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Old 12-01-2018, 07:35 AM
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Merlin66 (Ken)
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Accurate Polar alignment technique

Came across this link:
https://faculty.mu.edu.sa/public/upl...Final_ICAM.pdf

Interesting reading on polar alignment techniques.
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Old 12-01-2018, 10:31 AM
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A lot of complicated maths in there Ken, think I will stick to drift aligning.

As some mounts have DEC backlash, you want the polar alignment to be slightly out so you can guide DEC in one direction only. Therefore good accuracy is less important.
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Old 12-01-2018, 04:12 PM
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Thanks for the link Ken.



Having recently given a little talk about polar alignment I was naturally curious to read the paper that you mention. However I remain somewhat confused.


In the initial Abstract the paper simply refers to “... a new method to align and point small telescope systems”. But then the Abstract goes on to say that “This process solved the main problem of polar alignment”.


In its Introduction the paper discusses the nature of, and the need for, proper polar alignment of an equatorial mount. It specifically refers to the minimisation of both field rotation and declination drift as being among the expected benefits. It then displays the usual diagram showing an equatorial mount's polar axis aligned with the celestial axis and briefly describes the drift alignment method, referring specifically to the attendant adjustment of the mount's azimuth and elevation “screws”. So far, so good. At this stage one is expecting the paper to continue to describe some new method of polar alignment. But then things seem to go a bit awry because the Introduction continues by saying “Modern automated telescope systems use another method for polar alignment called three stars alignment.”. And this is followed by a description of the usual three star alignment routine that we are all familiar with. After listing some perceived shortcomings of the three star alignment method it states that “Getting accurate automatic pointing …. provides a check on the polar axis alignment”. It then refers to TPoint and, after briefly describing how it operates, lists perceived shortcoming of that method.


In the main body of the paper it states that “The whole process can be summarised … First the telescope polar axis is positioned towards the north celestial pole. Then it is automatically commanded through the serial port to slew from its home position to a selection of three reference stars all over the sky. Instead of asking the user to identify and center the reference star, this will be done automatically by the use of image synthesize.” Thereafter follows a description of how the method (presumably at each reference star) compares a field image taken by the camera being used with a synthesised image derived from the method's database. At no point is any reference made to adjusting the mount's azimuth and elevation “screws”. Thus, despite the earlier extensive references to polar alignment, it seems that the described new method does not involve polar alignment at all, being simply another method for star alignment.


But maybe I'm mistaken? In the final section Alignment and Pointing Performance the paper states “... after the alignment, the telescope is almost perfectly aligned to the north pole ...”.


Possibly just a storm in a teacup. But maybe someone better equipped to do so can make more sense of it?
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Old 12-01-2018, 04:25 PM
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The paper was published in 2004 - I guess no one implemented the required method/calculations in software in the proceeding 14 years?

Quote:
As some mounts have DEC backlash, you want the polar alignment to be slightly out so you can guide DEC in one direction only. Therefore good accuracy is less important.
This is only if you are guiding in DEC. If your polar alignment is excellent, then no DEC guiding is required, and hence backlash is irrelevant. Further, even a small PA error will invoke some drift (albeit slow) which can be handled by guiding in one direction (as you note Bill) --- it is all a matter of tuning the PHD2 (or equivalent) settings to get the interventions right. Basically, it's almost always good to be as close to the pole as is feasible.
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Old 12-01-2018, 09:59 PM
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Hi Barry,

When you say that when a perfect polar alignment is achieved then no DEC guiding is necessary, this is only true if the Earth did not have an atmosphere.

Due to refraction the stars appear slightly higher in the sky than they actually are in reality. And this difference varies as the star elevation changes.

If we were to put a laser pointer on our scope it would draw a perfect semi-circle in the sky as we move from the East horizon to the West. The problem is the stars as we observe them do not follow that semi-circle they follow a parabolic arc because of atmospheric refraction.

