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C11, how do I align the optics?

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#1 telfish

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Posted 09 January 2013 - 01:22 PM

I have my C11 apart for some modifications and wish to align the optics on reassembly. One part of the modifications is to install some edge style corrector plate adjustment screws and do away with the bits of cork and paper.

So As I understand it I need to center the secondary within the corrector plate and also center the corrector plate within the front assembly. This should in theory center the optics over the baffle tube and all should be well! Assuming the baffle tube is central to the whole shebang!

Am I wrong? Would this just give me mechanical alignment and not necessarily optical alignment, if so what else do I need to do?


I just measured the baffle tube within the rear casing and it's amazingly centralized for a massed produced product, not more than half a millimeter out measuring with a digital micrometer.

So as long as I get the casing onto the tube square then the baffle should be central to the front casing and corrector yes?
Thanks for your help.


Terry

#2 Eddgie

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Posted 09 January 2013 - 06:17 PM

It is best to center using an artifical star (or real star if you don't have or want to make an artificial star).

Measuring is not all that useful because it depends on the primary and baffle both being perfectly centered and perpindicular, and this is not always the case.

The good news is that it is not difficult to do.

The key is the Poisson point (though I may have miss-spelled it, forgive me).

The Poisson point is the tiny little point you see in the defocused image. At the very center of the Fresnel pattern, you will see this tiny little difraction artifact.

This point is a good indicator of the centering of the obstruction in a reflector. If it appears off to one side, of the shadow, this usually indicates that the secondary obstruction is not centered.

Just remove the diagonal and insert an eyepiece and put up a defocused image in the eyepeice. If the spot is at 3 O'Clock relative to the center of the shadow, then push to the left. If the spot is high, then push the corrector down. Pretty simple. The spot moves with the postion of the corrector/secondary.

ps. The "usually" above assumes that there is no other serious issue like primary baffle tilt.

#3 Steven

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Posted 10 January 2013 - 12:52 AM

You may find this helpful:
http://www.wilmslowa..._alignment.html

Steven

#4 frozen.kryo

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Posted 10 January 2013 - 01:24 AM

Please do check if the secondary mirror itself is centered to its backplate. I think the secondary mirror is attached to the backplate with double-sided adhesive tape.

On my C11, the mirror itself is not centered to its backplate. :mad: It has an offset of 0.5mm towards 7 o'clock (looking from the front of the scope with the secondary marking at 3 o'clock.)

I can confirm that the poission spot on my defocused stars are not centered and are slightly shifted downward to the 7 o'clock position despite very good collimation. In focus on-axis star images are also comatic and pointing to the 1 o'clock position on the clockface.

I have since tried shimming the secondary assembly (it has fastar). I left the secondary holder centered on the corrector for hyperstar use. This has helped a lot, but the poisson spot is still not fully centered. There is a massive improvement with on axis star images though.

I am planning to re-attach the secondary mirror, and center it myself. I am just not sure what type of adhesive to use. Anyone have any suggestions? I don't want the adhesive to fail and have the secondary mirror plonk down to the primary. :bawling:

#5 frozen.kryo

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Posted 10 January 2013 - 01:31 AM

Since you already have the the OTA apart, please check if the primary mirror locking collar is secure.

On my C11, the mirror lockring was loose, allowing the primary to flop about as you tracked and slewed the scope. It's impossible to keep focused/aligned/collimated with a loose primary mirror.

#6 telfish

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Posted 10 January 2013 - 02:37 PM

Thanks I used the method on that site and found the mirror was off of alignment to the baffle. I adjusted it so now all is square. I have centered the secondary in the corrector and will center that when I reassemble with the mods completed.

If anyone is looking to fit mirror locks to their scopes I have found some torque adjustable bolts with a large swivel delrin foot that looks ideal. You can adjust the torque point at which the bolts will just spin and tighten no further. The swivel foot will contour to the shape of the back of the mirror.

I have also found some delrin tipped spring loaded bolts to do a mod like the edge scopes witch have adjustable screws for corrector adjustment instead of cork spacers.

#7 telfish

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Posted 10 January 2013 - 02:38 PM

Since you already have the the OTA apart, please check if the primary mirror locking collar is secure.

On my C11, the mirror lockring was loose, allowing the primary to flop about as you tracked and slewed the scope. It's impossible to keep focused/aligned/collimated with a loose primary mirror.


It was loose!

#8 telfish

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Posted 10 January 2013 - 02:40 PM

I will take a closer look at the secondary mirror to make sure it's central to it's mount, thanks for the heads up.

#9 pgrunwald

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Posted 10 January 2013 - 04:50 PM

Steven,

I'm interested in the parts you mention, would you please send me the details.

Thanks,
Paul

#10 telfish

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Posted 10 January 2013 - 05:28 PM

Steven,

I'm interested in the parts you mention, would you please send me the details.

Thanks,
Paul


I got them from Carr Lane. Here are the part numbers




Line Line Unit Schedule
No Item Qty Price Date * Comment
---- -------------------- ----------- ----------- ----------- -----------------
1 30SSPN2 6 4.81 09-JAN-2013
CL-30-SSPN-2 SPRING
PLUNGER

2 630TSB 2 21.35 09-JAN-2013
CL-630-TSBN
ADJUSTABLE-TORQUE
THUMB SCREW SWIVEL
FOOT SCREWS ON/OFF
(LEFTHAND)

The N on the end of the thumb screws designates a Delrin foot, if you leave that off you get stainless steel.

Link to torque bolts.

http://www.carrlane....513E111D081B...


Terry

#11 frozen.kryo

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Posted 10 January 2013 - 11:15 PM

It was loose!



Well you'd want to tighten that up, but not too much as to pinch the mirror.

I've found that there is some lateral play with the lockring. Try to move the ring from side to side. Chances are, it'll loosen up and you'll have to snug it down again. Rinse repeat.

#12 telfish

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Posted 11 January 2013 - 08:35 AM

It was loose!



Well you'd want to tighten that up, but not too much as to pinch the mirror.

I've found that there is some lateral play with the lockring. Try to move the ring from side to side. Chances are, it'll loosen up and you'll have to snug it down again. Rinse repeat.


I had to adjust it when I added the shimming to straighten the mirror. I set it hand tight.

Just out of interest that collar covers a lot of mirror! It's about an inch wide. Has anyone ever reduced that width and gained some aperture?






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