In-travel focus problem

[samir_ansari]

Hi everyone, I've run into a bit of a problem with my new setup. I have a skywatcher 80ED and the 0.85x focal reducer to be used with a DSLR and 2" narrowband filters. The problem is that when adding the filter the CCD is just a little too far to achieve focus when the focus draw tube is all the way at the start, I don't have any more "in" travel.

I've tried to remove the focus tube locking ring but I still seem to be about 2/3mm away from getting the result in focus Has anyone come across this before or are there any ways I can get around it?

Thanks for your help,

Samir

[Merlin66]

Where are you positioning the filter? Before or after the reducer.

(Adding a filter adds about 1/3 the thickness of the filter (3mm thick?) to the focus)

[samir_ansari]

Im adding it after the reducer, just before the DSLR

[Merlin66]

Is it possible to add the filter before the reducer?

This will maintain the distance from the reducer to the CCD and should give you a bit more focus distance......

[samir_ansari]

Thanks Ken, I'll try that tonight

[samir_ansari]

Nope, the filter seems too small in diameter to screw directly in between the focus tube and reducer

[samir_ansari]

Does anybody have any ideas? There must be a way around it as I would've thought mine is a fairly standard setup

[Merlin66]

Running out of ideas for you...

I'm assuming you are using a DSLR behind the reducer ( exactly which reducer are you using?) - no other spacers etc in front of the reducer....

[samir_ansari]

Its a skywatcher 0.85x reducer:

First Light Optics - Skywatcher Focal Reducers

I dont have any other spacers infront of the reducer

Thanks for the help with this!

[Merlin66]

I suppose the obvious question is:

When you remove the filter, and only use the reducer/ camera set up - does it focus OK??

[samir_ansari]

Yep it does however I found I needed to use a 50mm extender as there wasn't enough back focus

[Merlin66]

Hmmm

You got me beat!

There's something not right here....you say without the filter you need to ADD 50mm extension ( I assume between the focuser and the reducer - the distance between the reducer and the camera should always be fixed and never changes)

So the difference is actually the amount to you wind out the focuser PLUS 50mm!!

No - can't be....

Can you take an image of the set-up with and without the filter....I'm obviously missing something.

[samir_ansari]

Sorry Ken I misread your original question, I only have to use the extension when Im not using the reducer. With the reducer and without the filter however I can focus just fine.

[Vega]

Silly question but I presume your not using a diagonal in the system?

Had to check

[samir_ansari]

Nope, no diagonal. Until I get home from work and can get a photo here's an outline of the setup I have:

Telescope>Reducer>Filter>T-adaptor>DSLR

[Merlin66]

On another thread

http://stargazerslounge.com/discussions-cameras/158880-how-fit-lp-filter-sw-reducer-flattener.html#post1985703

He seems to be using the same set up????

[samir_ansari]

I see the difference, I'm not screwing my filter straight into the t-ring (as its too small). Instead Im screwing into the end of a T nosepiece. I need an M48 adaptor for this to work I think! Thanks so much for the help - I was really worried this would never work