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Celestron VX Mount Tracking Test

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#26 Jeff2011

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Posted 08 April 2013 - 06:25 PM

In my opinion if you don't use a reticle EP you can forget about precision, in gotos or polar alignment.
Chris


I don't own a polar finder scope partly because my mount did not come with one and partly because many CN members like Chris do not think highly of them. A reticle EP is a must have. However, I would like to delve deeper in the polar alignment procedure itself. In my step one, I did an initial polar align on Polaris by centering Polaris in my reticle EP using the Alt/Az adjutments of the GEM. Since Polaris is approximately 0.7 degress from the NCP, my initial alignment was off. This was later corrected when I did a polar alignment using the mount software. This involved centering on a Star near the Meridian but not near the pole. Then executing the polar align function on the hand control which moved away from the star and then moves back but not exactly on the star. Then I centered the star in the reticle EP using the Alt/Az adjustments on the mount. Using the software polar align, the reticle EP is required for accuracy, but on the rough polar align that I did in step 1, the reticle is not. I am planning on changing step one as follows:

Dial in Polaris in Sky Safari and turn on the NCP marker and create a ring with crosshairs the size of 1.5 degrees. Place the center of the crosshairs on the NCP and see where Polaris falls. Of cource I will need to have Sky Safri flip the view horizontally to match the view through my refractor. I will then take an eyepiece that gets a 1.5 true field of view and place Polaris using the Alt/Az adjustments of the mount in the same place as that I see it in Sky Safari. If I then really wanted to get an accurate polar alignment, I could then skip the software Polar alignment in my step 3 and just do a star drift alignment from there using the reticle EP. If the clouds permit, I will try this tonight.

I remember reading that a polar alignment should only be as accurate as it needs to be. So if I am just trying to take 2 minute exposures, then my initial method will work just fine. If I need longer, then it looks like I will need to do a star drift alignment.

#27 cn register 5

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Posted 09 April 2013 - 01:38 PM

My main objection to the polar finder is that I'd have to demolish my house to use it :)

It may also be more difficult to use for us in the UK because Polaris is much higher than for most of the USA.

With ASPA it seems better to me to spend the money on a reticle EP, I think it will give a better quality alignment and this will help with ASPA.

Chris

#28 DaveJ

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Posted 09 April 2013 - 03:23 PM

With ASPA it seems better to me to spend the money on a reticle EP, I think it will give a better quality alignment and this will help with ASPA.


Hi Chris,

Truer words have never been spoken. That's exactly how I feel about ASPA, reticle EPs and polar alignment scopes in general. ASPA just plain works for me, and has since the first time I tried it. :waytogo:

#29 Raginar

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Posted 09 April 2013 - 06:38 PM

Chris,

This was corrected around firmware version 4.2 that you no longer needed to re-align after you did ASPA. Prior to that, it was a requirement for, at the very least, CGEMs.

Hope that clarifies that little issue. I was still doing it until I got rid of my CGEM because that was my habit routine from when I first got it.

Sometimes we all to be right when we should just ask if it matters :roflmao:

#30 cn register 5

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Posted 10 April 2013 - 06:27 PM

Strange, none of the beta versions needed this.

Chris

#31 Raginar

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Posted 10 April 2013 - 07:21 PM

Beta versions from 2010? It was a common thing to do up until v4.2.

Chris

#32 cn register 5

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Posted 11 April 2013 - 03:13 AM

I was testing this in 2006/7 and a realign after doing the polar align was never essential. This is what the developer told us. I tried it and it was correct. I can't speak for the manual, we didn't have one, just the instructions in the HC.

Yes, a lot of people thought a realign was essential. They were wrong. It would have taken them a few seconds to check this, all they needed to do was a goto after doing the ASPA and see what they got.

Chris

#33 freestar8n

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Posted 11 April 2013 - 03:34 AM

I think there may be some confusion of unsyncing with recalibrating - after polar align. The first step in ASPA is to sync on the star - and that star is still sync'd after the alignment completes. Any time you sync on a star you improve pointing at that star - but may lose accuracy far away from that star. So it may or may not be desirable to unsync after ASPA - depending on how far away you intend to wander, and how bad the pointing is.

