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C5 Star test horror (look at your own risk!)

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#1 Starman81

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Posted 23 March 2013 - 12:38 AM

Hi all, I recently received a C5 in a seemingly favorable trade. It was kinda banged up, a couple dents on the tube and what looked like some glue at the edge of the corrector plate. In a rush, I didn't ask too many questions and took the scope. Star testing it tonight, I will admit, I haven't star tested SCT's much but from my quick research I see that I had to pick a bright star and crank up the power and defocus and study the patterns, pretty much same as on a newtonian. The scope was decently cooled down (~45 minutes), but bright stars were hit or miss with the recurring partial cloud cover. I did try Sirius which was too low in the sky and then Capella and I could already tell that getting pinpoint stars was not happening, not even at 63x. Bright stars even at this low power were exhibiting a triangle-like shape even at focus. The defocused star image was showing broken diffraction rings, just scary looking (see below)! For high power testing, I settled on Polaris for convenience. At 63x, even with the highly non-pinpoint stars, the scope was able to split the Pole star. At 250x, the defocused star image showed an 'aberration' that I don't think I have ever seen in the typical pictures for aberrations (see next post). What do you guys think? Is it salvageable? Or should I have asked for ScopeFax report (duh)? :question:

Obviously, I should have been a little suspicious when the other party was willing to part with the scope for 1/2 the trade value! :foreheadslap:

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#2 Starman81

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Posted 23 March 2013 - 12:40 AM

Obviously, these crude drawings are not super accurate, I'm just trying to draw attention to the 'aberration' in each drawing. Please do not pay attention to the concentricity of the diffraction rings or the lack thereof and spacing between rings.

Polaris defocused quite a bit more.

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#3 dweller25

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Posted 23 March 2013 - 01:34 AM

Well if you are sure the scope was cooled down....

You mention a triangular shaped star - that sounds like pinched optics and I would guess it's something to do with the glue around the corrector.

#4 wh48gs

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Posted 23 March 2013 - 06:37 AM

Looks like a portion of optical surface in the optical train is displaced vs. rest of it. Worst-case scenario, it could be due to a large internal crack in the glass. Also, the glue could be causing it, so it should be removed. Areal contamination/damage to one of the surfaces should be ruled out.

#5 Cotts

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Posted 23 March 2013 - 07:52 AM

Your first picture looks very much like a heat plume.

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#6 Eddgie

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Posted 23 March 2013 - 08:58 AM

The scope is uncollimated.

There appears to be a heat plume. It could be a trench zone in one of the mirrors, but I think this is unlikely.
Just mark the point where it is showing by sticking your finger in front of the aperture so that it alignes to the pattern. Now, flip the scope to another bright star and check the position. If it moves away from were you originally observed it, it is a heat plume.

The lines would appear to be diffraction artifacts. Check for spider webs in the baffle or tube.

The only other thing that will cause these kinds of lines are trench zones (rarely appear on commercial SCTs with enough intensity to show in the star test) or scratches in one of the mirrors.

The heat plume will go away with more cooling. It can take hours if the temps are falling fast.

The mis-collimation is doing more damages than the lines you see.

The lines though should be investigated. Look for spider webs or scratches.

#7 davebl

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Posted 23 March 2013 - 06:32 PM

You should probably pick a star that is not a double to star test with. As you defocus on a double star, the diffraction rings from both stars can easily interfere with each other giving you a lopsided set of diffraction rings.

#8 Stelios

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Posted 23 March 2013 - 07:34 PM

45 minutes is not enough of a cool down. The top picture is a classic heat plume, and can happen to perfect optics. When the scope cools down, it'll disappear.

However, I have no clue what the horizontal lines in the 2nd picture are--I would check your optical train for scratches or maybe some foreign material left inside. Never seen such before.

Also, try to pick a 2nd or 3rd magnitude star that is *not* a double to collimate and/or star-test on. Very interested in what the lines in picture 2 turn out to be!

