19 replies to this topic

[lsintampa]

I'm not sure that I fully understand differential flexure and if one has it how to fix it.

 

What I understand it to be is if there is flexure between the guide scope and the imaging scope.  Meaning that the two scopes do not move in full tandem with each other.  In other words, the guide scope shifts a bit during imaging.

 

I just purchased the Apertura CarbonStar RC6 and it has a top and bottom rail (bottom for attaching to your mount and the top rail for support and other gear).  I have my guide scope attached to the top rail.  I'm using a Svbony 60 MM guide scope (seems to work well) but it is attached to a shoe which in turn is attached to the scope using rings.  I all seems to be rigid to me, but how much flexure can be occurring with that guide setup?

 

Hmm, I think I may have answered my own question.  Maybe a lot, he said guessing.

 

Another question... how important is it for the guide scope to be perfectly aligned with the main scope for guiding?  Example lets say on a horizontal level the guide scope is off five degrees from the horizontal level of the main scope, or say the guide scope if off to the right of horizontal from the main scope.  Does that matter?  Does this have anything to do with flexure?

 

I've heard that OAG is the way to go.  A few years ago I had the iOptron RC6 and tried an OAG, but never could get it to work for me. So I'm not convinced that OAG is a good thing or not?  For those that do use OAG, are there targets where you just can not get any star to guide on?  If so, what do you do then? 

 

See attached live stack using ASI Air Mini.

 

 

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[sharkmelley]

If you want to know if there is a flexure problem then it's easy to determine.  Point the scope so a bright star is central in the imaging camera and adjust the guidescope so the same star is central in the guide camera.  Now slew the scope to successive bright stars in various parts of the sky.  Are those stars always precisely central in the guide camera?

[duck]

tie up all camera cables so that gravity can't change the force on the camera when the scope tracks.  Lock down all focusers.  You need to guide on a star much closer than 5 degrees from the target.  There's no such thing as perfect polar alignment.  Field rotation due to imperfect polar alignment is centered on the guide star.  5 degrees of offset implies a large track on the plate if there is field rotation about the guide star.

 

My scope can handle 5 minute exposures before diff flex ruins them.  Quite limiting, but like you say, much easier to get a guide star.

[Alex McConahay]

I'm not sure that I fully understand differential flexure and if one has it how to fix it.

 

What I understand it to be is if there is flexure between the guide scope and the imaging scope.  Meaning that the two scopes do not move in full tandem with each other.  In other words, the guide scope shifts a bit during imaging.

 

I just purchased the Apertura CarbonStar RC6 and it has a top and bottom rail (bottom for attaching to your mount and the top rail for support and other gear).  I have my guide scope attached to the top rail.  I'm using a Svbony 60 MM guide scope (seems to work well) but it is attached to a shoe which in turn is attached to the scope using rings.  I all seems to be rigid to me, but how much flexure can be occurring with that guide setup?

 

Hmm, I think I may have answered my own question.  Maybe a lot, he said guessing.

 

Another question... how important is it for the guide scope to be perfectly aligned with the main scope for guiding?  Example lets say on a horizontal level the guide scope is off five degrees from the horizontal level of the main scope, or say the guide scope if off to the right of horizontal from the main scope.  Does that matter?  Does this have anything to do with flexure?

 

I've heard that OAG is the way to go.  A few years ago I had the iOptron RC6 and tried an OAG, but never could get it to work for me. So I'm not convinced that OAG is a good thing or not?  For those that do use OAG, are there targets where you just can not get any star to guide on?  If so, what do you do then? 

 

See attached live stack using ASI Air Mini.

Easy question first: 

 

>>>>>>how important is it for the guide scope to be perfectly aligned with the main scope for guiding?

 

It is not critical IF you have good polar alignment. The guide camera will keep the guide star properly positioned in the guide camera. However, what you really want is for your main camera to have its stars in the perfect position.  If the relationship between the guide camera and the main camera does not change, the left-right-up-down of the main and guide cameras will move together, keeping the guide star right where it belongs on the main camera chip. However, if you are not properly polar aligned, there will be a rotation AROUND the guide star. The further your guide star is away from the actual target, the more significant this drift (and apparent motion) will be. If the guide star is within the field of view, the rotation will be seen by all the stars circling the star being used to guide. If the guide star is outside the main camera field of view, the whole field will rotate around a point outside the field of view. It gets worse the further the guide star is away from the field of view, and the more misaligned the polar axis is. 

