Seondary Mirror

[rajiv 1972]

Does a secondary mirror of 50 mm makes a big difference VS a 30 mm for a primary mirror 188 mm newt f/6.53,and distance from the secondary to the focal plane is 185 mm? My main interest fully illuminate an APS-size DSLR chip ? secondary and primary mirror , both are 1/4 to 1/8 wavelengh.

[umadog]

Don't the APS chips come in a range of sizes? When I run the numbers it looks like you may want a 60 mm secondary, but then I don't know exactly what chip you have.

With photography, you care about illuminating your chip so use the right secondary for the job. For visual, secondary mirrors smaller than 20% by diameter will have no significant effect on the image and telescope with such a secondary will behave like an unobstructed scope (barring diffraction spikes from the spider, of course). The effect of the secondary may become noticeable at around 25%, but there are plenty of SCTs with secondaries over 30% that still produce great planetary views. So even a 50 mm secondary is not extreme for you scope. A 60 mm may be reasonable for photography and is still not a crazy size.

[rajiv 1972]

I have APS-C size DSLR .

[umadog]

According to the Wikipedia that's not enough to tell you the exact size since: "Sensor sizes range from 20.7×13.8 mm to 28.7×19.1 mm" for the APC-C chips. It should say on your camera's spec sheet. Then you can use http://www2.arnes.si/~gljsentvid10/diagonal.htm to figure out the size of the fully illuminated field you need. I imagine you want a fully illuminated field equal to the size of the chop across the diagonal. I also reckon that you can get away with not being quite fully illuminated across that axis. i.e. probably being within 1% or 2% is fine, but wait for someone who knows for sure to confirm that. You might also want to look here:

http://www.astronomyforum.net/ccd-imaging-forum/146906-ccd-sensor-size-vs-secondary-mirror-size.html