Stellarium accuracy

[Simon Dunsmore]

Hi Folks, 

So, after spending the last couple of cloudy weekends setting up my mount to work with Stellarium, I finally got a break in the clouds last night and decided to try it for real. 

Everything seemed to work well apart from the accuracy. The scope slewed to near the target but not to it. I realigned the star into the eyepiece and used ctrl 3 to sync it with stellarium several times but still not accurate. 

Before I started I used a polemaster to polar align. I was wondering, when Stellarium starts up it shows the scope centred on polaris, but actually it's the mount that is centred and not the scope. Does this mean that once I'm polar aligned that I need to put polaris in the eyepiece before using stellarium or are my inaccuracies down to something else or both? 

Thanks for your help 

Simon 

[Spile]

I am not sure if it works in the same way as I only really use Stellarium as a desktop application but my push to app of choice (SkEye) does "catch up" with itself occasionally. The insta-align function keeps things in check but sometimes you think the app has lost calibration but it hasn't. For example I was looking at Eta Cas last night. I then checked Polaris and it seems off beam. I ignored that and put Tegmine (Cnc) into the search. I followed the directions and hit the target first time without having to use the finder! 

[Tiny Clanger]

9 hours ago, Simon Dunsmore said:

Hi Folks, 

So, after spending the last couple of cloudy weekends setting up my mount to work with Stellarium, I finally got a break in the clouds last night and decided to try it for real. 

Everything seemed to work well apart from the accuracy. The scope slewed to near the target but not to it. I realigned the star into the eyepiece and used ctrl 3 to sync it with stellarium several times but still not accurate. 

Before I started I used a polemaster to polar align. I was wondering, when Stellarium starts up it shows the scope centred on polaris, but actually it's the mount that is centred and not the scope. Does this mean that once I'm polar aligned that I need to put polaris in the eyepiece before using stellarium or are my inaccuracies down to something else or both? 

Thanks for your help 

Simon 

Just a guess, but have you given Stellarium your position accurately, or just gone with the nearest town ?

Heather

[Simon Dunsmore]

Hi Heather, thanks for your reply. Yep, I used the coordinate from the GPS on my phone. 

[Skipper Billy]

Did you put your longitude in as a + value - Kent is East of the meridian line.

If you look on the location page of Stellarium is the red dot showing where you live??

 

[Tiny Clanger]

Well that eliminates one possibility ! One thing to be very careful of is (being not far from the meridian) it is easy to mix up W and E when inputting the longitude ...

Next simple checks ( beyond which I will have exhausted my knowledge, as EQ mounts and polar alignment are a foreign language to me 🙂  ) do you have your altitude set correctly ... not that you are up a mountain in Kent, but likely a few tens on m above sea level . Finally  is stellarium picking up the correct time from your computer , with the right time zone set ?

Heather

[Tiny Clanger]

2 minutes ago, Skipper Billy said:

Did you put your longitude in as a + value - Kent is East of the meridian line.

If you look on the location page of Stellarium is the red dot showing where you live??

 

Beat me to it by a whisker ! 🙂

Heather

[fifeskies]

How big is the error in the pointing accuracy. (full moon is 30 seconds wide)?

 

If it is a relatively small error , it may be down to using the J2000 co-ordinate system , while clearly in the sky you are setting to the current co-ordinates.

When you click on a target Stellarium will give both sets in the object description. (if they are both selected in the options)

A small offset may be showing the difference between the two.

(I think there is a setting somewhere to choose which co-ordinate system Stellarium uses)

 

Also you should be able to align the mount axis and the telescope axis by small adjustments to the mounting screws at the dovetail, loosen and nudge then retighten. Better quality dovetails have less wiggle error.

Edited January 26, 2021 by fifeskies

[Simon Dunsmore]

3 hours ago, Tiny Clanger said:

Beat me to it by a whisker ! 🙂

Heather

Hi, Really appreciate the collective brain power on this. I found one discrepancy. I had entered the lat and long in decimal format thinking that the degree and minute format would update automatically. 