#9 kansas skies

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Posted 23 March 2013 - 08:01 PM

I'm sure it's a long shot, but the second picture looks like something was between the scope and the star. I have some wires that are fairly high above my viewing location that will interfere and cause a certain "soft" look to objects in the field of view. When I defocus, I will see something similar to what you have drawn. If that's not the case, I would think spider webs inside the tube as mentioned might cause the same effect.

Bill

#10 jrbarnett

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Posted 23 March 2013 - 08:11 PM

That's a typical heat plume star test in the slightly defocused graphic.

The next graphic could be bad seeing or could be terrible optics. Triangular stars, though, suggests pinching somewhere; probably the corrector. The weird hashes in the second graphic may also suggest something in the baffle tube (like spider webs or similar).

I too have a C5. It has pretty mediocre optics. Sometimes it's the luck of the draw. That said, I would say that *any* SCT that looks beat up and has a few dents should probably trade at half the typical used price, even with decent optics. I can't abide by gear that is not taken care of. There's so much well-kept stuff out there, you can find most items in near new condition at very low prices if you're patient.

Take a daytime look up the empty baffle from the rear and see if there's anything in the light path. Also see if there's anything on the secondary or if the secondary is cracked.

Sorry for the dud, but maybe it's salvageable.

- Jim

#11 Eddgie

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Posted 24 March 2013 - 04:02 PM

There is one other possibility with respect to the lines in the defocused pattern.

Celestron uses float glass for the corrector. This glass is ground and polished on one side, but it appears that the other side is not.

As a result, there are cases where lines in the unfinished side of the glass (to subtle to be seen visually or felt) can be seen in the star test.

But in the cases where I have seen it, the lines have always been parallel. That does not be the case with the drawing provided here.

Here is a link to a page from Astro-Foren that shows this condition.

And it can indeed show in the star test. About two years ago on this same forum, someone noticed them in his star test and I directed him to the same forum as I am including here now. This is not Star testing, but I have ineed seen it show up in star testing done by Rohr at Astro-foren...

Again, these are almost always parallel though, but in this image you might see one where the lines are not quite parallel.

Float glass induced lines

#12 Eddgie

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Posted 24 March 2013 - 04:09 PM

Ah, here you go. Here is a example of how the float glass ripple can show up in the star test.

Everyone reading this needs to relize that the star test takes no prisoners. It shows every error no matter how small, and just because you can see an error doesn't mean that there is significant damage coming from that error.

Note that the testing on this C11 showed a Strehl of .95, so even with some minor defects like the float glass ripples, the effect is mostly in this case cosmetic. .95 Strehl is to me in the excellent catagory.

But here is what it can look like in a star test:

Float ripples in the star test

#13 Eddgie

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Posted 24 March 2013 - 04:20 PM

Another followup.

In my first note, I suggested thermal plumes as the most likely cause, and I believe that.

I also desribed how to diagonse it by making the direction and flipping the scope.

If one were to see that the error stayed in the same orientation, then the cause would be a trench zone. This is uncommon in SCTs but not at all unheard of.

In fact, there is a scope in the first link I showed a couple of posts above this where I showed examples of float glass ripple.

In those images are a couple of scopes what show distinct trench zones.

These tests (Lyot and Foucault) are like the star test, horribly sensitive, and can make a mole hill look like Mt Olympus. That doesn't mean there are no errors there. It means that the often do not do meaningful damage if they appear alone.

It has always been the case (in my experince) that most SCTs do not possess a single fatal error. Usually, it is the combined effects of many small errors that when acumulated together, can cause a given sample to appear to give a less than fully satisfactory image.

Anyone that wants to get really serious about star testing should read Suiters book on the topic, and if they don't have a lot of scopes to test, they can practic by using Astro-Foren. Once you have read Suiter's book, you can look at test after test on Astro-foren and see if you can diagonose the problems with each instrument tested before looking at the final results.

And what you will find out is that when done properly, star testing can indeed tell you a great deal about the errors in a given instrument.

A bit off topic because as I said in my first post, I think they problems are collimation, a thermal plume, and maybe spider webs or something, but the possibility exists that there are real errors on the glass, and my intent here is to show what star testing can show you.

