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WO Z61 - yet another backfocus/tilt analysis

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#1 bluewater

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Posted 03 February 2021 - 11:23 PM

Good Evening All,

 

I have a William Optics Zenithstar 61 with a 0.8x 61R flattener-reducer, imaging with a ZWO ASI 1600MM-Pro and EFW.  I have been experimenting and researching as much as I can and I still cannot figure out why I am getting elongated stars in the corners of my image.  I have tried starting at the recommended 56mm of backfocus and imaging in and out from that - yet still the stars are elongated, radiating from the centre as if the sensor and flattener are too close.  Below I have attached images from 55-60mm of backspace along with corresponding CCD Inspector maps in the reply (not sure if that's useful or not).

 

Current theories are optical pinching (it was -10C when these were taken), collimation or I just don't know what I'm doing.

 

Any help would be greatly appreciated!

 

-Blue

Attached Thumbnails

  • 55mm.jpg
  • 56mm.jpg
  • 57mm.jpg
  • 58mm.jpg
  • 59mm.jpg
  • 60mm.jpg

Edited by bluewater, 03 February 2021 - 11:29 PM.


#2 bluewater

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Posted 03 February 2021 - 11:24 PM

...and CCD Inspector maps...

Attached Thumbnails

  • CCDI 55mm.jpg
  • CCDI 56mm.jpg
  • CCDI 57mm.jpg
  • CCDI 58mm.jpg
  • CCDI 59mm.jpg
  • CCDI 60mm.jpg


#3 Tapio

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Posted 03 February 2021 - 11:28 PM

That last one looks good to me.
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#4 bluewater

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Posted 03 February 2021 - 11:34 PM

That last one looks good to me.

Tapio - thanks for your quick reply. The stars in the top left still look odd in that one, maybe my pic is too low res. Also when I look at the CCD inspector for that one (the last in that post) it looks skewed like crazy. I suspect I am so far back (+4mm) that I'm not getting things in focus that well.

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#5 Jinux

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Posted 04 February 2021 - 12:11 AM

Starts elongated to outside direction indicates flattener spacing is too close

 

https://www.cloudyni.../#entry10760465

 

Not centered field indicates that there is tilt. Most likely around focuser clamp to flattener unless F/F is thread in, in my experience.

Focuser tube sometimes tilt with weight though. In this case elongation will appear in different location per where it is pointing at.

After some trial on some area of sky and check, you may be able to identify the weakest link. Fixing it is another story though.

 

Hope it helps.

 

-Jinux 



#6 Jinux

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Posted 04 February 2021 - 12:17 AM

And I remember one trick to measure right distance for backfocus if typical 55mm doesn't provide you flat field.

I recall Roland of A-P tipped and I read somewhere in this CN forum.

Focus to center star as accurate as you can. (1)

Focus to the corner star as accurate as you can (2)

Distance difference of (2) - (1) is how much spacing you need to give or subtract for backfocus spacing.

I haven't tried this by myself but in theory looks like a plan.

 

-Jinux



#7 bluewater

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Posted 04 February 2021 - 07:53 AM

Starts elongated to outside direction indicates flattener spacing is too close

https://www.cloudyni.../#entry10760465

Not centered field indicates that there is tilt. Most likely around focuser clamp to flattener unless F/F is thread in, in my experience.
Focuser tube sometimes tilt with weight though. In this case elongation will appear in different location per where it is pointing at.
After some trial on some area of sky and check, you may be able to identify the weakest link. Fixing it is another story though.

Hope it helps.

-Jinux

Everything is screwed in and seems to be very solid. I can't feel any movement in anything from the focuser to the camera. I'll attach a photo when I'm home. I do also have the tilt adjuster for the 1600, I just haven't messed with it yet.

It does look like the 60mm starfield looks best, but that's +4mm from recommended. Is it common to be that far off? It may even need another mm perhaps.

How were you able to tell that the field wasn't centered?

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#8 bluewater

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Posted 07 February 2021 - 08:49 AM

Tried again last night - still slightly elongated stars at 60mm backfocus. At 61mm the draw tube wouldn't go in far enough to focus at all.

So it's not backfocus distance I guess - I'll see if I can get the tilt out and see. I contacted WO and as of right now they just keep telling me what the backfocal distance should be, which isn't hugely helpful.

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#9 Cfreerksen

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Posted 07 February 2021 - 10:55 AM

A picture of the imaging train would help.

