Polar alignment not even close

Using SharpCap's Polar Alignment feature
Post Reply
Luke Spacehopper
Posts: 20
Joined: Thu Jun 06, 2019 4:58 pm

Polar alignment not even close

Post by Luke Spacehopper »

Just spent two frustrating hours checking and rechecking everything.

I'm pretty sure I've specified the correct coordinates in the settings, and also tried using the settings from the ASCOM mount.

On every occasion, what Sharpcap was telling me was WAY out from what my mount's polar scope or apps were telling me.

I've been through the troubleshooting page, none of them apply.

In the end I've decided just to go with the polar scope which isn't terribly accurate but I have more confidence in it. But, I would really love to get Sharpcap to work because I think it's an amazing feature.

What's the first thing to check?
cuivenion
Posts: 225
Joined: Wed Feb 15, 2017 7:37 pm

Re: Polar alignment not even close

Post by cuivenion »

You have to make sure the polar scope is aligned with the mount to get any accuracy at all. What apps are you using that disagree with Sharpcap?
BlackWikkett
Posts: 377
Joined: Sat Jun 23, 2018 8:48 pm
Contact:

Re: Polar alignment not even close

Post by BlackWikkett »

Hi Luke Spacehopper,

First thing I'd check is that your PC and Mount are on the same and correct time. Can't tell you how many times this has bitten me. Usually all is well since the mount will sync from GPS but every now and then the mount will have trouble locking onto GPS. And I never check until there's a problem. Also the guide scope doesn't have to be perfectly colinear with the mount. This is resolved when you slew the mount 90 deg in RA the PA tool calculates the center of rotation and determines the adjustments provided. Have a look at what I'm working with from home in this thread. viewtopic.php?f=23&t=2524 This tool works very well.

-Chris
cuivenion
Posts: 225
Joined: Wed Feb 15, 2017 7:37 pm

Re: Polar alignment not even close

Post by cuivenion »

BlackWikkett wrote:
Sat Jul 18, 2020 1:28 am
....... Also the guide scope doesn't have to be perfectly colinear with the mount. This is resolved when you slew the mount 90 deg in RA the PA tool calculates the center of rotation and determines the adjustments provided.....

-Chris
Hi, if this is about what I was saying I was referring to the polar scope built into the mount. I agree what you were saying about times, I'd add that making sure all the software being used for astro should have the same longitude/latitude. Another thing that might make a difference is is the refraction compensation in Sharpcap if that is being used.

It'd be good to know what results Sharpcap is giving and what he getting from his polar scope, because Sharpcap has always been much more accurate for me.
User avatar
admin
Site Admin
Posts: 5233
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Polar alignment not even close

Post by admin »

Hi,

the most common cause of incorrect results is the movement of something relative to the amount as you rotate – perhaps the guide scope or camera can flex slightly. The way to check this is to rotate gradually in 15° increments. You will see horizontal dark red crosses appear as you rotate further – they should all be within a few pixels of each other if everything is secure. If something is loose then they will form a trail across the screen.

If that all looks good then perform your polar alignment in SharpCap, then rotate them out by hand after you've completed the alignment routine. The stars should all appear to rotate in circles around the point that SharpCap marked as the NCP. If that's happening then your polar alignment is correct – if they are rotating about some other point that obviously some way away from the NCP then something odd is going on which will have to investigate further.

Cheers, Robin
Luke Spacehopper
Posts: 20
Joined: Thu Jun 06, 2019 4:58 pm

Re: Polar alignment not even close

Post by Luke Spacehopper »

Thanks everyone for the suggestions. I really appreciate it.

Robin, I will go through your recommendations when the clouds next part which, in the UK, will probably be around November 2021 at this rate...

When I say WAY out for the polar alignment, I mean that, after going through the routine, then looking through the polar scope, Polaris is at the very edge of the polar scope's field of vision, almost out of it sometimes, nowhere near where it should be. I'm using this app to tell me where it should be: https://play.google.com/store/apps/deta ... w&hl=en_GB (yes, I do have the option ticked to invert the image for looking through a scope). I don't believe my polar scope could be that far out of whack, surely?

I think there could be something in the mention of time and location.

