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Refractor Collimation? SV70 -- How do you fix?

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#1 ks__observer

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Posted 31 May 2020 - 04:19 AM

I think I have a refractor collimation issue.

I tried increasing the spacing and it only created cross shaped stars at the corner.

My last shot from a few months ago looked pretty clean.

Not sure what is going on.

Help needed . . . . 

Link to Fits file as jpeg is a little hard to see oblong stars at corners:

https://www.dropbox...._Light.fit?dl=0

 

gallery_263201_9741_3865526.jpg


Edited by ks__observer, 31 May 2020 - 04:29 AM.


#2 james7ca

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Posted 31 May 2020 - 04:50 AM

You'll probably have to contact Stellarvue to see whether this scope even has a lens cell that offers adjustments for collimation. I think there are several different versions of the SV70 and they may not be all the same (in terms of both the optics and the lens cell).


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#3 james7ca

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Posted 31 May 2020 - 05:32 AM

I'd look at tilt in the focal plane and spacing for the reducer/flattener before I'd worry about collimation.



#4 ks__observer

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Posted 31 May 2020 - 05:53 AM

It appears in all four corners -- so i don't think tilt.
Also, my OAG stars were oblong and i rotated the OAG 180deg and still oblong. Only the reducer and OTA in front of OAG.
As noted in first post, i even adjusted the spacing outward by about 1mm and that didn't help. In fact it created cross shapes which indicates i went too far in spacing; plus i still had the radial oblong abberation, which I presume is collimation.

#5 ChrisWhite

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Posted 31 May 2020 - 06:10 AM

There is some asymmetry going on so I think there is a little bit of a tilt component, although could be caused by a miscollimated cell.  Whetver is going on it's really bad.  Even stars in the center have a slight elongation to them. 

 

When you say your last shot looked "pretty clean" is that code for... not good but not too bad?  For your last shot was it the same camera, or has anything changed in the imaging train? 

 

You can easily do a test to see if your spacing is playing a role. 

 

1) Achieve perfect focus on a center star.  Note the focuser position.

2) Now focus on a corner star.  It will probably "round out" and look much better with a little focus adjustment.  Note whether you needed to move the focuser IN or OUT to get better focus. 

3) If you needed to move the focuser OUT to get better stars in the corner, you need to REDUCE your spacing between the flattener and your sensor.

    If you needed to move the focuser IN to get better stars in the corner, you need to INCREASE your spacing between the flattener and your sensor. 

 

Based on what I am seeing, there is a problem with spacing.  BEFORE I would start playing with adjusting a lens cell, perform this test to see if you can get it better with spacing.  I'm not saying this is the only problem you have, but I do think it is a problem that you have. 

 

I would first dial in the spacing, second look for a source of tilt and third if you have been able to dial in the previous two... start suspecting the flattener and/or the scope.

 

These issues can be maddening!  Good luck!


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#6 ks__observer

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Posted 31 May 2020 - 07:45 AM

Here is a link a to pics from prior two SV70 outings:

17 min of Lum in March 2020

2 Ha Shots from October 2019 -- I think my spacing was a little off on these

https://www.dropbox....d9Nr1NkBYa?dl=0

 

Pixel peeing I can see things are far from perfect -- but not as bad as last night.

 

Tilt:

I don't think tilt because I have the issue is at all four corners.

 

Spacing:

I don't think spacing because i added a 1mm spacer and that did not help. 

In fact I got oblong stars perpendicular to radial indicating I added too much spacing.

If you are too far in you have radial oblong stars -- stars point toward center.

Too far out you have sagittal plane oblong stars (circular pattern perpendicular to the radial lines).

 

My last two outings with my 8in Newt, I think I had very good spacing with my 55mm back-focus coma-corrector, and my APO FR is 55mm back-focus -- I only added 0.7mm for the larger thickness narrowband filter.

