Topics

consistent goto error on meridian flip w/1200


Steven Panish
 

My venerable 1200 has a large and consistent goto error after a meridian flip.  It is roughly 2 deg in RA and a couple minutes in Dec, I haven't actually measured it.  Very consistent, so in spite of being way off the sensor I know how to move the scope E/W to get the target back in sight.  After a recal, goto accuracy is restored.  This isn't mirror flop, the finder scope also shows the issue and I can use that for centering as well.  Mostly I avoid the problem by starting an imaging run counterweight up and avoiding a meridian flip altogether.  I bought this 1200 3 years ago and it has always had the issue.

Does anyone have a clue as to what is wrong?  Orthogonality?  The scope is a C9.25 and the finder is mounted on the scop so the dovetail could be off, everything else is AP and should be precise.   I can work with it but it would be nice to not have to think about it.  

Steve


Christopher Erickson
 

Orthogonality. A.K.A. cone error.

Carefully collimate your optics then evaluate and adjust your error by shimming one end or the other of your dovetail bar on the OTA radius blocks.


-Christopher Erickson
Observatory engineer
Waikoloa, HI 96738
www.summitkinetics.com
   


On Sat, Nov 14, 2020, 3:31 PM Steven Panish <scpanish@...> wrote:
My venerable 1200 has a large and consistent goto error after a meridian flip.  It is roughly 2 deg in RA and a couple minutes in Dec, I haven't actually measured it.  Very consistent, so in spite of being way off the sensor I know how to move the scope E/W to get the target back in sight.  After a recal, goto accuracy is restored.  This isn't mirror flop, the finder scope also shows the issue and I can use that for centering as well.  Mostly I avoid the problem by starting an imaging run counterweight up and avoiding a meridian flip altogether.  I bought this 1200 3 years ago and it has always had the issue.

Does anyone have a clue as to what is wrong?  Orthogonality?  The scope is a C9.25 and the finder is mounted on the scop so the dovetail could be off, everything else is AP and should be precise.   I can work with it but it would be nice to not have to think about it.  

Steve


Steven Panish
 

Thanks.  Collimation is perfect so looks like I need some shim stock...

Steve


Christopher Erickson
 

I find that the sides of aluminum pop cans make great shim stock that can be cut with regular scissors and a paper punch. Just don't use your S.O.'s favorite cutting tools...

The aluminum near the top of the can is usually the thinnest the part near the bottom is usually the thickest.

Whatever your pointing error is, your ortho error will be half that much. Flipping sides of the pier/tripod doubles the error value.

Carpe Noctem!

-Christopher Erickson



On Sat, Nov 14, 2020 at 3:52 PM Steven Panish <scpanish@...> wrote:
Thanks.  Collimation is perfect so looks like I need some shim stock...

Steve


Roland Christen
 

Typically the mount is very accurate as far as orthogonality. I have measured the gearwheel accuracies of a number of mounts and they consistently fall below 15 arc seconds for the entire 360 degree rotation. In other words, the gearbox and worm wheels are extremely accurate and would not impact the orthogonal error that you experience.

The main culprit is usually the optical tube assembly. The optical axis of a telescope does not always line up precisely with the mechanical mounting of the tube. A 1 degree error of the mounting system will produce a 2 degree pointing error when flipping from one side to the other. The solution is to measure the pointing error an shim the tube assembly or mounting rings to eliminate the mechanical error. The other solution is to compensate for it with software. We now have incorporated an automated routine in the CP4 and new keypad software that measures the ortho error and compensates for it.

Roland Christen



-----Original Message-----
From: Steven Panish <scpanish@...>
To: main@ap-gto.groups.io
Sent: Sat, Nov 14, 2020 7:31 pm
Subject: [ap-gto] consistent goto error on meridian flip w/1200

My venerable 1200 has a large and consistent goto error after a meridian flip.  It is roughly 2 deg in RA and a couple minutes in Dec, I haven't actually measured it.  Very consistent, so in spite of being way off the sensor I know how to move the scope E/W to get the target back in sight.  After a recal, goto accuracy is restored.  This isn't mirror flop, the finder scope also shows the issue and I can use that for centering as well.  Mostly I avoid the problem by starting an imaging run counterweight up and avoiding a meridian flip altogether.  I bought this 1200 3 years ago and it has always had the issue.

Does anyone have a clue as to what is wrong?  Orthogonality?  The scope is a C9.25 and the finder is mounted on the scop so the dovetail could be off, everything else is AP and should be precise.   I can work with it but it would be nice to not have to think about it.  

