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CA at native f/10 on EdgeHD 800

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#1 gfunkernaught

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Posted 12 October 2021 - 07:42 PM

I've been doing my best trying to collimate this scope and I have my camera's sensor 133.5mm away from the edge of the rear baffle thread.  For some reason I cannot get rid of or correct chromatic aberration in the image.  See the image below.  Any ideas?  Does being .15mm too far have that much of an affect?
 
Hopefully this image is hi res enough to see the color fringing.
PREVIEW 20211012 20h17m02s038ms

 

 


Edited by gfunkernaught, 12 October 2021 - 07:48 PM.


#2 Migwan

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Posted 12 October 2021 - 08:30 PM

Don't know, but it looks something like vignetting. 



#3 luxo II

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Posted 12 October 2021 - 08:39 PM

 

Does being .15mm too far have that much of an affect?

Not on chromatic aberration, no. SCT's do have a trace of spherochromatism (the corrector is one element and not achromatic) if you look really closely but it shouldn't be objectionable.

 

I see slight vignetting, though.


Edited by luxo II, 12 October 2021 - 08:44 PM.


#4 gfunkernaught

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Posted 12 October 2021 - 10:45 PM

So this spherochromatism is very obvious when I have a bright star centered on the frame.  I was using Vega to collimate and when I got it into focus, there was very obvious blue and red fringing on both sides (left and right) of the star.  I know that image I uploaded isnt a good example.  I ended my session for the night because I cannot get good stars at all.  They're all elongated.  I take short, high ISO preview shots and they look like small comets all pointing in the same direction.  I took 3sec 40000ISO shots.  Could it be that the mount (AVX) can't keep the image stable for 3 seconds?



#5 luxo II

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Posted 13 October 2021 - 02:56 AM

Spherochromatism… since the index of the corrector isn’t the same for all wavelengths the degree of correction it provides for spherical aberration varies slightly, by wavelength and the aberration is the same across the field of view.

On-axis a bright star shows a slight colour-tinged halo. Off-axis the halo may be slightly displaced radially from the centre of the star.

In a typical Celestron or Meade it’s visible with a 7mm eyepiece.

The only SCT I’m aware of that didn’t show it was a 48” at Siding Spring, for which the corrector was an achromatic doublet.

Edited by luxo II, 13 October 2021 - 03:00 AM.


#6 gfunkernaught

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Posted 13 October 2021 - 08:31 AM

I thought the edge was supposed to provide a flat field. If the colors are separating, forming odd shaped stars, that isn't flat is it? Could spacing be an issue? Maybe 133.5mm isn't right for my setup.

#7 luxo II

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Posted 13 October 2021 - 08:41 AM

Let's just say many SCT's including the EdgeHD aren't as perfect as some here think, when you start to look closely. Tinkering with spacing is not going to cure what you're seeing.

 

A lucky few have good SCT's, many are "ordinary" and some are real dogs; your pic suggests yours is "ordinary".

 

The only scopes that show no chromatic aberrations - none whatsoever - are pure reflectors such as Newtonians, classical classes, or RCs.

 

As soon as you add any refractive element - a corrector or lens - including reducers, flatteners or coma correctors - you will have chromatic aberrations to some extent. 

 

APO's are just reflector-wannabe's. The moment anyone asks about polychromatic strehl the refractor types go very silent.

 

There are scopes that solve this dilemma, but not at amateur pricing. Add a nought - or two.


Edited by luxo II, 13 October 2021 - 08:53 AM.


#8 gfunkernaught

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Posted 13 October 2021 - 10:52 AM

I noticed this effect in my ed80. Without a flattener, there is no CA. With the flattener CA is visible, but not as bad as my edge 8. Guess I should have lowered my expectations, coming from a standard sct.

But other than the color artifacts, I'm still seeing issues with star shapes, which I'm assuming has to do with spacing. Hopefully I can find the right spacing to get round stars. These oval stars are driving me nuts.

Edited by gfunkernaught, 13 October 2021 - 10:54 AM.


#9 gfunkernaught

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Posted 13 October 2021 - 04:56 PM

Here is a comparison between my old 8SE+celestron f/6.3 reducer on the left and the EdgeHD 800 at f/10 native  on the right.  I could see small, negligible artifacts being a thing, but it is too obvious on the Edge.

Attached Thumbnails

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Edited by gfunkernaught, 13 October 2021 - 04:56 PM.


#10 luxo II

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Posted 13 October 2021 - 09:16 PM

I'm also intrigued by the flares from the bright star, in the Edge.


Edited by luxo II, 13 October 2021 - 09:16 PM.


#11 Rasfahan

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Posted 14 October 2021 - 12:50 AM

Here is a comparison between my old 8SE+celestron f/6.3 reducer on the left and the EdgeHD 800 at f/10 native  on the right.  I could see small, negligible artifacts being a thing, but it is too obvious on the Edge.

I‘ve look very hard at the EdgeHD scopes in the past for AP and decided against them but not because of their native optical qualities. I‘ve seen quite a bit CA in all images with the reducer but not native, so I think something is wrong with your scope. Especially since you say the star shapes are off. You sure you‘re collimated correctly? Can you post a defocused star image on axis? The flares look very strange, too, I‘ld inspect the coatings on the corrector plate and the in-baffle corrector. If the scope is new, take it up with the vendor and return.



#12 gfunkernaught

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Posted 14 October 2021 - 07:41 AM

Unfortunately it is not new. I recently bought it used from CN classifieds. I made sure to collimate the best I could. I even checked the donut at different levels of defocus. I already packed the thing up since I don't want to deal with it because it sounds like something is definitely wrong and could end up costing more to fix. Looks like the person I bought it from felt the same.

