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Weird shaped stars

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#1 blackrose

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Posted 26 May 2020 - 06:00 PM

Dear Astrofriends,

 

I have a relatively old EQ5goto, a TSQ 65 telescope, a D800 standard imaging camera and an ASI120MM mini guidecamera with Orion 50mm guidescope controlled via ASIAIR. Stars in every photo I take with this setup, turn out to be elongated in the same general direction. Since I'm using ASIair there aren't any log files I can provide, but the curve is relatively smooth and reproducible. It generally resides under 2 arc-second total and RA Error is normally around 0.8 arc-seconds. I know that guiding curve is not everything, but I don't think my stars should look like as bad as this at the end of the day. I have a very good balance and neither dec nor RA axis fall, when clutches are released. I even tried being a little east heavy to no avail. I kinda ran out of options to figure out the problem. Since I'm an electrical engineer I have the necessary skills to open the mount and tune it and grease it, if you guys think that would solve my problem. The photo looks as attached. Would appreciate any kind of insight. Thanks in advance,

 

Clear skies,

Kamyar

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#2 D_talley

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Posted 26 May 2020 - 07:10 PM

Make sure that your scope is not suffering pinched optics, which that model is known for.  


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#3 imtl

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Posted 26 May 2020 - 07:24 PM

Differential flexure because you are using a guide scope


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#4 blackrose

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Posted 26 May 2020 - 07:26 PM

Make sure that your scope is not suffering pinched optics, which that model is known for.  

Which I assume the only way to figure that out is to take a very short exposure to eliminate mount error. If the optics are bad it should be sent to a professional if I'm not mistaken, which would cost ton of money?



#5 blackrose

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Posted 26 May 2020 - 07:27 PM

Differential flexure because you are using a guide scope

What does it mean? Guidecam sensor and main camera sensor are not parallel or they are not pointing to the same target?



#6 imtl

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Posted 26 May 2020 - 08:04 PM

What does it mean? Guidecam sensor and main camera sensor are not parallel or they are not pointing to the same target?

No. As temperature during the night changes, the imaging and guide scope thermally contract and expand and their alignment to each other varies as well. So, even if the guidescope is showing nice numbers of RMS etc, it depends where is it mounted in relation to the imaging scope. There are locations that have minimum differential flexures (such as close to the focuser of the imaging scope according to AP for example). In any case there are many posts here about the subject. Bottom line is, even if you secure your guidescope as tight as possible and in a place of minimal flexure, still there is always going to be some left. The question is, does your image scale show it. 

 

According to what you posted on your gear, your image scale is ~2.37"pp which is quite tolerant to seeing and tracking errors etc. However if you have a really bad differential flexure it might show. You wrote that you are getting usually below 2" guiding. Just below 2" is actually not that good. Getting around 1" is not difficult but it might be that your mount is not at its best shape. 

Make sure you secure the guide camera to the guidescope properly. And that you mount the guide scope very well. A picture of your setup will help.

 

Last thing I am thinking about is that you are showing a crop image of your full frame camera. Which part of the image is this? The top left corner? Can you upload the full frame image somewhere so we could take a look?

 

Eyal


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#7 drd715

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Posted 26 May 2020 - 08:08 PM

What does it mean? Guidecam sensor and main camera sensor are not parallel or they are not pointing to the same target?

It means that the guide image and the main camera image are not tightly (rigidly) attached to each other. Various reasons this could happen. Focuser or extension tube flex is most likely, but single point connections between scopes can be less than rigid enough for solid tracking - at least for narrower field tracking which can be more sensitive to flexure.

Sent from my SM-N950U using Tapatalk
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#8 blackrose

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Posted 26 May 2020 - 08:23 PM

No. As temperature during the night changes, the imaging and guide scope thermally contract and expand and their alignment to each other varies as well. So, even if the guidescope is showing nice numbers of RMS etc, it depends where is it mounted in relation to the imaging scope. There are locations that have minimum differential flexures (such as close to the focuser of the imaging scope according to AP for example). In any case there are many posts here about the subject. Bottom line is, even if you secure your guidescope as tight as possible and in a place of minimal flexure, still there is always going to be some left. The question is, does your image scale show it. 

 

According to what you posted on your gear, your image scale is ~2.37"pp which is quite tolerant to seeing and tracking errors etc. However if you have a really bad differential flexure it might show. You wrote that you are getting usually below 2" guiding. Just below 2" is actually not that good. Getting around 1" is not difficult but it might be that your mount is not at its best shape. 

Make sure you secure the guide camera to the guidescope properly. And that you mount the guide scope very well. A picture of your setup will help.

 

Last thing I am thinking about is that you are showing a crop image of your full frame camera. Which part of the image is this? The top left corner? Can you upload the full frame image somewhere so we could take a look?

