#41  
Old 30-08-2013, 07:57 AM
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Hi Josh,

Do you use CCDInspector? It might tell you something your tests up to now are not seeing and could be worth a go. There is a free trial if you don't yet own this program. If you shoot a nice star field it will give you field curvature info as well as optical alignment. Another function will give you a plot of star aspect ratio over the whole field. It would be easy to compare shots in different portions of the sky. This "might" give you something more to work with.

Peter
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  #42  
Old 30-08-2013, 02:06 PM
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Thanks Peter, yes i have ccdinspector and i can give it a go.
I spoke to Dave Rowe last night and he's given me some tests to do so he can see further whats going on.

Josh
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  #43  
Old 30-08-2013, 03:31 PM
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Out of interest, is the star in the guide chip round?
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  #44  
Old 30-08-2013, 03:34 PM
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I dont think so for memory, but i will confirm (or not) that.
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  #45  
Old 06-09-2013, 10:35 AM
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Confirmed that the star on the guide chip is the same shape as the star on the imaging chip. Ive been speaking to Planewave and with the test images Ive provided them with, they said as the scope slews across the sky from one horizon to the other, there is a minute amount of focus shift. this occurs from the optics moving appart or closer together which is magnified with the secondary mirror amplification, this produces some astigmatism in the image as the image goes out of focus slightly (about < 100micron) and hence the oval stars.
They recommend to refocus in various parts of the sky.
Has anyone else experienced this?
Im going to reenforce my secondary support system and see if that makes a difference.

Josh
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  #46  
Old 06-09-2013, 02:36 PM
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Quote:
Originally Posted by Joshua Bunn View Post
They recommend to refocus in various parts of the sky.
...
Im going to reenforce my secondary support system and see if that makes a difference.

Josh
That doesn't sound right Josh. What they've provided is a workaround, not a solution. A scope of this calibre should hold the optical train rigidly regardless of the azimuth or altitude it's pointing.

Is it the secondary support system moving or the slack on the hedrick focuser? I can understand refocusing the focuser if there was slack, but refocusing due to secondary support problems does not make sense. The fact that the primary and secondary are fixed in the CDK12.5 make me think its the focuser.

Granted, we refocus our telescopes at lot due to temperature or filter changes, but to correct for optical alignment problems is pushing beyond what is 'normal' imho.

I don't think I'd be taking the problem into my own hands and re-enforce the secondary myself. Box it up and ship it back.
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  #47  
Old 06-09-2013, 03:20 PM
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Thanks Jase,

I agree, it should hold. I recently put some more tension on the spider vein screws so i will see if that makes a difference.

having to refocus due to temperature changes - yes, but not focuser movement, thats not right. i mean the focuser is always pulled down by gravity.

im going to do what PW say and refocus where i want to image and see what the results are. There are also 2 alen head bolts on the focuser that are used as hard stops for focuser travel. im going to clamp them down lightly on the focuser draw-tube so it cant move and try the images again.

Jase, i really cant be sure whats moving, the secondary support or focuser. The above paragraph should eliminate the focuser and i was going to strengthen the secondary supports so i could tighten the spider veins more, but i can see you point, and for now, you probably right. however i can see where the secondary could do with some strengthening just to totally eliminate the secondary - but for now i will leave it.

there is evidence of astigmatism in the scope. see the zipped folder bellow for the document which shows a series of images on one side of focus, through focus then to the other side. what this is due to is another matter. it could be from shifting collimation from the secondary as i slew to a low altitude. I should perform this test at the zenith hey.

PW said its probably less than 1/4 wave astigmatism but hard to tell with the test images i provided. they would need to have it in their workshop to be certain of a result.

Josh
Attached Files
File Type: zip defocus checks2.zip (130.1 KB, 2 views)

Last edited by Joshua Bunn; 06-09-2013 at 03:34 PM.
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  #48  
Old 06-09-2013, 04:14 PM
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Originally Posted by Joshua Bunn View Post
Confirmed that the star on the guide chip is the same shape as the star on the imaging chip.
Josh
This 100% supports the fact that it is not guiding related. I think the astigmatism idea fits all the criteria.
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  #49  
Old 06-09-2013, 04:17 PM
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Originally Posted by Joshua Bunn View Post
it could be from shifting collimation from the secondary as i slew to a low altitude. I should perform this test at the zenith hey.
Could well be Josh. I thought that may well be the case if the secondary was spring loaded (more tension needed), but referencing documentation highlights that the 12.5" and 17" are not, only the 20" is has a spring loaded secondary. http://planewave.com/wp-content/uplo...re-v112712.pdf

I agree it would be good to perform the tests at zenith.

