Is this sensor tilt?

[Iem1]

Morning guys,

Tried shooting NGC 5906 last night, but the session was cut short due to clouds, managed 30 minutes of useable data. 

I have been trying to improve overall image quality, and my back spacing is a little off, working on it by testing out in the field. However, when adjusting, I seem to be able to get only some of the outer edges looking ok at any given time. I can never seem to get the stars in all four corners to be round.

Here is an untouched 120 second sub from tonight, check out the bottom left corner compared to the top right.

 

None of the stars are perfect, struggled to get pin point stars tonight for some reason, guiding seemed ok to me (< 1.0" total), but to my eyes the edges are behaving differently? Im not sure how well you will be able to see them pasted here, so I will attach the image as a tif below for easier closer inspection too.

IMG_6701.tif

 

Final result, x15 120 s subs under a bright moon...Think we are going to need a bigger boat..  Would love a more appropriate galaxy hunting scope in the future!

 

 

 

 

[newbie alert]

To my eyes  on my phone the top one is worst than the bottom, seems to some improvement to the bottom one but I'd say you're too far away on your spacing

[Iem1]

Agreed, bottom one is stacked and processed and it seems to have helped a bit, but man those stars are terrible no idea why it was such a chore to get decent shaped stars last night...bad night at the office maybe. PA should have been good. I forgot the total error..but "Happy green face" from the plus and close to the next tier of accuracy for the PA, Guiding seemed OK too. 

Is it normal to have one corner worse than the other? Would this be solved by simply adjusting the spacing or is it indicative of something else perhaps. Concerned that if I adjust for the worst stars the slightly better stars will be thrown out of shape in the process. Or perhaps there is a fine line of balance to be struck somewhere.

Definitely noticing the spacing issue more now that I am guiding though. Its like all other boxes are generally ticked in the quality department, so the remaining flaws stand out like a sore thumb. It is no longer masked by the general instability that comes with an unguided sky guider pro for example

Can not wait to get a dedicated cooled astro camera next!

 

[Iem1]

3 hours ago, newbie alert said:

To my eyes  on my phone the top one is worst than the bottom, seems to some improvement to the bottom one but I'd say you're too far away on your spacing

Agreed, bottom one is stacked and processed and it seems to have helped a bit, but man those stars are terrible no idea why it was such a chore to get decent shaped stars last night...bad night at the office maybe. PA should have been good. I forgot the total error..but "Happy green face" from the plus and close to the next tier of accuracy for the PA, Guiding seemed OK too. 

Is it normal to have one corner worse than the other? Would this be solved by simply adjusting the spacing or is it indicative of something else perhaps. Concerned that if I adjust for the worst stars the slightly better stars will be thrown out of shape in the process. Or perhaps there is a fine line of balance to be struck somewhere.

Definitely noticing the spacing issue more now that I am guiding though. Its like all other boxes are generally ticked in the quality department, so the remaining flaws stand out like a sore thumb. It is no longer masked by the general instability that comes with an unguided sky guider pro for example

Can not wait to get a dedicated cooled astro camera next!

[michael8554]

The 120 sec sub has elongated stars in the middle.

Inspect a short exposure taken with high ISO, to minimise tracking/guiding errors.

It looks like there may be a tiny amount of tilt too.

Michael

[Adam J]

7 hours ago, Iem1 said:

Morning guys,

Tried shooting NGC 5906 last night, but the session was cut short due to clouds, managed 30 minutes of useable data. 

I have been trying to improve overall image quality, and my back spacing is a little off, working on it by testing out in the field. However, when adjusting, I seem to be able to get only some of the outer edges looking ok at any given time. I can never seem to get the stars in all four corners to be round.

Here is an untouched 120 second sub from tonight, check out the bottom left corner compared to the top right.

 

None of the stars are perfect, struggled to get pin point stars tonight for some reason, guiding seemed ok to me (< 1.0" total), but to my eyes the edges are behaving differently? Im not sure how well you will be able to see them pasted here, so I will attach the image as a tif below for easier closer inspection too.

IMG_6701.tif 51.29 MB · 2 downloads

 

Final result, x15 120 s subs under a bright moon...Think we are going to need a bigger boat..  Would love a more appropriate galaxy hunting scope in the future!

