focus or lack of it

[Pip]

Hi folks

I have a C9.25 and I'm having a problem focusing for photography. I bought a focuser with a 10:1 step down fine focus and get great focus when viewing through the ep but when I attach the camera I can't get a clean focus.

I did wonder if it's the laptop screen definition.

Forgot to say the camera is a DSI II colour.

Any thoughts/tips would be greatly appreciated

Cheers Pip

[radioactive]

Pip I am very new to this here imaging lark myself but one thing is for certain the focusing is essential you just have to keep trying mate you will I promise you get there in the end keep trying and trying then try some more think about what is happening when you go to focus one way ponder over it if need be its something I have found is singularly very difficult to master but then suddenly the penny drops and you start to figure it with more ease its a harsh learning curve but stick at it is my advice

such as it is :shock:

[beamish]

I agree with Martyn, it really is a bit of trial and error, re focusing. I had frights with new EP's not coming to focus but then you start experimenting . This has lead to me having 3x different length draw-tubes and just trying to remember which are likely to work as I need them. You could always try just pulling the adapter out of the focuser a tad see if that gets you any closer. I'm still trying to figure out which combo will allow my 300D to come to focus with Barlow having been told by an expert "it will" (not an in focus problem) but that's another story.

Keep playing around, you'll get there.

Karlo

[CELESCOPE]

Pip are us using it at f 10 if so that is really hard to focus at , you need to use a reducer to get that down a bit ,its always hard to get spot on focus at that long focal length and takes some practise , can try using a Hartman Mask ,that would make things alot easier for u.

Rog

http://www.astropix.com/HTML/I_ASTROP/FOCUS/METHODS.HTM

[Pip]

Thanks for the responses.

It sounds like I have a long way to go before I hit on the right combo.

Thanks for the link Rog, I have a focal reducer gets it down to f6.3 and used it the last time I had a go. I did find that taking the star diagonal out of the equation helped a bit.

I'll have to get going with hubby on producing a small selection extension tubes to see if that helps, I've got a feeling it might as I did seem to be almost at the stop on the main focus mirror before I got anywhere near and then quite a way on the attached focuser.

That will give me something useful to do on these grotty cloudy nights.

Thanks again folks

[Sparrow]

I was interested in Celescope (Rog)'s posting - I spent a few hours yesterdya making a Hartmann mask with a colar so that it fitted neatly and snugly on the end of the scope - I was amazed how a piece of card and some glue could make such a useful instrument - tried it last night and it really did help with the focusing. I just made a straighforward one with two holes - I'm that impressed I'm going to make one now with the three triangles to see if that works even better.

My biggest problem is the focusing is so fine - presumably because my scope is also f10 - I'd be interested if you could expand on how a focal reducer works and the effect it has - does it increase the field of view?

[cygnusx1]

The focal ratio determines the critical focusing zone of a scope.

I am just reading Ron Wodaski's book, and his chapter on focusing and methods.

...a f/10 SCT scope has a larger critical focus range than a say a F/4 SN or F/6 refractor.

The exact formula is:

CFZ = (focal_ratio )^2 * 2.2 , is critical focus zone in microns.