Please bear with me here: I'm about to go through everything I can think of to determine whether there could be something set up incorrectly. If it helps, my NEQ6 is connected directly via a Lynx astro cable (https://www.firstlightoptics.com/lynx-a ... ounts.html), using EQMOD, on a Windows 10 laptop, and a Skywatcher 130PDS for the scope.

EQMOD
- Location: set as Lat N 51 46 0.31, Long W 00 55 52.8, elevation 79m (I thought long and hard about whether it's a security risk listing the location here, but figured it's ok - my name isn't really Luke Spacehopper btw!)
- Time: I can't see anywhere to set this? This thread makes interesting reading: https://stargazerslounge.com/topic/1135 ... -in-eqmod/. As a result of going through it, I noticed it mentioned Windows settings so I've set my Windows time zone to automatic although it was already showing the correct time.

SHARPCAP
- Location: set in the Polar alignment tab as Long 0.93 W, Lat 51.77 N (it would be SO useful to be able to specify this as hours/mins/secs and degrees btw)
-- I also tried using the location from the ASCOM mount (would this be EQMOD?) with no success
-- I didn't use the option to correct for atmospheric refraction - would this really make such a difference?
- Time: I can't see anywhere to set this?

ASCOM
I can't see anywhere I would set the time or location in the bewildering set of ASCOM modules? I've been through the excellent Light Vortex guide at https://www.lightvortexastronomy.com/tu ... -ciel.html, which I used to set all this up to begin with, and also through the ASCOM site looking for ideas, but still can't find anywhere I would set the time or location.

So, my questions are:
* Is it possible for a polar scope to be so far wrong? If so, what would be a good check for this?
* Are my location settings correct in EQMOD and Sharpcap?
* Is there anywhere else I should be setting time and location? Especially in Windows or ASCOM?
* If I set Sharpcap to get location settings from ASCOM, would this mean EQMOD? Because if it really does mean ASCOM, that begs the question, where would I set this in ASCOM?
* Would atmospheric refraction make such a difference?

Any/all recommendations welcome!

Thanks.
cuivenion
Posts: 225
Joined: Wed Feb 15, 2017 7:37 pm

Re: Polar alignment not even close

Post by cuivenion »

Hi, You need to set your location in EQMOD and whatever planetarium software you're using, they need to be exactly the same. You input the longitude and latitude for EQMOD in EQMOD Setup, not in ASCOM, it's in the light vortex guide.

If you weren't using the refraction correction in Sharpcap then you didn't need the location inputted for Sharpcap. You should use it though. The nearer to the horizon Polaris is the more difference it makes.

If you set Sharpcap to get the location from an ASCOM mount its getting it from EQMOD in your case because thats the mount thats using ASCOM.

It is possible your polar scope is badly out of alignment, here's a guide to calibrating it:

https://www.myastroscience.com/polarscopecalibration
User avatar
admin
Site Admin
Posts: 5233
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Polar alignment not even close

Post by admin »

Hi,

the maximum error caused by refraction at UK latitudes is about one minute of arc, so that isn't going to be the cause of the problem.

From the point of view of polar alignment, having your latitude and longitude accurate in SharpCap is unnecessary – the coordinates get used the two things, firstly the refraction correction if that is turned on and secondly working out whether the movement should be up/down/left/right. Neither is necessary for the arrow to point to the correct alignment position. Having your position accurate to within 1° will be quite sufficient.

Similarly the time will come from your computer so there's no need to enter it. If the time on your computer was hours out then SharpCap might tell you to move the mount left when it should be asking you to move it down, but as long as you make adjustments so as to put the star in the target you will still get the correct polar alignment (you will just need to ignore the directional prompts).

It's certainly worth checking the alignment of the polar scope. I'd also be tempted to try a third method of alignment (drift alignment in PhD? The alignment tool in AstroTortilla) to see whether that agrees with SharpCap or the polar scope.

Thanks, Robin
Luke Spacehopper
Posts: 20
Joined: Thu Jun 06, 2019 4:58 pm

Re: Polar alignment not even close

Post by Luke Spacehopper »

Excellent suggestions, thanks Robin. I found the troubleshooting page that I think your previous reply was based on so I will be going through that first. Thanks again.
Post Reply