Prime focus with fl1200 and f6

[jcdundon]

I am using a bintel 8inch dobsonian that has been put on a friends spare mount. I am trying to do some prime focus photography, will i need to move the mirror closer, and if so how is this done (will this negatively impact visual observing). or will i just be able to replace the focuser with a shorter one. which would be easier...

Thanks heaps

[gbeal]

Ah, the joys of a Newt. Perfect for imaging though, and seriously under-rated.
I set mine up specifically like that. I used a shortish focuser, and placed the primary where it needed to be, so my Canon would just come to (prime) focus. I then use eyepieces at the outer end of the focuser travel, and to date have not required an extension tube (but a very short one is an option).
In your case, where you are not building from scratch, both choices are possible. Place the camera in the focuser, and make sure it is as low as possible (in an earlier scope I actually lathed the T ring down to 2" so it too slipped into the focuser, saving a few mm's). Tie the tube down and point it at a distant object. Now with the camera in place, have someone carefully slip the primary in it's cell forward a bit. You may find, if you are lucky that it doesn't need to move at all, but most likely it will need to move an inch or two. When the distant object is perfectly in focus, mark the new position of the attachment bolts/screws and this is where the primary needs to be.
This requires you make new holes in the tube, and perhaps you don't want to do this.
The other alternative is to change focusers. If you do, first check the minimum height of your existing focuser. Measure from the tube wall to the top of the focuser with it in it's min position. Let me know what this height is. We can then cast the eyes around for a shorter version, and I have been down this route as well. Really it all depends on how keen you are to hack the tube, and what the budget is. Also, if your current focuser is very short already then this isn't a viable option.
Gary

[jcdundon]

With the focuser all the way in, it is 55mm (just under 2.25inch) from the tube it seems like it needs to go a fair bit further in though. How low profile a focuser can you get, seems like i'll prob have to move the mirror.

[jcdundon]

just in case i do have to move the mirror, it is attached to an end plate, does this mean i will have to cut the tube shorter to move it, or can i get a new way to hold the mirror in the tube

[gbeal]

Hmmmmm, a couple of pictures would help, they are worth a thousand words you know.
The focuser can be as low as you want or can get. 55mm is about midway, if my memory serves correct. Maybe someone with a Moonlite could measure theirs but I recall them being about 35mm, so you gain about 20mm. (and a nicer focuser as well) My Feather Touch is about 40mm, still a gain.
If the mirror is indeed part of the tube end system, we may be stumped. As a last resort trimming the tube will do what you want, and if anything it will not leave a set of mirror mount holes in the tube. Maybe if you have the mirror cell off the tube, see how much forward adjustment is possible with the collimation screws. Heck, once you have the cell on the workbench, you may be able to adapt something to stand the mirror out about an inch or so, enough to get to where you want.
Really though a couple of things. First establish how much "in-focus" you really need. If it is only 10mm or so, then adjusting out the mirror, and a shorter focuser will more than likely do it.
Bear in mind as well that photography will be easier with a top quality focuser, as will ordinary visual viewing, so the replacement focuser has definite benefits.

[Lee]

There is a focuser around that sort of telescopes into the OTA isn't there? I'm sure I saw it on this site once..... would that help?
I have the same problem with my newt, I have given up on trying to photograph anything with it, and am getting an ED80!