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Thread: Astrophotography with Azimuth Mounts

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    Default Re: Astrophotography with Azimuth Mounts



    Have a Canon T3i and Nexstar 6SE. Just waiting on needed accessories and clear skies, and I'm in!

    Question: What would be a good first target for someone imaging on an az/alt mount with a DSLR? Should I start with a bright star? The moon? Or a brighter Messier, like M13? Does it depend on the time of year; ie what's in the sky 30* from the zenith right now?
    I'm always learning, so PLEASE correct me if I sound misinformed. I will not be offended if you tell me I'm wrong. Thanks for your patience!
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    Wants: Less Light Pollution, and Clear Skies!

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    Default Re: Astrophotography with Azimuth Mounts

    It's probably best to start with brighter objects. I would expect field rotation to be at its worst near the zenith (or perhaps the meridian), so I would avoid that area of the sky.

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    Default Re: Astrophotography with Azimuth Mounts

    The rate of field rotation increases as an object approaches the Meridian and as it increases its altitude angle approaching infinity at the zenith. Conversely the rate of field rotation slows to infinity as an object approaches due east or due west and its altitude angle approaches zero. These two quirks are mathematical oddities as they only occur for an instant in time as the earth continues to rotate and the object's altitude and azimuth angles change thus departing from the singularities. What this does mean is that if you have a good azimuth mount that can track without periodic error, etc. (the SLT is not one of them) you can obtain exposures of several minutes for objects low in altitude and that are near due east or due west. It also means that regardless of how good your mount is, exposure times become very short approaching zero near the zenith..

    Fortunately, if you stick with 20 second exposures you can photograph objects in the night sky that are 70 degrees or less in altitude regardless of their azimuth angle. Also, the SLT mount is good enough to statistically produce about a 60 to 70% retention rate with a 20 second exposure (exposures that do not have star trails due to mount movement). The SE mount can easily achieve the same rate with 30 second exposures if the object is located where 30 second exposures are possible.

    I started off doing piggy back photography with my DSLR and AZ goto mount. From there I went to prime focus and photographed some bright open clusters. As I became more skilled (obtaining images as well as processing) I went to nebulae then last to galaxies. There is a lot to learn and it can be overwhelming. However taking it one step at the time makes the process fairly simple.

    With the SLT you will need to either replace the tripod with one much more sturdy or modify the tripod to make it more sturdy. The tripod modifications are not difficult and are explained in a sticky on the SkyWatcher telescope forum as it uses the same tripod as the SLT.

    Currently many objects that are fairly easy for azimuth photography are located in the constellations Sagittarius and Scuptum. These object are fairly low in the and but are near the Meridian. I'd go with 20 second exposures. Take about 150 exposures. You will end up with about 30 minutes total time after stacking.
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    Default Re: Astrophotography with Azimuth Mounts

    Wow ! great pictures

    === Here is a link to my flickr sight showing what can be done by a novice with entry level goto mounts (SkyWatcher SynScan AZ and NexStar 4/5SE mounts). Flickr: Sxinias' Photostream ===

    I am not sure how to interpret this... Uplifting because it shows I have enough equipement to do the same (celestron nexstar 6SE). Depressing because I have a long way to go to find the exact recipe that works in my case

    Anyway,thanks a lot for such an informative message

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    Default Re: Astrophotography with Azimuth Mounts

    Thanks,

    I never did build my wedge, this information is great!

    Alan


    Quote Originally Posted by sxinias View Post
    We get a lot of inquiries from people who want to photograph with an azimuth mount. I thought that I would document my experience with azimuth photography and hope others will do likewise. Hopefully the quality of this thread will be sufficient for a sticky so we can easily answer the question each time it comes up. My purpose here is not to discourage anyone from photography with an azimuth mount but simply to describe the techniques needed as well as their limitations and leave up to each individual to decide if they wish to travel the road of AZ photography.

    Astrophotography with azimuth mounts. Yes, you can but field rotation, mount movement, and the location of objects in the sky will be your major limiting factors. Here is a link to my flickr sight showing what can be done by a novice with entry level goto mounts (SkyWatcher SynScan AZ and NexStar 4/5SE mounts). Flickr: Sxinias' Photostream Here is another link showing what an experienced person can accomplish with an advanced azimuth photography system: Messier Images 1-36

    Field rotation. If you limit your photography to areas of the sky approximately 30 degrees distant from your zenith, you can have exposure times of 30 seconds or more with 0.10 degrees or less field rotation (0.10 degrees of rotation will not be noticeable in a photograph). This essentially means except for objects on your southern or northern horizon stay 30 degrees away from your meridian. For an excellent explanation of field rotation, see: Field Rotation

    AZ mount tracking movements. AZ mounts can track objects and, if properly set up, keep them literally centered in the eyepiece for hours at least as far as the human eye can tell. With a photograph, the story is different. AZ mounts constantly make small adjustments in azimuth and altitude as they track an object. While not noticable with the eye, they produce movement that result with stars looking like jagged lines. You will need to experiment with your AZ mount to determine how long of an exposure you can make with it before mount movement ruins the shot. For example, with my SkyWatcher SynScan AZ mount (essentially the same as the Celestron SLT mount) the time is 20 seconds if I want 90% of my exposures to be acceptable or 30 seconds if I am willing to discard about half my exposures. The extra ten seconds of exposure time sometimes makes a 50% failure rate acceptable. With the more expensive 4/5SE mount the statistics result in a 30 second exposure for 90% keepers and 45 to 60 seconds for 50% keepers. Note: to get these times, I take pains to make sure the mount is orthogonal to the plane of the earth and use a reticle eyepiece to center alignment stars. While eyeballing these parameters is good enough for visual work, it is not for photography.

