Why shift the mirror up.

[omnivorr]

..when you could shift the secondary and focuer down..

Another of those conventions that seemingly go unquestioned, ..when to my novice view, the logic defies the convention.

To wit.. when desiring to attach a heavy photo-apperatus to the focuser of a Dob, thus shifting the CoG above the fulcrum, ...why would you decide to shift the primary forward, to achieve photographic focus, ..thus further shifting the CoG above the fulcrum ???

...surely the repositioning of secondary&spider and focuser can not present greater difficulty than shifting the primary, ..with its pursuantly greater requirement for balance remediation ???? (....minor issues of overlapping holes and suchlike can be averted by swapping sides, patches, inserts, whatever..)

is it a matter of so little difference in outcomes, and shifting primary is easier, ..or is it in some way more efficient to shift the primary, than I've perceived?

Cheers
Russ

[skwinty]

Hi Russ
When I modded my 12" I cut from the bottom.
Why?
1.Easier
2.Looks neater
3.Less Work

Do it from the top and you many more holes to patch and the OTA looks like Sh-one-t.
The balance issue doesnt go away either:thumbsup:

[omnivorr]

Thanks Steve, ..fair'nuff :) ..that's all I wanted to know...

...but mebe someday I'll look into it, for my own particular case, should that day arrive ;)
Cheers
Russ

[Blue Skies]

Sure, it can be done and has been done. But it has a more disadvantages than shifting the the mirror up, apart from it looking ugly. Well, it does look ugly - you have to cut long slits in the tube to slide the spider up and down. Makes them look sick. (I don't think I have any photos to show you but I saw some at a SPSP in the past).

The biggest yuk about it in my eyes is the large amount of potential error in collimation you introduce - I hope that makes sense, but let me try to explain. While newtonians have a generous amount of adjustment space with the various components of the scope, the more flexibility you build into them the harder it is going to be to get good collimation. The spider should be squared in the tube so your secondary cell will have room to be adjusted to see the primary so you can achieve collimation. If you just shift the primary up, you can more or less get the primary square in the tube again while the secondary and spider hold their alignment. Less things to go wrong, or to get right.

[davewaldo]

I too moved my primary mirror up the tube of my old dob. But at the same time I drilled 1 new hole on each side of the tube and moved the altitude bearing mounts up the tube by and inch as well. The mounts are large enough that they hide the old hole and this made my scope balance perfectly. Very easy to do too! :)

[omnivorr]

Thanks Blue Skies, .."ugly" wouldn't worry me so much, ..but collimation would!!
..and davewaldo, ;) thanks for anticipating my next Q :D

[Garyh]

Hi there, well most people do this to get prime focus on there newtonian with a DSLR and usually not on a dob but on a GEM mount so the balance is a non issue in that case.
Moving the focuser 20-30mm further down shall leave a big hole to patch over as well as 4 holes from the spider...to ugly for me and heaps more work.
Just move the mirror and to get a better balance with your dob, just make up a counterweight, wouldn`t have to be big..
cheers Gary