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star shape issue


Erquy
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Hi,

 

I have a little star shape defect in my images which starts bothering me: See below, an example of a very bright star, from a 10 min raw image (non processed) and they all appear the same where ever their position (center, corners etc..): all stars are pretty well round, but the light intensity seems always more on the right side and softer on the left side. It is taken through a C8 and SX694 camera, bin 2 

Untitled.thumb.jpg.f7632db98928dc403fdf9390c2d90951.jpg

 

I have done multiple checks and I am pretty sure, this is not related to

- guiding (a very short exposure of 1 s or much longer one does not matter, there is always exactly the same defect and my guiding is very good with my Avalon m-uno <0,6 rms arcsec typically, so well below my image scale), nor flexure -I use OAG- . No condition related (0m/s wind, good atmospheric condition, the problem is there)

- collimation is pint point (checked in many different ways, on the basic of diffraction rings they are very fine -visually like through imaging)

- My polar alignment is very good (below sub arc, it is permanent set up based on very long drift alignment procedure) 

- nothing in the imaging train (FR or filters or not does not make any difference, the defect is always there)

- to the camera itself: if I rotate the camera by 90 degrees, this defect is always towards the right of the picture -> would this be specific camera? (but a bit strange as I have another telescope and I do not see this issue -a WO GT81 refractor but maybe I do not see if as totally different focal length and scope?). There is a possible adjustment on SX camera to tilt the sensor, but for what I read, it would affect more about the focus plane, and in my case, the focus seems the same wherever on the picture, so I am not sure this could solve the issue (I have not tried to play with that)

 

Any ideas what could be the problem?

 

Now I can say it is not the end of the day, as when I fully process the picture, the defect is hardly if not at all seen on the final results . As example, the star I picked as example above is the main center star of 1* 10 min sub which give the following results once processed http://www.astrobin.com/263197 

 

Thanks for your input

 

Ha_Unk_250_221429_007_Preview01.jpg

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I still think it might be collimation, I know you checked your collimation but have a look at a bright star at near focus through an eyepiece, try tweaking your secondary toward the direction where the spill is. You might find that there is a slight spill visible as the star nears focus but your diffraction rings look spot on.

 

 

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Hi,

 

I cannot see how collimation come into play, as I mention turning the camera 90 degree, still give the defect in same way on the picture (higher intensity of the star on the right, softer on the left of the star, what soever I rotate the camera).  If it was a collimation issue, shouldnt then the position of the defect change on the picture itself as when I rotate the camera ? (rotating the camera 180, shall then turn the high intensity on the left and the soft on the right side?)

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I did look at the flat field from camera to scope and it was flat, I do not think a takahashi  would be out and the 694 chip is only 2200x2750 . I think my was to the Heq5 pro mount not up for the job . you have a good mount, so I think we can remove scope and mount as to why. as you put you can not see it in the photo.  you was doing 2x2  how do 1x1 look.

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1*1 is also giving this problem (just more detailed of course), but I typically use 2*2 when imaging with the C8, no need to get down to that image scale/using 1*1

I am now checking further other shots and I would say it seems also I have this with my WO81 (1*1), but much much less pronounced than with the C8. Also it is the strongest with the C8 without FR (comparing so 2000 vs approx. 1300 vs the WO81 at 380 with FR).

Also I notice this defect is stronger when I image in RGB (but it is not coming from the filter, as without any filter the problem is there) than in narrow band (and I image mostly narrow band, hence I am not bothering much)

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To add to the previous post, see below the case of 1*1 bin using the WO81: more intensity on the right than the left. I am quite picky I know, this is very small difference on a raw non processed image (and in this case, I choose I would say a "worst case" -on a large stars, it is really hard to see this slight higher intensity on right than left-, not like in my original post and case for the C8 example which I would say is typical and always easy to see-

 

 

Untitled.jpg

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My camera have been back and have some work done and Terry at Starlight give it a good look over as it have a line running down the left side from new . So  I never put this down to the camera . I got a new mount last year still not use it as I give up trying astro. To much like hard work. I have got the mount and camera still set up if  clear I run a test again to see what I come with.

Edited by Starlight 1
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and checking another set of data with the WO and an even clearer case this time (here again imaged in RGB -while I typically image in NB with my WO and hardly see such effect in NR, I guess the NR take away so much on the soft side, giving non noticeable defects (does that make sense??) . As always, once I processed my images, it is hardly shown on final processed image (apart of course if you go down to look at the pixel size as I show here). So I can live with it (but in any case it could still be good I am sure it comes from the camera or it comes from scope or something else)

Untitled2.jpg

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Clouds come over. I got one in lum pull it to 400 times and I say its may be on the left side more. So I have to do a lot more to tell . Steve very good on cameras he help my out a lot when I started out. PS just look out side  I not going to get any more to night so shut down shed.

