Pointed Stars?

[PeterEde (Peter)]

This maybe be a dumb question that I may answer in the question.
Typically we use fast glass or as fast as we can afford. In my case f7.5
In "Normal photography" if you take a long exp with lights at the largest aperture you have typically ie 4.5-5.6 or faster you wont get pointed lights without a filter. But if you stop down to f10 or less you get points without a filter.
So some astrophotos I see have pointed stars and some don't. Some have a mix
Now since we can't change apertures like on a camera and f7.5 is open enough to not get points and I doubt anyone puts a star filter on. My question is. What's going on?

hmmm f7.5 is without eyepiece. adding eyepiece stops down by a factor determined by magnification of attached eyepiece?
Which brings me to an f number 10-?

Am I on the right trail?

Just found this handy calculator http://www.stargazing.net/naa/scopemath.htm

[Screwdriverone (Chris)]

Hi Peter,

Typically, the photos with the points or spikes are caused by the secondary vanes of a newtonian reflector telescope being the device connected to the camera. The diffraction spikes are caused by the X pattern of the thin supports holding the secondary in the middle of the tube.

Conversely, the round star shots are normally done with a refractor or even a cassegrain design scope, which in the case of the refractor, has a clear aperture lens with no secondary and therefore no spikes occur. The cassegrain design has the secondary attached to a corrector plate which does obscure the total amount of light coming in the front of the tube by a degree, but as there is no vanes holding the secondary, you dont get the diffraction spikes with this scope either.

Assuming this is what you are referring to with "pointed stars" ?

Cheers

Chris

[PeterEde (Peter)]

Thanks Chris. Ok I didn't consider the different types and their construction