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my 17.5" Coulter ronhci pic TDE?

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#1 MessiToM

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Posted 06 April 2013 - 01:48 PM

I had trouble taking a picture. I couldnt get my dslr "close" enough and on top of that either tube currents or seeing was making some turbulence. It made the picture turn out with wavy lines but to the eye they were straight EXCEPT the turned down edge

What I could interpret (I think)Is a smooth/turned down edge. Again, lines were straight to the eye except toward the edge
Posted Image

#2 GlennLeDrew

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Posted 06 April 2013 - 01:52 PM

If this pic was taken inside focus, then there is most definitely turned down edge. And to a significant degree.

#3 MessiToM

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Posted 06 April 2013 - 02:08 PM

I sort of thought.

#4 kfrederick

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Posted 06 April 2013 - 04:08 PM

A ronchi at RC would tell me more .If the edge is bad easy fix .Mask it off .

#5 MessiToM

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Posted 06 April 2013 - 07:50 PM

Forgive my newbness to testing mirrors. "at RC". ? ^

#6 kfrederick

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Posted 06 April 2013 - 08:05 PM

Radius of the curvature It is twice the focal length of the primary

#7 MessiToM

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Posted 06 April 2013 - 09:50 PM

Oh ok. Primary would need removed wouldn't it?

#8 GlennLeDrew

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Posted 07 April 2013 - 04:00 AM

Testing at the radius of curvature adds the additional complication of curved Ronchi bands, due to testing away from the infinity focus conjugate, and will show the edge issue no better than we do now.

#9 Mirzam

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Posted 07 April 2013 - 07:51 AM

Can you give us a similar photo slightly outside of focus? Besides the edge, the correction looks a bit off. But this could be a thermal issue so be sure your mirror has had time to cool to the outside air temps.

JimC

#10 Mirzam

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Posted 07 April 2013 - 08:28 AM

Possibly worth mentioning is that you are testing the full optical system, so that any flaws present may not necessarily be caused by the primary.

JimC

#11 MessiToM

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Posted 07 April 2013 - 10:58 AM

Ok. Iam using a Coulter 4.25" secondary too. Just to be sure on my part. With the focuser racked all the way out advancing the ronchi ep in untill I only have about 4 lines is outside of focus. When I keep racking the ep in past the point of focus that is inside focus correct?

#12 Mirzam

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Posted 07 April 2013 - 12:57 PM

Yes. And to my eye the lines in the photo above do not look quite as hooked on the bottom as on the top. Maybe an indication of a diagonal edge problem?

JimC

#13 MessiToM

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Posted 07 April 2013 - 04:49 PM

Ok, Ive got a bit of a pickle. In looking through my pics on my camera card I cant remember what was inside or outside of focus

Iam "pretty" sure these first two pictures are OUTSIDE focus
Posted Image
Posted Image

Inside (pretty sure) focus and its not a good quality shot
Posted Image

So if my inside/outside focus is correct I think she is under corrected with maybe a turned down edge

#14 Mirzam

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Posted 07 April 2013 - 05:18 PM

It's pretty hard to see from your photos, but to me things do not look too bad. If the last photo could be a little lighter maybe the edge could be looked at more closely. Also, you need to be sure about the photos to determine if you are slightly over or under corrected. It's not off by a whole lot.

Does the scope come to a decent focus?

JimC

#15 DAVIDG

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Posted 07 April 2013 - 05:32 PM

Are these images taken at prime focus using a star as the light source ? If so this an insenitive test and 1/4 wave of error shows up as barely any curvature to the Ronchi lines. So to see the amount of curvature that is in your images, the turned edge and the spherical aberration is pretty large. If the images were taken outside the focus then the mirror is undercorrected by a fair amount. Typical of Coulter 17" mirrors.
An issue with the diagonal would show up as astigmatism and have not effect the showing a turned edge or spherical aberration of the primary. The Ronchi lines would be asymmetrical from the astigmatism which I'm not seeing.

- Dave

#16 Mirzam

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Posted 07 April 2013 - 05:50 PM

Hi Dave,
I understand that a convex or concave secondary will cause astigmatism at the focal plane. But what about a secondary that is mostly flat but with a bad edge? Specifically, if the secondary is collimated in such a way that one (bad) edge is involved with the light path but not the other?

JimC

#17 MessiToM

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Posted 07 April 2013 - 06:29 PM

Yes this was pointed at a star. Not prime focuse but eye piece projection. I held the camera up to the EP.

Yes stars focus just fine. I cant seem to eclipse about 180X on planets (mushy image. There is no astigmatism. I always let this primary cool down for a good wile and I run a rear exhaust fan and a foundry layer fan above the primary.

Also, next time it is clear I will re-do this test.

#18 Pinbout

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Posted 07 April 2013 - 07:29 PM

i modified your really dark ronchigram outside focus and it does show one side curve and the the other side straight.

Attached Files



#19 dave brock

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Posted 08 April 2013 - 01:45 AM

i modified your really dark ronchigram outside focus and it does show one side curve and the the other side straight.


His really dark image is supposedly inside focus.

Dave

#20 dave brock

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Posted 08 April 2013 - 01:55 AM

Ok, Ive got a bit of a pickle. In looking through my pics on my camera card I cant remember what was inside or outside of focus

Iam "pretty" sure these first two pictures are OUTSIDE focus

Inside (pretty sure) focus and its not a good quality shot

So if my inside/outside focus is correct I think she is under corrected with maybe a turned down edge


If you are correct with the inside/outside then you have a Turned Up Edge. I would have thought overcorrected with a Turned Down Edge is more likely. Either way, with those images it can't be undercorrected with a Turned Down Edge.

Dave

#21 Pinbout

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Posted 08 April 2013 - 06:59 AM

His really dark image is supposedly inside focus.



the file name is "outside_zpsfa8f0f56.jpg"

#22 kfrederick

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Posted 08 April 2013 - 07:05 AM

I would send Mark Harry the secondary to test . And go from there .Mainly use it and have fun .

#23 saemark30

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Posted 08 April 2013 - 11:45 AM

I don't understand how you can do a Ronchi with eyepiece projection, Where do you place the Ronchi screen relative to the eyepiece?
I can see adding adding a barlow and placing the Ronchi screen after the barlow close to the focal plane would work.

#24 GlennLeDrew

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Posted 08 April 2013 - 11:56 AM

I think it was actually afocal, with a lens on the camera and the Ronchi screen placed just in front of the camera lens.

#25 MessiToM

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Posted 08 April 2013 - 11:56 AM

I used a ronchi eye piece. Tuesday night I may have a chance at re testing.

I will know for sure then what my inside/outside images are like.






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