I guess the point I am making is you will still have to guide in DEC no matter how good your polar alignment is. However I do agree with you that the closest our polar alignment is the better off we are.

Cheers
Bill

Last edited by billdan; 12-01-2018 at 10:10 PM.
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Old 13-01-2018, 03:53 PM
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When I read that article it seemed to me the author was jumping around a bit - pointing out some shortcomings in other methods that still applied to their own method !
I just assumed that there was an implied adjustment being made.

A cheap mount system with backlash and large PE is going to be a problem no matter how your align it !

But the accuracy they claimed (from memory now as that was days ago) was around 2 arc minutes !
I dont see the point of that
That isnt accurate, but for some amateurs is probably more than accurate enough.

There is also no mention of having and maintaining the alignment of the main imaging telescope coaxial with their polar aligning scope and camera.

But since with modern tools (software and integration platforms like ASCOM), its easily possible to achieve polar alignment with the main scope and a camera using Tpoint down into the very low arc seconds -

Tpoint offers a polar alignment report with as little as 6 mapped stars (8 is better)
Do that twice or three times to verify you didnt make a mistake the secpnd time and you are pretty good to go.

This sort of alignment accuracy is only ever relevant if you are also using a system that is already using Tpoint as part of the tracking and pointing process because atmospheric refraction is already potentially introducing errors from 0 to 2 arc minutes anyway ! - depending on where you are are on the planet and where you are pointing.
For us at around 35S, just the error looking at the SCP is between 1 and 2 arc minutes.

If I can map 12-16 stars in about 15-30 minutes (running two models) and make the necessary physical Alt/Az adjustments on my mount.
Then do a fully automated run of say 20-40 points in the next 20 minutes while I have a drink and nibbles so that Tpoint has a more complete model of the other mechanical pointing system errors in my system.
And because I can do all of this well before atmospheric twilight (because I am using a camera not my eyes) - I havent lost any imaging time at all.

I really can't see the point of their process and system - at least not as something you would have to add to your won existing system.

I think the intent was to produce a theoretical system that could be fully integrated and included in future telescopes as part of the basic package that alleviated the novice user from understanding what was necessary.
But they use an equatorial mount as their example, but fail to explain how the required automation of adjustment of Alt and Az could work.

When really their system probably is better suited to an Alt/Az mount

I felt it was pooorly worded and constructed - but the authors arent native English speakers either.

It was interesting and rather than use ASCOM and existing Plate Solving and Image acquisition programs that could be scripted they used an entirely custom written and custom designed system.
I guess ASCOM wasnt around back then !
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Old 13-01-2018, 04:10 PM
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I think it represents the state of the art at the time, but recognises the issues involved.

Why do you say:
""There is also no mention of having and maintaining the alignment of the main imaging telescope coaxial with their polar aligning scope and camera.""

Based on the current plate solving methods - SharpCap, PPS etc it is the mount's rotational axis which is critical - the actual imaging scope doesn't have to be aligned with polar axis......
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Old 15-01-2018, 06:57 PM
rally
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Hi Ken,

Their system as described uses a separate aligning scope and camera to that of the main scope

If this second scope is not coaxial with the main image scope all that means is that their aligning system is now polar aligned but the main system is not necessarily polar aligned !

The number of ways that it could be misaligned is quite a few !
But Differential flexure being an awkward one without an easily computed solution since it varies across the sky.
Add in some camera rotation and different mechanical/optical centres of the two telescopes into the equation and you have a mess.

Its no big deal but it involves more hardware and more alignment issues

But that is where Tpoint does its magic - it corrects in software almost everything else left that you couldnt quite achieve in hardware - except for freeplay/backlash in cheaper mount designs and PE (which is corrected with other software and/or mount integrated firmware)
And this is all done with the main scope and main camera (no need for duplication)

So today there are simply better tools available than existed for amateurs at the time they wrote that paper, that not only do the job much better - ie into the very low arcseconds instead of their claimed 2 arc minutes, but doesn't require a separate system and other hardware and the resultant differential flexure problems that most dual telescope systems will suffer from.

