Paracorr focus issues

[Marcus10]

Hi all,

I have Paracorr Mk I which I'm using with a 12 inch f/5 dob and am experiencing some issues with eyepieces not reaching focus.

Of my eyepieces, the 21 mm Ethos doesn't reach focus in that I can't rack the focuser tube in far enough to achieve focus. I have the same problem with a 20mm Plossl. My 31mm Nagler will only just reach focus.

I'm guessing because it is an in-travel issue there is nothing I can do about this?

Your help would be much appreciated.

[sasup]

what about using a spacer? I use a 2"x 2.5" spacer (when visual -observing), as my Focal length is set up for photography. Would a spacer give you that extra bit of length needed to reach focus or is it all in the focus-er draw length? A ask as I have the same issue sometimes with high power, say 6mm +.
P.S. nice eye piece collection

[Marcus10]

Thanks Stacey.

Would a spacer/extension tube make things worse? At the moment, the eyepieces won't reach focus as the focuser tube won't go far enough in. If I add an extension, wouldn't it take the eyepiece further away from where it needs to be to focus?

[gb_astro]

Yes, spacers will make it worse.
Try moving the primary mirror up the tube equally with all three collimation bolts.
Move it up as far as it will go then back it off enough to give it room for normal collimation adjustments.
This may be enough to give you focus if you only need 10mm or so.

gb.

[ausastronomer]

Hi,

You need more infocus to reach focus with a paracorr, so spacers are NOT the answer.

The first thing to try is recollimating the scope with the primary sitting as "high" in its cell as you can whilst the scope retains collimation. This may give you enough to reach focus. Depending on the scope and type, longer collimation bolts may even give you enough extra collimation travel. If this doesn't work move on.

If your scope is a "truss" style dob with aluminium truss poles it's a simple job to dock 20mm off each of your poles. You need to do this properly, not just hacksaw away. You need to set up a jig with a stop and use a drop saw with an aluminium cutting blade in it. This way you ensure all the poles are cut to the same length with a neat square edge. A rotating pipe cutter will also work but does not do anywhere near as neat, or as accurate a job.

If your scope is not a truss dob then it gets a little more complicated.

Moving the focuser and secondary mirror holder down the tube closer to the primary mirror is a pain in the butt and if you don't know what you're doing there's a fair chance you will get it wrong. Plus it will leave some really ugly holes in the scope. The best way to fix the problem in this case is to shorten the tube and move the primary up the tube by about 20mm (at the first attempt). If you do this properly no one will ever know the tube has been shortened. Take all the optics spider and focuser out of/off the tube, as you don't want them near flying metal filings. Remove the primary mirror cell from the tube. Use an adjustable carpenters scribe to mark a line all around the tube for you to use as a cutting guide. If you don't have one of these scribes you can mark all around the tube measuring up 20mm from the bottom. About 12 to 20 marks around the tube is good. Cut the bottom off the tube with a metal cutting blade in an angle grinder. You will need someone to hold the tube steady while you cut. Do it slowly and neatly. If your not confident go to a metal fabrication shop and ask them to do it. Probably will cost about $20 to $30 or so. You then need to re drill the bottom of the tube to fit the mirror cell back in. Make sure the tube is thoroughly cleaned and all metal filings and burrs removed before you start to re fit the optics and focuser. If you know what your doing it's pretty simple and probably 1 hrs to 2 hrs work. If you don't it might seem intimidating but it's not all that hard. Good luck with it.

Cheers,
John B

[multiweb]

In travel issues can be resolved by a small barlow to increase your FL, low profile focuser or moving your primary closer to your secondary. Sometime it doesn't need much adjustement.

[asimov]

Yep. Try the moving of the mirror closer on the collimation bolts trick first, & if that fails you'll have to move the mirror cell & all closer.

[Marcus10]

Guys, thank you very much for all your help. I will let you know how I go.