Edited by Starlight 1
spelling no lights on
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Hi,

 

I did contacted Terry from SX and based on the input given, indeed he strongly suspect the antiblooming filter and a voltage issue leading to overflow to adjacent cells typical to the Sony sensor used (sounds then also confirming what I see: effect is strongest when I do not have any filter, a bit less with RGB and basically almost none in narrow band as saturated pixel are much less in NB).  I am not an electronic and sensor expert (but engineer physics background),  it all make sense to me.

He gave instructions how to fix it which seems simple to adjust and I will give it a try (as it involves my own risk of opening the camera, I do not want to give the instructions, if someone feel to have the same problem, then just contact SX to get instructions)

I will report after testing, I guess it will take a while, no clear skies forecasted ;-)

Thanks Steve for the tip! And as always great support from SX. I love my SX694, a great camera I can only recommend (even if I have this issue, I do not complain)

regards

 

David

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21 hours ago, steppenwolf said:

This is the anti-blooming gate out of adjustment. This can be adjusted by the factory (or end user under direction from SX) - adjust too far the other way and a black 'bite' will be taken out of the stars.

Steven, what do you mean as "a black bite will be taken out of the stars" if djusted too far the other way?

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33 minutes ago, Erquy said:

Steven, what do you mean as "a black bite will be taken out of the stars" if djusted too far the other way?

Hi David, here is a simulation image showing what happens if you increase the anti-blooming gate adjustment too far, you get 'suppression' of the star disk - the aim is for a fully saturated star to have no artefact either white or black.

star_bite.thumb.png.8e64a3559d6803fe4e67db3fb728b58e.png

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13 hours ago, steppenwolf said:

Hi David, here is a simulation image showing what happens if you increase the anti-blooming gate adjustment too far, you get 'suppression' of the star disk - the aim is for a fully saturated star to have no artefact either white or black.

star_bite.thumb.png.8e64a3559d6803fe4e67db3fb728b58e.png

Steve,

thanks! I think you may also have solved the mystery with the black "arrows" messing up bright stars in Liverpool Telescope data that I and @wimvb have been working on this spring. Here is an example from their own gallery (Wim and I have done our best to take these artifacts out in our processing). Wonder if their super advanced CCD camera (an 4096x4112 pixel e2v CCD 231) can be adjusted, or maybe they have not cared about it. At least the problem has not gone away over the years when I look at their data.

NGC6888

 

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Ouch, Göran, those are particularly bad!

Blooming is a pain to control and scientific-use cameras just accept the streaks which is why you often see them on images produced at research establishments as anti-blooming control produces non-linear results which is undesirable in research. In the amateur market where image appearance rather than linearity is required, anti-blooming control is very common although you can specify non-controlled cameras from some manufacturers as an option. Personally, I'd take the anti-blooming every time!

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I hope this thread gets wide attention because I can imagine some poor souls struggling to collimate their optics or tune out tilt when the culprit was the ABG. I knew about the bleeds from scientific cameras because I see a couple down here from time to time and, a while back, one of the SBIG scientific cameras was quite popular.

Olly

Edited by ollypenrice
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Hi again

 

I did adjustment according to instruction, and i had a little window to test yesterday evening (unfortunately high clouds and humidity). i tested on vega with my C8 and focal reducer, at different binning an short time. it looked all perfect, i then run on smaller stars and longer time to saturation and it also looked perfect (and no black bite). I did not use my dew heater to avoid any heat current defects, and the corrector plate quickly turn totally wet and more clouds came. So it looks the issue is adressed but i will need more good nights to confirm.

I am only concerned the sensitivity or other performance is  changed (in my test i tried one of my latest target and it looked like sensitivity went down but due to high clouds and humidity, of course the snr i got was far to be great, that was not conditions for imaging. those conditions were certainly the reason.

Steve would you know if this setting change on voltage of the antiblooming have any other negative influence?

Once i will have proper conditions, i will report again

thks again

David

 

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10 hours ago, Erquy said:

Steve would you know if this setting change on voltage of the antiblooming have any other negative influence?

Having the anti-blooming gate setting at the correct point will not have any adverse effect on the camera's operation - however, the very fact that your sensor does have anti-blooming structures built in does result in a small loss of sensitivity over a sensor without anti-blooming.

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Hi,

 

I had proper conditions tonight and happy to report the issue is definitively much better. Below are luminance at bin 1 (right) and bin 2 (left) for resp. 5 and 10 min after adjustment. Clearly no problem for bin 1 while for bin 2 I would say there is still a slight higher intensity on the right, but surely no more bleeding out of the star of 1-2 pixels as I have had so far

I have then taken shots of a previous targets and by analysis from pixinsight, I got lower FHMW, lower excentricity, to levels I have never reached so far . So to my satisfaction

Untitled.jpg

So thanks Steve and Terry from SX!

Edited by Erquy
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