Tpoint interaction with Hand control alignment choices question

[g__day (Matthew)]

I wonder if anyone could provide some advice.

Background

My hand controller (Vixen SkySensor2000-PC) can support a one, two or three star pointing model. If (and only if) I chose a 3 star model then the mount corrects both its pointing and its real time tracking quite accurately for any computed polar mis-alignment (up to 20 degrees). This feature is used primarily in case you wish to image near the Celestial Pole! I have tested this feature with a 5 degree polar misalignment - stars track beautifully at long focal for up to 10 minute individual shots (but field rotation occurs) even when you are miles off polar alignment. I am not sure how the 3 star model's tracking control interacts with my favourite guiding programs - as I expect it is periodically altering the real time DEC and RA motor rates + allowing for altitude based refraction correction to the tracking algorithm. But I routinely image at 2.3 metre focal length for 10 - 20 minute guided shots and get great results.

My hand controller's in-build clock isn't great - it seems to lose about 12 - 15 seconds a day (so I reset this every day as the first step - maybe the battery is getting really old or the crystal in the time piece just wasn't set accurately enough)!

My gear is permanently mounted.

In the past Tpoint has on a 120 star model reported my gear's pointing ability is around 90 arc seconds and with Tpoint engaged around 30 arc seconds. Since then I have improved the polar alignment substantially.

My polar mis-alignment I believe is now around 0.3 arc minutes.

Whenever I turn on the SS2K for a night's imaging - I check the time and then re-centre the alignment stars to preserve my one, two or three star pointing model from the previous evening.

My Questions

Is the Sky6 PE + Tpoint's pointing model sensitive to which alignment model (one, two or three stars) I have set at my hand controller, and if so do I need three pointing models - one for each of the SS2K's three possible initial alignment models?

So in essence I am asking should I have a preference for whether I select a one, two or three star model at the hand controller; given I need to consider both creation of the initial (say 120 star sky model) and then its continued re-use each subsequent evening?

Let's suppose I choose a simple one star alignment model (say I pick Antares) and then create a 120 point Tpoint sky model. I assume each subsequent night I should simply align the scope at power on to only Antares, then connect the Sky6 + Tpoint and do my normal night's work flow.

If I choose a two star model (say Antares and Spica) and created say a 120 point model of the sky; then on each subsequent evening I should re-align the SS2K on both of these two stars before I use the SKy6 + Tpoint.

Similarly for a 3 star model alignment.

So my over head each night is to confirm my one, two or three star model is still correct at the SS2K to the original alignment model I selected at the time I created the Tpoint model. After 3 months I might chosse one, two or three next alignment stars and rebuild the entire sky model before I start using the Sky6 + Tpoint.

Assumption

I never presume I can mix and match my one, two or three star model ideally with the Tpoint model if the original pointing model isn't the same. I assume if I do this I will get a less than optimal pointing model.

* * *

What advice can folks give on:

1. how many stars to select before I create my pointing model?
2. how stringently I should confirm my pointing model before connect Sky6 + Tpoint?
3. how stringently to avoid alter my alignment stars or their number without re-creating my Tpoint model?

More rambling thoughts

To me Tpoint is creating a pointing correction model of the sky. So regardless of what's visible - if I have a pointing correction measure at say when I am aiming due East at 30 degrees elevation - let's say its 5 arc seconds West and 3 arc seconds low - Tpoint always knows to adjust the aim by that amount. To me that is if not a sky model say but rather its an elevation / azimuth matrix of mechanical correction model. So it's the Sky6 and my hand controller) job to know where things are in the Sky. This then overlays onto Tpoint's correction model to improve my aim.

I guess (but I'd like confirmed) that the better the hand controller's model of the sky, the better the raw pointing and in my case tracking ability of the mount. If there is any differential change in pointing at the hand controller end than I posit that I need to be careful with having a matching Tpoint model; it may not be completely agnostic of the hand controller model used to create it.

So in a way I am guessing the Tpoint doesn't care where you look - it just measures deflections at the point you think you are at. But the Hand controller and Sky6 should improve the better sky alignment model one has at the Hand controller.

Many thanks!

Matthew

[White Rabbit]

My understanding of T point, is that you shouldn't do it in conjunction with a three point sync, as your 3 point model that you have built will be correcting slews, giving the tpoint run inaccurate positional information. It's an either or type of deal. After a 100 point tpoint run there really should be no need for a three point sync because you have 100 points of data already.

Sandy

[g__day (Matthew)]

The advantage of the three point model is the hand controller is adjusting track (real time pointing) to improve (minimise the need for guiding corrections).

I imagine no matter which model I use at the hand controller - results should be consistent within each model. So long as I don't mix and match pointing should improve. At the end of the day when I goto a RA / Dec position - Tpoint should know how far that position is out... Its only if I am not at that position is the correction model wrong!

Wish the Bisque developers would answer posts on their own forums promptly - rather than allow users to simple take up the role with guesses of unknown quantity before they eventually give an often incomplete reply to the questions asked!

[Tandum (Robin)]

Matt, You can't run 2 modeling systems at the same time. You either use your 3 star model in the handset or do a tpoint run with lots of points. What FL are you using that pointing is such an issue?

[g__day (Matthew)]

Robin,

Focal length 2.3 metres, Tpoint pointing model on 110 points give RMS pointing error of 15 arc seconds - not too bad!

* * *

I would view that the truth is the Handset needs a model of the sky, that Tpoint then corrects for.

