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Oblong stars near the edge

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#1 Jireh

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Posted 23 October 2019 - 02:03 PM

Hello-

 

I was trying out my new WO Zenithstar 61 with WO flattener last night. Pictured here is a section near the center and a section near the edge of a 30s lum frame. Is this a guiding issue? A flattener issue? What is going on here? Wouldn't guiding effect the center as well as the edge?

 

stars.jpg



#2 ehanes7612

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Posted 23 October 2019 - 02:16 PM

Not a guiding issue. The field is not flat all the way to the edge and therefore not focused. If the Stars are oblong at every edge in a circular pattern then you need to focus at the edges. If the stars are not oblong in a circular pattern at all edges (or you get stars that become oblong radially or pointing to the center on some edges) you have a tilt issue.  Stars around the edge will go through stages of focus from starry disk with black centers to oblong circular (around center)... to focused... to oblong radial (pointing to center)...back to airy disk.This is how most refractors and camera lenses stage through focusing around the edges as most are not truly flat field.  If you are using SG Pro to focus ..it's a waste of time...it is limited in its ability to focus this type of field. I always focus at the edges manually and check until the scope has stabilized with the dropping temps ...the center stars will be slightly more bloated (you cant tell with the naked eye) but the edges will have pointed stars. If you have a tilt issue then that's a whole new issue. You should post pics of all the edges in the correct orientation.


Edited by ehanes7612, 23 October 2019 - 02:26 PM.

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#3 psandelle

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Posted 23 October 2019 - 02:20 PM

If it's radial along the edge, pointing in, I usually find that's a spacing issue betwixt reducer and camera. Even if a company says the backfocus is, say, 55mm, I've had it be 55.2, 54, 53.7, etc. Spacing is the bane of my existence. I have M54 (since I use that opening size) spacers ranging from .1mm to 7mm and mess about...when I have time. Which I rarely do.

 

If it's not radial, but just one edge, yeah, probably tilt.

 

Paul


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#4 ehanes7612

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Posted 23 October 2019 - 02:33 PM

Even with proper back focus I doubt the WO field flattener will flatten the field all the way to the edges...but could improve it and make it easier to use with SGP



#5 Jireh

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Posted 23 October 2019 - 02:37 PM

Not a guiding issue. The field is not flat all the way to the edge and therefore not focused. If the Stars are oblong at every edge in a circular pattern then you need to focus at the edges. If the stars are not oblong in a circular pattern at all edges (or you get stars that become oblong radially or pointing to the center on some edges) you have a tilt issue.  Stars around the edge will go through stages of focus from starry disk with black centers to oblong circular (around center)... to focused... to oblong radial (pointing to center)...back to airy disk.This is how most refractors and camera lenses stage through focusing around the edges as most are not truly flat field.  If you are using SG Pro to focus ..it's a waste of time...it is limited in its ability to focus this type of field. I always focus at the edges manually and check until the scope has stabilized with the dropping temps ...the center stars will be slightly more bloated (you cant tell with the naked eye) but the edges will have pointed stars. If you have a tilt issue then that's a whole new issue. You should post pics of all the edges in the correct orientation.

 

 

If it's radial along the edge, pointing in, I usually find that's a spacing issue betwixt reducer and camera. Even if a company says the backfocus is, say, 55mm, I've had it be 55.2, 54, 53.7, etc. Spacing is the bane of my existence. I have M54 (since I use that opening size) spacers ranging from .1mm to 7mm and mess about...when I have time. Which I rarely do.

 

If it's not radial, but just one edge, yeah, probably tilt.

 

Paul

Thanks for the responses. Looks like it may be a spacing issue.

 

stars2.jpg



#6 ehanes7612

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Posted 23 October 2019 - 02:47 PM

So what is happening here is that you are not focused around the edges (too far focused out I think, if the stars were to focused in, they would appear circular around the center )...there might be a slight tilt (as the degree of oblong is not the same all around) but not too serious if there is. If the back spacing change doesnt resolve the issue ..you are not far from critical focus ...maybe a two or three mm turns of the focuser.


Edited by ehanes7612, 23 October 2019 - 03:14 PM.

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#7 kyle528

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Posted 23 October 2019 - 06:16 PM

I get a flat field from my z61 with my 183mc's 1" sensor. You have a bit larger sensor, but it can certainly get much better than that. Double check your backfocus and make sure you have the flattener set to 12.9mm, this measurement has nothing to do with the backfocus, it *must* be set to 12.9mm for the z61, and backfocus is an additional 55mm from the flattener. 


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#8 Stelios

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Posted 23 October 2019 - 06:46 PM

This post may help you debug if it's a spacing issue for your flattener.


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#9 Jireh

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Posted 23 October 2019 - 10:05 PM

I get a flat field from my z61 with my 183mc's 1" sensor. You have a bit larger sensor, but it can certainly get much better than that. Double check your backfocus and make sure you have the flattener set to 12.9mm, this measurement has nothing to do with the backfocus, it *must* be set to 12.9mm for the z61, and backfocus is an additional 55mm from the flattener.


