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Pinched stars on RC8

astrophotography reflector imaging cassegrain
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#1 syxbach

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Posted 31 December 2019 - 12:52 PM

Dear folks

 

I am starting my RC8 imaging and start to see its power and resolution. It provides much more details than any of my previous refractors. I do have some problems with it. For collimation, I use the DSI method and it is very effective. The real problem is pinched 'triangle' stars (you can clearly see them when out of focus). Originally, I thought this was due to my reducer ccdt67. After I removed it, the pinched stars still showed up. 

 

The problem showed up during the second day of imaging, not the first day. More specifically, I set up everything in my backyard and the first day imaging was fine, I got pretty round stars. Then I left the setup there for the whole day and started to image on the second day night. Then the pinched 'triangle' stars were all over the imaging chip. I wonder if the camera was too heavy to cause the problem, or the temperature was too cold to exert pressure or squeeze on the primary mirror. 

 

The attached is my setup. RC8 with moonlite 2.5 and SBIG 8300 camera. My first image of this setup is Thor's Helmet. I have not done any post process with it. Just a HOO combination and it is really good. This image was done during the first day of the imaging so that there were no pinched stars. Thanks for any useful suggestions.

 

Best

 

Yuexiao

 

49305962432_f21b264b7f_k.jpg

 

 

49305962402_5590fbed77_k.jpg

 


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#2 sonny486

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Posted 31 December 2019 - 01:04 PM

Try taking a very short exposure on a bright star.

See if it's still there, I had flat stars due to my mount firmware (CEM60EC) , however that was corrected with the latest firmware.


Also I only take a guide image once every 8 seconds that really helped.



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#3 syxbach

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Posted 31 December 2019 - 01:18 PM

I took 5-10 second exposure, still there. The imaging was guided

 

Yuexiao 

Try taking a very short exposure on a bright star.

See if it's still there, I had flat stars due to my mount firmware (CEM60EC) , however that was corrected with the latest firmware.


Also I only take a guide image once every 8 seconds that really helped.



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#4 sonny486

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Posted 31 December 2019 - 01:21 PM

thinking about it now triangular means there are three points where there's a problem I would check your push pull alignment screws and see if maybe your screws are pushing against the primary mirror

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#5 syxbach

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Posted 31 December 2019 - 01:41 PM

I also think I tighten those screws too much. Temperature did go down below 0 C, which I think it may also affect. I will test it again. Thanks!

 

Yuexiao

 

thinking about it now triangular means there are three points where there's a problem I would check your push pull alignment screws and see if maybe your screws are pushing against the primary mirror

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#6 sharkmelley

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Posted 31 December 2019 - 01:55 PM

You said you see triangle stars all over the imaging chip.  It's not obvious at the scale of the image you've provided.  Can you show a 1:1 scale crop so we can see what you are referring to?

 

Mark


Edited by sharkmelley, 31 December 2019 - 01:59 PM.

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#7 einarin

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Posted 31 December 2019 - 02:01 PM

Yeah, would be better to see the image with the triangle stars.



#8 syxbach

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Posted 31 December 2019 - 02:04 PM

Mark those stars are over the entire images. The attached Thor's Helmet are stacked from good subs with round stars.  Here are some bad subs. 5-10 sec exposure. You can see whether small or bright stars, all stars are triangular. The target is double cluster. 

 

Yuexiao

 

 

You said you see triangle stars all over the imaging chip.  It's not obvious at the scale of the image you've provided.  Can you show a 1:1 scale crop so we can see what you are referring to?

 

Mark

 

Attached Thumbnails

  • IMG_1331.JPG
  • IMG_1329.JPG


#9 syxbach

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Posted 31 December 2019 - 02:05 PM

I attached those images. The bright star is in the center of the image.

 

Yuexiao

 

Yeah, would be better to see the image with the triangle stars.