 

Eyal

Thank you for the very detailed answer and your time. I finally managed to find somewhere to upload it. Link as follows:

 

https://ibb.co/r4yxzTT

 

Yeah exactly and factoring in the sensor size and pixel width of my camera, that 2 arcsecond error should translate into 1 or maximum two pixels of movement. What we are seeing is way more. I actually have seen some stuff about pinched optics on this particular telescope and it looks promising. As soon as I get clear skies, I'll look into it. Because if you look at my full res photo carefully, you would see that color channels are also misaligned, which means there is something bigger than mount error and guide error going on. Blue and red colors are bleeding out of starts, which should never even be a case in a triplet design.Wors case scenario would be blue bleeding (fringing) all around the star, not in a general direction.



#9 blackrose

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Posted 26 May 2020 - 08:25 PM

Thank you for the very detailed answer and your time. I finally managed to find somewhere to upload it. Link as follows:

 

https://ibb.co/r4yxzTT

 

Yeah exactly and factoring in the sensor size and pixel width of my camera, that 2 arcsecond error should translate into 1 or maximum two pixels of movement. What we are seeing is way more. I actually have seen some stuff about pinched optics on this particular telescope and it looks promising. As soon as I get clear skies, I'll look into it. Because if you look at my full res photo carefully, you would see that color channels are also misaligned, which means there is something bigger than mount error and guide error going on. Blue and red colors are bleeding out of starts, which should never even be a case in a triplet design.Wors case scenario would be blue bleeding (fringing) all around the star, not in a general direction.

In this link, the weird shape of stars kinda looks like mine.



#10 D_talley

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Posted 26 May 2020 - 08:41 PM

Which I assume the only way to figure that out is to take a very short exposure to eliminate mount error. If the optics are bad it should be sent to a professional if I'm not mistaken, which would cost ton of money?

 

If it is pinched optics this link will help.  

 

https://www.cloudyni...pinched-optics/

 

Most likely as mentioned above, flexure between the guide scope and main scope. 



#11 imtl

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Posted 26 May 2020 - 08:52 PM

I looked at your link. This is not the raw file but I managed to take a close look.

 

There are several things that I see. There is definitely elongation direction top left bottom right. I do not know the orientation of this image in relation to RA and DEC but in any case there is more than one thing going on.

1. You are not in focus.

2. You got some differential flexure.

3. I appears to also have some radial elongation as well (in "all directions"). Are you using a reducer? It could also very well be that your quadruplet is out of collimation. That can cause something like this.

4. Since it is not in focus it is hard to tell if there is pinched optics. I would leave that for now.

 

If you really want to investigate this. Make sure you are properly focused (Baht mask) and take an image of a rich star field.

Upload the raw file and we'll take a look.

 

A picture of your setup can also help identify what is going on


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#12 imtl

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Posted 26 May 2020 - 08:53 PM

Thank you for the very detailed answer and your time. I finally managed to find somewhere to upload it. Link as follows:

 

https://ibb.co/r4yxzTT

 

Yeah exactly and factoring in the sensor size and pixel width of my camera, that 2 arcsecond error should translate into 1 or maximum two pixels of movement. What we are seeing is way more. I actually have seen some stuff about pinched optics on this particular telescope and it looks promising. As soon as I get clear skies, I'll look into it. Because if you look at my full res photo carefully, you would see that color channels are also misaligned, which means there is something bigger than mount error and guide error going on. Blue and red colors are bleeding out of starts, which should never even be a case in a triplet design.Wors case scenario would be blue bleeding (fringing) all around the star, not in a general direction.

 

Sensor size has nothing to do with it. And what you wrote about translating into 1 or two pixels is also not correct. It depends on exposure time and on differential flexure


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#13 drd715

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Posted 26 May 2020 - 10:43 PM

So i put your download picture in my desktop for a better view. 

 

I  don't think you have a tracking problem. Some hot pixels and a wierd dark spot lower left side, but tracking seems fine for your equipment. 

 

I  can see the color offset in the  brighter larger stars. This could be a focus issue, but it could be a shifted certering between lens elements. Could it be pinching  -  maybe, but there is more than one kind of pinching. Pinching from too tight of threaded ring hold down and tab spacers (probably not in your scope, but check)  Or possibly the centering side push set threaded allen bolts (tiny) are too tight. Usually a set of 4  at 90* apart around the circumference of the cell at each individual lens level. Used to center the optical centers together so all the lenses work evenly to correct the color ca and focus centered. A star with color showing out opposite sides red-green-blue and not perfectly white is showing some optical miss alignment (or deficiency in design) .  The  non round edge on one side could be caused by optical miss alignment, but it could be from pinching one of the lenses when opposite centering screws are tightened against the lens instead of just barely or very slightly not touching the lens. Side loads on a lens will ever so slightly deform it at the  contact point. Notably if it is cold out and the cell shrinks more than the lens.

 

Also one of the lenses or the whole cell could be tilted slightly  - as in the optical cell is not 90* to the light path as it should be (center of the light cone through center of the focuser).

 

I  don't know the details of how your particular scope is engineered or to what level it is corrected for CA.

 

It does appear to be in need of some optical adjustment. You should probably contact the manufacturer for instructions or have an expert do the adjustments. 

 

Some optical systems are extremely sensitive to fine tuning adjustments especially low focal ratio scopes.


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#14 blackrose

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Posted 27 May 2020 - 03:37 AM

I looked at your link. This is not the raw file but I managed to take a close look.

