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Help on collimating a C6 SCT please!!

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#26 KerryR

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Posted 23 February 2013 - 02:07 PM

I once tried doubling the distance with a bathroom-type mirror. The mirror was nowhere near flat enough to keep the pin-poit star I started with pin-pointy. If memory serves, it was so large and misshapen I couldn't use it for anything. But then, for collimation, you only really need to be able to discern centering, so if the rings are fairly round, it might still work. It didn't for me, but I didn't try very hard to find a flatter mirror.

You might get lucky, though-- I know that every once and a while, astoundingly flat mirrors of the hardware store variety can be found, or so I've heard.

I bet you could use an optical flat from a newt, though alignment might be a bit of a pain...

I don't know, though, that doubling the distance would solve the focus issue. And, you'd still have to solve the mirror flop issue, as Edgie mentioned. And, on top of all that, the scope would have to keep it's collimation upon moving. Some scopes do so without fail, others seem to loose collimation in transport. My C6, and CPC11 hold collimation extremely well. MY LX90 needs collimation every time I set up.

#27 Alfredo Beltran

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Posted 23 February 2013 - 04:52 PM

Well, first let me thank you all for the very useful information that has been discussed in this thread. I think As you can see, I could collimate my telescope!

Some conclusions:

1. When I asked here for help I had done collimation wrong because I didn't re-center the "star" on the frame.

2. As Eddgie said, I did have the corrector uncentered. I checked it with this video suggested from Dean of Starizona: . It explains how to center the corrector. I must say that when I took the corrector two years ago, I had done the suggested markings. It seems I assembled it in the wrong way. Last night I watched the video again I saw my mistake bevause the markings were not aligned. When they were and checked centering of the corrector, only a minor adjusted was needed.

3. After centering the corrector I did rough collimation on a bright star (Sirius). Then I moved to Pollux to check it (see image below). You can see the Posion point centered in the secondary shadow. Also you can see the diffraction rings and the shadow centered.

4. Then I moved to a low magnitude star to do fine collimation, slightly defocused just to let the ring being seen. I needed to do a few adjustments with one of the collimation screws. In this site I learned how the turning of a collimation screw moves the secondary. Simple, but very effective explanation.

After that, I had the stars looking like you will see in the next image (see next message)

Even though I think I collimated the scope, I'd appreciate to have your feedback to see if something still needs to be done.

Best regards

Alfredo

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#28 Alfredo Beltran

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Posted 23 February 2013 - 04:56 PM

Here is the image on focus on a dim star:

In your opinion, am I finished collimation my scope?

Best regards

Alfredo

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#29 Eddgie

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Posted 23 February 2013 - 08:43 PM

Happy to hear that you were able to get the scope back together properly.

The images are not so easy to see but clearly the Poisson point looks centered now, and the in focus images do indeed look to be in excellent collimation.

Did the diffraction artifict go away?

The defocused image above still seems to show some odd characteristic of having a bright horseshow circle in the fresnel pattern, but It could just be the image.

Most important things to check are that the in-focus star looks to have an evenly illuinated first ring, and as you pass though focus, the secondary shadow should stay centered and not move from one side to the other.

The Poisson point is where it belongs though.

Hope it is working well for you now.

#30 Alfredo Beltran

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Posted 23 February 2013 - 09:23 PM

Hi Edgie

I'll have to check if when passing through focus the shadow remains in the same place. If it doesn't, what would be the solution?

On the other hand, the image was a little afected by dew because I chose not to use the dew heater to avoid any disturbance during collimation.

Thanks in advance

Alfredo

#31 DesertRat

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Posted 23 February 2013 - 10:46 PM

You asked "In your opinion, am I finished collimation my scope?"

Looking at the last outside of focus image you posted I would answer no. See how the bottom of the pattern is brighter than the top? This indicates a squeezing of the diffraction rings near the bottom. If you were to focus that star and be able to capture the first in focus diffraction ring about the central spot (not easy to record in average seeing, and requires imaging with a barlow because its very small) you would see the brighter part of the ring at the top, it reverses position. This effect is the same whether you are inside or outside of focus - just to be clear.

Now how to fix that? Well the star needs to be moved up in this case by adjusting the appropriate collimation screw. As you get closer to focus the necessary adjustments get smaller and smaller. The test for collimation becomes more sensitive. In good seeing you can even out the ring with very small adjustments, almost too small to quantify.

Good luck Alfredo, wish you the best!

Glenn

#32 REC

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Posted 24 February 2013 - 10:15 AM

I have been watching this post and learning a lot, but can someone tell me what a "Poisson point" is?

Thanks!

Bob

#33 Alfredo Beltran

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Posted 24 February 2013 - 10:30 AM

Hi Glenn

At the end of the session there was dew forming on the corrector because I chose not to use te dew strap to avoid any thermal interference. That's why you see a brighter part on the image.

Nevertheless, I will re-check and post here.

Thanks for your help

Alfedo

#34 Alfredo Beltran

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Posted 24 February 2013 - 10:36 AM

Hi Bob

The Poisson point is the small white dot you see inside the secondary shadow and must be centered relative to it. Tells you if your secondary is centered or not.

My secondary was not centered, and thus I couldn't get collimation.

Best regards

Alfredo

#35 Eddgie

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Posted 24 February 2013 - 04:19 PM

Most if the time, if colliation is perfect when a star is at best focus and the Poisson Point is centered when slightly defocused, if the secondary shadow shifts when passing through focus using counter-clockwise movement of the knob (always approach final focus or move though best focus using counter-clockwise motion of the focuser to ensure consistent mirror angle) the most likley cause is that the diagonal mirror is not perfectly collimated (tilted +/- to a perfect 45 degree angle). This hurts nothing unless the tilt is extreme.

