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Flat Field Issues with WO Spacecat 51. Should it be returned?

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#1 benringel22

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Posted 23 May 2020 - 12:17 AM

Hi everyone,

 

Last night I spent about four hours imaging the North America Nebula with my new WO Spacecat 51. I used my stock Canon 6D (full frame) with an Orion Atlas EQ-G and had okay guiding for this image scale (1"-1.4"). I also used the WO Canon EOS DSLR adapter to ensure the right back focus was achieved. 

 

I noticed at the top and bottom right corners of my frames the stars were slightly out of focus, having a "doughnut" shape. This concerned me, since the center of the frame looked razor sharp, and I am worried the scope might be faulty. I read online that others have had similar problems, but do any of the more experienced astrophotographers out there know if this is a scope defect or user error? I'm wondering if I should consider sending the scope back, or if maybe my expectations are too high considering I am using a full frame sensor. Is their a known way to fix this? I have attached a single frame and a cropped image of the top right corner of the sub for reference.

 

Thanks in advance for the help; I appreciate the wealth of knowledge everyone here brings!

Ben

 

4 minute sub of North America Nebula:

North America Nebula - 4 Minute Sub - Spacecat 51

 

Top right corner of sub:

North America Nebula Sub Corner - Flat Field Issues - Spacecat 51


#2 rockstarbill

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Posted 23 May 2020 - 12:21 AM

I get permission errors looking at your images. Hard to say what the issue is, but it's safe to say that the edges are likely to be affected by mechanical vignetting in addition to mirror box issues. What % of the frame was solid?

Also could be tilt. That's why they include a tilt plate.

Can you upload raw frames somewhere?

Edited by rockstarbill, 23 May 2020 - 12:22 AM.

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#3 benringel22

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Posted 23 May 2020 - 12:35 AM

That's a great suggestion. Do I not risk severely messing up the scope if I play with the tilt adjuster?

 

Sorry about the permission error. I hopefully just fixed this. Here they are again if it didn't fix the previous images. I am unable to upload the RAW files to CN, but I put a few in a drive folder. Feel free to follow this link to find them.

 

4 minute sub of North America Nebula:

North America Nebula - 4 Minute Sub - Spacecat 51
 
Top right corner of sub:
North America Nebula Sub Corner - Flat Field Issues - Spacecat 51

 

Thanks so much for the help!

Ben



#4 Hobby Astronomer

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Posted 23 May 2020 - 12:37 AM

If you do not like the quality send it back. Life is too short.


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#5 jerahian

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Posted 23 May 2020 - 12:38 AM

 I believe you have a tilt issue.  Here are the measured Eccentricity and FWHM values for your image.

 

gallery 310611 13050 161521 fwhm
gallery 310611 13050 161521 Ecc

 

 


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#6 benringel22

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Posted 23 May 2020 - 12:41 AM

 I believe you have a tilt issue.  Here are the measured Eccentricity and FWHM values for your image.

 

Thank you for checking that. I wouldn't have known how to do this (I am still very much a beginner).

 

Is this able to be fixed using the tilt adjuster or is this an issue with the optics?

 

Thanks!

Ben



#7 Becomart

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Posted 23 May 2020 - 12:47 AM

Thank you for checking that. I wouldn't have known how to do this (I am still very much a beginner).

 

Is this able to be fixed using the tilt adjuster or is this an issue with the optics?

 

Thanks!

Ben

It can be fixed with tilt screws. Adjust in small increments to compensate for the above pattern, shoot again and compare the subs. Will likely take you several goes to get it right. 


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#8 benringel22

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Posted 23 May 2020 - 12:51 AM

It can be fixed with tilt screws. Adjust in small increments to compensate for the above pattern, shoot again and compare the subs. Will likely take you several goes to get it right. 

This is a huge relief! Thank you so much for the help. I will happily spend a few hours tuning the tilt.

 

Thank you all for the help, and if you have any other suggestions or comments please share them. I can use all the help I can get...

 

Ben



#9 jerahian

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Posted 23 May 2020 - 12:54 AM

Thank you for checking that. I wouldn't have known how to do this (I am still very much a beginner).

 

Is this able to be fixed using the tilt adjuster or is this an issue with the optics?

 

Thanks!

Ben

I really doubt it's an issue with the optics.  High likelihood it's tilt and fixable with the adjuster, which is nicely included with the SpaceCat.  Definitely take your time and use small steps, as Becomart said.



#10 rockstarbill

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Posted 23 May 2020 - 12:55 AM

Significant tilt from left to right in this frame. Nice thing is, if you solve the tilt, I bet the frame will look pretty good. 

 

Use the tilt adjuster. Get a copy of CCD Inspector, install that (you get to use it for free for 30 or 45 days) and that should help guide you in solving it.

 

Example of what CCDI shows:

 

CCDI-RedCat_CN.JPG

 

I have never seen a number that large for tilt %. I wish I had a prize to give you. grin.gif lol.gif

 

Adjust the tilt plate opposite of the tilt you see here. You will know if you are going the right way, if that massive number goes down. Once it gets near 0, you should be good to test again. Be sure to focus the scope after each change.


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#11 AstroBrett

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Posted 23 May 2020 - 12:56 AM

If all four corners look like that and they all point toward the center, in other words they are radially symmetric, then most it is field curvature, and the most probable culprit is improper back spacing between your sensor and the field flattener. Mine was about a millimeter off the manufacturer's suggested spacing, but once I got it set, my stars are flat across the full width of the frame. 

