can i damage my 20D cmos with long exposures

Discussion in 'Australia Photography' started by boris, Jan 20, 2006.

  1. boris

    boris Guest

    I did an astro shot the other night for an hour and got good results with
    star trails but a friend mentioned that i may be harming the cmos (he talked
    about voltage/heat)

    Is it possible that it could harm the cmos by doing this ?

    Thanks Boris
     
    boris, Jan 20, 2006
    #1
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  2. boris

    Jasen Guest

    Not at night it won't. Shoot for as long as you want!
     
    Jasen, Jan 20, 2006
    #2
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  3. boris

    Jasen Guest

    P.S. Just don't do Astro shots of the big star we call the sun, otherwise
    you'll be buying a new camera VERY soon.
     
    Jasen, Jan 20, 2006
    #3
  4. boris

    Jeff R Guest


    Your friend was almost certainly referring to the buildup of noise you will
    have noticed in the image - such noise which would be avoided/reduced if the
    sensor was cooled. Sounds like he had misunderstood or misreported the
    distinction between temporary noise and permanent damage.

    How does a star trail look in digital? Did you do a dark frame to remove the
    noise?
     
    Jeff R, Jan 21, 2006
    #4
  5. boris

    boris Guest

    Tks mate....i do shoot directly into the early morning & late afternoon sun
    sometimes but i prefocus on infinite & never put my eye to the camera. I
    assume whilst its an AV then it will look after itself. I have done this for
    a year now off & on & the camera still takes great photos. No way would i
    hold the shutter open against it though.



    "> P.S. Just don't do Astro shots of the big star we call the sun,
    otherwise
     
    boris, Jan 21, 2006
    #5
  6. boris

    boris Guest

    Very little noise to speak of jeff. I was very impressed with the outcome.
    Needed no work for my eye.

    My only regret is that i didnt let it go for longer as it was a moonless
    night. For some odd reason though...the camera took the same amount of time
    to write to the 80x card in RAW as did the shutter was open. ie if it was a
    15 min shot then 15 min write time...56 min exposure...56 min write time.


    "> How does a star trail look in digital? Did you do a dark frame to remove
    the
     
    boris, Jan 21, 2006
    #6
  7. boris

    Jeff R Guest

    Aha!
    That would be the camera doing automatic dark frame/noise reduction.
    Did you say the camera was a 20D? (too lazy to restore thread)
    I didn't know they did that.
     
    Jeff R, Jan 21, 2006
    #7
  8. boris

    boris Guest

    Yep just the standard 20D ...i didnt know about it & i was quite taken back
    watching such a long write time. Now i remember...i set it to that 'noise
    removal' in the functions.

    Very very little noise...i get more at ISO 400 with normal shooting if you
    can call that noise. They are a great camera.



    "Jeff > Aha!
     
    boris, Jan 21, 2006
    #8
  9. That's because the camera is making a dark exposure to adjust for noise
    in the sensor. If your exposure is 15 minutes the camera takes another
    15-minute exposure with no light on the sensor and subtracts the dark
    sensor values from the exposure. (This is probably not electronically
    correct but hopefully you get the gist.)

    -Michael
     
    Michael Strasser, Jan 21, 2006
    #9
  10. boris

    Ken Chandler Guest

    Sounds like 'Long exposure noise reduction' Custom function 02 is set to on.

    In this mode the 20D will take a second shot of the same length, a dark
    frame with the shutter closed, to aid in noise removal.

    When the camera is actually writing to the card, the red light will be on
    and it should be about the same as any other shot.

    Kind regards, Ken
     
    Ken Chandler, Jan 21, 2006
    #10
  11. boris

    boris Guest

    Thanks ken/michael. That explains it. Great function as i have CS2 & noise
    ninja & when i opened the shot i just sat there looking at it & didnt have
    to touch it at all. Im certainly no PS guru but i didnt change the photo at
    all for my eye.
     
    boris, Jan 21, 2006
    #11
  12. boris

    boris Guest

    Make sure you batttery is good though as a fully charged one only just did
    an hour open & an hour writting.
     
    boris, Jan 21, 2006
    #12
  13. boris

    kosh Guest

    I have also come across a peice of software which does double null
    ixposures for astro shots to reduce noise.

    kosh
     
    kosh, Jan 21, 2006
    #13
  14. boris

    kosh Guest

    heard of the 20DA for astro?
     
    kosh, Jan 21, 2006
    #14
  15. boris

    boris Guest

    Yep...
     
    boris, Jan 21, 2006
    #15
  16. boris

    Jasen Guest

    I did a short 5 minute exposure at night with the KM 7D and it came out
    rather well. From memory, I did a little tweaking in PhotoShop with the
    brightness, contrast and gamma I think, but it was amazing how many stars
    were picked up and with very little noise. I did this on the outer edge of
    Brisbane where the city lights still impede the view. I zoomed in on the
    image and I could see hundreds and hundreds more of tiny star trails that
    couldn't be attributed to the effects of noise. The brightest stars also had
    a variety of different colours too which you don't really pick up easily
    simply looking at the night sky.
     