 

There are several things that I see. There is definitely elongation direction top left bottom right. I do not know the orientation of this image in relation to RA and DEC but in any case there is more than one thing going on.

1. You are not in focus.

2. You got some differential flexure.

3. I appears to also have some radial elongation as well (in "all directions"). Are you using a reducer? It could also very well be that your quadruplet is out of collimation. That can cause something like this.

4. Since it is not in focus it is hard to tell if there is pinched optics. I would leave that for now.

 

If you really want to investigate this. Make sure you are properly focused (Baht mask) and take an image of a rich star field.

Upload the raw file and we'll take a look.

 

A picture of your setup can also help identify what is going on

Thanks for the answer.

 

1) I'm pretty sure I'm in focus(or as much as I can be with this scope), because I used a Bhatinov mask just seconds before this picture and the middle diffraction was bang in the middle of the X.

2) Yeah I think that might be also the case, so I'll read about it more and try to fix every problem source regarding this topic.

3) I'm not using a reducer. It's a bare scope with a IDAS LP Filter D1. This scope was advertised to grant edge to edge correction even for FF sensor.

4) Now that I told, you I was completely in focus, or rather as much as I could, what do you think?

 

I'll def do that, and let you know the results. Thanks again.

 

CS

Kami



#15 blackrose

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Posted 27 May 2020 - 03:52 AM

So i put your download picture in my desktop for a better view. 

 

I  don't think you have a tracking problem. Some hot pixels and a wierd dark spot lower left side, but tracking seems fine for your equipment. 

 

I  can see the color offset in the  brighter larger stars. This could be a focus issue, but it could be a shifted certering between lens elements. Could it be pinching  -  maybe, but there is more than one kind of pinching. Pinching from too tight of threaded ring hold down and tab spacers (probably not in your scope, but check)  Or possibly the centering side push set threaded allen bolts (tiny) are too tight. Usually a set of 4  at 90* apart around the circumference of the cell at each individual lens level. Used to center the optical centers together so all the lenses work evenly to correct the color ca and focus centered. A star with color showing out opposite sides red-green-blue and not perfectly white is showing some optical miss alignment (or deficiency in design) .  The  non round edge on one side could be caused by optical miss alignment, but it could be from pinching one of the lenses when opposite centering screws are tightened against the lens instead of just barely or very slightly not touching the lens. Side loads on a lens will ever so slightly deform it at the  contact point. Notably if it is cold out and the cell shrinks more than the lens.

 

Also one of the lenses or the whole cell could be tilted slightly  - as in the optical cell is not 90* to the light path as it should be (center of the light cone through center of the focuser).

 

I  don't know the details of how your particular scope is engineered or to what level it is corrected for CA.

 

It does appear to be in need of some optical adjustment. You should probably contact the manufacturer for instructions or have an expert do the adjustments. 

 

Some optical systems are extremely sensitive to fine tuning adjustments especially low focal ratio scopes.

Thank you very much. I think my strategy would be to check if the problem is very tight ring leaning on the lens element (pinching) which is my scope is notorious for and also is the cheapest way. If that and flexure reduction didn't help in the end, it should go to the professional maintanance.

 

CS

Kami



#16 blackrose

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Posted 27 May 2020 - 03:55 AM

Sensor size has nothing to do with it. And what you wrote about translating into 1 or two pixels is also not correct. It depends on exposure time and on differential flexure

I'll try to fix existing issues and see if the problem is solved. Thanks.

 

CS

Kami



#17 happylimpet

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Posted 27 May 2020 - 04:12 AM

I think thats entirely an optical alignment issue. The clue is how the stars fade out in one direction like a comet - rather than just being elongated as would tend to occur with differential flexure. It could be that though.

 

The easy test is to do short exposures that you know arent subject to diff flexure (like 5 seconds) and look at the star shapes. If they look the same its optical alignment/collimation. I bet it is.


Edited by happylimpet, 27 May 2020 - 07:24 AM.

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#18 imtl

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Posted 27 May 2020 - 07:32 AM

Do a star test. Look at the Airy rings. If they are concentric then at least that is something. If they are not then you got an optical misalignment between the elements.



#19 blackrose

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Posted 31 May 2020 - 01:40 AM

I think thats entirely an optical alignment issue. The clue is how the stars fade out in one direction like a comet - rather than just being elongated as would tend to occur with differential flexure. It could be that though.

 

The easy test is to do short exposures that you know arent subject to diff flexure (like 5 seconds) and look at the star shapes. If they look the same its optical alignment/collimation. I bet it is.

I talked to TS Germany and asked them if they can fix my telescope. They asked for a raw photo and at the end, they said it is perfectly collimated and they can do nothing to improve it and gave me this tip, which I have never heard:

 

He said I should focus on my stars at my worst corner to get a more well-rounded focus. Is that really a thing?

 

All in all, it is either flexure or pinched optics and I hope when I improve on these two, I solve my problem or I should just sell the telescope, because this shape of stars is not acceptable for me, since I'm putting this much effort and I get bad results, because of telescope design imperfection.

 

CS

Kami




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