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Stars not round at corners

astrophotography beginner
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#1 Moorefam

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Posted 29 October 2020 - 08:25 AM

I am fairly new to astrophotography and been at it about 6 months.

 

I use an 8 inch 1000mm F.L. Skywatcher reflector, with an AZ-EQ6 pro mount on a Skywatcher pillar resting on a concrete patio. Strangely the pillar moves a little from the vertical after a few days and needs resetting. This requires new polar alignment which I think I am doing properly. After polar alignment Polaris is near the top of the frame which is puzzling? I have checked for cone error and can't see any but as I said I haven't been doing this long so have an awful lot to learn.

 

I also use a Baader MPCC Mk III coma corrector and Sony A7 III full frame camera with MGEN III auto tracking with a 60mm guide scope and attached is a  sub of 2 mins duration. It just shows the corners and centre.   The stars at centre are round but not at the corners  and they differ at each corner. I rotated the camera 90 degrees with no change. The focusser looks square to the scope. Everything seems tight and I am pleased with my results but not near the corners. The sub was taken on a windless night.

Basically I am scratching my head as to what the problem is and any help would be much appreciated. I hope the attachment comes out OK.

_DSC6930_4 corners centre_2.jpg   



#2 PirateMike

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Posted 29 October 2020 - 08:46 AM

There must be a million threads here about flattening the field for AP which is what you need to do to get round star in the corners. I'm sure a quick search will provide you with more than enough information.

 

Polaris is not at the NCP, yes it is close but... Your scope could be centered exactly at the NCP, and if it were then Polaris would not be there.

 

NCP.png

 

Miguel   8-)

 

 

.


Edited by PirateMike, 29 October 2020 - 08:48 AM.


#3 mxpwr

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Posted 29 October 2020 - 09:17 AM

I'm not sure the Baader MPCC Mk III is capable of coma correction for full frame.

 

Here is a comparison using a 4/3 sensor:

https://www.astrofotoblog.eu/?p=856


Edited by mxpwr, 29 October 2020 - 09:39 AM.


#4 06AwzIyI

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Posted 29 October 2020 - 09:34 AM

Have you tried using a mirror lockup delay on your camera yet?

Nevermind, looks like the a7 is mirrorless

Edited by 06AwzIyI, 29 October 2020 - 09:39 AM.


#5 erictheastrojunkie

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Posted 29 October 2020 - 09:50 AM

Given that all the stars appear to be radiating away from the center my first inclination is that there is a backspacing issue, in this case the camera is ever so slightly too far from the corrector, is there a way you can move the camera closer by 0.5-1mm? 


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#6 deansjc

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Posted 29 October 2020 - 10:03 AM

Now I am  confused.  I took from a very experienced CN'er advice that states that if the stars are "radiating away" from the center that one should add distance, not take it away.

 

Which advice is correct?  (Or perhaps I mistakenly interpreted the advice, but I don't think so.)

 

Of course trial and error would be definitive.

 

Further input is most welcome here :-)

 

John



#7 DubbelDerp

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Posted 29 October 2020 - 10:09 AM

Same here.. I have a flattener/reducer that produces similar stars radiating away from the center, and my understanding was that indicated more spacing was necessary between the flattener and sensor. However, adding distance made the elongation worse, not better. I'm waiting on delivery of a zero-length t-adapter to test reduced spacing, but I can say with confidence that I don't know what is going on here.



#8 michael8554

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Posted 29 October 2020 - 10:57 AM

There is anecdotal evidence that both answers are correct, it may be dependant on the scope or the FF design.

 

Add a bit and see if it improves !



#9 Kevin_A

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Posted 29 October 2020 - 11:24 AM

Here is your typical chart for spacing... but in some instances your flattener may not be able to correct enough for the amount of edge elongation vs the sensor size you are using... and that I think comes down to the cost of equipment. Some scopes that are rated for fullframe cameras don't actually do it very well... that may be the case if moving both ways doesn't work. My cheaper WO scope has a fullframe image circle but still has edge elongation with its dedicated field flattener in the recommended setting... so I use a smaller sensor. 

images.jpg


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#10 Moorefam

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Posted 29 October 2020 - 12:17 PM

Thanks for the replies so far. I have been looking into backspacing and from Kevin's charts it could be the camera is too close to the coma reducer, though Baader specify the correct distance and that is what I have used. I contacted them and they tell me that a full frame camera should be fully corrected. Even if that is the problem I still have the asymmetry between each corner so why is that? Is it some tilt of the focusser or something else?  I have the 5" Skywatcher and the problems are worse with that. More ideas appreciated.

Is there some way of sorting this out in the daytime as 5 sec or 120 sec exposures give the same result and the centers are sharp so I not convinced it happens due to tracking issues?



#11 mxpwr

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Posted 29 October 2020 - 12:28 PM

It took me several attempts to get my Newtonian dialed in and following this tutorial did the trick.

https://youtu.be/8ROvNH5uwDo

It's a bit cumbersome, but I only had to do it once.

#12 Kevin_A

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Posted 29 October 2020 - 03:06 PM

On second look I do think your collimation is definitely not great (coma in 2 corners).... and even your center stars are a bit off looking (donuts) 

but it is hard to tell if in perfect focus from the low res CN image.

 

You may have multiple issues besides spacing.


Edited by Kevin_A, 30 October 2020 - 11:05 AM.


#13 deansjc

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Posted 29 October 2020 - 06:26 PM

A second pair of Ontario eyes might not help here: to me the stars are either very fine in the center or fine enough that if you can achieve the same roundness across the field, edge to edge, you will probably be very satisfied :-)  Regardless, your path to happiness (in the short term ;-) is to get round stars near to the edges.

