Confusion

[raymo]

I have a couple of interlinked questions for gurus out there.
When a 2 or 3 star alignment is completed on my HEQ5, and the error is
shown as just a few seconds for both axes, does that mean that the polar
alignment is also very good? or just that the mount knows where it is pointing?
If a polar alignment and a star alignment are needed, which should be done first?
Whichever is done second, wouldn't the adjustments stuff up the one
done first?
If one did a drift align first, how would one access the Go To? then doing
a star align would surely stuff up the drift align adjustments.
raymo

[billdan]

Hi Raymo,

I would do the 2 star alignment first that gets the mount tracking and goto's working.
After that drift align ( meridian and low on horizon) adjust your AZ/EL bolts and when you are satisfied, park the scope, turn it off.
Turn back on and redo your 2 star alignment.

In my situation (mount fixed in the Obs) I always align the first star by hand with the clutches loose, looking through the Telrad. Then just a minor tweak with the arrow keys after locking the clutches.
This way the second star is always spot on and my SPEC and the Skywatcher internal sky model is correct.

Regards
Bill

[raymo]

Thanks Bill. When doing the second star align after restarting the mount,
wouldn't the adjustments made stuff up the drift alignment?
raymo

[Camelopardalis]

raymo, the star alignment just affects the goto accuracy. If you've got good polar alignment having drift aligned, just try a 1-star alignment (after parking and reboot) and slew around to a few objects around the sky and see if the object is in the FOV...

[billdan]

What Dunk said Raymo, the only thing that affects polar alignment is the AZ/EL bolts or someone kicking the tripod by mistake.

The two star alignment is only for goto's and the three star alignment compensates for cone error. Cone error affects how accurate a meridian flip is.


Bill

[raymo]

Thank you both.
raymo

[Camelopardalis]

Bill makes a good point wrt the cone error...you can see how much you're may be when you've drift aligned on one side of the meridian, then flipping it to the other side. With any luck, the difference will be negligible.

Btw, another technique to consider raymo when drift aligning is the DARV method. You'll find much better descriptions in a Google search but essentially you start your camera running (say) a 30 second exposure and slew your scope west for 15 seconds followed by 15 seconds going east (or vice versa). When you are polar aligned, the star trails will be lines...i.e. the Stars have retraced their steps when the slewing is reversed. If your polar alignment is off, you'll get a V-shape. You need to do this both near the horizon and meridian to highlight the error in altitude/azimuth as with conventional drift alignment of course.

[raymo]

Hi Dunk, As you may know, I 've been in this game a very long time, and
know my way around drift aligning, and I keep my cone error negligible
by checking it from time to time. It's the Go To era that has stumped me.
My problem is it seems to me that a star alignment followed by a drift alignment is doomed to failure. Any adjustment made to either axis
during a drift align immediately negates all the work put in to get a
decent star alignment. On top of that, if a single star align is attempted
at the end, and the target is found to be outside a low power EP F.O.V.
any adjustment made to bring it back into the field will also stuff up the
polar alignment.
I followed Billdan's suggestions to the letter last night three times.
Each time, after rebooting, a further 2star align ruined the polar alignment. I tried checking the 2 star alignment after doing a drift align, and it was a mile out, as I expected it would be.
On all three tests the target ended up way outside the F.O.V. of my
lowest power EP [40mm]. It's a bit like the uncertainty principle; I can
have great P.A. or great Go To, but not both.
raymo

[kens]

You are right that a drift alignment will destroy the star model. But a subsequent star alignment cannot change the polar alignment if you don't touch the alt/az bolts. The star model might want to tell you that you have a PA error (Maz and Mel) but you can ignore that if you trust your drift alignment.
Its just as (or more) likely that the reported PA error after a star alignment is due to inaccurate centring of the stars in the eyepiece during the star alignment (including not clearing any backlash).

[raymo]

Thanks for your observations Ken. I understand all that you are saying.
I am careful with centreing stars, and always approach using up and right movements to minimise backlash. If after all routines are completed
I end up with Go To where the target is outside the F.O.V. so be it. The only time that it might be a real problem is with very faint targets. I would have to rely on the inaccuracy being always the same amount, and in the same direction.
raymo

[Camelopardalis]

I only suggested it as a means to test your polar alignment after completing the star alignment, so you don't need to dismantle your optical train.

I've not had much luck with the synscan polar alignment, though it got me started on the road. Before long frustration set in with wildly different figures being returned with subsequent iterations, and this was while using a reticle eyepiece. I ended up seeking alternatives.

From what I understand, the 1-star alignment just applies a time/RA offset and activates tracking. If you have good polar alignment and give the handset accurate time and coordinates, it shouldn't miss by far.

Certainly is a weird one raymo.

[Eden]

Even at conservative focal lengths (900mm), I've never been able to achieve absolutely spot-on GoTo using the SynScan alignment routine.

Using your guiding camera in conjunction with the bullseye overlay in PHD will get you closer than doing it visually. You can do this by defocusing the star so that it fills the inside circle of the bullseye overlay -- that'll get you right in the middle assuming your optical train is centered on your objective.

