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Old 02-07-2016, 02:14 PM
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PRejto (Peter)
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Field Rotation? (not the usual thread you might expect!)

I could use some help in figuring out my results to date experimenting with an SX-AO on telescope #2 of my dual scope setup.

The largest unresolved issue I face with my system is that of differential flexure between the two scopes. I can fairly reliably go 5 min on scope #2 whilst guiding scope 1 through an ONAG. Sometimes I can go 10 min but too many subs show substantial movement in that time period depending on scope orientation.

After thinking about the issue for a long time (and after a suggestion by another IIS member) I decided to try to eliminate DF on scope #2 by using AO on scope 2 (but without any mount corrections). So, scope 1 would guide and scope #2 would have the benefit of AO corrections. Yikes, easier said than done!

My first experiments were total and complete failures no matter what combination of guide settings in TSX (scope 1) or AO settings (scope 2). The results were always a very poor image and one much worse than not using the AO. But, I know the AO is working great and properly calibrated as it will guide scope #2 perfectly if guiding is turned off in scope #1.


Thus began the trouble shooting. I decided to guide scope #1 and just watch what scope #2 shows when tracking a star (but applying no AO or other corrections). The result is uniformly the same regardless of the scope orientation unlike what I might expect to see from DF (where DF ought to be different in different sky locations, etc). Attached is a screen capture of 6 min of "watching" scope #2 in PHD2. The drift is ca -1 pix in 6 min and always in the same direction. This is an ST-i through my TEC140 = 1.56 arc-sec.


EDIT: Just to be perfectly clear I'm seeing this constant drift while I am guiding in TSX. There is zero drift in TSX with great guiding. The drift is just showing up in scope #2.

So, the question is what is this if it isn't DF? Am I just measuring field rotation because I have selected 2 different guide stars between the two guide cameras? I've yet to positively try guiding on the same star. Last night I was fairly sure both stars were quite close together but I still saw the same amount of drift. Tonight I will plate solve both guiders and try again. I think if I get no drift using the same guide star the idea of field rotation will be confirmed. And, if so, it's no wonder my intial tests using the AO failed.

Any other ideas?

Thanks for reading!

Peter
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Last edited by PRejto; 03-07-2016 at 08:43 AM.
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Old 02-07-2016, 02:42 PM
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billdan (Bill)
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Gooday Peter,

Looking at the graph I would immediately suspect polar alignment error if it was a single scope.

I think the more likely cause is cone error, i.e. the optical axis of both scopes are not exactly parallel to each other.

My 2c worth.

Cheers mate
Bill
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Old 02-07-2016, 10:25 PM
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PRejto (Peter)
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Hi Bill,

I appreciate your reply! I'm no expert at the subject of cone error but what I understand is that it is primarily a pointing issue, not a tracking issue.

Whatever is going on I'm having a hard time believing it is effecting scope #2 when I am actually imaging. I say that because I don't see any star distortion or rotation of one image vs the other when combining. Hopefully the issue will go away when I'm able to test this by guiding on the same star with both scopes. It was clear tonight but of course I had some equipment issues.

Peter
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Old 03-07-2016, 09:15 PM
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Tonight I was able to guide with both guide cameras on the same star. The results are interesting. There is much reduced drift in declination but still drift in RA - perhaps 50% of what I was previously measuring. I've attached a screen shot of that test in PHD2.

Guiding on the same star I was able to get a very good result with the SX-AO on camera #2. I used long exposures (7 sec) and low aggression to avoid any over correction. I'm pretty sure I can refine these settings for a better result.

What I don't understand is why there is drift in RA on guide camera #2 when there clearly isn't RA drift showing up in the imaging camera #1 sub of 10 minutes. It's clear from the final position of the AO (star) that it has been compensating for the drift. The image from camera #2 shows no drift in RA. I've attached the two 10 min registered images.

Can anybody reason out this drift?