Given mirror spacing is critical for this design, one would expect it to hold collimation extremely well.
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  #50  
Old 07-09-2013, 02:35 PM
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Thanks Peter and Jase,

last night i ran some more tests and for a moment there i thought i found the issue. When i turned the cooling fans off on the scope, the elongation went (in a few images) then when i turned them on, (without slewing) the elongation re appeared. this was rather repeatable. However, then the elongation re appeared with the fans off. So that was like ....aahhhh, taking candy from a child.

I do believe however that running the fans made it easier to collimate while in focus. Dont know why but star shapes just settled down a little. weather this was due to vibrations or tube currents??? the scope had been cooling for hrs aswell before i found this out.

I think my collimation maybe shifting somewhat judging by the images i get from my defocused stars that i periodically check. however that sensitivity varies depending on how defocused you are, so i need to go back to the same position all the time.

I did the test for astigmatism at the zenith and there is some evident as the scope gets close to focus (focus is around 18400). here is a dropbox folder with images at the zenith in the middle of the chip (file name is the focus possition). what does one make from these. Has anyone with anothe PW CDK seen similar results?

I did tighten up the spider veins before i started last night and some images were better, but still got some bad ones and in particular a horrendous one after flipping the meridian while pointing at the SCP. this is it here. its only 10 sec so there isnt much data.

So in summary, i need to talk with PW agian. Im of the opinion something is moving and its not always repeatable - the focuser or mirror??? and i need to decide weather to send it back (after talks with PW) or work on it myself. the 12 month waranty has passed by nearly 2 months, but ive been working on this for 4 months, so ... i dont know. It would be really good to have another focuser that i knew would be sollid so that part of the train could be eliminated. I can flex the current focuser with some slight hand force, but thats pretty subjective.

thanks
Josh
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  #51  
Old 07-09-2013, 02:43 PM
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Originally Posted by Joshua Bunn View Post
the 12 month waranty has passed by nearly 2 months, but ive been working on this for 4 months, so ... i dont know. It would be really good to have another focuser that i knew would be sollid so that part of the train could be eliminated. I can flex the current focuser with some slight hand force, but thats pretty subjective.

thanks
Josh
I would not have thought you would have any warranty issues with PW, surely they would rather have a happy customer producing images for others to aspire too than an angry one telling people his scope is flexing all over the place with the images to back it up . Goodluck getting it fixed, I understand how frustrating it must be.

As for the fans, wouldn't have thought it would make an elongation in one direction maybe blurred the stars slightly but not along one axis. Ofcourse if it fixes the problem I am happy to be wrong.
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  #52  
Old 07-09-2013, 06:12 PM
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Quote:
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I would not have thought you would have any warranty issues with PW, surely they would rather have a happy customer producing images for others to aspire too than an angry one telling people his scope is flexing all over the place with the images to back it up . Goodluck getting it fixed, I understand how frustrating it must be.

As for the fans, wouldn't have thought it would make an elongation in one direction maybe blurred the stars slightly but not along one axis. Ofcourse if it fixes the problem I am happy to be wrong.
Thanks Peter,

I would think the same. PW have been great to talk to. its just about acknowledging that i have a problem here. thats probably the next step unless they ask for some more tests.

the fans, well yes i wouldnt have thought the fans would introduce odd star shapes, but... Bellow are some examples of what im seeing. Hover over the image for the description. One set was in the south and the other in the north, no slewing or focusing was done between the south and north sets. They were all within minutes of each other.

thanks
Josh
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  #53  
Old 08-09-2013, 07:00 AM
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There's a point and you decide when it has arrived, when you ship the scope back at their expense and say please fix or replace. To have a solid case for that you will need to have done a thorough and reasonable set of tests to show its not user error but manufacturing error and that the error is not able to detected out in the field.

I think you are close to this point if not already there.

As you say obviously something is moving. Did you try to replicate the elongation by putting some light pressure on the focuser? I don't recall having any movement in my focuser.

Greg.
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Old 08-09-2013, 07:55 AM
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Hi Josh,
I really don't think it has anything to do with the focuser or image train behind the focuser. A tilted focal plane can only cause defocus by itself. If other aberrations such astig are present they can be more easily observed under defocus, however a tilted image plane will have an axis in focus somewhere and in front of and behind focus elsewhere. You will not see the same aberration pattern across the entire image.

