Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

PA: IPolar vs Sharpcap

  • Please log in to reply
10 replies to this topic

#1 ippiu

ippiu

    Vostok 1

  • -----
  • topic starter
  • Posts: 182
  • Joined: 06 Dec 2010
  • Loc: Italy

Posted 27 June 2019 - 02:56 AM

Hi.

 

I checked carefully last night PA with SC and IPolar and did some tests.

 

IPolar mounted inside Cem25P with no adapter. Calibrate more times, checked in every RA mount position (and obviously while i was rotating RA it slightly changed PA precision), changed gain up to 7X (at gain 0 just polar star can be seen).

 

SC: did a multi polar alignment routine. I checked PA from east to west through 15° step: as always happened, PA on east side is quite different from west side at about >1'...

 

Anyway...

 

I carefully polar aligned with SC (excellent: <30") in one position: IPolar way out of alignment (red circle outside of the edges of red cross).

I carefully polar aligned with IPolar (the red circle perfectly matched the red cross): SC says more or less 2.5-3' PA misalignemnt... 

 

Who says the truth?

 

I took some 2 min shots with the above 2 different PA methods: the shots taken after PA with SC are little little slightly elongated than those one taken with IPolar (but only when viewed at 100%)..

 

I have this idea: IPolar is mounted inside the RA axis, while SC did a PA with main scope and camera. So IPolar seems to be more solid and united with the mount: so it should be better to indicate the right PA than SC.

 

Any idea or tests?

 

Thanks



#2 DuncanM

DuncanM

    Mercury-Atlas

  • -----
  • Posts: 2657
  • Joined: 03 Nov 2009
  • Loc: Arizona Sky Village or the rain forest

Posted 27 June 2019 - 04:35 AM

Your SC guidescope and camera is probably moving a bit in it's mount. Which version of SC were you using and was it compensating for refraction?



#3 ippiu

ippiu

    Vostok 1

  • -----
  • topic starter
  • Posts: 182
  • Joined: 06 Dec 2010
  • Loc: Italy

Posted 27 June 2019 - 04:52 AM

Your SC guidescope and camera is probably moving a bit in it's mount. Which version of SC were you using and was it compensating for refraction?

The last version of SC: of course it was compensating for refraction.

 

I never used my guidescope and camera to PA with SC: i always used my main scope and main camera (it's obvious for me: less flexure and more stable)...


Edited by ippiu, 27 June 2019 - 04:53 AM.


#4 Dynan

Dynan

    Gemini

  • *****
  • Posts: 3186
  • Joined: 11 Mar 2018
  • Loc: NOLA

Posted 27 June 2019 - 06:08 AM

It may take some time, but can you verify each different PA with SC Drift Align, or PDH2's  Drift/Polar/Static Polar Align? I haven't done a check with SC DA, but my SC PA gives good images. (I have discovered innumerable other ways to ruin an image thusfar...which is why I have a dearth of images in my gallery, and no Astrobin.)

 

When I run SC PA and get a result under 10" error in SC (both sides of pier/mount, twice each side), my PHD2 Guiding Assistant sometimes says I'm off by a minute plus! I don't know which is lying, but the images are well guided/tracked.

 

Third method verification will at least give some validity to one method or the other. I haven't had sufficient, consistent clear sky to do any testing, since it's so rare anymore. I never felt like spending it on tests. I've relied on the image quality.

 

I only got my CEM60EC yesterday, but from what I seen and read here over the past year+, I trust SC PA more than I do a polar scope. Of course, I haven't used one yet. Nor have I run iPolar. wacko.png

 

Also, have you checked for Cone Error? SC has a free program called ConeSharp:

https://www.sharpcap.co.uk/conesharp

 

Might help...


  • AhBok and ippiu like this

#5 ippiu

ippiu

    Vostok 1

  • -----
  • topic starter
  • Posts: 182
  • Joined: 06 Dec 2010
  • Loc: Italy

Posted 27 June 2019 - 06:26 AM

When I run SC PA and get a result under 10" error in SC (both sides of pier/mount, twice each side), my PHD2 Guiding Assistant sometimes says I'm off by a minute plus! 

Sure, of course. Often i'm under 30" (excellent) in SC and after 5 minutes of PHD2 guiding assistant it tells me that i'm off 2' or 3' minutes.

I can't understand why and i don't mind it: maybe it's a phd2 guiding assistant error...


  • Dynan likes this

#6 DuncanM

DuncanM

    Mercury-Atlas

  • -----
  • Posts: 2657
  • Joined: 03 Nov 2009
  • Loc: Arizona Sky Village or the rain forest

Posted 27 June 2019 - 02:17 PM

Sure, of course. Often i'm under 30" (excellent) in SC and after 5 minutes of PHD2 guiding assistant it tells me that i'm off 2' or 3' minutes.

I can't understand why and i don't mind it: maybe it's a phd2 guiding assistant error...

The guiding assistant can be fooled by atmospheric refraction and seeing conditions. I suspect that a longer drift test is required.



#7 OldManSky

OldManSky

    Vanguard

  • *****
  • Posts: 2182
  • Joined: 03 Jan 2019
  • Loc: Valley Center, CA USA

Posted 27 June 2019 - 08:14 PM

I know this probably won't be a popular opinion, but...

It's my opinion that both SC and iPolar provide false accuracy in their readings.

The platesolving can't be any more accurate than the pixel scale of the camera being used, and neither can the positioning of the "put the star here" user interfaces.

In most cases they're much less accurate than the pixel scale of the camera being used (because of seeing, rounding in calculations of star centroids, and lots of other sources of error that all compound the total error).

 

The thing is, in most cases it doesn't matter.  They'll get you close enough so you have very little drift, and what drift there is can be easily guided out.  

Just don't take the amount they say you're off as "gospel."  It's an approximation from large error bar measurements.

 

If you really want to get really close to "nuts on" polar alignment, multiple iterations of drift alignment (and time) are the best ways to do it.

Most of the time, especially if your setup isn't permanent, you don't care about being "nuts on."  Close enough is good enough.  And that's what those tools give you.


  • psandelle and Dynan like this

#8 AhBok

AhBok

    Vanguard

  • *****
  • Posts: 2385
  • Joined: 02 Dec 2010
  • Loc: Lakeland, TN

Posted 27 June 2019 - 10:59 PM

Agree with Dylan. When I first began using SC PA, I tested it by drift aligning afterwards. I found I could not improve on SC with a 10-15 minute run. This was more than accurate enough since I guide. I would test both methods against a pre-determined drift time and see which is more accurate. If they are the same, try for 30min. For me, I’m confident in SC for field use, though if I were setting up a mount on a permanent pier, I would definitely want to drift align. It is still the gold standard for the most accurate PA, IMO.
  • psandelle and Dynan like this

#9 ippiu

ippiu

    Vostok 1

  • -----
  • topic starter
  • Posts: 182
  • Joined: 06 Dec 2010
  • Loc: Italy

Posted 28 June 2019 - 06:12 AM

Thanks to everybody.

 

I agree with you all.

 

I now starting using more IPolar only because in any scope position, in any mount position, i could check quickly polar alignment and refine it if necessary, even after any goto, before starting guiding or taking photo.

With SC you have to start the procedure from beginning, restore home position and always rotate the mount: that is, you cannot check PA every X goto or time...



#10 ippiu

ippiu

    Vostok 1

  • -----
  • topic starter
  • Posts: 182
  • Joined: 06 Dec 2010
  • Loc: Italy

Posted 30 June 2019 - 05:48 PM

 I would test both methods against a pre-determined drift time and see which is more accurate. If they are the same, try for 30min. 

Which of 3 phd2 drift methods do you suggest?

 

Static polar align?

Polar drift align?

Or simply drift align?



#11 PilotAstronomy

PilotAstronomy

    Vostok 1

  • *****
  • Posts: 100
  • Joined: 25 Sep 2017

Posted 28 August 2019 - 12:12 AM

I used to HAVE to do drift align to get good results because I couldn't see the pole from my spot. Started being able to get it done to "good enough" in 15 minutes of effort. It's one of those things that REALLY benefits from more time so if you're a permanent setup take a whole night to do it.


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics