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Field flattener and color artifacts

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#1 barrabclaw

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Posted 19 September 2019 - 08:40 AM

Hi all,

 

I've been reading quite a bit, mostly here but wherever I can find some good info. I have an Explore Scientific 102mm refractor, Explore Scientific field flattener, QHY 183C imager, and Sirius mount. I bought them all used on CN. I've had some luck imaging, but my results could use improvement. Previously, I did not pay attention to the distance between the ES field flattener and the camera. I still got images, but they had lots of artifacts and I'm sure some of them were due to that mistake. I've now tried to correct for that.

 

My main concern is if I have everything setup correctly, especially with the field flattener. From what I can find out, the back focus of the ES field flattener is 55 mm. QHY seems to be telling me that the backfocus of the 183C is 17.5 mm. I have 38 mm of extension tubes between the flattener and the camera. There seem to still be some funny artifacts in my images. I linked a picture of my setup with the ES field flattener and extension tubes labeled.

 

I also linked my image of the Dumbbell Nebula. This image is a stack of 105 15s subs. I am not guiding, hence the short exposures. I used SharpCap for polar alignment until I got an "Excellent" result. I think it was within 10-20", the best I could get with the knobs on the Sirius. I stacked in Deep Sky Stacker and processed in AstroTools.

 

Does it look like my setup is approximately correct? The backfocus is off by 0.5 mm by my calculation, but it could be more if the measurements I have aren't precise.

 

Also, any obvious issues with the Dumbbell Nebula image? It looks like the colors aren't lined up - blue is shifted up/left, red is down/right. I would appreciate any feedback on equipment or the image itself.

 

Image link: https://imgur.com/a/omvp5yg

 

Thanks,

Brendan

 

P.S. How does everyone get their images down to 500KB to meet the size limit for attachments?

 

Edit: I started writing this last night and posted this morning. I see now there was another topic posted about the same FF last night. I think we're seeing different image issues, though.


Edited by barrabclaw, 19 September 2019 - 08:43 AM.


#2 drmikevt

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Posted 19 September 2019 - 10:53 AM

Something is up with the alignment of your system.  In the image you posted, you can see how the star cores are not centered in their stars.  This is an issue.  

 

 - I don't know how you are processing, but if you are splitting the color image into RBG images and then realigning then something may be happening there. 

 

 - But, another issue for you is how far you have to rack out the focuser to achieve focus.  With the focuser you are using, it is highly likely that you are getting significant sag in the focuser and that is is affecting the alignment of your system - and will change as the angle of the scope changes as it moves through the sky.  You will have many less headaches going forward if you add some extension rings BEFORE the focuser so that you only have to extend the focuser a short distance.  Smarter people then me can tell you if this might be playing a role in your alignment issues but, regardless, I would consider doing this.  



#3 barrabclaw

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Posted 19 September 2019 - 11:41 AM

Thanks for the tip. The imager is an OSC CMOS. I'm not splitting the colors up explicitly into RBG. I have tried using StarTools' ...tool... that lets you shift the color channels. That helps a bit. But I posted an image without that shifting done because I'd rather fix the problem upstream than during processing.

 

So you typically see more sag from an extended focuser than from extension tubes? I suppose that makes sense. So in the end, the chip may not be orthogonal to the long axis of the scope, and that could cause the color alignment issue?



#4 barrabclaw

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Posted 19 September 2019 - 12:09 PM

Actually it's probably even worse than that. What you're seeing is not the extended focuser, but two 2" extension tubes. Then the FF, then the smaller tubes, then the camera. I need those 2" tubes to be able to achieve focus.  I'm not sure how to put extension tubes before the focuser. Would they be very large extension tubes?

 

I also have more 2" extension tubes. Would I be better off using another of those so the focuser doesn't have to extend as far to achieve focus?



#5 astronate

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Posted 19 September 2019 - 01:29 PM

I have a Explore Scientific 127 and one of the ES Field flatteners and I’ll share what I have learned.

Extension tubes are a part of life with Explore Scientific scopes. And the focuser is ok, but not great. I think you might get a little less sag if you use 3 extensions (if possible). That way more of the focuser tube is being supported by the rest of the focuser mechanism. Physics is working against you though, that’s a large moment arm from the focuser to the camera. Check the tightness of all the screws including the grub screws that allow the focuser to rotate with respect to the main tube.

The 2” ES Field Flattener backfocus is a subject of a lot of discussion because they don’t have a history of documenting it well. I’ve seen 55mm and 45mm. For example the web page says it “now has 55mm of backfocus”. But I don’t know if that includes the one you have or mine. I use 45mm. If the stars in the corners are elongated then try the other distance.

