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Thread: Stacking images from alt-azimuth mount (image shift)

  1. #31
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    Default Re: Stacking images from alt-azimuth mount (image shift)



    Quote Originally Posted by STEVE333 View Post
    Paul,

    Consider an example where you wanted to view M33. The declination for M33 is about 30.7 deg. Putting that value into my program along my latitude of 37 deg will produce a graph (on my laptop) showing the Max Exposure vs. Time Before and Time After transit. At my location (Santa Cruz, CA which is at longitude 121 W) M33 will transit (be at it's highest elevation in the sky) at 39 minutes after midnight. Three hours before transit would be 9:39 pm, two hours before transit would be 10:39 pm and so on. The graph shows the Max Exposure for all times when the object is above the horizon. See an example of this in the picture below



    I hope you can see the picture as this is my first attempt to include a picture.

    Hopefully this makes sense.

    Steve
    Hi Steve... I'm not clear how the results from your program translate into the real world because an alt/az mount will stuggle to achieve more than 30 seconds without trailing, regardless of where the observer and obsrved object are located... And for the majority of targets 10-20 seconds is more realistic.



    Am I missing something?
    regards

    Phil
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  2. #32
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    Default Re: Stacking images from alt-azimuth mount (image shift)

    Also, any such calculation would need to take account of the focal length (not aperture) of the scope and the size of the camera pixels...
    Telescope: SWEquinox ED80 Pro (Schott/Ohara Fluorite)+Baader Steeltrack, C8 XLT Mount: Skywatcher HEQ5 Pro, Rowan belt drive on Pillar mount+EQDIR+ADM saddle/dovetails Camera: JTW1100D mono , CentralDS 600, ASI120MM. ASI120MC, IDAS & Baader NB 2-inch filters, TS Filter Drawer System Accessories: TV Nagler T4 12 mm, Baader 8-24 MkIII, Ortho 5mm, Aspheric 31mm, Meade 12mm reticle, TV Powermate 2x & 2.5x, JMI+BAST Motorfocus+FCUSB, TV 0.8x FR/FF, Celestron 0.63FR, Orion RACI & Mag Mini, Canon IS 15x50 Bins
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  3. #33
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    Default Re: Stacking images from alt-azimuth mount (image shift)

    Phil,

    I had many of the same opinions when I started looking into this problem of star trails and Az El mounts. However, it turns out that there are some combinations of observer location and object position that can lead to much longer possible exposures. I also thought that telescope focal length and camera pixel size would be required information for these calculations. However, Michael Covington in his book "Digital SLR Astrophotography" says that an image rotation of about 0.1 degree is acceptable in most cases. I have not tried this yet, but, in a private communication with him he reiterated that this amount of image rotation is tolerable in most astrophotographs. In my program I leave the amount of allowed image rotation as a value that can be changed if the user desires "sharper images".

    In Covington's book in Chapter 9 "Tracking Stars" he has a Figure showing the maximum allowed exposure time for Az El tracking of objects at various location in the sky and my calculations agree exactly with his. Of course, all of this assumes that the Az El mount is well aligned initially so that only image rotation needs to be of concern.

    In the picture below I show the results for M33 from my location at Latitude 37. It shows that I could take 60 sec exposures any time between 6 1/2 hours and 1 1/2 hours before transit. It also shows that I could take 120 sec exposures any time between 4 hrs and 2 hrs before transit.



    I know these exposure times are much greater than what most people are recommending, but, for this combination of observer latitude and object location in the sky, the image will only rotate 0.1 deg or less for the 60 sec or 120 sec exposures mentioned above.

    Not all sky objects yield such favorable results and some can only be exposed for 15 to 30 seconds before trailing becomes an issue. However, these shorter times are exactly what my program shows for these less favorable objects.

    I have just ordered an Orion ST 80 and hope soon to attach it to my Celestron 4 SE mount and be able to test this out for myself. Right now I have a NexStar 4 SE and the F/13 optics makes it difficult to gather much light.

    Clear skies,

    Steve
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  4. #34
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    Default Re: Stacking images from alt-azimuth mount (image shift)

    There are two aspects of exposure time with an altazimuth mount. One has to do with the ability of the mount to track keeping the image steady and centered; the other is field rotation.