    Jasen, Jan 21, 2006
    #16
  17. boris

    boris Guest

    Jasen i went out into the bush near wivenhoe dam on a new moon night. I did
    score a satellite going over flashing in the shot.

    Black as a bats arse it was.
     
    boris, Jan 21, 2006
    #17
  18. boris

    Jasen Guest

    excellent. I might have to do the same and try it out. How long has been
    your longest exposure? I don't have much experience with night sky
    photography, so is there a particular compass point that is best to point
    at? I assume to get the nice curved trails you have to point towards the
    poles, but at this latitude it might not be so evident??
    I was impressed with the night sky out at Carnarvon Gorge late last year,
    but for some reason didn't try out capturing the stars there.
    Have you a link to your sky pics?
     
    Jasen, Jan 21, 2006
    #18
  19. boris

    boris Guest

    If you want that 'circular' look with a centre star then point to a star
    about 3 mans hand widths under the southern cross & in a line at 90 degrees
    perp to the right of the 2 pointers & imagine where they intersect.

    Thats how i find it & i only hope someone reading this can explain it better
    for you. That point is quite low on the horizon though so you will need a
    nice uninterrupted view to the lower sourthern sky. I would love to go into
    the dam at night & incorp the water & stars...i have no idea what it would
    look like.

    Do remember to have a fully charged battery & use that 'noise ruduction'
    function if you have a 20D.
     
    boris, Jan 21, 2006
    #19
  20. boris

    Jeff R Guest


    Star trails are fun. There's no doubt about that.

    Boris' advice is fine: Point towards the South Celestial Pole if you want to
    see the "central" point of the rotation, and point in the opposite direction
    (due North) if you want to maximise the movement (but miss out on the "axis"
    point.)

    Boris' description of how to find the SCT is good too, but simpler than
    that:
    Aim due south (azimuth) and your _local latitude_ high (altitude). In my
    case
    (Pennant Hills NSW) that's 33.7° high. There's lots of sites that will show
    you graphically how to find the SCT using the Southern Cross and the
    pointers, and I'm lousy at ASCII art, so won't try here.
    http://www.i4at.org/surv/directon.htm isn't bad.

    Some more fun:
    Start your star trail, run it for a few minutes, then *carefully* and very
    gradually unfocus the lens - just little-bit-by-little bit. Do it so that
    it takes at least 10 minutes to go from sharp focussed to fully out of
    focus.

    The result is spectacular. The trail starts out sharp and defined, then
    turns gradually softer and wider (well, duhhh), but what is really cool is
    the way it emphasises the "true" colour of the star. When focussed, the
    star tends to overexpose and image as "white". When unfocussed, it doesn't
    overwhelm the film (or sensor) and it exposes "correctly", revealing
    beautiful shades of blue and orange and red. I haven't looked, but there's
    bound to be examples on the net.
    (p.s. I looked. I *love* Google.
    http://pages.prodigy.net/pam.orman/joetrails/Trails_050312_17.html )
    (not my image, obviously)

    After star trails come tracked shots, where the intention is to show the
    star as a point, not a trail. In other words, the camera is set to rotate
    with the stars - against the earth. Google up "barn-door platforms" for a
    cheap and very fun
    way to do this.
    Piggy-backed on an equatorial-mounted telescope is the next (more expensive)
    method.

    Here is one of my favourites:
    http://faxmentis.org/html/science9.html
    Not technically brilliant, (wracked with light pollution) but:

    On that night, (it was years ago) I distinctly remember that I could only
    *just* see the fifth star in the Southern Cross, with the naked eye - yet
    look how much was revealed in a simple 5 minute exposure.
    I *think* this might have been a barn-door shot - but it might have been a
    piggy-back. Not sure. Barn door can certainly do as good - or better - than
    this.

    I could rave on, but...
     
    Jeff R, Jan 22, 2006
    #20
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