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Equipment Discussions >> Cats & Casses

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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Help on collimating a C6 SCT please!!
      #5691879 - 02/20/13 10:31 PM Attachment (127 downloads)

Hi

I was trying to collimate my C6 SCT and got everything worse . I cannot get pinpoint stars, but instead I got some flares around them. You can see it here:

What can I do to fix that? I mean, is there a sequence to do the collimation (for example one knob at a time or something)

In the next message you can see how it looks when defocused.

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]
      #5691884 - 02/20/13 10:34 PM Attachment (132 downloads)

Here it is defocused:

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rmollise
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Reged: 07/06/07

Re: Help on collimating a C6 SCT please!! new [Re: Alfredo Beltran]
      #5692293 - 02/21/13 07:37 AM

You've defocused way too far. Defocus just enough to show diffraction rings.

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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Re: Help on collimating a C6 SCT please!! new [Re: rmollise]
      #5692317 - 02/21/13 07:58 AM

Thanks Rod

I have a few questions:

1. Can collimation be afected if the corrector plate isn't perpendicular to the optical axis of the telescope? If it can, how do I make sure the corrector is perpendicular?

2. Is it correct to get the central obstruction centered with the adjustment of one knob? I ask this because at some point I thought I had it collimated but when I tried to focus the stars, I couldn't (obviously the telescope was not collimated). I used Sirius all of the time and the tracking always kept it on the FOV (very high in the sky here in Bogotá since we are 4.8 deg North and Polaris is too low when you can see it). Software was PHD and the images were 1 sec taken with a SSAG. Is that an ok procedure, or should I do something different?

3. I made the process working with the Celestron f6.3 focal reducer. Would it be better to use the telescope at f10 or even at f20 with a barlow?

4. Using a star implies that I will have to wait for the next clear night. I could use a point light of the red illuminator of the finding eyepeice at low intensity inside the house, a few meters away of the telescope. Will this produce good results?

I know thes are a lot of questions but I really need some help on this matter.

Thank you in advance

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]
      #5692377 - 02/21/13 09:08 AM

First, the scope doen not appear collimated.

However I believe that the flaring you see is not an artifact of mis-collimation.

Mis-collimation causes coma with is an asymmetric abberation. Your in-focus image shows a radial projection from one one side of the Airy Disk htat is off center in the in-focus image. This is a diffraction artifact.

Also, based on the fact that the tiny point at the center of the secondary shadow is not centered under the secondary shadow, I think the problem with our telescope causing the flaring is that the secondary is not centered.

Have you had the corrector off? Has did you install Bob's Knobs and somehow knock or bump the secondary mirror out of position?

In the defocused image, the tiny point with the flares coming of of it is called the Poisson Point. It is a defraction artifict that appears in all obstructed telescopes when the image is defocused.

When the secondary mirror is centered exactly over the center of the primary mirror, the Poisson Point should appear to be very near the center of the secondary shadow.

If it is not perfetly centered, it can cause the Poisson Point to appear away from the center of the secondary shadow.

This has nothing to do with the concentric alignment of the outside of the shadow and the outside of the Fresnel Pattern. The scope can be out of collimation but the Poisson Point should still be a the center of the secondary shadow.

Notice that the flares are arcs. You can easily see this. This is to me looks like some kind of diffraction, and if it were from something straight, they would appear straight. But they are curved, and that indicates that they are being causeed by somehting curved, and this is most likely the light that is grazing the secondary and primary baffles because the light from one side is hitting the edge of these baffles on the way to the focal plane.

I could be wrong. It happens. But I think your primary problem here is that the secondary mirror is not centered over the primary mirror bafffle.

Check to see if the secondary is loose or has moved (or the entire corrector). Just grasp the secondary mirror housing and tug it in different directions. If it moves in any direction with even a little pressure, it has most likely shifted.

If you removed the corrector or if you find shift, then you have to fix it.

The good news is that it is very easy to adjust centering.

Remove the diagonal and put in an eyepeice and focus on a star a comfortable altitude above the horison.

If the spot is high and right, move the secondary low and left. Just push the seconary in a manner that would push the Poisson Point back to center it. Very simple.

But the bigger question is why did it move to start with.. Is the secondary mirror loose, or did the corrector shift?

