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Tilt plate with digitial readout

Nathan Myhrvold
 

I got tired of trying to guess how many turns to twist the adjustment screws on my Octopi-Astro tilt adapter.  So in our shop we added 4 digital dial indicators.   Each one reads to 1 micron, but claims accuracy to 5 microns.   They were only $50 each on Amazon.



The idea is simple: before testing tilt I can zero out the indicators, and then use Hocus Focus to figure out how far out of tilt I am.   HF will tell you the number of microns you are out of square at each corner of the frame (well, sort of).   Ideally I would translate those into the number of microns out of tilt at the adjustment screws, but that will require some support in HF that is forthcoming.  However, at present the adjustment needed at the screws is approximately 3X the number of microns that HF tells me. 

The adjustment screws are 200 threads per inch, which means one turn is 127 microns.  Judging a partial turn is hard to do accurately.  Hence the dial indicators.

Then I just twist the adjustment screws until I get the right numbers on the dial indicators.  Since there is about a 3:1 ratio, I should in theory be able to get the sensor square to within 1.7 microns, which is more than enough.  This is on a very fast system (Planewave DR 350) and others with the same OTA say I need to be square to within 10 microns, so this is comfortable better than that.

The clearance between the edge of the Octopi-Astro adapter and the camera is such that I had to use an extension probe on the indicators.  That meant mounting them via stainless tubes so that the body of the indicator would not conflict with the QHY 600M camera.   I can remove them after adjusting tilt, or just leave them there.   Removal is no problem because they get zeroed before use.

This may seem like overkill but I use a mobile set up so I have to check tilt every time I move and relocate the scope.   Maybe it will turn out to be robust and stay square between being taken apart, moved and set up, but so far I am assuming that is not the case.     

--------------
Note that above I say "sort of" above because HF currently does not actually know where it's measuring the tilt - it is an average across about 1/9 of the frame near the corner.  A better approach would be to calculate the plane that the sensor is in, then from the equation output the number of microns at each screw location directly.  HF will implement that, but it isn't quite there yet.

Nathan
Marc Blank
 

This assumes that each turn of a screw precisely moves the camera the expected amount.  Given how the Octopi works, there’s no guarantee that the screw you’re turning is actually moving anything; it may well be free-floating.  So I don’t think this method is nearly fool-proof, without some additional work. 

Marc
Nathan Myhrvold
 

I already wrote a program to convert the 4 numbers that HF gives me to the numbers at the screws.    That is simple geometry.  It isn’t a simple multiplier.

 

But it isn’t perfect for several reasons.  The main one being that HF does not measure the offsets in a great way – there is some error in knowing where the offset is taken, and currently it does not output the equation of the plane.  The right answer is for HF to have you select your tilt adapter and just tell you the correct number at each screw.  George says he will add this. 

 

Nathan

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Marc Blank via groups.io
Sent: Wednesday, March 29, 2023 5:15 PM
To: main@ap-gto.groups.io
Subject: [EXTERNAL] Re: [ap-gto] Tilt plate with digitial readout

 

This assumes that each turn of a screw precisely moves the camera the expected amount.  Given how the Octopi works, there’s no guarantee that the screw you’re turning is actually moving anything; it may well be free-floating.  So I don’t think this method is nearly fool-proof, without some additional work. 

Marc
ROBERT WYNNE
 

How was the fixture calibrated? -Best, Robert

On 03/29/2023 4:46 PM Nathan Myhrvold <nathanm@...> wrote:


I got tired of trying to guess how many turns to twist the adjustment screws on my Octopi-Astro tilt adapter.  So in our shop we added 4 digital dial indicators.   Each one reads to 1 micron, but claims accuracy to 5 microns.   They were only $50 each on Amazon.



The idea is simple: before testing tilt I can zero out the indicators, and then use Hocus Focus to figure out how far out of tilt I am.   HF will tell you the number of microns you are out of square at each corner of the frame (well, sort of).   Ideally I would translate those into the number of microns out of tilt at the adjustment screws, but that will require some support in HF that is forthcoming.  However, at present the adjustment needed at the screws is approximately 3X the number of microns that HF tells me. 

The adjustment screws are 200 threads per inch, which means one turn is 127 microns.  Judging a partial turn is hard to do accurately.  Hence the dial indicators.

Then I just twist the adjustment screws until I get the right numbers on the dial indicators.  Since there is about a 3:1 ratio, I should in theory be able to get the sensor square to within 1.7 microns, which is more than enough.  This is on a very fast system (Planewave DR 350) and others with the same OTA say I need to be square to within 10 microns, so this is comfortable better than that.

The clearance between the edge of the Octopi-Astro adapter and the camera is such that I had to use an extension probe on the indicators.  That meant mounting them via stainless tubes so that the body of the indicator would not conflict with the QHY 600M camera.   I can remove them after adjusting tilt, or just leave them there.   Removal is no problem because they get zeroed before use.

This may seem like overkill but I use a mobile set up so I have to check tilt every time I move and relocate the scope.   Maybe it will turn out to be robust and stay square between being taken apart, moved and set up, but so far I am assuming that is not the case.     

--------------
Note that above I say "sort of" above because HF currently does not actually know where it's measuring the tilt - it is an average across about 1/9 of the frame near the corner.  A better approach would be to calculate the plane that the sensor is in, then from the equation output the number of microns at each screw location directly.  HF will implement that, but it isn't quite there yet.

Nathan

Nathan Myhrvold
 

You don't need to calibrate because the indicators are zeroed before testing tilt

From: ROBERT WYNNE <robert-wynne@...>
Sent: Wednesday, March 29, 2023 5:51:24 PM
To: main@ap-gto.groups.io <main@ap-gto.groups.io>; Nathan Myhrvold <nathanm@...>
Subject: [EXTERNAL] Re: [ap-gto] Tilt plate with digitial readout
 
How was the fixture calibrated? -Best, Robert
On 03/29/2023 4:46 PM Nathan Myhrvold <nathanm@...> wrote:


I got tired of trying to guess how many turns to twist the adjustment screws on my Octopi-Astro tilt adapter.  So in our shop we added 4 digital dial indicators.   Each one reads to 1 micron, but claims accuracy to 5 microns.   They were only $50 each on Amazon.



The idea is simple: before testing tilt I can zero out the indicators, and then use Hocus Focus to figure out how far out of tilt I am.   HF will tell you the number of microns you are out of square at each corner of the frame (well, sort of).   Ideally I would translate those into the number of microns out of tilt at the adjustment screws, but that will require some support in HF that is forthcoming.  However, at present the adjustment needed at the screws is approximately 3X the number of microns that HF tells me. 

The adjustment screws are 200 threads per inch, which means one turn is 127 microns.  Judging a partial turn is hard to do accurately.  Hence the dial indicators.

Then I just twist the adjustment screws until I get the right numbers on the dial indicators.  Since there is about a 3:1 ratio, I should in theory be able to get the sensor square to within 1.7 microns, which is more than enough.  This is on a very fast system (Planewave DR 350) and others with the same OTA say I need to be square to within 10 microns, so this is comfortable better than that.

The clearance between the edge of the Octopi-Astro adapter and the camera is such that I had to use an extension probe on the indicators.  That meant mounting them via stainless tubes so that the body of the indicator would not conflict with the QHY 600M camera.   I can remove them after adjusting tilt, or just leave them there.   Removal is no problem because they get zeroed before use.

This may seem like overkill but I use a mobile set up so I have to check tilt every time I move and relocate the scope.   Maybe it will turn out to be robust and stay square between being taken apart, moved and set up, but so far I am assuming that is not the case.     

--------------
Note that above I say "sort of" above because HF currently does not actually know where it's measuring the tilt - it is an average across about 1/9 of the frame near the corner.  A better approach would be to calculate the plane that the sensor is in, then from the equation output the number of microns at each screw location directly.  HF will implement that, but it isn't quite there yet.

Nathan

Chris White
 
Edited

Nice work, Nathan!  It's cool idea to quantify the adjustments. 

HF determines the micron adjustment recommendation by deriving the delta between the optimal focus position of a given corner with an average of the optimal focus position of all the corners. This is essentially a step measurement between the optimal focus positions, converted to microns.  So the recommendation is not likely to correlate with how much to adjust the screw.  I have found there to be relationship though as long as you are close to optimal backspacing.  With my Epsilon system if I turned the 200tpi screw a half a turn (for a 64 micron adjustment) a Hocus Focus run would yield about 15 microns in measured movement of that corner.  Keep us posted on how your test works out.
ROBERT WYNNE
 

Wonder how much play there is between threaded female components and threaded male components? Most of the off the shelf industry unless otherwise specified makes do with 75% threads allowing quite a bit of play between the two. Unless you're working with mil spec 100% threads I doubt either component comes close to 100% and thus the play between the parts and the inexplicable delta between the two. -Best, Robert 

On 03/29/2023 7:25 PM Chris White <chris.white@...> wrote:


Nice work, Nathan!  I cool idea to quantify the adjustments. 

HF determines the micron adjustment recommendation by deriving the delta between the optimal focus position of a given corner with an average of the optimal focus position of all the corners. This is essentially a step measurement between the optimal focus positions, converted to microns.  So the recommendation is not likely to correlate with how much to adjust the screw.  I have found there to be relationship though as long as you are close to optimal backspacing.  With my Epsilon system if I turned the 200tpi screw a half a turn (for a 64 micron adjustment) a Hocus Focus run would yield about 15 microns in measured movement of that corner.  Keep us posted on how your test works out. 

Nathan Myhrvold
 

The Octopi Astro adapter uses 200 TPI threads which are finer than most micrometers use. 

 

If you just measure the displacement, as with the dial indicators then play in the threads does not matter.

 

But in any event there will be some inaccuracy – the goal is to get the sensor flat to within 10 microns, and that seems quite feasible.  Optically slower systems could probably tolerate more.

 

Nathan

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of ROBERT WYNNE via groups.io
Sent: Wednesday, March 29, 2023 7:35 PM
To: main@ap-gto.groups.io; Chris White <chris.white@...>
Subject: [EXTERNAL] Re: [ap-gto] Tilt plate with digitial readout

 

Wonder how much play there is between threaded female components and threaded male components? Most of the off the shelf industry unless otherwise specified makes do with 75% threads allowing quite a bit of play between the two. Unless you're working with mil spec 100% threads I doubt either component comes close to 100% and thus the play between the parts and the inexplicable delta between the two. -Best, Robert 

On 03/29/2023 7:25 PM Chris White <chris.white@...> wrote:

 

 

Nice work, Nathan!  I cool idea to quantify the adjustments. 

HF determines the micron adjustment recommendation by deriving the delta between the optimal focus position of a given corner with an average of the optimal focus position of all the corners. This is essentially a step measurement between the optimal focus positions, converted to microns.  So the recommendation is not likely to correlate with how much to adjust the screw.  I have found there to be relationship though as long as you are close to optimal backspacing.  With my Epsilon system if I turned the 200tpi screw a half a turn (for a 64 micron adjustment) a Hocus Focus run would yield about 15 microns in measured movement of that corner.  Keep us posted on how your test works out. 

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