Telescope Reviews: Celestron VX mount

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DaveJ
Pooh-Bah

Reged: 01/07/05

Loc: NE Ohio

Re: Celestron VX mount [Re: bunyon]
#5649953 - 01/29/13 10:01 AM

Quote:

This is off-topic at this point, I think, but my experience was, I'd do a polar alignment with the polar scope and be very close. I'd then do an ASPA and be a little closer or a little further, depending. Either way, I inevitably had to do a drift alignment.

Unless you've also done a 2+4 star alignment before the ASPA, then the ASPA can't possibly work correctly. An initial polar alignment with your polar scope is fine, if you want to take the time & effort to do it, but it must be followed immediately with the 2+4 alignment THEN the ASPA which will get you closer to polar alignment than the polar scope.

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bunyon
Carpal Tunnel

Reged: 10/23/10

Loc: Winston-Salem, NC

Re: Celestron VX mount [Re: DaveJ]
#5649996 - 01/29/13 10:26 AM

Dave that may be it. I don't do 2+4 - I just do the 2 star alignment. I'll do the full 2+4 next time out before trying ASPA. Thanks.

But my larger point is this: all that aligning and getting the computer stuff takes me far longer than sighting Polaris through the polar scope and making the necessary and trivial adjustments to go from Polaris to the pole.

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RTLR 12
Post Laureate

Reged: 12/04/08

Loc: Capistrano Beach, Ca

Re: Celestron VX mount [Re: bunyon]
#5650162 - 01/29/13 12:03 PM

A 2 + 4 star alignment followed by an "All Star" polar alignment takes less than 10 minutes. The "All Star" take less than 10 seconds.

Stan

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cn register 5
sage

Reged: 12/26/12

Re: Celestron VX mount [Re: RTLR 12]
#5650623 - 01/29/13 03:47 PM

I'd have to demolish my house to see Polaris.

I'm not sure that a full 2+4 alignment is needed but if you do a 2* alignment use a star on the same side of the meridian as the alignment stars.

Chris

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DaveJ
Pooh-Bah

Reged: 01/07/05

Loc: NE Ohio

Re: Celestron VX mount [Re: cn register 5]
#5650958 - 01/29/13 07:05 PM

Quote:

I'm not sure that a full 2+4 alignment is needed but if you do a 2* alignment use a star on the same side of the meridian as the alignment stars.

No, a 2+4 is required FIRST to give the mount the information it needs to build an accurate sky model in order to know the cone error. The cone error computed from the sky model is necessary for an accurate ASPA.

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orlyandico
Carpal Tunnel

Reged: 08/10/09

Loc: Singapore

Re: Celestron VX mount [Re: DaveJ]
#5651134 - 01/29/13 08:44 PM

i dont think you can even do an ASPA unless you do a 2+4... the menu option doesn't become available until enough stars have been added to the model.

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frozen.kryo
member

Reged: 01/28/11

Loc: East of the Sun, West of the M...

Re: Celestron VX mount [Re: orlyandico]
#5651182 - 01/29/13 09:16 PM

I am in a balcony type situation and my view is limited to the east.

I do an accurate 2-star alignment only, and ASPA works fine with good results as long as the star is near the celestial equator and the meridian.

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Ken Hutchinson
sage

Reged: 04/24/03

Re: Celestron VX mount [Re: EManT2200]
#5651427 - 01/29/13 11:55 PM

Quote:

Does anyone know if the AVX is designed to accept a polar scope, perhaps a CG-5 polar scope ?

I don't have one so all I can say is probably. The polar axis is hollow and the non-polar end is threaded. I take the caps off, center Polaris in the hole, and I am done. The only time I do an All Star is when I am testing for Celestron. You do not have to do a 2+4 to do a polar alignment. I never do more than 2+3 and I think you can do a polar with a 2+0 because I think the polar error is pulled from the initial two star alignment. I've never done any testing to see if the polar alignment is improved by adding calibration stars. Goto performance is, of course, but the polar alignment is based on a sync to the alignment star and I don't know if the polar alignment is affected by the general goto performance. It could be, I just don't know.