If you can't easily determine this, and the scope is a new scope, the easiest thing to do would be to return it. Normal user collimation would not cause the Poisson Point to move.

If the corrector has slipped though (dropped in shipping), you can re-center simply be loosening the corrector retaining rings and shifting the corrector in the direction required.

Anyway, your scope does show mis-collimtion, but it is also showing what I believe to be a decentered seconary mirror.

The Poisson Point not being centered in the secondary shadow (that is the way it looks to me) and the curved diffraction spikes both point to this condition.

This is not a serious condition from the standpoint of the adjustment required to correct it. I have recentered several seconary mirrors over the decades, but the quesiton once again is "How did it get decentered?"

If the secondary is not centered, you get some weird behaviors. Most notably, if you collimate on one side of focus and check collimation on the other side, you might find that the secondary shadow will show miscollimation the other side. Collimate on that side and move back, and now the first side will show perfect collimtion.

If you collimate in an in-focus star, you can get it collimmated, for that eyepeice, but you still have the diffraction from the edges of the baffles.

I could be wrong of course. It happens. But the image to me along with my own experience with the behavior of an uncentered secondary mirror point to this as a component of your problem.

My advice is to try to determing if the scondary really is decentered and fix this first if it is. Otherwise, as you may actually be experiencing, collimating a scope with a decenterd seconary is a weird thing to do because it will seem impossible to get the socpe collimated for concentric patterns on both sides of focus.

If it is a new scope, consider returning it if you feel uncomfortable moving things like this. The Secondary Mirror should NOT decenter from normal collimation. It was either shipped that way, or it was jarred out of position during shipping, or if used, the previous ownwer (or perhaps the current ownwer) made it move.

Let us know what you find out.

Edited by Eddgie (02/21/13 09:48 AM)


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spencerj
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Re: Help on collimating a C6 SCT please!! new [Re: Alfredo Beltran]
      #5692387 - 02/21/13 09:16 AM

For now, don't worry about the corrector. Give Celestron the benefit of the doubt and assume the scope is machined to a good tolerance and put together properly.

Sirius is too bright for fine collimation. I prefer something around 2nd to 3rd magnitude. On a good night of seeing this will allow you to see a decent diffraction ring around a focused star and really allow you to dial in final collimation. Sometimes this will adjustment will only be one knob, but often times you need to adjust two knobs to get the desired movement.


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REC
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Reged: 10/20/10

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Re: Help on collimating a C6 SCT please!! new [Re: spencerj]
      #5692411 - 02/21/13 09:38 AM

Lot of people suggest using Polaris to do the test on.

Good luck and I need to do mine as well, so appreciate this post and all the advise from the experts:)


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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]
      #5692429 - 02/21/13 09:49 AM

Hi Edgie

Thanks for your very detailed explanation. It gives me a lot of things to check.

I did have to remove the corrector about two years ago because I was trying if the scope was Fastar compatible and the secondary baffle became loose. I regreted to be that curious! I've been the only owner of this wonderful OTA, which has given me so much fun.

It seems to me like I have two issues: first I have to center the secondary, and then adjust collimation.

Is it possible to do that indoors?

Best regards

Alfredo


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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: Alfredo Beltran]
      #5692495 - 02/21/13 10:39 AM

I had success centering the secondary by measurement alone (indoors, daytime), using a Vernier caliper and measuring the distance between the periphery of the secondary housing and the inner periphery of the top tube ring/corrector cell.

My process was:
Point the ota straight up. Remove the corrector retainer and shims. Shove the corrector plate against the edge of it's cell, deliberately off center. Take 3 measurements with the calipers, all 120* apart. Average the measurements. Set the calipers to the average and use it as a feeler guage to center the secondary housing. Shim the space between the corrector and cell edges with 3 shims 120* apart, using thick card stock. Affix the retaining ring. Collimate and star test to (hopefully) confirm centering. If it worked, after collimation, the secondary shadow should stay centered on both sides of focus, as Edgie mentioned. I didn't notice the Poisson's Spot thing that Edgie mentioned, but I didn't look for it, either, so check that out.

Hopefully this mechanical centering works on your ota-- it did on my 8" LX90 and CPC11, necessary after disassembly for adding active venting.

