Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

DIY refractor focuser alignment

DIY collimation refractor
  • Please log in to reply
3 replies to this topic

#1 AlphaGJohn

AlphaGJohn

    Mariner 2

  • *****
  • topic starter
  • Posts: 253
  • Joined: 12 Jun 2013
  • Loc: Wasatch Front, Utah

Posted 18 January 2020 - 06:52 PM

Hi Folks,

 

I have one of the Lunt-branded APM 152mm ED APO refractors that came to me second hand. It's a really nice scope for the money, IMO, but I've got a frustrating problem: the focuser seems to be angled slightly with respect to the light path. With my Howie Glatter (RIP, good brother!) laser collimator in the focuser, I can't get the spot on the objective to be in the same position when I rotate the focuser (in the different rotational positions, it ends up describing about a 10mm circle--that just seems like way too much). Rotation of the focuser and German equatorials kind of go together, so I don't want to give up on the rotation option.

 

It really seems like somewhere in its shipping, that the focuser got whacked and it or the backplate is now effectively bent just a bit. It's also possible that the focuser just isn't up to maintaining alignment when rotated regardless of whether it sufferred a whack or not.

 

So, I'm looking at a new Moonlight focuser (with their cool bearing-cam adjustment for centering the focuser tube to the light path). I suppose I could try to find out if APM could supply a new backplate (which seems the most likely culprit) and perhaps a new focuser (I haven't investigated what they might be able to do yet--or even tried to detach the focuser from the backplate).

 

If anyone has any DIY suggestions, I'd be grateful to hear them. Also, if someone has experience with the focusers on these babies, perhaps you'd know if it's worth fussing with or if replacing it is likely the way to go.

 

I can supply photos of the thing if that'll help with the suggestions.

 

Thanks for looking!

 

John


Edited by AlphaGJohn, 18 January 2020 - 06:55 PM.


#2 Beeham

Beeham

    Vostok 1

  • *****
  • Posts: 168
  • Joined: 08 Nov 2019
  • Loc: Oakland County, Michigan

Posted 18 January 2020 - 08:11 PM

I would first make sure the laser is collimated.  If that's off, then there's nothing on the scope that needs fixing, just collimate the laser and you're done.

 

If the laser checks out, then (assuming this scope is something like 4' long for a 152mm f7.9), then your theory that the focuser being is off-axis seems reasonable.  If the laser makes a 5mm radius circle, and the focuser is maybe the last 5 inches of the 48 inch overall length, then it would only need to off by a half millimeter to cause the problem.  (5/48 fraction of a 5mm radius error is about 0.5mm...right?)

 

I don't know how your focuser attaches to the tube, but on my scope it's with screws that go radially inward through the tube into threaded holes in the focuser, and the focuser aligns to the tube with a shoulder that goes up flush to the edge of the tube once you slip the focuser into the tube.  If yours is similar, I would think you'd be able to see a 0.5mm gap between the shouder and the tube (or check with feeler gauges), and if you find that the shoulder of the focuser isn't square up against the tube, loosen the screws, square it up, and then snug everything back up.

 

That being said...I admittedly am not first-hand knowledgeable about the APM 152 so I may be wrong here.

 

I hope this is helpful.



#3 AlphaGJohn

AlphaGJohn

    Mariner 2

  • *****
  • topic starter
  • Posts: 253
  • Joined: 12 Jun 2013
  • Loc: Wasatch Front, Utah

Posted 19 January 2020 - 12:20 AM

Thanks for replying,  Beeham.

 

I definitely should have included some photos.

I don't have any reason to mistrust the laser, but I'm open to suggestions for testing it.

The backplate on this scope threads onto the tube (very fine threads). I assume the flange the focuser body rotates around threads into the backplate,  although I have not removed it or seriously attempted to get enough torque applied to uunscrew it.

There's  not much in the way of adjustment options beyond the screws that bear against the flange that allows the rotation. fusing with the screws has only narrowed the circle the laser spot describes.

 

I'll post some pictures and maybe that'll spark some thoughts.



#4 AlphaGJohn

AlphaGJohn

    Mariner 2

  • *****
  • topic starter
  • Posts: 253
  • Joined: 12 Jun 2013
  • Loc: Wasatch Front, Utah

Posted 08 February 2020 - 06:51 PM

Gave up on fussing with the essentially non-adjustable focuser and bought a Moonlite focuser. Its adjustments allowed good alignment with the optical axis. Looking forward to a clear night or two; be nice if wasn't too cold either!

John


Edited by AlphaGJohn, 08 February 2020 - 06:52 PM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: DIY, collimation, refractor



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics