Low profile focuser instead of moving primary up tube.

[mrsnipey]

Hi all, I have a 12" dob and I want to do Astro-Photography with it using my Canon DSLR.
I've read that people move their mirrors up the tube but would I be able to just replace
my focuser with something like a "Bintel Crayford Focuser Low Profile 10:1" to get to focus?

I don't really want to move the mirror if I can avoid it.
If I do have to move the mirror, how do I calculate how much I have to
move it up the tube? Any help would be appreciated.

(Unsteady hands + bad metal working skills = recipe for disaster)

Thanks,
Denis.

[gbeal]

Have you tried your DSLR at the end of the focuser yet?
If not, simply remove the eyepiece adaptor, and hold the camera body (no lens) on the end of the focuser opening. The diameter of the focuser, and the camera lens throat will be a similar size.
Aim the scope at something like the moon (OK not tonight) or Sirius, anything as long as it is waaaaaaaaay out there and bright. See what the image looks like. It may be in focus, but likely not.
Try that first. If it isn't then find a piece of tracing paper or similar and affix it to the focuser opening. Rack the focuser in and out until you get the image of the moon or Sirius or whatever to come to a rough focus, this is like a ground=glass screen, which us older turkeys used all the time.
Measure the distance out that the focuser has to move to come to this rough focus and come back to me/us.
If you are impatient like me, and have a solar filter use the sun, or a distance lamppost.
Moving the primary isn't as bad as it sounds, but once we establish how much if any you need to change things we can go from there, as there are a few tricks we can employ.
Gary

[davewaldo]

Hi Denis,

I have been through the process of cutting my Dob tube and moving the mirror up. Its not a simple task, but if your careful, not too hard.

I have a few threads on this site detailing the procedure. Do a search for Dob reborn.

I also looked into the possibility of using a Bintel Low-profile focuser. However I have heard these focusers suffer a lot from sag (the focuser tube drops) when using heavy EPs or cameras. Also for my scope the lowprofile focuser alone would not have been low enough.

Good Luck!

Dave.

[mrsnipey]

Thanks Gary, I'll try the tracing paper tonight - weather permitting, and get back to you with the measurements.

I tried focusing with the camera late last year but only got a faint halo.

Thanks Dave, good to know about the sagging of the low profile focus tube. I hadn't thought about that. The blue looks really good.

Just the thought of going near the tube with an angle grinder sends shivers down my spine. Angle grinders hate me for some reason.

[iceman (Mike)]

Try this article, it might give you some hints.

[mick pinner]

what are you going to try and photograph with a DSLR on a Dob?

[mrsnipey]

Thanks for the pointer Mike.
I've been eyeing off your setup for a while now.
It's probably what I'm heading towards.

Mick, I'm hoping to get an EQ6 in the near future and I
already have an 80mm refractor so I was going to try
my hand at some very basic ap on anything really just
to start learning. I want to be able to do ap through
both the refractor and the reflector though.
I'm also building an Equatorial Platform.

Eventually I'd imagine that I'll just be using the dob as
a guide scope, but that's a loooong way in the future,
once I can take photos like EzyStyles.

[iceman (Mike)]

Deep space photography with a DSLR on a 12", will be tonnes more difficult than on an ED80 - because of the focal length and weight involved.

At long focal lengths, guiding, focus, seeing, collimation all become more critical and make it more difficult to get good images.

Start with the ED80 on your EQ6 - but you'll still need a separate guidescope. I have mine side by side mounted. I posted some pictures of my setup about a month ago.

[mrsnipey]

Advice noted Mike.
I might just start with the 80mm like you said.

I'm probably trying to do too much too fast.