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APEX-L Coma at the corners?

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#1 Ethanwyh

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Posted 05 March 2021 - 11:21 AM

This is a little frustrating, so long story short I recently accquired my Esprit 100 and of course, came the usual challeneg of nailing backspacing right for an extremely fast scope (down to F/3.6)

First night: since i'm using filters, the sugested backspacing would be 58.67mm (58 + 1/3*2mm filter thickness)

The metal washers that i have are either in 1mm or 0.5mm increments, thus i set it at 58.5mm

 

Results:

1614958684281.png

 

 


Verdict:
a) still not enough backspacing 
b) coma at the bottom left??
HUGE question mark, but i decided to solve the backspacing issue first

 



 



#2 Ethanwyh

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Posted 05 March 2021 - 11:23 AM

Second night: The next logical step would be to add 0.5mm spacer right? So i did that, and it amounted to 59mm now
Results:

 

1614960231155.png


Verdict: Corner elongation has significantly improved BUT, i'm seeing the same exact 'flaring' which looks like coma. And this is only obeserved at the top right and left corners. 

Has anyone with this exact/similar set-up experienced something like this before?
 


Edited by Ethanwyh, 05 March 2021 - 11:25 AM.


#3 Ethanwyh

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Posted 05 March 2021 - 11:27 AM

An example of the star profiles with 'coma' 
anydesk00037 (2).png



#4 fetoma

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Posted 05 March 2021 - 01:52 PM

I would make sure your imaging train is square with the focal plane. CCD Inspector would show something like that although I'm not too experienced with it.



#5 RossW

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Posted 05 March 2021 - 11:13 PM

This is a little frustrating, so long story short I recently accquired my Esprit 100 and of course, came the usual challeneg of nailing backspacing right for an extremely fast scope (down to F/3.6)

First night: since i'm using filters, the sugested backspacing would be 58.67mm (58 + 1/3*2mm filter thickness)

The metal washers that i have are either in 1mm or 0.5mm increments, thus i set it at 58.5mm

 

Results:

attachicon.gif1614958684281.png

 

 

Verdict:
a) still not enough backspacing 
b) coma at the bottom left??
HUGE question mark, but i decided to solve the backspacing issue first

 


 

 

The flaring you see is common to the Esprit 100 + reducer when you don't have the backspacing and/or image tilt nailed. I believe what you are most likely seeing here is a combination of incorrect backspacing and, most importantly, tilt caused by the camera sensor or somewhere in the image train. If there is tilt in the imaging train it could be somewhere between the reducer and camera, or it could be between the reducer and scope, so it becomes quite complicated as to how to correct it. If it is between the reducer and camera, and if you are using an OAG, then you probably won't be able to correct it because there won't be enough backfocus for a tilt corrector. I successfully modified my ZWO OAG to be both an OAG and a tilt corrector, so you might be able to take that approach if you don't mind a little DIY.

 

Your images are unfortunately a little too small to determine what components of the aberration belong to tilt and backspacing though. 

 

Your smallest spacer, being 0.5mm, is probably too coarse for accurate backfocus adjustment, but until you obtain thinner spacers (i.e., 0.2mm) I suggest you concentrate on removing the tilt so that the aberration appears equal in all four corners of the image.

 

Good luck!

 

Ross


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#6 Ethanwyh

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Posted 06 March 2021 - 01:42 AM

Thanks Ross and Frank, really appreciate both of your help and chiming in. Seems like i'll have to ditch the rotator in favour of a sturdier one (William optics or Tak...oouch indeed)

@Ross, do you know of any shops that sells custom spacers? I can't seem to find any online retailer which carry such exotic spacers lol.gif



#7 ks__observer

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Posted 06 March 2021 - 05:12 AM

Your corners look very good compared to mine -- i heard the Apex-L works better than the S.

ZWO sells the paper thin spacers, which might be needed for f3.6  -- i think the Apex may have cone with some.



#8 Ron (Lubbock)

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Posted 06 March 2021 - 05:39 AM

I have the same telescope & reducer.  After a few months of fiddling with the back-focus, I had the same issues... imperfect corner stars, and one corner that was usually worse than the others.  It helps a lot if you aren't using any adapters that have short threads.  Nice, long threads reduce the chances of any flexure or tilt  of the camera due to imperfect machining of the threads.

 

When I moved the camera to my C11 recently, I was surprised to see that some of the problems with star shapes came along for the ride. The culprit was a short adapter (4 mm) that was still attached directly to the camera face plate, which I failed to remove.  It had very short threads.  I now think that adapter caused some of the optical issues with the Esprit as well. 

 

I still haven't gotten the Esprit + reducer combination perfect, though I now have some hope after realizing that one of the adapters didn't work with my familiar C11 either.  I did produce some nice images with the Esprit + reducer over the winter, but if you insist on perfect stars in the corners, you wouldn't be completely happy with them.

 

FWIW, your star shapes in the corners look the same as mine did when the camera sensor was too close to the optics.  I think you need to add a little bit more spacing and the corner stars will  improve.



#9 sn2006gy

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Posted 06 March 2021 - 10:11 AM

If this is on the QHY 294, you should have no problem getting the reducer to be flat across the entire sensor.  I would advise to use CCD Inspector and have it measure the tilt/center of your sensor.

 

I couldn't get APS-C fully corrected on my 120 with an apex-l, i think asp-c is just a tad too big for this reducer but the qhy 294 should fit right in the beautifully flat center



#10 fetoma

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Posted 06 March 2021 - 10:25 AM

Thanks Ross and Frank, really appreciate both of your help and chiming in. Seems like i'll have to ditch the rotator in favour of a sturdier one (William optics or Tak...oouch indeed)

@Ross, do you know of any shops that sells custom spacers? I can't seem to find any online retailer which carry such exotic spacers lol.gif

https://www.cloudyni...ics-custom-etc/



#11 RossW

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Posted 07 March 2021 - 12:32 AM

@Ross, do you know of any shops that sells custom spacers? I can't seem to find any online retailer which carry such exotic spacers lol.gif

The product that @fetoma posted seems to be good quality, so if you live in the States that may be the way to go, but as you (and I) live in Asia you might like to try this:

 

https://www.aliexpre....27424c4dWQEpD1

 

Thicknesses down to 0.1mm, but he might not have the diameter you're after, and you'll probably need to smooth them a little before use (I think he uses a small lathe to manufacture them, so there may be a few remnant burs).

 

For larger sizes I usually purchase these aluminium spacers (4mm to 9mm) for M42 and M48 diameters:

 

https://www.aliexpre....27424c4dWQEpD1

 

I usually request those that have a knurled outer surface, because they can be very hard to separate when connected together (cold welded).

 

There is also Precise Parts in the States; expensive but highly recommended.

 

I think I need to invest in a mini lathe :-)

 

Cheers,

 

Ross


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