The moderm alternative also means that Tpoint is correcting for atmospheric refraction when pointing and other software such as ProTrack can actually make all the same Tpoint calculated corrections in real time during actual tracking, you can be relatively certain of excellent tracking right across the sky

Add in some regular guiding and of course your set !

Of course if this was really cheaply integrated into a telescope and or mount at the point of manufacture (say less than $100 which the Chinese could potentially do), I can still see that it might have some benefit for novices to get them started and remove some of the initial pain of polar alignment.

But 2 arc minutes of accuracy and another 1-2 arc minutes of atmospheric refraction and maybe some more in all the rest of a typical amateur system (flexure, backlash, mechanical errors, PE, loose fittings, sleeved nose adapters instead of threaded adapters, balance problems etc) suddenly we're looking at maybe 4++ arc minutes of error in Polar Alignment depending on where you end up pointing to after the initial alignment

But maybe the novice should just start with an Alt/Az mount and use 3 star alignment and then once they have the determination and understanding move up to an equatorial mount !

Its still interesting, but in many ways we have everything they proposed embedded within some of the more modern software - but we still dont have an automated mount adjustment system for EQ style mounts to set Alt/Az - but that would need two more drives/controllers on top of RA/Dec !
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Old 16-01-2018, 10:34 AM
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Quote:
Originally Posted by rally View Post
Its still interesting, but in many ways we have everything they proposed embedded within some of the more modern software - but we still dont have an automated mount adjustment system for EQ style mounts to set Alt/Az - but that would need two more drives/controllers on top of RA/Dec !
I know we're a niche market, etc, but I am surprised that no one has done this (or at least afaik). It would be a "killer" portable system then.
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Old 16-01-2018, 06:09 PM
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Chris,

Yes !

A worm and wheel for Az and a Ballscrew and nut for Alt would do it plus motors, controllers and extra housings.

But given the rigidity you would need for Az - since the entire mount, counterweights and telescope system would be sitting on top of this - I could imagine the mechanical side adding at least another $1-2000 to a top end mount, plus the motors, controller system and software development.


Quote:
Originally Posted by lazjen View Post
I know we're a niche market, etc, but I am surprised that no one has done this (or at least afaik). It would be a "killer" portable system then.
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Old 16-01-2018, 08:17 PM
bigjoe (JOSEPH)
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The method talked about in 2... On page 97 very interesting indeed.
bigjoe.
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Old 16-01-2018, 08:26 PM
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I've got a polar alignment question I was hoping someone here might be able to help me with it.

I would like to know if it is feasible to determine polar alignment error by taking a series of plate solved images, two near the horizon maybe 15-30 minutes of arc apart and then two near the meridian with similar spacing.

My hypothesis is there would be constant dec in each of the plate solves if the mount only moves in RA and the polar alignment is perfect. Is that logic sound or am I missing something? I get that atmospheric effects can shift the apparent location of the star and this might be the show stopper. Is there a way to account for this?

If my first hypothesis holds true the my second hypothesis is that a polar alignment error would result in a change in the plate solved dec result between subsequent images. Furthermore the change in dec divided by the change in RA between the pairs of images would enable you to determine the rate of drift and hence the magnitude of the alignment error in both altitude and azimuth.

I've been a bit sick and have taken some cold and flue medication so please forgive me if I'm talking nonsense.

Cheers

Peter
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Old 17-01-2018, 09:29 PM
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Quote:
Originally Posted by peter_4059 View Post
I've got a polar alignment question I was hoping someone here might be able to help me with it.

