Is this a bad idea?

[stevewanstall]

I have a 200mm F5 NewtonIan. If I attach a DSLR to the focuser , I cannot achieve focus; I have to screw the focuser right in but it just will not go in far enough. So, at the moment, I use a Barlow which solves the problem by moving the point of focus further out, but reduces the field of view. So, I am wondering about moving the secondary mirror/spider assembly closer to the primary by about a centimetre, plus moving the focusing assembly the same distance . Is this a good idea and will it solve the problem?

[Kropster]

Most people just use a Barlow.... but some move the primary up the tube a little.

I think this causes less problems than moving the secondary and the focuser..... usually just a few extra bolt holes in the tube.

[Joseki]

It should work, but I think most people move the primary further up the tube - perhaps by replacing the mirror cell bolts with longer ones or by cutting a section of tube off.  You could also replace your focuser with a low profile one.

[Herra Kuulapaa]

I think you can introduce tilt to your setup with that operation. What about moving the primary?

[rl]

The old problem..lack of in-travel.

You might find it easier to move the primary up a bit. There might be enough adjustment in the collimation bolts. Do you mind drilling the tube? Presumably not if you're contemplating moving the secondary. If you don't mind then it might be possible to move up the whole primary cell an inch or so. This is what I have done on 2 orion-optics scopes...it's much easier than cutting new holes for the focuser and spider. It's simple enough to move it back if necessary and the extra holes can be blanked off.

RL

[Joseki]

For astrophotography you really don't want to change your f5 scope into a f10 scope with a barlow (as far as I understand it anyway) - unless you're only doing solar system stuff.

[Stu]

How much travel do you think you need? It may be a case of the collimation springs/adjustments are at their furthest adjustment down the tube. By pushing the mirror up the tube a little could you get there?

Otherwise, either a low profile focuser or moving the primary up the tube are the best options

[ollypenrice]

For astrophotography you really don't want to change your f5 scope into a f10 scope with a barlow (as far as I understand it anyway) - unless you're only doing solar system stuff.

This is absolutely correct. F10 is not where you want to be for DS imaging, especially with a DSLR. You'd be on a 2 metre focal length and an absurdly fine pixel scale. Whatever the solution is, it isn't a Barlow!

Olly

[Superdavo]

Try re-collimating the telescope, moving the primary further up the tube. This should give you a little bit more room to maneuver. 

David

[Astromaster130]

Is this a skywatcher explorer 200p? If so i use a dslr with my 200p and attach it using a t ring and screw it straight to the focus tube by removing the 1.25 adapter! I found that i couldnt get focus using a barlow!

[Bukko]

My Orion Optics Newt actually came with two sets of holes for the primary, so I could choose which set to use. (The shortest Primary/secondary distance won.)

So I would recommend you drill three additional holes, say an inch in from the existing and re-locate. But do some work to make sure an inch is enough; is the focus almost acheived with the focuser fully in? Any slight error in the drilling can be taken up with the mirror adjustments.

And good luck.

Gordon.

[Scott]

my 200p (sw) reaches focus just. how close are you? is it possible that a low profile t-ring? or perhps that and longer primary mirror adjustment screws/springs? as mentioned, a barlow is not the answer

[Joseki]

OP's sig lists a  Altair Astro 8" F5 - I think you'd have to actually shorten the tube (based on photos available online).

[beamer3.6m]

My 200p reaches focus with the dslr just screwed directly to the focuser.

Is this how your attaching it?

[stevewanstall]

Thanks for the advice. I will have a good look at the primary and see what is the best way to get it a bit closer to the secondary.

[glennbech]

I replaced my stock focuser with a Moonlite focuser. Better focuser and barely achieves focus with a DSLR.

[laser_jock99]

One possibility is to move the primary up the tube by installing longer, stronger springs and longer adjuster screws. I've done this on a couple of scopes and both gained about 10mm back focus but more importantly made collimation easier and more stable.

Some details on this thread.

[Purplehayze104]

not sure of your focuser but my standard one on the 150P I could reach focus with a DSLR fine, what I had to do was the bit that sticks out where the eyepeices go had a 2nd part the enlarged to 2" that went in focuser an tightened by 3 screws, well I found that the pair unscrew to leave you with the 2" adapter that had a thread on it which in turn screwed into my DSLR t ring then refitted that back into scope, so it brought the whole camera forward around 1 1/2 inches left me with around 5-7mm of travel to play with.

clear skies

john

[laser_jock99]

One thought- do you have a coma corrector? I wouldn't start dismantling your scope until you try the scope with a coma corrector fitted. You will need a coma corrector to take decent DSO pictures anyway - and your scope may well reach focus without further adjustment?

[Scooot]

Yes as above, I can achieve focus with my dslr on my dobs with my paracorr attached whereas I can't without it. So I think laser jock might well have the answer.