You know the old saying: "Just because you're paranoid, it doesn't mean people aren't out there tyring to get you."

Well, just because your diffraction circles are round doesn't mean that your scope is free of optical problems.

Everyone with a telescope should learn the proper methods for star testing.

#14 GlennLeDrew

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Posted 24 March 2013 - 05:05 PM

I suspect the float glass ripples are not so much due to surface errors but instead are variations in refractive index due to a not ideal annealing or material uniformity. I'm pretty sure these correctors are fully worked on both sides; if one side were left 'as is', the optical harm would pretty surely be awful.

#15 Eddgie

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Posted 24 March 2013 - 05:17 PM

Well, I am sure you are correct. I did not know the exact mechanism whereby the abberation was being introduced.

Have seen it though, and the exact mechanism is not so important I guess as the potential to diagnose the problem exists.

This rarely shows up with enough effect to be easily seen though. I have looked at a lot of star tests and not ever seen it in person at the eyepeiece.

But I have seen images that show it on Astro-Foren and the tests showed it was coming from the corrector, so we know that it is a possiblity in this case.

There was also an instance maybe a year or two ago where somone posted a picture of these here on CN taken from a C11 wondering what they were.

Almsot always though the lines have been much more parallel though at least one image from the above links hinted at a sample where the lines were converging.

#16 orion61

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Posted 25 March 2013 - 09:11 AM

#1 Heat Plume,
#2? ever looked in the baffle tube for Spider webs?
#2.. power lines/tree limbs EXTERNAL...are possible (good catch on that one!)
If the optics have been removed and the origonal sealer reused,(Reddish Orange, rubbery looking) the primary may have been rotated and the retaining ring overtightened, the SN of the primary will be written in Blue marker on the back of the Primary, which will be horizontal 9:00 to 3:00.
Usually the SN# will be on the focuser side of the Mirror holder. You would have to be pulled apart to check.
There should NOT be any adhesive on the Corrector.. if there is
all bets are off.... good luck!
Larry

#17 spencerj

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Posted 25 March 2013 - 10:31 AM


Also, try to pick a 2nd or 3rd magnitude star that is *not* a double to collimate and/or star-test on. Very interested in what the lines in picture 2 turn out to be!


+1 on the first image being a heat plume in a scope that needs collimation.

While I agree that a double star is not typically a good candidate, I use Polaris a lot (exclusively on a scope with an undriven mount). Sure it is a double, but the second star is faint and far enough away that it does not impact the star test.

#18 Starman81

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Posted 25 March 2013 - 06:01 PM

Thanks to all for the valuable feedback. I do admit that the scope was not adequately cooled and it was probably 'chasing thermals' even towards the tail end of the session.

So the next night I left it out for a good hour to hour and a half before doing my testing. Beforehand, I checked the baffle tube for spider webs or any other debris, but did not find anything. I made sure not to use a double star to confound the issue, so I used Arcturus as it was ~55-60 degree high at that time since I was out quite late at night (1:30am - 3:30am). Stelios, you suggest using a mag 2-3 star--how come? I thought that you should try to choose the brightest star that's reasonable high in the sky, so I went with Arcturus (mag 0.2)... Anyways, at 63x the scope gave a decently tight image of the star, but still not perfect. The star had a little arc-like appearance to it. Slightly defocusing this time around, did not produce that same artifact that has been noted by many of you as a heat plume. There was some uneveness in the rings though, I wish I had maybe sketched it, but overall things were looking better.

Upping the power to 250x and defocusing a lot more on the star, those weird lines that were showing up were nowhere to be seen! Maybe some air got in there and displaced some unseen cobwebs--I don't know. With the star in the center of the FOV highly defocused, the secondary shadow was reasonably well-centered.