Thanks,

Peter

The G2-8300 sub is from the TSX guided scope. The Trius-C is on scope #2 and corrected by the AO. Both subs are 10 minutes.
Attached Thumbnails
Click for full-size image (July 3 10 min on Meridian.jpg)
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Click for full-size image (July 3 10 min test exposures both cameras.jpg)
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Click for full-size image (G2 -8300 10 min test.jpg)
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Click for full-size image (TRIUS-C_IC_4628.00000039.jpg)
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Last edited by PRejto; 04-07-2016 at 01:55 PM.
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Old 04-07-2016, 04:04 PM
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My opinion for what its worth is the concept of 2 separate scopes on the same mount with a tandem bar type mounting is engineering-wise never really going to work.

To get tight stars the guiding has to be what the sensor sees and the more removed it is from that the more variable and error can be added in from the many sources of flex in a system that the TPoint model shows.

I presume you are using Protrack corrections as well? That may help.

Personally I believe you would be better off using 2 mounts rather than trying to make a tandem system work. Are there examples of others using tandem systems like this that work well? Usually its a smaller scope riding on top of the larger scope. Like a guide scope. Even then there are likely to be small differences.

I think you've got a great result already from a setup I would not have thought would have worked at all. The TEC180 is a large scope and quite long. Its angular momentum will be quite a bit more than the TEC140 perhaps that could be part of it. Are you sure your setup is balanced at different angles? It would be tricky to get 2 scopes to be balanced at all imaging angles which may be part of why its been erratic in certain parts of the sky. The weight load is not even perhaps.

When I had a TE180 a Tak NJP could not quite handle it but the PMX could with MMOAG guiding. Guide scope guiding worked sometimes but was less certain and often eggy stars. A PME11 should handle it easily but then a TEC140 which is also quite a large scope as well?

Greg.
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Old 04-07-2016, 09:16 PM
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Hi Greg,

I hear you! If I had this exercise to do over again I probably wouldn't do it. But, I'm pretty stubborn. And, once you start down a path like this it's hard to quit when there is already so much invested in it. Then there is also the factor of trying to succeed at something others say won't work! It's all been done before (except this!) so it's kind of fun (and very masochistic!) to persist.

Anyway, there is no factor of weight with the two scopes on the MEII. Yes, balance is quite tricky but actually very good as I've worked hard to find a method to go about it. I won't say perfect but I think it isn't a significant factor.

The "drift" I observe between the two scopes when guiding through one and observing the other's guide camera is curious. Obviously some component (at times) must be DF, but on the whole I think I'm observing something that some very smart person (with initials P.W. perhaps) could explain. If what I'm observing were purely DF then it would be unreasonable for the drift to pretty much always look the same no matter where I am pointing - even with the telescope on the east side of the mount pointing -2 hr east (in other words upside down!). I think there is a rational explanation. Maybe it's somthing along the lines of our ability to see a stereo image where the separation of our eyes allows us to see depth. In my case I have two scopes that are separated but looking at the same point in space. They cannot be seeing the same thing even though the separation compared to infinity is quite miniscule. But then so is the drift when one comes down to it. .5 pix in 10 minutes. I'm probably wrong about this idea but I think there is a logical explanation.

Peter
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Old 04-07-2016, 09:59 PM
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Could it be parallax error? 2 scopes slightly different views with different focal lengths and different curvature of the optics. When you overlay an image from the TEC180 does it match the TEC140 image or are there different geometries? Some alignment routines will rotate and scale for example and without that using multiple telescope images won't align properly.

With my RHA I notice at F3.8 it shows up tilt tip errors very clearly as elongated stars in one corner or side etc. For a long time I thought I had tracking errors but it was tilt/tip errors and when I developed a procedure to correct them I got lovely round stars to the corners. So I understand if you look at something from the wrong view you don't get a resolution of the problem.

