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FORUMS Photo Sharing & Visual Enjoyment Astronomy & Celestial 
Thread started 27 Oct 2010 (Wednesday) 15:45
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You don't need a telescope

 
tkerr
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Feb 08, 2012 10:14 |  #751

VisualUniverse wrote in post #13848644 (external link)
Thanks. It is an f/1.4 lens and I shot at f/2.2. When shooting stars, it is normal practice to stop down a bit from wide-open.

It is?
I was always under the impression that the more light you can allow the better..
The more light that is allowed to hit the sensor usually means you can reduce the length of exposure needed which also means less noise, and a larger aperture means that you can capture more detail in that shorter amount of time.


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archer1960
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Feb 08, 2012 10:41 |  #752

It varies. I tend to stop down 1 stop from wide open to get a bit more sharpness. With tracking, longer exposures usually aren't an issue. If I'm shooting on a camera tripod, then I'll shoot wide open.


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luigis
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Feb 08, 2012 11:28 |  #753

tkerr wrote in post #13848739 (external link)
It is?
I was always under the impression that the more light you can allow the better..
The more light that is allowed to hit the sensor usually means you can reduce the length of exposure needed which also means less noise, and a larger aperture means that you can capture more detail in that shorter amount of time.

The problem is that lenses tend to work badly for the stars wide open. The borders will show elongated stars (comma) and a lot of chromatic aberration.

The drill is to take an exposure, check the stars at the border, change the F number, take an exposure, repeat until you are fine with your stars at the border of the frame.


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SteveInNZ
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Feb 08, 2012 13:12 |  #754

VisualUniverse wrote in post #13848644 (external link)
Thanks. It is an f/1.4 lens and I shot at f/2.2. When shooting stars, it is normal practice to stop down a bit from wide-open.

That'll save me some money then. I already have a f/1.4 (Takumar) that I stop down a click or two for astro use. :)


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Beardy
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Feb 08, 2012 19:06 |  #755

luigis wrote in post #11176966 (external link)
Who needs a telescope anyway?
Canon 550D + 400mm F5.6L + TC 1.4x

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IMAGE NOT FOUND
HTTP response: NOT FOUND | MIME changed to 'image/gif' | Redirected to error image by FLICKR

Sorry, I'm a total greenhorn here. What's a TC 1.4x? Also, can you provide more details on your camera settings for the shot?

Thanks




  
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archer1960
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Feb 08, 2012 19:16 |  #756

That means a 1.4x TeleConverter. That's an adapter that (usually) goes between the lens and the camera body, and increases the focal length by a factor of 1.4x, turning his 400mm f/5.6 into a 640mm f/8.0. You can also easily find 2x TCs and there are even a few 3x ones out there. The good ones don't affect your image quality very much, and the bad ones can make a decent lens totally useless.


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Beardy
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Feb 08, 2012 19:32 |  #757

Ah, okay then. Thanks Archer.




  
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VisualUniverse
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Feb 08, 2012 22:31 |  #758

tkerr wrote in post #13848739 (external link)
It is?
I was always under the impression that the more light you can allow the better..
The more light that is allowed to hit the sensor usually means you can reduce the length of exposure needed which also means less noise, and a larger aperture means that you can capture more detail in that shorter amount of time.

Yes, but when you open up the lens all the way, star distortion occurs (spherical distortion), especially toward the edge of the field of view, along with chromatic aberration on brighter stars. By giving a little bit of depth of field, these issues are minimized. So for f/1.4 lens, I do 2.0 to 2.5. For f/2.8 lens, I'll do f/3.2 to f/3.5.


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VisualUniverse
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Feb 08, 2012 22:34 |  #759

saiph wrote in post #13752292 (external link)
I just stumbled upon this thread while looking for some nice astro images. Just Got into the whole astrothing and bought myself an astrotrac for christmas :-)

Here's the very first result, looking forward for more once the clouds went away...

Eos 7D | 200mm 2.8 L at f/3.5 | 20 x 60s stacked

Very nice and congrats on the AstroTrac. Just got one last month and recently used it. Is your 7D modified to pick up the reds?


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Feb 08, 2012 23:20 |  #760

VisualUniverse wrote in post #13841904 (external link)
Single exposure tracked with an AstroTrac.

Awesome shot! How easy is the AstroTrac to set up and get going? Lot of cables involved? I assume it needs visibility to Polaris for polar alignment? I live in a place with a LOT of snow in the winter, and it's really not pleasant to be standing in thigh-deep snow trying to fiddle around with all the cables on my eq mount + camera + computer in subzero temps. Would love something that's (relatively) quick and easy to set up to be able to shoot in the winter.


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VisualUniverse
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Feb 09, 2012 07:59 |  #761

FuzzSummit wrote in post #13853602 (external link)
Awesome shot! How easy is the AstroTrac to set up and get going? Lot of cables involved? I assume it needs visibility to Polaris for polar alignment?

It is very quick and easy. Only cables were for the programmable remote (intervelometer) and AstroTrac battery compartment (holds 8 AA batteries). I don't have the battery pack showing in this photo set:

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tkerr
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Feb 09, 2012 08:35 |  #762

VisualUniverse wrote in post #13853402 (external link)
Yes, but when you open up the lens all the way, star distortion occurs (spherical distortion), especially toward the edge of the field of view, along with chromatic aberration on brighter stars. By giving a little bit of depth of field, these issues are minimized. So for f/1.4 lens, I do 2.0 to 2.5. For f/2.8 lens, I'll do f/3.2 to f/3.5.

DOF Isn't an issue when shooting the night sky. But I can see where distortion and CA could be a problem with a short focal length lens at such fast focal ratios.


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FuzzSummit
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Feb 09, 2012 14:48 |  #763

VisualUniverse wrote in post #13854811 (external link)
It is very quick and easy. Only cables were for the programmable remote (intervelometer) and AstroTrac battery compartment (holds 8 AA batteries).

Thanks, that is helpful. I may have to seriously look into getting the AstroTrac.


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VisualUniverse
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Feb 09, 2012 15:09 |  #764

tkerr wrote in post #13854980 (external link)
DOF Isn't an issue when shooting the night sky. But I can see where distortion and CA could be a problem with a short focal length lens at such fast focal ratios.

DOF is an issue if you want tight stars.


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tkerr
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Feb 09, 2012 16:08 |  #765

VisualUniverse wrote in post #13857219 (external link)
DOF is an issue if you want tight stars.

How is it an issue when you're shooting a very distant object at infinity?
What is the issue in order to get tight stars is perfect sharp focus, not DOF..
DOF is the range of acceptable focus sharpness from a point starting in front of the object to a point behind it.
When you shoot the dark night sky, as far as the camera is concerned you're shooting a flat object with no foreground or background. Everything at that distance is either in focus or none of it is.
DOF is not an issue of concern for astro imaging. Focus as well as clear steady dark skies are the issues of concern for nice sharp stars.
When using a short fast lens a concern is the field of view(FOV) and how flat that FOV is. Typically very fast short lenses don't have a flat field and you run into distortion problems, especially as you near the edge of the field.


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You don't need a telescope
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