Secondary mirror obstruction

Is it possible (or advisable!) to raise your Newtonian spider a few MM, install a smaller secondary, and realise the effects of a smaller obstruction? I know this will not change the focal length, and the focuser will "rake-in" to compensate, but is it viable? Chris Brown

Depends on your telescope. If you make your secondary too small some of your precious light will get reflected straight back into space, or at least not reach your eye Try using some design software like "newt" to do the math.

When you say raise your Newtonian spider, do you mean raising the whole secondary cage? If you only raise the spider but the focuser remains in the same position, it will no longer be centered on the secondary (or the secondary will no longer be centered in the focuser). With a truss-type Dob you should move the whole secondary cage. In a sonotube Dob you'd need to move the focuser. As Dan mentioned, make sure you don't go too small (and if you do, make sure you get a smaller primary mirror as well ;-)

Doug Tanaka

If you raise the secondary, you have to move the focus in. this will probably mean that the focuser will impinge on the light path. All this assumes that the proper size secondary was installed in the first place...

Pat

Other factor.
Size of the secondary should be changed depending on the Newtonian telescopes intended use. For optimum performance using the telescope visually, a smaller secondary is approriate, since what you see with your eye cannot be digitally processed and there is some light loss and loss of contrast with a larger secondary. Obstruction, however, is not as much an issue as some people make it out to be. The drop in contrast and total light pickup is pretty minimal unless you are really talking big %'s. The difference between an f10 and an f6.3 SC for instance is about 3%.
For imaging with camera/ccd you should install a larger secondary.
I've done this with my 8"f3.5 Newt..although it's a bit of a hassle..haven't really noticed a big difference in the visual performance myself.
Better, you might talk to Protec about their 3 vane dew-heated spiders, which I've found to be excellent products. You get less of a diffraction affect off the 3 vane.