You can improve on the accuracy of the Synscan alignment even further by using the EQMOD n point alignment model in conjunction with AstroTortilla plate-solving. By syncing the mount half a dozen or more times this way, slewing to a target will see it very close to the middle of the FoV if done correctly. The more points you add to your model in various parts of the sky, the more accurate your GoTo will be.

This is probably not applicable to you Raymo, but it should be said that an overloaded or poorly balanced mount will throw your GoTo accuracy.

[raymo]

Thankyou all for your contributions, all welcome. My confusion was
started by the fact that the error shown at the end of each iteration
of a star alignment is described in the Synscan manual as "Polar
Alignment Error". It would appear that that is not what it actually is.
Eden, my HEQ5 generally carries around 10.5kg, and you're right, I
balance it carefully with a little weight acting against the drive.
Well. I'll try out some of the suggestions and see how I go.
raymo

[raymo]

Well, I'm still confused. I have spent three sessions doing nothing else but "darving" and star aligning, and still cannot achieve both polar alignment and reasonable "Go To" simultaneously. I have tried several
different routines. Start off with a 1 or 2 star align, then do a precise
DARV on both axes, resulting in really good tracking, and then do another
I or 2 star align [ignoring the error readout by not adjusting the Alt or Az] and thereby destroying the DARV]. After the final star align
the Go To can't put a target close to my finder's F.O.V. let alone an
eyepieces one. If the target outside the finder's F.O.V. is bright enough to be visible, then obviously I can centre it, but if it is not visible, then just as obviously, I can't locate it, especially as the target is not always
off in the same direction, or by the same amount. So, in summary, I can get great tracking, or good Go To, but not both.
raymo

[glend]

Personally I do not obsess about alignment perfection. Guiding cures all ills.

[multiweb]

Good PA will minimise field rotation and improve guiding as every correction in DEC does impact on your RA guiding so getting it right is important. This is like good focus. Also underestimated.

[raymo]

Hi Glen and Marc, I heroically :lol: manually guided for nearly 50 years, so I've been around the traps, as the saying goes. My current gear is the last I shall use before having to finally give it all up. I don't have an autoguider, or an O.A.G. and don't have enough back focus to use an O.A.G. anyway, so am limited to the mount's tracking capability. When I have good P.A. I can manage 100 sec subs with some longer acceptable ones interspersed among them. I'm happy with that, but just can't get useable Go To as well, just one or the other.
raymo

[barx1963]

Ray
Just reading this post, I have underlined a part. When Synscan reports a polar alignment error, it is a Polar Alignment Error. The Synscan looks at how accurate the alignment was compared to its ideal and males an estimate as to how much PA is out. It is then up to you to decide if you want to adjust the PA.

Malcolm

[raymo]

Thanks Malcolm, Being as Synscan calls the error "Polar Error" that was what I thought it was in the first place. I think that the reason I started to
wonder, was that I got significantly different max tracking times after
2 star alignments; maybe 40 secs after one alignment, and 75 after another, even though the final error readouts were down to a few
seconds each time.
I'll do a really accurate 2 star alignment, and then do a DARV routine.
In theory the DARV should show really good P.A. [that assumes that the
Synscan readout is accurate]. It never occurred to me before that you
could use either routine to check the accuracy of the other.
raymo

[barx1963]

The way I have been doing it (and this seems to work for me, others may do it differently)
1 Setup the mount as accurately as I can.
2 Do a 2 star alignment. This is only to check how close the Synscan thinks my Polar Alignment is by checking the result at the end.
3 Go through the Synscan Polar Alignment routine. I only do this step if the error is over 30' on both RA and Dec otherwise I skip to step 5.
4 Do a 3 Star alignment. This is really only to get reasonable accurate pointing so I can be confident the scope is pointing where I want.
5 Drift align using the PHD routine (obviously you cannot do this unless you have a suitable autoguiding or similar camera.
6 Do a final 3 star alignment. Ignore what Synscan says about my Polar Alignment at the end of it, if PHD drift is OK, I am happy with that.
Doing this I seem to get pretty good PA and targets are always on the chip of my DSLR and usually very near the centre.

Malcolm

[raymo]

Thanks Malcolm, I'll try that routine, using Darv in place of PHD.
raymo

[barx1963]

Ray
Just to clear, I am no expert, dobs are more my thing but I am having fun learning all about these EQ beasties.

Malcolm

[raymo]

I've been using EQ mounts for 57 yrs, but am not handling the
digital/electronic/ computer age at all well.
raymo

[kens]

I wonder if the problem lies in doing a 2-star rather than a 3-star alignment? A 3-star also accounts for cone error, so if you do have any significant cone error a 2-star will give poor results.

[raymo]

Hi Ken, I am well aware of cone error, and check mine regularly, as I think my 8" f/5 plus extras is close to the limit the vixen size dovetail bar can handle without a bit of distortion. I do find I have to make minor adjustments each time I check it. I will however do a 3 star, and see if it helps.
raymo