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Gemini Closed Loop tracks in DEC too?

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#1 DeanS

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Posted 26 June 2009 - 10:10 AM

Ok, Alph made a comment that I would like to know for a fact. He states his Gemini will track in DEC as well as RA.

I owned an MI-250 for a couple years, loved the Gemini pointing model and how it corrected my GOTO's for an inaccurate polar alignment. Key word here is GOTO's as I never once thought it did anything more than that.

Since this is not an ALT/AZ mount then the dec motor does not track when on an object, only the RA?

I would like to hear other comments about the capabilities of the pointing model. Me experience says it does not track in DEC.

Oh and without starting an AP vs Gemini agrument please :grin:

And as a side note from actual user experience of owning and using both mounts, I find that the polar scope alone does just fine for the AP mount in locating objects, I may have to re-synch after a flip but not always. I was very concerned about this at first but it has become a complete non-issue. My Genimi did not always handle a flip perfectly either so it was about the same, even after building a model. I do alot of star parties, imaging only, and many times only use the polar scope and do not drift align and have no problem getting objects on my ccd chips.

I liked my MI-250, and I like my AP mounts. Its all good :D

#2 rsbfoto

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Posted 26 June 2009 - 10:15 AM

Hi,

Yes Gemini has a function of comet tracking and in that case it tracks in DEC too :grin:

You can train the comet tracking or there is the possibility to input some coordinates (it is explained in the instruction manual).

#3 DeanS

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Posted 26 June 2009 - 10:24 AM

But does it do it during normal operation? Again I never noticed mine doing it as I would get drift if I was not polar aligned very well. If it does track, then it should not drift much in DEC?

AP comes with a neat piece of software called Pulse Guide that says:

Pulse guiding. Corrects for drift due to mispolar alignment and atmospheric refraction. Pulse guiding is an idea originally suggested by Roland Christen of Astro-Physics and implemented in this application. It works by sending precisely timed serial commands that make the mount move faster in RA and/or Dec for a short period (hence “pulses”).

Bascially what you do is take an image, let the mount track (not guide) for 5-10 minutes. Then take another image and it determines what the drift has been and will adjust your mount accordingly. Not sure how it handles PE but I can see how it would help with DEC drift.

#4 rsbfoto

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Posted 26 June 2009 - 10:29 AM

But does it do it during normal operation? Again I never noticed mine doing it as I would get drift if I was not polar aligned very well. If it does track, then it should not drift much in DEC?


Hi,

No, it does not track in DEC when using Sidereal or King Rate tracking nor does it do it when using Solar or Lunar tracking.

In the normal tracking mode Gemini behaves as any other tracking system for a GEM.

#5 DeanS

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Posted 26 June 2009 - 10:51 AM

Ok, I think this statement made by Alph will confuse people when comparing mounts and controllers.

>>The Gemini controller can track an object both in Dec and RA for hours without an accurate polar alignment. The AP-GTO controller does not have that capability.<<

Perhaps if you set it up for comet tracking but not for normal visual or imaging settings.

Thanks

#6 rsbfoto

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Posted 26 June 2009 - 11:24 AM

Hi,

That is correct. OK, you could turn on Comet Tracking and train the mount for an hour or so(maybe in this time you can detect a DEC drift :grin:) and then leave it like that but you will have to do it for any position on the Sky :grin:

#7 Alph

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Posted 26 June 2009 - 04:10 PM

Ok, Alph made a comment that I would like to know for a fact. He states his Gemini will track in DEC as well as RA.

This feature is called closed loop tracking. See section 2.4.1.1 Rough Polar Alignment of the gemini_manual

#8 Charlie Hein

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Posted 26 June 2009 - 04:32 PM

Also Section 3.3.1.5, page 25.

#9 DeanS

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Posted 26 June 2009 - 06:58 PM

HMMMMM I say! I had no idea it did that!

I never did anything other imaging with mine so was always polar aligned very well. Never looked at any other tracking modes.

This sounds very similar to the AP Pulse Guide. Do you have to follow an object for a period of time to set up the correct tracking?

Have you actually tried this?

#10 Alph

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Posted 26 June 2009 - 08:02 PM

This sounds very similar to the AP Pulse Guide


No, not at all (Does anyone use pulse guide?)
The Gemini closed loop tracking is based on the same principles as the Alt-Az tracking.

#11 Tom and Beth

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Posted 26 June 2009 - 08:39 PM

Just a note that I believe the Polar scope AP sells comes from Losmandy.

I've never had a Losmandy mount so can add nothing to the Losmandy side of the discussion.