 

Chris



#10 bluewater

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Posted 07 February 2021 - 11:43 AM

Here it is. Apologies, I thought it attached.

Thank you,

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57497a147aecb977d90d7a8b4f149461.jpg

Edited by bluewater, 07 February 2021 - 12:21 PM.


#11 Cfreerksen

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Posted 07 February 2021 - 01:48 PM

Here it is. Apologies, I thought it attached.

Thank you,

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57497a147aecb977d90d7a8b4f149461.jpg

So it's:

 

61R  adjusted to 1   mm:

 

 

1600MM             6.5mm

ZWO T2 tilter ?  11  mm

ZWO wheel       20  mm

spacer               11  mm

-----------------------------------

Total                  48.5mm

 

Does this explain your imaging train?

 

Chris


Edited by Cfreerksen, 07 February 2021 - 01:58 PM.


#12 bluewater

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Posted 07 February 2021 - 02:04 PM

What I have right now is...

1600MM - 6.5
T2 Tilter - 11
54-48 reducer -2
ZWO EFW - 20
54-48 reducer - 2
Spacer - 10
61r set to 4.5
‐------------------
Total = 56mm

I've tried playing with the 61r spacing to achieve a range from 55-60mm with the results in the original post.


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#13 Cfreerksen

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Posted 07 February 2021 - 02:41 PM

when did you get the reducer? There has been some posts here about issues with the first batch but I thought they have been sorted out. I hope so since I bought one about a month ago and have yet to get it out under the stars. Been too busy with my GT102 for the last month.

 

Chris



#14 bluewater

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Posted 07 February 2021 - 02:45 PM

when did you get the reducer? There has been some posts here about issues with the first batch but I thought they have been sorted out. I hope so since I bought one about a month ago and have yet to get it out under the stars. Been too busy with my GT102 for the last month.

Chris

I got mine back in Nov 2020. I also got the Flat61 for it as well as the 61r above and get the same results.

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#15 Cfreerksen

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Posted 07 February 2021 - 11:52 PM

I got mine back in Nov 2020. I also got the Flat61 for it as well as the 61r above and get the same results.

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I have the Flat61 and when I set it up it was dead on per directions. Stars looked perfect to the edge of a FF camera.

 

Chris



#16 bluewater

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Posted 08 February 2021 - 09:03 AM

Yeah that's certainly not the case with mine. Last night I got some clear skies and tried with 3x flatteners, 2x astro cameras and a DSLR to try to eliminate the flatteners or sensor tilt as the issue. It was the same no matter what.

I'm thinking the scope is a lemon. WO has been less than helpful and just keeps telling me what the backfocus should be. I've contacted my retailer to look into a warranty replacement.

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#17 Kiwi-bloke

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Posted 08 February 2021 - 06:03 PM

I can't make heads or tails out of all that gear apart from a couple items. For my ZS103 i got the matching field flattener for it at the same time. I believe the combo needed a spacing of 5.3 or 5.4mm. I worked this out not by the measurement markings on the field flattener.Instead, as I worked in an engineering company I had access to Verniers.I measured the gap from the locking ring to the front edge and set it there. My images is rounded stars to the edge of my Canon 7D shots. Secondly, you did take the rear locking ring, or rotalock off? and have the field flattener screwed directly to the draw tube? like that in my picture?, because that's going to give you a lot more back focus if not. If it is on, and you need to remove it. You unscrew the end, that's what they kept telling me at WO , but their were 3 small hex screws, be careful.You will need to loosen them off before it's able to unscrew.

20210209 115057[1]

 



#18 Cfreerksen

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Posted 08 February 2021 - 09:08 PM

Yeah that's certainly not the case with mine. Last night I got some clear skies and tried with 3x flatteners, 2x astro cameras and a DSLR to try to eliminate the flatteners or sensor tilt as the issue. It was the same no matter what.

I'm thinking the scope is a lemon. WO has been less than helpful and just keeps telling me what the backfocus should be. I've contacted my retailer to look into a warranty replacement.

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Please update on what you find out. I will update on mine when I get it under the stars. I know that I have seen one other person with a similar issue. He said that he could not get focus with the 61R (Facebook?). He said nothing about having star issues or having the backfocus not to spec to get good stars. He just said he could not get the enough inward travel on the focuser to get to focus. I am wondering if it has something to do with the camera rotator. You could try removing it? That would get you like 30mm more inward travel.