My recommendation - based on experience with cge and cge-pro - plus the basic principles of ASPA is:

1) Start with a good 2+4 alignment so the mount is calibrated

2) Use a star near the meridian and maybe 30 degrees above the horizon - i.e. ignore the equator here - you want a star lowish in the sky and opposite the pole. This may differ from the manual.

3) Don't spend too much time fiddling with the alt/az motion to center the star - just do one move in az and one in alt.

4) When ASPA completes, check the accuracy in an area of sky you want to study. If pointing is a bit off, first try unsync to see if it improves

5) If it is still a bit off and you want to improve it, replace the two alignment stars with the same ones, or new ones.

That should be all you need to do, and you definitely shouldn't power down. Unsyncing is simple and may improve pointing.

The main thing is - after ASPA everything may be fine and you don't have to do anything more, but if you want to improve it, there are two simple things to try.

Frank

#34 DaveJ

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Posted 11 April 2013 - 08:30 AM

I was testing this in 2006/7 and a realign after doing the polar align was never essential. This is what the developer told us. I tried it and it was correct. I can't speak for the manual, we didn't have one, just the instructions in the HC. Yes, a lot of people thought a realign was essential. They were wrong. It would have taken them a few seconds to check this, all they needed to do was a goto after doing the ASPA and see what they got.


I'm in complete agreement with Chris on this. I, too, have been using this exact same technique from day one with ASPA with success. I even told the Celestron reps how the ASPA worked following its installation in my CGE prior to NEAF that year. They, quite frankly, were baffled by it all and were explaining its function in a most bizarre (and incorrect) way to some potential customers.

#35 Stew57

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Posted 11 April 2013 - 09:21 AM

The need for realignment will depend on how far off you original polar alignment was before doing aspa. Do a test yourself and you will see an increase in pointing accuracy. The need of a realignment or lackthereof has been discussed on teamcelestron.com. If you objects are landing on you camera chip with the same accuracy as before the aspa your fine. If not redo the alignment. It only takes a few minutes and could save some frustration.

#36 DaveJ

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Posted 11 April 2013 - 11:16 AM

The need for realignment will depend on how far off you original polar alignment was before doing aspa. Do a test yourself and you will see an increase in pointing accuracy. The need of a realignment or lackthereof has been discussed on teamcelestron.com. If you objects are landing on you camera chip with the same accuracy as before the aspa your fine. If not redo the alignment. It only takes a few minutes and could save some frustration.


Oh, I HAVE done the test - a zillion times. I never pushed it to see how far off I had to be to get it NOT to work, but I've intentionally misaligned from the pole by at least 5 degrees, did the 2+4 alignment using an illuminated cross-hair reticle eyepiece, selected a star near the intersection of the meridian and celestial equator, performed the ASPA using that star, and went about my business with every object - either side of the meridian - falling dead center. Once centered, I could leave the object with no drift whatsoever from the center of a high-powered eyepiece for as long as I chose. In other words, pretty darned perfect behavior with both goto centering and subsequent tracking. That's what has worked for me. I've never performed another alignment following ASPA. HOWEVER, the 2+4 with cross-hair reticle is required FIRST before the ASPA so the firmware has an accurate model of the sky. The ASPA uses that model to perform its task. Any discrepancy between what I've explained and what was actually done by the user will result in an inaccurate internal model and render ASPA far less accurate and probably require another alignment. Hey, it works perfectly for me - what can I say? Math is doing the magic and if the rules are followed, it'll do it every time.

#37 Jeff2011

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Posted 11 April 2013 - 11:19 AM

My interpretation of their recomendation in the manual to resync after the ASPA was in case your scope shifted while making the Alt/Az adjustment. I assume they did not want people to call them up and whine about ASPA not working. I think if you are careful when making the Alt/Az adjustment and have your scope attached tightly to the mount, the resync of the alignment stars is not needed after the ASPA.

#38 Stew57

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Posted 11 April 2013 - 12:06 PM

My experience is a bit different. I use a mallincam in my C11 with cross box enabled (more accurate than a reticular eyepiece), do a 2+4 alignment followed by an aspa. If the mount needs little adjustment the objects will fall on the chip at F10. If the mount needs much adjustment then the objects will no longer fall on the chip. If I redo the 2+4 they again start falling on the chip. Derik from celestron acknowledges that sometimes an alignment will need to be redone. If your objects are landing where you want them, go with it, if not a redo is in order. BTW how long an object stays in an eyepiece has nothing to do with pointing accuracy but it does speak to polar alignment. I had one CGEM that would keep the object in the eyepiece all night with a couple of arc minutes of PE. The object just oscillated back and forth roughly centered all night. Was not a factor (you couldn.t tell) visually but for use with the mallincam it was a disaster.