You'll want to be sure the secondary assembly/baffle is no longer loose, so reverse whatever you did that loosened it, or your scope may never hold centering or collimation.

Edited by KerryR (02/21/13 10:40 AM)


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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Re: Help on collimating a C6 SCT please!! new [Re: KerryR]
      #5692552 - 02/21/13 11:23 AM

Hi Kerry

This is a very good option. Thanks for sharing.

Best regards

Alfredo


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Eddgie
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Re: Help on collimating a C6 SCT please!! new [Re: Alfredo Beltran]
      #5692577 - 02/21/13 11:44 AM

Hello Alfredo,

It is possible and perhaps desirable to do so indoors if you can place an artificial star at a distance far enough away to focus. For alignment, the distance of an artifical star is not at all important. Only for checking spherical abberation.

Before you actuallly do any adjustments, make sure you re-test without the diagonal eyepeice directly in the visual back) to elimnate any issue with an out of collimation diagonal.

But knowing that you removed the corrector in the past is indeed consistent with my belief that the secondary is not centered.

You can make an artifical pin-hole buy layering 3 or 4 sheets of aluminium foil and tapeing them at the edges to keep them from shifting. Place the foil on glass and use a sewing needle to "Drill" a fine hole. Cut a hole in a cardboard box and tape the foil the the inside over the hole, and place a lamp or other light source to shine though the hole (assuming that you don't have an artificial star).

Bring the scope back as required to get the point is focus and that is all you need.

Again, you simply use the eyepeice right in the visual back. The seconary needs to be pushed in the direction that logically would move it toward the center of the shadow.

Doing it with a diagnal is more difficult becuase in the lef/right axis, the image is reversed. Just easier to do it straight though.

And now that we know that the corrector has been removed already, you have nothing to loose, and are clearly willing to do the admustment since you have already been in this area in the past.


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Eddgie
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Re: Help on collimating a C6 SCT please!! new [Re: KerryR]
      #5692581 - 02/21/13 11:49 AM

This method (vernier) is not as accurate. If the central baffle and OTA are not concentric, or if there is some tilt in the system, it will not result in a properly centered secondary.

Another reason why the star test is so important. It is very sensitive and will allow for very quick and very accurate adjusment.

Not saying that the caliper won't work in all cases, becuase it might get you close.

But the star method can get you reliably close every time.

The reason I know is that I have encountered this before. My C5 can have the secondary perfectly centered using a vernier, but show decentering at the Poisson Point (with the resulting apparence of secondary mirror shadow decentering when out of focus). The only way to get it centered was using a star.

Not saying it is not worth a try to use this method, but I would rely on the star test to confirm it...


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Eddgie
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Re: Help on collimating a C6 SCT please!! new [Re: Eddgie]
      #5692604 - 02/21/13 12:00 PM

I should have mentiond this earlier....

When you set up to do the indoor test, do try to collimate the scope as best as possible on a focused star.

The image above seems to show though that the scope is in fact in good collimation at best focus already.

This is one of the possible clues to decentered secondary. You can have a star that shows good colliation when the scope is in perfect focus, but as you move away from best focus, the secondary shadow will appear to shift in one direction as you move inside of focus, and in the opposite direction when you move outside of focus.

It frustrates you becuase it appears that the collimation is always shifting.

It is acatually a proccess you my have to repeat a few times... Collimate, check the Poisson point, move it as necessary , check collimation, check Poisson point.

A tilted secondary can move the Poisson point a small amount but no matter how far you tilt it, you should not see diffraction..

So, you may have to repeat the proceedure a couple of times.

But it is not hard and does not take a long time.

Again, knowing the corrector has been removed gives me every confidence that your problem is indeed secondary centering (or at least some component of the problem anyway). All of the symptoms pointed to it, and your confirmation that it has been reomoved puts it at number one in the "What could be wrong" short list.


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rmollise
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Re: Help on collimating a C6 SCT please!! new [Re: Alfredo Beltran]
      #5692723 - 02/21/13 01:07 PM

Quote:

Thanks Rod

I have a few questions:

1. Can collimation be afected if the corrector plate isn't perpendicular to the optical axis of the telescope? If it can, how do I make sure the corrector is perpendicular?