I would like to know if it is feasible to determine polar alignment error by taking a series of plate solved images, two near the horizon maybe 15-30 minutes of arc apart and then two near the meridian with similar spacing.
It is feasible... the polar alignment error is a constant offset in altitude and an offset in azimuth. Mathematically the offsets can be computed from ONE short image taken near the east/west horizon and ONE short image near the zenith, provided you accurately know

a) the time the images were taken, and

b) the precise pointing of the telescope for each axis when each image was taken, ie from encoders or at worst, setting circles.

c) it assumes the mechanical axes of rotation of the mount and the optical axis of the scope are precisely orthogonal. They never are.

Notes:

1. the accuracy is fundamentally limited by the accuracy of the measurements of the telescope position in each axis. For example, if you used setting circles with 1-degree resolution the best accuracy you might achieve is +/- 0.5 degree. With encoders you can do significantly better, but there's another limitation...

2. orthogonality of the mount and optical axis - if the RA and dec axis aren't exactly perpendicular, nor the optical axis perpendicular to the dec axis, these introduce more errors in the calculation. Errors up to 1 degree or even more are not uncommon in commercial budget-grade mounts for the amateur market.

There are ways to measure these errors and correct them - this is fundamental to the precision of theodolites used in surveying - where accuracy of 1 arc second is considered common.

3. two images are not required.

The maths is also known as the "surveyors equation" as this is also the same as the method used for a 2-star astrometric fix to determine latitude and longitude in absolute coordinates using a theodolite. It is also the same maths in the heart of GOTO mounts to determine how to calibrate the mount using 2 stars, and to do the GOTO calculations.

Quote:
My hypothesis is there would be constant dec in each of the plate solves
Incorrect, study the spherical trig in say Smart's book. The the polar alignment error is a constant offset in altitude and an offset in azimuth, and is not a constant error in dec. Atmospheric refraction can be approximated using some formulae that take into account your site position, site altitude, air pressure and humidity but its not perfect, there are other variables in play (eg weather systems).

Quote:
if ... the polar alignment is perfect
but it cant be perfect. It never is, because...

Quote:
atmospheric effects can shift the apparent location of the star
Yup, atmospheric refraction produces a vertical shift (ie in the apparent altitude) of a star - it appears higher in altitude than it really is. This produces an offset in its hour angle (ie RA) AND its declination - and worse, these offsets vary as the star moves across the sky.

So wherever you aim your polar axis, its really only an approximation and you DO need to guide in RA and dec to account for the changing offsets as the stars altitude changes.

Quote:
If my first hypothesis holds true
Don't hypothesize - do the maths. It's describe in Smart's book "Spherical Trigonometry" and it isn't trivial - it is a significant challenge for someone with good highschool year 12 trig and calculus. I did all this 35 years ago to produce a program for determining position from 2-star alignments using a HP41CX... but don't ask me now.

Cheers, indeed.

Wavy.

Last edited by Wavytone; 17-01-2018 at 09:55 PM.
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Old 17-01-2018, 09:53 PM
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Thanks for the feedback Wavy. I did some actual testing tonight. Took two images near the eastern horizon, slewing only in RA about 30 mins between the two. I plate solved each image and then also did a 5 minute drift using PHD2 in this location. PHD2 reports an alignment error of 0.30'. I need to take a look at the reference you mention and do some calcs.
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Old 18-01-2018, 12:39 AM
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I think I'm understanding what Peter is getting at, but suspect others aren't? He seems to be suggesting a way of accelerating drift alignment by taking 2 images - one at where you would start drift aligning, the other at the end of drift aligning. Plate solving each image and comparing the difference in dec is effectively the same as watching a star drift off the dec axis like we do when drift aligning. No?

Alt - drift aligning you'd point somewhere above horizon and near celestial equator (dec around 0). Turn off dec guiding and watch over some time the stars drift off axis. Adjust alt until they remain on axis. With Peter's method you'd do the same thing, only you'd take one image, plate solve and note the centre of image dec. You'd slew in RA only some angle, Peter is suggesting 30 arcmins? Image and plate solve again. Compare the dec to previous, and any difference would surely be the same as if you'd watched a star drift off axis over the same time mount would go that angle in sidereal time. This would assume the plate solve is pretty accurate, and that the difference in atmospheric refraction over such small angle is negligible. You'd adjust alt until the difference in dec between the 2 images is acceptably low.