I then took down the power to a much more manageable 125x to see how 'pleasing' the view of Saturn (which was about 30* high at this point) and the Moon would be. It was nearly 3am at this time and the air had settled and the seeing improved. I was very satisfied with the view of Saturn in these conditions at 125x; banding was apparent on the planet and a tiny moon or two visible. The CD was not glimpsed but overall the image was very decent. Ditto for the Moon at 125x. I know the stellar views could not be near perfect, as focusing the scope did not bring them to nice little points, but as far as the Moon and Saturn were concerned, they were satisfactory.

I need to read up more on star testing SCTs and interpreting the results of the defocused star images. Next time out, I will 'sketch' my findings and present them here.

#19 Stelios

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Posted 25 March 2013 - 06:49 PM

I must admit I was parroting what I learned from Ed Moreno's excellent "Guide to SCT collimation" on that "other site". He recommends a 2nd to 3rd magnitude star and a 6mm to 8mm EP.

I note that others differ; Thierry Legault suggests a 0 to 1 magnitude star. Our own Uncle Rod advocates a 'medium bright star'. So two out of three advocate a 2nd/3rd mag star. Personally I like that because I find 0/1 mag stars rather blinding when only slightly defocused (as they should be).

Incidentally, the arc-like appearance sounds like mis-collimation to me. I suspect that when you get the collimation perfect, you will be totally wowed by your scope.

#20 Starman81

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Posted 25 March 2013 - 08:11 PM

I must admit I was parroting what I learned from Ed Moreno's excellent "Guide to SCT collimation" on that "other site". He recommends a 2nd to 3rd magnitude star and a 6mm to 8mm EP.

I note that others differ; Thierry Legault suggests a 0 to 1 magnitude star. Our own Uncle Rod advocates a 'medium bright star'. So two out of three advocate a 2nd/3rd mag star. Personally I like that because I find 0/1 mag stars rather blinding when only slightly defocused (as they should be).

Incidentally, the arc-like appearance sounds like mis-collimation to me. I suspect that when you get the collimation perfect, you will be totally wowed by your scope.


Thanks for the follow up Stelios and the tip to check out Ed's guide--I'll definitely do that. I haven't used my Nexstar mount in several months but it will prove very useful with its tracking capability to keep the star centered while doing the collimation.

#21 spencerj

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Posted 27 March 2013 - 03:39 PM

If seeing is not that good or if you are not trying to get perfect collimation, then the magnitude of the star does not matter.

For the first round of collimation, the brightness of the star is not that big of deal. But after that when you are trying to really fine-tune collimation on an IN-FOCUS star, you really need something that is not super bright.

You are looking for an airy disk surrounded but a well defined diffraction ring. If a star is too bright, the airy disk and first diffraction ring just get blurred together and you have a bigger, fuzzy star.

#22 Starhawk

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Posted 27 March 2013 - 09:49 PM

Star test generates new false indictment on scope, yet again.

Consider looking at something in focus. My own C5 does best with a sharp collimating and sharp focus.

-Rich

#23 Starman81

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Posted 28 March 2013 - 11:32 AM

If seeing is not that good or if you are not trying to get perfect collimation, then the magnitude of the star does not matter.

For the first round of collimation, the brightness of the star is not that big of deal. But after that when you are trying to really fine-tune collimation on an IN-FOCUS star, you really need something that is not super bright.

You are looking for an airy disk surrounded but a well defined diffraction ring. If a star is too bright, the airy disk and first diffraction ring just get blurred together and you have a bigger, fuzzy star.


That makes a lot of sense--thanks Jason.

#24 Starman81

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Posted 28 March 2013 - 11:36 AM

Star test generates new false indictment on scope, yet again.

Consider looking at something in focus. My own C5 does best with a sharp collimating and sharp focus.

-Rich


Well... Maybe, maybe not. Like I said, stars are not yet very close to pinpoint just from getting the scope cooled down properly; it'll have to wait for some proper collimation. The somewhat 'banged-up' condition of the scope and the glue around the corrector plate still have me concerned.

#25 Geo.

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Posted 29 March 2013 - 07:19 PM

Looks OK to me you'll just have to tune it. If this is a post 93 C5+ the land for the corrector is excessively large and difficult to keep centere(see discussion at http://tinyurl.com/co552v8 )






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