If the scope can deliver good images near zenith then the optics are probably ok, that is good news. As you slew lower in altitude things go pear shape literally, star dispersion increases making then look like little rainbows (try it with a OSC camera), the dispersion axis vertically aligned, this is really obvious at altitude below 30ish degrees. What becomes difficult to judge is the relative contribution of dispersion compared with OTA misalignment, and how much you're willing to put up with. High resolution imaging can't be done below 30 deg altitude, so maybe that's a bad place to be testing things. But the image should hold up well between 30 and 45deg

Best,
EB
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  #55  
Old 08-09-2013, 09:53 AM
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Quote:
Originally Posted by ericwbenson View Post
As you slew lower in altitude things go pear shape literally, star dispersion increases making then look like little rainbows (try it with a OSC camera), the dispersion axis vertically aligned, this is really obvious at altitude below 30ish degrees. What becomes difficult to judge is the relative contribution of dispersion compared with OTA misalignment, and how much you're willing to put up with.

Would have thought chucking a narrowband filter in the train would test this theory pretty thoroughly. If it is an effect where the colours are being smeared at low altitudes then the easyest way to test that would be to filter all but a small range of the colours out.

Get your narrowest filter and take an image low on the horizon, any elongation in that image has to be from the scope.
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  #56  
Old 08-09-2013, 12:22 PM
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Thanks Guys, appreciate your comments.

Quote:
Originally Posted by gregbradley View Post
There's a point and you decide when it has arrived, when you ship the scope back at their expense and say please fix or replace. To have a solid case for that you will need to have done a thorough and reasonable set of tests to show its not user error but manufacturing error and that the error is not able to detected out in the field.

I think you are close to this point if not already there.

As you say obviously something is moving. Did you try to replicate the elongation by putting some light pressure on the focuser? I don't recall having any movement in my focuser.

Greg.
Greg, i didnt put pressure on the focuser but i did loosen off the bearing block and nothing changed in terms of star elongation, it was elongated tight and the same elongation loose, although the image just shifted sideways. I would agree with Eric's comments bellow about the focuser.
Your scope has a bigger focuser than mime although its the same design.

Quote:
Originally Posted by ericwbenson View Post
Hi Josh,
I really don't think it has anything to do with the focuser or image train behind the focuser. A tilted focal plane can only cause defocus by itself. If other aberrations such astig are present they can be more easily observed under defocus, however a tilted image plane will have an axis in focus somewhere and in front of and behind focus elsewhere. You will not see the same aberration pattern across the entire image.

If the scope can deliver good images near zenith then the optics are probably ok, that is good news. As you slew lower in altitude things go pear shape literally, star dispersion increases making then look like little rainbows (try it with a OSC camera), the dispersion axis vertically aligned, this is really obvious at altitude below 30ish degrees. What becomes difficult to judge is the relative contribution of dispersion compared with OTA misalignment, and how much you're willing to put up with. High resolution imaging can't be done below 30 deg altitude, so maybe that's a bad place to be testing things. But the image should hold up well between 30 and 45deg

Best,
EB
Eric and Peter,
All my test images have been around the 30 - 40 degrees altitude range, and all of them have been with a red filter to minimize the chromatic dispersion, is this sufficient?? I was told to do my tests with a red filter by PW. I could try with my 5nm Ha though.

thanks
Josh
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Old 08-09-2013, 12:34 PM
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Originally Posted by Joshua Bunn View Post



Eric and Peter,
All my test images have been around the 30 - 40 degrees altitude range, and all of them have been with a red filter to minimize the chromatic dispersion, is this sufficient?? I was told to do my tests with a red filter by PW. I could try with my 5nm Ha though.

thanks
Josh
Cant hurt to try, I mean your 5nm HA filter is essentially a "red" filter anyway. It just lets through less red!
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Old 08-09-2013, 12:37 PM
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yes, quite right Peter.
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Old 09-09-2013, 04:00 PM
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Well, last night i took some images and to my supprise, the optics produced round stars pointing low in the sky. First i used the Ha filter, things looked good. then i switched to the red filter and there was no change from the Ha. I think the Lum also produced round stars.

So im a little confused about whats going on here - i didn't change anything on the scope. I determined my collimation held from one horizon to the other by slightly defocusing just enough so i could see the poisson spot in the centre. this remained centre from one side of the sky to the other. that's fantastic news. see bellow. one at zenith, one at low south and one at low north. beats me why they wont all go as thumbnails?

Does anyone think it could be something to do with atmospherics? there are no buildings around me, i shoot out in the bush.

thanks
Josh
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