 

The harder one:\

 

>>>>>>>I'm not sure that I fully understand differential flexure and if one has it how to fix it.

 

Let's not talk just about differential flexure, but  all the things related to it. Namely, anything that makes it so that the mechanical relationship between position of the star in the guide camera  and the position of that same star in the main camera can change. 

 

These issues include actual bumps, slippage, flops, sags, imbalances, and so forth. 

 

Bumps is when you bump one camera or another and move them in their mountings. 

Slippage is where over time something that was loose moves to a different position. 

Flops are where something that is loose (perhaps a mirror in an SCT or RC) moves in mountings.

Sags are when, the mechanical support of the optics, camera montings, and all that, bend over time. 

Imbalances are when the equipment mounted all over the telescope is imbalances. As a result, through the night, the stresses that hold the cameras in one relationship are shifted and the equipment moves. 

 

(That is my classification----Somebody smarter than me probably has a better named list.) 

 

I do not mean to say these are all different things. A sag, for instance, changes the relationship between the sensors over time. So, it may be considered a slippage of sorts. 

 

At any rate, you have an RC. The big mirror and the secondary are usually mounted securely in these things. Better than an SCT, but hardly as firmly as a refractor. 

 

Are all your fittings tight? If not, they can slip as  the scope moves through the evening. You have to make sure everything is tight and stable, and, of course stable. 

 

If you are having shifts, it is just a series of dicoveries. You have to think about the possible causes and isolate and test each one. 

 

Alex

[TOMDEY]

Differential flexure can be difficult or practically impossible to troubleshoot/isolate when the ~load path~ twixt the two is long and/or convoluted. The most common way to thwart that problem is to use an Off Axis Guider (OAG) where the load path is short and rigid. A second approach is my invention that we use on some professional ground based telescopes and also orbital imagers. My patent shows a ~whitened~ version in the context of a traditional astronomical telescope. Easily achieves the required co-alignment of 1/10 arc-sec for ten years, with 1/100 are-sec precision and 1/000 arc-sec noise floor.    Tom

 

PS: The biggest pointing maintenance challenge with earth-orbiting imagers (e.g. Hubble) is the thermal shock when the sat goes in and out of eclipse. This causes "microquakes" that impulse the scope, including the three degrees of freedom defining its line of sight and clocking about that line. This happens thirty times a day for Hubble. It's original cantilevered solar panels were the biggest contributor to this oversight. These were upgraded/replaced in one of the heroic repair missions.    Tom

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[PIEJr]

I've done all I can, short of welding things together, to make sure my equipment is all in agreement of where everybody is pointed.

And I've always practiced very good cable management.

 

But I like threads like this to learn more of what others can share.

 

My main telescope and my guide scope are on Losmandy D bars which are very ridgid, especially with the guide rings on my main telescope.

If there is any flecture, I can't see it

 

 

 

[fmendes]

Between OAG and guidescope, I'm between the few that prefer the guidescope. On telescopes wth long focal length, almost always the OAG is the only viable solution. However, even with my SCT (an Edge HD8), I tested first a guidescope. While guiding was not too bad, the image would shift on the course of the night. 

 

Then what is the better way to set an OAG? You need a guide camera with a big-ish sensor. The 120MM can only be used if you're lucky, as the chance of capturing any stars with such a small sensor is not great. Then, the prism size should somewhat match the size of the guide camera sensor size. If you plan to use a camera like the ASI174, then you need a large prism, and that means less space for the main sensor, or the prism will be in front of the sensor.

 

Finally, make sure there is enough back focus on your telescope. It was not easy for me to get the right back focus with an external focuser, OAG and filter holder. Not something I can do with my Newtonians.

[Spaceman 56]

I have seen pictures of OAG systems, and sometimes the cameras  hang off the side a large distance.

 

I cant see how its possible for these cameras not to SAG, and therefore introduce the problem they are supposed to fix.

 

experts might explain this for me ?

[michael8554]

An OAG is recommended for a  scope with 1377mm focal length - what problems did you have ?

Too small a sensor?

Camera not sensitive enough ?

 

Differential Flexture of a few microns will cause problems at that FL.

 

For comparison a human hair is on average 75um in diameter.

 

Causes of Diff Flex:

 

Primary mirror movement in SCT's.

