secondary mirror specs?

[ebdons]

Hi, sorry if this question is an obvious one, but is a 70mm 2nd mirror from a F3.9 the same as one from a F4.0 up to a f5 or 6? are they generally the same apart from the coating and mirror material? so the only difference would be the parabolic ratio of the primary to make the difference in total F ratio and focal length? also is an astropgraph newt the same as a imaging Newtonian? looks very similar. thx tony.

[Astrobits]

If the 2ndry mirror is from a Newtonian then it is flat and has no part in the focal ration of the telescope. It is therefore usable in any other Newtonian that requires a 70mm dia minor axis diagonal.

If the Astrograph and Imaging Newtonians are true Newtonians then they should be the same optical configuration. Unfortunately today, names are being used for designs which are different to the original meaning often to give some additional attraction to the product.

Nigel

[ebdons]

Hi Nigel, thankyou for the quick reply, yes it seems that retailers are using the "astrograph" term more widely now and so there is now the "true astrograph" which could be a f4 or f3 etc scope focal ratio? tony

[alexbb]

The astrograph term refers mostly to a telescope being designed for imaging, not for visual. Therefore it is possible that you cannot achieve focus with the original focuser by simply popping an eyepiece in.

A secondary mirror diameter should increase for 3 reasons:

1. You want a wider fully illuminated circle
2. You have a wider primary mirror
3. You have a faster f-ratio
4. You moved the primary closer to the focuser and you want to keep the same fully illuminated circle. This is equivalent to 1.