Field of View Between Two Telescopes

I have two telescopes, a Skywatcher 80ED (identical to the Orion 80ED – 600mm focal length at F7.5) and a Williams Optics Gran Turismo 71 APO with 420mm focal length at F5.9. Just looking at the numbers it’s easy to see that the GT71 is a smaller and faster telescope, and because of the shorter focal length it should have a larger field of view.

Comparing size with Skywatcher 80ED

Comparing size with Skywatcher 80ED

Now I’ve photographed the same part of the sky with both telescopes, and can now overlap the images to see exactly what is the difference between the field of view between these two telescopes.

First I need to say that that GT71 NEEDS a field flattener when imaging with DSLR. The distortions off-center are terrible.  Don’t get me wrong, as a three objective lens telescope (including 1 fluorite for color correction), it has provided me with the best lunar photos, however it has issues when using the large DSLR sensor. The SW80ED provides a much flatter field of view for photography out of the box.

The flattener for GT71 is in the plans…

So how does both telescope compare?  Below is a photo of open star cluster Messier 38 taken with my GT71 and I’ve overlapped as a brighter box an image taken with the SW80. For those wondering, I used IRIS to register and align both photos using the coregister command.

Messier 38 - Field of view with William Option GT71 and Skywatcher 80ED (brighter box)

Messier 38 – Field of view with William Option GT71 and Skywatcher 80ED (brighter box)

Both telescopes deliver just about the same field of view with the GT71 providing 1 degree more of horizontal field. But the difference is much less on the vertical.

What did surprise me is how much light the GT71 gathers. Inspecting the photos showed me that even with the smaller setup, the GT71 has great light gathering capabilities.  I got down into magnitude 12 with only 15 seconds of exposure, which is nearly similar to the SW80ED at 30 seconds.

WO GT71 vs SW80ED Optics

WO GT71 vs SW80ED Optics

In conclusion I would say the GT71 has good photographic potential, but requires a field flattener if it will be used with DSLR.  Stay tuned…

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