So, if you observe it, try using an eyepeice straight through (no diaongal).

If there is still shift, it is because there is tilt in the system.

The most likely cause of tilt in the system is when someone removes the Corrector housing or the primary mirror housing (or both) from the metal OTA and does not put it back together properly.

This tilt does not have to be all that much to make a difference at the focal plane.

In visual use, most people will never see it. It will usually show at the extreme corners of a big CCD chip image.

And that is the ultimate test if you have a big CCD camera.

For exampe, lets say you imaged a frame and at North and South, the stars at the edge of the frame were in perfect focus. Then it would stant that the stars at the east and west side of the frame would also be in perfect focus.

But if they were out of focus (one would be inside of best focus and the other would be outside of best focus) then that indicates tilt in the tube. The effect is to tilt the focal plane.

I diagnosed this problem (tilted tube assembly) for someone on the forum a couple of years ago... I asked him if the telecope had been taken apart and he indicated that it had, during the process to install flocking. When he re-assmbled the tube, he just slippped it into the cell, put in the screws, and tightend them.

He had to go back and loosen the OTA tube from the rear cell and re-space it to eliminate the tilt. Remember, we are dealing with wavelegths of light here. 1mm of Tilt in an f/10 system could throw the edges of the focal plane out of focus by 3 or 4 wavelenghts of light. His symptom was that if he focused on one side of the field, the other side was not infocus.... Dead giveaway that there is tilt in the system (though an off center seconary can do this, the tell would be the Poission Point not being centered).

So, if at best focus the stars are collimated and when defocused the Poission Point is centered, that is enough visually. If imaging though, and you see the secondary shadow shift across to the other side when passing focus, then you have to look at your frame to see if the focus is the same at all cardinal points of a circle imposed on the field. If all appear the same sharpness, then there is not enough tilt to be concerned with.

I would imagine that during manufacture, Celeston has a fixture that automatically squares the end of the optical tube to the optical axis. I know that they have a fixture that they use to center the seconary assembly.

There should be no way in normal use and handling for it to ever change, but if dis-assembled, care must be taken to ensure that it goes back toghther properly.

So, even if you see a tiny amount of travel, don't worry about it for visual use.

But if you have had the corrector cell off of the telescope (unlikely to just pull the corrector itself off) then you could get it tilted when you replace it. An accurate carpenter square across the top of the corrector and along the tube wall at the cardinal compass points can tell you if this condition is present.

Much harder with the new rear cell designs to do this because the carpenter square doesn't lay flat on the curves at the back anymore. Anyone that removed the rear cell from a new model needs to take very special care to mark the OTA very precisely before removing the rear cell because it is no longer easily possible to do with a carpenter's square.

Thanks for clarifying the Horsehoe being dew. I could not for the life of me tell what in the telescope itself could have caused that.

So you are looking good my friend.

Let me know how the test goes.

#36 Alfredo Beltran

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Posted 24 February 2013 - 05:57 PM

Hi Ed

I still have to check it when the weather cooperates, but when I took the corrector plate off I didn't took its housing apart. So this makes me think that, since the Poisson point is centered, there shouldn't be any tilt present (hopefully).

As soon as I can I will check this and post the findings.

Thank you very much for your help

Alfredo

#37 Alfredo Beltran

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Posted 24 February 2013 - 10:53 PM

Well, it satisfies me to say that now I think I succesfully collimated my scope, as you can see here in the next image!

As you can see, the fine collimation shows the Poisson point centered and the profile of the defocused star is symmetric. I just needed to do very small adjustments this time. I also checked and the secondary shadow didn't shift when passing through focus. So, good news!

Thank you very much to all of you who helped me, and specially to Edgie for his very detailed explanations which were key to get this result.

Best regards,

Alfredo

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#38 Eddgie

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Posted 25 February 2013 - 09:52 AM

Beautiful.

Glad it worked out, and happy to help with the diagnosis and alignment instructions.

Regards,
Ed

#39 orion61

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Posted 25 February 2013 - 03:07 PM

I have also had the issue with either a lip of grease or on the new Celestrons a screw that is used on the focus baffle as a stop, instead of a retaining ring.
protruding into the light path inside the baffle.
If you own an older C6 you might want to check it out.

#40 KerryR

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Posted 25 February 2013 - 03:17 PM

Alfredo,

How did you precisely move and then test the secondary position? (referring to centering the secondary on the primary, not adjusting it's tilt via the collimation screws)

#41 Alfredo Beltran

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Posted 25 February 2013 - 04:55 PM

Hi Kerry

I just centered it using the method suggested by Starizona in this video

After that, the Poisson point was centered, relative to the secondary shadow. It wasn't because when I took the corrector off I made a mistake during the re-assembly process and I didn't align the marking I made before de-assembling it with the notch on the secondary holder.

Once aligned, I had the secondary centered again.

Best regards

Alfredo

#42 KerryR

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Posted 25 February 2013 - 04:59 PM

Thanks. I was wondering if you used the method in the video. Good to know that method is easy and effective.

#43 Alfredo Beltran

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Posted 25 February 2013 - 05:31 PM

Hi Kerry

That method worked for me because I made the markings when I took the corrector off.

If there are no markings, I don't know if it will work. I've read somewhere that new OTAs have markings made at factory. Mine didn't have any.

Best regards

Alfredo






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