The back focus probably isn’t causing the issue in your image.

Can you post one frame, not stacked, not cropped? That way we can see the stars without wondering about stacking artifacts.

-Nathan

#6 barrabclaw

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Posted 20 September 2019 - 07:55 AM

I have a Explore Scientific 127 and one of the ES Field flatteners and I’ll share what I have learned.

Extension tubes are a part of life with Explore Scientific scopes. And the focuser is ok, but not great. I think you might get a little less sag if you use 3 extensions (if possible). That way more of the focuser tube is being supported by the rest of the focuser mechanism. Physics is working against you though, that’s a large moment arm from the focuser to the camera. Check the tightness of all the screws including the grub screws that allow the focuser to rotate with respect to the main tube.

The 2” ES Field Flattener backfocus is a subject of a lot of discussion because they don’t have a history of documenting it well. I’ve seen 55mm and 45mm. For example the web page says it “now has 55mm of backfocus”. But I don’t know if that includes the one you have or mine. I use 45mm. If the stars in the corners are elongated then try the other distance.

The back focus probably isn’t causing the issue in your image.

Can you post one frame, not stacked, not cropped? That way we can see the stars without wondering about stacking artifacts.

-Nathan

Hi Nathan,

 

Thanks for your experience. I'll try using more extensions and the least possible focuser extension. I got this telescope used, and there is only one screw holding the focuser in. It seems pretty stable, but maybe it isn't straight. I'll see what I can do about that; not sure what size the threads are.

 

I've seen a lot of discussion about that FF also. I'd like to just some images without the FF at all, but I don't have the adapted I would need. I think I would need a 2" to M42 adapter that would connect directly to the 2" extension tubes on one side and to the M42 extension tubes that are also in my current setup. Then I would bypass the 2" tightening friction adapter (what do you call that piece?) and the FF itself. I could try at 45 mm too.

 

I put up one of my subs at the link below. It's 40 MB, so I wasn't sure how else to share it. CN and imgur have pretty small limits. I didn't want to mess with it to make it smaller, because that might have removed some useful info for diagnosing the issue.

 

https://drive.google...iew?usp=sharing

Thanks,

Brendan



#7 RJF-Astro

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Posted 20 September 2019 - 09:42 AM

Hi Brendan, this looks a lot like tilt. The elongation is not radial, so that does not make a backfocus problem likely. Instead is goes in one direction. The lower right corner seems to be worse compared to the upper left.

 

ZWO have a quick check on their site to test if its optical or sensor tilt. They use CCD Inspector, but I'd say visual can learn you a lot. CCD Inspector is expensive, although there is a free trail period if you have not used it yet.


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#8 drmikevt

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Posted 20 September 2019 - 09:46 AM

What this shows are significant tracking errors and/or tilt.  Your stars are all very oblong, and all in the same direction.  This implies that its' not the flattener, but the tracking and guiding that is the main issue here (or tilt - and probably a combination of both).  If the issue was coma from the flattener, all the stars would be 'pointing' to whatever edge was closest.  


Edited by drmikevt, 20 September 2019 - 09:47 AM.

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#9 barrabclaw

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Posted 20 September 2019 - 09:58 AM

Hi Brendan, this looks a lot like tilt. The elongation is not radial, so that does not make a backfocus problem likely. Instead is goes in one direction. The lower right corner seems to be worse compared to the upper left.

ZWO have a quick check on their site to test if its optical or sensor tilt. They use CCD Inspector, but I'd say visual can learn you a lot. CCD Inspector is expensive, although there is a free trail period if you have not used it yet.


Thanks for your thoughts. It's nice to have the issue potentially identified so I can try to work on it. I was at a bit of a loss as to what to do next. If I rotate the camera and see if the direction of the artifact changes, I should know if the camera is tilted. I could also try gently nudging the imaging train to see if there is some give.

I'll check out ZWOs website later tonight/this weekend and see what they have to offer. We're supposed to have storms this weekend but maybe I can figure out what's tilting just by manually inspecting everything indoors.

#10 barrabclaw

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Posted 20 September 2019 - 10:00 AM

What this shows are significant tracking errors and/or tilt. Your stars are all very oblong, and all in the same direction. This implies that its' not the flattener, but the tracking and guiding that is the main issue here (or tilt - and probably a combination of both). If the issue was coma from the flattener, all the stars would be 'pointing' to whatever edge was closest.


