5 Degree limit - Suggestions

[AdsOrion]

Hiya's,

Been using SharpCap for a good while now - I've just moved to a new place with nice views to the skies... until I noticed where the few trees are placed..

image_2025-11-16_173204981.png (356.86 KiB) Viewed 1122 times

Quick, dirty sequator trail with dslr on tripod

My last place had a similar issue, trees and power lines obstructing the pole - SharpCap was able to work around that as expected, but it's significantly more obstructed here.

The 5 degree limit of SharpCap's polar alignment - what's the limitation?
I'm yet to have a go at PHD drift aligning etc - but would be interested to see perhaps an 'experimental' feature that would allow for a wider platesolve-polar align range
I expect that results may not be as good, but anything would be an improvement from the current situation.

I've seen a few forum posts mentioning other programs that platesolve between RA offsets etc to try and figure out the alignment, but all seem to be deprecated and gone now.

Would there be a decent solution here? PHD drift guiding will be the next attempt, and I've seen the [name forgotten] technique of slewing back and forth in an axis during an exposure to see the angle subtended by star trails ....

Extending the 5 deg limit of the platesolver would be amazing if possible to be implemented.

I usually do the PA with a zwo120 on my guide scope, which already gives a limited field, but I'm running a 6" RC for the primary imaging so changing the setup for alignment wouldn't be deal.

Would appreciate any tips or ideas!

Thanks all,

Clear Skies


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[admin]

Hi,

my belief is that the errors in the polar alignment calculations (at least the way SharpCap does it) will build up significantly as the distance from the pole increases far beyond the current 5 degree limit. To be honest, I haven't worked through the mathematics to prove it, but since you are essentially working out the center of rotation of the image as seen by the camera, having that a *long* way out of view would be expected to make the result less accurate and therefore the polar alignment less accurate.

There are alternative approaches that work the mathematics in a different way and can give valid results without a view of the sky - they usually involve 3 GOTO movements and subsequent re-aligns or a measurement of declination by plate solving at various points as you rotate the scope in RA at a lower dec position. Unfortunately the former is only accurate to the extent that the original GOTO movements are accurate and the latter is made harder by the need to flip the mount when crossing the southern (or northern) meridian.

I don't really have plans to add either of those other approaches to SharpCap - I think there is a plug in to NINA that offers one or both of those approaches, and that's free software so easily available to everyone.

cheers,

Robin

[AdsOrion]

Hi Robin,

Thanks for your advice.

I was thinking it would be something along the lines of increasing error wrt dec.

I'll have to check out the NINA plug-ins, thanks for that!

I've only tried a 3-star align with my mount (CEM-26) once, due to SharpCap's PA being so robust - I'm pretty sure the hand controller returns values for PA-error and maybe Alt/Az-error.. It would be very time consuming to try and dial that in, very iterative..
The way that iOptron mounts communicate between handcontroller/mount and PC is certainly a downside, seems very hard to tell whats happening, and they don't seem to sync on all operations - eg. no option to do alignments via the software, the program generally being pretty clunky etc

I can fully appreciate your hesitation to implement my earlier suggestions, I'm sure it would result in a lot of hassle for users and become a fair amount of work - although I will still express my interest in such an idea ('advanced setting -> unlock/change PA dec limit' [?])

I might have a go at building a script to work out the d.RA/PS/err technique - Thanks for your suggestions!


Much appreciated!

All the best,

Aidan

[admin]

Hi Aidan,

I forgot to mention that there is already at least one script for SharpCap that does this - see this forum threads

viewtopic.php?t=2331

Unfortunately that is a bit old and uses an external plate solver (rather than SharpCap's internal one) and the R programming language. There is a discussion here about a possibility to update it to avoid R and use the internal plate solver - viewtopic.php?p=50264#p50264

cheers,

Robin

[Jean-Francois]

Hello Robin and Aidan,

Yes, I'm preparing a script for the polar alignment "fast everywhere"
But, I was the last 10 days not at home and I did no progress

One important point concerning the 2 or 3 stars alignment ... that is NOT a polar alignment.
The 2 or 3 star alignment finds a primitive pointing model, so that the telescope can perform a GOTO to a target.

Concerning the old script "no_polaris_no_problem" ... it was working in the past, but in the meantime some changes appear in ASCOM.
And it is a little bit annoying to install R-Studio and an extern plate solve software only for the calculation.

I found a solution to linearise the cosine function fit that does not need extern mathematical library.
Now I'm programming the "live" view of the deviation.

Regards,
Jean-Francois