#10 Peter Besenbruch

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Posted 31 December 2019 - 05:52 PM

I have trouble picturing a push-pull collimation setup doing this, as the screws typically push against the mirror cell, not the mirror. None-the-less, something is pinching a mirror. I have seen this on Newtonians when mirror retaining clips were too tight. Likewise, if you use a mirror cell that uses centering screws for the primary mirror, that can be a source of pinching.


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#11 syxbach

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Posted 01 January 2020 - 12:02 AM

Yes, this is what I did today. I disassembled my RC8 and found the centering ring holding the primary mirror was really tight. I adjust it and will see the star when the sky is clear. Thanks for the suggestion!

 

Best

 

Yuexiao 

I have trouble picturing a push-pull collimation setup doing this, as the screws typically push against the mirror cell, not the mirror. None-the-less, something is pinching a mirror. I have seen this on Newtonians when mirror retaining clips were too tight. Likewise, if you use a mirror cell that uses centering screws for the primary mirror, that can be a source of pinching.



#12 Peter Besenbruch

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Posted 01 January 2020 - 03:40 PM

Yes, this is what I did today. I disassembled my RC8 and found the centering ring holding the primary mirror was really tight. I adjust it and will see the star when the sky is clear. Thanks for the suggestion!

Here's hoping it works out.



#13 syxbach

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Posted 04 January 2020 - 02:35 PM

Hi Peter

 

Update some progresses on my RC8. I adjusted the primary retaining ring and recollimate the scope. Yesterday night I took it out for imaging. Originally I wanted to collect more data of the Thor's Helmet. However, seeing over that region was terrible. I got ~10 more O3 but the data was too soft. Early morning, I pointed the scope to those galaxies and did some test shots (this is the main reason why I got this RC8). Here are some single 10-min L subs, after moonset and calibrated with the flats I took early morning. What do you think of the collimation? Some subs are better and the others are not in terms of star shape. Do you think it is due to guiding or just collimation? Thank you!

 

Best

 

Yuexiao

 

49329336962_efcd0456a8_b.jpg

 

 

49328644928_5b8b4a2a73_b.jpg

 

49329115586_e9839aa36b_b.jpg

 

49328643943_c19d3238b6_b.jpg

Here's hoping it works out.


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#14 Peter Besenbruch

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Posted 04 January 2020 - 04:17 PM

Update some progresses on my RC8. I adjusted the primary retaining ring and recollimate the scope. Yesterday night I took it out for imaging. Originally I wanted to collect more data of the Thor's Helmet. However, seeing over that region was terrible. I got ~10 more O3 but the data was too soft. Early morning, I pointed the scope to those galaxies and did some test shots (this is the main reason why I got this RC8). Here are some single 10-min L subs, after moonset and calibrated with the flats I took early morning. What do you think of the collimation? Some subs are better and the others are not in terms of star shape. Do you think it is due to guiding or just collimation? Thank you!

Nice shots. At this point and resolution, I can detect a bit of out of roundness in the top right, and a light leak at the bottom left. I took the liberty of downloading the images and playing with them. Here is a snippet from the top, right corner of the Antennae image with the black level set and scaled 2x.

ant-tr.jpg

 

This may be a collimation issue, but there is no longer any pinching. It's also not much of a collimation issue to my mind.

 

Here is a larger section from the bottom left of the same image with the same black leveland scaled 2x.

ant-bl.jpg

 

Again, note the very slightly out of round stars, and the presence of noise/light. I tried the same black level settings with several of your photos, and the bottom left was always brighter, regardless of object. Not sure what's causing that. Actual astrophotographers should weigh in on that. Overall, the images look better. I think you found the cause of the pinching.



#15 syxbach

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Posted 04 January 2020 - 05:17 PM

Hi Peter 

 

Thanks for your comments, Here is the link for all images. Please be free to download them. The light leak at the bottom left is probably due to the light pollution from the town. I imaged them in my backyard. The Antennae is pretty low so the guiding was not as good as those galaxies up my head. M63 and M106 were very high when I imaged them. M51 and Antennae were relatively low. I also find another thread about RC non-round star. Do you think that is attributed to collimation?

 

Thank you!