#12 Chris Curran

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Posted 27 June 2009 - 07:09 AM

That's the irony... :) I use a Losmandy PAS to get aligned in a matter of seconds, while most of my buddies are fiddling with GPS units or looking for enough alignment stars. One thing everyone seems to miss (and no one seems to mention) about the Closed Loop tracking rate. It's in the doc (pg 25):

Although Closed Loop tracking will keep an object centered for a long time, it is not a
substitute for a good polar alignment


and

Because Gemini uses modeling parameters to calculate coordinates, Closed Loop
tracking is only available after a pointing model has been established


Sure, you don't *have* polar align a Gemini unit, but it's recommended, and, you have to perform other tasks (instead of a polar align) to achieve any result at all. I can see where this would be very useful to someone who doesn't have access to Polaris (trees, houses, etc), but, if you can see Polaris, why not just polar align? It's easier and faster...

#13 Charlie Hein

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Posted 27 June 2009 - 07:32 AM

Sure, you don't *have* polar align a Gemini unit, but it's recommended, and, you have to perform other tasks (instead of a polar align) to achieve any result at all. I can see where this would be very useful to someone who doesn't have access to Polaris (trees, houses, etc), but, if you can see Polaris, why not just polar align? It's easier and faster...


I'm pretty sure that you'll get universal agreement on this... but Closed Loop tracking looks like a nice feature to have *when you need it*.

#14 neptun2

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Posted 27 June 2009 - 08:04 AM

Yes but in my opinion something like all star polar alignment in celestron's recent software will be best for people which can not see polaris from their observing site. I hope more manufacturers include this in their firmwares. If you can see polaris i also prefer to use the polar scope. Even the polar scope of my heq5 which is not the easiest to use is enough to achieve good polar aignment if you know how to work with it.

#15 Chris Curran

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Posted 27 June 2009 - 08:17 AM

Charlie,

I agree 100%. I'm just having trouble figuring out the "need" part. The only GEM systems I have any experience with are G11/Gemini and AP. Both use the same PAS so there's a huge comfort factor for me with that method. Both mounts offer methods of alignment that do not need Polaris, so I just don't see the "need" for closed loop tracking.

#16 JAT Observatory

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Posted 27 June 2009 - 08:27 AM

This Closed Loop tracking feature sounds like the ProTrack feature that is available for the Paramount.

I think the need part is it increases unguided tracking times by using the pointing model to make corrections in Ra & Dec as the mount tracks. The more points in the model, the more accurate the unguided tracking will be.

While I am only familiar with the Bisque process I only use it after the scope is aligned. The initial alignment is refined after a limited pointing model run, then a full pointing model is created (100+ points). At that point the Ra & Dec unguiding tracking works very well.

I am surprised this is not available for more mounts as it would correct a lot of tracking issues that some mounts seem to have when used on long focal length OTA's.

#17 Chris Curran

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Posted 27 June 2009 - 08:42 AM

Marcus,

They are similar in nature, but in practice they are somewhat different animals. The Bisque method relies on a TPoint model and decent polar alignment. The purpose of that software is to improve goto operations and unguided tracking for imaging. The purpose of the Gemini setup is to allow decent visual use in the goto/tracking department with poor alignment - it is not intended for use in imaging applications (e.g. field rotation).

BTW, the AP 3600 improves on the Paramount model with it's ultra-precision encoders, but it comes at a price!

#18 DeanS

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Posted 27 June 2009 - 09:06 AM

Good thread, I have learned something, and will try my pulse guide next chance to see how well it works.

#19 Charlie Hein

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Posted 27 June 2009 - 09:20 AM

Charlie,

I agree 100%. I'm just having trouble figuring out the "need" part. The only GEM systems I have any experience with are G11/Gemini and AP. Both use the same PAS so there's a huge comfort factor for me with that method. Both mounts offer methods of alignment that do not need Polaris, so I just don't see the "need" for closed loop tracking.


Ah... I think I can help here. When you take into account that on the Losmandy the PAS is an option (so you might not have one), and you could also find yourself in situations where you cannot see the pole even if you do have a PAS, then in certain scenarios - mostly where you're operating visually, quick setup time is a must and the G-11 doesn't have a PAS (or when you cannot *see* the pole) - the Closed Loop tracking could be quite handy. Given what they say about it in the manual all you'd need to do is a quickie bore-hole sighting on the pole (or just take your best shot if you can't see it), run two or three stars on one side of the meridian and one on the other, turn it on and you're done for the night.