I did consider that as well. That's why I took short 15s exposures. My polar alignment was quite good using SharpCap, but of course the mount could be doing something funny. I'm not using any guiding.

Would a tracking issue show up as the colors being shifted, though? But like you said, it could be both things happening together.

#11 RedLionNJ

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Posted 20 September 2019 - 12:30 PM

 

. I used SharpCap for polar alignment until I got an "Excellent" result. I think it was within 10-20", the best I could get with the knobs on the Sirius. 

 

No way are you getting 10-20 arcsec alignment, particularly (as you qualified) with the adjustments possible on a stock Sirius.  This number is bogus.

 

I suspect your alignment is off quite considerably.



#12 drmikevt

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Posted 20 September 2019 - 12:43 PM

No way are you getting 10-20 arcsec alignment, particularly (as you qualified) with the adjustments possible on a stock Sirius.  This number is bogus.

 

I suspect your alignment is off quite considerably.

Also, if it was just tilt, we would (I think) be seeing the star shapes differ considerably across the image, but we don't.  The stars are all about the same size and length and shape.  If this was just tilt, the tilt would be quite considerable and we should see obvious differences from one side of the image to the other in terms of star shape.



#13 RJF-Astro

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Posted 20 September 2019 - 01:07 PM

Also, if it was just tilt, we would (I think) be seeing the star shapes differ considerably across the image, but we don't.  The stars are all about the same size and length and shape.  If this was just tilt, the tilt would be quite considerable and we should see obvious differences from one side of the image to the other in terms of star shape.

There is a difference, although more stars in the corners would help:

 

Knipsel.PNG

 

Maybe try some short (tracked) exposures (a few seconds) on a rich starfield. If the effect is gone, the chance of a tracking error is much slimmer. 


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#14 barrabclaw

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Posted 20 September 2019 - 05:09 PM

No way are you getting 10-20 arcsec alignment, particularly (as you qualified) with the adjustments possible on a stock Sirius.  This number is bogus.

 

I suspect your alignment is off quite considerably.

That's certainly possible. I was just going off what SharpCap was giving me using its polar alignment tool (two plate solves, before and after a 90 deg rotation). I could take some longer exposures to see how much worse the stars look compared to a short exposure. Although qualitative, that would help determine if it's a polar alignment issue. I probably won't have a good chance for a few nights though.



#15 barrabclaw

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Posted 20 September 2019 - 05:32 PM

I should also mention that I imaged for about an hour. I had a crosshair/circle on the display to help centering. The circle was about as big as the dumbbell nebula in the image. After an hour, the nebula was still within the circle. So that amount of streaking can't all be from tracking, or I would have quickly lost the nebula from my field of view. Those were only 15s images, so that much motion over an hour would have been quite considerable.



#16 barrabclaw

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Posted 22 September 2019 - 11:04 AM

Hi Brendan, this looks a lot like tilt. The elongation is not radial, so that does not make a backfocus problem likely. Instead is goes in one direction. The lower right corner seems to be worse compared to the upper left.

 

ZWO have a quick check on their site to test if its optical or sensor tilt. They use CCD Inspector, but I'd say visual can learn you a lot. CCD Inspector is expensive, although there is a free trail period if you have not used it yet.

 

So I was able to determine that when I turn the camera, the direction of the color shift in the image changes. So say red is on the left, blue on the right at "0 degrees." If I turn the camera 90 degrees, red will be on the top, blue on bottom. If I turn the camera a total of 180 degrees, red is on the right, blue on the left.

 

By "turning the camera" I mean everything past the compression ring - the FF, the small extension tubes, and the camera.

 

That indicates to me that the issue is upstream - the telescope itself, or a sagging focuser. However, when I did this test, the scope was almost perpendicular to the ground, so that should have been minimal. Could the issue be with misaligned optics? This is a telescope I bought used, and it got sent through the mail.

 

I just received an adapter that will let me attach the camera without the FF. I'm going to try that as well, just to really rule out that piece.



#17 RJF-Astro

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Posted 23 September 2019 - 01:58 AM

That indicates to me that the issue is upstream - the telescope itself, or a sagging focuser. However, when I did this test, the scope was almost perpendicular to the ground, so that should have been minimal. Could the issue be with misaligned optics? This is a telescope I bought used, and it got sent through the mail.

Could be! But I am not an expert on this. You could try reposting this issue on the refractor forums.

 

If you have a cheshire eyepiece you can use it to check the collimation of a refractor. I found this pdf a while back, can be helpfull. This thread about collimating ES refractors might also be usefull.




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