#13 Borodog

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Posted 14 October 2021 - 09:19 AM

Here is a comparison between my old 8SE+celestron f/6.3 reducer on the left and the EdgeHD 800 at f/10 native  on the right.  I could see small, negligible artifacts being a thing, but it is too obvious on the Edge.

Looking at the image on the right, it appears that something may be pinched?



#14 gfunkernaught

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Posted 14 October 2021 - 11:13 AM

Looking at the image on the right, it appears that something may be pinched?


Which screws would have to be adjusted to alleviate the pinch?
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#15 Borodog

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Posted 14 October 2021 - 11:36 AM

Which screws would have to be adjusted to alleviate the pinch?

Honestly I'm not sure. I don't even know if anything is pinched. Just a guess looking at the strange curving shape of the flare off of that central star. It has the look of a stress in the glass somewhere; either a pinch or a warp.



#16 gfunkernaught

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Posted 14 October 2021 - 01:51 PM

Honestly I'm not sure. I don't even know if anything is pinched. Just a guess looking at the strange curving shape of the flare off of that central star. It has the look of a stress in the glass somewhere; either a pinch or a warp.


Can glass warp from rapid temperature or huge variances in climate? I'm wondering if the previous owner kept it in a garage or something. I never leave any glass in my garage. I left my mount (in its case) in my garage for few nights and the CW bar screw that stops the cw from falling had become thread locked. That never happened before, so it led me to believe that leaving it outside must have done something to the metal.

#17 jjack's

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Posted 14 October 2021 - 04:21 PM

seems like a tilted lens into the inner baffle tube. are you shure this scope was not dismentled ?



#18 freestar8n

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Posted 14 October 2021 - 05:13 PM

If you're looking at Vega in mid-October, I assume it was low-ish in the sky.  So you are probably just seeing atmospheric dispersion.

 

Did you try a star directly overhead?

 

Frank



#19 gfunkernaught

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Posted 14 October 2021 - 09:50 PM

seems like a tilted lens into the inner baffle tube. are you shure this scope was not dismentled ?

I certainly did not dismantle it.  I don't know what the previous owner did.



#20 gfunkernaught

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Posted 14 October 2021 - 09:51 PM

If you're looking at Vega in mid-October, I assume it was low-ish in the sky.  So you are probably just seeing atmospheric dispersion.

 

Did you try a star directly overhead?

 

Frank

From my yard, in New York, Vega is only visible at high altitudes.  When that picture was taken, it was close to zenith.



#21 freestar8n

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Posted 15 October 2021 - 01:25 AM

From my yard, in New York, Vega is only visible at high altitudes.  When that picture was taken, it was close to zenith.

Sorry - I lived for many years in New York, but I guess I have a bad memory of how the sky works.  Moving to the southern hemisphere really changes your view of the sky.

 

But your image does look like textbook atmospheric dispersion.  I guess there could be something out of alignment in the optics.  One thing to confirm is that it's the same orientation with other objects in the sky in different directions.

 

And without seeing the actual in-focus view of a star, it's hard to tell how well it's collimated.  So it may just need tweaking of the secondary screws.

 

Frank



#22 Borodog

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Posted 16 October 2021 - 12:39 PM

Forgive me, but that doesn’t look anything like atmospheric dispersion other than the superficial similarity of a red-blue color shift.

My money is indeed on the internal lens being the root of the problem.

#23 freestar8n

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Posted 16 October 2021 - 05:50 PM

Forgive me, but that doesn’t look anything like atmospheric dispersion other than the superficial similarity of a red-blue color shift.

My money is indeed on the internal lens being the root of the problem.

This site has an image of Polaris at 52 degrees altitude showing the same prismatic effect but in a more extreme way:

 

https://skyinspector...-corrector-adc/

 

And this site has more elaborate examples along with simulations:

 

http://paquettefamil...tro/star_study/

 

You can start seeing this effect even high up and not far from the Zenith.  One way to rule out dispersion vs. collimation is to image in different parts of the sky and see if the stretching of colors is always on a line pointing to the zenith, or is it fixed relative to the camera.

 

But the overall appearance is exactly what dispersion would look like in a star fairly high up in the sky.

 

Frank



#24 gfunkernaught

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Posted 17 October 2021 - 06:39 PM

This site has an image of Polaris at 52 degrees altitude showing the same prismatic effect but in a more extreme way:

 

https://skyinspector...-corrector-adc/

 

And this site has more elaborate examples along with simulations:

 

http://paquettefamil...tro/star_study/

 

You can start seeing this effect even high up and not far from the Zenith.  One way to rule out dispersion vs. collimation is to image in different parts of the sky and see if the stretching of colors is always on a line pointing to the zenith, or is it fixed relative to the camera.

 

But the overall appearance is exactly what dispersion would look like in a star fairly high up in the sky.

 

Frank

Did you see my comparison pic?  The 8SE+f/6.3 reducer does not have the same effect as the EdgeHD 8.



#25 freestar8n

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Posted 17 October 2021 - 09:42 PM

Did you see my comparison pic?  The 8SE+f/6.3 reducer does not have the same effect as the EdgeHD 8.

If it's atmospheric dispersion it would require good optics and a long focal length for it to be visible - so I'm not surprised if it doesn't show with the f/6.3 reducer.

 

It is all consistent with dispersion but it may be collimation.  Both things are easy to check.

 

Frank




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