Need some advice on objective

[Mayank]

Hey there stargazers..

     I'm trying to construct my own Newtonian telescope. The f-length of my objective is around 900mm ( planed..not bought yet). 

Can i use Concave mirror as an objective.. As parabolic mirror isn't available around here anywhere in my city.   And if yes.. What aberrations can I face.?

 

 

Please do help guys 

Thanks

[ronin]

There are a number of reflectors made with spherical mirrors, Skywatcher produce a few. Will say they are not overly common, perhaps becasue it would appear not too difficult to parabolise a mirror these days using production machinery.

The "key" factor is that the the mirror will need to be "slow" to reduce the aberrations. In my estimate you are looking at an f/8 or slower mirror. So if 900mm focal length that means around 120mm diameter (all numbers are approximate, just not worth giving you a value to 2 decimal places). Something like f/10 would be better but then the diameter (90mm) is getting small, likely too small to be of great use.

Not sure what the aberrations are I would say they are simply termed Spherical Aberration BUT in mirrors the names seem to get altered a little, so half expect someone else to appear with a different term. It is a lot easier to keep it general and just refer to "aberrations".

 

[Louis D]

Read the Spherical aberration article on Wikipedia for a primer on it.  A much more in depth description can be found here.