You don't need a telescope

tkerr wrote in post #13857603
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.

What I'm stating comes from common knowledge amongst astrophotographers, is taught by astrophotography instructors, and is a proven concept. I simply stated the facts and since my experience has affirmed what everyone before me in this activity has stated, I will continue the practice. If you want to debate this from your theoretical viewpoint, I suggest taking it up on the Cloudy Nights forum in the DSLR section where the experts hang out. At this point, I am done.

tkerr wrote in post #13857603
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.

I recommend reading this article by Jerry Lodriguss on using lenses for astrophotography...he talks about the effects:
http://www.astropix.co​m/HTML/I_ASTROP/LENSES​.HTM

The issue is not stopping down to reduce vignetting, CA, and other distortions (as noted by both Tim and Jerry) -- that goes without saying. The issue is your assertion that depth of field is important in getting sharp stars. As far as I know, whether a star is 500 light years or 5,000 light years, it's all the same to the camera - infinity. A fast lens should be stopped down not to increase DoF, but rather to reduce distortions.

VisualUniverse wrote in post #13858051
I recommend reading this article by Jerry Lodriguss on using lenses for astrophotography...he talks about the effects:
http://www.astropix.co​m/HTML/I_ASTROP/LENSES​.HTM

He does mention vignetting and optical aberrations, but I didn't see anything about DOF.

VisualUniverse wrote in post #13858051
I recommend reading this article by Jerry Lodriguss on using lenses for astrophotography...he talks about the effects:
http://www.astropix.co​m/HTML/I_ASTROP/LENSES​.HTM

I already have his astrophotography pages bookmarked, and I have read much of Jerry's stuff long ago.

You must be referring to this which has absolutely nothing to do with Depth Of Field (DOF).
Quote:
All lenses will also exhibit "vignetting", or more properly, "geometric light falloff." This is an uneven circular field illumination when used wide open, which continues to improve as the lens is stopped down down 3 to 4 stops. Optical aberrations such as astigmatism and coma are generally much improved by the time the lens is stopped down two stops.

Camera lenses, however, can be used for shooting the stars! In most cases they must be stopped down a stop or two from wide open to improve coma, astigmatism and chromatic aberrations enough to produce usable stars. Even lenses that work very well wide open, such as the Nikon 180mm f/2.8 ED and Canon 200mm f/2.8, will get sharper if they are stopped down one or two stops.

No mention of nor does it have anything to do with the DOF. It does however have to do with, among other things, something that is called Petzval Field Curvature. The inability for a lens to bring a distant object to focus evenly throughout the entire FOV all the way to the edge on a flat image plane.

FuzzSummit wrote in post #13858094
The issue is not stopping down to reduce vignetting, CA, and other distortions (as noted by both Tim and Jerry) -- that goes without saying. The issue is your assertion that depth of field is important in getting sharp stars. As far as I know, whether a star is 500 light years or 5,000 light years, it's all the same to the camera - infinity. A fast lens should be stopped down not to increase DoF, but rather to reduce distortions.

Exactly! ^^^^^^^^^^

Allow me to clarify: if you care only about the center of the FOV (e.g. a planetary nebula) at a very short exposure where the lens is at ambient temperature and the temperature will not change over time, then concern about DOF is unnecessary. But if you want the entire FOV to contain tight stars and account for focus shift from temp changes, then yes...DOF is a concern.

TKerr was initially attacking my assessment about wide-open lens shooting. Sorry for distracting with the DOF reference.

VisualUniverse wrote in post #13858196
Allow me to clarify: if you care only about the center of the FOV (e.g. a planetary nebula) at a very short exposure where the lens is at ambient temperature and the temperature will not change over time, then concern about DOF is unnecessary. But if you want the entire FOV to contain tight stars and account for focus shift from temp changes, then yes...DOF is a concern.

TKerr was initially attacking my assessment about wide-open lens shooting. Sorry for distracting with the DOF reference.

No I was initially asking about your statement that "it is Normal Practice" To stop down a bit from wide open. Which could be a misleading statement for someone just getting started if not clarified.

It might be a practice that some people do stop down, but it is not a practice that everyone does. therefore it is not Normal Practice to stop down the aperture.

And then you replied:

VisualUniverse wrote in post #13858196
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.

I replied that DOF isn't the issue, and I will still argue that it isn't. Because it is not.. DOF has nothing to do with those distortions or aberrations.

There is no DOF at those distances against the dark night sky..
DOF is a non-issue for astroimaging.


What you seem to be concerned with however is an "Even Field" or Flat Field as well as other issues that cause aberrations such as CA, Coma and Astigmatism. Focal ratio is only one, which by increasing can help decrease the effects of some of those problems that are inherent to the lens design and build.

Focus shift is another issue all together. Unless you have a rapid and drastic temperature change, as long as you allow your camera lens to acclimate properly, the amount of focus shift on a small camera lens due to the expansion and contraction of the glass elements is very insignificant if there is any. And the amount of time for a small camera lens to acclimate is very short.
Those things are more of an issue with larger telescopes that are more prone to temperature variances. Otherwise focus shift is a mechanical issue which should be a non issues once set.
Mechanical Focus shift is also a problem using a camera on a telescope when the focuser isn't or doesn't lock down tight enough. When that happens it will gradually start slipping out of focus as the telescope increases its angle while tracking.

