A 300mm telephoto vs a 6 inches refractor

The Tamron SP 54 is a 300mm telephoto lens, with an aperture of 54mm. I took a 60 seconds of exposition image of M42 in Orion. A few months later I took a 15 seconds image of the same field through a 6 inches Sky-Watcher non-apo refractor.

The comparison throws some interesting conclusions:

1.- A 150mm lens gives more resolution than a 54mm lens. Stars are fainter in the 6 inches refractor picture.

2.- The light captured by a 54mm lens in 60 seconds is nearly equivalent to the one captured by a 150mm lens in 15 seconds. Nebulosity appears practically equal in brightness in the two pictures. The lower image is a full frame, meanwhile the upper one is cropped at 25%.

Both images were taken using an unmodded Canon DSLR camera, EOS 450D (or Rebel XTi) and an EQ6 equatorial mount.

4 Responses to “A 300mm telephoto vs a 6 inches refractor”

  1. Hi,
    nice images of Orion nebula, i have few questions. i already have Sky watcher’s 6 inch dobsonian telescope. what type of tracking mount is required for taking long exposure images like above without introducing star trails?
    can you recommend some budget mounts that can be used for long exposure astrophotography?


    • computerphysicslab Says:

      For taking long exposure pictures you need to mount your optical tube assembly over a mount. You may get a cheap SkyWatcher HEQ5 that would be enough to track at sideral rate for short exposures, like 30 seconds or 1 minute and large focal lengths, like 1000mm or so. For longer expositions an autoguider would be necessary. I don’t use autoguider, and a way to take longer expositions is digitally stacking through a computer a set of short exposures. I use to take a lot of 30 seconds expositions.

  2. Julie Says:

    I wanted to thank you for the most amazing last 45mins as I have lingered through your images! I was online, looking for maps of the lunar surface and stumbled upon your site. I am neither an astronomer nor a scientist although I love to take photographs (Macro) of more terrestrial subjects! It is not my usual MO to leave comments however the intensity of your love and dedication obvious in the execution of your work was more than worth my time to encourage you in your pursuits..kudos and thank you.

    • computerphysicslab Says:

      Thanks to people like you I have enough encouragement to share my modest pictures. I hope you enjoy them as much as I did when I took them.

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