2. Is it correct to get the central obstruction centered with the adjustment of one knob? I ask this because at some point I thought I had it collimated but when I tried to focus the stars, I couldn't (obviously the telescope was not collimated). I used Sirius all of the time and the tracking always kept it on the FOV (very high in the sky here in Bogotá since we are 4.8 deg North and Polaris is too low when you can see it). Software was PHD and the images were 1 sec taken with a SSAG. Is that an ok procedure, or should I do something different?

3. I made the process working with the Celestron f6.3 focal reducer. Would it be better to use the telescope at f10 or even at f20 with a barlow?

4. Using a star implies that I will have to wait for the next clear night. I could use a point light of the red illuminator of the finding eyepeice at low intensity inside the house, a few meters away of the telescope. Will this produce good results?

I know thes are a lot of questions but I really need some help on this matter.

Thank you in advance

Alfredo




1. NO.

2. Your goal is to center the star and observe the diffraction rings. If they are concentric, you are collimated. If they are squished on one side, you are not.

3. I generally recommend people collimate with the optical accessories they normally use in place.

4. It can. Usually it's easier with a star. A very small pinhole works best, but yes, you can do that.


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Ed Holland
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Reged: 06/16/10

Loc: San Jose, CA and Oxford, UK
Re: Help on collimating a C6 SCT please!! new [Re: rmollise]
      #5693211 - 02/21/13 05:43 PM

The reflection of the sun by a distant, shiny object can also be a substitute "star". I have used the reflection from a porcelain power insulator on a pole located about 1/8 mile away from our house.

This provided a very clear diffraction pattern for fine collimation. There is one potential difficulty, because the 'scope is likely to be horizontal when viewing such an object. Mirror shift may occur when the 'scope is elevated t view the sky, such that a final fine adjustment is needed. This last effect wasn't too pronounced on my C8, and the first attempt at collimation was very close indeed. It also has the advantage that one is not trying to work in the dark, and that the artificial star is free

Good luck,

Ed


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KerryR
Carpal Tunnel
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Reged: 12/05/07

Loc: SW Michigan
Re: Help on collimating a C6 SCT please!! new [Re: Eddgie]
      #5693352 - 02/21/13 07:11 PM

Quote:

This method (vernier) is not as accurate. If the central baffle and OTA are not concentric, or if there is some tilt in the system, it will not result in a properly centered secondary.

Another reason why the star test is so important. It is very sensitive and will allow for very quick and very accurate adjusment.

Not saying that the caliper won't work in all cases, becuase it might get you close.

But the star method can get you reliably close every time.

The reason I know is that I have encountered this before. My C5 can have the secondary perfectly centered using a vernier, but show decentering at the Poisson Point (with the resulting apparence of secondary mirror shadow decentering when out of focus). The only way to get it centered was using a star.

Not saying it is not worth a try to use this method, but I would rely on the star test to confirm it...




Agreed-- the star test method is more definitive. (That's why I suggested following up my method with the star test.)

The caliper method is probably a little easier because the ota points straight up, so it's very easy to nudge the corrector around by very small increments. And, of course, no star is necessary.

On my ota's, the calipers worked well enough that I can't detect the remaining error.

How'd you nudge the corrector small amounts, then hold it in place while testing?


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Eddgie
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Re: Help on collimating a C6 SCT please!! new [Re: KerryR]
      #5693484 - 02/21/13 08:19 PM

I think that if one has calipers of course it does not hurt to try it and follow up with star testing to confirm that the secondary centering is right.

It is actually a finicky measurement to make using a caliper though. But I agree that if someone has one available it can be a good place to start.

As for the corrector.. Yes, it can be very finicky to move the corrector. You have to put just enough pressure on the retaining ring so that you can still move the corrector.

I know that people will think I am a barbarian, but I have used a small rubber mallet in the past to tap it. In most cases it only needs to be moved a couple of millimeters.

About a year ago, I posted that I had to pull the corrector of my EdgeHD 8" because it had clouded up on the back. The EdgeHD has four radial screws that bear against the edge of the corrector. I had to back these out to get the corrector free.

Of course when I put it back together, it was no longer centered. It showed in the position of the Poisson Point.

The radial screws made it an absolute breeze to push/pull the corrector into position. And it took only a tiny amount of movement.

