Quote:
Originally Posted by astro744
Use the formula
Angular diameter (deg) = field stop diameter of eyepiece (mm) x 57.3 / focal length of telescope.
See http://www.televue.com/engine/TV3b_page.asp?id=214 for Tele Vue eyepieces. For 13mm T6 Nagler, FSD=17.6mm and for telescope focal length of 1500mm;
Angular diameter = 17.6 x 57.3 / 1500 = 0.67deg.
With the Moon at perigee you can have close to 0.55 deg so that can leave (0.67-0.55)/2 = 0.06 deg all around the edge of the Moon. (Note Moon is not at perigee at present but getting closer these past few days. This is still 3.6 arc min or 216 arc sec or about 7 Jupiter diameters (present diameter) so have a look at Jupiter at 115x and approximate what seven look like. Is the the gap between the Moon and the edge of field? If not and from how you describe it then something else is at play and the most obvious is that the telescope is not working at f10 and your effective focal length is greater than 1500mm. This can happen with an SCT as it is a complex curved two mirror system but it won't happen by itself. What can affect the effective focal ratio/length is if you had a longer path to the eyepiece such as adding a 2" diagonal or using a binoviewer require the SCT primary to be focus differently by considerably shifting its position, (this is how SCTs focus).
I would trust the Tele Vue eyepiece field stop diameter specifications and use these for TFOV calculations as the AFOV/magnification is close but doesn't take into account angular distortion.
I'm not sure why you are getting a larger image than the focal length would give you and the only reason I can think of is that your effective focal length is greater.
Perhaps do a search for "calculating SCT effective focal length or focal ratio". (Note aperture is fixed). |
Thank you very much. I will do some research