Greg.
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Old 05-07-2016, 06:28 AM
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Quote:
Originally Posted by gregbradley View Post
Could it be parallax error? 2 scopes slightly different views with different focal lengths and different curvature of the optics. When you overlay an image from the TEC180 does it match the TEC140 image or are there different geometries? Some alignment routines will rotate and scale for example and without that using multiple telescope images won't align properly.
Greg.
Well, maybe!!! I use CCDStack and it automatically does rescale the TEC140 images to the TEC180 (.95 to .88 arc-sec). As far as other adjustments (it does rotate) I'm not sure. Generally my images have properly registered even though I don't have a flattener on the TEC140.

However, the effect I'm observing with the last experiment is drift observing the same guide star between the two scopes. Both scopes are looking at the exact same point in space.....only that point is being measured so as far as the fundamental principal going on here I think what might or might not be going on in the corners is perhaps a side issue (?).

Here is a though experiment I came up with. What would one observe if two guide cameras could be set up to observe two different stars through the SAME scope? In my initial experiment (first post) I observed two different stars and got a consistent drift no matter where I pointed. If the same experimnt were carried out with one telescope would we see the same drift? If "yes" then I fail to understand how "multistar" guiding can ever possibly work! But it does work as best I know, so perhaps the drift only exists between two separated scopes.

Any thoughts about this?

Peter
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Old 05-07-2016, 03:24 PM
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Astrodon brought out a MMOAG with either 2 or 4 ports for guide cameras (I take it to make it easier to get a guide star). So that would be the tool for that experiment. It would be interesting if a multiport MMOAG would be used with more than 1 camera at the same time. I think you would have to have 2 guiding software running at the same time, one for each camera. That may get overcorrection as both try to correct at the same time. But I suppose you can turn down the aggressiveness to compensate.

Greg.
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Old 05-07-2016, 07:37 PM
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Although I cannot help with most of what is being discussed, I can say with assuring that parallax definitely doesn't come into consideration!

As for two cameras guiding through the same scope, the only instance I can think of there being issues is if you have one guiding in the centre and the other guiding from a corner when the PA is off. Error here will be caused by one camera detecting rotation around the centre while the other is guiding on the centre.
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Old 06-07-2016, 07:57 AM
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Although I cannot help with most of what is being discussed, I can say with assuring that parallax definitely doesn't come into consideration!

As for two cameras guiding through the same scope, the only instance I can think of there being issues is if you have one guiding in the centre and the other guiding from a corner when the PA is off. Error here will be caused by one camera detecting rotation around the centre while the other is guiding on the centre.
Why are you so sure?

With digital cameras and panoramas great care is taken to have the camera lens rotate around the nodal point. The nodal point is the point where the rays of light converge. If you are fore or aft of this point you get parallax error. That's with a short focal length lens of 14-24mm let alone 1260mm of the TEC180. They are different focal lengths for one thing and the nodal point for each will be different won't it? Or do I have this wrong?

http://www.hugha.co.uk/NodalPoint/In...he_Nodal_Point

The first thing to check is to make sure your PA is perfect though. After a heavy rain etc I find sometimes PA goes off as a permanent pier can shift a little over time from changes in the soil.

Greg.
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Old 06-07-2016, 09:34 AM
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I think I can add clarity now to this discussion.

Andy Galasso (of PHD2) has been kindly discussing all of this with me for which I'm really grateful. As far as scope separation or parallax error he said it isn't a factor and presented this argument:

"we can forget about the separation since arcsin(15"/10ly)=0 "

However, I know from following Patrick Wallace's discussions of T-Point over at the Software Bisque forum that all polar alignment is a compromise and that field rotation can only be minimized. One only need look at the T-Point PA recommendations in the advanced settings to see how to compensate with ME or MA adjustments to get a PA that suits a particular need. So, I'm left with the conclusion that all fields rotate and it's just a question of how much one can tolerate.

Andy confirmed the following.

