16 inch GSO Ritchey Chretien collimation.

[tegwilym]

I know this topic has been beaten to death already and I'm studying the posts of ideas.  Just a little venting - I have access to our club's new RC 16 inch GSO Truss scope that I've been working on collimating.  I've tried the Howie Glatter Grid pattern laser (seems the round pattern has been discontinued), a single spot laser to center the focuser, cheshire eyepiece, an Orion laser mirror collimator (the one with the 45 degree target - not sure the proper name at the moment), and a lot of visual attempts.   I've spent HOURS working on this thing and have yet to get a decent round star. 

I have a good idea what I'm looking for and just have this question to toss out. 

When I get everything centered nicely yesterday, but the outer "daylight" ring of the primary wasn't even.   I know that I need to adjust the primary, but then the secondary is messed up again.  I know the order to collimate these is:

• 1) Focuser line up laser with center of secondary.
• 2) Adjust secondary
• 3) Finish up with Primary.....but then secondary is messed up again.   *ugh!*

Do I roughly center the primary with an even "ring" around it then go back to the secondary even if this isn't the correct order?  But then I'm seeing the reflection of the primary in the mis-aligned secondary.  Do I go back and forth between the primary and secondary, then do the proper order if the primary is somewhat lined up?

Any good tips on this?  I've been reading a lot about how to do this, tried many things, but just not getting it working right.  I'm understanding the RC Collimation "nightmare" that I keep reading about is true!

Thanks,

Tom

Door Peninsula Astronomical Society

 

This is what mine looked like after lining things up symmetrically - but then the primary looked like this:  (View through cheshire eyepiece to keep my eye in the middle)

 

This is the scope that I'm working on...and it keeps beating me up!  

 

[lukebl]

Sorry, I can't help you with this but it may be useful to know that the Howie Glatter circular pattern adapter is being made again. In the US I believe that Agena Astro, Starlight Instruments and Skies Unlimited have them in stock.

I've just got one myself, although it doesn't seem to be helping me with my RC collimation.

[B4silio]

Hey Tom!

First of all, what a monster of a scope!

Following a tip from David on the other thread on RC collimation (seems to be a popular sport these days!):

1. To skip the back and forth between secondary and primary you can take out the secondary entirely (mark the position of the screw so you can get back to the right spot, and do a rough count of the turns you do when unscrewing) and aim the laser to the center of the screw hole where the secondary would be (the real center of the mirror, not where the small ring happens to be right now). This means that once that is done you won't have to go back to changing the primary and can focus entirely on the secondary
2. Attach the secondary back on and realign the screw to its marker
3. With the laser on, adjust the secondary so that the pointer is reflected back to the Howie Glatter (you'll notice the beam become a lot brighter when the two are superposed). The advantage of the truss design is that you can stick your head inside to have a better look at the pointer getting projected back to the source
4. Put the camera in and either get an artificial star or a real star and re-adjust the secondary, as the optical axis and mechanical axis might not be identical (less likely on such a high quality scope)

A tip to test how good it is: you'll need a crappy filter that does huge halos, when your primary is well collimated, the halo will be centered on the star when it's in the center of the image, when the secondary is well collimated, the ring will be evenly thick all around. You need a bright star to do that! (here it's actually venus right over the pleiades!)

[tegwilym]

On 06/04/2020 at 09:53, lukebl said:

Sorry, I can't help you with this but it may be useful to know that the Howie Glatter circular pattern adapter is being made again. In the US I believe that Agena Astro, Starlight Instruments and Skies Unlimited have them in stock.

I've just got one myself, although it doesn't seem to be helping me with my RC collimation.

Good to know!  I did buy the grid pattern for the Glatter but seem that the round one  would be easier.  Then again...t.his seems to be a beast to tweak no matter what!

 

[tegwilym]

Thanks for the tips!  Removing the secondary seems a little scary, but i did come across that .pdf file out there that someone showed that with a smaller 8 inch or similar.   The other day I was out again messing with it.  I think I made some progress since the stars were more round than before.  I think part of the problem was that I was messing with the secondary incorrectly trying to even out the dark boundary around the view through a cheshire.   I realized my mistake and now worked on centering the center dot of the secondary in the view of the cheshire - then carefully worked the primary a bit.  I think with all this fiddling with this thing, if nothing else, I'm learning with way to twist things to move the mirrors where I want them. 

It was getting too dark (I used a sky background) when I was working on it, and will have to check again during the day and see how close I got it. 

Wow....these things are a pain, but I'll eventually get round stars!

See our club page here:  https://www.doorastronomy.org/

Tom

[tegwilym]

Update:  I did some more work on it yesterday using the sky through the open dome as an even background.  Using the cheshire eyepiece I got everything as perfectly lined up as I could.  I adjusted the focuser to point the laser at the center of the secondary, the reflected eyepiece hole appears in the middle of the secondary center circle after I adjusted the secondary mirror.  Then I made everything centered again by moving the primary  mirror.  No matter what I do, I still see an uneven spacing around the primary and the baffle tube edge.  I tried viewing Sirius but it clouded over before I could get spacing correct for focus.

My latest question:  Is it possible that the baffle tube could be slightly off center from the factory?  I don't see that being a huge problem as long as mirrors are lined up.  Then again, I see diagrams showing the same thing and it says the secondary needs to be adjusted. Gaaahh!!!!   I'll need a clear night to see how my latest adjustments work to test out my baffle theory.

[Buzzard75]

Just wondering how you're getting along with this. I only have an 8", but am having a bit of a time getting mine right as well. It's definitely close, but not perfect. My MoonLite focuser has an adjustment ring on it, which is attached via a spacer ring to the telescope which also happens to be the part that adjusts the primary. My secondary mirror is center spotted, but it would seem to me that any adjustment you do to the primary throws off any adjustment you've done to the focuser. I'm not quite sure how to get them co-planar without removing the primary assembly from the telescope and doing the kitchen table collimation that is referenced in another thread.