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FORUMS Photography Talk by Genre Astronomy & Celestial Talk 
Thread started 16 Jan 2011 (Sunday) 07:34
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Finding objects with long slow DSLR lenses

 
weeatmice
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Jan 16, 2011 07:34 |  #1

I often shoot the sky with just my camera and tripod and I am thinking about trying an Astrotrac at some point.

I have recently been practicing with just my 400mm f5.6 and attempting to get an image of Andromeda. I realise that I wont get decent detail but I wanted to see what the framing was like.

Whilst I can find it with wider lenses, It was impossible with this one. With wider lenses I can see it with a few seconds exposure, but never naked eye and never through the viewfinder or liveview as it is too dim.

Anyone have any tips for finding objects like this at longer focal lengths? A friend has a 100-400mm f5.6, I suppose that would be easier but I'd rather use my own lens.


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Nighthound
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Jan 16, 2011 12:11 |  #2

While I haven't used the 400 (5.6) telephoto I have imaged Andromeda at 407mm with my refractor. It's actually a 500mm refractor but with the focal reducer/field flattener it works out to 407mm.

I'm not sure an Astrotrac is robust enough for the 400 and your DSLR, but maybe so. I know they have heavier duty mounting available now.

400mm is not extremely high as astro focal lengths go but I can certainly understand having trouble locating and framing it up without the means to track it. Attempting to photograph Andromeda at 400mm without the means of tracking will be an exercise in futility. Even glass in the f/2 to f/4 range would not eliminate the bigger beast and that's Earth's rotation which is made very apparent at 400mm. You'll see movement after a couple of seconds and that's no where near the exposure length required. The darker the skies are obviously the easier it becomes to see with the naked eye so I would start by seeking very dark skies for the sake of locating it when you're set up with a tracking mount or device.

Here's what 407 mm looks like with a slight crop using a Canon 20D, Its around 3 hours of total exposure time at f/4. Individual exposures in the staked file were 4 minutes each:
http://i3.photobucket.​com …lery/andromedax​cg3cxc.jpg (external link)

Here's a single 1.5 minute exposure of Andromeda under dark skies using a 100-400L/20D piggy backed on top of a telescope that was on a tracking mount:
http://i3.photobucket.​com …iggy%20Back/m31​pb2xsx.jpg (external link)

Here's a step-by-step way to locate Andromeda:
http://www.wikihow.com​/Find-the-Andromeda-Galaxy (external link)

I use the "W" shape of Cassiopeia to quickly find it. I locate the highest peak, actually the lowest side in the "W" shape of the constellation and draw an imaginary line straight down from it. If the skies are dark enough that line will lead you right to the fuzzy glow of Andromeda.


Steve
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weeatmice
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Jan 16, 2011 12:55 |  #3

It wasn't really for the purpose of imaging, but to see if I could locate it within the narrow FOV, I realise the images would be useless.

Astrotrac probably isn't up to 400mm but I'll try regardless :)

Thanks for the links and info, I guess its just a case of really becoming very familiar with its location.


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SteveInNZ
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Jan 16, 2011 13:08 |  #4

One method I have used fairly successfully is to take a slightly out of focus image with a shorter lens centered on the object just before swapping to the longer lens and taking an out of focus image with that. The reason for using it out of focus is that it removes the dim stars, makes the bright stars bigger and adds color. You've made your own star chart in camera that matches your test exposures and has color.

Steve.


"Treat every photon with respect" - David Malin.

  
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weeatmice
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Jan 21, 2011 09:53 |  #5

Thanks Steve, thats a nice idea the more I think about it. I did many wider shots but never thought to use them as a star map.


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Finding objects with long slow DSLR lenses
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