The Handset can have a one, two or three star model of the sky. The more sophisticated a model - supposedly the better the handcontrollers pointing (and in case of a 3 star model - tracking too).

One each night of use I presume I have to re-sync the handcontroller to the alignment stars originally chosen - becuase 1) the clock is out and 2) some information may be lost during power down (e.g. is the mount level?).

I presume each Tpoint model is valid for one and only one hand controller model - and so long as you don't mix and match - everything should work okay.

Can you see any flaws in my thinking?

Ciao, Matt

[g__day (Matthew)]

Bisque folks answered all my questions, interesting and unexpected!

http://www.bisque.com/sc/Themes/hawa...icon-quote.gif g__day:
Is the Sky6 PE + Tpoint's pointing model sensitive to which alignment model (one, two or three stars) I have set at my hand controller, and if so do I need three pointing models - one for each of the SS2K's three possible initial alignment models?

No, how the mount is aligned in inconsequential.

http://www.bisque.com/sc/Themes/hawa...icon-quote.gif g__day:
So in essence I am asking should I have a preference for whether I select a one, two or three star model at the hand controller; given I need to consider both creation of the initial (say 120 star sky model) and then its continued re-use each subsequent evening?

No.

http://www.bisque.com/sc/Themes/hawa...icon-quote.gif g__day:
1. how many stars to select before I create my pointing model?

Inconsequential..

http://www.bisque.com/sc/Themes/hawa...icon-quote.gif g__day:
2. how stringently I should confirm my pointing model before connect Sky6 + Tpoint?

Sorry, I'm not sure I understand this question.

• Step 1: Establish a pointing model (TPoint for Windows users please see this document: http://www.bisque.com/sc/media/p/29538.aspx).
• Step 2: For subsequent sessions, perform a Short Mapping Run (see the TPoint for Windows User Guide: http://www.bisque.com/sc/media/p/28002.aspx for details) to reestablish the TPoint model.

Does this answer your question?

http://www.bisque.com/sc/Themes/hawa...icon-quote.gif g__day:
3. how stringently to avoid alter my alignment stars or their number without re-creating my Tpoint model?

Inconsequential.

* * *

And I finally read the part that on re-power on the next day - you open the model and select a quick model with 6 points as this re-sync's the three variables most likely to be lost on a power cycles, without changing the underlying detailed model... Wish I knew about that 2 years ago!

[Logieberra (Logan)]

Matthew, I saw this post on the SB forum as well.

I think you need to dismiss the pointing-type model that the Vixen can provide (1, 2 and 3 star). You need to think of T-point as doing it all. Forget the Vixen modelling, in fact, don't even refer to it as modelling (it just confuses the matter). It's more of a 'sync' for positioning.

Coming from a MX, you 'home' the mount electronically to a set position each use. Because that position is set, fixed, the model is ready to go each night and accurate.

For you, would you not do a 'synchronize' into a single star or image link into a star field each night? That would be your starting point, NOT the Vixen's 1, 2 and 3 star stuff. Don't even use that if possible.

From that initial 'sync' (via single star or image link), you would THEN proceed to do T-point modelling. How does this sound?

Cheers, Logan.

[Tandum (Robin)]

I agree with logan. Ignore the handset and model with tpoint.

[g__day (Matthew)]

Happy to do this, so if I understand correctly - this means:

1. Power on telescope but press ESC to revert to last pointing setup
2. Manually set the hand controllers clock (or even ignore this too?)
3. Use the Sky6 to go to a star
4. Centre on this star and from the Sky6 issue a sync

* * *

5. Next I have to refresh the Tpoint model, to do this I presume I
6. Have the current 110 star pointing model loaded, else load a new one (not yesterday's short run 6 star model) and import the 110 star model in
7. start a short imaging run of 6 stars
8. close this and return to the Sky.

Note when I do this I see the IH and ID terms change and all the rest are locked (good) however I see the reported pointing RMS go from 18 arc seconds to 50 arc seconds when I trialled this yesterday using the scope simulator on another PC.

But have I got the step of how to execute your advice correctly set out guys?

Many thanks,
Matthew

[White Rabbit]

You only need to refresh the t point model with a minimum 12 point calibration run, there should be no need to do a sync.

[g__day (Matthew)]

I do the sync so if I wish to do a goto from the hand controller itself things align moderately well.

So two questions:

Software Bisque document 6 points on a short modelling run - you suggest 12; why?

Do I care that a short modeling run reports far worse pointing then my detailed sky model map?

Thanks,
Matthew

[White Rabbit]

Correct, the tpoint manual says 6 but I read a post on TSX forum with Daniel suggesting 12, so I do 12. I think it may have been in relation to a portable mount (i have a semi permanent mount) as a-posed to a permanently mounted mount as tpoint needs to refit more terms back into the model.

I'm not sure about the last question, but have noticed the same thing. I think it has something to do the fact that your re calibration run has far fewer points than the super model and the tpoint module only shows the re calibration data not the super model but the super model is running underneath. I'm way out of my depth on this one.

[g__day (Matthew)]

That's really good to know - big thanks for that one guys!

With Remote Liveview using BYE - a 12 star sync isn't much effort.

One thing that did surprise me when building the original pointing model; at around the 30+ stars mark the target star would very reliably appear in the Remove Liveview cross hairs about an inch to the right and half and inch up - every time. The reliability was stunning, but I expected it to be centred. Each one was roughly the same amount and angle out - ponder why this occurred?

Cheers,

Matthew