Az - same as above but instead of near horizon, you're near zenith and adjusting az knobs.
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Old 21-01-2018, 01:46 AM
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Great answer Wavy !

Now add in a meridian flip and the problems are worse.

But again Tpoint has already been designed to consider these things as mentioned a, b, c and 1, 2, 3 and includes a total of nearly 30 repeatable mechanical errors in mounts and optical systems that affect telescope pointing.

Tpoint calls them Terms and anyone who has used it will be familiar with them. There are 6 basic terms that cover the basics of most mount and optical systems and will get you very close to perfect if you have a refractor thats rigid and collimated its often hardly worth persevering further, but the other two dozen or thereabouts will cover almost everything else.
Many terms are only likely to apply to one particular type of system or another - eg Refractor or Mirror or Nasmyth so you never need to use all of them.

Its not that Im trying to be a fanboy of Tpoint ! but the beauty is someone has done all the hard work already and its so easily implemented by an amateur without any additional hardware.
Any other method to truly yield high accuracy will necessarily have to make an attempt to cover all the very same Terms that Tpoint deals with - so why bother if its already done.
But if you are just seeking sub 2-4arcmin PA accuracy then of course its not needed and for many amateurs this is probably adequate for even medium to long length exposures with guiding as the field rotation is very minimal.

It just relies on doing some star mapping each time you setup your mount and system - once every so often if you have a fixed observatory setup and each time you setup in the field or if you drunkedly stumble into your telescope at night or the ground moves (eg heavy rain or passionate night - joke - we're amateurs)

But it really doesnt take that long unless you are also trying to solve all of the extra terms, and you dont need to understand how and why it works under the hood - just that it does work - it was originally developed for professional observatories to use - in fact I think it was originally developed for or in conjunction with the AAT at Siding Springs in NSW.

The accuracy of the results is dependent on the accuracy of your mapping and gear, although even poor mapping (eg visually or with equipment with lots of errors and instability - remember the errors have to be mechanically repeatable) can still yield reasonable results if you have enough mapped stars.
But mapping on screen using a camera at say 400% magnification means you are invariably within a pixel of the star's centre during mapping and any errors then get statistically "averaged out" of the equation anyway.

It doesnt deal with varying camera rotation angle and any inherent coaxial induced offset of the camera centre (ie the centre pixel of the camera image moves as the camera is rotated) - so you want a method for either keeping that fixed at least when initially mapping and preferably have the camera angle mechanically calibrated to some reference position when you first setup (preferably astronomical zero)

Dont mean to harp on about it but using the best tool for the job will give you great results and anything else is just an approximation because it comes with exceptions and all sorts of excluded cases.

One of the real benefits of using it (apart from accuracy) is that in two or three years time assuming you are using the same gear - you can go back to an old target you want to add more data to and start imaging again and you can use the same guide star in the identical same positons and this can be pointed by using one of the old images as the reference - it makes life so easy.
Your new images will then all register perfectly with the old ones (assuming you also have camera rotation either automated using ASCOM or manually set) since that will be recorded in the FITS header and can be translated directly into your planetarium software - at least with TheSky it does.
So you dont suddenly lose half your data after registraton becasue camera angle and image centres have shifted too much.
And you can save lots of time setting up and getting going.

I have to thank Eric for showing me how to use MaxIm to best extent for this !

Quote:
Originally Posted by Wavytone View Post
It is feasible... the polar alignment error is a constant offset in altitude and an offset in azimuth. Mathematically the offsets can be computed from ONE short image taken near the east/west horizon and ONE short image near the zenith, provided you accurately know

a) the time the images were taken, and

b) the precise pointing of the telescope for each axis when each image was taken, ie from encoders or at worst, setting circles.

c) it assumes the mechanical axes of rotation of the mount and the optical axis of the scope are precisely orthogonal. They never are.