 

Chris


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#19 bluewater

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Posted 08 February 2021 - 10:00 PM

WO is shut down for a few weeks due to Chinese New Year. I will keep messing around with it until then and let you know how I make out when they get back to me.

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#20 RogerM

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Posted 08 February 2021 - 10:22 PM

Here it is. Apologies, I thought it attached.

Thank you,

Sent from my SM-A715W using Tapatalk
 

I have my imaging train set up as follows:  61A/61R --> M42-M48 x 16.5m --> EFW --> camera

 

61R Required backfocus:  56.0mm

    M42-M48: -16.5mm

   EFW: -20.0mm

        Camera: - 6.5mm

=================

    13.0mm

 

Accounting for filter thickness:  + 0.4mm

=================

   13.4mm <-- this is what I have my 61R set to.

 

If you were to reconfigure your set up like mine:

 

    M42-M48: -16.5mm
            EFW: -20.0mm

         Tilt Adj: -11.0mm
        Camera: - 6.5mm
=================
                         54mm ---> 56mm - 54mm = 2mm  Important, Add your filter thickness (approx. 1/3 of actual) here and then set your 61R to this number.

Attached Thumbnails

  • Flat61R_Image_Train.jpg

Edited by RogerM, 08 February 2021 - 10:26 PM.


#21 Kiwi-bloke

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Posted 09 February 2021 - 02:06 AM

Please update on what you find out. I will update on mine when I get it under the stars. I know that I have seen one other person with a similar issue. He said that he could not get focus with the 61R (Facebook?). He said nothing about having star issues or having the backfocus not to spec to get good stars. He just said he could not get the enough inward travel on the focuser to get to focus. I am wondering if it has something to do with the camera rotator. You could try removing it? That would get you like 30mm more inward travel.

 

Chris

Having had a look on the WO site, I second that, the rotator comes off, and the focuser rotates anyway. He's also done what I would have, take back to stock and add part by part and try other gear. Same result. Leaves a crooked drawtube or glass not aligned, damaged in transit? Perhaps a eyepiece test next? 



#22 bluewater

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Posted 09 February 2021 - 07:45 AM

I have my imaging train set up as follows: 61A/61R --> M42-M48 x 16.5m --> EFW --> camera

61R Required backfocus: 56.0mm

M42-M48: -16.5mm

EFW: -20.0mm

Camera: - 6.5mm

=================

13.0mm


Accounting for filter thickness: + 0.4mm

=================

13.4mm <-- this is what I have my 61R set to.


If you were to reconfigure your set up like mine:

M42-M48: -16.5mm
EFW: -20.0mm

Tilt Adj: -11.0mm
Camera: - 6.5mm
=================
54mm ---> 56mm - 54mm = 2mm Important, Add your filter thickness (approx. 1/3 of actual) here and then set your 61R to this number.

Thank you for this - this is pretty similar to how I had mine set up initially before I started messing around with the spacing. I was a bit unsure about how the filters would impact things so I tried +/- 5mm from the stated backfocus and no joy.

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#23 bluewater

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Posted 09 February 2021 - 07:46 AM

Having had a look on the WO site, I second that, the rotator comes off, and the focuser rotates anyway. He's also done what I would have, take back to stock and add part by part and try other gear. Same result. Leaves a crooked drawtube or glass not aligned, damaged in transit? Perhaps a eyepiece test next?

I will try removing the rotator on the 61r - it is huge. I'll see if that solves things, but even then I would be 6+mm longer than the stated backfocal distance which seems like a lot and may be too long to even get the benefit of the flattener.

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#24 bluewater

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Posted 06 April 2021 - 06:56 AM

Update (finally). It was the scope. My vendor kindly sent me a different scope to test side by side and was perfect. FFs worked fine. I was able to keep the good scope and he sent the other back to WO.

Thanks for all of your help!

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#25 Stevemr2t

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Posted 05 May 2021 - 11:54 AM

when did you get the reducer? There has been some posts here about issues with the first batch but I thought they have been sorted out. I hope so since I bought one about a month ago and have yet to get it out under the stars. Been too busy with my GT102 for the last month.

 

Chris

I've been testing a 61 R flatner that I picked up late Dec 2020 and with the Nikon D850 full frame camera I have elongated stars at the edges. Tried all sorts of back focus amounts. I will try another scope (surprised that was the issue on this thread), but hoping to get a replacement from my vendor (Mile High Astro) when they respond.




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