Confused.com, why won't my vixen frac focus?

[stan26]

I'm a bit confused as to why my vixen 60L refractor won't focus using my one and only skywatcher star diagonal....

A few months back i was given a vixen 60mm refractor with wood tripod AZ mount BUT without any star diag or EP's, just the basic tube. Today I fitted it with my Skywatcher 45 degree erect prism used on my ST102, but when I tried to focus it on a distant object (a building 1/2mile away) the focuser ran out of inward travel...?

If I put an eyepiece directly to the (draw tube?) I can get it to focus perfectly with loads of travel to spare either way....

Why is this?

Thanks

Stan.

[Bizibilder]

The diagonal "uses up" some of the focal length (probably around 40-50mm). Therefore you can no longer focus the scope. There is no solution apart from cutting a bit off the main tube of the scope if you really want to use the diagonal! Then, of course, you have the opposite problem and will have to buy a 50mm extension tube to get back to where you were in the first place in order to use your eyepieces without the diagonal!!! Your scope wasn't designed to use a diagonal - hence the problem.

Hope this helps.

[stan26]

Thanks bizbuilder.

Now, where did I last see the hacksaw.....im sure its on a shelf in the garage..........

stan

[John]

Before you start cutting .......

A 45 degree diagonal uses up more in focus travel than a 90 degree one because it uses prisms rather than a mirror - the light path though it is longer. You may find a plain 90 degree mirror diagonal does enable the scope to focus.

[stan26]

ah rite, thanks jahmanson.

I suppose I really need to find out what the scope came with from new. I don't really want to buy a 90* diag to find it dosen't work, and it needs to be the old 0.96" to 1.25" conversion to allow me to use modern age EP's.

can't seem to find much info about old vixens.

hmm

[stan26]

Just found this, the 1985 catalogue for vixen!

Although I can't translate jap, the pic clearly shows it with a 90* diagonal.....

[AndyH]

Yup. 45 degree prisms need a hell of a lot of in travel. Next best is a mirror diagonal, but best for lack of in travel needed, is the 90 degree prism diagonal.

My 90 degree Takahashi prism diagonals use waaay less 'travel' than any mirror diagonal. That includes my low profile Tal one. I was rather surprised when I did some measurements, some time back.

As said, a mirror diagonal should bring your eyepieces to focus. If not, a 90 degree prism, will defo work. Especially as vixen used to sell their scopes with the latter(24.5mm/0.965" ones).

Cheers,

Andy.

[AndyH]

Most 1.25"/0.965" mirror diagonals need/use up, about 3"/75mm of in travel.

Using an eyepiece straight in the focuser, measure how much distance the focuser tube can still go in, when focused on a star. If there's still 3"/75mm to go, then a mirror diagonal will work.

Your vixen 60L scope came with a 24.5mm/0.965" prism diagonal, according to my vixen catalogue (no date, but sometime in the 2000's)

The hybrid diagonals are readily available, BUT, again according to the catalogue, standard 1.25"/31.75mm diagonals and accessories can be fitted to the 'custom 60L' focuser with an adapter. Is there a piece that can screw off the end of your focuser tube?

Cheers,

Andy.

[stan26]

Hi Andy cheers for your help.

Yes there is a piece on the end of the focuser tube, ive already unscrewed it. I have ordered a cheapo 90* mirror 0.965 to 1.25" SD from scopes n skies today to see if i can get it to work. The two problems i can see that il be up against is...

1, will there be enough travel left, going by what you say above it'll be close....

2, the bit I took off the end of the focuser also secures the toothed rail for the focuser gear, without it the rail just flops about, BUT i suspect Il be able to bodge somthing to get it all together!

Thanks

Stan