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

Collimation Idea for GSO RC Scopes

  • Please log in to reply
1 reply to this topic

#1 WadeH237

WadeH237

    Skylab

  • *****
  • topic starter
  • Posts: 4030
  • Joined: 24 Feb 2007
  • Loc: Snohomish, WA

Posted 12 January 2019 - 07:52 PM

So I have a 6" Astro-Tech RC scope that's been sitting on my shelf for the last couple of years.

 

I spent a year or two imaging with it, and it was a fun scope - but collimation is pretty difficult.  In the past, I have collimated it using the "hall of mirrors" effect to adjust the primary.  It took some time for me to figure out that you can't actually see the reflections properly unless you remove the baffle tube, which requires taking off the front of the telescope to get the secondary mirror out of the way.

 

Anyway, I had a thought the other day that it might be possible to collimate one of these scopes with a barlowed laser collimator.

 

I happen to have a nice AstroSystems barlowed collimator, so I pulled the scope off of the shelf and played with it for a while today.  To start the process, I put the laser into the focuser without the barlow lens.  This sent the beam directly at the secondary mirror.  looking through the front of the scope, I was able to see the secondary mark reflected in the primary mirror.  I could also see that the beam was not quite centered in the secondary mark.  To center the laser beam, I used the primary mirror collimation screws.

 

This method of adjusting the primary depends on what (in my opinion) is one of the weaker design points on these scopes.  Namely, it works because the focuser is attached to the primary mirror mount.  If that were not the case, then adjusting the primary collimation screws would not affect the beam at all.  I still think that this is a weak point in the design, because a heavy imaging train could slightly pull the primary mirror out of collimation as the scope points in different directions - but hey, this was an experiment, and perhaps I could mount a light weight imaging train for this scope.

 

After adjusting the primary, I attached the barlow lens to the laser and put it back in the focuser.  What this does is diffuses the laser beam such that a reflection is cast back towards the laser emitter with a shadow of the center mark from the secondary.  I could use the seconary mirror collimation screws to adjust this, but I didn't need to.  The laser emitter was already perfectly centered in center of the reflected mark.

 

So at this point, I believe that the scope is pretty close to collimated.  We don't get many clear nights around here, so I doubt that I'll be able to do a full evaluation anytime soon.  But we are forecast for a clear night tomorrow night, so I might throw an eyepiece onto the scope and see what it looks like.

 

Anyway, I just wanted to throw this out there to see what people think of the idea.  It does depend on the secondary mark being accurately centered.  It also depends on the focuser being square with the primary mirror.  But if those two things are true, then the two mirrors should be pretty accurately aligned with each other at this point.



#2 WadeH237

WadeH237

    Skylab

  • *****
  • topic starter
  • Posts: 4030
  • Joined: 24 Feb 2007
  • Loc: Snohomish, WA

Posted 13 January 2019 - 02:19 AM

So I think that this might have some merit.

 

It actually cleared off a bit tonight, so I had a chance to take a look through the scope visually.

 

The moon was prominently placed at dusk, so I pointed there.  I was actually pretty disappointed with the view.  It was softer than I expected.  Mars was nearby and it did not look so good either.  After checking both sides of focus, it was apparent that the scope was not acclimated.  So I went back inside for a couple of ours.

 

When I went back outside later, the views were much more stable, but still not what I expected to see from the scope.  At this point, though, it was acclimated sufficiently to really check the collimation.  Working with Caph, it was clear that the laser hadn't gotten things perfectly aligned.

 

The way the barlowed laser works, I had a lot more confidence in the secondary mirror collimation than the primary.  This is because checking the alignment of the primary involves ensuring that the beam itself is centered in the mark of the secondary.  The laser is bright enough, and not quite a point spot, that there is some slop here.  Aligning the secondary mirror involves centering the white face of the emitter in a donut shadow of the secondary mark.  It is very accurate.  (When I use the same laser to collimate a Newtonian, it is the opposite; aligning the primary is more accurate than aligning the secondary - since the primary is the marked mirror).

 

Anyway, I decided to tweak the collimation by using the primary mirror screws only.  This turned out to be pretty easy to do.  Since the primary screws are easily accessible, I was able to look through the eyepiece while making very small adjustments to the screws.  After about 2 or 3 minutes, I had very concentric fresnel rings just outside of focus, and nice diffraction rings at focus.

 

After looking at a few more stars with different eyepieces, I went back inside for a couple more hours.  By that time, M42 had cleared the trees in my yard, so I spend about an hour there.

 

I started with a Pentax 21mm eyepiece for a broad view of the brightest parts of M42.  I switched to a Delos 17.3 for a slightly higher magnification.  But I spent most of the time with my Pentax 7mm, which is my shortest eyepiece.  On the AT6RC, this gives my just under 200x.

 

Transparency was pretty poor (most of my light pollution is to the south, and there have been high, thin clouds).  Seeing was pretty steady and the 4 stars of the Trapezium were nice and stable.  There were two stars nearby that were faint, but obvious (I believe that one was V1230 and the other was V410).  Finally, the E star in the Trapezium popped in and out of my averted vision.  It's been a while since I spent much time in that area of the sky, so I verified the position in a chart, and it was exactly where I saw it.

 

At this point, I think that the scope is dialed in pretty well.  I'd like to put a camera on it to see how the imaging field looks.  Sooner or later, I will need to change the configuration of my imaging rig, and I might just mount the 6" at the end of galaxy season to see how it works.

 

In the meantime, I'll spend some more time with it tomorrow night.  I'm leaving it set up, so it will be acclimated by dusk so I can go back to the moon and Mars now that I've finished tweaking collimation to my satisfaction.


Edited by WadeH237, 13 January 2019 - 02:21 AM.



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






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics