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Walking on the Moon

PA on mount direct vs via camera on OTA


powerlord
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So.. for us asiair users and more and more maybe, users of other software we are PAing via our main cameras and OTA. And mostly this works great.

What I do find though is that when I swap to a new OTA, I have to do it again. At first I wondered why - has the mount moved somehow, etc.

Now I realise that its basically just that each OTA and camera as mouted on EQ mount is going to be a tiny wee bit probably not pointing 100% in line ?

I suppose questions are - well - ok so we end up PAing the optical path effectively - which is probably not what we should be doing, but it's close enough ?

Certainly that's what I've been doing for 2 years and get .4 rms with the EQ6r pro so works fine.

With the AM5 though it definately needs to be as good as you can get, and so, I mounted a guidescope I'd made 3d printed brackets for that ensure its as parallel to the mount as possible, and PAed with that. But to be honest, when I then put an OTA and camera on, I found the guiding was not very good until I re-PAed with the OTA.

Which just confused me if I'm honest. I mean in an ideal world, you have a 100% accurate polarscope in your mount and you do a perfect PA, and provided the mount doesn't move - you are done for ever (as per a concrete fixed pier) right ? So even if your OTA and camera are a bit squint it doesn't matter - because your mount is aligned to the earth's rotation which is the whole point.

One would thing therefore that PAing with some sort of super parallel OTA would be the next best thing ?

stu

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This is a good point actually. I had this thought a couple of times recently. I use my guidescope to plate solve but it is always ever so slightly out from the main tube (being a portable rig I can't get it perfect). 

I get guide figures similar to yours and haven't had to toss any subs yet except for clouds or satellites etc. As the old saying goes, if it ain't broke... 

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With PA software you are aligning the mount not the scope. It works by calculating the centre of pivot of the mount which works as long as the camera is fixed and stays at the same distance from the pivot point.

So even if your camera used for PA was a foot from the axis of the mount it would still work.

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10 minutes ago, StevieDvd said:

With PA software you are aligning the mount not the scope. It works by calculating the centre of pivot of the mount which works as long as the camera is fixed and stays at the same distance from the pivot point.

So even if your camera used for PA was a foot from the axis of the mount it would still work.

it's not the distance from the axis, it's how colinear it is with the mount. As Craig says - apparently this is called Cone error (thanks Craig).

Imagine something like your OTA mounted so badly onto the mount that it points upwards 45 degrees from the direction the mount is pointing. Clearly you'd have some issues PAing.

I think the thing is, with something like my big EQ6r-pro you can basically be miles off, but it'll still guide it out no bother at all. But with something like the AM5, it's already having to 'guide' out all the periodic error, so if you don't have an OTA that is very colinear to the mount, you might end up getting a good PA in software, which is actually off enough that the extra work the AM5 has to do is too much for it while also trying to flatten that periodic error out.

At least that's my theory. My AM5 had other issues - massive 5-10' spikes intermittently while generally otherwise running nicely at .5 rms or so... hence its back with FLO to be looked at.

stu

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3 minutes ago, powerlord said:

it's not the distance from the axis, it's how colinear it is with the mount. As Craig says - apparently this is called Cone error (thanks Craig).

 

No as long as the cone error is constant during the PA it would make no difference. For example a lot of PA cameras are not on the OTA but on brackets sticking out from the mount.

 

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34 minutes ago, StevieDvd said:

It works by calculating the centre of pivot of the mount which works as long as the camera is fixed and stays at the same distance from the pivot point.

Stu raises an interesting question here which has played on my mind also over recent time.

I operate two rigs, (a) a RedCat 51 mounted on a AZEQ6 which is pier mounted and aligned using a PoleMaster and (b) a Samyang 135 mounted on a CEM25-EC which is tripod mounted.

I use a PoleMaster with associated QHY software to PA the CEM25 but on occasion I have used (a) SharpCap and (b) the ASIair PA routine.

On more than one occasion I have PA'ed the mount using the Polemaster and then checked it out of curiosity with the ASIair. The ASIair reported the PA was well off and needed adjustment which I completed to the point where I got the huge smily face and firework display and was told I was in the top n% of PA results. I felt very chuffed! Re-checking with the PoleMaster it told me my PA was well off the mark. I re-aligned with the PoleMaster and then thought I will check it with SharpCap using the PoleMaster camera. Low and behold SharpCap reported the PA was well off and needed adjustment.

