6" F8 rebuild/upgrade question

[groot]

hello folks! i just desided to upgrade an olde 6" F8 dob and need some advice on placing the focuser. i am going from a helical to a crayford focuser. and the new crayford's profile is a good bit higher than the old focuser. ( see pic) should i shorten the distance the same amount as the difference between the two? and if so, will this mean i need a larger secondary? thanks for any help i can get on this. 

 

Rick

 

Edited July 19, 2020 by groot

[Peter Drew]

Yes, you're correct, you will need to move the new focuser down the tube by the difference in order to reach focus.  Whether you need need a larger secondary depends on how generous the existing one is, slightly too large is preferable than too small.    🙂 

[groot]

thank you very much! this had me scratching my head. lol. the difference was 1.25 inches. not a huge difference but enough i thought it wiser to ask. 

[andrew s]

Google "Newt for the web". It will let you do all the sums needed for the design.

Regards Andrew 

[groot]

thank you very much Andrew! that's what i've been looking for. seems it's fine. i should have a diagonal obstruction of 18% which is under the 20% they want. the obstruction of the primary surface area by the diagonal is 3% so all good! 

Edited July 20, 2020 by groot

[Moonshane]

It might be easier to move the primary up. Then no cutting,  just (presumably) new holes for the screws.

You will probably be OK re the secondary. assuming you can see all of the primary in it when in position.

[groot]

right. i went ahead and got a sonotube and doing a whole rebuild. if i'm reading it right the "newt for the web" said it's good. if not i'll get a new tube and learn some more hahaha. good project to play with though! thanks for the help! i understand the mechanics, putting them into practice is another thing all together!

[groot]

hello folks! ok so. rebuild went well. so far i'm happy with it. had to rework the mirror cell ( broke in shipping ) still a few things left to do. took it out for a look at jup. it did very well! but here's is the basic results. 

 

[groot]

hello folks! well the clouds have moved out for a few hours so i think it's time for a test! keep your fingers crossed.