So, I agree that it can be a bit of a pain getting it right, but I believe that in the OPs case, he really doesn't have a choice but try moving it to see.

And if he can't (or does not want to) tap it into place, he can just use trial and error. Since the point moves in the direction that the secondary moves, he can adjust, tilt the scope up, loosen the ring, move, snug up, test and repeat.

I think that once he moves it the first time and sees the result, if he misses, he will know better how much to move it the second time.

I am sure that he is at least 2mm or 3mm out. Less than this and the baffle is not in the path of the light cone. The baffles are not that tight. Only if the secondary were off about this much would he see diffraction from the baffle.

Looking forward to his report.


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bremms
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Re: Help on collimating a C6 SCT please!! new [Re: Eddgie]
      #5693600 - 02/21/13 09:31 PM

Don't try and move the corrector without changing shims around the edge.
Under NO circumstance hit the secondary holder. In fact, unless you are experienced at mucking with these things don't try centering the corrector. You can check it with calipers. just worry about the secondary for now. most SCT only give good images when tweaked after set up. Bobs knobs are your friend


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zAmbonii
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Reged: 01/19/08

Loc: Ypsilanti, MI
Re: Help on collimating a C6 SCT please!! new [Re: bremms]
      #5693652 - 02/21/13 10:09 PM Attachment (46 downloads)

Sorry to piggyback on a thread, but after reading the thread I am wondering if I have a collimation problem, secondary out of center, or both. I think it is just collimation, but just wanted to see what you guys say.

I've had my C8 for about 3 years and for almost all of that time I have never collimated it. My Jupiter images were fine, so I never messed with it. Couple of weeks ago I attempted collimation because a star test didn't look as good as it should be. Last week I got a set of Bob's knobs and installed them.

When installing the knobs, the outside ring around the secondary seemed a little loose, when tightening the entire secondary seemed like it turned a little. Could this cause the secondary to get out of center or does the whole corrector plate need to move?

I had the scope out that night and tried collimating, it was way out of whack, and got it close. Was a little tough since the tube still hadn't completely cooled. When I slewed to Jupiter I could see Io had a couple of wings on it and you can see what the stacked image looks like at the bottom of this post. After I imaged Jupiter, I slewed to Betelgeuse and took a 10s video, stacked the star and applied a little sharpening. Betelgeuse is in the top right inside the square. I didn't nail collimation , but do you think there is any hints of the secondary not being centered? I wasn't expecting to see any type of arcs around Io. There is a lighter arc on the opposite side of the two brighter arcs....If I was in perfect collimation would all of those arcs disappear?

I was wondering about the secondary not being centered because of the possibility of the arcs being caused by the secondary getting into the light path off center and causing some diffraction.


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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Re: Help on collimating a C6 SCT please!! new [Re: rmollise]
      #5693710 - 02/21/13 10:35 PM

Quote:


4. It can. Usually it's easier with a star. A very small pinhole works best, but yes, you can do that.




Hi Rod

Thanks for your suggestion. I'll try if sky doesn't cooperate

Alfredo


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Eddgie
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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
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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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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
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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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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
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Re: Help on collimating a C6 SCT please!! new [Re: Eddgie]
      #5696965 - 02/23/13 04:52 PM Attachment (29 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
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Re: Help on collimating a C6 SCT please!! new [Re: Alfredo Beltran]
      #5696972 - 02/23/13 04:56 PM Attachment (39 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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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
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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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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
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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
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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
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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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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
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Reged: 08/01/10

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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
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Reged: 08/01/10

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Re: Help on collimating a C6 SCT please!! new [Re: Eddgie]
      #5699068 - 02/24/13 10:53 PM Attachment (27 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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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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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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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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Alfredo Beltran
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Reged: 08/01/10

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Re: Help on collimating a C6 SCT please!! new [Re: KerryR]
      #5700234 - 02/25/13 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


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KerryR
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Re: Help on collimating a C6 SCT please!! new [Re: Alfredo Beltran]
      #5700240 - 02/25/13 04:59 PM

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

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Alfredo Beltran
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Reged: 08/01/10

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Re: Help on collimating a C6 SCT please!! new [Re: KerryR]
      #5700283 - 02/25/13 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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