    Exposure times and stacking Often you will read that stacking many short exposures produces the same results as one long exposure with the same exposure time. This is true but not true at the same time. There is a minimum exposure time where this works; below that things get iffy as the shots are so underexposed. Actually it's not exposure time but the exposure that is important. When you are doing 30 second and shorter exposures, only so much information can be captured. For bright objects like M8, M16, M20, M42, etc. significant information can be obtained in a short exposure. But for very dim objects like M51, the shot can be too underexposed for stacking programs to recognize enough stars needed to stack. Short exposures require a lot of shots ... to get one hours total exposure time with 20 second exposures, you will need to take over 200 shots to have a statistical probability of enough good shots to equal one hour.

    Unlike visual work, in photography the focal ratio of a telescope is important NOT ITS APERTURE. Two telescopes of the same aperture but having different focal ratios. The telescope with the lower focal ratio will have the brighter image and shorter exposure times while the telescope with the larger focal ratio will have a larger but dimer image and the longer exposure time. For two telescopes with the same focal ratio but different apertures, both telescopes will have the same exposure times but the telescope with the larger aperture will have the larger image because it has the longer focal length. For azimuth photography, telescopes with low focal ratios are better as they can capture more information in the short exposure times that are available.

    Your camera's ISO setting also comes into play. With 20 second exposures, low ISO settings often produce shots too under exposed for stacking programs. This can be countered by using a high ISO setting. I use 1600 and this produces noise that detracts from my photos. I use a SCT which is a f/10 telescope. An f/7 telescope would almost double the exposure efficiency giving approximately the same exposure at 800ISO without the extra noise. May SCT owners simply use a focal reducer to shorten the focal length of their telescopes and lower the focal ratio to f6.3 (a good focal reducer cost new from $60 to $100)

    Time to expose. Taking 200 exposures at say 20 seconds duration per exposure equates to 4000 seconds of time. But, there is a dead time between shots that increases this to 5000 seconds of time. However, you really need to let the camera's sensor cool a bit between shots. I use 15 seconds which results in 7000 seconds needed for 4000 seconds of time. After that comes the darks which for 200 exposures can easily add another 1000 seconds of time. All in all for an hour's total exposure you are looking at approximately 1 hr 40 minutes to 2 hrs 15 minutes of camera work. However, keep in mind, AZ photography depends upon the location of the object in the sky and the earth is constantly rotating .... this has a major impact upon how much time you have to capture the object as the earth rotates 15 degrees every hour. (Note: this is written from the perspective of using a DSLR)

    The influence of atmosphere and an object's position in the sky has a big impact with AZ photography. Keep in mind the earth is rotating as you work. For AZ photography, you have to stay about 30 degrees from the zenith or, except for objects on the northern or southern horizon, essentially 30 degrees away from your meridian. To get 30 seconds of exposure time for objects on your meridian, they need to be almost on the southern or northern horizon. This means if you start with an object in the east it needs to be fairly low on the horizon when you start as it will be constantly approaching your meridian as your photograph and the time you have for exposing will become shorter and dependent upon its altitude can become very short ... a few seconds at best. If you start with an object to the west, it will be setting out of sight. Since you can not photograph near the zenith, you are shooting through a lot of atmosphere which prevents images from being as sharp as they could be. Photography with an equatorial mount does not have these restrictions. You can start in the east and go most of the night with an object. You can also photograph objects high in the sky where the influence of the atmosphere is at a minimum.


    Time to process photos. With 200 to 400 exposures, stacking programs will need a two to four hours processing time to stack. This time is also dependent upon your computer. A computer with lots of memory and a fast chip will take less time. After stacking, you will then need to process the image with a processing program such as photoshop elements, CS5, PhotoPaint, Gimp, etc.

    I am not trying to discourage anyone from using an azimuth mount but did want to give you a brief description of the process and techniques and limitations. Keep in mind, with an equatorial mount one simply does not go out at night, point their telescope at the sky and snap away. There are issues there too but you are not limited by very short exposures times or the position of objects in the sky.

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    Default Re: Astrophotography with Azimuth Mounts

    Hi Terry,
    Would you be able to share the link of your work?
    I'm about to buy a Nexstar 8i and will try to learn from this scope the basics staffs and to see if my passion for the AP will grow....
    Would love to see your work and equipment used to reproduce some okay images!!
    Thanks a lot!
    Robert

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    Default Re: Astrophotography with Azimuth Mounts

    My pleasure Robert. The main page of my website is:

    The Astronomy Stop

    The direct link to my images is:

    Deep Space Images

    The direct link to my Image Processing page is:

    Image Processing

    While my Image Processing page describes how I capture and process my images with a Meade DSI, some of the information is pretty generic to anyone creating an image. It's kind of basic and doesn't get into a lot of detail but should provide an overview of the process I go through.

    Terry

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    Default Re: Astrophotography with Azimuth Mounts

    Hi Terry!!
    Many thanks for sharing your site and info!!
    It will be a long reading for me!
    I've saved them to my Favourite, so whenever I have chance, it will be at one click!!

    Thanks again!
    Robert

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    Default Re: Astrophotography with Azimuth Mounts

    very helpful thanks

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    Default Re: Astrophotography with Azimuth Mounts

    Wow,
    All that with an AZ mount? Very impressive. It's something thing to strive for. I hope when I get my scope I can get close to snagging one of those objects. Great work!
    George

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