Thanks - can you provide details on how to set that? I’ve been searching but I can’t find any info.

#10 kyle528

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Posted 24 October 2019 - 06:05 AM

Thanks - can you provide details on how to set that? I’ve been searching but I can’t find any info.


There are markings on the barrel of the flattener, and while not necessary, inside jaws of calipers will get you even closer than the markings.


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#11 OldManSky

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Posted 24 October 2019 - 08:34 AM

https://williamoptic...-flat61-for-z61

Scroll down the page a bit, it shows you how to adjust the distance on the flattener.

 

My Z61 with the old-style, non-adjustable flattener gave a nice flat field all the way out to the edges of an APS-C sized chip, which is larger than your 1600's, so with adjustment you should be able to get everything flat all the way to the edge.


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#12 Jireh

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Posted 24 October 2019 - 09:19 AM

There are markings on the barrel of the flattener, and while not necessary, inside jaws of calipers will get you even closer than the markings.


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Thanks - it arrive screwed most of the way in so I never saw the markings. I'm not sharpest pencil in the pencil box!

 

 

https://williamoptic...-flat61-for-z61

Scroll down the page a bit, it shows you how to adjust the distance on the flattener.

 

My Z61 with the old-style, non-adjustable flattener gave a nice flat field all the way out to the edges of an APS-C sized chip, which is larger than your 1600's, so with adjustment you should be able to get everything flat all the way to the edge.

Thanks - I wish I had seen this a few days ago. Got it set correctly now. I'm hoping the forecast changes and I get at least one night out before the moon comes back.

 

 

This post may help you debug if it's a spacing issue for your flattener.

Wow, this is very helpful! Yes, my spacing between scope and FF was definitely way off.


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#13 kyle528

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Posted 24 October 2019 - 09:39 AM

This may help you, I posted this in a different thread, same problem.

If this helps at all, my train is camera(asi183mc pro)>21mm spacer>OAG (16.5mm)>11mm spacer>FF>Z61.
If you aren’t using an OAG, substitute your 16.5mm spacer


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Edited by kyle528, 24 October 2019 - 09:41 AM.

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#14 Jireh

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Posted 24 October 2019 - 11:25 AM

https://williamoptic...-flat61-for-z61

Scroll down the page a bit, it shows you how to adjust the distance on the flattener.

 

My Z61 with the old-style, non-adjustable flattener gave a nice flat field all the way out to the edges of an APS-C sized chip, which is larger than your 1600's, so with adjustment you should be able to get everything flat all the way to the edge.

Do you have any issues with the weight of the FF/camera/efw on the Z61 focuser?

 

 

This may help you, I posted this in a different thread, same problem.

If this helps at all, my train is camera(asi183mc pro)>21mm spacer>OAG (16.5mm)>11mm spacer>FF>Z61.
If you aren’t using an OAG, substitute your 16.5mm spacer


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Yes, thank you. I have the ZWO OAG but I've not added it as a variable just yet. My train is camera>efw>t2-to-t2 adapter>11mm spacer>16.5mm spacer>FF>Z61. So that's 6.5+20+1+11+16.5=55mm so I think I'm good. And now, with the help of you and OldManSky with the FF I think I'm ready for some clear skies!



#15 kyle528

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Posted 24 October 2019 - 11:41 AM

Do you have any issues with the weight of the FF/camera/efw on the Z61 focuser?


Yes, thank you. I have the ZWO OAG but I've not added it as a variable just yet. My train is camera>efw>t2-to-t2 adapter>11mm spacer>16.5mm spacer>FF>Z61. So that's 6.5+20+1+11+16.5=55mm so I think I'm good. And now, with the help of you and OldManSky with the FF I think I'm ready for some clear skies!


That sounds right, some folks have had to do some tweaking either side of that 55mm, but it worked fine for me. Good luck!


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#16 DubbelDerp

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Posted 24 October 2019 - 11:53 AM

I did the same thing recently... I had my backspacing set to 55mm from the shoulder of the flattener to the imaging plane, and had the same elongated stars at the edges of the field. If you click on the "more information" tab on the William optics page referenced above, it shows you the shoulder to imaging plane should be 67.7mm (reference A), or 69.2mm (reference B). Which is basically the 12.9mm adjustment (more or less) plus 55mm. This is how I set mine, and it now flattens nicely on a crop sensor DSLR. So hit 67.7mm as referenced from the shoulder in datum A from the drawing with some combination of the adjustable flattener and spacers and you should be good to go!


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#17 OldManSky

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Posted 24 October 2019 - 12:29 PM

Do you have any issues with the weight of the FF/camera/efw on the Z61 focuser?

 

Not really.  ASTAP image analysis in different orientations showed a tiny bit of orientation-dependent tilt, but it was always on the order of 1% or so, and wasn't visible in the images.  The focuser is quite robust for a little scope.


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