    The lower tier of altazimuth mounts (8/6SE, 4/5SE, SLT, DS2000, Cubes, ETX, SW AZ, etc.) are excellent for visual work but do not have the precision gearing to support long exposures. Having said that, most are good enough to statistically allow 30 second exposures about 60% of the time before something moves or vibrates spoiling the exposure. This ~30 second capability is good enough for photographing deep space down to around an apparent mag 11 if a fast telescope or camera lens is used so that subs with a usable signal to noise ratio are obtained.

    Field rotation is dependent upon the position of the observer on our planet, the altitude angle of object, and the azimuth angle of the object. It is not dependent upon telescope focal length. I think Covington is conservative with his 0.1 degree being not detectable but even so with 0.1 degree allowed exposure times range from many minutes to a couple of seconds. The formula for calculating exposure time is:

    T=Rcos(alt)/(0.004157cos(lat)cos(az)) where

    R is the amount of field rotation tolerated in degrees (Covington uses 0.1 degree)
    alt is the altitude angle of the object
    lat is the latitude of the observer
    az is the azimuth angle of the object
    0.004157 is the rotation rate of earth in degrees per second (sidereal day)

    There are two singularities, one at an azimuth of 90 and 270 degrees and another at an altitude of 90 degrees.

    The 20 to 30 seconds is a rule of thumb that is generally doable for objects that are not near the zenith. The image size in a short focal length telescope or camera lens is small; thus, movements, including field rotation, are not as "magnified" for want of a better term and not as noticeable as the same movement in a long focal length telescope. In this respect, a short focal length lens or telescope is a better match for altazimuth mount photography than a long focal length telescope. Also, a telescope or camera lens with a short focal length generally has a low focal ratio which is also desirable from a signal to noise perspective.
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  6. #35
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    Default Re: Stacking images from alt-azimuth mount (image shift)

    Nice clarifications. I especially appreciate the information about the "30 sec limit" for my 4 SE mount because of it's inherent tracking accuracy. Now I won't waste too much time trying for 2 min exposures.

    You may have noticed that I finally gave in and have purchased an Orion ST 80-A. I hope to get it before winter gets too cold. At 72 I don't appreciate the cold nights very much any more! Also, M33 and M31 are both in good positions now for my observations from home. Time will tell if the lights in our mobile home park will allow for any reasonable images of these and other DSO objects.
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  7. #36
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    Default Re: Stacking images from alt-azimuth mount (image shift)

    OK - got it, thanks Steve & Joe.

    As Joe points out the gearing mechanism on the Nexstar mount is just not designed for AP and will struggle to give you usable subs over 40 seconds or so regardless of the alt/az of target and latitude of the observer.


    Also, if using a DSLR or any camera with a lot of pixels the corners of the image (where the effects of field rotation become apparent) are quite a long way from the centre. You would expect to have to crop the images a fair bit. Finally, shooting objects very high in the sky is problematic with the Nexstar mount because of clearance... and shooting very low is usually a problem because of atmosphere/LP.

    However, don't let me put you off trying!
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    Default

    Focal length doesn't matter for field rotation - because it's a rotation by the same angle, any star that ends up at the same distance from the centre will move by the same amount - the focal length / image scale just alters what ends up where - so a particular star may be closer to the centre and affected less, but overall it's the same.

  9. #38
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    Default Re: Stacking images from alt-azimuth mount (image shift)

    Quote Originally Posted by jerryTheC View Post
    Focal length doesn't matter for field rotation - because it's a rotation by the same angle, any star that ends up at the same distance from the centre will move by the same amount - the focal length / image scale just alters what ends up where - so a particular star may be closer to the centre and affected less, but overall it's the same.
    So the smaller the sensor area, the closer everything is to the centre and the less likely it is to detect rotation at the periphery. Which is why DSLR sensors are typically more prone to showing field rotation than CCD sensors...?
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  10. #39
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    Quote Originally Posted by Phil Leigh View Post
    So the smaller the sensor area, the closer everything is to the centre and the less likely it is to detect rotation at the periphery. Which is why DSLR sensors are typically more prone to showing field rotation than CCD sensors...?
    Sounds sensible to me . A smaller sensor is like only looking at the central bit of a DSLR image - you can rotate more before it becomes noticeable at pixel level - although I'm not sure what happens if you enlarge the result to the same final size as the DSLR one.

    You probably need to factor in pixel size as well - at a given distance from the centre, smaller pixels will pick up a shift before larger ones.
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    Default Re: Stacking images from alt-azimuth mount (image shift)

    Good point re: pixel size.
    My DSLRs have pixel sizes half that of full-frame cameras so will pick up any rotational movement > 1 pixel ~twice as quickly...
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