I suppose I could spend the entire evening playing PA and chasing my tail. For the sake of getting some imaging done I used the PoleMaster PA camera and associated software.

The SharpCap routine works with either the imaging OTA or the PoleMaster camera which is on the mount axis but gives different results to the QHY software and the ASIair.

I do not guide the CEM25-EC - using the Samyang 135 there are no discernible star trails. I do guide the AZEQ6 and achieve guiding r.m.s. typically in the range 0.35 to 0.75 dependent on 'seeing'.

Just to add I didn't bother getting down on my hands and knees, twisting my neck into an unnatural position, removing my spectacles and looking up the polar-scope with its associated parallax problems.

So which to believe? To say "It works" is fine if it does! I don't know whether it does or it doesn't but one thing is clear - all three methods give different results.

 

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If you check Sharpcap user manual here under 'What is required'  it says:

"It is not required to have perfectly aligned the guider scope or main scope as the polar alignment process is not affected by this sort of misalignment."

Sharpcap PA was inspired by PhotoPolarAlign by Themos Tsikas who posted it here on SGL for us to test. I did some of the early testing and even added a small bit of code to allow colour markers with mono cameras as I used a dslr back then and had a cheap mono guide camera.*

Each PA software has it's own way of doing essentially the same job but don't always produce the same accuracy so they will vary from one another.

SGL thread on PhotoPolarAlign  to give credit to Themos Tsikas

Edit: I had to find the thread to correct my memory. It was to make the overlays colour for mono cameras I changed.

Edited by StevieDvd
Corected ref to my code
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Exactly, and it would seem to be that the algorithm that is used may or may not be affected by cone error.

As I understand it asiair requires alignment on polaris. it shoots a still at home position, rotates 60 degrees west and takes one more picture. it then calcuates error and leaves you ti fix (you can set to auto refresh picture and watch error go down).

There is therefore surely an expectation that there is no cone error.

consider that there was, and that it is massive: 10 degrees  in ALT. So you're mount is at say the right 53 degrees for your latitude, but the OTA is pointing at 63.

asiair doesn't know that - it will surely end up requesting something like a reduction in ALT of 10 degrees till it looks right.

But of course, it isn't - you're now 10 degrees off being aligned to the polar axis.

I've not tested this btw, I might be talking mince.

But for other PA methods like PHD2s that don't need to be PAed to polaris, they might be pretty much immune to cone error ? sharpcap is probably the same ?

I mean it might not be that at all, I've not done any tests as I say - it could be that it is something else, but it does seem weird that such a simple thing can not match up between different software.

and IF cone error didn't make any difference with asiair it wouldn't need re-PAed for a different mount - but it certainly does.

All fun and games.

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FYI

Asiair now has an all sky alignment option which needs to be activated under Advanced

Embedded PA software uses a cut-down catalog for PA/plate-solving so it can work without internet access.  This is probably why the earliest software used NCP or SCP as a first capture and nearby regions with a small rotation for the subsequent captures.

Today there seems to be a PA solution with most astro software but I doubt many people have tried more than a couple.

To be honest cone error has never been an issue for me. As I understood it, if your mount was 100% accurately aligned and your date/time/location were correct, then a goto could still be off due to 'cone error' . The mount is pointing at a certain target but the ota is a bit skew-wiff.

 

 

 

 

 

 

 

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2 hours ago, powerlord said:

As I understand it asiair requires 0alignment on polaris.

My understanding (😉) is that when AAP does a PA alignment, it calculates where the centres of the circles are that stars would draw out as the mount rotates as part of its PA routine. The aim then would be to adjust the mount pointing in order to align that common centre, (i.e. the pointing direction of the mount's polar axis), with the NCP. Presumably, AAP knows where the NCP is in relation to the stars it sees during plate solving. If there is cone error, it just means that the circles drawn would be either smaller or larger, but their centres would still be at the same position(s). I'm open to challenge on this presumption if you think otherwise.

1 hour ago, StevieDvd said:

As I understood it, if your mount was 100% accurately aligned and your date/time/location were correct, then a goto could still be off due to 'cone error'

Yes, that's what I think too. So far as I can see, the necessity to establish cone error is only required if you are using the mount's GOTOs. I understand that this is why you need to do a 3-star alignment so that the mount's software can work out the cone error in order for GOTOs to be accurate. If you are aligning by plate solving, I don't think the cone error matters.