 

In all honesty it looks more like miscolimation of the scope or focuser to me than tilt. Best way forward is to perform a star test to see if the scope has a colimation error. Appart from that what I would suggest is to rotate the camera and see if the distortion pattern stays the same or if it rotates as you rotate the camera. 

Tilt is the goto for everyone when they see any optical aberration but to be honest its hardly ever tilt and much more often turns out to be a optical / colimation issue. 

Unless the Z73 now has a colimatable lens cell then there may well be no way to correct this. 

Adam

Edited August 15, 2022 by Adam J

[michael8554]

The OP's astro modded DSLR may have been reassembled with a small amount of sensor tilt...........

Michael

[Iem1]

Thanks for the input guys, I have just been looking at my data from the other night. Unfortunately I do not have any short exposure times saved to compare. So I am not sure how relevant this comparison is;

3 minute untouched exposure

 

NA tif.tif - tif for closer inspection

 

To my eyes these stars are a lot better, still some warping around the edges, but the stars look a lot more uniform in their imperfections I think. Though it could just be a longer exposure with more bloated stars hiding the issue.

Only difference with this image would be a change in back spacing and camera orientation.

[Iem1]

Another recent night (90 seconds I believe)

Pillars tif.tif

Edited August 15, 2022 by Iem1

[Adam J]

11 minutes ago, Iem1 said:

Thanks for the input guys, I have just been looking at my data from the other night. Unfortunately I do not have any short exposure times saved to compare. So I am not sure how relevant this comparison is;

3 minute untouched exposure

 

NA tif.tif - tif for closer inspection

 

To my eyes these stars are a lot better, still some warping around the edges, but the stars look a lot more uniform in their imperfections I think. Though it could just be a longer exposure with more bloated stars hiding the issue.

Only difference with this image would be a change in back spacing and camera orientation.

Its not back spacing, at least its not just back spacing or all corners would be out, the fact that the camera orientation is different is suspicious of a colimation error, however, it doesn't have to be just one issue, it could be DSLR sensor tilt and colimation combined. Only a star test will reveal the true problem.  One thing is certain and that is that you cant correctly adjust tilt until you have corrected the other aberrations first.  Another strong possibility is that the corrector is tilted relative to the optical axis, this essentially causes very similar issues to the objective being miss colimated. I am not really a fan of any scope that cant have its colimation adjusted. 

Adam 

Edited August 15, 2022 by Adam J

[Adam J]

Take a look at the discussion between me and @alcol620 here and note that his stars look similar to yours prior to him adjusting the objective colimation. 

Look on page 9. 

 

Adam 

[michael8554]

All those earlier subs show the same elongation in the centre of frame.

If you have eliminated collimation, then it's PE and PA.

A single short exposure should settle the argument.

Michael

[Iem1]

Found a 1 second exposure from a recent night in my recycling bin

 

1 sec tif.tif

 

Not sure if this was straight after moving, so not sure if it particularly credible 

 

[newbie alert]

Looking at all the images they're all inconsistent, so it could be guiding errors, tilt, spacing, focuser sag, un- solid fixings to the camera, delrin spacers...  Lots of reasons that can cause these errors so need to tackle them one at a time 

Edited August 15, 2022 by newbie alert
Changed

[michael8554]

I've stretched the 1 second exposure to show the stars, okay would you say ? 

A 5 sec or 10 sec exposure may be clearer.

Michael

[Iem1]

7 hours ago, newbie alert said:

Looking at all the images they're all inconsistent, so it could be guiding errors, tilt, spacing, focuser sag, un- solid fixings to the camera, delrin spacers...  Lots of reasons that can cause these errors so need to tackle them one at a time 

I agree. It is hard to pinpoint (pun intended) the exact cause, but i do think it is a number of things. No doubt back spacing is off, just hoping I can rectify the worst of it by making adjustments. I will try my best to solve that first.

1 hour ago, michael8554 said:

I've stretched the 1 second exposure to show the stars, okay would you say ? 

A 5 sec or 10 sec exposure may be clearer.

Michael

I have stretched the original file and converted to png and cropped each corner, hopefully make it easier to see. But to me, they look at lot better than the long exposures, which if I am not mistaken is a good sign? 

Top right

 

Top Left

 

Bottom right

 

Bottom Left

[newbie alert]

On my phone it looks like you're slightly too long to me