 

Guidescopes held in adjustable rings, particularly those with soft-tipped adjustment screws.

 

Guidescopes on wobbly Finderscope mounts.

 

Having said that, the stacked image looks good on the right hand side.

 

But elongated stars on the left side suggest some tilt.

[fmendes]

I have seen pictures of OAG systems, and sometimes the cameras  hang off the side a large distance.

 

I cant see how its possible for these cameras not to SAG, and therefore introduce the problem they are supposed to fix.

 

experts might explain this for me ?

 

It may, or it may not... The guide camera is usually much lighter than an imaging camera. 

 

Still, my OAG is a cheap one bought at Amazon, and I needed to cut some of the tube that holds the prism so I could achieve focus. The threaded adapter for the guide camera focuser is as close to the body of the OAG as it can be. But if I used a filter wheel, I'd have problems.

[Alex McConahay]

I have seen pictures of OAG systems, and sometimes the cameras  hang off the side a large distance.

 

I cant see how its possible for these cameras not to SAG, and therefore introduce the problem they are supposed to fix.

 

experts might explain this for me ?

You have a point. But as has been pointed out, guide cameras are light. And the OAG's are usually very short mechanically, with not much room for movement/.

 

If it is way off to the side, it is extension tubes. Make sure you use good, tight tubes..

 

Alex

[kathyastro]

The most rigid connection is to put the guide scope in rings that are attached directly to the imaging scope or its rings.  The finder-shoe guide scope mounts are a very common source of flexure.

 

It doesn't matter if the guide scope's FOV is not centred on the imaging scope's FOV.  I find it useful to have them aligned, because then I can use the guide scope as a finder.

 

An OAG is traditional on SCTs where mirror-flop is an issue.  It also ensures that the guiding system's focal length is long enough for the imaging scope, sine they will be the same.  However, it is not 100% necessary.  What matters is the respective focal lengths.  Once, just to see it it would work, I rigged up my C-90 with a reducer to make it f/7, and bolted it onto my C-11.  It actually worked quite well as a guide scope!

[lsintampa]

The most rigid connection is to put the guide scope in rings that are attached directly to the imaging scope or its rings.  The finder-shoe guide scope mounts are a very common source of flexure.

 

It doesn't matter if the guide scope's FOV is not centred on the imaging scope's FOV.  I find it useful to have them aligned, because then I can use the guide scope as a finder.

 

An OAG is traditional on SCTs where mirror-flop is an issue.  It also ensures that the guiding system's focal length is long enough for the imaging scope, sine they will be the same.  However, it is not 100% necessary.  What matters is the respective focal lengths.  Once, just to see it it would work, I rigged up my C-90 with a reducer to make it f/7, and bolted it onto my C-11.  It actually worked quite well as a guide scope!

That seems like my setup.  My guide scope is in rings and is attached to the top rail of the telescope.  Still think a better guide scope might work better.  I'm still sorting it out.

[Spaceman 56]

guide cameras are light. And the OAG's are usually very short mechanically, with not much room for movement/.

 

Alex

Hey Alex. I am still questioning this.

 

guide cameras are cheap things, that drop into an eyepiece slot. generally there are no screwed connections, so the camera is technically floating in a hole that has mechanical slop (by design) and therefore must have flexure.

 

I see some peoples OAG systems that look visually as if they hang 250 or 300mm at least away from the prism, and I would expect flexure as the camera, cables, and the connection mechanism to the camera is not Balanced in any way, and cable tension and gravity would  come into play, as the night goes on and the orientation of the Camera changes.

 

I would think the closer to the prism the camera is the better. 

 

Experts would know more about this.  

[John Rogers]

With modern CCD cameras, a lot of problems go away with relatively short exposures and stacking.   I recall that Software Bisque made a claim a few years ago, that any sort of guiding may be a thing of the past.

[Alex McConahay]

>>>>>>>I see some peoples OAG systems that look visually as if they hang 250 or 300mm at least away from the prism,

 

Can't recall any I have seen set up like that---and I have been to New Zealand!!!!!!!

 

I'd like to know what they have between the OAG and the main camera sensor that takes ten or twelve inches!

 

Alex

[TelescopeGreg]

The most rigid connection is to put the guide scope in rings that are attached directly to the imaging scope or its rings.  The finder-shoe guide scope mounts are a very common source of flexure.

