View Full Version here: : Primary alignment and collimation with Hotech Laser
22-12-2011, 11:09 AM
This is a quick series of pics I took of the setup I used to align the optics on my C11 the other night.
I purposely place the cross laser far enough for added precision. The light path with the secondary in place folds four times.
Because I can now tilt the primary all I have to do is match the inner cross with the outer cross. This ensures my primary is square with my back focus which is really what I care about.
Then standard collimation follows which is just tilting the secondary and finally star test, hopefully tomorrow night.
I have taken the scope apart twice since I originally tilted the primary and locked it and I'm happy to say it hasn't moved a bit since.
Yesterday I removed the corrector and flipped it around then recollimated the secondary and everything was still inline so it looks like it's rock solid now. :thumbsup:
With the old primary mounting, moving the scope from A to B would shift collimation. :rolleyes:
22-12-2011, 06:08 PM
Pictures' worth a thousand words! My only question is in Pic 7, which shows the shadows on the corrector plate as not being symmetrical with the circles on the Hutech. Why is that, and does it even matter?
Seems like a really neat way to collimate a Hyperstar. Bored, I did my C8 and checked my (post Tak scope coll) RC today using an artificial star, they both seemed OK, but once I get the courage to rip the C8 to pieces and do the Aragnou mod, it looks like a Hutech would be justified. Can't say I see many used ones come up, so it looks like another big purchase next year. Sigh.
Anyway, congrats Marc, looks like it's all worked out well for you. Now if I can only find a good machine shop....
22-12-2011, 07:01 PM
Yes - that's the offset of my baffle tube. It is very evident isn't it? If I had no problems the shadows would be concentric. Because I chose to tilt the primary square to the baffle it's slightly off the aperture. The secondary being spherical can be slightly offset and still aligned by tilt so it's co-axial with the primary but offset mechanically. The corrector tilt is not an issue as far as I can see. It's minimal.
I'll try this configuration first. If it doesn't work I'll center the corrector and the secondary mechanically then tilt the primary to match. This means I should have a slight shadow of the baffle tube. I'll see what works best.
The kit has come down a lot. It's in the $300 ballpark now. It was well over $500 when I bought it. I think it's something you want to have if you play with CATs.
It will be interesting to see the final star test pic?
Hopefully you will have better luck than me, as my laser alignment and star test where miles apart!
Although in my case I can't rule out the nut behind the laser, was the actual problem. :D
I will try again one day or sell the kit.
22-12-2011, 08:45 PM
Drop me PM if you decide to do that! :)
22-12-2011, 09:11 PM
I'm not out of the woods just yet. Still experimenting. In the time I've used the Hotech board I don't think it's practical at all to use it with the scope on the mount as advertised and at the recommended distance just in front of the aperture. It takes me routinely a good 15min to square the scope on the bench before I can start any measurements. And that's with the set up on the pics. Very accurately and slowly.
22-12-2011, 09:40 PM
Noted the board with the wheels to rotate the OTA, what I can't see is if you have any way of adjusting the board rear elevation and azimuth or if these are really that critical.
22-12-2011, 10:14 PM
The wheels are there so I can rotate the tube and align its orientation to the board cross. I have 4 marks at the edge of the primary with white tipex. When the laser hits the white paint it really shines so you know you're on the registration marks. Then I 'shim' the whole wooden plank up or down. I usually slide a dovetail under forward or backward for level then shift it sideways to get the cross lateraly. The registration of the board to your primary must be spot on. If it's not you'll set your scope out of whack in no time.
23-12-2011, 12:28 AM
Oh no, Marc says, another question from him! .......
I have a small obs, so have the luxury of a fixed pier and EQ6 to locate the OTA, and a vertical internal wall about 70cm in front of the horizontal OTA .
In this case, in your experience, is it practical to contemplate setting the OTA perfectly horizontal with a level and at right angles to the wall with a carpenter's steel angle, (then memorising that as a home point in EQMOD), then putting solid mounting points at the appropriate points on the wall for the laser board?
Any fine adjustment could then be done with the EQ6. Reason? Beats lugging it all inside and setting up in the living room!
23-12-2011, 08:50 AM
I don't know about that. I tried once to register it with the scope on the mount when I first bought it, even used the external markers. I remember it was a real pain. But I guess with a bit of practice it would work. Regardless you'll have to mount the board on a bracket because everytime you move the scope you have to move the board again and vice versa and iterate to the right orientation. I see this as a tool to align and diagnose mechanical problem with optics. Not a collimation tool because the result will depend on how good you register the board to the scope. It'll get you very close but a star test is still the finishing touch.
23-12-2011, 10:03 AM
OK Marc, Thanks for that advice, I promise I'll leave you in peace now.
Thanks for the board idea. The markers you have on the ota, do they line up with the four screws at the back of the primary mirror(the ones with the nuts on them)?
Once you start normal collimation I would be interested to hear how much the ota moves on the rollers, when you start adjusting the collimation screws.
23-12-2011, 12:57 PM
No at all mate - keep firing away. :thumbsup:
I marked the primary directly. It has a small bevelled edge. I aligned the first mark with the black line at the back of the primary. Then I wrapped a paper tape to mark the other three. Don't rely on any screws or mechanical features on your scope tube. The holes are not evenly spaced.
I stick a carton business card on the roller. They're rubber castor wheels and that's enough grip to stop the OTA from turning. I remove the dovetail so it doesn't weigh the scope out of balance too.
23-12-2011, 08:32 PM
Well, since you put it that way.......Before I can attack the fabrication of the pieces for a C8, I need to locate an engineering shop that doesn't charge rocket-science prices. I have one nearby, who are happy to do small work, but their precision is not super great. (They basically import and integrate bush fire-pumps to steel chassis, etc.)
My question is: It does not seem like there is any need for great surface precision in either of the two plates, basically the collimation screws provide the final precision.
That being the case, if I purchase a couple of pieces of thick (5mm or more) aluminium stock from the local supplier, and have them reamed/drilled/milled etc to fit, taking into account how they work as a sandwich with the existing focuser plate, do you see any need for high precision in that work? Also, it's only an 8" mirror so thread loadings should be less, allowing all aluminium stock.
Second question, do you think I can keep the plate thicknesses such that I can use normal dovetail-size hex bolts for the collimation function, of which I have many surplus?
23-12-2011, 09:06 PM
That is correct.
Everything out of alu will me more than adequate for a C8. It's a small piece of glass. The front ring is pretty simple, the back ring will need steps to be machined so you can mate it to the back of the existing focusing arm. I suspect 5mm is fine. I araldited mine to the existing piece and used a dremel to finish the holes.
The plate thickness will depend on how much room you have when the mirror bottoms out. You still want to be able to focus at infinity or you'll have to move the corrector further out.
23-12-2011, 09:57 PM
Thanks again Marc, Looks like it may be simpler than I first envisaged. All I need now is for these darn holidays to finish so I can go to the machine shop. Bah humbug! :)
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