I can envision a couple of scenarios where this would be quite handy, especially in the summertime when the sun goes down so late in the day. I hear it all the time - in the summer, setup time can often be the deciding factor for a lot of guys in either having an observing session or just leaving the scope packed up on a weeknight, particularly with a larger and more complex mount like the G-11. If the ability to use the Closed Loop tracking is what gets you out under the stars when you might not have bothered otherwise then that's a good deal in my book.

We do a lot of public outreach in our club and a lot of it is done in county or municipal parks where the employees aren't always disposed to stay too late. If we need to be wrapped up by say 10:00pm then this doesn't give you a lot of time at all to actually show things to folks, so speed of alignment is very important to a successful event. I completely agree with you that the PAS is the fastest way to get there if you can see the pole, but if you don't have one or you can't see the pole then IMO this Closed Loop tracking would likely be the second fastest way to get to a decent enough result to allow long lines of the public to observe without having to continually jump in and recenter your target like you do when you aren't tracking well.

#20 Tom and Beth

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Posted 27 June 2009 - 11:06 AM

May I note that quick setup of AP mounts using the Meridian Delay alignment procedure allow very good (precise?) polar alignment during the day, which makes the PAS redundant and Solar outreach programs effective? Is it possible that this procedure would work on other GOTO mounts?
This is a good thread, folks.

#21 rsbfoto

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Posted 27 June 2009 - 11:30 AM

Hi,

After reading this thread I went to read the Gemini manual again. I knew something about closed loop but honestly did not understand it quite well. Now afterreading about it it looks a little bit clearer for me. Will test the next time I am in the observatory.

One thing is not clear still.

If I use closed loop and go to an object I assume I must do a sync when I have the object in the center of the FOV so Gemini knows what the actual coordinates are the mount is pointing and so make the corresponding calculation for the closed loop tracking ¿ or ?

#22 Alph

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Posted 27 June 2009 - 12:46 PM

One thing is not clear still.

If I use closed loop and go to an object I assume I must do a sync when I have the object in the center of the FOV



Sync'ing is optional. It is a good practice though. It will improve tracking. I do it with my Alt-Az mount too.

#23 David Pavlich

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Posted 27 June 2009 - 01:58 PM

Gemini's Polar Axis Correction doesn't involve Polaris at all. Make sure that the mount is pointing North. Do at least a five star align, I do more, then pick a star close to the meridian and the celestial equator.

Goto the star, center it with the HC, select Polar Axis Correction and the mount slews to where the star would be if the mount were polar aligned. Center the star mechanically and voila! It's polar aligned.

David

#24 Chris Curran

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Posted 28 June 2009 - 05:40 AM

Charlie,

It's refreshing that this has turned into a civil, instructional thread. It may start to restore my lost faith in CN....

To be clear: I think the G11/Gemini combo is a wonderful visual rig. Probably the best bang/buck out there in that segment.

PAS not included... Yea, ditto with AP. Everything is ala carte over there! :D However, I do a good bit of public settings and I consider the PAS a required tool for such endeavorers.

I guess I don't see how CLT gets you under the stars any quicker. You have to wait for (at a minimum) dusk so you can do those alignments stars. Our club also does a lot of outreach - we have two sidewalk astro events every month like clockwork, plus other "special" events. Getting setup and being ready at dusk is a distinct advantage. Wouldn't it be more useful if the mount provided a method of polar aligning in the daylight instead? You'd be ready at dusk, instead of looking for a handful of stars to sync on.

This isn't to say that I think CLT is "junk". It's very clever, but, it has always felt like a "gimmick" to me. It's a GEM - don't give me Alt-Az solutions. I would prefer to see routines that provide for daytime alignment, and the ability to move the meridian. Those two features are huge for public outreach, personal use and imaging. CLT just never did anything "useful" for me. Even a rough polar alignment has always been "good enough" for any goto/tracking operation that I ever perform in a visual setting.

And no, I don't think AP is perfect... None of these systems are perfect. It bugs the you know what out of me that a $10k mount doesn't have a bubble level built in... How lame is that!?

#25 Chris Curran

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Posted 28 June 2009 - 05:45 AM

Actually, the Meridian Delay method needs dusk/dark. I think you're referring to the Park1&2/Level method for daytime alignment (which is stinking brilliant).

IMHO this method is a little overkill for public settings. And no, to the best of my knowledge, no other mount provides a Meridian Delay type feature.

May I note that quick setup of AP mounts using the Meridian Delay alignment procedure allow very good (precise?) polar alignment during the day, which makes the PAS redundant and Solar outreach programs effective? Is it possible that this procedure would work on other GOTO mounts?
This is a good thread, folks.








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