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Thread: Favorite all-around focal length for "smaller" sky objects

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    Default Favorite all-around focal length for "smaller" sky objects



    What focal lengths (please specify full frame or crop) do people like for various "smaller" astro objects...Andromeda, Rho Ophiuchi, globs, Orion nebula, others? I realize there is a large range of apparent sizes of objects, but I'm looking to upgrade to a faster lens for astro, and wondering what a good all-around choice might be on my Fuji crop sensor. TIA!

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    Default Re: Favorite all-around focal length for "smaller" sky objects

    I wouldn't include Andromeda in that list: it is six times the size of the full Moon!

    I find that 800mm is a good all-round focal length for most DSO targets. It is a bit too long for large targets like M31, the Rosette, or the Heart Nebula, and a bit too short for smaller targets like more distant galaxies or planetary nebulae. But for a large number of intermediate-size objects, it is just right. For large targets, you probably want around 400-500mm. For smaller DSOs, 2000mm is a good size. For planets, I use 5600mm. My camera's sensor is 18.1mmx13.1mm.

    What focal length is going to work depends on how the rig is mounted. You are talking about camera lenses, I think, so you might be using a small tracker suitable for cameras. You will have to look at its manual to see what its weight limits are. For lenses or scopes suitable for DSOs, especially with the longer exposure times you will need, you should be looking at good quality motorized equatorial mounts.

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    Default Re: Favorite all-around focal length for "smaller" sky objects

    KathyNS, I just had a look at your albums here...wow! Amazing, inspiring. I should have said, I am talking about using an EQ tracker for the camera itself, with lenses. When you are talking about focal lengths of 2000mm and 5600mm, can I assume you are talking about total optical train focal lengths when imaging with a telescope?

    I see folks on Flickr and other places doing AP with camera and lenses of as short as 105mm and such getting great images of nebula and galaxies. Were your initial recommendations based on telescope AP? I observe with a 10" Dob, which I love, and really just want to take photos with my DSLR. Thanks for your information and advice!

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    Default Re: Favorite all-around focal length for "smaller" sky objects

    Quote Originally Posted by 4thpage View Post
    Were your initial recommendations based on telescope AP?
    Focal length is focal length. A 400mm focal length telescope will give you essentially the same image as a 400mm lens of the same aperture. The examples I gave were based on telescopes that I use or am familiar with. If you choose to try the longer focal lengths, chances are that a telescope will be more practical than a camera lens of the same focal length.

    A 105mm lens is a bit short for most targets. However, you can crop the image and get good results, depending on image resolution. Most camera-lens-based astrophotographers that I know operate in the 200-400mm range.

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    Default Re: Favorite all-around focal length for "smaller" sky objects

    A camera on a tracking mount for me is one of my more favorite forms of AP.

    Here are some examples from my Canon T3i with the kit 55-250mm lens on a Celestron GT goto mount on a wedge. These are unguided images.

    This is M31 at 200mm, (22 x 1-minute exposures)total time of 22 minutes at ISO 1600.




    This is M27 "the dumbbell nebula" at 200mm 20 minutes (20 x 1 minute) at ISO 1600


    This is M104 at 250mm 35 minutes (35 x 1 minute) at ISO 1600.


    And don't forget wide field Milky Way images. A single 3-minute exposure. Lens @ 25mm ISO 6400


    This is the mount the camera is attached to.


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    Default Re: Favorite all-around focal length for "smaller" sky objects

    Thanks for the reply! I'm not seeing the images for some reason, though.

 

 

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