Ken

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cn register 5
sage

Reged: 12/26/12

Re: Celestron VX mount [Re: Ken Hutchinson]
#5651577 - 01/30/13 03:26 AM

From a quick test the polar align option is available after a 2* alignment, the ASPA option is active and can be done.

At least one calib star is required to get the cone error but that's not the same as the polar align error.

After all the fork mounts only have 2* with no option of adding calibration stars and they can do ASPA.

Of course more stars should give a more accurate alignment but that's not the same as it being essential.

Chris

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orlyandico
Carpal Tunnel

Reged: 08/10/09

Loc: Singapore

Re: Celestron VX mount [Re: cn register 5]
#5651580 - 01/30/13 03:32 AM

thanks Chris. i will have to revisit that menu structure again..

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freestar8n
Carpal Tunnel

Reged: 10/12/07

Re: Celestron VX mount [Re: orlyandico]
#5651727 - 01/30/13 07:43 AM

I don't know the details of this mount, but I do know some things about how the calibration affects AllStar. A key point is that if a mount is ever calibrated with a 2+n alignment - that calibration will overwrite the previous one. If you just do a 2-star alignment, it will use the calibration info from the previous session - but that info will be old and possibly stale. A key issue is if this mount has index switches like a cge, or just index marks. In order for the calibration to be reusable, the index marks would need to be set very accurately.

In short, the AllStar polar alignment accuracy is dependent on the calibration accuracy and there would likely be a win with a fresh 2+4 alignment. 2+1 alignments are probably much less accurate than 2+3, and may even be worse than doing a simple 2-star alignment and relying on old 2+4 calibration data.

So - this is somewhat convoluted, but these are some of the factors at work. You should be able to tell something about the expected AllStar accuracy based on the GoTo accuracy using the different methods.

Frank

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David Pavlich
Transmographied

Reged: 05/18/05

Loc: Mandeville, LA USA

Re: Celestron VX mount [Re: freestar8n]
#5651805 - 01/30/13 08:45 AM

It really doesn't take that long to do a 2+4, so you may as well do it and get a good polar alignment along with a good pointing model.

David

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bunyon
Carpal Tunnel

Reged: 10/23/10

Loc: Winston-Salem, NC

Re: Celestron VX mount [Re: David Pavlich]
#5651812 - 01/30/13 08:51 AM

David, I appreciate that for many it doesn't take long. For me it takes much longer (okay, minutes) than using the polar scope and, again, in my hands, didn't get better accuracy. I'm completely willing to believe that is user error. I have been polar aligning Super Polaris and later CG5 mounts with the scope since 1983. I gave it a half dozen tries and a few weeks with ASPA.

As for pointing accuracy, I don't use the Goto. I only upgraded from the SP so that I had an autoguide port.

I'm old is what I'm saying.

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WadeH237
scholastic sledgehammer

Reged: 02/24/07

Loc: Snohomish, WA

Re: Celestron VX mount [Re: freestar8n]
#5651892 - 01/30/13 09:37 AM

Quote:

If you just do a 2-star alignment, it will use the calibration info from the previous session - but that info will be old and possibly stale. A key issue is if this mount has index switches like a cge, or just index marks. In order for the calibration to be reusable, the index marks would need to be set very accurately.

This would be true if NexStar actually had a pointing model...but it doesn't.

The 2 alignment stars orient the mount to the sky. The calibration stars are used to determine cone error, which is a single value that represents orthogonality error. In an ideal world, a single calibration star would be sufficient. In practice, adding calibration stars just refines the single cone error value by averaging the results.

Since orthogonality error is determined by the physical difference between the scope's axis and mount's axis, it only changes if you physically change or move the scope relative to the mount.

There is no need to redo the calibration stars if you have not removed the OTA since the last calibration.

I routinely do a 2+4 alignment when I first set up the mount. And then if I power cycle without removing the OTA, I just power up and do a 2 star alignment with no calibration.

