AVX and ZeQ25: PE/goto accuracy info
Posted 31 January 2014 - 11:05 PM
For whoever, like me, was wondering how these mounts compare, here are responses from Ioptron and Celestron technical support.
Anyone with actual field data can challenge or verify the numbers?.
Both mounts seem in the "ballpark"...but interestingly the AVX got a solid response vs a vague one from the "makers" of the Z.
If these numbers hold true the AVX has a bit better "spec"... but both seem to be in the same ballpark.
There has to be a benchmark or baseline to judge these specs for the consumer's benefit, like "Goto accuracy for our XYZ mount is < "abc" at 50% maximum load and +/- x arc sec maximum polar alignment error..." similarly for PE. Such important specs?. Sure, there is variability in the manufacturing process, components, etc but there has to be a "maximum".
Ioptron's response about the ZEQ25:
PE : "...we don't spec PE for ZEQ25...from customer's feedback or
posts in forums, the best PE reported was 9 arc sec peak peak,
and average PE is about 15-30"
Goto accuracy : "...goto accuracy should be a few minutes".
Celestron's response about Advanced VX:
PE : +/- 5 arc seconds
Goto accuracy : +/- 10 arc minutes
"PEC will further improve the above numbers"
Knowing there was no issue with the mount's accuracy at the spec level, it was easy for me to decide for one of the mounts basically because I had the chance to "play" with one in advance of the purchase and got the confidence I was going to like it....hope this post helps another one too!.
Posted 31 January 2014 - 11:18 PM
Posted 31 January 2014 - 11:34 PM
+/-5 arc second PE, that's a spec for a few thousand dollar mount.
Yes. And 10 arcsecond goto is a spec for no mount ever built. Whoever answered your question doesn't have the information to answer it correctly.
Ioptron's response is probably correct for both mounts. The thing is, those are NOT specs. Specs are promises. If yours doesn't meet them, that's your problem because neither manufacturer promises even those larger figures.
Posted 01 February 2014 - 12:26 AM
The AVX is in fact quite a good mount, but that Celestron rep did a disservice by quoting numbers like that.
Going back to the original question you tried to get answered I suspect you would find that for PE the results will be similar enough between both mounts as to not worry about it. For Go-To accuracy I am a big fan of the NexStar go-to system (so long as you give it a proper 2+4 star calibration). I found it very accurate indeed on my CG5 (the predecessor to the AVX).
Even so I like the mechanical design of the ZEQ25 better. It is my opinion that for visual observing or planetary imaging the AVX is the better mount, for long exposure deep-sky imaging the ZEQ25 is the better mount (for mechanical design reasons). What is your intended use for the mount? I would guess by the hunt for PE numbers you want to do long exposure AP....?
Posted 01 February 2014 - 12:35 AM
The charitable view could be that the Celestron rep was confused about what mount you where asking about perhaps.
Posted 01 February 2014 - 11:06 AM
Anyway, that's the CGEM DX. There is the CGEM below that, and the AVX below the CGEM. I'd found a couple statistics over the last week that indicated the CGEM P2P PE w/o pec was anywhere from 15-60 (no numbers with pec, sorry), so I would imagine the AVX can't be any better than that. That is, however, without PEC. I did find some numbers that showed the Atlas EQ-G drops to ~10 P2P PE w/ PEC, and since that mount is largely the same as a CGEM, I imagine you would get something similar with PEC programmed. Throw in guiding, and your performance should be even better.
Posted 01 February 2014 - 12:06 PM
PE : +/- 5 arc seconds ( 10 peak to peak)
Goto accuracy : +/- 10 arc minutes ( 20 peak to peak)
Average PE: +/-7.5 to 15 arc sec (15 to 30 peak to peak)
Best reported PE: +/- 4.5 arc sec (9 peak to peak)
Goto accuracy: "a few minutes"
Apologies for the confusion!!!!
Posted 01 February 2014 - 12:19 PM
It does seem that you can greatly mitigate PE by programming PEC, though.
Posted 01 February 2014 - 01:40 PM
Celestron doesn't even quote sub-arcsecond for the CGE Pro with PEC. That's a mount with a guaranteed +/- 3" before PEC. They only claim "may be reduced to +/- 1" with PEC." This is a mount with Pittman motors that don't have the dreaded 8/3 gear harmonic.
Anyone who claims their guiding graph "flatlined" simply is using too short a guide scope. That is not an indication of PE accuracy.
Bottom line there are no magical diamond-in-the-rough bargains in the mount business, particularly in the Chinese mount business. I have learned this good and hard. Any wishful thinking or attempts to prove otherwise are doomed to fail. One must accept these mounts for what they are.
Posted 01 February 2014 - 05:01 PM
EDIT: Actually, let me correct that. It isn't +/-1 with PEC, it is "flatlined" with PEC and guiding. I'm honestly not sure what the guy means by flatlined, however it is better than the intrinsic +/-5 PE the mount starts out with.
I received an email from Celestron today. I asked a few questions, one of them regarding PE for the CGEM and CGEM DX. According to Celestron, they claim +/-5 PE for both mounts, as well as the CGE Pro. Now, I read that to mean 10arcsec P2P PE, not 5arcsec P2P PE. So, +/-1arcsec with PEC+guiding is still ~2arcsec P2P PE... (just to make sure were on the same page.)
Posted 01 February 2014 - 10:18 PM
Posted 01 February 2014 - 11:42 PM
Its easy to get low PE. Declination guiding is a whole other ball of wax..
In any case I'd really love to see where Celestron states - in writing - that they rate the CGEM for ±5". That would probably be grounds for a class action lawsuit
Posted 02 February 2014 - 01:01 AM
(The other issue is backlash but that is more easily manageable regardless of the mount - there just tends to be more of it in mounts of this price range vs multi thousand $ mounts)
too bad there isn't some sort of "spec" for declination performance. If I have a problem guiding with the Chinese mounts (and I've owned them all practically) it has always been in declination. Magically, never had any problems with a Tak.
I am sure you could document things like typical backlash amount and a measure of static friction for DEC (if that is the correct thing to measure for stiction). Of course that type of thing gets into rather technical detail and given manufacturers do not tend to want to measure typical PE it seems unrealistic to expect those DEC figures. One could dream though.