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Equipment Discussions >> Mounts

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freestar8n
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Reged: 10/12/07

Re: Celestron VX Mount Tracking Test new [Re: DaveJ]
      #5792419 - 04/11/13 01:48 PM

Quote:

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


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DaveJ
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Reged: 01/07/05

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Re: Celestron VX Mount Tracking Test new [Re: freestar8n]
      #5792440 - 04/11/13 02:01 PM

Quote:

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.


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freestar8n
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Re: Celestron VX Mount Tracking Test new [Re: DaveJ]
      #5792487 - 04/11/13 02:19 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


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DaveJ
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Re: Celestron VX Mount Tracking Test new [Re: freestar8n]
      #5792557 - 04/11/13 02:51 PM

Quote:

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. 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.


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freestar8n
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Re: Celestron VX Mount Tracking Test new [Re: DaveJ]
      #5792584 - 04/11/13 03:10 PM

Quote:

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]

Edited by freestar8n (04/11/13 03:48 PM)


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Patrick
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Re: Celestron VX Mount Tracking Test [Re: Raginar]
      #5792585 - 04/11/13 03:11 PM

Quote:

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

Edited by Patrick (04/11/13 09:29 PM)


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Jeff2011
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Reged: 01/01/13

Loc: Sugar Land, TX
Re: Celestron VX Mount Tracking Test new [Re: Patrick]
      #5794064 - 04/12/13 10:04 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?

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Whichwayisnorth
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Reged: 07/04/11

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Re: Celestron VX Mount Tracking Test new [Re: Jeff2011]
      #5794167 - 04/12/13 10:53 AM

Quote:

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?


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mpgxsvcd
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Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: Celestron VX Mount Tracking Test new [Re: cn register 5]
      #5794294 - 04/12/13 11:57 AM

Quote:



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.


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Jeff2011
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Reged: 01/01/13

Loc: Sugar Land, TX
Re: Celestron VX Mount Tracking Test new [Re: Whichwayisnorth]
      #5794306 - 04/12/13 12:02 PM


Quote:

At what focal length?




430mm


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zawijava
scholastic sledgehammer
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Reged: 10/06/07

Loc: Wells, Maine 04090
Re: Celestron VX Mount Tracking Test new [Re: Jeff2011]
      #5794469 - 04/12/13 01:31 PM

Did you "unsync" after ASPA? I haven't yet had the opportunity to run ASPA on my AVX due to bad weather but from what I've read this is a critical piece...I'm just not sure if the ASPA routine prompts you to unsync after polar alignment or if you need to remember to do it??? -Tim

Quote:

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?




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Jeff2011
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Reged: 01/01/13

Loc: Sugar Land, TX
Re: Celestron VX Mount Tracking Test new [Re: zawijava]
      #5794662 - 04/12/13 02:34 PM

Quote:

Did you "unsync" after ASPA? I haven't yet had the opportunity to run ASPA on my AVX due to bad weather but from what I've read this is a critical piece...I'm just not sure if the ASPA routine prompts you to unsync after polar alignment or if you need to remember to do it??? -Tim





No. It just says it is done and does not prompt for anything else. I then go to replace a calibrated star and all calibrated slots are empty when I try to replace one. It is like an ASAP clears all of them. The previous alignment stars however are still there when I go to replace them.


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zawijava
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Reged: 10/06/07

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Re: Celestron VX Mount Tracking Test new [Re: Jeff2011]
      #5794849 - 04/12/13 04:25 PM

On page 30 of my AVX Manual under the heading "Updating your Star Alignment", directly following the ASPA routine it states: "Since the Polar Alignment process requires you to "Sync" the telescope on a bright star before you begin, it will be necessary to undo the sync before realigning". I would guess that since Sync is only helpful within a small area of the sync'ed star that if you DON'T unsync after ASPA then things won't be accurate as you move on to other areas of the sky. -Tim

Quote:

Quote:

Did you "unsync" after ASPA? I haven't yet had the opportunity to run ASPA on my AVX due to bad weather but from what I've read this is a critical piece...I'm just not sure if the ASPA routine prompts you to unsync after polar alignment or if you need to remember to do it??? -Tim





No. It just says it is done and does not prompt for anything else. I then go to replace a calibrated star and all calibrated slots are empty when I try to replace one. It is like an ASAP clears all of them. The previous alignment stars however are still there when I go to replace them.




