Getting a telescope, have some questions about taking pictures through it.

Discussion in 'Astrophotography' started by colbycheese, Jun 5, 2015.

  1. colbycheese

    colbycheese Mu-43 Veteran

    378
    May 1, 2012
    Way up there.
    So i have decided to take up astronomy. I was going to buy (maybe rent first) a dobsonian telescope. I have little idea what i would need to be able to connect my camera to it to take photos. I was planning on buying either the Orion XT6 (http://www.telescope.com/Telescopes...sonian-Telescope/pc/1/c/12/sc/13/p/102004.uts) or the Sky watcher 8 inch dobsonian (http://ca.skywatcher.com/_english/01_products/02_detail.php?sid=361)

    The one my local telescope store rents out is the Sky watcher one. I was wondering other than a t mount adapter, what else would i need to take pictures through it. Also if anyone here is knowledgeable on telescopes, which one would you recommend? I know the aperature is important for how much light gathering there is and that a dobsonian is a good cost to performance ratio. Thank you
     
  2. PMCC

    PMCC Mu-43 Regular

    132
    Feb 18, 2013
    The #1 problem is what do you expect to shoot.
    If you want to take picture of faint nebulae, etc. then you need a good equatorial mount to track the movement of stars. The Dobsonian mount is not going to do it. Also you may want a shorter focal length telescope.
    If your target is planets, you need a video camera. An equatorial mount is still useful. On top of that large aperture.
     
  3. dwig

    dwig Mu-43 Top Veteran

    624
    Jun 26, 2010
    Key West FL
    For most astrophotography, you need an equatorial mount with a motor driven polar axis to compensate for the rotation of the earth during the long exposures needed. The most common type of equatorial mount is sometimes called a "German mount".

    Alt-Azimuth mounts don't work well for astrophotography, even the computer driven tracking versions, though they can be very suitable for terrestrial work. There are quite a number of such designs with one popular type being the Dobsonian mount. With the computerized tracking versions the target object will remain centered, but the image field will rotate during a long exposure.

    You should also note that telescopes tend to be spec'd with slightly different terms. The "size" of a scope is usually spec'd with the diameter of the primary optic (lens or mirror) and not the focal length. For point like objects (stars and distant planets that don't show a significant disk) the area, which is directly proportional to the diameter, of the primary is the only controlling factor for light grasp. Also, the scope spec "Focal Ratio" is what is referred to as the maximum f/stop when discussing camera lenses. This is important only when photographing diffuse objects (nebulae, ...) or objects that resolve a significant disk (the moon, ...) but not significant when photographing stars.
     
    • Like Like x 1
  4. colbycheese

    colbycheese Mu-43 Veteran

    378
    May 1, 2012
    Way up there.
    I guess i never though of it like this. I guess you would need longer exposures to get a better image. My question is, is it like the star trail equation, which i dont remember exactly but something about focal length and exposure time. Is there an equation like this that works for telescopes?
     
  5. dwig

    dwig Mu-43 Top Veteran

    624
    Jun 26, 2010
    Key West FL
    At its root, it is the same equasion, but the desired outcome is different. With star trails you want the stars to move across the image plane and calculate the amount of time needed to get the desired degree of motion. When you want crisp images with a fixed telescope you calculate the maximum exposure time that will have less movement than the blur size of the star image.

    Generally, you can't make an acceptable image with a fixed scope. The image is too weak to make an acceptable exposure in the brief time that is required to avoid the motion blur with the possible exception of a very few subjects that are bright enough (e.g. the Moon).

    There are ways of adding a proper tracking drive to a Dobsonian mount. Google "Poncet Platform" for more information.
     
  6. Machi

    Machi Mu-43 Veteran

    208
    May 23, 2015
    Taking images of celestial objects with Dobsonian is difficult but not entirely impossible.
    I have SW8 and those are few examples obtained through stacking multiple shots (from 10s to 100s).

    M104 "Sombrero" galaxy:

    M104.

    M13 globular cluster:

    M13.

    M97 "Owl nebula":

    SUM_P4011081-P4011213s-6.

    And Dobsonian isn't limited only to the objects of night sky but it's usable also in daylight.

    GulfstreamG450b.
     
    • Like Like x 3
  7. Holoholo55

    Holoholo55 Mu-43 All-Pro

    Aug 13, 2014
    Honolulu, HI
    Walter
    I have an old Celestron C90 I acquired from a friend, and also had his Celestron C8 w/German equatorial mount that I ended up selling. The C8 would have been good for astrophotography. I bought a MFT T-adaptor for the C90, but have not yet tried astrophotography. There's an impressive amount to learn as I found just by searching the web.

    A good equatorial mount is really needed to get deep sky photos. I have heard of DIY mounts. When I view the moon through the C90, it moves so fast that I have to keep adjusting my alt-azimuth mount (video fluid head) to keep it in view, although one can certainly get a usable photo of the moon with a good telephoto lens and a standard tripod.

    I have nothing particular to recommend, but I suggest doing your research thoroughly before purchasing anything. It would be worth looking for an astronomy club in your area. I'm sure that by going out with them on a viewing and just talking with them, you'd get a lot of good advice.

    BTW, the guy I sold the C8 to said that one of the reasons he bought it was because of the wooden tripod. He was advised that wooden tripods absorb vibration better than metal tripods and that it was well suited for astrophotography. If it wasn't for the difficulty of finding dark skies here and me not likely to put it to good use, I might have kept the C8 kit. It was really a nice setup.
     
  8. MadMarco

    MadMarco Mu-43 Veteran

    298
    Oct 30, 2014
    Guildford, England
    What everyone else said:

    Without a good tracking equatorial mount, the only objects that you will be able to photograph will be either the moon or maybe Mars/Saturn/Jupiter. Most of my astronomical photos are taken at ISO800 with multiple exposures between 30-60 seconds and then stacked in Deep Sky Stacker. I have recently bought a tracking scope to add feedback to my tracking mount, this increases my maximum exposure length to around 10 minutes.

    You should be looking to spend about 2x the cost of the telescope on the mount/tracking solution, it's that important!!! A refractor is generally a better bet for astrophotography than a reflector type, there is less maintenance, they are less prone to accidental damage and they are smaller and easier to handle for you and the tracker. That said; you can get amazing photos from a reflector, but it's more of a labour of love.

    30x 30s + 30x 60s stacked @ISO800
    16195599966_453fca4781_b.
     
    Last edited: Jun 9, 2015
    • Like Like x 1