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Equipment Discussions >> Cats & Casses

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Eddgie
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Reged: 02/01/06

Re: Help on collimating a C6 SCT please!! new [Re: zAmbonii]
      #5694843 - 02/22/13 02:23 PM

These don't look at all like diffraction arcs.

They look like the first ring being comatic. This is a collimation error.

The OPs images showed diffraction spikes that radiated out quite far from the image blur of the star.

It looks like your scope just needs fine collimation.

If you collimated using a defocused Fresnel pattern, this only gets you close.

After rough collimation, you should focus the star as best as you can and use the first diffraction ring as your final guide to best collimation. Collimate until the first ring appears evenly bright around the Airy disk.

If it looks like an "O", you are in collimation. If it looks like a "C" it is close enough. Your scope looks to be more out than this but it is close to being a "C".


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zAmbonii
Carpal Tunnel


Reged: 01/19/08

Loc: Ypsilanti, MI
Re: Help on collimating a C6 SCT please!! new [Re: Eddgie]
      #5696384 - 02/23/13 11:13 AM

Thanks for the info.

I had seen the same type of arcs around Io imaging a couple of weeks ago and figured I really need to do the collimation thingy, but my first attempts at trying to use a screwdriver in 10F cold left me feeling like I didn't know what I was doing .

I have an indoor star that I can use to get it straight and then confirm the collimation with a star the next time out.


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Eddgie
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Re: Help on collimating a C6 SCT please!! new [Re: zAmbonii]
      #5696528 - 02/23/13 12:38 PM

Don't be surprised if using an indoor star does not result in getting good collimation outside.

The reason is that if you collimate with the scope in a horizontal position, the mirror will want to lay flat against the baffle.

When you take the scope out and point it at the sky, the mirror will usually wind up at a very slight angle because the focuser rod will support it on one side and the mirror carrier will settle on the baffle at a slight angle.

This shift can be very small and not all telsecopes will do it, but it can cause the collimation to get as far out from perfect as your scope currently is right now.

If you must to it inside, try to get the artificial source as high as possbile so that the scope is pointing up if at all possible.

I am not saying that you can't be successful doing it inside. It depends on how much play there is in the mirror carrier/baffle

More like "Don't be surprised if you get perfect collimation indoors, but find out that it has changed under the stars."

I personally think that it is best to always colliate using a star, and remembering to alwasy approach final focus using counter-clockwise movement of the focuser when testing your collimation. This helps ensure that the prmiarmy mirror alwasy stops at the same angle relative to the baffle.


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KerryR
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Reged: 12/05/07

Loc: SW Michigan
Re: Help on collimating a C6 SCT please!! new [Re: zAmbonii]
      #5696541 - 02/23/13 12:42 PM

Here's an issue I've had with using an artificial star indoors at 25 feet: The focus was so different that perfect collimation at 25 feet was no longer perfect when focused on infinity. Close, but off enough that further star collimation yielded better Jovian observation.

This suggests, I guess, some sort of mechanical misalignment, maybe a tilt error of the baffle vs. secondary housing (?). Or, of course, that the collimation shifted upon moving the ota from the basement to the observing site, but, in general, the scopes on which I've observed the phenomenon otherwise hold collimation very well, so I don't know for sure what the cause was.

In any case, I now only use artificial (close) stars when diagnosing mechanical issues, and use real stars for fine collimation when observing, and all's well.

EDIT: Upon posting the above, I see Edgie addressed a similar issue. No duplication intended...

Edited by KerryR (02/23/13 12:43 PM)


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zAmbonii
Carpal Tunnel


Reged: 01/19/08

Loc: Ypsilanti, MI
Re: Help on collimating a C6 SCT please!! new [Re: KerryR]
      #5696654 - 02/23/13 01:53 PM

I'll keep note. I know what the pattern looked like the last time I had the scope out and will see if it is close to the same when doing the indoor check. If it is a bit different, I may just skip the indoor collimation totally.

I was worried about the difference in focus also when working indoors. Think I am going to put a mirror at the other end of the house to double the length from the star, we'll see what happens


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KerryR
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Reged: 12/05/07

Loc: SW Michigan
Re: Help on collimating a C6 SCT please!! new [Re: zAmbonii]
      #5696694 - 02/23/13 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.


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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Re: Help on collimating a C6 SCT please!! new [Re: Eddgie]
      #5696965 - 02/23/13 04:52 PM Attachment (30 downloads)

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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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Re: Help on collimating a C6 SCT please!! new [Re: Alfredo Beltran]
      #5696972 - 02/23/13 04:56 PM Attachment (41 downloads)

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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Eddgie
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Reged: 02/01/06

Re: Help on collimating a C6 SCT please!! new [Re: Alfredo Beltran]
      #5697284 - 02/23/13 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.


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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Re: Help on collimating a C6 SCT please!! new [Re: Eddgie]
      #5697368 - 02/23/13 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


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DesertRat
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Reged: 06/18/06

Loc: Valley of the Sun
Re: Help on collimating a C6 SCT please!! new [Re: Alfredo Beltran]
      #5697502 - 02/23/13 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


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REC
Post Laureate
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Reged: 10/20/10

Loc: NC
Re: Help on collimating a C6 SCT please!! new [Re: DesertRat]
      #5697992 - 02/24/13 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


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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Re: Help on collimating a C6 SCT please!! new [Re: DesertRat]
      #5698007 - 02/24/13 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


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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Re: Help on collimating a C6 SCT please!! new [Re: REC]
      #5698021 - 02/24/13 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


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Eddgie
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Reged: 02/01/06

Re: Help on collimating a C6 SCT please!! new [Re: Alfredo Beltran]
      #5698528 - 02/24/13 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.


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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Re: Help on collimating a C6 SCT please!! new [Re: Eddgie]
      #5698713 - 02/24/13 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


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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Re: Help on collimating a C6 SCT please!! new [Re: Eddgie]
      #5699068 - 02/24/13 10:53 PM Attachment (28 downloads)

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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Eddgie
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Reged: 02/01/06

Re: Help on collimating a C6 SCT please!! new [Re: Alfredo Beltran]
      #5699520 - 02/25/13 09:52 AM

Beautiful.

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

Regards,
Ed


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orion61

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Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Help on collimating a C6 SCT please!! new [Re: Alfredo Beltran]
      #5700008 - 02/25/13 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.


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KerryR
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Reged: 12/05/07

Loc: SW Michigan
Re: Help on collimating a C6 SCT please!! new [Re: orion61]
      #5700031 - 02/25/13 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)

Edited by KerryR (02/25/13 03:18 PM)


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