super telephoto or telescope for astrophotography?

Discussion in 'Astrophotography' started by pood, Oct 5, 2012.

  1. pood

    pood Mu-43 Rookie

    12
    Aug 27, 2012
    At the moment, i have a meade 125EXT telescope on loan and I'm thinking about buying the adapter to attach to my EM5, but then I discovered I could get a Sigma 600mm f/8 for around $200. On my EM5, that would put it at 1200mm.

    I'm wondering if I would get better quality images with a super telephoto or the meade telescope?
     
  2. Region2

    Region2 New to Mu-43

    8
    Sep 30, 2012
    New York, NY
    Since it's the 125 ETX, it has really good tracking. Personally I prefer a tracking telescope over a telephoto for astrophotos since you'll need a long exposure time and to compensate for the Earth's rotation. If you're shooting the moon, a telephoto will do since exposure time doesn't need to be long and the moon is terribly bright.
     
  3. Geoff3DMN

    Geoff3DMN Mu-43 Veteran

    The 125 ETX has an alt-azimuth fork mount which will cause field rotation during longer exposures, I would suggest shorter exposures and stacking for better results.
     
  4. RobWatson

    RobWatson Mu-43 Hall of Famer

    F/10 for the ETX? A bit slower than the Sigma F/8.

    I find the ETX very hard to focus. I have both manual and motorized focus controls.

    Field rotation (as was mentioned) is worse at the ecliptic and gets more managable towards the poles but will still be a problem in some cases. Track and stack is almost always called for except for moon & planet shots. 1250mm is not quite enough for planets so you will need to do eye piece projection to get reasonable magnification. Note that even alt-az mounted on a wedge (and tripod) will track without field rotation.

    I believe everyone should have a telescope (or 3) so I'm biased.

    I suggest two adapters: one for direct (aka prime) use and one for eyepiece projection - and you will need some eyepieces ...


    Image quality ... tough call. I suspect unit variation will be dominant. I think I'd have to give the edge to Sigma on no firmer grounds than it was nominally designed specifically for photography while the ETX not so much. Lots of lovely photos taken with every model of the ETX ... and plenty of garbage, too. I bet the same with the Sigma!
     
  5. Togna Bologna

    Togna Bologna New to Mu-43

    6
    Sep 4, 2012
    I used the OM 500mm reflex lens + EC-14 on my E-30 to photograph the annular eclipse last May and the Venus transit in June. Like the Sigma lens, the mirror lens aperture is f/8, reduced to f/11 by the telextender. Fine for solar photography through that super-dark filter, slow for night sky photography. A homebuilt barn door tracking mount allowed me to keep the camera pointed at the eclipse and the Venus transit with only occasional repointing by hand.

    1206050468-The-photo-rig-XL.

    At 1400mm effective focal length, the sun or moon occupies about a quarter of the frame. I got unexpectedly good images, limited by atmospheric haze and turbulence rather than lens optics. Remember that earthbound astrophotographers are always shooting through a poor 'accessory lens', the atmosphere. We always want the best lenses we can afford, but only rarely will a night sky be clear and calm enough to show the highest optical quality.

    The instant of annularity, only a slight crop to correct the celestial angle, to show you the field of view at 1400mm efl:

    1205200848-XL.

    Venus in transit, plus sunspots:

    1206053041-Clear-moment-L.

    For a hundred dollar investment in parts and my old camera gear, this combo worked well. However at that magnification it was almost impossible to focus the lens, even with Live View magnification. Not only did the image vibrate when the focus ring was touched, but I couldn't make small enough turns to avoid overshooting the focus point. In the first photo you can see my Mickey Mouse solution to the problem: a wire coathanger coiled around the the focus ring with two arms projecting out, to act as lever arms so I could make small adjustments.

    Pointing the lens accurately at 1400mm focal length using a ballhead was very touchy too - hit or miss. A telescope and a good mount are designed for ease of focusing and pointing. I hope to buy a telescope and mount for astrophotography next year, with the ultimate goal to photograph the North American total eclipse in 2017.
     
    • Like Like x 3
  6. pood

    pood Mu-43 Rookie

    12
    Aug 27, 2012