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Help on collimating a C6 SCT please!!

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#1 Alfredo Beltran

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Posted 20 February 2013 - 10:31 PM

Hi

I was trying to collimate my C6 SCT and got everything worse :foreheadslap:. 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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#2 Alfredo Beltran

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Posted 20 February 2013 - 10:34 PM

Here it is defocused:

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#3 rmollise

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Posted 21 February 2013 - 07:37 AM

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

#4 Alfredo Beltran

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Posted 21 February 2013 - 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

#5 Eddgie

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Posted 21 February 2013 - 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.

#6 spencerj

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Posted 21 February 2013 - 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.

#7 REC

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Posted 21 February 2013 - 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:)

#8 Alfredo Beltran

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Posted 21 February 2013 - 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! :foreheadslap: 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

#9 KerryR

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Posted 21 February 2013 - 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.

#10 Alfredo Beltran

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Posted 21 February 2013 - 11:23 AM

Hi Kerry

This is a very good option. Thanks for sharing.

Best regards

Alfredo

#11 Eddgie

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Posted 21 February 2013 - 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.

#12 Eddgie

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Posted 21 February 2013 - 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...

#13 Eddgie

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Posted 21 February 2013 - 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.

#14 rmollise

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Posted 21 February 2013 - 01:07 PM

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.

#15 Ed Holland

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Posted 21 February 2013 - 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

#16 KerryR

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Posted 21 February 2013 - 07:11 PM

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?

#17 Eddgie

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Posted 21 February 2013 - 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.

#18 bremms

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Posted 21 February 2013 - 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

#19 zAmbonii

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Posted 21 February 2013 - 10:09 PM

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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#20 Alfredo Beltran

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Posted 21 February 2013 - 10:35 PM

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

#21 Eddgie

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Posted 22 February 2013 - 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".

#22 zAmbonii

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Posted 23 February 2013 - 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.

#23 Eddgie

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Posted 23 February 2013 - 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.

#24 KerryR

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Posted 23 February 2013 - 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...

#25 zAmbonii

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Posted 23 February 2013 - 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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