 

Yuexiao

 

https://www.flickr.c.../150464529@N03/

 

https://www.cloudyni...ith-pain/page-2

 

 

 

 

 

Nice shots. At this point and resolution, I can detect a bit of out of roundness in the top right, and a light leak at the bottom left. I took the liberty of downloading the images and playing with them. Here is a snippet from the top, right corner of the Antennae image with the black level set and scaled 2x.

attachicon.gifant-tr.jpg

 

This may be a collimation issue, but there is no longer any pinching. It's also not much of a collimation issue to my mind.

 

Here is a larger section from the bottom left of the same image with the same black leveland scaled 2x.

attachicon.gifant-bl.jpg

 

Again, note the very slightly out of round stars, and the presence of noise/light. I tried the same black level settings with several of your photos, and the bottom left was always brighter, regardless of object. Not sure what's causing that. Actual astrophotographers should weigh in on that. Overall, the images look better. I think you found the cause of the pinching.



#16 syxbach

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Posted 05 January 2020 - 10:37 PM

Hi Peter

 

Tonight pinched triangle stars returned. Do not know what to do. The only thing I can summarize is the three nights when pinched star showed up, really cold. Need to do more trouble shooting:(

 

Yuexiao

Nice shots. At this point and resolution, I can detect a bit of out of roundness in the top right, and a light leak at the bottom left. I took the liberty of downloading the images and playing with them. Here is a snippet from the top, right corner of the Antennae image with the black level set and scaled 2x.

attachicon.gifant-tr.jpg

 

This may be a collimation issue, but there is no longer any pinching. It's also not much of a collimation issue to my mind.

 

Here is a larger section from the bottom left of the same image with the same black leveland scaled 2x.

attachicon.gifant-bl.jpg

 

Again, note the very slightly out of round stars, and the presence of noise/light. I tried the same black level settings with several of your photos, and the bottom left was always brighter, regardless of object. Not sure what's causing that. Actual astrophotographers should weigh in on that. Overall, the images look better. I think you found the cause of the pinching.



#17 Peter Besenbruch

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Posted 06 January 2020 - 12:24 AM

Tonight pinched triangle stars returned. Do not know what to do. The only thing I can summarize is the three nights when pinched star showed up, really cold. Need to do more trouble shooting:(

That's too bad.



#18 syxbach

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Posted 06 January 2020 - 01:50 AM

Solved, Peter. Just need to equlibrate for a while. At least 1-2 hour so that all material shrinking was done. Star became round again. I think this is why I got pinched star before as well.  All those days were really cold. Temperature dropped very quickly.

That's too bad.


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#19 sharkmelley

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Posted 06 January 2020 - 03:59 AM

Solved, Peter. Just need to equlibrate for a while. At least 1-2 hour so that all material shrinking was done. Star became round again. I think this is why I got pinched star before as well.  All those days were really cold. Temperature dropped very quickly.

That explanation makes sense.  However, it's worth contacting the retailer/manufacturer to find out if this is expected behaviour or if some adjustment is required.

 

Mark



#20 syxbach

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Posted 06 January 2020 - 11:28 AM

I got my answer from them. 'What you are seeing is pretty normal , especially when the temps are dropping quickly during a session'. Need more subs to average out!

 

Yuexiao

 

 

 

 

That explanation makes sense.  However, it's worth contacting the retailer/manufacturer to find out if this is expected behaviour or if some adjustment is required.

 

Mark



#21 syxbach

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Posted 08 January 2020 - 02:44 PM

Here is the first successful image from this RC8.

 

49352576166_1f95298af3_k.jpg

 

 

That's too bad.

 

That explanation makes sense.  However, it's worth contacting the retailer/manufacturer to find out if this is expected behaviour or if some adjustment is required.

 

Mark


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#22 sharkmelley

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Posted 08 January 2020 - 03:15 PM

Here is the first successful image from this RC8.

Beautiful result!

 

Mark


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#23 Peter Besenbruch

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Posted 08 January 2020 - 11:24 PM

Here is the first successful image from this RC8.

Looking good.


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