#39 DaveJ

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Posted 11 April 2013 - 12:24 PM

BTW how long an object stays in an eyepiece has nothing to do with pointing accuracy but it does speak to polar alignment.


Hi Stew57, I'm fully aware of that - but it's a common mistake I've seen made here on the forum. There's a gigantic difference between a goto alignment and a polar alignment. A mount can be set up to have neither, one or both. Both is, of course, preferred. I've been setting up GEMs for fifty-three years and know the difference. :grin:

#40 jrcrilly

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Posted 11 April 2013 - 12:34 PM

BTW how long an object stays in an eyepiece has nothing to do with pointing accuracy but it does speak to polar alignment.


Hi Stew57, I'm fully aware of that - but it's a common mistake I've seen made here on the forum. There's a gigantic difference between a goto alignment and a polar alignment. A mount can be set up to have neither, one or both. Both is, of course, preferred. I've been setting up GEMs for fifty-three years and know the difference. :grin:


BUT in the context of ASPA, the polar alignment is dependent on the goto alignment - and thus, the tracking is also. A weak goto alignment will result in a weak polar alignment - and poor tracking.

#41 freestar8n

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Posted 11 April 2013 - 12:48 PM

Any discrepancy between what I've explained and what was actually done by the user will result in an inaccurate internal model and render ASPA far less accurate and probably require another alignment. Hey, it works perfectly for me - what can I say? Math is doing the magic and if the rules are followed, it'll do it every time.



At your latitude using the equator may work ok but it is probably not optimal, and it would work worse for people closer to the equator. So again I would use a star lower in the sky to improve the azimuth alignment accuracy.

Also - people are talking about realignment after ASPA to improve accuracy. There is another thing to try, which is just unsyncing. Syncing is usually only good for a part of the sky, and since the mount is still sync'd after ASPA, it may lead to inaccurate goto's across the sky. It may also be fine - but the point is there is something else to try and it is faster than realignment.

Frank

#42 DaveJ

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Posted 11 April 2013 - 01:01 PM

There is another thing to try, which is just unsyncing. Syncing is usually only good for a part of the sky, and since the mount is still sync'd after ASPA, it may lead to inaccurate goto's across the sky. It may also be fine - but the point is there is something else to try and it is faster than realignment.


The unsyncing is part of the ASPA, assuming the ASPA is done correctly. I left that part out thinking the reader would know that it's included as part of the ASPA. That's the only way to get dead-centered gotos on both sides of the meridian following ASPA.

#43 freestar8n

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Posted 11 April 2013 - 01:19 PM

Unfortunately there are now many documents on ASPA and I don't know what is official. But I would not consider unsyncing "part of" ASPA because you would NOT want to unsync if you intend to observe near the star you used for ASPA. You are only prompted to unsync if you choose to start a new alignment. My point is - you can unsync without doing a new alignment - and it may improve all sky pointing.

I'm pretty sure many people don't realize this is a step that could help them - and avoid the need for realignment. It's something easy to try anyway.

I don't know of any celestron document that has corrected the mistaken notion that the equator is somehow optimal - but I hope people can realize it from common sense - e.g. by realizing it would fail completely for someone near the Earth's equator since such a star would be nearly overhead.

Frank

#44 DaveJ

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Posted 11 April 2013 - 01:51 PM

I don't know of any celestron document that has corrected the mistaken notion that the equator is somehow optimal - but I hope people can realize it from common sense - e.g. by realizing it would fail completely for someone near the Earth's equator since such a star would be nearly overhead.


You are, obviously, exactly correct. I'm at 41° 11" latitude so the intersection of the celestial equator and meridian is 48° 49" in altitude due south so the AZ adjustment is still effective. I'll admit that the recommendation in the manual to use a star as close to the celestial equator & meridian has overly influenced me to the point of being an obsession. :grin: I can't see any reason to not follow your advice and use a star closer to the horizon and intersecting the meridian. It would seem that the AZ and the ALT adjustments would have their highest sensitivity there - which is exactly what we're after. For those that are confused by this discussion, imagine you're located at the equator. A star at the intersection of the celestial equator and meridian would be directly overhead. Thus, the AZ manual controls would be totally worthless since you could turn and turn and turn until you're blue and the star would remain centered. Not so if the star is much closer to your local horizon where the AZ manual control has its greatest effect per adjustment.

