Need help for processing night sky

Discussion in 'Beyond the Basics' started by fazhar, May 24, 2015.

  1. wfooshee

    wfooshee TPF Noob!

    Joined:
    Oct 28, 2014
    Messages:
    663
    Likes Received:
    182
    Well, OK, then! Yeah, three stops is probably too far. And I'm going on my experience with my own 1.4 lens, which was awful wide open. I had MUCH better results stopping it down and raising the ISO.


     
  2. fjrabon

    fjrabon TPF Noob!

    Joined:
    Nov 3, 2011
    Messages:
    3,644
    Likes Received:
    755
    Location:
    Atlanta, GA, USA
    What f/1.4 was that?
     
  3. fazhar

    fazhar TPF Noob!

    Joined:
    Jan 29, 2014
    Messages:
    19
    Likes Received:
    0
    Location:
    United Arab Emirates
    I actually tested a F8 when I took those photos and it came out underexposed. Couldn't see much in the photos. And after all the readings I've done on astrophotography, I'm quite convinced that F should be lowest (or slightly above lowest depending on lens' sweet spot).
     
  4. fjrabon

    fjrabon TPF Noob!

    Joined:
    Nov 3, 2011
    Messages:
    3,644
    Likes Received:
    755
    Location:
    Atlanta, GA, USA
    Yeah, the only way I wouldn't shoot at the most open aperture is if it was comically bad wide open or had major vignetting. But even then I'd never stop down slower than f/2.8.
     
  5. TCampbell

    TCampbell TPF Noob!

    Joined:
    Mar 31, 2012
    Messages:
    3,615
    Likes Received:
    1,556
    Location:
    Dearborn, MI
    Faraz, you'll want to patiently work on focus and get that refined before you begin shooting the images you plan to keep. You can shoot at f/1.4, but the lower focal ratio, the fussier the camera will be if you miss perfect focus (it's harder to nail it.) Very wide-angle lenses tend to be a bit more forgiving on focus. I have a 14mm f/2.8 lens -- but even still, I do have to be careful to refine my focus (Take lots of test shots. If you can tether to a computer so you can truly inspect the focus accuracy first, that really helps.)

    Generally speaking, do not trust the "infinity" mark on your lens' focus ring.

    The good news is... if "anything" in space is accurately focused... then "everything" in space is accurately focused. When we want to shoot "faint fuzzies" (deep space objects that barely show up against the background and are often diffuse so there's nothing with high-contrast to use to nail the focus) what we really do is point the telescope (or camera) at at a bright pin-point star (even though that's not the object we plan to image), and use that to refine focus. Once we know it's focused... we point back to the object we really plan to shoot and begin collecting our images. Those images will be accurately focused (that's just the nature of astrophotography... it doesn't matter if the object is 250,000 miles away... or 25 million light years away... it'll all be in focus even at very low focal ratios.)

    When shooting images from a stationary tripod, time is not your friend. You want to keep the exposures short enough to so that the motion of the sky doesn't cause stars to elongate and form "star trails". There's a guideline for this.

    That guideline was designed for 35mm film cameras and is called the "Rule of 600". It suggests that if you use 600 as a base value and then divide that by the focal length of your lens, then the result is the number of seconds you can leave the shutter open without stars elongating to form star trails. With a 50mm lens, that'd be 600 ÷ 50 = 12 (12 seconds). BUT... that's for a 35mm camera and you have an APS-C camera. So you actually have to divide 600 by the crop factor for you camera sensor (for your camera that's 1.6). 600 ÷ 1.6 = 375. That means you'd use 375 as your base. 375 ÷ 50 = 7.5 (seconds). That means you'd want to limit exposure times to 7.5 seconds or less. It turns out many accomplished imagers think that 600 is too generous and they prefer a more conservative base... such as 500 or even 450. When you divide that by your crop factor, your adjusted base is nearer to 300 (which would put you at a 6 second exposure when using a 50mm lens).

    To saturate the milky way and nebulae, it's nice to go longer. But to go longer you would want the camera on a tracking head. I have a number of computer-guided "equatorial" mounts designed for telescopes (I almost never just use a camera and camera lens. I have at time, but usually I'm using a telescope.) These things range in price from about $800 (for a low-end mount) and can exceed $10k (just for the mount -- no telescope, no camera in that price tag.)

    You can also get "tracker" heads that can be attached to a solid camera tripod (emphasis on "solid"). The tracking head has a rotational axis (motorized) and this is pointed at the pole (Northern hemisphere observers use the star "Polaris" to align it -- though Polaris is technically about 2/3rds of a degree away from the true pole, there are alignment aids that help you refine the pointing accuracy.) You can attach a ball-head to the rotation axis so that you can point the camera to any section of sky you want. As the Earth rotates from west to east (creating the illusion that stars move from east to west) the tracking motor rotates in the opposite direction and at the same speed as the Earth's rotation. This cancels out the rotation of the Earth and holds everything in place so you can take very long images. Just "how long" depends on how well you aligned the mount.

    The models I know of are (from cheapest to most expensive)

    1) iOptron SkyTracker (normally $399, but appears to be on special right now for $299)
    2) Vixen Polarie ($399)
    3) AstroTrac ($619)
    4) Losmandy StarLapse ($695)

    Here's a link to OPT -- probably the largest dealer of astronomy equipment in the country.

