Hi Youssef,
Welcome to IIS!
Here is a link to the instruction manual for an Orion 114mm Reflector. Different brand to yours but it will be similar and may be some help in getting things right. Also the focal length is only 450mm where I think you said yours is 900mm.
http://www.hometrainingtools.com/ima...IN-TSTAREQ.pdf
Hope this helps,
Molly.