I've attached a screenshot of the coordinates, where do I find the + and - 

Also still not sure about polaris needing to be in the eyepiece when stellarium is showing the telescope rectical on polaris. 

Thanks again folks 

[Tiny Clanger]

3 minutes ago, Simon Dunsmore said:

Hi, Really appreciate the collective brain power on this. I found one discrepancy. I had entered the lat and long in decimal format thinking that the degree and minute format would update automatically. 

I've attached a screenshot of the coordinates, where do I find the + and - 

Also still not sure about polaris needing to be in the eyepiece when stellarium is showing the telescope rectical on polaris. 

Thanks again folks 

OK, I know this !

In my daytime guise as a geocacher I often need to convert all manner of co-ordinate types to solve puzzles, and this is the page I generally use:

https://www.gpscoordinates.eu/convert-gps-coordinates.php

not least because it usually can automatically work out what system your input co-ords are , which saves brain cells for a fool like me.

Heather

[Tiny Clanger]

Oh, and your screenshot doesn't show ?

[Simon Dunsmore]

Hi thanks, I'll take a good look at this. Let's try again with the screen shot 😁

[Simon Dunsmore]

21 minutes ago, Tiny Clanger said:

OK, I know this !

In my daytime guise as a geocacher I often need to convert all manner of co-ordinate types to solve puzzles, and this is the page I generally use:

https://www.gpscoordinates.eu/convert-gps-coordinates.php

not least because it usually can automatically work out what system your input co-ords are , which saves brain cells for a fool like me.

Heather

Heather, 

Thanks for that link, looking at my gps coordinates and what I have entered in Stellarium, I need to amend things. 

My GPS coordinates 

51.0881474 Lat

1.1256632 Long

Therefore am I right in thinking these should be entered as:

51 5 17.3

 1 7 32.4

The only problem is that the website list them both as N. 🤔 

[Simon Dunsmore]

OK so I answered my own stupid question. Durrrr

The screenshot below shows the options for my location. Can I ask which format I use? 

[Tiny Clanger]

That's the app version is it? Ive always use the desktop version which actually tells you what you are looking at!

I think stellarium uses the middle one degrees, minutes and seconds, then decimals of seconds, that your conversion here only has one tenth of a second while stellarium has two digits after the decimal point shouldn't matter ... stick a '5' on the end of both N and E if inputting it to stellarium requires an extra digit, the difference is tiny .

By the way, I'd suggest deleting your image showing the co-ords (and I'll do the same when you have got your problem sorted) , if you click on the show location on map button on the converter you will see how accurately it could lead someone to your door !

Heather

[Simon Dunsmore]

8 hours ago, Tiny Clanger said:

That's the app version is it? Ive always use the desktop version which actually tells you what you are looking at!

I think stellarium uses the middle one degrees, minutes and seconds, then decimals of seconds, that your conversion here only has one tenth of a second while stellarium has two digits after the decimal point shouldn't matter ... stick a '5' on the end of both N and E if inputting it to stellarium requires an extra digit, the difference is tiny .

By the way, I'd suggest deleting your image showing the co-ords (and I'll do the same when you have got your problem sorted) , if you click on the show location on map button on the converter you will see how accurately it could lead someone to your door !

Heather

Hi Heather, 

That's really helpful, thanks for your time and effort on this. 

I just need a clear night to test it out 😁. 

Good idea on deleting the images. Will do once I've set up Stellarium. 

Thanks again and stay safe 

Simon 

[Spile]

The SkEye developer has said that accuracy issues relate to the magnetometer and the fact that they are sensitive to "noise" and need calibration before use. This would apply to Stellarium.

Quote

"One thing to note is that the magnetometer is prone to a lot of noise. SkEye filters the noise using algorithms similar to a moving average. This introduces some delay between OTA movement and the position shown on the screen. That is why you need to wait for the drifting to settle down, before making an alignment or observation."

He recommends carrying out the figure of 8 dance and have released a calibration app with instructions.