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Jeff2011
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Loc: Sugar Land, TX
Re: Celestron VX Mount Tracking Test new [Re: zawijava]
      #5794990 - 04/12/13 05:30 PM

I guess I don't understand the need for the unsync. If I replace an alignment star with another, does that not imply an unsync of the star being replaced?

I did find out one interesting thing last night. If after an ASPA, I just do the calibration stars again without first replacing the alignment stars, the mount was way off when slewing to a target. It then would not let me replace an alignment star and I had to reboot the mount and start over. Did not try the unsync. Perhaps that is what the unsync command is for. If you get it so screwed up that you can't replace an alignment star anymore.


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zawijava
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Reged: 10/06/07

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Re: Celestron VX Mount Tracking Test new [Re: Jeff2011]
      #5795039 - 04/12/13 05:41 PM

My understanding is that Sync is a way to make goto's within a small area of the sky more accurate and ANY time you sync you should unsync prior to leaving that area and slewing to another area of the sky. For the time it takes to "unsync" after ASPA it's likely worth the added key strokes. -Tim

Quote:

I guess I don't understand the need for the unsync. If I replace an alignment star with another, does that not imply an unsync of the star being replaced?

I did find out one interesting thing last night. If after an ASPA, I just do the calibration stars again without first replacing the alignment stars, the mount was way off when slewing to a target. It then would not let me replace an alignment star and I had to reboot the mount and start over. Did not try the unsync. Perhaps that is what the unsync command is for. If you get it so screwed up that you can't replace an alignment star anymore.




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Jeff2011
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Re: Celestron VX Mount Tracking Test new [Re: zawijava]
      #5795223 - 04/12/13 07:01 PM

I am making the assumption that a synced star is one of the two alignment stars. Is that correct or is a sync something different?

What about the calibration stars? Do I need to recalibrate after changing the alignment stars? Do I need to recalibrate after a ASPA?

Thought I had this down. The more I think about it the more I confuse myself.


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zawijava
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Re: Celestron VX Mount Tracking Test new [Re: Jeff2011]
      #5795264 - 04/12/13 07:27 PM

I believe that AFTER doing the ASPA the only star that you've Synced on is the one used for the Polar Alignment. The alignment you did prior to the ASPA should not contain any Synced stars. After unsyncing from the ASPA star you can then recheck and/or redo your alignment and calibration stars. And from what I've read in this Thread, using a cross-hair centering eyepiece makes everything more accurate. I just picked one up on AM today and looking forward to using it. IMHO the "Sync" option causes more problems than not, possibly because users don't really understand it's shortcomings. -Tim

Quote:

I am making the assumption that a synced star is one of the two alignment stars. Is that correct or is a sync something different?

What about the calibration stars? Do I need to recalibrate after changing the alignment stars? Do I need to recalibrate after a ASPA?

Thought I had this down. The more I think about it the more I confuse myself.




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cn register 5
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Reged: 12/26/12

Re: Celestron VX Mount Tracking Test new [Re: Jeff2011]
      #5795275 - 04/12/13 07:33 PM

AIUI Sync is an offset in Ra and Dec that's applied to the position calculated from the mount model. You can sync on anything and it is independent of the align or calib stars.

There's no need to redo any calibration stars after sync.

Chris


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zawijava
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Re: Celestron VX Mount Tracking Test new [Re: cn register 5]
      #5795289 - 04/12/13 07:46 PM

But....what I understand from this Thread and the AVX Manual, if you do not unsync prior to slewing to another part of the sky, then your initial alignment and calibration, as good as they might have been, will likely be [much] less accurate and will get worse the further away from the "synced" star you go. Hence the requirement that you unsync. I believe Users are misunderstanding and misusing the "Sync" option. -Tim

Quote:

AIUI Sync is an offset in Ra and Dec that's applied to the position calculated from the mount model. You can sync on anything and it is independent of the align or calib stars.

There's no need to redo any calibration stars after sync.

Chris




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Jeff2011
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Re: Celestron VX Mount Tracking Test new [Re: zawijava]
      #5795330 - 04/12/13 08:26 PM

Tim,

Thank you very much for clarifying this. It makes sense to me again. Hopefully it will stay that way.

I can also say that I have seen the behavior you describe. The further away I slewed from the synced star used for ASPA, the less accurate it was.

Edited by Jeff2011 (04/12/13 08:32 PM)


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