AVX Mount I found out something interesting
Posted 26 March 2014 - 11:25 PM
I did a Two Star alignment on Dubhe and Mizar, then 4 calib stars on Dubhe, Mizar, Alioth, and Alkaid all stars in the Big Dipper. So far so good everything was hitting pretty much dead on in the 6mm EP. So my target was M101 (Pinwheel Galaxy) and with the wind gusts not as strong from the porch was a good chance to see how the Autoguider would operate under a little duress. I got 3 minute subs without trails, even with some wind gusts. I then got 5 minutes without star trails. Minor egg shapes on larger stars. I then got 10 minutes with egg shapes and no trails due to the wind gusts. So this mount can track just fine doing alignments near the intended target. I didn't slew all over the place of course, but I went out back and tried the same operation on Spica, Sirius, and Calib stars all near Spica and once again it worked. So as long as I'm out imaging a certain area I don't have to spend time slewing all over the place just for initial alignment, and ASPA worked using Dubhe out front, and Algorab out back. From M101 I was able to slew over to Mars without issue. Then down to Saturn once she rose over our shed.
So pretty happy with learning my way around the AVX mount so far. Now if I could just figure out how to drift align.
Posted 29 March 2014 - 02:30 PM
Posted 01 April 2014 - 02:12 AM
The GOTO accuracies on these mounts probably need to be measured in multiple units of arc minutes. IMO, that means that any form of polar alignment that you perform using simple GOTO pointing is NOT going to get you back night-after-night to the same alignment (if you switch among different sets of stars). So, no matter how well or how badly (within limits) you perform your ASPA you're almost always going to be off by multiple arc minutes (say +/- 5 arc minutes). So, it's entirely possible to throw up an example of polar alignment that has been done either badly or well and still (by luck) end up with an acceptable alignment.
However, go out at a different time or on a different date or use a different set of stars and you'll probably find that your alignment is either worse or better than last time (even if you do just as good of a job in aligning the stars).
IMO, the only way of getting around this variation is to do a very precise drift alignment OR by using plate solving software. With drift alignment you completely eliminating the errors that are inherent in your GOTO system (since it doesn't depend upon the GOTOs). With plate solving software you are actually calculating the true centers and pointing of your optical tube, not some point that is a DELTA from an approximate position driven to by an estimate provided by you GOTO mount.
Of course, as always, this is just my opinion and YMMV.
I don't think that ASPA is any more accurate that an alignment done with a GOOD polar scope. The only difference is that a polar scope will probably be more repeatable (if adjusted and used correctly). Of course, the calibration of your mount still needs to be done even if you use a polar scope and pointing accuracies will be affected by cone error so in the end it probably makes sense to combine all of those requirements into a single process and that's why ASPA is still a good thing to have (i.e. it combines mount calibration with an approximate but often entirely adequate polar alignment).
This isn't meant to be a criticism of your experience, I don't doubt what you saw. However, perhaps you should think of it as having won the lottery rather than as an absolute testament to the accuracy of ASPA.
Posted 01 April 2014 - 08:10 AM
Okay, but may I suggest that you may have just gotten lucky? Did you feel lucky?
The GOTO accuracies on these mounts probably need to be measured in multiple units of arc minutes. IMO, that means that any form of polar alignment that you perform using simple GOTO pointing is NOT going to get you back night-after-night to the same alignment (if you switch among different sets of stars).
Not my experience. My experience is that ASPA is remarkably consistent night after night assuming you don't do something funky during the goto alignment. Is it as good as a drift alignment? No. Is it good enough for five minute subs at least? Yes.
Posted 01 April 2014 - 05:43 PM
Bad polar alignment will cause a rotation around the guide star which will vary depending upon the distance from that guide star. As an example, if you have a polar alignment error of around 16 arc minutes and you are imaging an object at +35 degrees declination and your expose for 10 minutes using a 500mm focal length then you will "suffer" about one pixel of rotation on any star that is located up to 2 degrees away from your guide star (where I assume that one pixel is about 4.5 microns).
As you move closer (in the image) to the guide star the rotation will be less and as you move your image field nearer to the celestial equator (i.e. 0 declination) the error will also become less. In fact, given this same setup and when imaging near to the celestial equator the error will be less than one pixel (or the exposure time could be extended to about 12 minutes to produce the same one pixel of displacement). However, if you are imaging near to the pole (say at 80 degrees declination) then the error over a ten minute exposure time would be fully four times worse).
The above example was prepared by reference to the following link ("Determining Polar Axis Alignment Accuracy"): http://celestialwond...entAccuracy.pdf
So, yes, you can do good photography using just ASPA. However, the evidence of that (alone) doesn't prove ASPA's accuracy, nor does it prove ASPA's inaccuracy (it could either be "good" or "bad", however you want to define those terms).
In any case, as I said earlier, ASPA is a good thing to have so I'm not suggesting that it shouldn't be used.
Posted 02 April 2014 - 05:35 AM
Is it possible to get say 20 arcminute pointing with a Celestron mount? if not, then ASPA would not be able to get you closer than 20 arcminutes to the pole (other than luck).
Celestron claims 5 arcminute pointing (and 1.5' with a sync and precision slew). So that would be the limiting factor. Of course getting 5' off the pole is already pretty good.
Posted 02 April 2014 - 07:09 AM
If you want good GoTo accuracy in a small region of the sky, all you have to do is select two alignment stars in that region and it should do pretty well. But the OP is using stars in a local part of the sky not for goto accuracy in that area, but for polar alignment - and that is different.
So I agree there may be some element of good fortune here if the resulting polar alignment was good. I'm not surprised goto was good in the region, but ASPA needs a good model of the sky to work well. You don't need a good model of the sky to get good goto in a small section of it.