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I have a William Optics Zenithstar 61 teleacope. I'm using it with a 0.6x flattener-reducer. What is this doing, and what magnification would I get with a 25mm eyepiece?
The William Optics Zenithstar 61 is a portable, well-made refractor with a short focal length of 360mm making it ideal for wide-field observations. The multi-coated, doublet optics use lowdispersion FPL-53 glass so the scope is also suitable for imaging deep-sky objects. As with all refractors, a field flattener or field flattener-focal reducer is recommended for imaging purposes but it is unusual to use such an adaptor for observational use. There’s no reason why you shouldn’t use one for this purpose as it will allow you to observe even larger swathes of the night sky than the instrument already displays.
The magnification of a telescopeeyepiece setup is calculated by dividing the focal length of the telescope by the focal length of the eyepiece, which in the case of your Zenithstar gives 360÷25 = 14.4x magnification. This type of magnification is in the binocular realm and will deliver great views of large asterisms, constellations and many deep-sky objects, but planets and the Moon will be very small indeed. The field of view will be 4.7° across, or roughly the equivalent of nine Moon widths.
Inserting a 0.6x focal reducer produces an effective focal length of 360x0.6, which equals 216mm, yielding a magnification of 8.64x, which equates to a field of view of approximately 15 Moon widths.
When I look at Jupiter with my Celestron CPC I see a huge black spot in the centre. What could be causing this?
The Celestron CPC range of telescopes are of the Schmidt-Cassegrain design, with a large concave primary mirror at the base, a corrector plate at the front and a convex secondary mirror attached to the corrector plate on the inside of the tube.
The light they collect follows a complex path: it travels through the corrector plate to the primary mirror; back up the telescope’s tube in a converging beam to the secondary mirror; back down the tube again and through a hole in the middle of the primary mirror; and finally through the focuser and star diagonal to reach the eyepiece. All in the name of an effectively long focal length.
Tha black spot you are seeing is the shadow of the secondary mirror, indicating that you have not achieved correct focus.
The Zenithstar 61 is ideal for wide-field observing and you can make its view even wider
A photon needs a GPS to navigate through an SCT Primary mirror Starlight Eyepiece Corrector plate Secondary mirror Starlight