Notes:

1. the accuracy is fundamentally limited by the accuracy of the measurements of the telescope position in each axis. For example, if you used setting circles with 1-degree resolution the best accuracy you might achieve is +/- 0.5 degree. With encoders you can do significantly better, but there's another limitation...

2. orthogonality of the mount and optical axis - if the RA and dec axis aren't exactly perpendicular, nor the optical axis perpendicular to the dec axis, these introduce more errors in the calculation. Errors up to 1 degree or even more are not uncommon in commercial budget-grade mounts for the amateur market.

There are ways to measure these errors and correct them - this is fundamental to the precision of theodolites used in surveying - where accuracy of 1 arc second is considered common.

3. two images are not required.

The maths is also known as the "surveyors equation" as this is also the same as the method used for a 2-star astrometric fix to determine latitude and longitude in absolute coordinates using a theodolite. It is also the same maths in the heart of GOTO mounts to determine how to calibrate the mount using 2 stars, and to do the GOTO calculations.

Incorrect, study the spherical trig in say Smart's book. The the polar alignment error is a constant offset in altitude and an offset in azimuth, and is not a constant error in dec. Atmospheric refraction can be approximated using some formulae that take into account your site position, site altitude, air pressure and humidity but its not perfect, there are other variables in play (eg weather systems).

but it cant be perfect. It never is, because...

Yup, atmospheric refraction produces a vertical shift (ie in the apparent altitude) of a star - it appears higher in altitude than it really is. This produces an offset in its hour angle (ie RA) AND its declination - and worse, these offsets vary as the star moves across the sky.

So wherever you aim your polar axis, its really only an approximation and you DO need to guide in RA and dec to account for the changing offsets as the stars altitude changes.

Don't hypothesize - do the maths. It's describe in Smart's book "Spherical Trigonometry" and it isn't trivial - it is a significant challenge for someone with good highschool year 12 trig and calculus. I did all this 35 years ago to produce a program for determining position from 2-star alignments using a HP41CX... but don't ask me now.

Cheers, indeed.

Wavy.

Last edited by rally; 21-01-2018 at 01:57 AM.
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Old 21-01-2018, 08:00 AM
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I had a look at Tpoint. Looks like it is and add on for TheSkyX. Universal license for the suite was $1200. I assume this is US $ - that's a significant amount of money to spend on improving polar alignment.
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Old 21-01-2018, 12:46 PM
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I agree Peter - $1200 is a lot, especially if you dont need it.

But in fairness the $1200 is for lot more than just Tpoint !

My version of TheSky and Tpoint came free with two of my Mounts !
So from my perspective there was no outlay

Tpoint was a stand alone program back then too, but it fully integrated into TheSky if that was how you wanted to use it or I believe it could be used stand alone but not sure how.

Now it is designed and sold as an addon - I am sure for financial reasons.

BTW I just looked up the cost of Tpoint on SB website - it is $249US as an online download but you still need the TheSky to make it work.

Anyone who is doing anything at moderate to high level will need some sort of planetarium package that is similar in capability to this - star databases, NEO trajectory support, PEC support, rotator and guidecamera positioning and field of view indicators, ascom compliancy, measurement and analysis etc etc - so I guess what you start off buying in the first place depends on your analysis of where you see yourself going in the future and what your needs are.
I started off with TheSky even before I bought my good mount.

Knowing that its an addon may be a reason to sway your decision toward the TheSky or maybe the other way around !

The full suite obviously includes all sorts of extra things - if you need them all then its what you pay I guess - its not a cheap or a free exercise thats for sure - but its a well supported sophisticated platform.

