Refractor can't reach focus with DSLR attached

[vanwonky (Dave)]

I finally got to setup my new AP gear last night with the keeness of a boy scout getting his first badge. Mostly downhill from there . Well not really. Alignmaster (another post) gave me the poops. My Orion Autoguider gave me the willys and when I gave up on them and just thought bugger it I will just do a manual find and shoot M42 just to see that something will work I see that my refractor would not reach focus.. argh!

So I guess I need an extension tube of some sort. Or does a field flattener or (which I will get sooner than later) a field reducer double as an extension in this sort of situation? Either way what length of extension tube do I need to get?

I did use the 2" diagonal in the end and managed an OK first light of sorts which I will post later.

Cheers
Dave

[keni (Ken)]

Hi Dave,
You're right you will need some other things.
And here's a surprise you get to spend more money.
You'll need a Focal Reducer at least.
Here's a link to one at Bintel to give you an idea.
http://www.bintel.com.au/Astrophotog...oductview.aspx

Although you'll have to buy the correct size for your refractor.
This attaches directly to the focussing tube.
If you read the link it says you'll need a "wide" T ring adaptor because the thread to attach it to the reducer is wider...bummer..more money.
http://www.bintel.com.au/Astrophotog...oductview.aspx

From my very limited experience I think this is the minimum you'll need.
Good luck and good hunting.

Cheers,
Ken.

[vanwonky (Dave)]

Thanks Ken. It felt as if it only needed another 10mm or so back and it would be able to focus. I was wondering if one of these would help.

[rcheshire (Rowland)]

That should do if you don't need a field flattener, or as Ken mentioned, a focal reducer. The focal reducer will be handy to increase FOV and should also flatten the field. No egg shape stars in the corners.

[leon]

I would just get a short extension tube, that should get you plenty of length to focus.

Leon

[vanwonky (Dave)]

I will go the reducer / flattener path when I have taken more images and know what I need exactly I think.

Since the diagonal allowed me to come to focus is it just simple maths to work out what length extension I need i.e - the 2" diagonal gives me 110mm + what I had the drawtube extended out to which I need to check again but was about 20mm. So the drawtube will do 80 on its own (max) + a 50mm extension will get there maybe.

I guess I wouldn't want to leave it too exact and have some play so a bit more wouldn't hurt but am I right in thinking to much could cause vignetting?
Cheers
Dave

EDIT: the drawtube was only out 7mm when I used the diagonal so 117mm-ish needed to reach focus.

[vanwonky (Dave)]

The 80mm extension tube arrived a while back but I have only just managed to get out to test it. All good! The DSLR will now reach focus with the drawtube out only 35mm. So in theory I actually could of gotten away with a 50mm (to allow a bit of play) extension. I don't think there is any detrimental effect to having the extra drawtube inside the scope as far as astrophotography goes but maybe some more knowledgeable could tell me if there is. When I start adding reducers / flatteners then I imagine the shorter extension will be necessary.
Cheers
Dave