Ian

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@The Admiral you seem to have posted exactly my thoughts on the matter. 😶

In my coders mind I envisage the polar alignment working out the co-ordinates/axis  for each image platesolved and an algorithm to calculate their offset from NCP/SCP and each further iteration showing the change.

With star alignment my mental image is of a correct circle perfectly horizontal and pointing True north.  And a computer circle that floats above this - created by the alignment process. So the internal mount map then knows how to correct the users goto to allow for the errors on a 360degree 3d mapping.

Not that I could code that, I would do the functional specs and get a better coder.🤫

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hmm... very interesting thread which has triggered a question for me...
I use an NEQ6 on which I mount a SW130PDS with DSLR for imaging, and a SW ST80 with ASI120MM Mini for guiding. I use Sharpcap to PA, but connected to the ASI120MM Mini on the guidescope. Is that the right thing to do? should I be connecting to the DSLR (or ideally a camera connected to the mounts Polar scope?) I'm questioning whether any misalignment between the guidescope, main scope and mount would all be adding up when I PA, effectively putting the mount in the wrong place to correct for any error the guidescope has?

Thanks
Ed

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1 hour ago, edarter said:

hmm... very interesting thread which has triggered a question for me...
I use an NEQ6 on which I mount a SW130PDS with DSLR for imaging, and a SW ST80 with ASI120MM Mini for guiding. I use Sharpcap to PA, but connected to the ASI120MM Mini on the guidescope. Is that the right thing to do? should I be connecting to the DSLR (or ideally a camera connected to the mounts Polar scope?) I'm questioning whether any misalignment between the guidescope, main scope and mount would all be adding up when I PA, effectively putting the mount in the wrong place to correct for any error the guidescope has?

Thanks
Ed

  • Unless  it's changed Sharpcap PA was not a simple task using a DSLR - the guide cameras supported are fine however.
  • Don't use the polarscope
  • As long as the camera used does not flex/wobble or is otherwise loose the PA can be done even at a distance from the mount axis. The camera must describe a circle when the mount is rotated for the software to be accurate.

 

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Interesting thread...  When Polemaster came out I was lucky enough to be loaned one for my HEQ5 from Modern Astronomy to review for our society newsletter.  My mount is fixed in an observatory so once set PA shouldn't need adjusting.  I like the logic that having a camera in the RA axis should give the best accuracy of PA.  For ages after the review kit was returned I got various guiding results, and a lot of people suggested my PA was out.  I would always disagree as I had used what was then claimed to be the most accurate method of PA.  Prior to this I had polar aligned using EQMOD which would rotate the mount to the correct polar hour, but then required some manual input which given the tolerance of the polar scope could be hit an miss, but my guiding errors were normally better than after I used Polemaster.

When Sharpcap released the version with their PA tool I tried that.  But this time I used my finder guider.  My work flow was to centre a bright star in the main scope, and then centre the same star in the finder.  I then ran through the alignment routine until I received an "excellent" result. However when I first ran the steps to calculate the initial alignment after the mount had been aligned with Polemaster  it was way out (as Adreneline encountered).  My guiding graphs after PA'ing  with Sharpcap were better, but I felt could be improved upon.  I began questioning myself over using the guidescope as it's probably around 5-8" off the same axis of the main scope.  But then any cone angle formed using this would have been calculated as the software (as far as I recall) had me rotate the scope through 180 degrees E to W.    My thoughts were that in reality the angle over the millions of miles to the targets we are imaging would be so insignificant it's hardly worth worrying about.  Having close PA is better than having hardly any at all, and certainly miles better than using the polar scope and procedure in the SW manual !

I guess the best option would be to use a Polemaster camera fitted to the mount, but use the Sharpcap software.... something that apparently isn't possible as the camera is locked into to the software thus preventing it being used with any other applications. 

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Just out of interest, I noted that my guiding performance has dropped off over the summer, so I did a SharpCap PA and it was ‘poor’ about 10’ adrift. 
Now I have a permanent setup with a steel pier securely bolted to a cubic metre of reinforced concrete so what’s going on? Has the block shifted in the ground after a very dry summer? I have bumped into the scopes a couple of times, but I wouldn’t have thought that would shift the PA by that much.