 

If it is sensor tilt, as suggested above, the pattern will not be radially symmetric, but will vary systematically across the frame in the direction of the tilt.

 

As suggested above, posting a downloadable frame makes it a lot easier to diagnose.

 

Brett


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#12 rockstarbill

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Posted 23 May 2020 - 12:58 AM

If all four corners look like that and they all point toward the center, in other words they are radially symmetric, then most it is field curvature, and the most probable culprit is improper back spacing between your sensor and the field flattener. Mine was about a millimeter off the manufacturer's suggested spacing, but once I got it set, my stars are flat across the full width of the frame. 

 

If it is sensor tilt, as suggested above, the pattern will not be radially symmetric, but will vary systematically across the frame in the direction of the tilt.

 

As suggested above, posting a downloadable frame makes it a lot easier to diagnose.

 

Brett

Its a Petzval telescope, there is no backspacing nor flattener. :) Like an FSQ, put your camera on, get in focus and go. There is an ideal place to put the camera, but that isnt the problem here. Its significant tilt. So bad, that judging the symmetry of the stars is not even possible.



#13 benringel22

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Posted 23 May 2020 - 01:00 AM

Significant tilt from left to right in this frame. Nice thing is, if you solve the tilt, I bet the frame will look pretty good. 

 

Use the tilt adjuster. Get a copy of CCD Inspector, install that (you get to use it for free for 30 or 45 days) and that should help guide you in solving it.

 

Example of what CCDI shows:

 

attachicon.gifCCDI-RedCat_CN.JPG

 

I have never seen a number that large for tilt %. I wish I had a prize to give you. grin.gif lol.gif

 

Adjust the tilt plate opposite of the tilt you see here. You will know if you are going the right way, if that massive number goes down. Once it gets near 0, you should be good to test again. Be sure to focus the scope after each change.

I will definitely look into CCD inspector. It would be great to have a robust and quantitative way of checking tilt. Thank you for pointing this out! Also, if there was an award I would gladly accept. lol.gif



#14 rockstarbill

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Posted 23 May 2020 - 01:02 AM

I will definitely look into CCD inspector. It would be great to have a robust and quantitative way of checking tilt. Thank you for pointing this out! Also, if there was an award I would gladly accept. lol.gif

Its a good tool, although I am not entirely sure I think its $150 good. I paid for it, and have gotten some good mileage out of it, but PI technically tells you the same thing (Post #5 above), albeit in a less friendly manner.



#15 benringel22

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Posted 23 May 2020 - 01:06 AM

Its a good tool, although I am not entirely sure I think its $150 good. I paid for it, and have gotten some good mileage out of it, but PI technically tells you the same thing (Post #5 above), albeit in a less friendly manner.

Oh wow! A trial license sounds like music to my ears then. I could see myself using PI in the future so it may be worth considering. Thanks again!


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#16 benringel22

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Posted 23 May 2020 - 01:17 AM

As suggested above, posting a downloadable frame makes it a lot easier to diagnose.

Also, here is a link to a few of the RAW files if you would like to take a look for yourself. Sorry for not including these in the original post.

 

Thanks for the help!



#17 ChrisWhite

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Posted 23 May 2020 - 05:45 AM

That corner crop you show in your original post shows that you are significantly out of focus.  Maybe thats caused by such severe tilt, but in order to root out any optical or mechanical gremlins you MUST get as perfect focus as you can.  Good enough focus... isn't good enough.  wink.gif


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#18 james7ca

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Posted 23 May 2020 - 06:32 AM

....and the most probable culprit is improper back spacing between your sensor and the field flattener. Mine was about a millimeter off the manufacturer's suggested spacing, but once I got it set, my stars are flat across the full width of the frame. 

 

If it is sensor tilt, as suggested above, the pattern will not be radially symmetric, but will vary systematically across the frame in the direction of the tilt...

 

Brett

As rockstarbill noted, there is NO backfocus requirement for the WO Spacecat 51. This lens/scope comes to focus where it comes to focus, there is no optimum spacing in terms of optical performance. That said, you probably want as little extension of the focus mechanism as possible, but that's just for mechanical stability not for some required spacing between the lens and sensor.


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#19 benringel22

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Posted 23 May 2020 - 11:20 AM

That corner crop you show in your original post shows that you are significantly out of focus.  Maybe thats caused by such severe tilt, but in order to root out any optical or mechanical gremlins you MUST get as perfect focus as you can.  Good enough focus... isn't good enough.  wink.gif

I think you are 100% right. If you look at the three RAW images I posted to the drive you will see the stars in the corner get slightly worse later in the night, probably implying that the focus shifted very slightly. I will keep an eye on this next time as well.

 

Thanks for everyone's help!



#20 rockstarbill

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Posted 23 May 2020 - 02:16 PM

I think you are 100% right. If you look at the three RAW images I posted to the drive you will see the stars in the corner get slightly worse later in the night, probably implying that the focus shifted very slightly. I will keep an eye on this next time as well.

 

Thanks for everyone's help!

The focus will shift *significantly* on this scope. You cannot just fire and forget with the focus, you need something to maintain automatic focusing of this system, and it should do it roughly every 0.5C change in temperature. These Petzval systems, while nice in some design regards, are very, very, very sensitive to temperature change.

 

My suggestion would be to get the Astro Dad AF3 focusing system for the Cats, although mount it on top of the ring and not under the scope (that is begging for problems). 


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