 

I appreciate yours and James' response :)



#7 Salty_snack

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Posted 31 May 2020 - 09:05 AM

Looks like your sv70 is performing about as well as mine after multiple trips back to stellarvue. When the reducer/flattener was poorly collimated it was producing oblong stars across the entire fov. It was replaced by stellarvue. I still had problems in all corners with one corner being worse than the other three. Back to stellarvue my sv70 went. I learned that the lens cell is glued in place and is not able to be collimated. The focuser has basically no adjustments either and at best you can shim up the felt pads the focuser tube rides on.

I never did get a straight answer from them as to what was wrong and what they “fixed” but my sv70 still isn’t a great performer. Sorry to say the best way to deal with the sv70 that I’ve found is to sell it and not buy stellarvue products again.

#8 Madratter

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Posted 31 May 2020 - 09:16 AM

I don't see any clear collimation issue based on the image. But to really know, you should use a cheshire and check it. This is an easy/cheap test done during daylight.

 

See for example this thread:

 

https://www.cloudyni...or-collimation/



#9 james7ca

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Posted 31 May 2020 - 09:41 AM

...

Tilt:

I don't think tilt because I have the issue is at all four corners...

 

...I appreciate yours and James' response smile.gif

As Chris also noted, your corners are not equal. The upper right is much worse than the lower left and the right side (in general) is worse than the left side. That said, I'd look at tweaks to the spacing before trying to fix any potential issue with tilt, but you may have to adjust both.

 

Here is a 1:1 AberrationInspector matrix for your image that I created in PixInsight. Compare the upper right to the lower left.

Attached Thumbnails

  • L_2020_05_31_01_59_03_Bin1x1_120s__10C_G139_NGC_6969_Ha_Light_mosaic.jpg

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#10 ks__observer

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Posted 31 May 2020 - 10:28 AM

As Chris also noted, your corners are not equal. The upper right is much worse than the lower left and the right side (in general) is worse than the left side. That said, I'd look at tweaks to the spacing before trying to fix any potential issue with tilt, but you may have to adjust both.

 

Here is a 1:1 AberrationInspector matrix for your image that I created in PixInsight. Compare the upper right to the lower left.

Wow -- never knew about Aberration Inspector -- awesome stuff.

I will play with the spacing and take the image train apart and reconnect to watch for any tilt issue

 

Thank you!


Edited by ks__observer, 31 May 2020 - 10:28 AM.


#11 james7ca

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Posted 31 May 2020 - 01:06 PM

Here is an AberrationInspector matrix from my most recent H-alpha integration using a Stellarvue SV80ST2 and a Stellarvue 0.8X reducer with a ZWO ASI183MM Pro camera. This is from a stack of 30 subs that were exposed for 180s each at ZWO's lowest read noise gain setting. A simple auto stretch in PixInsight with no other processing (other than the DF calibration).

 

You can see that the stars aren't completely round, but they don't look that bad and this is with the standard 55mm spacing recommended for Stellarvue's reducer (no spacing optimizations or tilt corrections). There is also some vignetting and a little pattern walking noise, but those shouldn't cause any problems in my finished image (HaRGB).

 

The ASI183MM Pro is a little smaller than the OP's ASI1600MM, but the ASI183MM has smaller pixels so it's even more unforgiving in terms of optical problems.

Attached Thumbnails

  • M101_Ha_30x180s_Clip_Low_Range_Enabled_mosaic.jpg

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#12 ks__observer

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Posted 31 May 2020 - 11:41 PM

I think problem solved.

I will send pic tomorrow.

I think it was a loose camera rotator.

I noticed a lot of screws around the rotator, and which were all loose.  The rotator also has a compression ring type screw.  I have always been using the compression ring.

Now I made sure, as best as could, that the rotator was flush with the OTA tube and tightened everything down -- as you tighten the compression ring or screws you can see the rotator shift.

My corners are not perfect -- but far better than what I had last night.

Thanks for all the input!

Hope to have a pic out tomorrow.


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#13 ks__observer

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Posted 01 June 2020 - 07:52 AM

There is still some funkiness going on at the corners, but I think much more manageable than the prior night.

Maybe it will take a lot of subtle tweaking to get right -- and they say Newts are tough -- or maybe it is just this APO . . . .  