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3 hours ago, malc-c said:

I guess the best option would be to use a Polemaster camera fitted to the mount, but use the Sharpcap software.... something that apparently isn't possible as the camera is locked into to the software thus preventing it being used with any other applications. 

Interesting to read your experiences.

I have used the PoleMaster camera with SharpCap - it gave a different result to using the PoleMaster camera with its own software - hence my confusion and doubting as to which is best/better.

I did try using the PoleMaster software with another camera but the QHY software will only connect with the QHY (PoleMaster) camera.

I can't help but feel that a camera mounted on the mount RA axis is a good starting point so differences must (?) be in the software and the way the underlying mathematics is performed.

@StevieDvd's comment that "Each PA software has it's own way of doing essentially the same job but don't always produce the same accuracy so they will vary from one another." is I am sure correct. The problem is in the central words "don't always produce the same accuracy" which begs the question "how accurate do you need to be?". It also makes me wonder how many ways are there of "doing essentially the same job". It seems it would be preferable if all the different software did exactly the same job (perfect polar alignment) to the same degree of accuracy - but what do I know?

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I recall reading that Shapcap pro polar alignment somehow takes cone error into account in its calculations, cant point to any writing about that though.

I do have significant cone error in my AZ-EQ6 with its axis of RA rotation being nowhere near 90° to DEC and also in my newtonian with its rings/plate/mangled tube. I guess i have at least a degree, probably more. I always aim for 2 arcmin PA or better and in the end PHD2 agrees with that number pretty closely. Usually the measured drift is between 0-8 arcmin at the end of a session so sharpcap does a pretty good job of it.

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Interesting stuff ! Don't forget that with a polemaster it has to be calibrated on first use and again if it's ever knocked or moved.

Possibly why differing results to sharpcap.

I'm using an ipolar now and it's usually very close to sharpcap result.

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So none of above though (and I'm sure @StevieDvd is on the money) explains why one night asiair can tell me it's within 0.00.20, and the next night it's the same, but I change the OTA to something else and now it's miles out and I need to move to get it nice and low again. That is my process now, as it is consistent in this behaviour - I don't PA unless I change the OTA. I mean sure new OTA, different FL, etc - but its claims of accuracy shouldnt be affected by that really, and anyway.. youd expect that to them result in the sort of 'if I PA with a 1000FL then it'll be good with 250FL sort of deal - and that's not the case at all)

The fact it CAN stay the same calculating over several nights with the same OTA suggests the software in consistent, and the fact it is almost always different with a new OTA suggest  something about the way it's doing it is affected with this ?

I'm afraid asiair is the only thing I can PA with. I tried phd2s drift thing and gave up as it was all over the place. Maybe sharpcap is easier ? but I don't have the paid version of that. So I admit this could be asiair funkyness.

Tomorrow night looks good, so going to take the 300PDS outside and mount it on the EQ6r-PRO. So that's always fun....and since I need to move the tripod to centre of obsy to fit it in, I'll definately need to PA then!

 

@tomato - I assume that is the same software (and version of it) that you used originally to PA ? If not might be as Stevie says - different sofware different results ?

 

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IMO there’s no way you’ll be able to swap one OTA for another and maintain an accurate PA, just the act of changing the tubes over is going to disturb it. You PA first when the mount is loaded up and when you unload it by taking weights/tube off and the mount is going to mechanically rebound (for want of a better word), next OTA goes on and the mount won’t then settle back into exactly its previous position. That’s my thoughts anyway. 
 

If you think about heave of the soil during dry/wet spells, thermal expansion/contraction of all the metal parts of the pier and mount, not to mention the concrete itself (CoTE of concrete is roughly the same as steel - depending on the aggregate used) and it’s no wonder that the most carefully adjusted PA doesn’t last long. 

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I use Sharpcap’s PA tool, I’m not sure if Robin makes changes to it between versions.

It does make sense then to check the PA regularly even if you have a permanent setup, although the giveaway is seeing the guiding performance change. 

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16 hours ago, Adreneline said:

Interesting to read your experiences.

I have used the PoleMaster camera with SharpCap - it gave a different result to using the PoleMaster camera with its own software - hence my confusion and doubting as to which is best/better.

 

That's interesting, and something must had changed in either the camera firmware or Sharpcap for them both to work together.  As I mentioned, this was in the days when both options had just emerged onto the market.

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