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  1. #1
    velasoraptor's Avatar
    velasoraptor is offline Main Sequence
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    Question Upgrade or not to upgrade is the question?



    Hi,
    i am having a Sky-watcher's 6 inch Dobsonian telescope for about a year now. i am completely satisfied with its performance and optics quality.
    since, i am now comfortable with using the telescope & also done some good lunar and planetary astrophotography using it.
    The only problem i have now is to have some good tracking mechanism. I have seen this new range of motorized Dobsonian telescopes with GoTo mechanism with moderately accurate tracking mechanism.
    Since i could not have a separate auto-tracking Dobsonian mount for my current 6 inch OTA, i am thinking of upgrading to the new 8 inch autotracking telescope model.

    I want to ask does it worth upgrading?
    1] how much will i gain in terms of resolving power & aperture gain by comparing the 6 inch & new 8 inch apertures?
    2] could i be able to do DSO astrophotography by autotracking mechanism with long exposures say upto 10 sec without introducing any star trail defects?
    3] does anyone have additional recommendations about best deals for per aperture etc. ?

    i have very limited budget so i can't afford to invest in EQ mounts for serious astrophotography. I just want to perfect my moderate astrophotography needs using auto-tracking dobsonian mounts.

    Thanks
    Scopes: Sky-Watcher's 6" Dobsonian Telescope (Sold to upgrade)
    GSO 6 inch F/4 Newtonian
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  2. #2
    sxinias's Avatar
    sxinias is offline Super Moderator
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    Going from a 6 to 8 inch telescope of approximately the same focal ratio. Images in the 8 inch telescope will be a little brighter and you will see slightly more details. While not earth shaking, the difference is there and is easily noticeable. Whether or not the difference is worth several hundred dollars (ten to twenty thousand rupees) is difficult to say.

    Using an auto tracking DOB for photography. It really depends upon how smoothly the dob tracks. A dob mount is an azimuth mount so field rotation is a problem. However, for objects 30 degrees or more from the meridian, you can get 30 second or longer exposures with 0.1degrees or less of field rotation. 0.1 degree of field rotation is insufficient to produce noticeable star tracks. Thirty seconds is more than sufficient to take multiple exposures and stack using Deep Sky Stacker. However, and there is a however, the question is how smoothly will the tracking DOB move? You will need at least 15 seconds, preferable 20 seconds where there is no vibration caused by tracking movement, motor/gear induced vibration, or wind induced vibration as exposures shorter than about 20 seconds are too often underexposed for stacking programs to process.

    While I don't really know, I suspect that a tracking DOB will have difficulty producing a 20 second period with no movements; thus, photography with it, other than solar system objects, will be problematic. A decent photograph of a deep sky object like M8, M16, M17, M20, etc. needs about 30 minutes total exposure and longer is better. I suspect the failure rate for exposures with an automatic Dob will be in the 60 to 80 percent range, meaning to get 30 minutes worth of good exposures, you will need to take between 75 and 150 minutes worth of exposures. That's a lot of work and time. To take a twenty second exposure requires about 30 of seconds of camera time if you push it; thus, one 30 minute shot will take you approximately 2 to 4 hours of time on site with a camera so you can statistically end up with 30 minutes worth of decent exposures. Also, processing the hundreds of exposures into a final photograph will take you another 4 to 6 hours. Thus, for one photograph, you will be spending 6 to 10 hours of time. I base the above on my experience using a SkyWatcher SynScan AZ goto mount for astrophotography. I could be wrong about the DOB regarding its ability to take a 30 second exposure with no movement, but feel confident enough that I would not risk my money buying a tracking DOB, or a goto DOB for that matter, for photography. Dobs are designed for visual work and they do that very well.

    The SkyWatcher Newtonian telescopes on the SkyWatcher SynScan AZ goto mount, unlike their other Newtonians like your 150P, do not have a DSLR mounting and do not have sufficient back focus for photography. The SkyWatcher 102 and 127MAKs do have sufficient back focus but their high focal ratio means they will need long exposure times .... something that is not compatible with photography with an low cost azimuth mount. You could purchase a SkyWatcher SynScan AZ goto mount and put your 150P on it. If you carefully balance the 150 and do not photograph during windy conditions, it may work decent enough. You will need to strengthen the tripod that comes with the SynScan AZ mount and could possibly obtain a high percentage of successful 20 to 30 second exposures (less than a 10 to 20% rejection rate). This greatly simplifies the process. The concern with a 150P on the SkyWatcher SynScan AZ mount is its length and the resulting moment of inertia which may amplify movements made by the mount while tracking or by wind gusts. The mount can more than handle the weight of your 150P. If you go this route, I estimate you have about a 50% chance of success.

    Really, the best road is to enjoy your 150P viewing the night sky and save your money until you have sufficient funds for a proper mount and telescope needed for astrophotograhy.
    SXINIAS

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    ZEQ25 mount;
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    SynScan AZ GOTO Mount;

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    ST-80A 80mm Refractor (OTA); Meade
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    203mm SCT (OTA);
    Meade DS2090AT 90mm Refractor; Meade 2045LX3 102mm SCT;

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    4 SE 102mm MAK; Celestron Advanced Series C6S (XLT) 150mm SCT
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  3. The Following 2 Users Say Thank You to sxinias For This Useful Post:

    BobDob (07-02-2011),velasoraptor (07-04-2011)

  4. #3
    Cladinator's Avatar
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    Good advice above.
    -Nick

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