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Stellarium Accuracy in Alt Az


Dellis

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I've added a levelling base to my Dob to get better accuracy using the azimuth setting circle, I'd previously had accuracy issues and attributed this to the base not being level hence the modification. Anyway I get out this evening, the first semi clear sky since I finished the base, carefully levelled the base turning the spirit level to ensure everything was done to reduce any chance of errors all good so far. I then fired up Stellarium and looked up the Alt Az for Betelgeuse and adjusted the pointer on my scope base to match the azimuth then spun the scope round to Merak read off the azimuth referred back to Stellarium only to find it was a couple of degrees out. I went back in and double checked the time and location I had set on Stellarium and it was and still is spot on. I repeated this throughout the sky and it was the same story.

I'm back inside now having spent the evening trying to understand the problem, re levelling the base, re checking Stellarium, re aligning and all to no avail, I've even checked the accuracy of my degree circle which is spot on. I then came to the conclusion Stellarium was wrong opened up Skychart checking location and time against Stellarium and both are identical. I then looked at a few alt az coordinates for various objects and found a discrepancy between the two programs with Skychart showing 4 degrees higher azimuth reading than Stellarium exactly what I'd seen earlier with the scope after aligning to Polaris. Thus Stellarium appears to be showing incorrect alt az coordinates.

The obvious answer is to use Skychart but I find the program very clunky when compared to Stellarium so would prefer to find out what's going wrong.

I have another machine running Stellarium and that shows exactly the same alt az as machine number one????

Any ideas?

Off to bed now....

Dave

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It could be that your home co-ordinates are not spot on in Stellarium to give an accurate AZ position , and the fact that objects track across the AZ grid with time.

EQ co-ordinates are fixed with ref to the sky whereas AZ are fixed relative to the terrestrial position of the observer.

HTH.

Steve.

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It could be that your home co-ordinates are not spot on in Stellarium to give an accurate AZ position , and the fact that objects track across the AZ grid with time.

EQ co-ordinates are fixed with ref to the sky whereas AZ are fixed relative to the terrestrial position of the observer.

HTH.

Steve.

Hi Steve,

The GPS coordinates are spot on and are set exactly the same in both Skychart and Stellarium.

Dave

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Skychart showing 4 degrees higher azimuth reading than Stellarium

Sorry but can you clarify that statement, azimuth is the 0-360deg reading around the horizon, altitude the reading from horizon to zenith. So when you say 'azimuth is higher' I take it you mean 'azimuth is greater' and not literally 'higher' in the sky?

There don't appear to be any current bugs relating to incorrect Alt-Az on the Stellarium bug tracker, so might be worth searching/posting on their support forum, http://sourceforge.net/p/stellarium/discussion/search/?q=azimuth , which also has a link to the bug tracker if that doesn't help.

I'd be very surprised if such a major bug had gone unnoticed, but who knows? I'd be more inclined to assume it is something more subtle about the configuration of one or other of the two applications that is producing different results. As someone who has dealt with a lot of 'bug' reports in software down the years, I can tell you that if you really think it is a bug, the more evidence you can provide that it is not human error or misconfiguration, the better. Things you would be best off proving to your own satisfactionL

- Have set your altitude properly?

- Have you made an error in lat/long, e.g. if you needed to convert between decimal degrees and dd mm ss or vice versa that is any easy way to get it wrong, but I'd have thought it unlikely with a 4 degree error.

- Start by aligning the az circle to Polaris as that should be fairly close to zero (obviously not exactly) rather than aligning to one star you have looked up and then navigating to another. If the error is different (you can mentally account for the current az offset of polaris easily enough), then that might point you in the right direction as to the source/

- Try using Skychart to calibrate the dob and then look for some other targets with it. At least that 100% proves that Stellarium is giving you duff information rather than operator error.

- Are you running the two programs on the same computer? If not, check that they have the correct date, time and daylight savings settings.

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Sorry but can you clarify that statement, azimuth is the 0-360deg reading around the horizon, altitude the reading from horizon to zenith. So when you say 'azimuth is higher' I take it you mean 'azimuth is greater' and not literally 'higher' in the sky?

There don't appear to be any current bugs relating to incorrect Alt-Az on the Stellarium bug tracker, so might be worth searching/posting on their support forum, http://sourceforge.n...arch/?q=azimuth , which also has a link to the bug tracker if that doesn't help.

I'd be very surprised if such a major bug had gone unnoticed, but who knows? I'd be more inclined to assume it is something more subtle about the configuration of one or other of the two applications that is producing different results. As someone who has dealt with a lot of 'bug' reports in software down the years, I can tell you that if you really think it is a bug, the more evidence you can provide that it is not human error or misconfiguration, the better. Things you would be best off proving to your own satisfactionL

- Have set your altitude properly?

- Have you made an error in lat/long, e.g. if you needed to convert between decimal degrees and dd mm ss or vice versa that is any easy way to get it wrong, but I'd have thought it unlikely with a 4 degree error.

- Start by aligning the az circle to Polaris as that should be fairly close to zero (obviously not exactly) rather than aligning to one star you have looked up and then navigating to another. If the error is different (you can mentally account for the current az offset of polaris easily enough), then that might point you in the right direction as to the source/

- Try using Skychart to calibrate the dob and then look for some other targets with it. At least that 100% proves that Stellarium is giving you duff information rather than operator error.

- Are you running the two programs on the same computer? If not, check that they have the correct date, time and daylight savings settings.

1. Yes I mean greater - it was late...

2. Set at 38 metres which is correct for my location

3. Checked lat/long

4. I did set to Polaris initially then susequently set to other objects - should have made that clear

5. I haven't used Skychert to calibrate the dob but the differences I was seeing between Stellarium and Skychart were in line with those I was seening between the dob and Stellarium - I will try with Skychart but the earliest, cloud permitting, will be the weekend

6. Stellarium and Skychart are running on the same machine a Macbook Air - all settings are identical; lat/long, time, date, altitude (Ihave a second Macbook which has exactly the same settings as the Air which I looked at later on to compare Stellarium on both machines and they were in sync)

Thanks for the input and if anyone else can think of anything else to check please let me know.

Dave

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Just typed the above and realised that I’m typing on my work laptop which is a Windows machine and I have one of my Macs with me so I’ve cross referenced the machines and found where the problem is.

Stellarium on the Windows machine agrees with Skychart on the Mac.

Stellarium on the Windows machine DOES NOT agree with Stellarium on the Mac.

Before I did this comparison I double checked the setting in all three programs and they are all identical so it looks like the issue is with Stellarium on my Macs and I’ve no idea how I sort that one!

Any ideas?

Dave

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Dave,

Don't know why my font is so small for this reply (looks small as I type) but I'll chip in anyway, for the dedicated or for those with great eyesight. I always found Stellarium and CdC to be spot on for the Alt/Az. However, when i used my Wixey with the new 16" I found it did not work well. I then realised becase the location of various objects such as RACI and Telrad were positioned such that i placed the Wixey off center, it threw it off. This was probably not helped, mind you, by not levelling the 16" well. When I then found a less convenient spot on the on the Dob, but on the 'crest' of the curve of the OTA, it worked.

Worth trying maybe.

Barry

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So, sounds like a problem with Stellarium on the Mac - sorry can't help much (i use windoze and linux).

I would check around the area of the time / timezone settings, though. A few degrees might be due to being one hour out.

If you have the Timezone plugin, it might be worth checking/playing there (Configuration panel, Plugins).

Callum

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