#45 freestar8n

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Posted 11 April 2013 - 02:10 PM

I can't see any reason to not follow your advice and use a star closer to the horizon and intersecting the meridian.


Thanks - please help spread the word. I think ASPA will work very well for most people as long as the star is not too high in the sky and it is near the meridian. Many of these threads focus on fine details to finesse the results - but there are people who talk of it not working as well as they hoped - and I think using a star lower down may help.

And that will help both the polar alignment accuracy and the GoTo accuracy - since GoTo after ASPA assumes the mount is perfectly polar aligned.

Frank

[edited for clarity]

#46 Patrick

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Posted 11 April 2013 - 02:11 PM

Nothing will beat a drift alignment. The software gets you close but it's based on an imperfect model. A drift alignment is based on observation.



Hmmm...that has not been my experience. When comparing image drift from subframe to subframe, ASPA has always done a better job of controlling it than any drift alignment I could do. I would typically do 5 minute observations without drift before changing the axis, then work on it in the other direction until I could go 5 minutes, and then recheck the first axis, etc. That was typically at high power.

As John mentioned, the key is to do a good goto alignment first, and then do the ASPA. Without a good goto alignment the hand controller model may not be accurate enough. I know that by the time I do a 2+2 or a 2+3 alignment, the controller is putting objects dead center in the eyepiece.

That's when you want to do ASPA. So, I am a believer that it's more accurate than I can be with a manual drift alignment, and it's certainly a lot faster.

Patrick

#47 Jeff2011

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Posted 12 April 2013 - 09:04 AM

Had the mount out again last night and decided to give ASPA another try. When I do my alignments I use a 12.5mm reticle eyepiece with a 2x barlow. I noticed after the polar alignment, the calibration stars were gone. By gone, I mean that when I went to recalibrate a star after the polar align, there were none to replace. All four calibraton slots that I had filled before the polar align were now open. This is what I saw last week, but I wanted to repeat the test to make sure. After dinking around with it for over two hours, basically repeating the ASPA serveral times to make sure it was working well I decided to take a few shots. Out of 9 frames that were 90 seconds each, 4 had well rounded stars and 5 had slightly oval stars. I did mave the mirror lockup set for the DSLR to prevent vibration from the camera shutter. Not sure if the slightly rounded stars were caused by periodic error or some other cause. It was not windy and the scope was well balanced. Are my results typical for unguided shots?

#48 Whichwayisnorth

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Posted 12 April 2013 - 09:53 AM

Had the mount out again last night and decided to give ASPA another try. When I do my alignments I use a 12.5mm reticle eyepiece with a 2x barlow. I noticed after the polar alignment, the calibration stars were gone. By gone, I mean that when I went to recalibrate a star after the polar align, there were none to replace. All four calibraton slots that I had filled before the polar align were now open. This is what I saw last week, but I wanted to repeat the test to make sure. After dinking around with it for over two hours, basically repeating the ASPA serveral times to make sure it was working well I decided to take a few shots. Out of 9 frames that were 90 seconds each, 4 had well rounded stars and 5 had slightly oval stars. I did mave the mirror lockup set for the DSLR to prevent vibration from the camera shutter. Not sure if the slightly rounded stars were caused by periodic error or some other cause. It was not windy and the scope was well balanced. Are my results typical for unguided shots?


At what focal length?

#49 mpgxsvcd

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Posted 12 April 2013 - 10:57 AM


In my opinion if you don't use a reticle EP you can forget about precision, in gotos or polar alignment.

Chris


Or a camera with a really nice 10x lossless crop mode and centering lines on the display. No eyepiece would ever give you a 10x magnification and still allow you to see the stars. However, certain cameras can do that easily.

There is also the added bonus that you won't have to align with something other than what you are actually using for observing/imaging.

#50 Jeff2011

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Posted 12 April 2013 - 11:02 AM


At what focal length?



430mm






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