C11 Primary collimation

multiweb (Marc) · #1 17-12-2011, 08:44 PM

Here's a project that I've just completed. I've been thinking about it for a long time now and two weeks ago I finally bit the bullet and got the bits machined.

The challenge was to fit everything between the back of the mirror and the focusing arm and not loose too much back travelling. I've got two rings made. One made out of 8mm stainless steel threaded at M10 in 3 points at 120 degrees. The other is made out of aluminium 13mm and just drilled at 10mm.

I bonded the stainless steel ring to the mirror glass directly. The glass is very thick at the back and there's no problem getting a good hold on it. It's almost like a conical mirror. I used stainless steel because I was a bit worried that aluminum threads may strip with the weight of the glass.

So the first ring pulls the mirror back. The aluminium ring at the back has small 4mm locking screws. So it's a push/pull system basically. The focusing arm is sandwiched in between the two rings.

The back cell was drilled to make 6 access holes. 3 for the 3 M10 hex bolts and 3 for the smaller 4mm philips heads (locking).

The whole assembly slides up and down the baffle tube like a normal C11. The only difference is that you can tilt the primary.

There's a small animation here that I used to block the components and work out the volumes in MAX. The mirror is grey, the dark and light blue bits are the existing focusing tube. The green ring is the stainless ring. The light blue semi-transparent ring at the back is the aluminium ring machined to match the steps of the back of the focusing arm.

I've also attached a couple of photos of the various components below. That might be of interest to a few SCT users although I suspect most wouldn't need that option it's still pretty cool to have.

mldee (Mike) · #2 17-12-2011, 09:25 PM

Well Marc, Nice approach. Thanks for the very nice internal pics of the C11 rear assembly. I'm hoping to do something similar to my C8, but for different objectives. Your concept adds a lot of incentive.

You were the one who first got me interested in the Hyperstar approach to the Celestrons, and my scope is outside right now in Hyperstar mode busy on the Pleiades.

To cut a long story short, after the Hyperstar shooting season finishes in a couple of weeks, I intend to disassemble the OTA and put a similar type of assembly at the rear of the mirror to allow 3-point focussing via a rear-mounted Crayford and tube inside the baffle tube. Details at 11.

I wont go into gory details at this stage, but your effort has sure inspired me to push onwards with my mods. I think the older Celestrons are a really cost-effective machine amenable to such mods and experimentation.

DavidTrap (David) · #3 17-12-2011, 10:08 PM

I'll bite - why do you need to tilt a spherical mirror?

DT

multiweb (Marc) · #4 18-12-2011, 08:50 AM

Quote:

Originally Posted by mldee

Well Marc, Nice approach. Thanks for the very nice internal pics of the C11 rear assembly. I'm hoping to do something similar to my C8, but for different objectives. Your concept adds a lot of incentive.

You were the one who first got me interested in the Hyperstar approach to the Celestrons, and my scope is outside right now in Hyperstar mode busy on the Pleiades.

To cut a long story short, after the Hyperstar shooting season finishes in a couple of weeks, I intend to disassemble the OTA and put a similar type of assembly at the rear of the mirror to allow 3-point focussing via a rear-mounted Crayford and tube inside the baffle tube. Details at 11.

I wont go into gory details at this stage, but your effort has sure inspired me to push onwards with my mods. I think the older Celestrons are a really cost-effective machine amenable to such mods and experimentation.

That's good to hear. A lot of the vintage scopes in the celestron range used to have various primary mirror mounting and most could be tilted. Something that got lost down the road and the assembly line, primarily because people didn't understand how to collimate them. You have to start the other way round with the secondary and do the primary last, not unlike the iterative procedure of a newt. I suspect also it brought cost down dramatically.

Quote:

Originally Posted by DavidTrap

I'll bite - why do you need to tilt a spherical mirror?

DT

Hi David, having the ability to tilt both mirrors will allow you to always get them on axis regardless of how well centered they are.

DavidTrap (David) · #5 18-12-2011, 09:36 AM

Thought it might be something like that.

How have you worked out you primary needed adjustment?

DT

multiweb (Marc) · #6 18-12-2011, 12:10 PM

Quote:

Originally Posted by DavidTrap

Thought it might be something like that.

How have you worked out you primary needed adjustment?

DT

With the hyperstar.

DavidTrap (David) · #7 18-12-2011, 12:39 PM

Ahhhhhh - makes sense.

DT

multiweb (Marc) · #8 18-12-2011, 09:33 PM

Well I'm happy to report the mods did work a treat. I got the scope out today and had a quick peek at Jupiter early evening. It was just awesome. Pin point moons, snapped into focus. I could clearly make up the bands and also some of the cloudy wisps. I called the kids and they went wow! So test passed with flying colors.

The collimation worked surprisingly well. I got the scope on the bench without the corrector or the secondary. Registered it to the Hotech cross laser and tweaked the primary tilt until I aligned both the cross reflected from the primary with the cross reflected by the small mirror in the back focus. Took a few iterations to get it done and close in the sweet spot. Once it was aligned I locked the primary and placed the corrector back in then proceeded with the secondary collimation which was nearly spot on out of the box as everything was already aligned.

Then took the scope out and the views were splendid. Can't wait to stick a camera in there now. Big relief it all worked out so well in the end too.

DavidTrap (David) · #9 19-12-2011, 10:19 AM

Just reading this thread and your comments on Peters thread about the corrector on his Edge14.

What did you bond the ring to the mirror with?

How would you mirror lock via the focuser alone, rather than you 3 point system?

I'm currently selling my SCT, but may come back to a C11 in due course for some photometry work. A C11 is probably the largest scope I can put on my current mount.

DT

multiweb (Marc) · #10 19-12-2011, 10:27 AM

Quote:

Originally Posted by DavidTrap

Just reading this thread and your comments on Peters thread about the corrector on his Edge14.

What did you bond the ring to the mirror with?

Selley's Steel Polymer Knead it. Dries in 2min, bonds in 5 and you can drill, tap, machine it within 1h.

Quote:

Originally Posted by DavidTrap

How would you mirror lock via the focuser alone, rather than you 3 point system?

Drill and tap a hole/bush in the casting right at the back of the focusing arm so you can push against it.

DavidTrap (David) · #11 19-12-2011, 12:50 PM

Sounds a bit like JB Weld - a metalised epoxy that can be drilled, etc.

And the mirror lock sounds like you are creating a push-pull system?

Thanks,

DT

multiweb (Marc) · #12 19-12-2011, 01:19 PM

Quote:

Originally Posted by DavidTrap

Sounds a bit like JB Weld - a metalised epoxy that can be drilled, etc.

And the mirror lock sounds like you are creating a push-pull system?

Thanks,

DT

In your case you want a simple push mechanism againt the back of the focusing arm. The weight of the mirror will push down as your scope is mostly pointing up so you only need to hold it there.