Otherwise as long as you have your focus set and locked in you should be fine.
Still Nothing to do with DOF. There is no circle of confusion, nor area of acceptable sharpness.
At those distances, "provided you have a flat field", all the stars are sharp and in focus or they are all out of focus. And they will stay that way throughout your total exposure, or the night if you're taking multiple exposures for stacking.

Even by stopping down, when using a short fast lens you probably won't totally reduce the distortions all the way out to the edge of the field. But you can significantly reduce the effects and more so the amount of CA which will help make your stars appear much sharper and less bloated.

Stopping down IS a normal practice to avoid comma.
DOF is not an issue and never is.
Vignetting is easy to correct with software.
Chromatic aberration is also something you can correct.

Comma is the big problem and even the best lenses exhibit comma wide open. There are a few exceptions of course but since the majority exhibit the problem then we can say that stopping down is a fairly common practice.

luigis wrote in post #13858480
Stopping down IS a normal practice to avoid comma.
DOF is not an issue and never is.
Vignetting is easy to correct with software.
Chromatic aberration is also something you can correct.

Comma is the big problem and even the best lenses exhibit comma wide open. There are a few exceptions of course but since the majority exhibit the problem then we can say that stopping down is a fairly common practice.

It would be nice if you could buy accessories for camera lens like you can for a telescope. Such as a Field Flattener and Coma Corrector.
You should be able to by a filter to help with the CA such as an anti-fringing or minus violet filter. Some companies that manufacture them for telescopes Eyepieces and CCD Cameras also make them for camera lenses.

As for astigmatisms, that is another problem that usually indicates a more serious problem with your lens. I.e bad lens grinding or imperfect glass element.

Another way to correct vignetting is to shoot and use Flat Frames. A subject that could be discussed elsewhere on it's own.
http://www.cyanogen.co​m …eld_Frame_Calib​ration.htm

tkerr wrote in post #13858538
It would be nice if you could buy accessories for camera lens like you can for a telescope. Such as a Field Flattener and Coma Corrector.
You should be able to by a filter to help with the CA such as an anti-fringing or minus violet filter. Some companies that manufacture them for telescopes Eyepieces and CCD Cameras also make them for camera lenses.

As for astigmatisms, that is another problem that usually indicates a more serious problem with your lens. I.e bad lens grinding or imperfect glass element.

Another way to correct vignetting is to shoot and use Flat Frames. A subject that could be discussed elsewhere on it's own.
http://www.cyanogen.co​m …eld_Frame_Calib​ration.htm

Flat frames are probably not needed if you have a good RAW processing software. I think they were the solution for FITs files from CCD cameras were there wasn't a tool to automatically remove vignetting.

What do you think?

This is comical.

luigis wrote in post #13858730
Flat frames are probably not needed if you have a good RAW processing software. I think they were the solution for FITs files from CCD cameras were there wasn't a tool to automatically remove vignetting.

What do you think?

For vignetting alone you shouldn't need them if you have something like Photoshop or Lightroom. But Flats are also used to help remove some noise and other inconsistencies in the image caused by anything in the optical path that shouldn't be there, such as dust spots. However, If the dust isn't very bad, that too can be removed quite easily in Photoshop.

Damn i wish i could get good into here and not wee wee measuring a contest.is the sigma 8-16mm a good choice for wide angle sky shoots? Or would the 17-50 be a better option?

VisualUniverse wrote in post #13858783
This is comical.

Glad you are enjoying it.
FWIW, When you refer to Depth Of Field, are you sure you are not actually thinking of Depth Of Focus, as in the symmetry of focus throughout the image plane?

S.n.a.f.u. wrote in post #13858817
Damn i wish i could get good into here and not wee wee measuring a contest.is the sigma 8-16mm a good choice for wide angle sky shoots? Or would the 17-50 be a better option?

That depends on what you are after. If you want a real wide field of view to cover more area of the sky then the 8-16mm would be a good choice. But then at those focal lengths, especially the shorter, you will start running into the problems that have been discussed above such as field distortions, coma and vignetting.

However, the wider you go the more tolerant it is to the movement of the stars across the sky. IOW you will be able to get a longer exposure which will collect more details in the night sky.
Typically a good starting figure for shutter speed before trailing will be noticeable is to divide 500 by the lens focal length. Or if you're using a Full Frame Camera try 600 by the focal length. It's not perfect but it will get you close.
For Example with the 8-16mm Sigma @ 8mm on a camera with an APS-C crop sensor you can go as long as 62 seconds
With the other lens at 17mm you can only go as long as about 30 seconds.

guys what is the cheapest atrotrack (im not sure if atrotrack is a brand or the actual object. but i meant the tool rly)?
i say that because here in brazil i cant find anywhere that sells it. and to import would be very expensive for me, so im afraid id have to lose a bit on quality in order to have one.
i see here some equatorial mounts. perhaps it would be cheaper to buy one of those and later try to find the motor for it?