You don't need a telescope

VisualUniverse wrote in post #13848644
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.

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.

tkerr wrote in post #13848739
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.

VisualUniverse wrote in post #13848644
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.

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

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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

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.

Ah, okay then. Thanks Archer.

tkerr wrote in post #13848739
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.

saiph wrote in post #13752292
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?

VisualUniverse wrote in post #13841904
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.

FuzzSummit wrote in post #13853602
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:

IMAGE LINK: http://www.flickr.com …sual-universe/6844993261/

VisualUniverse wrote in post #13853402
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.

VisualUniverse wrote in post #13854811
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.

tkerr wrote in post #13854980
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.

VisualUniverse wrote in post #13857219
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.