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發表於 2008-1-4 16:53:22
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原帖由 ianchung 於 2008-1-4 00:53 發表
一直都有個想法,賣左d takahashi鏡,買番支600/f4或400/f2.8,
用途多d唔洗禁浪費
There are advantages and disadvantages. Based on my experiences so far:
1) Its very light and compact compared to same aperture telescope; the 600/4 is only about 6kg and I can hand carry it on an airplane with a photopack, yet it has 6" aperture
2) You can autofocus on bright stars with a DSLR, very convenient in the field and no need to carry laptop and/or focuser for fine focusing
3) field is very flat up to 35mm format
4) can use teleconverters to extend focal length, they are parfocal too
5) contrast and sharpness not as good as an APO scope
6) bright stars tend to flare with peculiar shapes
7) back focus distance is very short (44mm for Canon, Nikon slight longer); may not be enough for large CCD cameras with filter wheels
8) fine manual focus by hand very difficult; I use a RoboFocus which is satisfactory
9) stars not round if not critically focused; pronounced aberrations of various kinds will appear even when focus is slight off; so accurate focus is very critical
Overall, I still think it is excellent for astro-photo on travel. You only need the mount, the lens + DSLR to make the simplest setup for unguided photog. During the day, you can use the lens & DSLR for birds and other stuff too. |
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