 

I suspect that there is a utility out there that can scrutinize stars for roundness down to the pixel image.  If not, there should be, and if a utility can diagnose, that would be great.

 

I think that more difficult feats have been accomplished.

 

Are any CNers aware of such a tool??



#14 06AwzIyI

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Posted 29 October 2020 - 07:39 PM

The only one I know of (other than Photoshop) is startools, using the repair tool.

#15 Kevin_A

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Posted 30 October 2020 - 10:09 AM

A second pair of Ontario eyes might not help here: to me the stars are either very fine in the center or fine enough that if you can achieve the same roundness across the field, edge to edge, you will probably be very satisfied :-)  Regardless, your path to happiness (in the short term ;-) is to get round stars near to the edges.

 

I suspect that there is a utility out there that can scrutinize stars for roundness down to the pixel image.  If not, there should be, and if a utility can diagnose, that would be great.

 

I think that more difficult feats have been accomplished.

 

Are any CNers aware of such a tool??

Not sure how this helps at all fixing the issue here bud?

 

Unfortunately the image posted is a low resolution and on my screen it shows round center stars but with a slight donut effect (slightly darker in the centers) which could be a focusing issue that would amplify the corner issues further than normal. 

 

All corners in your picture show slight elongation with 2 of them showing definite coma. This would to me show a few areas to check and collimation is an easy one to check with a reflector.

Any coma issues at all (with a properly adjusted field flattener/coma corrector) should be re-verified by simply verifying collimation is good to start with. Once you know its good then you can rule it out. It is not uncommon for all reflectors to lose collimation at any time... it just happens when moving them etc.

 

I just had a very similar experience with a refractor but unfortunately it was unfixable (bad collimation and no adjustability) and it was replaced by the manufacturer. I adjusted the back spacing on the FF back and forth and even sent them a Bahtenov mask focus photo to verify to them the scope was indeed in perfect focus when taking photos accompanied by the example star shots.

 

So, really what I am saying is... first check collimation to rule it out, then when imaging, look at your target and make sure it is in perfect focus and not just round stars in the center... because, stars can be out of focus slightly and still be round which will amplify the corner star issues further...  and then adjust back focus to get rid of any elongation and what is left is maybe some residual coma or a perfectly flat image. You may have a bad coma corrector or multiple issues that reduce the ability of the corrector to do its job perfectly.

 

There is no tool available for checking everything, but you can get expensive software that can tell you if your sensor is tilted and you can use software or masks to see if you are in perfect focus.... coma can be checked by a $30 tool and elongated stars are variables that are trial and error based on back spacing.... but alas...no magic tool for everything. It can be a frustrating hobby when adjusted properly isn't perfect!

 

Cheers, I hope this free checking tool helps!


Edited by Kevin_A, 30 October 2020 - 11:08 AM.


#16 Moorefam

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Posted 31 October 2020 - 06:34 PM

Kevin,

Thanks for that advice. I looked at an image with my 5" Newtonian instead of the 8" and the problems were very similar in the same corners. I am going to again try different backfocus even though the spacing is per coma reducer spec.The results are similar on different nights with new collimations each time, which look good with my Cheshire eyepiece.I wish I had a different coma reducer to try. CCD Inspector reckons I have a 5 degree tilt in the focusser but it must be roughly the same on both scopes to give similar results with different scopes. I think focussing is good as the Sony allows *11 mag on the image on rearview and I also have a Bahtinov mask.  FWHM is usually above >4 pixels though on any night? I wonder if the secondary can be out a bit and the focusser also out a bit to compensation throwing everything off?



#17 Kevin_A

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Posted 01 November 2020 - 01:08 PM

Kevin,

Thanks for that advice. I looked at an image with my 5" Newtonian instead of the 8" and the problems were very similar in the same corners. I am going to again try different backfocus even though the spacing is per coma reducer spec.The results are similar on different nights with new collimations each time, which look good with my Cheshire eyepiece.I wish I had a different coma reducer to try. CCD Inspector reckons I have a 5 degree tilt in the focusser but it must be roughly the same on both scopes to give similar results with different scopes. I think focussing is good as the Sony allows *11 mag on the image on rearview and I also have a Bahtinov mask.  FWHM is usually above >4 pixels though on any night? I wonder if the secondary can be out a bit and the focusser also out a bit to compensation throwing everything off?

If the problems are exactly the same on both scopes then you can rule out collimation as the odds of that are basically zero.

Focus issues just amplify any issues further and are not the cause... just make matters worse.

I have had a bad coma corrector and some correctors just don't work well with some scopes.

Full-frame cameras cannot be corrected perfectly on some scope flatteners/correctors and....... does your scope info indicate it is good for full-frame cameras??? Some are not!



#18 SANDRO78

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Posted 01 November 2020 - 01:15 PM

Good morning.

I also had problems with my corrector. The most important thing is to respect the BF. 

And if that doesn't work, make a not-for-not-to-do BF.

 

Bye.

alex.



#19 tloebl

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Posted 01 November 2020 - 03:50 PM

I found the older MPCC required more than 55mm spacing. I wound up  closer to 60mm. Thing to do is get an assortment of spacers, available through Baader and I think I also saw the package on Amazon from 0.6 to 1.3mm. I think there were 15 in a plastic package.

 

Tom


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#20 Moorefam

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Posted 01 November 2020 - 05:44 PM

Thanks all, I will try increasing backfocus when the rain decides it's had enough! 




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