 

It doesn't matter if the guide scope's FOV is not centred on the imaging scope's FOV.  I find it useful to have them aligned, because then I can use the guide scope as a finder.

What Kathy said.

 

The key is that you need to have the guiding optical path and the imaging optical path move exactly (within microns) in step.  The guiding system can only command the mount's rotisserie to keep the guide stars stationary, as seen by the guide camera, and it trusts that the imaging system is exactly following along.  

 

But imagine for a moment that all the guide scope mounting hardware were made from foam rubber.  Wiggle the OTA and you'll see the guide scope wiggle too, but not at the same time.  There's a lag between them, right?  The mount's gears and bearings hit a bit of manufactured imprecision and jerk the OTA to the left, faking out the guide scope that inertia is still pushing to the right.  That's differential flexure amplified to where we can see it.  But on the scale of micron-sized pixels, even metal mounting bars have flex.

 

So you need to assemble things such that you don't depend on torsional rigidity, instead use the fact that metal doesn't compress at all (at least not on the time and temperature scales we deal with).  The way to do this is to have two points of support, widely spaced, all the way from the OTA to the guider.  Flex is minimized with that arrangement, since we're not depending on metal not to bend.

[Spaceman 56]

I'd like to know what they have between the OAG and the main camera sensor that takes ten or twelve inches!

 

Alex

unsure myself Alex.  

 

my RCs camera sensor is about 260mm from the back of the primary mirror, so perhaps that distance has to be replicated to the OAG camera sensor ?

[lsintampa]

What Kathy said.

 

The key is that you need to have the guiding optical path and the imaging optical path move exactly (within microns) in step.  The guiding system can only command the mount's rotisserie to keep the guide stars stationary, as seen by the guide camera, and it trusts that the imaging system is exactly following along.  

 

But imagine for a moment that all the guide scope mounting hardware were made from foam rubber.  Wiggle the OTA and you'll see the guide scope wiggle too, but not at the same time.  There's a lag between them, right?  The mount's gears and bearings hit a bit of manufactured imprecision and jerk the OTA to the left, faking out the guide scope that inertia is still pushing to the right.  That's differential flexure amplified to where we can see it.  But on the scale of micron-sized pixels, even metal mounting bars have flex.

 

So you need to assemble things such that you don't depend on torsional rigidity, instead use the fact that metal doesn't compress at all (at least not on the time and temperature scales we deal with).  The way to do this is to have two points of support, widely spaced, all the way from the OTA to the guider.  Flex is minimized with that arrangement, since we're not depending on metal not to bend.

 

What Kathy said.

 

The key is that you need to have the guiding optical path and the imaging optical path move exactly (within microns) in step.  The guiding system can only command the mount's rotisserie to keep the guide stars stationary, as seen by the guide camera, and it trusts that the imaging system is exactly following along. 

 

But imagine for a moment that all the guide scope mounting hardware were made from foam rubber.  Wiggle the OTA and you'll see the guide scope wiggle too, but not at the same time.  There's a lag between them, right?  The mount's gears and bearings hit a bit of manufactured imprecision and jerk the OTA to the left, faking out the guide scope that inertia is still pushing to the right.  That's differential flexure amplified to where we can see it.  But on the scale of micron-sized pixels, even metal mounting bars have flex.

 

So you need to assemble things such that you don't depend on torsional rigidity, instead use the fact that metal doesn't compress at all (at least not on the time and temperature scales we deal with).  The way to do this is to have two points of support, widely spaced, all the way from the OTA to the guider.  Flex is minimized with that arrangement, since we're not depending on metal not to bend.

 

So I adjusted a few things.

 

First, the Svbony 60mm guide scope has a thumb screw to lock down the focus tube.  But it alone doesn't stop flexure.  There are two additional grub screws that are just tension tight to allow the tube to move as you focus.  If they are not tightened you can easily flex the focus tube.  So I locked them down tightly.

 

I did better cable management making sure there were no camera cables that could move during imaging.

 

Made sure all connections and bolts were firm.

 

New target tonight and with one hour of live stacking my flexure artifacts are not an issue.

 

[Alex McConahay]

unsure myself Alex.  

 

my RCs camera sensor is about 260mm from the back of the primary mirror, so perhaps that distance has to be replicated to the OAG camera sensor ?

Nobody should place an OAG at the back plate of an RC. That would be in front of the focuser. Every time you focused you would change the pickoff mirror to main camera sensor. 

 

Alex