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Quote:I am making an 8" Newtonian telescope. I would like to use it for planetary viewing and for as many deep sky objects as possible. The primary mirror is an f6 (47.3"). As I see it, my choices are probably between a 1.52" or a 1.83" secondary mirror. Which would you recommend and to what advantage? I have a 1.25" focuser and don't want an unnecessarily narrow field of view but also want to maximize light gathering and contrast. I am considering a 3 vane spider to reduce visible diffraction.
Ed "You have to grow old, you don't have to grow up."
Quote:+1 for what Pinbout said. 10" diameter tube. 1/2 of that is 5". Add 2" for a low profile focuser. 5" + 2" = 7". Divide 7" by the focal ratio. 7" / 6 = 1.16" point illuminated field diagonal size. Add some additional size for a bit of 100% illuminated field and positioning tolerance. Next standard size up is 1.3". Hardcore planetary stickler only! Very tight on the positioning. For me, the next standard size up 1.5" is pretty much perfect for general observing. 1.83" or 2.1" are good if widefield photography is your top priority.
Quote:I currently have a 1.25" University Optics focuser and it is rather tall. From the top of the focuser tube to the top of the 10" scope tube with the focuser racked all the way in would be approximately 3.75". It also has a lot of extra outward travel that is probably not necessary. With the "Newt for the web" program this ends up requiring non standard secondary sizes and the 1.83" one I have would cause vignetting at the 75% ray.Focal length of the mirror is 47.3", f ratio 5.9125.I may have to change my focuser to a 2" low profile one to allow a standard secondary size and a smaller secondary size. If so, I would like an economical option and something that will work with a 48" overall tube length. I haven't bought a tube yet but 48" tubes should be available.Comments, further recommendations?
Quote:There is no reason why you shouldn't go larger but it's a fact that planetary contrast will suffer.
Quote:It would be very good on a 10in as it's less than 1/5th of the dia of the scope. That's not a bad idea for planetary work and similar. What's degrading the image in this area is pure simple physics. It wouldn't matter if the scope was swinging from a tree.
Quote:Here's the deal with the secondary. I purchased the parts as a group and 3 vanes of the spider were damaged in shipment. I have to replace the spider so I figure, maybe I should replace the mirror and holder too to optimize the scope. If the thread tap in the new spider would also accept my current secondary mirror I could possibly switch between the secondaries.
Quote:If you model your scope using software or even drawing it out on a long piece of paper, you will see that you can't swap secondaries without also moving the mirror.
Quote:I guess that wasn't very well stated. Of course the two secondaries can be swapped, but if one models it then it is clear that to take full advantage of the secondary size change some of the other dimensions ought to change as well, including the distance between the primary and the secondary and the width of the tube. The same configuration can't be optimal for both secondaries.
Quote:If you optimize for a smaller secondary, then that includes optimizing the entrance pupil (i.e. the tube diameter). If you then swap in a larger secondary, then you will vignette at the entrance pupil. On the other hand, if you optimize for a larger secondary, then a smaller secondary will not illuminate as wide a fov as it otherwise would. I guess that a compromise design could be used that was acceptable for both, depending on what "acceptable" means. It seems to me that, in talking about using a smaller secondary in a scope that is optimized for "planetary", that compromising in this way defeats the purpose.
Quote:I'm not advocating for the smaller secondary, but rather pointing out that simply swapping secondaries is not going to produce an optimized "planetary" scope. Specialized "planetary" scopes are designed to get as much light into the small secondary as possible, and so they cannot take advantage of a larger secondary. Again, I am not advocating for such a scope, and I am certain that the 1.83" secondary is capable of producing knock your socks off planetary views, so long as everything is working properly.
Quote:Maximizing the diameter of the fully illuminated field for a small secondary means minimizing the distance from the focal plane to the center of the secondary.
Quote:Quote:Maximizing the diameter of the fully illuminated field for a small secondary means minimizing the distance from the focal plane to the center of the secondary.It sure does, and the only simple way to do that (assuming the focuser height is already minimized) is to shrink the entrance pupil/tube diameter. Your design holding the tube diameter constant at 10" means that the resulting scope is not an optimized planetary scope. In fact, you might reject this entrance pupil optimization as there is a cost (there always is), but if you do then you have rejected what I think most folks would consider one of the defining characteristics of a newtonian optimized for planetary observing - trading entrance pupil for small secondary obstruction.