Sounds like a problem of nomenclature, an adapter can mean a lot of different things to different people, personally I enjoyed the large scale of the photo and seemed sharp enough that an enlargement would suffice if you wanted to change the scale. For sure different optical assemblies have their pro's and con's, try before you buy and get as much info as you can. Erik
Daniel, this was mentioned earlier in the thread.
The fact is that Joe, or anyone shooting with a SCT, never shoots using direct objective because the secondary mirror multiplies the objective's f-ratio. You are correct, the secondary in his telescope is effectively a reflective Barlow, and I did mention that. For that reason, Joe always shoots with the SCT equivalent of configuration number 4 in the chart I posted. Projection using a negative lens, or, in his case, a convex mirror.
It may be splitting hairs to you, but any time you modify the f-ratio of the objective before the prime focus, it is no longer a true direct objective system optically. Claiming that the secondary in SCT doesn't count is optically the equivalent of saying that a Barlow in a Newtonian doesn't count.
Optically speaking, these configurations are not simply a matter of what extension tube or adapter you have in place between the camera and telescope.
On Sun, Aug 11, 2013 at 12:45 PM, daniel turner <outwest112@yahoo.com>wrote:
I believe that Joe is correct and that Chuck and Brent are wrong.
Remember that Joe is using an SCT which is NOT a fixed focal length telescope like a refractor or a Newtonian. Joe can put extension tubes on his optics and then adjust the focal plane to meet his camera where he wants to.
He does this by adjusting the distance between the primary and the secondary, that is how focus is achieved on an SCT, and in the process of moving the focal plane, he also changes the final focal length as well as the image scale and the focal ratio.
Remember that the magnification achieved with a barlow depends strongly on the spacing between the primary optics and the barlow lense. That is why the SCT works like it does. The secondary mirror is actually a barlow and moving it during focusing in relation to the primary changes the focal length and image scale of the entire assembly.
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