DYI newtonian

[mcmalloy]

Hello there!

Last fall I purchased the popular Orion 8'' F/3.9 Astrograph, and have since had many great AP sessions with the scope. However a few weeks ago I was so unfortunate that a powerful gust of wind threw a small rock/pebble in to my telescope, damaging the primary mirror. Since I can't use the telescope optimally anymore, I was looking at the possibility of replacing the 203mm primary mirror with a new one. 

I have been looking at getting a 203mm primary designed for F/5. Since I wouldn't mind working at just a tad bit of a higher focal length. The one issue I have though is that I am not sure if the secondary mirror will be compatible with the F/5 primary.

Can anyone tell if a 203 F/5 is compatible with the original secondary for the F/3.9 astrograph? If not then i'll just get the F/4 primary. 

https://www.teleskop-express.de/shop/product_info.php/info/p6039_TS-High-Quality-Newton-Primary-203mm-f5---Pyrex-with-Zygo-Test-Report.html( This is the mirror I have been looking at getting ).

Thanks everyone!

[Joseki]

Well the secondary is flat so it's not specific to any focal length but you can check if it's too large or small for even illumination by putting your details in here:

https://stellafane.org/tm/newt-web/newt-web.html

however I would have thought it would be a bigger problem that your telescope tube is going to be too short.

[michael.h.f.wilkinson]

Sorry to hear of the accident. I assume the defective mirror is an F/4. Typically this will use a slightly larger secondary than an F/5, but using a slightly oversized secondary will not change the image much (you are already used to this much central obstruction, after all). You will need a new tube, as stated above. I personally would think about getting an F/4 mirror

[nmoushon]

Once you have to buy a new tube then you are basically making a brand new scope which at that point you might as well just buy a new scope. Since you are now replacing about 75% of the scope. Unless you want to build a new scope from scratch DIY style. But if you are going to do that you might as well build the whole thing from scratch new and not piece together bit from the other scope. That way you can get everything exactly the way you want it. 

Plus you then have a decent newt you could do some observing with while imaging.