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torr
12-02-2016, 10:41 PM
Hi folks,

If I replaced my 300mm f4.9 mirror in my xt12g with a 300mm f5 mirror, would that give me more back focus.
I don't mind if I have to use extension tubes.
I guess this will introduce other questions like will my secondary mirror be big enough to reflect all the light to the eyepiece.

Its just a thought as I'm about 20mm short of in-focus for my canon d700.
I did construct a t-ring adaptor that screws directly onto the focuser , this got me an extra 10mm in-focus, but I'm still short 10mm :shrug:
Even with the primary mirror adjusted up as far as I dare on the collimating screws/bolts


thanks

James

raymo
13-02-2016, 12:51 AM
Hi James, The cheapest and relatively simple way of fixing your problem
is to move the primary mirror up the tube the requisite amount, as many others have done.You will almost certainly find informative video tutorials on youtube.
It is unfortunate that this issue surfaces over and over again.
Obviously some retailers are not informing their customers that their
Newtonians do not have sufficient back focus for astrophotography.
For the benefit of anyone reading this post, Skywatcher Newts are all
suitable for photography. I have no connection with SW other than having
owned several of their Newts.
raymo

billdan
13-02-2016, 12:52 AM
Hi James,

In theory you will gain an extra 30mm of back focus by going to F5 but that is an expensive fix. The secondary would be OK.

Another slightly less expensive fix is a low profile focuser that is an inch shorter in height to the one you have, just make sure the mounting holes line up without having to use an adapter.

Before spending heaps of money why not try getting some longer collimation bolts and springs.
Alternatively take the OTA tube to a machine shop and get them to cut off an inch off the bottom and re-drill the mounting holes for the primary cell.

Another thing to be aware of is, you don't want the focuser racked all the way in as the focuser tube will cast a shadow on the primary.

Regards
Bill

torr
13-02-2016, 08:23 AM
Thanks for the responses guys.
If I did the mod I would probably go for a new mirror as I prefer going up to f5.
I did look at a focuser and they are around half the cost of a mirror.
Just thinking out loud for now.

Thanks again
James

Merlin66
13-02-2016, 08:39 AM
Just a thought...
Instead to the standard T2 adaptor which is 11mm wide for the Canon, you can get a very narrow version only 1-2mm wide....
http://www.firstlightoptics.com/adapters/baader-ultra-short-canon-eos-t-ring.html
This would give you that bit more opportunity.
(I've also seen similar on eBay -T thread to Canon OES adaptor)

torr
13-02-2016, 11:31 AM
Thanks Ken :thumbsup: ,
Fortunately I have access to a lathe an knocked this little baby up.
I removed everything from the focuser tube including the 2" eyepiece holder and screwed this directly into the focuser tube.
This ended up giving me near on extra 15mm in focus. still not quite enough though.

http://i191.photobucket.com/albums/z66/torr001/cannon%20adapter.jpg

For now I'm still excited with visual astronomy, so much to see and learn.

James

Shiraz
13-02-2016, 12:44 PM
be careful. most mirrors of this size seem to have around 1500mm fl with slight variation in the diameter. You want a longer fl and it might be hit or miss if you actually get it by changing mirrors based on fnumber. For example, your XT mirror is 305mmx1500mm f4.9, but an f5 GSO seems to be 300mmx1500mm f5 (depending on where you look).

It would be much better to move the mirror up the tube as others have said as an interim solution.

The best solution for stability when imaging is to cut 25mm(eg) off the tube and redrill the mounting holes - fine blade hacksaw, Dremel, or even an fine disc on an angle grinder will do the job. I guess you will have to add a bit of weight to the back end to compensate for the changed CofG, but that should be a minor mod.

Of course, there are problems imaging with a Dob, but that is another issue.

torr
13-02-2016, 03:21 PM
Thanks Ray,
I see mine is 305 mm vs gso at 302 according to bintel stats.
I was presuming 1500/305= F4.9 therefore 302*F5= 1510mm and realistically that's not going to give me enough in focus at this time anyway.
I find my scope awesome for visual, so if I mod it at all it would probably only be a low profile focuser.
For now I'll just play with my SW120 at F5 for photography untill I grasp the basics.

Regards

James