EQ3-2 with Synscan - Accuracy

[OzDave]

Hi,

I've had an EQ3-2 mount with Synscan for nearly a year now and I'm starting to wonder about the general accuracy of the thing. I am fairly certain that it has a woeful periodic error but have not set about measuring it yet, though I am getting towards that.

What I'm finding a bit more annoying though lately is the general accuracy of the GOTO slews.

Generally speaking, I do a polar alignment using the polarscope and I'm pretty certain that I've done a reasonable job here as I can see polaris tracking around the polarscope circle over the course of a night.

I then do a 3-star alignment which mostly is successful. Last night I used a webcam with a graphically overlaid reticle to do the alignment. Chosen stars were Arcturis, Vega, Pollux. I started from a manually set HOME position and of course the first slew to Arcturis was off by a fair amount. After centring on the reticle, I would have expected the next slew to Vega to be a bit more accurate, which it was, but it still needed a bit of adjustment. Ok, that happens. Finally, the last slew to the opposite meridian to Pollux and again it was quite a bit off. After centring I thought the alignment would most likely fail, but it reported success.

After this, I swapped my webcam for my Canon EOS and did a slew back to Vega to attempt focus with the EOS. Alas, I could not even find Vega in the wider field of the EOS (compared to Webcam). I then slewed around to find Mars instead. Again, nowhere in sight. So I popped in a 25mm eyepiece and corrected the slew to get Mars in the middle, then put the EOS back. OK, I was able to focus on Mars.

Finally, I slewed to Bode's Galaxy, which I'd never seen before. I didn't check what the EOS was seeing, I just took a 60sec exposure as everything was too dim to see on the live view. Checking the exposure, afterwards I saw some fuzzy stuff resembling a DSO and figured that the slew must have been relatively accurate, although the placement of the DSO was not dead centre...more like North-East of centre on the 2/3rd division of the frame if that makes sense.

The whole purpose of the night was to test my autoguiding setup on various exposure lengths, so I took a few different subs and then packed up when some high level clouds blew over and put an end to things.

I then get inside and check my subs and I find that the slew to Bode's had missed completely and what I was actually looking at was the Cigar galaxy, which was very obvious form the shape and location of surrounding stars.

So what I'm wondering is, is this kind of slew "accuracy" typical for Synscan? To completely miss objects? Or does it happen because the FOV of the EOS is quite small on my 1500mm scope and it doesn't take much to miss? FYI, the EOS/Webcam are much like putting in a 5-10mm eyepiece in terms of FOV.

I have the capability to try out EQ-MOD, which I'll probably do next time I get an opportunity, but can anyone report whether EQ-MOD is likely to be any more accurate?

Any ideas why the Synscan slews might not be very accurate?

What can I check to locate the source of the problem?

Any advice much appreciated.

Regards,

David

[sgazer]

Have you tried aligning on 3 stars around (and reasonably close to) the DSO you are interested in? I find this works wonders with the HEQ5 with EQmod, but your mount and control is quite a bit different so it may or may not improve the situation.

[Macavity]

I always sensed the EQ3-2 was capable of recovering settings to ~0.1 Deg or better. I wonder if the GoTo version suffers from the same "problem" as it's Azimuthal siblings. Do you have it on a tripod (on grass etc.). I found that attaching my Az Synscan a SOLID pillar transformed the whole GoTo-ing experience... Only a thought though.

[beamish]

You could try making sure that the polar scope is correctly aligned to the mounts RA axis, tho it sounds okay, when you move from HOME to your first alignment star do you centre using the H/set or undo the clutches. If the former, try undoing the clutches and manually moving the star to centre( or as close as..), then proceed. Personally I've always found better accuracy with a single or two Star alignment rather than 3, tho I too now tend to use EQmod almost exclusively.

I used a single alignment with the h/set just a fortnight ago and the targets were just bang on every time right round the sky ! - I was very impressed !

[OzDave]

I've not tried aligning with stars specifically around an object of interest, although I guess I could try that. I thought the point of 3-star alignment was to provide reasonably accurate slews around the whole sky, overcoming any slight polar misalignment and cone error and other mount defects. But perhaps that is what I'm getting but I'm expecting too much in terms of accuracy.

The mount is on a solid concrete footing, so its not suffering from a soft surface like grass.

When I recenter slew targets during alignment, I use the H/C. I figured that if I use the clutches, the mount has no way to measure how far off the slew was. And isn't that the whole point of alignment - to slew to some targets and have the user "tell" the mount via correction how far it was off? Clearly, that isn't working that well, so I'll give the clutches a shot on the first correction, which I guess would essentially be pretending that the mount got it bang on from the first slew.

I think I'll give EQMOD a shot also and see if that is any more accurate. I guess there is a lot more you can do with a whole computer as opposed to a fairly simple microcontroller (in the SynScan h/c).

Has anyone done any comparisons of SynScan vs EQMOD performance / accuracy in the past? I guess there must be good reasons why many people use EQMOD, so it must be a lot better than the SynScan stuff ?

David

[beamish]

Eqmod will and does win hands down !! the accuracy improves with use !

Join the group here and find out more cos there's so much more to eqmod then merely improved accuracy

EQMOD : EQMOD - open source EQ mount control