Steve


Steven Panish
 

Thanks for the input, Roland.  It always seemed most likely to be orthogonality related to the dovetail bar on the C9.25.   All the AP stuff I trust.  The software compensation feature sounds great...but I have a CP3.  I'll try shimming first, which will be a bit of a random walk combined with a binary search.  

Steve

On Sun, Nov 15, 2020 at 12:55 PM uncarollo2 <chris1011@...> via groups.io <chris1011=aol.com@groups.io> wrote:
Typically the mount is very accurate as far as orthogonality. I have measured the gearwheel accuracies of a number of mounts and they consistently fall below 15 arc seconds for the entire 360 degree rotation. In other words, the gearbox and worm wheels are extremely accurate and would not impact the orthogonal error that you experience.

The main culprit is usually the optical tube assembly. The optical axis of a telescope does not always line up precisely with the mechanical mounting of the tube. A 1 degree error of the mounting system will produce a 2 degree pointing error when flipping from one side to the other. The solution is to measure the pointing error an shim the tube assembly or mounting rings to eliminate the mechanical error. The other solution is to compensate for it with software. We now have incorporated an automated routine in the CP4 and new keypad software that measures the ortho error and compensates for it.

Roland Christen



-----Original Message-----
From: Steven Panish <scpanish@...>
To: main@ap-gto.groups.io
Sent: Sat, Nov 14, 2020 7:31 pm
Subject: [ap-gto] consistent goto error on meridian flip w/1200

My venerable 1200 has a large and consistent goto error after a meridian flip.  It is roughly 2 deg in RA and a couple minutes in Dec, I haven't actually measured it.  Very consistent, so in spite of being way off the sensor I know how to move the scope E/W to get the target back in sight.  After a recal, goto accuracy is restored.  This isn't mirror flop, the finder scope also shows the issue and I can use that for centering as well.  Mostly I avoid the problem by starting an imaging run counterweight up and avoiding a meridian flip altogether.  I bought this 1200 3 years ago and it has always had the issue.

Does anyone have a clue as to what is wrong?  Orthogonality?  The scope is a C9.25 and the finder is mounted on the scop so the dovetail could be off, everything else is AP and should be precise.   I can work with it but it would be nice to not have to think about it.  

Steve


Steven Panish
 

Last night I used the ConeSharp program with spectacular results.  This is a free product from the SharpCap team (Robin Glover et. al.).  It uses a semi automated procedure to put a target star offset from center and you then adjust the source of the orthogonality error (the dovetail rail on the C9.25 in my case) to recenter the image.  Shimming is kind of miserable and crude.  At some point when/if the scope is off the mount I'll put adjustment screws on the rail.  Even so, the results are impressive.  The program reported my initial error as 14+ minutes.  I'm not sure how that relates to the error that was on the sensor, which was much larger.  After a lot of careful shimming and 3 iterations of the procedure, reported error was 37 seconds.  A goto halfway across the sky including a meridian flip had the target on the sensor and off-center by about 8 minutes (guestimate).  

My thanks for the advice.

Steve


Christopher Erickson
 

Congratulations!


On Sun, Nov 22, 2020 at 4:51 AM Steven Panish <scpanish@...> wrote:
Last night I used the ConeSharp program with spectacular results.  This is a free product from the SharpCap team (Robin Glover et. al.).  It uses a semi automated procedure to put a target star offset from center and you then adjust the source of the orthogonality error (the dovetail rail on the C9.25 in my case) to recenter the image.  Shimming is kind of miserable and crude.  At some point when/if the scope is off the mount I'll put adjustment screws on the rail.  Even so, the results are impressive.  The program reported my initial error as 14+ minutes.  I'm not sure how that relates to the error that was on the sensor, which was much larger.  After a lot of careful shimming and 3 iterations of the procedure, reported error was 37 seconds.  A goto halfway across the sky including a meridian flip had the target on the sensor and off-center by about 8 minutes (guestimate).  

My thanks for the advice.

Steve


Manusfisch
 

Steven P, thanks for that informed analysis of your problem and your solution.  these kinds of posts really help us pilgrims.  This whole thread is one that both informs and teaches quite well.  I appreciate the incredible brain power and innovative problem solving that occurs in amateur astronomy and am in complete awe of the Astro-Physics crew for always perfecting their craft. Innovation, freedom of thought, a bit of dreaming and wishing all add up  to a great experience for a hobby relatively new to me.   Tom Fischer, Indy


Steven Panish
 

Thanks!  We're all pilgrims and learning is what the forums are all about.  How the AP folks get any actual work done is a continuing puzzle, their helpfulness and dedication are exemplary.  As it is for other experts who often lend a hand to we-who-have-really-a-lot-to-learn!

Keep safe, everybody!

Steve