1. When a guide star is selected the guide star becomes the center point for the rotating field.

2. Multistar guiding forces a compromised center of rotation where the center is actually the mean position of all the guide stars.
a. So my question of looking at two different guide stars through a single scope (one guiding, one not guiding) will give the same result (except for DF!) as using two telescopes. The camera not guiding will detect the field rotation exactly as I did in my first post of this thread.

3. My own observation from #1 + #2 is that people with wide field guiders such as an ONAG + SX Ultrastar or ATIK 440EX need to be careful to select the same guide star from night to night! If this isn't done there will be multiple sets of images that have different centers of rotation. Registration might be compromised! Notice in my first experiment that the field rotated up to 1 - 1.5 pix in 5 minutes. Longer exposures would be even more problematic.

4. I was able to experiment further last night by guiding through one scope and just watching the same star through scope #2. When I first did this experiment I only had a sample of 1 successful image (of the guiding graph in PHD2) that I posted earlier. I speculated incorrectly that I couldn't believe that everything I was observing was DF. After repeating this in different areas I think I was quite wrong as I see totally different results for different areas along with greater deviations exactly where one might expect them....such as when the mount would be tracking west at around 45 degrees altitude. In this position gravity would be acting more and more on the focuser/camera combination. The attached photo demonstrates this quite well. The SX AO was able to successfully eliminate the DF in a second run at the same position.

If there was a mystery in my mind I think it is all nicely resolved!

Peter
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Old 06-07-2016, 10:45 AM
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Quote:
Originally Posted by gregbradley View Post
Why are you so sure?

With digital cameras and panoramas great care is taken to have the camera lens rotate around the nodal point. The nodal point is the point where the rays of light converge. If you are fore or aft of this point you get parallax error. That's with a short focal length lens of 14-24mm let alone 1260mm of the TEC180. They are different focal lengths for one thing and the nodal point for each will be different won't it? Or do I have this wrong?

http://www.hugha.co.uk/NodalPoint/In...he_Nodal_Point
I've never done any real panos so I don't have any experience with it but consider the distance. The parallax of our nearest star can be done in the back yard over a 6 month period as we have the Earths orbit as a baseline. Having two telescopes let's say, one metre apart, does not have the resolution to be able to detect parallax. If it did, any star in our stellar neighbourhood would not be registerable from one minute to the next due to its movement due to the Earths orbit and close stars moving against the stagnant background.
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Old 06-07-2016, 10:52 AM
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I've never done any real panos so I don't have any experience with it but consider the distance. The parallax of our nearest star can be done in the back yard over a 6 month period as we have the Earths orbit as a baseline. Having two telescopes let's say, one metre apart, does not have the resolution to be able to detect parallax. If it did, any star in our stellar neighbourhood would not be registerable from one minute to the next due to its movement due to the Earths orbit and close stars moving against the stagnant background.

Yeah you are most likely right there.
Best guess is PA is off if the guiding is good but there is drift, that is the usual culprit, 2nd guess would be differential flexure which is hard to handle. Differential flexure raises its ugly head in a simple camera on the focuser all by itself without a tandem bar, a 2nd set of rings, a second focuser, a 2nd guide camera mounting, pickup prism. Also how square is the pick up prism for that matter. They are merely glued onto a metal frame so not 100% orthogonal anyway.


Also guide stars with OAG often show distorted stars to some degree off axis as scopes generally have poorer performance the further away from the centre the star is.

3rd guess would be differences in curvature of the optics and focal length causing some interplay that is not ideal.

You'd have to eliminate each of the above with some tests. A fresh Tpoint model to check PA accuracy for starters. Flex is harder to measure but if you got round stars on the scope the OAG is on and not on the other there is your answer, its flex.

Another simple approach is to use PHD2 or Pempro to do Polar drift alignment and you can adjust the drift out in real time until its stable and then it does not matter what was causing it unless the flex is worse at different angles that is!

Greg.
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