Overall, I am pleased with where I am.

 

Really appreciate everyone's input and help

 

gallery_263201_9741_61850.jpg

 

3 hour, 45min of Ha 5nm.

 

gallery_263201_9741_271163.jpg


Edited by ks__observer, 01 June 2020 - 08:30 AM.

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#14 Madratter

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Posted 01 June 2020 - 08:07 AM

The wider the field the tougher it is to correct the whole frame. Think of it this way - you are trying to cram a part of a sphere onto a flat surface. The wider the field of the view, the more obviously curved the part of the sphere, and the tougher it is to do without distortion.

 

Anyway, that is a very nice image of the crescent and its surroundings. :)


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#15 ChrisWhite

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Posted 01 June 2020 - 09:05 AM

Looking at that latest, you clearly have a spacing issue. 

 

I've recopied this on from my previous post to help you sort this out:

 

1) Achieve perfect focus on a center star.  Note the focuser position.

2) Now focus on a corner star.  It will probably "round out" and look much better with a little focus adjustment.  Note whether you needed to move the focuser IN or OUT to get better focus.

3) If you needed to move the focuser OUT to get better stars in the corner, you need to REDUCE your spacing between the flattener and your sensor.

    If you needed to move the focuser IN to get better stars in the corner, you need to INCREASE your spacing between the flattener and your sensor.

 

Once you dial in spacing, you can determine if you still have a little bit of tilt component. 

 

As MR says... nice image!


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#16 ks__observer

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Posted 01 June 2020 - 09:21 AM

It seems like a tilt issue because the right-side looks a lot worse than the left side.  But I could be wrong.



#17 ChrisWhite

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Posted 01 June 2020 - 10:31 AM

It seems like a tilt issue because the right-side looks a lot worse than the left side.  But I could be wrong.

No, it looks like there is *also* a tilt issue.  Spacing is definitely wrong. 

 

I've spent a lot of time dialing in spacing and working on tilt.  First you need to get your spacing correct... then you can address tilt.    You could try going the other way, but evaluating bad stars caused by bad spacing to see if you have improved tilt is kind of a fools errand.  Not easy. 

 

Fix the spacing, then fix the tilt.  You might be surprised at how much the tilt issue goes away once the spacing is correct. 


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#18 ks__observer

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Posted 01 June 2020 - 11:29 AM

To determine spacing you don't need to adjust focus -- you can look at the star shape at the corners:

 

post-37568-0-14196000-1531337976.png

 

 https://www.cloudyni...ocus/?p=8696478



#19 ChrisWhite

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Posted 01 June 2020 - 12:10 PM

To determine spacing you don't need to adjust focus -- you can look at the star shape at the corners:

 

 

 

 https://www.cloudyni...ocus/?p=8696478

In theory, but that chart seems to be a point of contention for many here on CN, so I don't recommend it as a reference anymore.  Some people say they are observing the opposite.  So I don't know if it is applicable across all optical systems.  Some people have native flat fields, some people use flatteners, others use reducers, some are coma correctors, etc...

 

The VERY SIMPLE process I recommend you do is the very process that Roland from Astro-Physics suggests people use. He knows one or two things about optical systems.  wink.gif

 

EDIT-  I will add that you can easily get a red herring if you are simply taking a photo and visually inspecting.  If you are in-focus or out-focus just a little bit it can also reverse what you might see in the corners.  The method of focus can also present red herrings.  I advise that when examining for spacing and/or tilt that you always focus on a SINGLE star in the center of the image.   Don't do a focus routine that is looking for best average focus across the entire field.  So use a single star routine, or focus manually on a single star in the center. 


Edited by ChrisWhite, 01 June 2020 - 12:15 PM.

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#20 ks__observer

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Posted 02 June 2020 - 04:02 AM

The above diagram is classic astigmatism plot.

https://www.handprin.../ASTRO/ae4.html

I suspect it should work across most optics.


Edited by ks__observer, 02 June 2020 - 04:02 AM.



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