    Shop at OPT - Tracking Mounts Camera Mounts Tripod Heads for Sale at OPT Telescopes - OPT Telescopes

    Naturally the most expensive one turns out to be the best and most versatile. Their mount is designed to allow you to neutrally balance the camera (so weight won't throw off tracking accuracy), it can be pointed anywhere, and it can track as long as you want (it also has multiple tracking rates.) Losmandy is all precision machined components with exceptionally good quality. The iOptron and Vixen are the least expensive (I don't know how well they handle the weight -- they don't have the ability to neutral balance the camera load.)

    You will probably want to invest in a gadget called an "intervalometer". This is a "wired" remote shutter release ... except it has a programable timer in it.

    Here's a link to one: Shop at OPT - Tracking Mounts Camera Mounts Tripod Heads for Sale at OPT Telescopes - OPT Telescopes

    You program the number of images you want to collect, the duration of each image (and you can specify any duration ... not just the ones the camera allows because the device works with the camera in "bulb" mode) and lastly you program the time interval to wait between images.

    IMPORTANT: You probably want to make sure you turn off "Long Exposure Noise Reduction" in the camera. Otherwise the camera takes TWO images for every image... the first with the shutter OPEN, the second with the shutter CLOSED. The second image is used to gather noise (it's called a "dark frame") and the dark frame is subtracted from the "light" frame when shooting JPEG to produce a lower-noise (but not noise-free) image. I had a friend that accidentally left this on while shooting multi-minute exposures. The camera would finish the exposure (what we refer to as the "light" exposure because the shutter is open) and the camera would remain "busy" for several minutes -- he was locked out of doing anything and thought his camera was defective. He later figured it out... the camera had long-exposure noise-reduction enabled and it was actually taking the "dark" (shutter closed) image.

    Everything I've written up to this point is all part of what we call "image acquisition". There is more.

    You can shoot multiple images of the same object to reduce noise. The ability to reduce noise is based on a Poisson distribution. The extent to which you can reduced noise is based on the square root of the total number of images you shoot. If I shoot 9 images of the same object and then use computer software to integrate them, I can reduce the noise by a factor of 3x (square root of 9 is 3). If I shoot 16 images, I can reduce noise by a factor of 4x. Most astro-imagers I know think that beyond 20 images you start hitting diminishing returns... you spend time collecting vastly increasing numbers of images for just marginal gains in noise reduction.

    But IF you do collect multiple images, then the next step is to learn how to combine those images ("registration and integration") and this is another learning process. You ultimately end up with a "master" image (the result of all the image data after it has been combined.)

    This master image now needs to be processed ("adjusted") artistically.

    There are a number of video tutorials on how to do this, but typically we "stretch" the data in the image to bring out details. Imagers will sometimes work in "zones" similar to the Ansel Adams "Zone System" -- but a bit more simplified. Instead of 10 zones, they use 4. The darkest "zone" usually doesn't contain any interesting data and tends to mostly be just "noise" and can safely be discarded... so we send that to "black" and make it all background. The next zone up contains a lot of noise, but it also contains faint nebulosity that we don't want to lose. The nebulosity usually doesn't have good detail though -- so we don't try for detail. Instead we apply noise-reduction to that zone (which softens the data in that zone as a side-effect.). The next zone usually has less noise and more detail... that zone usually can stand "as is" (doesn't need noise-reduction (which has the side-effect of softening) but also doesn't need sharpening. The final zone -- the bright zone -- has very little noise and it also offers the best detail. This zone can stand sharpening to refine the detail. All of this is also "stretched" (meaning if you were to use a "levels" adjustment tool, you'd get rid of the empty space on the left and right to boost contrast.

    Every imager I know spends years learning and refining their techniques to artistically adjust the images.
     
    • Winner Winner x 1
  6. fazhar

    fazhar TPF Noob!

    Joined:
    Jan 29, 2014
    Messages:
    19
    Likes Received:
    0
    Location:
    United Arab Emirates
    Wow that is a mighty detailed information. You've pointed out some things I never realized before, like focusing one thing and everything is in focus. I've recently learnt the 500 rule, I'll definitely give it a shot. Makes absolute sense to me since my sensor is APS-C. However getting all that motorised equipment is overkill for me since I'm just starting off in astrophotography. :) But thanks for all the immense information, I know I will need to look up this again when I get more and more into night skies.
     
  7. wfooshee

    wfooshee TPF Noob!

    Joined:
    Oct 28, 2014
    Messages:
    663
    Likes Received:
    182
    Sigma 30mm f:1.4 EX DC HSM. I got it used for a song, hoping for the best, but I should have been paid to take it. I'm not sure it's really that bad a lens, I think it had just seen a rough life. I knew going in that it had issues, but I was able to have it cleaned locally and it was WAY better after that. Still, it focuses poorly on distant objects, even in daylight, although within 6 to 8 feet or so it's actually quite nice. The distance issue may be something structural, as in not all the elements are where they ought to be, maybe.

    My only wide lenses at the moment are my original Nikkor 18-55 kit (non-VR) and the Rokinon 8mm fisheye, so right now I don't have a good night-sky lens. The 18-55 is usable but not ideal, but I've borrowed a couple of nice lenses from local club members.
     
  8. fjrabon

    fjrabon TPF Noob!

    Joined:
    Nov 3, 2011
    Messages:
    3,644
    Likes Received:
    755
    Location:
    Atlanta, GA, USA


    ah that makes more sense
     

Share This Page