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Inline tilt corrector vs. integrated corrector in focuser base

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#1 John Tucker

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Posted 21 November 2019 - 06:06 AM

I purchased the inline tilt corrector below for my F4 Newt.  It is mounted inline between the corrector and the camera and am not having much luck with it. 

 

https://www.firstlig...t-adjuster.html

 

It occurs to me that maybe the problem is this:

 

  • The optical tilt in the system is probably due to non-orthogonality of the focuser and the primary mirror
  • The device attempts to compensate for this tilt by tilting the camera only, and in doing so, introduces tilt between the sensor and the coma corrector
  • All of which is probably fine if the amount of tilt is small and the Newt is not especially fast, but neither of these presuppositions holds in my case.  It seems that as I adjust the corrector to bring the sensor into the focal plane I end up with some parts of the sensor the wrong distance from the corrector.

Thus far I have not been able to track down the source of the tilt, and given that the system is sensitive to 0.1mm changes in focus or spacing, I suspect I won't be able to.  So I'm wondering if I need to swap out the focuser for a Moonlite or some other unit that offers tilt adjustment integrated into the base of the focuser.


Edited by John Tucker, 21 November 2019 - 06:09 AM.


#2 ChrisWhite

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Posted 21 November 2019 - 09:09 AM

John,

 

A few things to think about:

 

1) Adding a tilt corrector between the corrector and the camera is only useful for address a sensor that is tilted.  It will not help you to resolve tilt issues that exist due to focuser or OTA collimation error.   Have you ruled out whether the sensor is the issue or not?  You can do this by rotating the camera 180 degrees and leaving everything else the same.  Does the tilt follow the camera or does it remain where it was?

 

2) How are you collimating your scope?  Are you using a laser and/or a cheshire?  Newtonian collimation is tricky in the first place.  Try to add a reducer and image at f2.8 and that amplifies everything.  A laser/Chechire is not accurate enough to collimate a Newt for imaging at this focal ratio.  If you are using a catseye collimation set with hotspot on the primary, Autocollimator with offset pupil and Cheshire... you have a prayer of collimating the scope to be able to image at f2.8.  Collimation of the OTA is the number one issue when people experience issues like this.  It is less likely to be tilt in the focuser, but that is always a possibility.  Collimation must be perfect to rule out whether there is any collimation error due to the focuser.  Re-read that last sentence.  Then re-read the sentence that I bolded

 

3) If you are able to achieve perfect collimation and you still have tilt you can start to investigate the focuser.  With a moonlite focuser (or any that allow collimation of the drawtube and tilt of the base) you will be able to address issues that reside in the focuser.  There are two types of collimation error in a focuser . Tilt of the entire body, as well as axial alignment of the drawtube.  If the drawtube is not collimated accurately, you can cancel that out by tilting the base, but you will be unable to rotate the focuser to maintain collimation.  Collimating a focuser drawtube is tricky, but with a high quality laser and by removing the primary cell you can shoot a wall target off the secondary mirror and dial this in. 

 

The main thing here, is that most issues truly lie within the the collimation of the OTA.  If you are not using a system that amplifies collimation error, such as the catseye system, there is no way you are going to get perfect collimation with traditional tools and be able to image at f2.8... unless maybe you are Vic Menard.   That said, can you guess what tools Vic uses for collimation?  And as far as I know, he does not try to image at f2.8.  wink.gif

 

Lastly, the reducer you are using is not designed to produce pin-point stars across the frame.  It's just not up to the task.  Whether you have the ASA version, or the TS version, odds are if you correct all the error I discussed above, you will not get perfect stars.  If you want speed, you will have to deal with less than perfection.  If you want perfect stars, get a paracorr and image at f4.5.  If you want very good stars, get a GPU/Quattro and image at f4. 

 

All of this is stated, without seeing your issues in an image.  These are just my thoughts having imaged with three different Newts, three different focusers, 4 different correctors (including the reducer).  Good luck. 



#3 Alex McConahay

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Posted 21 November 2019 - 09:13 AM

If you have tilt in the focuser, get rid of it. 

 

As Chris pointed out, a laser collimator is not adequate to really collimate a scope. It is only really accurate after you have made sure the focuser is level and in the right position. For most of us this means we must collimate first with the whole set of collimation tools. Then we can touch up with a laser collimator. 

 

Alex


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#4 John Tucker

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Posted 21 November 2019 - 09:58 AM

John,

 

A few things to think about:

 

1) Adding a tilt corrector between the corrector and the camera is only useful for address a sensor that is tilted.  It will not help you to resolve tilt issues that exist due to focuser or OTA collimation error.   Have you ruled out whether the sensor is the issue or not?  You can do this by rotating the camera 180 degrees and leaving everything else the same.  Does the tilt follow the camera or does it remain where it was?

