Celestron VX mount
Posted 30 January 2013 - 07:43 AM
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.
Posted 30 January 2013 - 08:45 AM
Posted 30 January 2013 - 08:51 AM
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.
Posted 30 January 2013 - 09:37 AM
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.
Posted 30 January 2013 - 10:18 AM
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.
Posted 30 January 2013 - 12:30 PM
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.
Posted 30 January 2013 - 12:48 PM
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.
Posted 30 January 2013 - 01:02 PM
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.
Posted 30 January 2013 - 01:24 PM
Posted 30 January 2013 - 01:33 PM
Posted 30 January 2013 - 02:21 PM
My suggested test is a response to Frank's assertion that the dec offset is both set by calibration and important to goto.
His statements do not match what I've been told and observed, at least for the CGE and CG5. I am curious to see if it's possible to demonstrate that the calibration affects more than just cone error.
I am planning on trying it myself to satisfy my curiosity. But like I said, it probably won't be any time soon. If anyone else has an interest, it could be something to try.
Understanding how these mounts actually work seems like a good thing.
Posted 31 January 2013 - 03:58 AM
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.
I actually disagree with this assessment. The dec. offset is very important because if it is wrong, the error in dec. couples into the measurement of RA - and both are wrong. On the other hand, an error in the RA index does not couple into dec., so it is less critical. The dec. error causes the measured separation of two stars to be way off - which would make the two-star alignment very unhappy. And the calibration stars don't act as a single star - they sample the error around the sky just like a more elaborate mount model.
On the topic of how to get best ASPA with this mount - if it doesn't have index switches I would align it to the marks carefully, and for best results I would use a fresh 2+3 or 2+4. If you don't want to take much time - it may be better to use a 2-star alignment that relies on a prior 2+4, rather than a fresh 2+1, which may be inaccurate because it didn't sample enough across the sky.
Posted 31 January 2013 - 07:37 AM
Posted 31 January 2013 - 08:27 AM
The marks give a starting position. BUT...it doesn't even matter what that position is _as long as you use it every time_ you can make your marks wherever you want them.
+1 to that, Unc! I've proven that with my CGEM DX - both in RA and DEC.
Posted 31 January 2013 - 12:30 PM
That said, the last 4 points in a 2+4 alignment each measure cone error against the "backdrop" of the two star alignment. In that sense, this is really just a 3 point alignment with the last point being an average of 4 measurements. Why would the 4 measurements of the cone error vary? One cause is mount flexure as the OP eluded to. In principle you could use some of the extra data points in the 2+4 to back out the mount flexure terms. The legacy handcontrol did not have the computational prowess, the code space, or even sufficient RAM to do that kind of computation. The NS+ has enough of all three (I think) to do that. Look for something like this to be available in the future.
I was the original poster and I was questioning the ability of calibration stars to cancel out mount flexure.
Posted 31 January 2013 - 03:42 PM
Although the 2+4 doesn't have an explicit model for flexure and atmospheric refraction, it does accomodate more parameters than just cone, and a key one is the declination offset. The resulting pointing allows some "mush" and distortion across the sky to help reduce the error with the 4 stars. For my c11 on cge, and including stars down low and refracted, there would have to be some "mush" in the pointing to result in the 3-5' all sky accuracy that can be achieved. I have never seen a good write up of this stuff, but I studied it in detail some time ago.
So - I think there is already some accomodation for non-rigidity and refraction in the model, and it does very well with just 6 stars. I expect that when the new stuff comes out, with more horsepower, more stars, and an explicit model - it will do even better.
Posted 31 January 2013 - 11:55 PM
Posted 01 February 2013 - 12:05 AM
Posted 01 February 2013 - 09:15 AM
I have been looking at the EdgeHD 800 and AVX combination as I am hearing good things about both. It looks like the OTA, EQ Head and Tripod break down into more manageable peices for transport and I am assuming the AVX should be able to handle the 8 inch at F10 for reasonable exposure lenghts. (30 sec unguided and 2 min guided)
I would like to know if my assumptions are accurate. Would the Edge800 and AVX be a decent replacement for a fork mounted Nexstar 8 with a wedge?
Posted 01 February 2013 - 09:22 AM
Posted 01 February 2013 - 04:07 PM
Assemble on site. Line up marks. Point counterweight shaft over leg pointed north.
Power on and do 2+4calib
Do an ASPA if you are going to hook a camera up.
Re-do 2+4 from the alignment menu (not powering off).
Go-to your target.
Find nearby bright star.
Set speed to 6 or 7 and slew over to that star.
Set speed to 3 to get it right in the center
Sync on that star.
Go-to your original target.
Good to go for the night? Total elapsed time, 10 minutes or less.
Anyone here do it differently?
Posted 01 February 2013 - 04:30 PM
Posted 01 February 2013 - 05:59 PM
Level is put scope down where it was last time.
No need to do a second 2+4 alignment after ASPA.
No need to sync on bright star, use Precise align if the object isn't in the FOV.
It feels like 10 minutes but probably takes a bit longer, still it isn't a race.
Posted 01 February 2013 - 07:03 PM
As to setup time... You can transport it with the mount attached to the tripod as it is light enough to do so. That's how I do it. Rides nicely on the shoulder that way. Just take off the weight bar.
In the field from taking it out of the trunk, putting it together, balancing the scope, aligning to viewing is under 15 minutes for visual and about 3 trips. Trips being from trunk to viewing site with mount/tripod and counter weight being one trip, OTA the second trip, and battery plus EP's and folding table the third. This would be the same for AP setup.
As I am just now learning ASPA since I am getting into longer guided AP I can't say how long that would take for someone who's been there and done that.
The 8" Edge rides real smooth on it with the mount not even straining and that is with a Stellarvue 10x60 and SBIG ST-ic guider setup, Canon 450D DSLR, and a Stellarvue 8x50 RACI finder.
I am 100% sure I can side by side mount my 102mm ES APO and the 8" for visual with zero problem, 85% sure I can do the same for AP, 80% sure I could put up the 127mm APO and the 8" for SBS visual, and 50/50 that I could put up my 127mm APO and the 8" for AP. Since the CG5 will take a C11 for visual no problem. My only worry is that the 127mm is a sail in the wind and may not balance well with the 8".
Posted 01 February 2013 - 10:03 PM