DSLR focusing problem

[CChelle (Michelle)]

Before I start I will warn you that I have no idea what I'm doing. I have a Dobsonian and a Canon D30 DSLR. I bought a 2" Prime Focus Adapter to fit the camera to the scope. It winds in to get almost in focus, but then get's "stuck" and won't go any further to get a sharp focus. Is there some other adapter or extension tube or something that I need to fix this problem?
Michelle

[leon]

Michelle, yes you need to get a little out ward distance, so I suggest maybe a 2" or so extension tube.
Pretty well standard equipment these days and easily available from Bintel or the like.

Leon

[adman (Adam)]

Leon - I think it might be the other way. Newtonians often run out of inwards travel on their focusers. From the original post, it sounds like the focuser is being wound in towards the tube, but just doesn't quite get to focus.

There are a few fixes for this - it is a well known problem with newtonians. The first is to buy a low profile focuser so that the camera sits closer to the telescope tube. Another one is to get some longer collimation bolts to move the mirror (and hence the focal plane) towards the camera. And still another fix is to cut the tube and reattach the mirror cell to acheive the same thing.

Have a search on IIS, and you will find lots of ideas from people who have had the same problem.

Cheers
Adam

[Screwdriverone (Chris)]

Hi Michelle,

Firstly, I dont know what scope you have, but for this I am assuming its a Skywatcher 8" dob? (based on your comments on my beginner's package thread?)

My 1000D on my Skywatcher 8" has no trouble when the T ring is connected to the 1.25" adaptor (by unscrewing the 1.25" barrel and threading the remaining part of the focuser adaptor into the T ring.

If you have purchased a 2 inch adaptor, then I assume it is being inserted into the 2" eyepiece adaptor and then you will have a problem as the Skywatcher 2" adaptor is TOO LONG and the DSLR wont reach focus (by a smidgen).

Now, if you have a collapsible truss dob then you can try sliding the truss DOWN toward the primary by "a little bit" lock the truss tube screws and then try again. This effectively brings the primary UP the tube as you are lowering the secondary cage towards it.

Second, if the first idea doesnt apply (no truss tube) you can try winding the Primary screws OUT so the mirror is pushed away from the bottom of the tube (loosening) and this will bring the primary closer to the secondary and hopefully within grasp of the DSLR focus point. Dont wind them all the way out as the screws will drop off the primary and a bit of a bugger to get back on easily. One thing I found with my 8" tube is that the primary was ALL the way at the bottom of the tube when I unboxed it, causing all sorts of collimation issues. Check this....see if it works.

Finally, as Adam mentioned, you can replace the primary screws and springs with longer ones, but this may lead to you needing an extension tube on the 1.25" eyepieces to reach focus.....

Low profile focusers help also.

Hope you can find the best solution for you, give the suggestions above a go...

Cheers

Chris

[loki78 (Jon)]

I have the same problem with my GSO 8" dob and canon 600D using the 2" adaptor, so i use a 2x barlow lens I have, which changes the focal point and allows the DSLR to focus.

The other methods work as well but I didnt really want to, or was comfortable at the time in making alterations to the scope such as moving the primary mirror/replacing the focusser.

[alistairsam]

Hi,

I initially used longer springs and longer collimation bolts to move my primary up. that got it to focus.
But then I moved to a low profile focuser as I was able to move my primary back and that helps avoid the need for a bigger secondary.
when you move the primary up, you might lose some light if its the standard 50mm secondary. so its common to change the secondary to a 62mm or so.

Low profile focusers are a very good alternative, but they're a bit hard to find and tend to be a bit expensive.
there are a few on ebay, andrews sells the long perng focuser that has a low profile, but easiest fix would be the springs and longer collimation bolts.
just get some broad enough springs.

here's a pic of my collimation bolts and a comparison of the normal and low profile focuser. yours could be different, but just giving you an idea.
that 30mm or so that you save with the low profile will mean you don't need to touch your mirror as the slr can go 30mm inward.

edit: you will most likely need an extension tube to use an eyepiece once you move your mirror up or use a low profile focuser.

[leon]

Opp's wrong answer, sorry, should have read it a bit more clearly, not thinking.

Leon