 

2) How are you collimating your scope?  Are you using a laser and/or a cheshire?  Newtonian collimation is tricky in the first place.  Try to add a reducer and image at f2.8 and that amplifies everything.  A laser/Chechire is not accurate enough to collimate a Newt for imaging at this focal ratio.  If you are using a catseye collimation set with hotspot on the primary, Autocollimator with offset pupil and Cheshire... you have a prayer of collimating the scope to be able to image at f2.8.  Collimation of the OTA is the number one issue when people experience issues like this.  It is less likely to be tilt in the focuser, but that is always a possibility.  Collimation must be perfect to rule out whether there is any collimation error due to the focuser.  Re-read that last sentence.  Then re-read the sentence that I bolded

 

3) If you are able to achieve perfect collimation and you still have tilt you can start to investigate the focuser.  With a moonlite focuser (or any that allow collimation of the drawtube and tilt of the base) you will be able to address issues that reside in the focuser.  There are two types of collimation error in a focuser . Tilt of the entire body, as well as axial alignment of the drawtube.  If the drawtube is not collimated accurately, you can cancel that out by tilting the base, but you will be unable to rotate the focuser to maintain collimation.  Collimating a focuser drawtube is tricky, but with a high quality laser and by removing the primary cell you can shoot a wall target off the secondary mirror and dial this in. 

 

The main thing here, is that most issues truly lie within the the collimation of the OTA.  If you are not using a system that amplifies collimation error, such as the catseye system, there is no way you are going to get perfect collimation with traditional tools and be able to image at f2.8... unless maybe you are Vic Menard.   That said, can you guess what tools Vic uses for collimation?  And as far as I know, he does not try to image at f2.8.  wink.gif

 

Lastly, the reducer you are using is not designed to produce pin-point stars across the frame.  It's just not up to the task.  Whether you have the ASA version, or the TS version, odds are if you correct all the error I discussed above, you will not get perfect stars.  If you want speed, you will have to deal with less than perfection.  If you want perfect stars, get a paracorr and image at f4.5.  If you want very good stars, get a GPU/Quattro and image at f4. 

 

All of this is stated, without seeing your issues in an image.  These are just my thoughts having imaged with three different Newts, three different focusers, 4 different correctors (including the reducer).  Good luck. 

Fantastic and thank you!

 

Just to be on the same page, my standards of what is acceptable are probably far less demanding than yours. At this point I'm not looking for the sorts of pictures I would post here and get oohs and ahs from a sophisticated AP audience.  I just want to get rid of the donuts and the stretched donuts.  Footballs are fine as long as they aren't in the middle 50% of the picture.

 

To address a few questions:

  •  Yes, I knew F2.8 would be hard when I went into this.  I also bought a non-reducing corrector, which puts me at F4.  Also quite difficult.  I'm actually having more trouble with the non-reducing corrector than with the reducing one at this point, but this probably reflects the relative amount of time I've put into each. 
  • The tilt I am seeing is independent of the camera orientation, and for that matter, the identity of the camera
  • I am collimating with a laser, chesire, and autocollimator from FarPoint.  The $400 or so I spent on this combo was pretty much what was left in the budget after buying the scope. The laser is correctly collimated.
  • I've been reading about star collimation and will give it a go tonight, but most of the time the stars in the middle of my field are nicely round.  Much worse as we move toward the edges, and some edges much worse than others. 
  • The identity of the worst corner varies consistently with the amount of spacing.  In general, and using my default orientation of the camera relative to the focuser, the stars on the bottom of the frame look best with about 9.2mm of spacing added and those at the top with about 9.7mm.  I understand from your post that this probably isn't the best way to look at the issue, as spacing is adding both mirror-sensor and corrector-sensor distance, and they may conflict with each other. 

Identifying and eliminating the source of the tilt seems quite challenging here, as I see substantial changes in my image with 0.1mm changes in spacing and with what are probably even smaller changes in focus.  Its just not clear to me that one can line things up well enough to eliminate 0.1 mm of tilt from a 400mm light path using the methods I have seen online ("Wrap a piece of paper around the inside of the tube.  Make a hole for the laser to shine through and mark where it hits the other side of the tube with a pencil.  Measure the distance from the edge of the paper to the hole and to the pencil mark.  They should be equal").  

 

Isn't this why they sell focusers with tilt compensation built into the base?

 

Thanks again!


Edited by John Tucker, 21 November 2019 - 10:05 AM.


#5 John Tucker

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Posted 21 November 2019 - 10:10 AM

If you have tilt in the focuser, get rid of it. 

 

As Chris pointed out, a laser collimator is not adequate to really collimate a scope. It is only really accurate after you have made sure the focuser is level and in the right position. For most of us this means we must collimate first with the whole set of collimation tools. Then we can touch up with a laser collimator. 

 

Alex

Thanks. 

 

I'm not sure the tilt resides in the focuser itself.  Inspecting the OTA, I suspect that the slight deformation of the carbon tube that occurs when a curved focuser base is bolted to a carbon tube of ever-so-slightly-different curvature at four points is a more likely culprit. 


Edited by John Tucker, 21 November 2019 - 10:10 AM.


#6 ChrisWhite

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Posted 21 November 2019 - 10:18 AM

John,

 

Can you post a RAW file that shows off what you are dealing with.  Also, what is the camera model?

 

EDIT-  FWIW, the reducer in question notoriously has one "bad" corner.  If you are able to get everything else looking pretty good and still have that one corner, that would be considered a very good job. 


Edited by ChrisWhite, 21 November 2019 - 10:19 AM.

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#7 John Tucker

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Posted 21 November 2019 - 10:28 AM

John,

 

Can you post a RAW file that shows off what you are dealing with.  Also, what is the camera model?

 

EDIT-  FWIW, the reducer in question notoriously has one "bad" corner.  If you are able to get everything else looking pretty good and still have that one corner, that would be considered a very good job. 

Thank you. At work now, but will post when I get home.




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