rotation and is the only practical way to get long exposures.

    What Sxinias and Jennifer are saying is that if you use short exposures (Jennifer has success with ~30 seconds or less) then you can rotate each individual image and then stack the images and get good results. Sxinias says that DSS does the image rotation and stacking for him and it does a good job.

    I also have an AZ/EL mount and am just starting in astrophotography. I have created a program that tells me the maximum exposure allowed for my AZ/EL setup before star tailing becomes a problem (always worst at the edge of the FOV). The maximum exposure depends on telescope aperture, FOV of the camera/telescope combination, observer Latitude, and object (galaxy, etc.) Declination. I am looking forward to testing the program results against actual images of deep space objects.

    Clear skies and clear images,

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