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Help needed please - Skywatcher ED72 + OVL Field Flattener Strange star shapes at the edges.


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Hi everyone,

I've just taken delivery of the above and was anticipating a nice flat field with round stars all the way to the edges of images but frustratingly things are never that easy.

Some corners are worse than others but to various degrees, the stars all appear to point towards / away from the centre of the image which leads me to believe the DSLR camera sensor is too close but too close to what, the OVL field flattener? If this is indeed the issue, what spacers are needed please and how / where do I put them?

 

Thanks in advance

🙂

M51-ED72-FITS-DSS-STARTOOLS-GIMP.JPEG

ED72.jpg

ED72 BACK FOCUS.JPG

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This image has confused me even further, where exactly am I supposed to add extra spacing? I thought it was between the flattener and the camera, not the flattener and the telescope?

 

OVL-Field-Flattener-Adaptor-20225-2.jpg

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The FF corrects the image from the telescope, so the spacing from the rear of the FF to the camera sensor is crucial.

However, your stars are elongated top left to bottom right even in the centre of frame, which is due to guiding errors.

There is a sniff of real coma in the bottom left corner, as per the "too close" image, suggesting a little tilt too.

Michael

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Thanks Michael, I've ordered some spacers to see if they improve things. Even if I jump from too close to too far, at least this will hopefully tell me that "just right" is somewhere in between.

 

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On 01/06/2021 at 15:37, FaB-Bo-Peep said:

This image has confused me even further, where exactly am I supposed to add extra spacing? I thought it was between the flattener and the camera, not the flattener and the telescope?

 

 

The distance between the flattener and the telescope can be (and is) adjusted by the focuser so the distance in question is between the flattener and the camera.

Olly

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Posted (edited)

Ok so I bought a set of T2 / M42 extension tubes of 4/5/6/7/8/9mm to go between my camera and the OVL field flattener. Last night I had a play with the 9mm, figuring that should show a difference even if it ended up being too much. Anyway in my opinion this has significantly improved things but to me it still looks like a little more is needed so I will keep increasing by using combinations of the spacers until I feel like it's peaked.  I also used a Bahtinov focus mask which clearly showed that focus on my previous attempt was a fair way off.  Attached is a processed image with only the very edges of the frame cropped to remove stacking artefacts and nothing applied to the star shapes.  Do you also think it's moving in the right direction?

 

M51-BAT-EXT-DSS-STARTOOLS-JPEG.JPEG

Edited by FaB-Bo-Peep
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What spacing was that, have you tried at the recommended 55mm ?

Experiments like this should be based on a single sub, not a stacked image that contains guiding and stacking artefacts. 

Take a single sub at max ISO and short duration, then stretch that to look for coma etc.

Michael

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Posted (edited)

I think the Canon DSLR + "T"-ring pretty much gives the suggested 55mm and that was where my initial image was taken. 

Last night I added a 9mm spacer which obviously takes me way passed the recommended but I think the results speak for themselves but also think I still need to add a little more. 

The below snip is a side by side comparison of the bottom left part of a single sub from my original image vs the same area of last nights with the 9mm spacer ring.

OVL spacing.JPG

Edited by FaB-Bo-Peep
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