What is the cost of going on a weekend or week long astro imaging trip and finding out all your subs are substandard due to issues that the software would have fixed or identified so you could fix it ?
Or wasting many hours over many nights drift aligning or trying to sort out the mount or pointing issues.
Or the lost opportunities of deciding not to setup at all because the weather forecast says it might be cloudy or it might rain and SkippySky says the seeing is going to be crap but its ends up sub arc second - you make the decision knowing that its going to take you an hour or more to drift align and choose not to do it and then find the sky is great and it never rains ! havng a tool that allows you to do everything earlier in the evening and more accurately means you have more options.
Ive often wondered what the real cost of our hobby is when we divide the costs by the amount of useful astro time
Then using that approach we suddenly get a number that in some ways represents a return on our investment !
Having these tools makes that a much better number

Or wanting to track the ISS or NEO and your mount and software not being able to do it becasue you cannot even get the latest ephemeris orbit into your planetarium package let alone control the mount with anything but sidereal or lunar tracking rates - so there is some benefits in having the right software - if that is the sort of thing you want to do or at least try and have a go at.

I just see it as part of the toolbox you need to get the job done - that includes the car we travel in, the car fridge, the camping gear, the laptop and the computer and other software licences to do our processing and the cold weather clothing we need to handle freezing cold nights at 3.00am, the table, chair and everything else that collectively is needed to do the hobby from start to finish - and there is bucket load of stuff !
The costs of doing this include the meals for the trip, accomodation, fuel and other wear tear costs, entry fees and site fees etc - we're happy to pay for this because its part of the job.

I wasted countless hours drift aligning and in the early days messed it up so badly that at 1.00am in the morning I was worse than when i started, tired and making silly mistakes and at the point of giving up for the night ! - until I started using Tpoint - then I was setup before atmospheric twilight and I could kick back and relax and not stress.
In my case once I had drift aligned I then had to disturb the entire telescope system rebalancing by removing the visual system and adding the camera system - so PA was slightly affected anyway and I never really knew how good my PA was after doing this (i guess I could have checked with the camera but never bothered) - so for me it solved a lot more problems than maybe for others.

The other option that this thread sort of started off with is the Plate Solve and Goto approach and for that you dont have to reinvent the wheel you can use PlateSolve2 and Astrotortilla I think they are still free, but have no personal experience using them.

I thing pretty much all telescopes around the world above 3m use Tpoint and plenty of satellites and other space based systems for antennae - but they use the Tpoint software librarys which are then integrated into their own custom control systems by full time programmers
SB provides its own version of Tpoint which is what we buy because it integrates into their planetarium suite of programs and addons.
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  #19  
Old 21-01-2018, 01:01 PM
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peter_4059 (Peter)
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Rally,

I agree with you. It is a bit like buying photoshop and all the processing add-ons these days when you can purchase PI that does it all and better.

If I was in your situation where the software came with the mount then I'd be saying the same thing. I've gone down a different path where I've been able to justify the cost of mainly Chinese hardware and have over time accumulated a suite of software at little or no cost that does pretty much everything I need.

My gear is permanently set up for most of the year and I take it to astrofest once a year for a 10 day stint. At astrofest I can use sharpcap and be polar aligned in minutes. At home I can't see the pole due to trees so resort to drift aligning. I checked it last week after many months since I did the initial alignment post last astrofest and it was remarkably good so I assume my obs footings are not moving much despite all the rain and clay in our area.

The use of SGP and platesolving allows me to return to the same object within a few pixels without having an amazing pointing model and I'll soon be adding camera rotation when the LiteCrawler arrives in a few weeks time so both position and angle should remain constant between sessions.

My qustions regarding polar alignment really started with why plate solving couldn't be used to speed up the drift process. I believe it can but I just need to spend the time working through the spherical trig maths and then working out how to automate the mount movement, camera and platesolving.

Cheers,

Peter
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  #20  
Old 21-01-2018, 01:04 PM
rally
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Peter,

Sounds good.
Why dont you look at the Tpoint documentation and have a look at the maths they use behind their Terms - pretty sure its mostly published
Go to Patrick Wallaces (original author) official Tpoint site not the Sb site for this information
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