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EDGE HD @ F7 Field Curvature in CCD Inspector

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#1 Janco

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Posted 02 December 2019 - 04:24 AM

Good day everyone, 

 

After many nights of trouble shooting and fine tuning I've managed to get my scope to a point where I am pretty happy with its performance but just want to check with some other EDGE HD owners.

 

My telescope with the F7 reducer attached seems to be giving its flattest field at 154 mm back focus not 146 mm as specified. Below is a stretched stack of 6x 30 sec exposures. Visually the stars look pretty good to me. CCD inspector says it has a fields curvature of 11%. Is this pretty much in line with what everyone else is getting at F7? 

 

This being said, its about the first time in 2 years I am truly happy with this scope's performance. 

 

Autosave.jpg

 

 



#2 Benni123456

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Posted 02 December 2019 - 06:39 AM

hi,

 

with your mono camera, can you try to shoot r,g,b and show the different channels? 

 

Also, I noticed that with longer exposures, e.g. 120sek with guiding, I see image problems that are not visible at 30 sec.

 

This may help for fine tuning.

 

But Im glad for you. 

 

With my edge 8, I am still experimenting,  but Im also somewhat hopefull. When Ive found the flattest field, I will post the data here too...

 

We clearly need more people who measure these distances. I would bet they are all quite similar due to mass production.

 

Then we can post the data here perhaps make celestron update their specs and we all have good images finally.


Edited by Benni123456, 02 December 2019 - 06:47 AM.


#3 Janco

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Posted 02 December 2019 - 07:19 AM

hi,

 

with your mono camera, can you try to shoot r,g,b and show the different channels? 

 

Also, I noticed that with longer exposures, e.g. 120sek with guiding, I see image problems that are not visible at 30 sec.

 

This may help for fine tuning.

 

But Im glad for you. 

 

With my edge 8, I am still experimenting,  but Im also somewhat hopefull. When Ive found the flattest field, I will post the data here too...

 

We clearly need more people who measure these distances. I would bet they are all quite similar due to mass production.

 

Then we can post the data here perhaps make celestron update their specs and we all have good images finally.

Thanks Benni, 

 

This was shot using a red filter. I will however do the other channels when a get a chance. 

 

I don't know if I agree with you on with longer exposures revealing more in terms of optical aberrations. What you are most likely seeing is mount issues or  a not good enough polar alignment for your focal length. 



#4 Benni123456

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Posted 02 December 2019 - 09:31 AM

Hi Janco,

 

I wrote that with the different channels, because in mine, red is almost always perfect and blue is always worst. I'm now at a point where red and green is perfect and blue is still a bit out. But this fits the spot diagram of the edge 8. It may be different with your edge 9, which has a different spot diagram for each color.

 

Regarding longer exposures:

Well, (at least in my edge 8) the problem is that there is sometimes a bright center and some blue smear. And this smear is distributed elliptically. With short exposures, I won't notice that. But it can be seen in guided long exposure images.

 

But one must also say that I have an active optics which runs at 10hz and more. So I do not get mount errors and I can always suppress seiing to 0.54'' (the optical resolution of the scope).

 

If you do not have an active opticsm it may help to use a high gain value or guide with an off axis guider. In fact, a high gain value may be enough to capture the smear for each color.

 

At least you should make trials that are similar to a normal imaging situation. 

 

Also, it would be interesting if you now make an image with the pure edge and compare the FHWM.

 

Does the pure edge also deliver a better image quality at your new distance?

 

And finally, if you ask me, this problem is simply annoying. One has to make a series of images, remove the cables, screw out out the imaging train, add an 1mm spacer, screw the imaging train on, add the cables, recalibrate phd guiding, and repeat with imaging...

 

It just takes long time...

 

Celestron should measure this before delivery, not us by trial and error. 



#5 Benni123456

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Posted 04 December 2019 - 04:18 AM

I have had some success with a distance of 15mm out of the native spec with my edge 800. Stars at the edges are no more triangles. But in blue they are still comets, although just slightly

 

https://www.cloudyni...-800/?p=9812249

 

So i will now try 133.35+20mm and report the results.

 

I noticed this night that short exposures really do not work to find these problems. If it is round in 30 sec, it may still be an ellipse after 120 sec. Images with guiding errors can be removed simply by looking whether the stars in the center are round.


Edited by Benni123456, 04 December 2019 - 04:19 AM.


#6 Tapio

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Posted 04 December 2019 - 04:40 AM

Hi Janco,

 

But one must also say that I have an active optics which runs at 10hz and more. So I do not get mount errors and I can always suppress seiing to 0.54'' (the optical resolution of the scope).

 

 

 

I think you mean adaptive optics instead of active optics.

https://en.m.wikiped...i/Active_optics



#7 Benni123456

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Posted 04 December 2019 - 05:18 AM

adaptive optics would be 100hz and more. No, while the system could operate at 5ms reaction time, I would need a bright guidestar for that. Theoretically, one could try to project a laser to the sky in the off axis guider, but that is forbidden for an amateur. Imagine a plane going through this guiding laser..


Edited by Benni123456, 04 December 2019 - 05:22 AM.



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