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freestar8n
Carpal Tunnel

Reged: 10/12/07

Re: Celestron VX mount [Re: WadeH237]
#5651970 - 01/30/13 10:18 AM

Quote:

This would be true if NexStar actually had a pointing model...but it doesn't.

I stand by my story. The 2+4 does more than just cone error, and a key item overlooked is the declination offset - which is tied to the index accuracy. For some reason people focus on cone and orthogonality - while the dec. offset is critical to a good alignment and benefits from the additional calibration stars.

Even if you don't move the mount - if there is a slight change in the dec. offset, it can have a big effect on both pointing and ASPA.

Doing a fresh 2+4, as suggested above, takes care of all these concerns, and doesn't take too long.

Frank

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WadeH237
scholastic sledgehammer

Reged: 02/24/07

Loc: Snohomish, WA

Re: Celestron VX mount [Re: freestar8n]
#5652227 - 01/30/13 12:30 PM

When I've queried Celestron (and folks familiar with the NexStar code), they have told me that the calibration only addresses cone error. If you have any better source of information, I would welcome clarification.

Also, my own anecdotal experience (since NexStar 4.0 was released) matches my assertion that calibration only needs to be done after removing/installing the OTA. For example, skipping calibration after setting up the mount and OTA freqently results in poor pointing performance. Skipping calibration after power cycling, etc. without removing the OTA never results in poor pointing performance. This is true on both my CGEs and my CG5s, so I have examples both with and without home switches.

I will note that my information applies to the NexStar 4.x hand controller. The NexStar+ controller that ships with the VX has more capabilities (perhaps even to include a real pointing model someday), but I didn't think that the current version of the firmware takes advantage of that.

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freestar8n
Carpal Tunnel

Reged: 10/12/07

Re: Celestron VX mount [Re: WadeH237]
#5652254 - 01/30/13 12:48 PM

Again, I stand by my story. Any pointing system I know of, including a formal mount model, includes a term representing the dec. offset - and it is hard to get accurately from only a few stars. For some reason, this value is overlooked and people focus on Cone error. But right next to Cone error in the NexStar menu, is the declination offset. There is also RA offset, but it is less critical. If you want good pointing - you need both cone and dec. offset to be accurate. Try changing the dec. offset by 0.5 degrees...

The CGE has switches to mark the dec. offset - and they are very repeatable. I don't know what the VX has - which is why I mentioned it. If it has manual alignment marks to set the axes - they should be done with care so the dec. offset from the previous calibration is accurate.

Frank

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WadeH237
scholastic sledgehammer

Reged: 02/24/07

Loc: Snohomish, WA

Re: Celestron VX mount [Re: freestar8n]
#5652280 - 01/30/13 01:02 PM

Interesting.

It should be simple enough to test on a CG5. You could do a 2+4 alignment, then power off the mount. Release the clutches and move the dec axis 30 degrees or so from the index mark. Finally, start up the mount, do a 2 star alignment with no calibration and then test pointing accuracy.

I will try this when I get a chance. Unfortunately, I'm not expecting clear skies any time in the next month or two (gotta like that Seattle overcast). And when I do get a clear night, I have a new AP1600 that needs first light - and that one is my priority.

If anyone else wants to try it, it would be interesting to see the result.

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rmollise
Postmaster

Reged: 07/06/07

Re: Celestron VX mount [Re: WadeH237]
#5652321 - 01/30/13 01:24 PM

Not sure what not starting from the marks, which are only there do give the mount a start position reference--it has no switches--will do. But I can tell you the calibration stars make a considerable difference in go-to accuracy. Whether this is "always" or "only when you remove/replace OTA" depends on the particular mount and OTA. With an SCT on a mount that's used portably, the answer is "always."

Edited by rmollise (01/30/13 03:13 PM)

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Stew57
Carpal Tunnel

Reged: 05/03/09

Loc: Silsbee Texas

Re: Celestron VX mount [Re: rmollise]
#5652338 - 01/30/13 01:33 PM

The dec ofset is to get the initial stars closer. Once the model is mazde it has no relevance. The calibration (which is actually just one as the 4 are averaged together) will make a big difference as the mount WILL have cone error.

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