Trouble getting focus with an WO GT81

[rodgerK]

Hi

I have just purchased a new GT81 and  I am having trouble getting focus.

If  I use a 1 1/4" diagonal and 1 1/4" eyepiece, I cannot reach focus at all.

If I use a 2" diagonal and 1 1/4" eyepiece, I can only just reach focus.

If I directly attach a DSLR or ATIK CCD camera I cannot reach focus.

My only option to do astro-photography  is to add a 2" extension tube, but even then I am at position 7.5 out of 8.  In this I am concerned about flex using an extension tube when I use a DSLR or worse still when I use an ATIK CCD and Filter wheel attached.

Is this all to be expected ?

I have also ordered the Field Flattener but it has not arrived yet. Will it be easier or harder with the FF ?

Rodger

[steppenwolf]

Quote

Is this all to be expected ?

For imaging, this is indeed to be expected. I would be tempted to buy an 80mm extension for native focal length imaging. However, for imaging per se, you really do need to use a field flattener and if this includes focal reduction then your Camera focus issue will be resolved. If you are buying a flattener with no reduction then you will still have this issue.

[Alien 13]

This is a common problem with scopes that are a jack of all trades i.e. visual and imaging and if you add in binoviewers it gets worse but as said extensions are the way to go and a reducer/ff usually gets the spacing just right for a DSLR, it is a pain but far better than the alternative of too little back focus.

Alan

[rodgerK]

Hi the FF I have ordered is actually an 0.8 FR.  Will this make things a bit easier or worse ?

[Alien 13]

I have similar with the ZS71 and it puts the focus point with a DSLR about one third of the outward travel.

Alan

[steppenwolf]

Quote

Hi the FF I have ordered is actually an 0.8 FR.  Will this make things a bit easier or worse ?

Easier!

[rodgerK]

Hi -  that's good to know but I was hoping to use this new scope in both modes dependant on the object.  I wait for that to arrive.

I have done some measurements now.  The Scope's focal length is 478mm but the scope from primary objective lens to full travel of the focuser is only ~ 380mm - so that puts the focus point a full 4 inches beyond the scope at maximum focus.  Even with an Atik 314+ CCD and a Starlight focus wheel attached, I have to use a 4cm extension tube and the focuser is at 7.1cm out of 8.  So I can reach focus (just) but the centre of balance is now over the focus wheel and full 4 inches beyond the mounting shoe.  I am really concerned about stability of this setup for imaging, which is what this scope is promoted for.  Am I being too pessimistic ?